General Description
The MAX6650/MAX6651 fan controllers use an
SMBus/I2C-compatible interface to regulate and moni-
tor the speed of 5VDC/12VDC brushless fans with built-
in tachometers. They automatically force the fan’s
tachometer frequency (fan speed) to match a prepro-
grammed value in the Fan-Speed Register by using an
external MOSFET or bipolar transistor to linearly regu-
late the voltage across the fan. The MAX6650 regulates
the speed of a single fan by monitoring its tachometer
output. The MAX6651 also regulates the speed of a sin-
gle fan, but it contains additional tachometer inputs to
monitor up to four fans and control them as a single unit
when they are used in parallel.
The MAX6650/MAX6651 provide general-purpose
input/output (GPIO) pins that serve as digital inputs,
digital outputs, or various hardware interfaces. Capable
of sinking 10mA, these open-drain inputs/outputs can
drive an LED. To add additional hardware control, con-
figure GPIO1 to fully turn on the fan in case of software
failure. To generate an interrupt when a fault condition
is detected, configure GPIO0 to behave as an active-
low alert output. Synchronize multiple devices by set-
ting GPIO2 (MAX6651 only) as an internal clock output
or an external clock input.
The MAX6650 is available in a space-saving 10-pin
µMAX®package, and the MAX6651 is available in a
small 16-pin QSOP package.
________________________Applications
RAID Desktop Computers
Servers Networking
Workstations Telecommunications
____________________________Features
♦Closed/Open-Loop Fan-Speed Control for
5V/12V Fans
♦2-Wire SMBus/I2C-Compatible Interface
♦Monitors Tachometer Output
Single Tachometer (MAX6650)
Up to Four Tachometers (MAX6651)
♦Programmable Alert Output
♦GPIOs
♦Hardware Full-On Override
♦Synchronize Multiple Fans
♦Four Selectable Slave Addresses
♦3V to 5.5V Supply Voltage
♦Small Packages
10-Pin µMAX (MAX6650)
16-Pin QSOP (MAX6651)
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
MAX6650/MAX6651
For pricing, delivery, and ordering information, please contact Maxim Direct at
1-888-629-4642, or visit Maxim Integrated’s website at www.maximintegrated.com.
EVALUATION KIT AVAILABLE
19-1784; Rev 5; 12/12
Ordering Information
µMAX is a registered trademark of Maxim Integrated Products, Inc.
+
Denotes a lead(Pb)-free/RoHS-compliant package.
MAX6650
VCC
SCL
10kΩ
SDA
GPIO0 OUT
ADD
GND
FB
TACH0
VCC
3V TO 5.5V
VFAN
5V OR 12V
CCOMP
10μF
SMBus/I2C
INTERFACE
GPIO1
LED
FAN
FULL ON
ALERT
Typical Operating Circuit
Pin Configurations appear at end of data sheet.
PART TEMP RANGE PIN-PACKAGE
MAX6650EUB -40°C to +85°C 10 µMAX
MAX6650EUB+ -40°C to +85°C 10 µMAX
MAX6651EEE -40°C to +85°C 16 QSOP
MAX6651EEE+ -40°C to +85°C 16 QSOP
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
2 Maxim Integrated
MAX6650/MAX6651
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VCC = 3.0V to 5.5V, TA= -40°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C and VCC = 5V.)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
VCC to GND..............................................................-0.3V to +6V
FB, TACH_ ..........................................................-0.3V to +13.2V
All Other Pins..............................................-0.3V to (VCC + 0.3V)
Output Voltages..........................................-0.3V to (VCC + 0.3V)
Maximum Current
Into VCC, GND, VOUT ...................................................100mA
Into All Other Pins ..........................................................50mA
Continuous Power Dissipation (TA= +70°C)
µMAX (derate 5.6mW/°C above +70°C) .....................444mW
QSOP (derate 8.3mW/°C above +70°C).....................667mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature .....................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
Soldering Temperature (reflow)
Lead(Pb)-free..............................................................+260°C
Containing lead(Pb) ....................................................+240°C
Input Low Voltage
Input Hysteresis VHYS 200 mV
Tachometer Threshold VTACH_ VFB + 1.0 VFB +3 V
12V fan, 0 < VFB < 9V
PARAMETER SYMBOL MIN TYP MAX UNITS
Output Source Current ISOURCE 50 mA
Output Sink Current ISINK 10 mA
Output Voltage Range VOUT 0.3 VCC - 0.3 V
VFB + 0.5 VFB +1.5
Tachometer Input Impedance RTACH_ 70 100 150 kΩ
Supply Voltage VCC 3.0 5.5 V
Supply Current ICC 10 mA
DAC Differential Nonlinearity 5LSB
Useful DAC Resolution 8bits
Feedback Input Impedance RFB 70 100 150 kΩ
Output Sink Current IGPIO_ 10 mA
CONDITIONS
Guaranteed monotonicity on FB (Note 1)
VOUT = VCC - 1.8V
Measured at FB (Note 1)
VOUT = 0.5V
IOUT = ±100µA
5V fan, 0 < VFB < 4.5V
0 < VFB < 9V
0 < VTACH < 9V
VGPIO_ = 0.4V
Full-on mode, IOUT = 0
V
0.8VIL(GPIO_)
Input High Voltage V
2VCC ≤3.6V
VIH(GPIO_) 3VCC > 3.6V
Pullup Resistor RGPIO_ 100 kΩ
TACHOMETER INPUTS (TACH_)
FEEDBACK (FB)
GENERAL-PURPOSE INPUTS/OUTPUTS (GPIO_) (Note 2)
POWER SUPPLY (VCC)
OUTPUT (OUT)
kΩ9.5 10.5Selects slave address 3Eh (Table 1)RADD
ADD External Pulldown Resistor
to GND
Selects slave address 36h (Table 1) (Note 3) µA-1 0ILADD
ADD Input Leakage
ELECTRICAL CHARACTERISTICS (continued)
(VCC = 3.0V to 5.5V, TA= -40°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C and VCC = 5V.)
TIMING CHARACTERISTICS
(VCC = 3.0V to 5.5V, TA= -40°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C and VCC = 5V.)
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
Maxim Integrated 3
MAX6650/MAX6651
Selects slave address 96h (Table 1)
Selects slave address 90h (Table 1)
CONDITIONS
VVCC - 0.05VIH(ADD)
ADD Input High Voltage
V0.1VIL(ADD)
ADD Input Low Voltage
UNITSMIN TYP MAXSYMBOLPARAMETER
ADDRESS SELECT (ADD)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
µs500Minimum pulse durationGlitch Rejection
kHz254fCLK
Clock Frequency
kHz0 400fSCL
SCL Clock Frequency
µs1.3tBUF
Bus Free Time Between Stop
and Start Condition
Hold-Time Start Condition tHD:STA 0.6 µs
µs1.3tLOW
Low Period of the SCL Clock
High Period of the SCL Clock tHIGH 0.6 µs
µs0 900(Note 5)tHD:DAT
Data Hold Time
Data Setup Time tSU:DAT 100 ns
ns20 + 0.1CB(pF) 300(Note 6)tR
Rise-Time SDA/SCL Signal
(Receiving)
Fall-Time SDA/SCL Signal
(Receiving) tF(Note 6) 20 + 0.1CB(pF) 300 ns
ns20 + 0.1CB(pF) 250ISINK < 6mA (Note 6) tF
Fall-Time SDA Signal
(Transmitting)
%-10 +10VCC = 5VfCLK
Clock Frequency Uncertainty
TACHOMETERS
GPIO2 (Note 2)
SMBus/I2C INTERFACE (Figures 3, 4)
VADD = 0.5V (Note 4)
SMBus/I2C INTERFACE (SDA, SCL)
µA-80 -40IADD
ADD Pulldown Current
VSDA = 0.6V mA6ISDA
Data Output Sink Current
VCC ≤3.6V V
2
V0.8VIL
Input Low Voltage
0 < VIN < VCC µA±1Input Leakage Current
VCC > 3.6V 3
VIH
Input High Voltage
mV200VHYS
Input Hysteresis
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
MAX6650/MAX6651
4 Maxim Integrated
Typical Operating Characteristics
(TA = +25°C, unless otherwise noted.)
240
245
255
250
260
265
3.0 4.03.5 4.5 5.0 5.5
INTERNAL OSCILLATOR FREQUENCY
vs. SUPPLY VOLTAGE
MAX6650/51-01
SUPPLY VOLTAGE (V)
FREQUENCY (kHz)
200
220
260
240
280
300
INTERNAL OSCILLATOR FREQUENCY
vs. TEMPERATURE
MAX6650/51-02
TEMPERATURE (°C)
FREQUENCY (kHz)
-50 050
100
VCC = 5.5V
VCC = 3.0V
2.0
2.1
2.3
2.2
2.4
2.5
FEEDBACK VOLTAGE
vs. TEMPERATURE
MAX6650/51-03
TEMPERATURE (°C)
FEEDBACK VOLTAGE (V)
-50 050
100
1.9
1.8
VCC = 5.5V,
VFAN = 5.5V, VFAN = 12.0V
VFAN = 12.0V, VFAN = 5.5V
VCC = 3.0V
1.80
1.85
1.90
1.95
2.00
2.05
2.10
2.15
2.20
3.0 3.5 4.0 4.5 5.0 5.5
FEEDBACK VOLTAGE vs. SUPPLY
VOLTAGE (DAC SET TO 35)
MAX6650/51-04
SUPPLY VOLTAGE (V)
FEEDBACK VOLTAGE (V)
VFAN = 5.5V
VFAN = 12.0V
2.0
2.4
2.2
2.8
2.6
3.0
3.2
3.6
3.4
3.8
3.0 3.4 3.8 4.2 4.63.2 3.6 4.0 4.4 4.8 5.0
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX6650/51-05
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (mA)
1.5
2.0
3.0
2.5
3.5
4.0
-40 0-20 20 40 60 80 100
SUPPLY CURRENT vs. TEMPERATURE
MAX6650/51-06
TEMPERATURE (°C)
SUPPLY CURRENT (mA)
VCC = 5.5V
VCC = 3V
TIMING CHARACTERISTICS (continued)
(VCC = 3.0V to 5.5V, TA= -40°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C and VCC = 5V.)
Note 1: For proper measurement of VFB, connect OUT and FB as shown in the
Typical Operating Circuit
.
Note 2: GPIO2, GPIO3, and GPIO4 only in the MAX6651.
Note 3: Guaranteed by design and not 100% production tested.
Note 4: For RADD component test purposes only.
Note 5: Note that the transition must internally provide at least a hold time to bridge the undefined region (300ns max) of SCL’s
falling edge.
Note 6: CBis the total capacitance of one bus line in pF. Tested with CB= 400pF. Rise and fall times are measured between 0.3 x
VCC and 0.7 x VCC.
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
µs
ns050
0.6
tSPIKE
tSU:STO
Setup Time for Stop Condition
Pulse Width of Spike Suppressed
PIN
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
Detailed Description
The MAX6650/MAX6651 use an SMBus/I2C-Compatible
interface to regulate and monitor the speed of
5VDC/12VDC brush-less fans with built-in open-collec-
tor/drain tachometers. Regulating fan speed propor-
tionally with temperature saves power, increases fan
life, and reduces acoustic noise. Since fan speed is
proportional to the voltage across the fan, the
MAX6650/MAX6651 control the speed by regulating the
voltage on the low side of the fan with an external MOS-
FET or bipolar transistor.
The MAX6650/MAX6651 each contain two internal con-
trol loops. The first loop controls the voltage across the
fan. The internal digital-to-analog converter (DAC) sets
the reference voltage for an internal amplifier (Figure 1),
which then drives the gate of an external N-channel
MOSFET (or the base of a bipolar transistor) to regulate
the voltage on the low side of the fan. As the reference
voltage provided by the DAC changes, the feedback
amplifier automatically adjusts the feedback voltage,
which changes the voltage across the fan.
The second control loop consists of the internal digital
logic that controls the fan’s speed. The MAX6650/
MAX6651 control fan speed by forcing the tachometer
frequency to equal a reference frequency set by the
Fan-Speed Register, the prescaler, and the internal
oscillator (see the
Fan-Speed Register
section). When
the tachometer frequency is too high, the value of the
DAC’s input register is increased by the regulator.
Once the DAC voltage increases, the analog control
loop forces the feedback voltage to rise, which reduces
the voltage across the fan. Since fan speed is propor-
tional to the voltage across the fan, the fan slows down.
2-Wire SMBus/I2C-Compatible
Digital Interface
From a software perspective, the MAX6650/MAX6651
appear as a set of byte-wide registers that contain
speed control, tachometer count, alarm conditions, or
configuration bits. These devices use a standard
SMBus/I2C-compatible 2-wire serial interface to access
the internal registers.
Pin Description
Maxim Integrated 5
MAX6650/MAX6651
FUNCTIONNAME
PIN
MAX6650 MAX6651
Tachometer Input. Used to close the loop around the tachometer.TACH011
—2, 3, 16 TACH2, TACH3,
TACH1 Tachometer Inputs. Used to monitor tachometers only.
GroundGND42
3 5 SDA 2-Wire Serial-Data Input/Output (open drain)
2-Wire Serial Clock InputSCL64
5 8 ADD Slave Address Select Input (Table 1)
General-Purpose Input/Output (open drain). Configurable to act either as an out-
put or as an input (FULL ON or general purpose).
General-Purpose Input/Output (open drain). Configurable to act as a general
input/output line or an active-low ALERT output.
General-Purpose Input/Output (open drain). Configurable to act as a general
input/output line, an internal clock output, or an external clock input.
Output. Drives the external MOSFET or bipolar transistor.
+3.0V to +5.5V Power Supply
Feedback Input. Closes the loop around the external MOSFET or bipolar tran-
sistor.
FB
VCC
OUT
GPIO2
GPIO0
GPIO196
710
11—
138
914
1510
—7, 12 GPIO4, GPIO3 General-Purpose Input/Output (open drain)
HEX
96
36
3E
90
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
6 Maxim Integrated
MAX6650/MAX6651
The MAX6650/MAX6651 employ three standard SMBus
protocols: write byte, read byte, and receive byte
(Figure 2). The shorter protocol (receive) allows quicker
transfers, provided that the correct data register was
previously selected by a write or read byte instruction.
Use caution with the shorter protocol in multimaster
systems, since a second master could overwrite the
command byte without informing the first master.
Slave Addresses
The device address can be set to one of four different
values. Accomplish this by pin-strapping ADD so that
more than one MAX6650/MAX6651 can reside on the
same bus without address conflicts (Table 1).
SMBus/I2C
INTERFACE
SMBus/I2C
INTERFACE
VCC
3V TO 5.5V VCC
SCL
SDA
ADD
GND
FAN SPEED
CONFIGURE
ALARM ENABLE
ALARM STATUS
TACH COUNT
COUNT TIME
GPIO DEF
GPIO STATUS
DAC
ADDRESS
DECODE
TACHOMETER
COUNT
CONTROL
LOGIC
8-BIT
DAC
10kΩ
VREF
GPIO
BLOCKS
(FIGURE 5)
GPIO0
OUT
FB
TACH0
GPIO1
10kΩ
90kΩ
90kΩ
10kΩ
MAX6650
MAX6651
ALERT
FULL ON
FAN
VFAN = 5V OR 12V
VOFFSET
Figure 1. Block Diagram
Table 1. Slave Address Decoding (ADD)
BINARY
VCC 1001 011
No connection (high-Z) 0011 011
10kΩresistor to GND 0011 111
ADDRESS
ADD
1001 000GND
Slave Address Command byte: Selects
which register you are
writing to.
Data byte: Data goes into
the register set by the
command byte (to set
thresholds, configuration
masks, and sampling rate).
Figure 2a. SMBus Protocol: Write Byte Format
Slave Address Command byte: Selects
which register you are
reading from.
Slave Address.
Repeated due to
change in data-flow
direction
Data byte: Reads from
the register set by the
command byte.
Figure 2b. SMBus Protocol: Read Byte Format
Data byte: Reads data
from the register com-
manded by the last
read-byte or write-byte
transmission; also
used for SMBus alert
response return address.
Figure 2c. SMBus Protocol: Receive Byte Format
S = Start condition Shaded = Slave transmission WR = Write = 0
P = Stop condition ACK = Acknowledged = 0 RD = Read =1
A= Not acknowledged = 1
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
Maxim Integrated 7
MAX6650/MAX6651
COMMANDS
8 bits
PACKDATA
8 bits
WR
0
ACKADDRESS
7 bits
ACK
COMMANDS
8 bits
WR
0
ACKADDRESS
7 bits
ACK
S P
A
DATA
8 bits
RD
1
ADDRESS
7 bits
ACK
S P
A
DATA
8 bits
RD
1
ADDRESS
7 bits
ACK
Slave Address
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
8 Maxim Integrated
MAX6650/MAX6651
SMBCLK
AB CD
EFG H
IJK
SMBDATA
tSU:STA tHD:STA
tLOW tHIGH
tSU:DAT tHD:DAT tSU:STO tBUF
A = START CONDITION
B = MSB OF ADDRESS CLOCKED INTO SLAVE
C = LSB OF ADDRESS CLOCKED INTO SLAVE
D = R/W BIT CLOCKED INTO SLAVE
E = SLAVE PULLS SMBDATA LINE LOW
LM
F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO SLAVE
H = LSB OF DATA CLOCKED INTO SLAVE
I = SLAVE PULLS SMBDATA LINE LOW
J = ACKNOWLEDGE CLOCKED INTO MASTER
K = ACKNOWLEDGE CLOCK PULSE
L = STOP CONDITION, DATA EXECUTED BY SLAVE
M = NEW START CONDITION
SMBCLK
A = START CONDITION
B = MSB OF ADDRESS CLOCKED INTO SLAVE
C = LSB OF ADDRESS CLOCKED INTO SLAVE
D = R/W BIT CLOCKED INTO SLAVE
AB CD
EFG H
IJ
SMBDATA
tSU:STA tHD:STA
tLOW tHIGH
tSU:DAT tSU:STO tBUF
K
E = SLAVE PULLS SMBDATA LINE LOW
F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO MASTER
H = LSB OF DATA CLOCKED INTO MASTER
I = ACKNOWLEDGE CLOCK PULSE
J = STOP CONDITION
K = NEW START CONDITION
Figure 3. SMBus Write Timing Diagram
Figure 4. SMBus Read Timing Diagram
Command-Byte Functions
The 8-bit Command-Byte Register (Table 2) is the mas-
ter index that points to the various other registers within
MAX6650/MAX6651. The register’s power-on reset
(POR) state is 0000 0000, so that a receive-byte trans-
mission (a protocol that lacks the command byte)
occurring immediately after POR returns the current
speed setting.
Fan-Speed Register
In closed-loop mode, the MAX6650/MAX6651 use the
Fan-Speed Register to set the period of the tachometer
signal that controls the fan speed. The Fan-Speed
Register is ignored in all other modes of operation. The
MAX6650/MAX6651 regulate the fan speed by forcing
the tachometer period (tTACH) equal to the scaled reg-
ister value. One revolution of the fan generates two
tachometer pulses, so the required Fan-Speed Register
value (KTACH)may be calculated as:
tTACH = 1 / (2 x Fan Speed)
KTACH = [tTACH x KSCALE x (fCLK / 128)] - 1
where the fan speed is in rotations per second (RPS),
tTACH is the period of the tachometer signal, fCLK is the
internal oscillator frequency (254kHz ±10%), and
KSCALE is the prescaler value (see
Configuration-Byte
Register
). Since the fan speed is inversely proportional
to the tachometer period, the Fan-Speed Register value
(KTACH) does not linearly control the fan speed (Table
3). Select the prescaler value so the fan’s full speed is
achieved with a register value of approximately 64
(0100 0000) to optimize speed range and resolution.
The MAX6651 may be controlled by an external oscilla-
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
Maxim Integrated 9
MAX6650/MAX6651
tor that overrides the internal oscillator (see
General-
Purpose Input/Output
). When using an external oscillator
(fOSC), calculate the Fan-Speed Register value with fCLK
equal to fOSC. Codes above F8h (1111 1000) are
allowed, but will not significantly decrease the frequency.
Configuration-Byte Register
The Configuration-Byte Register (Table 4) adjusts the
prescaler, changes the tachometer threshold voltage,
and sets the mode of operation. The three least-signifi-
cant bits configure the prescaler division used to scale
the tachometer period. Select the prescaler value so the
fan’s full speed is achieved with a register value of
approximately 64 (0100 0000) to optimize speed range
and resolution (see the
Fan Speed Register
section). The
fourth bit selects the fan operating voltage.
The fifth and sixth bits configure the operating mode.
The MAX6650/MAX6651 have four modes of operation:
full-on, full-off (shutdown), closed-loop, and open-loop.
In closed-loop operation, the external microcontroller
(µC) sets the desired speed by writing an 8-bit word to
the Fan-Speed Register (see the
Fan-Speed Register
section). The MAX6650/MAX6651 monitor the fan’s
tachometer output and automatically adjust the voltage
Table 2. Command-Byte Assignments
COUNT
x
SPEED 0000 0000
xCONFIG 0000 0010
x
READ
GPIO DEF 0000 0100
0001 0110 x
x
x
x
WRITE
x02h Tachometer count time
FFh
0Ah
00h
POR (DEFAULT)
STATE
COMMAND
GPIO definition
REGISTER
Configuration
Fan speed
FUNCTION
DAC 0000 0110 x x 00h DAC
ALARM ENABLE 0000 1000 x x 00h Alarm enable
ALARM 0000 1010 x — 00h Alarm status
TACH0 0000 1100 x — 00h Tachometer 0 count
TACH1 0000 1110 x — 00h Tachometer 1 count
TACH2 0001 0000 x — 00h Tachometer 2 count
TACH3 0001 0010 x — 00h Tachometer 3 count
GPIO STAT 0001 0100 x — 1Fh GPIO status
Table 3. Fan Speed
*
0000 0000 1.0
*0000 0001 1.0
*
0000 0010 1.5
KSCALE (ms)
*
*
*
tTACH
KSCALE
330
500
500
*
*
*
KTACH
*
* *
FAN SPEED (RPS)
—
*
500
480
—
240
— — — — — —
—
0001 1110 16 3.9 *32 128
64
0001 1111 16 4.0 1.0 31 124
0010 0000 17 4.2
20,000
1.0 30 120
30,000
— — — — — —
30,000
0100 0000 33 8.2 2.1 15.3 61.1
— — — — — —
—
1111 1000 125 31 7.8 415.9
1900
1900
1800
—
910
—
240
KSCALE
*
*
*
—
7700
7400
7200
—
3700
—
960
*
*
*
FAN SPEED (RPM)
—
*
30,000
29,000
—
15,000
—
3830
416 11 4 16 1 4 16
*
The minimum allowed tachometer period is 1ms.
across the fan until the desired speed is reached. Open-
loop operation allows the µC to regulate fan speed direct-
ly. The µC reads the fan speed from the Tach-
ometer-Count Register. Based on the tachometer
count, the µC decides if the fan speed requires adjust-
ment, and changes the voltage across the fan by writ-
ing an 8-bit word to the DAC Register. Full-on mode
applies the maximum voltage across the fan, forcing it
to spin at full speed. Configuring GPIO1 (see the
General-Purpose Input/Output
section) as an active-low
input provides additional hardware control that fully
turns on the fan and overrides all software commands.
General-Purpose Input/Output
The GPIO pins connect to the drain of the internal N-
channel MOSFET and pullup resistor (Figure 5). When
the N-channel MOSFET is off (Table 5), the pullup resis-
tor provides a logic-level high output. However, with the
MOSFET off, the GPIO may serve as an input pin and
its state is read from the GPIO Status Register (Table
6). The MAX6650/MAX6651 power up with the MOSFET
off, so input signals may be safely connected to the
GPIO pins. When using the GPIO pin as a general-pur-
pose output, change the output by writing to the GPIO
Definition Register.
GPIO0 may be configured as an ALERT output that will
go low whenever a fault-condition is detected (see the
Alarm-Enable and Status Registers
section). GPIO1
may be configured as a FULL ON input to allow hard-
ware control to fully turn on the fan in case of software
or µC failure. GPIO2 (MAX6651 only) may be config-
ured as an internal clock output or as an external clock
input to allow synchronization of multiple devices.
Alarm-Enable and Status Registers
The alarms are enabled only when the appropriate bits of
the Alarm-Enable Register are set (Table 7). The maxi-
mum and minimum output level alarms function only
when the device is configured to operate in the closed-
loop mode (see the
Configuration-Byte Register
section).
The Alarm Status Register allows the system to deter-
mine which alarm caused the alert output (Table 8).
The set-alarm and alert outputs clear after reading the
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
10 Maxim Integrated
MAX6650/MAX6651
Table 4. Configuration Byte Register
Figure 5. General-Purpose Input/Output Structure
MAX6650
MAX6651
100kΩ
GPIO
STATUS
REGISTER
VCC
3.0V TO 5.5V
VCC
CBYPASS
GPIO_
GND
GPIO
DEFINITION
REGISTER
BIT NAME POR (DEFAULT)
STATE FUNCTION
5 to 4
7 (MSB) to 6 — 0 Always 0
Operating Mode:
00 = Software full-on (default)
01 = Software off (shutdown)
10 = Closed-loop operation
11 = Open-loop operation
00MODE
35/12V 1
Fan/Tachometer Voltage:
0 = 5V
1 = 12V (default)
2 to 0 (LSB) SCALE 010
Prescaler Division:
000 = Divide by 1
001 = Divide by 2
010 = Divide by 4 (default)
011 = Divide by 8
100 = Divide by 16
Alarm Status Register if the condition that caused the
alarm is removed.
Tachometer
The Tachometer Count Registers record the number of
pulses on the corresponding tachometer input during the
period defined by the Tachometer Count-Time Register.
The MAX6651 contains three additional tachometer
inputs, which may be used to monitor additional fans. For
accurate control of multiple fans, use identical fans.
The Tachometer Count-Time Register sets the integration
time over which the MAX6650/MAX6651 count tachome-
ter pulses. The devices can count up to 255 (FFh) pulses
during the selected count time. If more than 255 pulses
occur, the IC sets the overflow alarm and the Tachometer
Count Register reports the maximum value of 255. Set
the time register so the count register will not overflow
under worst-case conditions (maximum fan speed) while
maximizing resolution. Calculate the maximum measur-
able fan speed and minimum resolution with the following
equations:
Max Fan Speed (in RPS) = 255 / (2 x tCOUNT)
Min Resolution (in RPS) = 1 / (2 x tCOUNT)
where tCOUNT is the tachometer count time; 1kHz is the
maximum allowable tachometer input frequency for the
MAX6650/MAX6651.
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
Maxim Integrated 11
MAX6650/MAX6651
Table 5. GPIO Definition Register
Table 6. GPIO Status Register
BIT
POR
(DEFAULT)
STATE
MAX6650
PIN
MAX6651
PIN STATE FUNCTION
0 GPIO4 outputs a logic-low level.
7 1 N/A
(must be 1) GPIO4 1 GPIO4 outputs a logic-high level or serves as an input.
0 GPIO3 outputs a logic-low level.
6 1 N/A
(must be 1) GPIO3 1 GPIO3 outputs a logic-high level or serves as an input.
00 GPIO2 serves as an external clock input.
01 GPIO2 serves as an internal clock output.
10 GPIO2 outputs a logic-low level.
5:4 11 N/A
(must be 11) GPIO2
11 GPIO2 outputs a logic-high level or serves as an input.
00 GPIO1 outputs a logic-high level or serves as an input.
01 GPIO1 serves as a FULL ON input.
10 GPIO1 outputs a logic-low level.
3:2 11 GPIO1 GPIO1
11 GPIO1 outputs a logic-high level or serves as an input.
00 GPIO0 outputs a logic-high level or serves as an input.
01 GPIO0 serves as an ALERT output.
10 GPIO0 outputs a logic-low level.
1:0 11 GPIO0 GPIO0
11 GPIO0 outputs a logic-high level or serves as an input.
BIT NAME
POR
(DEFAULT
STATE)
7 (MSB) to 5 Always 0 0
4 GPIO4 (MAX6651 only) 1
3 GPIO3 (MAX6651 only) 1
2 GPIO2 (MAX6651 only) 1
1GPIO11
0 (LSB) GPIO0 1
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
12 Maxim Integrated
MAX6650/MAX6651
Table 8. Alarm Status Register Bit Assignments
7 (MSB) to 5
GPIO1
—
BIT
GPIO2 (MAX6651 only)
0
4
NAME
1MIN
3
0
0 (LSB)
0
0
POR
(DEFAULT)
STATE
MAX 0
Always 0
Minimum Output Level Alarm
GPIO1 Alarm. Set when GPIO1 is low.
GPIO2 Alarm. Set when GPIO2 is low (MAX6651 only).
FUNCTION
Maximum Output Level Alarm
Tachometer Overflow Alarm2TACH 0
The first 6 bits of the Tachometer Count-Time Register
are always zero, and the last 2 bits set the count time
(Table 9). The count time may be determined from the
following equation:
tCOUNT = 0.25s x 2KCOUNT
where KCOUNT is the numerical value of the two 2LSBs.
The 0.25 factor has a ±10% uncertainty.
Upon power-up, the Tachometer Count Registers reset
to 00h and the Tachometer Count-Time Register sets a
1s integration time.
Digital-to-Analog Converter
When using the open-loop mode of operation, the DAC
Register sets the voltage on the low side of the fan. An
internal operational amplifier compares the feedback
voltage (VFB) with the reference voltage set by the 8-bit
DAC, and adjusts the output voltage (VOUT) until the
two input voltages are equal. The voltage at the FB pin
may be determined by the following equation:
VFB = (10 x VREF x KDAC) / 256
and the voltage across the fan is:
where KDAC is the numerical value of the DAC Register
and VREF = 1.5V. The minimum feedback voltage is
limited by the voltage drop across the external MOS-
FET (RON x IFAN), and the maximum voltage is limited
by the fan’s supply voltage (VFAN). For linear opera-
Vk
k
KV
FAN DAC REF
–
90
10 1256
+
⎛
⎝
⎜⎞
⎠
⎟⎛
⎝
⎜⎞
⎠
⎟
Table 9. Tachometer Count-Time Register
(Assumes two pulses per revolution)
1 = Alarm condition
REGISTER
VALUE
(KCOUNT)
COUNT
TIME
(s)
MAXIMUM
FAN SPEED
(RPS)
MINIMUM
RESOLUTION
(Hz/COUNT)
0000 0000 0.25 512 2
12560.50000 0001
0000 0010 1.0 128 0.5
0.25642.00000 0011
Table 7. Alarm-Enable Register Bit Masks
7 (MSB) to 5
GPIO1
—
BIT
GPIO2 (MAX6651 only)
0
4
NAME
1MIN
3
0
0 (LSB)
0
0
POR
(DEFAULT)
STATE
MAX 0
2TACH 0
Always 0
Minimum Output Level Alarm Enable/Disable
GPIO1 Alarm Enable/Disable
GPIO2 Alarm Enable/Disable (MAX6651 only)
FUNCTION
Maximum Output Level Alarm Enable/Disable
Tachometer Overflow Alarm Enable/Disable
1 = Enabled
tion, use DAC values between 08h and B0h (see
Typical Operating Characteristics
). When using the
closed-loop mode of operation, the contents of the
DAC Register are ignored. When writing to the DAC,
wait at least 500µs before attempting to read back.
Power-on Reset (POR)
The MAX6650/MAX6651 have volatile memory. To pre-
vent ambiguous power-supply conditions from corrupt-
ing the data in the memory and causing erratic
behavior, a POR voltage detector monitors VCC and
clears the memory if VCC falls below 1.6V. When power
is first applied and VCC rises above 1.6V, the logic
blocks begin operating (though reads and writes at
VCC levels below 3V are not recommended).
Power-up defaults include the following:
• All alarms are disabled.
• Prescale divider is set to 4.
• Fan speed is set in full-on mode.
See Table 2 for the default states of all registers.
Applications Information
MOSFET and Bipolar Transistor
Selection
The MAX6650/MAX6651 drive an external N-channel
MOSFET that requires five important parameters for
proper selection: gate-to-source conduction threshold,
maximum gate-to-source voltage, drain-to-source
breakdown voltage, current rating, and drain-to-source
on-resistance (RDS(ON)). Gate-to-source conduction
threshold must be compatible with available VCC. The
maximum gate-to-source voltage and the drain-to-
source breakdown voltage rating should both be at
least a few volts higher than the fan supply voltage
(VFAN). Choose a MOSFET with a maximum continuous
drain current rating higher than the maximum fan cur-
rent. RDS(ON) should be as low as practical to maxi-
mize the feedback voltage range. Maximum power
dissipation in the power transistor can be approximat-
ed by P = (VFAN X IFAN(MAX)) / 4. Bipolar power transis-
tors are practical for driving small and midsize fans
(Figure 6). Very-high-current fans may require output
transistor base current greater than the MAX6650’s
50mA drive capability. Bipolar Darlington transistors
will work but have poor saturation characteristics and
could lose up to 2V to 3V of drive voltage.
Resistor Selection
The tachometer input voltages (VTACH_) and feedback
voltage (VFB) cannot exceed 13.2V (see
Absolute
Maximum Ratings
). When using a fan powered by a
13.2V or greater supply (VFAN), protect these inputs
from overvoltage conditions with series resistors. The
resistance required to protect these pins may be calcu-
lated from the following equation:
RPROTECT = [(VFAN(MAX) - 13.2V) x RIN] / 13.2V
where VFAN(MAX) is the worst-case maximum supply
voltage used to power the fan and RIN is the input
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
Maxim Integrated 13
MAX6650/MAX6651
MAX6650
VCC
SCL
10kΩ
SDA
GPIO0 OUT
ADD
GND
FB
TACH0
VCC
3V TO 5.5V
VFAN
5V OR 12V
CCOMP
10μF
SMBus/I2C
INTERFACE
GPIO1
FAN
FULL ON
ALERT
Figure 6. Fan Control with a Bipolar Transistor
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
14 Maxim Integrated
MAX6650/MAX6651
impedance of the tachometer input (150kΩmax) or the
feedback input (150kΩmax).
Compensation Capacitor
A compensation capacitor is needed from the fan’s low
side to ground to stabilize the analog control loop.
Typically, this capacitor should be 10µF, but depend-
ing on the type of fan being used, a value between 1µF
and 100µF may be required. The proper value has
been selected when no ringing is present on the volt-
age at the fan’s low side.
Fan Selection
For closed-loop operation and fan monitoring, the
MAX6650/MAX6651 require fans with tachometer outputs.
A tachometer output is typically specified as an option on
many fan models from a variety of manufacturers. Verify
the nature of the tachometer output (open collector, totem-
pole) and the resultant levels, and configure the connec-
tion to the MAX6650/MAX6651 accordingly. Note how
many pulses per revolution are generated by the
tachometer output (this varies from model to model and
among manufacturers, though two pulses per revolution is
the most common).
Table 10 lists the representative fan manufacturers and
the models they make available with tachometer outputs.
Low-Speed Operation
Brushless DC fans increase reliability by replacing
mechanical commutation with electronic commutation. By
lowering the voltage across the fan to reduce its speed,
the MAX6650/MAX6651 are also lowering the supply volt-
age for the electronic commutation and tachometer elec-
tronics. If the voltage supplied to the fan is lowered too
far, the internal electronics may no longer function prop-
erly. Some of the following symptoms are possible:
• The fan may stop spinning.
• The tachometer output may stop generating a signal.
• The tachometer output may generate more than two
pulses per revolution.
The problems that occur, and the supply voltages at
which they occur, depend on which fan is used. As a
Figure 7. Using the MAX6651 to Control Parallel Fans
MAX6651
VCC
SCL
10kΩ
10kΩ
10kΩ
SDA
GPIO0 OUT
ADD
GND
FB
TACH0
TACH1
TACH2
VCC
3V TO 5.5V
VFAN 5V OR 12V
CCOMP
SMBus/I2C
INTERFACE
GPIO1
FAN
0
FAN
1
FAN
2
FULL ON
ALERT
MANUFACTURER FAN MODEL OPTION
Comair Rotron
All DC brushless models can be
ordered with optional tachometer
output.
EBM-Papst Tachometer output optional on some
models.
NMB
All DC brushless models can be
ordered with optional tachometer
output.
Panasonic
Panaflo and flat unidirectional
miniature fans can be ordered with
tachometer output.
Sunon Tachometer output optional on some
models.
Table 10. Fan Manufacturers
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
Maxim Integrated 15
MAX6650/MAX6651
very rough rule of thumb, 12V fans can be expected to
experience problems somewhere around 1/4 to 1/2
their rated speed.
Predicting Future Fan Failure
In systems that require maximum reliability, such as
servers and network equipment, it can be advantageous
to predict fan failure before it actually happens, to alert
the system operator before the fan fails, minimizing down
time. The MAX6650 allows the user to monitor the fan’s
condition through the following modes.
Full-On Mode
By occasionally (over a period of days or weeks) turning
the fan on full and measuring the resultant speed, a
failing fan can be detected by a trend of decreasing
speeds at a given power-supply voltage. Power-up is a
convenient time to measure the maximum fan speed.
Open-Loop Mode
The fan’s condition can also be monitored using open-
loop mode. By characterizing the fan while it is new,
fan failure can be determined by writing a predeter-
mined value to the DAC and measuring the resultant
fan speed. A decrease over time of the resultant speed
may be an indication of future fan failure.
Closed-Loop Mode
The MAX6650 allows the system to read the DAC value
used to regulate the fan speed. For a given speed, a
significant change in the required DAC value may indi-
cate future fan problems.
Monitoring More than 4 Fans
Use the MAX6651 to monitor up to four fans at a time
(Figure 7). For systems requiring more than four fans,
Figure 8 shows an application using an analog multi-
plexer (mux) to monitor 11 fans. GPIO2, GPIO3, and
GPIO4 are connected to the mux’s address pins. By
writing the appropriate value to the GPIO pins, the
desired tachometer gets selected and counted by the
TACH3 input. Because the TACH inputs are double-
buffered, and only sampled every other time slot, it is
important to wait at least 4 times the tachometer count
time before reading the register after changing the mux
address. In the extreme case, a total of 25 fans can be
monitored using three multiplexers connected to
TACH1, TACH2, and TACH3. Do not connect TACH0 to
a mux if the MAX6651 is under closed-loop mode.
N + 1 Fan Application
As shown in Figure 9, if any MAX6650 cannot maintain
speed regulation, all other fans will automatically be
turned on full. This can be useful in high-reliability sys-
tems where any single fan failure should not cause
MAX6651
TACH0
TACH1
TACH2
TACH3
GPIO4
GPIO3
GPIO2
MAX4051
VCC
3V OR 5.5V
COM
ADDA
ADDB
ADDC
INH
GND V-
NO0
NO1
NO2
NO3
NO4
NO5
NO6
FAN TACH 4
FAN TACH 9
FAN TACH 5
FAN TACH 6
FAN TACH 7
FAN TACH 8
FAN TACH 1
FAN TACH 2
FAN TACH 3
FAN TACH 10
NO7 FAN TACH 11
TO FAN VOLTAGE
5V OR 12V
Figure 8. Monitoring Multiple Fans
downtime. The system should be designed so that the
number of fans used is one more than are actually
needed. This way, there is sufficient cooling even if a
fan fails. With all fans operating correctly, it is unneces-
sary to run the fans at their maximum speed. Reducing
fan speed can reduce noise and increase the life of the
fans. However, once a fan fails, it is important that the
remaining fans spin at their maximum speed.
In Figure 9, all the GPIO0s are configured as ALERT
outputs, and all the GPIO1s are configured as
FULL ON inputs. If any MAX6650 generates an ALERT
(indicating failure), the remaining MAX6650s will auto-
matically turn their fans on full.
Temperature Monitoring and Fan Control
The circuit shown in Figure 10 provides complete tem-
perature monitoring and fan control. The MAX1617A (a
remote/local temperature serial interface with SMBus)
monitors temperature with a diode-connected transis-
tor. Based on the temperature readings provided by
the MAX1617A, the µC can adjust the fan speed pro-
portionally with temperature. Connecting the ALERT
output of the MAX1617A to the FULL ON input of the
MAX6650/MAX6651 (see the
General-Purpose Input/
Output
section) allows the fan to turn on fully if the
MAX1617A detects an overtemperature condition.
MAX6501 Hardware Fail-Safe
Figure 11 shows an application using a MAX6501 as a
hardware fail-safe. The MAX6650 has its GPIO1 config-
ured as FULL ON input. The MAX6501 TOVER pin goes
low whenever its temperature goes above a preset value.
This pulls the FULL ON pin (GPIO1) low, forcing the fan to
spin at its maximum speed. Figure 12 shows the use of
multiple MAX6501s. The MAX6501 has an open-drain
output, allowing multiple devices to be wire ORed to the
FULL ON input. This configuration allows fail-safe moni-
toring of multiple locations around the system.
Hot-Swap Application
Hot swapping of a fan can be detected using the circuit
in Figure 13 where GPIO2 is configured to generate an
alert whenever it is pulled low. As long as the fan card
is connected, GPIO2 is high. However, when the fan
card is removed, a 2.2kΩresistor pulls GPIO2 low,
causing an interrupt. This signals to the system that a
hot swap is occurring.
Step-by-Step Part Selection
and Software Setup
Determining the Fan System Topology
The MAX6650/MAX6651 support three fan system
topologies. These are single fan control, parallel fan
control, and synchronized fan control.
Single Fan Control
The simplest configuration is a single MAX6650 for
each fan. If two or more fans are required per system,
then additional MAX6650 controllers are used (one per
fan). The advantage of this configuration is the ability to
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
16 Maxim Integrated
MAX6650/MAX6651
MAX6650
GPIO1
MAX6650
GPIO1
MAX6650
GPIO1
FAN
1
FAN
2
FAN
3
MAX6650
GPIO1
FAN
4
GPIO0
GPIO0
GPIO0
GPIO0
ALERT
FULL ON
ALERT
ALERT
ALERT
FULL ON
FULL ON
FULL ON
TO INT PIN
ON NC
Figure 9. N + 1 Application
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
Maxim Integrated 17
MAX6650/MAX6651
FAN
VFAN = 5V OR 12V
OUT
FB
GND
SDA
SCL
VCC
TACH0
GPIO0
DXN
DXP
GND
ADD1
ADD0
VCC
SCL
SDA
GND
ADD
VCC
SDA
SCL
GPIO1
μC
INTERRUPT TO μC
TEMPERATURE
SENSOR
STBY
ALERT
ALERT
FULL ON
MAX1617A
MAX6650
MAX6651
VCC
Figure 10. Temperature Monitoring and Fan Control
independently control each fan. The disadvantage is
cost, size, and complexity.
For single fan control, use the MAX6650 (unless addi-
tional GPIOs are needed).
Parallel Fan Control
If multiple fans are required but independent control is
not, then a single MAX6650/MAX6651 connected to
two or more fans in parallel may make sense (Figure 7).
The obvious advantage is simplicity, size, and cost
savings. If all the fans connected in parallel are the
same type, they will tend to run at similar speeds.
However, if one or more of the fans are wearing out,
speed mismatches can occur. The MAX6651 allows the
system to monitor up to four fans, ensuring any signifi-
cant speed mismatches can be detected.
For parallel fan control while monitoring up to four fan
speeds, select the MAX6651.
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
18 Maxim Integrated
MAX6650/MAX6651
MAX6650
VCC
SCL
10kΩ
SDA
GPIO0
OUT
ADD
GND
FB
TACH0
VCC
3V TO 5.5V
VFAN
5V OR 12V
CCOMP
10μF
SMBus/I2C
INTERFACE
GPIO1
FULL ON
ALERT
FAN
MAX6501
TOVER
Figure 11. MAX6501 Hardware Fail-Safe
MAX6650
VCC
SCL
10kΩ
SDA
GPIO0
OUT
ADD
GND
FB
TACH0
VCC
3V TO 5.5V
VFAN
5V OR 12V
CCOMP
10μF
SMBus/I2C
INTERFACE
GPIO1
FULL ON
ALERT
FAN
MAX6501
TOVER
MAX6501
TOVER
MAX6501
TOVER
Figure 12. MAX6501 Hardware Fail-Safe
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
Maxim Integrated 19
MAX6650/MAX6651
For parallel fan control while monitoring only a single
fan, select the MAX6650.
Synchronized Fan Control (MAX6651 Only)
In systems with multiple fans, an audible beat frequency
can sometimes be detected due to fan speed mismatch.
This happens in systems where fans are connected in
parallel or in systems with a MAX6650 controlling each
fan. In parallel fan systems, speed mismatches occur
because no two fans are identical. Slight mechanical
variations or loading differences can result in enough of
a speed mismatch to cause an audible beat.
Even in systems where there is a MAX6650/MAX6651
for each fan, there can still be speed mismatches. This
is primarily due to the oscillator tolerance. The
MAX6650/MAX6651 oscillator tolerance is specified to
be ±10%. In the worst case, this could result in a 20%
(one 10% high, one 10% low) speed mismatch.
The solution is to use a single MAX6651 for each fan,
and configure the parts to use a shared clock. The
shared clock can either be an external system clock or
one of the MAX6651’s internal clocks. If an external
clock is used, its frequency can range from approxi-
mately 50kHz to 500kHz.
For synchronized fan control, select the MAX6651.
Combination
In more complex systems, a combination of some or all
of the above control types may be needed.
Choosing a Fan
Once the topology is chosen, the next step is to choose
a fan. See the appropriate section.
Enter a zero in bit 3 of the configuration register for a
5V fan and 1 for a 12V fan.
Configuring this bit also adjusts the tachometer input
threshold voltage. This optimizes operation of the
MAX6650/MAX6651 for the operating voltage of the fan
being used.
Setting the Mode of Operation
The MAX6650/MAX6651 have four modes of operation
as determined by bits 5 and 6 of the configuration reg-
ister: full on, full off, open loop, and closed loop.
Full-On
The full-on mode applies the maximum available volt-
age across the fan, guaranteeing maximum cooling.
Full-on mode can be entered through software or hard-
MAX6651
VCC
SCL
10kΩ
SDA
GPIO0
OUT
ADD
GND
FB
TACH0
VCC
3V TO 5.5V
VFAN
5V OR 12V
CCOMP
10μF
SMBus/I2C
INTERFACE
GPIO1
GPIO2
FULL ON
ALERT
FAN
2.2kΩ
VID
HOT-SWAP SECTION
Figure 13. Hot-Swap Application
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
20 Maxim Integrated
MAX6650/MAX6651
ware control. To enter full-on mode through hardware,
see the
Setting Up the GPIOs
section. Note that a hard-
ware full-on overrides all other modes.
Configure the MAX6650/MAX6651 to run in software
full-on mode by entering 00 into bits 5 and 4 of the con-
figuration register.
Full-Off
The full-off mode removes all the voltage across the
fan, causing the fan to stop. Because the MAX6650/
MAX6651 work by controlling the voltage on the low
side of the fan, either 5V or 12V will be on both leads.
Enter full-off mode by entering 01 into bits 5 and 4 of
the configuration register.
Open Loop
In open-loop mode, the MAX6650/MAX6651 do not
actually regulate the fan speed. Speed regulation
requires an external µC. Although open-loop mode
allows maximum flexibility, it also requires the most
software/processor overhead.
In open-loop mode, the MAX6650/MAX6651 act as an
SMB/I2C-controlled voltage regulator. The µC adjusts
the voltage across the fan by writing an 8-bit value to
the DAC register. This gives the µC direct control of the
voltage across the fan. Speed regulation is accom-
plished by periodically reading the tachometer regis-
ter(s) and adjusting the DAC register appropriately. The
DAC value controls the voltage across the fan accord-
ing to the following equation:
VFAN = VFAN_SUPPLY - [((R2) / R1) + 1] x VREF x KDAC /
256
where VFAN = the voltage across the fan, VFAN_SUPPLY
= the supply voltage for the fan (5V or 12V), R2 = 90kΩ
(typ), R1 = 10kΩ(typ), VREF = 1.5V (typ), and KDAC =
the value in the DAC register.
Note several important things in this equation. First, the
voltage across the fan moves in the opposite direction
of the DAC value. In other words, low DAC values cor-
respond to higher voltages across the fan and therefore
higher speeds. Second, DAC values greater than 180
will result in 0V across a 12V fan. Similarly, DAC values
greater than 76 will produce 0V across a 5V fan. This
limits the useful range of the DAC from 0 to 180 for 12V
fans and 0 to 76 for 5V fans.
Remember that device tolerances can cause the output
voltage value to vary significantly from unit to unit and
over temperature. However, because this voltage is
within a closed speed-control loop, such errors are cor-
rected by the loop.
Below is a possible strategy for controlling the fan
under open-loop mode:
1) On power-up, put the device in open-loop mode with
a DAC value of 00 (full speed).
2) Allow the fan speed to settle.
3) Read the TACH register to determine the speed.
4) Gradually increase the DAC register value (in steps
of 1 or 2) until the desired speed is obtained.
In open-loop mode, any one of the four tachometer regis-
ters (MAX6651) can be used to measure and regulate the
fan’s speed. This is especially useful in parallel fan sys-
tems where up to four fans will be controlled as one unit.
Care must be taken with this mode to prevent instabili-
ty, which can be caused by trying to update the fan
speed too often or in increments that are too large.
Instability can result in the fan speeding up and slowing
down repeatedly. Determining the proper update rate,
as shown in the following steps, depends largely on the
fan’s mechanical time constant and the system’s loop
gain (DAC step sizes):
1) Enter open-loop mode by setting bits 5 and 4 of the
control register to 11.
2) Determine the speed of the fan(s) by reading the
TACH register(s).
3) Increase or decrease the DAC register to decrease
or increase the voltage across the fan, thereby
adjusting its speed.
Closed Loop
In closed-loop mode, the SMBus/I2C master (usually a
µC) writes a desired fan speed to the MAX6650/
MAX6651, and the device automatically adjusts the
voltage across the fan to maintain this speed. This
operation mode requires less software/processor over-
head than the open-loop mode. Once the desired
speed has been written, the MAX6650/MAX6651 con-
trol the fan’s speed independently, with no intervention
required from the master. If desired, the MAX6650/
MAX6651 can be configured to generate an interrupt if
it is unable to regulate the fan’s speed at the desired
value (see
Setting Up Alarms
). The MAX6650/MAX6651
can regulate only the speed of the fan connected to the
TACH0 input. Fans connected in parallel to the TACH0
fan will tend to run at similar speeds (assuming similar
fans). When going from full-off to closed-loop-mode, it
is recommended following this sequence:
1) Full-off mode
2) Full-on mode (with sufficient pause to initiate
movement)
3) Closed-loop mode
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
Maxim Integrated 21
MAX6650/MAX6651
The MAX6650 regulates fan speed in the following
manner. The output of an internal 254kHz oscillator is
divided by 128, generating a roughly 2kHz signal. This
signal is divided by 1 plus the value in the speed regis-
ter and is used as a reference frequency. For example,
02h in the speed register will result in a 667Hz [2kHz /
(02h+1)] reference frequency, which is then compared
against the frequency at the tachometer input divided
by the prescaler value. The MAX6650/MAX6651
attempt to keep the tachometer frequency divided by
the prescaler equal to the reference frequency by
adjusting the voltage across the fan. If the tachometer
frequency divided by the prescaler value is less than
the reference frequency, the voltage across the fan is
increased. Remember that the tachometer will give two
pulses per revolution of the fan. The following equations
describe the operation.
When in regulation:
[fCLK / (128 x (KTACH + 1))] = 2 x FanSpeed / KSCALE
where fCLK = oscillator frequency (either the 254kHz
internal oscillator or the externally applied clock), KTACH
= the value in the speed register, FanSpeed = the
speed of the fan in revolutions per second (Hz),
KSCALE = the prescaler value (1, 2, 4, 8, or 16).
Solving for all four variables:
KTACH = [(fCLK x KSCALE) / (256 x FanSpeed)] - 1
KSCALE = [256 x FanSpeed x (KTACH + 1)] / fCLK
FanSpeed = KSCALE x fCLK / [256 x (KTACH + 1)]
fCLK = 256 x FanSpeed x (KTACH + 1) / KSCALE
If the internal oscillator is used, setting fCLK to 254kHz
can further reduce the equations:
Equation 1: KSCALE = FanSpeed x (KTACH + 1) / 992
Equation 2: KTACH = (992 x KSCALE / FanSpeed) - 1
Equation 3: FanSpeed = 992 x KSCALE / (KTACH + 1)
Enter closed-loop mode by entering 10 into bits 5 and 4
of the configuration register.
Note that in equation 3, the fan speed is inversely pro-
portional to (KTACH + 1). This means the regulated fan
speed is a nonlinear function of the value written to the
speed register. Low values written to the speed register
can result in large relative changes in fan speed. For
best results, design the system so that small values
(such as 02h) are not needed. This is easily accom-
plished because an 8-bit speed register is used, and
fan-speed control should rarely need more than 16
speeds. A good compromise is to design the system
(by selecting the appropriate prescaler value) so that
the maximum-rated speed of the fan occurs when the
speed register equals approximately 64 (decimal).
Although 64 is a good target value, values between 20
and 100 will work fine.
The prescaler value also affects the response time and
the stability of the speed-control loop. Adjusting the
prescaler value effectively adjusts the loop gain. A larg-
er prescaler value will slow the response time and
increase stability, while a smaller prescaler value will
yield quicker response time. The optimum prescaler
value for response time and stability depends on the
fan’s mechanical time constant. Small, fast-spinning
fans will tend to have small mechanical time constants
and can benefit from smaller prescaler values. A good
rule of thumb is to try the selected prescaler value in
the target system. Set KTACH to around 75% of full
scale, and watch for overshoot or oscillation in the fan
speed. Also look for overshoot or oscillation when
KTACH is changed from one value to another (e.g., from
75% of full-scale speed to 90% of full scale). If there is
unacceptable overshoot or if the fan speeds up and
slows down with KTACH, set it to a constant value;
increase the prescaler value.
Enter the appropriate prescaler value in bits zero to 2 of
the configuration register.
Fan speed is a trade-off between cooling requirements,
noise, power, and fan wear. In general, it is desirable
(within limits) to run the fan at the slowest speed that
will accomplish the cooling goals. This will reduce
power consumption, increase fan life, and minimize
noise. When calculating the desired fan speed, remem-
ber that the above equations are written in rotations per
second (RPS), where most fans are specified in rota-
tions per minute (RPM).
Write the desired fan speed to the speed register.
Example:
Assume the following:
• 12V fan is rated at 2000RPM at 12V.
• Use the internal oscillator (fCLK = 254kHz).
• Desired fan speed = 1500RPM (25RPS).
First, calculate an appropriate prescaler value
(KSCALE) using equation 1. Attempt to get KTACH as
close to 64 as possible for the maximum speed of
2000RPM.
• Set FanSpeed = 33.3RPS (2000RPM/60).
• Set KTACH = 64.
• Solving equation 1 gives KSCALE = 2.18.
We will start with KSCALE = 2 (to increase stability, a 4
could be tried, or to improve response time, a 1 could
be tried).
Second, calculate the appropriate value for the Speed
Register (KTACH) using equation 2.
• Set FanSpeed = 25RPS (1500PRM/60).
• Solving for equation 2 gives KTACH = 78 for KSCALE
= 2, KTACH = 39 for KSCALE = 1, or KTACH = 158 for
K = 4.
Determining the Tachometer Count Time
To monitor the fan speed using the SMBus/I2C, the next
step is to determine the tachometer count time. In sys-
tems running in open-loop mode, this is necessary. In
closed-loop or full-speed mode, reading the tachome-
ter can serve as a valuable check to ensure the fan and
the control loop are operating properly.
The MAX6650/MAX6651 use an 8-bit counter to count
the tachometer pulses. This means the device can
count from 0 to 255 tachometer pulses before overflow-
ing. The MAX6650/MAX6651 can accommodate a large
range of fan speeds by allowing the counting interval to
be programmed. Smaller/faster fans should use smaller
count times. Although larger fans could also use small-
er count times, resolution would suffer. Choose the
slowest count time that will not overflow under worst-
case conditions. Fans are mechanical devices, and
their speeds are subject to large tolerance variations. If
an overflow does occur, the counter will read 255. The
MAX6650/MAX6651 can be configured to generate an
alert if an overflow is encountered (see
Setting Up
Alarms
). Note that the prescaler value has no effect on
the TACH0 register.
Enter the appropriate count-time value in the tachometer
count-time register.
Example:
Assume a 12V fan rated at 2000 RPM.
To accommodate large tolerance variations, choose a
count time appropriate for a maximum speed of
3000RPM; 3000RPM is 50RPS and generates a 100Hz
(2 pulses/revolution) tachometer signal. Table 9 indi-
cates a count time of 2s will optimize resolution. With a
2s count time, speeds as fast as 3825RPM can be
monitored without overflow. The minimum resolution will
be 15RPM or 0.75% of the rated speed of 2000RPM.
Setting Up the GPIOs
To increase versatility, the MAX6650/MAX6651 have
two and five general-purpose digital inputs/outputs,
respectively. These GPIOs can be configured through
the SMBus/I2C.
Digital Out Low
All GPIOs can be configured to output a logic-level low.
The MAX6650/MAX6651 are designed to sink up to
10mA. This high sink current can be especially useful
for driving LEDs.
On the MAX6651, for GPIO3 and GPIO4, write a zero to
the appropriate location in the GPIO definition register.
For GPIO0, GPIO1, and (MAX6551 only) GPIO2, write a
10 to the appropriate location in the GPIO definition
register.
Digital Out High
All GPIOs can be configured to generate a logic-level
high. An output high is generated using an open-drain
output stage with an internal pullup resistor of nominally
100kΩ. The MAX6650/MAX6651 power-up default state
is with all GPIOs configured as output highs.
On the MAX6651, for GPIO3 and GPIO4, write a 1 to
the appropriate location in the GPIO definition register.
For GPIO0, GPIO1, and (MAX6551 only) GPIO2, write
an 11 to the appropriate location in the GPIO definition
register.
Digital Input
Since a logic-level high output is open drain with an
internal pullup, an external device can actively pull this
pin low. The MAX6650/MAX6651 allow the user to read
the GPIO value through the GPIO status register.
• Configure the GPIO as an output logic level high (see
above).
• Read the state of the GPIO by reading the GPIO sta-
tus register.
Alert Output
GPIO0 can also serve as an ALERT output. The ALERT
output is designed to drive an interrupt on a µC. The
ALERT output goes low whenever an enabled alarm
condition occurs (see
Setting Up Alarms
).
Configure GPIO0 as an ALERT output by writing a 01 to
bits 1 and 0 of the GPIO definition register.
Full-On Input
GPIO1 can also be configured as a full-on input. When
the full-on pin is pulled low, the MAX6650/MAX6651
apply the full available voltage across the fan. This hap-
pens independently of the software mode of operation.
This is a particularly valuable feature in high-reliability
systems, designed to prevent software malfunctions
from causing system overheating.
Configure GPIO1 as a full-on input by writing a 01 to
bits 3 and 2 of the GPIO definition register.
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
22 Maxim Integrated
MAX6650/MAX6651
Synchronizing Fans
GPIO2 can be configured to allow multiple MAX6651s
to synchronize the speeds of the fans they are driving
(Figure 14). Synchronization is accomplished by having
one of the MAX6651s (or an external clock) serve as
the clock master by configuring one of the GPIO2s in
the system as a clock output. The remaining GPIO2s in
the system need to be configured as clock inputs:
• Electrically connect all MAX6651 GPIO2s together.
• Configure one of the MAX6651’s GPIO2s to be a
clock output, using the GPIO Definition Register (set
bits 5 and 4 to 01).
• Configure the rest of the GPIO2s as clock inputs, using
the GPIO Definition Register (set bits 5 and 4 to 00).
• Configure all MAX6651s in closed-loop mode.
• Configure all prescaler values to be equal.
• Write identical values to all speed registers.
Setting Up Alarms
The MAX6650/MAX6651 can be configured to generate
an ALERT output on GPIO0 whenever certain events,
such as control loop out of regulation, tachometer over-
flow, or GPI01/GPI02 being driven low, occur. This is
designed to enhance the “set and forget” functionality
of the fan control system.
Configure GPIO0 to be an ALERT output (see above).
Minimum/Maximum Output Level Alarm
The minimum/maximum output level alarms are
designed to warn the system when the MAX6650/
MAX6651 are unable to maintain speed regulation in
closed-loop mode. The MAX6650/MAX6651 maintain
speed regulation by adjusting the voltage across the
fan. If the desired speed can’t be attained, one of these
alarms will be generated. Possible causes for failure to
attain the desired speed include system programming
problems, incipient fan failure, and a programmed
speed that the fan cannot support.
The minimum output alarm occurs when the DAC out-
put is 00h. A DAC value of 00h means that the
MAX6650/MAX6651 have applied the largest available
voltage across the fan. This typically means the fan is
unable to spin as fast as the desired speed.
The maximum output alarm occurs when the DAC value
is FF h . A DAC value of F F h means the MAX6650/MAX6651
have tried to reduce the voltage across the fan to 0.
Although this would seem to indicate the fan is spinning
faster than the desired speed, this should rarely hap-
pen. If this alarm occurs, it probably indicates some
type of system error.
Enable the minimum/maximum output level alarm by
setting bits 0 and 1 of the alarm enable register to 11.
Tachometer Overflow Alarm
If any tachometer counter overflows (reaches a count of
255), this alarm will be set.
Enable the overflow output level alarm by setting bit 2
of the alarm enable register bit to 1.
GPIO1/2 Pulled Low
Enabling this alarm causes the ALERT output to go low
whenever GPIO1 or GPIO2 is pulled low. This will occur
independent of the configuration of GPIO1 or GPIO2.
Enable the GPIO1/GPIO2 output level alarms by setting
bits 3 and/or 4 of the alarm enable register bit to 1.
Clearing the ALERT
Once an ALERT is generated, determine which alarm
caused the ALERT pin to go low. Do this by reading the
Alarm Status Register. An ALERT output will stay active
(low) even if the condition that caused the alert is
removed. Reading the Alarm Status Register clears the
ALERT, if the condition that caused the alert is gone. If
the condition has not gone away, the ALERT will stay
active. Disabling the alarm with the Alarm Enable
Register will cause the ALERToutput to go inactive.
Read the Alarm Status Register.
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
Maxim Integrated 23
MAX6650/MAX6651
MAX6501
GPIO2
MAX6501
GPIO2
MAX6501
GPIO2
FAN
1
FAN
2
FAN
3
CLOCK OUT
CLOCK IN
CLOCK IN
Figure 14. Synchronizing Fans
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
24 Maxim Integrated
MAX6650/MAX6651
TOP VIEW
1
2
3
4
5
10
9
8
7
6
FB
VCC
OUT
GPIO0SCL
SDA
GND
TACH0
MAX6650
μMAX
GPIO1ADD
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
TACH0 TACH1
FB
VCC
OUT
GPIO3
GPIO2
GPIO0
GPIO1
QSOP
TACH2
TACH3
SCL
GND
SDA
GPIO4
ADD
MAX6651
Pin Configurations
Package Information
For the latest package outline information and land patterns (footprints), go to www.maximintegrated.com/packages. Note that a
“+”, “#”, or “-” in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the
drawing pertains to the package regardless of RoHS status.
PACKAGE TYPE PACKAGE CODE OUTLINE NO. LAND PATTERN NO.
10 µMAX U10-2 21-0061 90-0330
16 QSOP E16-1 21-0055 90-0167
Fan-Speed Regulators and Monitors
with SMBus/I2C-Compatible Interface
MAX6650/MAX6651
Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent
licenses are implied. Maxim Integrated reserves the right to change the circuitry and specifications without notice at any time. The parametric values (min and
max limits) shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.
Maxim Integrated 160 Rio Robles, San Jose, CA 95134 USA 1-408-601-1000
25
© 2012 Maxim Integrated Products, Inc. Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc.
REVISION
NUMBER
REVISION
DATE DESCRIPTION PAGES
CHANGED
Added lead-free parts to the Ordering Information 1
4 7/10
Updated Table 5 to include the pins for both the MAX6650 and MAX6651 11
5 12/12
Updated the ADD parameters in the Electrical Characteristics table; updated the
conditions notes for tHD:DAT, tR, and tF in the Timing Characteristics table; updated
Table 1
3, 6
Revision History
