MAX16909
36V, 220kHz to 1MHz Step-Down Converter
with Low Operating Current
General Description
The MAX16909 is a 3A, current-mode, step-down con-
verter with an integrated high-side switch. The device is
designed to operate with input voltages from 3.5V to 36V
while using only 30FA quiescent current at no load. The
switching frequency is adjustable from 220kHz to 1MHz
by an external resistor and can be synchronized to an
external clock. The output voltage is pin selectable to
be 5V fixed or adjustable from 1V to 10V. The wide input
voltage range along with its ability to operate at high duty
cycle during undervoltage transients make the device
ideal for automotive and industrial applications.
The device operates in skip mode for reduced current
consumption in light-load applications. Protection features
include overcurrent limit, overvoltage, and thermal shut-
down with automatic recovery. The device also features
a power-good monitor to ease power-supply sequencing.
The device operates over the -40NC to +125NC automo-
tive temperature range, and is available in 16-pin TSSOP
and TQFN (5mm x 5mm) packages with exposed pads.
Applications
Automotive
Industrial/Military
High-Voltage Input DC-DC Converter
Point-of-Load Applications
Features
S Wide 3.5V to 36V Input Voltage Range
S 42V Input Transients Tolerance
S High Duty Cycle During Undervoltage Transients
S 5V Fixed or 1V to 10V Adjustable Output Voltage
S Integrated 3A Internal High-Side (70mI typ)
Switch
S Fast Load-Transient Response and Current-Mode
Architecture
S Adjustable Switching Frequency (220kHz to 1MHz)
S Frequency Synchronization Input
S 30µA Standby Mode Operating Current
S 5µA Typical Shutdown Current
S Overvoltage, Undervoltage, Overtemperature, and
Short-Circuit Protections
Typical Application Circuit
19-5777; Rev 2; 8/12
Ordering Information appears at end of data sheet.
For related parts and recommended products to use with this part,
refer to: www.maximintegrated.com/MAX16909.related
EVALUATION KIT AVAILABLE
D1 COUT
100µF
CIN2
4.7µF
RCOMP
47kIRPGOOD
10kI
RFOSC
65kI
L1
10µH VOUT
5V AT 3A
CBST
0.1µF
LX
BST
VOUT
VBIAS
OUT
VBAT
FB
VBIAS
PGOOD
FOSC
CBIAS
1µF
CCOMP2
10pF
BIAS
CCOMP1
2.7nF
COMP
FSYNC
EN
SUPSWSUP
GND
CIN1
47µF
POWER GOOD
MAX16909
For pricing, delivery, and ordering information, please contact Maxim Direct
at 1-888-629-4642, or visit Maxim’s website at www.maximintegrated.com.
2Maxim Integrated
MAX16909
36V, 220kHz to 1MHz Step-Down Converter
with Low Operating Current
SUP, SUPSW, LX, EN to GND ...............................-0.3V to +42V
SUP to SUPSW .....................................................-0.3V to +0.3V
BST to GND ........................................................... -0.3V to +47V
BST to LX ...............................................................-0.3V to +6V
OUT to GND ..........................................................-0.3V to +12V
FOSC, COMP, BIAS, FSYNC, I.C., PGOOD,
FB to GND ............................................................-0.3V to +6V
LX Continuous RMS Current ...................................................4A
Output Short-Circuit Duration .................................... Continuous
Continuous Power Dissipation (TA = +70NC)
TSSOP (derate 26.1mW/oC above +70NC) .......... 2088.8mW*
TQFN (derate 28.6mW/oC above +70NC) ............ 2285.7mW*
Operating Temperature Range ........................ -40NC to +125NC
Junction Temperature .....................................................+150NC
Storage Temperature Range ............................ -65NC to +150NC
Lead Temperature (soldering, 10s) ................................+300NC
Soldering Temperature (reflow) ..................................... +260oC
TSSOP
Junction-to-Ambient Thermal Resistance (BJA) .......38.3NC/W
Junction-to-Case Thermal Resistance (BJC) .................3NC/W
TQFN
Junction-to-Ambient Thermal Resistance (BJA) ..........35NC/W
Junction-to-Case Thermal Resistance(BJC) ...............2.7NC/W
ABSOLUTE MAXIMUM RATINGS
Note 1: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a four-layer
board. For detailed information on package thermal considerations, refer to www.maximintegrated.com/thermal-tutorial.
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional opera-
tion 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.
PACKAGE THERMAL CHARACTERISTICS (Note 1)
ELECTRICAL CHARACTERISTICS
(VSUP = VSUPSW = 14V, VEN = 14V, RFOSC = 66.5kI, TA = TJ = -40NC to +125NC, unless otherwise noted. Typical values are at
TA = +25NC.)
*As per the JEDEC 51 standard (multilayer board).
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Supply Voltage Range VSUP,
VSUPSW 3.5 36 V
Load-Dump Event Supply
Voltage VSUP_LD tLD < 1s 42 V
Supply Current
ISUP ILOAD = 1.5A 3.5 mA
ISUP_STANDBY
Standby mode, no load, VOUT = 5V 30 60
FA
Standby mode, no load, VOUT = 5V,
TA = +25°C 30 45
Shutdown Supply Current ISHDN VEN = 0V 5 12 FA
BIAS Regulator Voltage VBIAS VSUP = VSUPSW = 6V to 36V 4.7 5 5.3 V
BIAS Undervoltage Lockout VUVBIAS VBIAS rising 2.9 3.1 3.3 V
BIAS Undervoltage-Lockout
Hysteresis 400 mV
Thermal-Shutdown Threshold +175 NC
Thermal-Shutdown Threshold
Hysteresis 15 NC
OUTPUT VOLTAGE (OUT)
Output Voltage VOUT VFB = VBIAS, normal operation 4.925 5 5.075 V
3Maxim Integrated
MAX16909
36V, 220kHz to 1MHz Step-Down Converter
with Low Operating Current
ELECTRICAL CHARACTERISTICS (continued)
(VSUP = VSUPSW = 14V, VEN = 14V, RFOSC = 66.5kI, TA = TJ = -40NC to +125NC, unless otherwise noted. Typical values are at
TA = +25NC.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Skip-Mode Output Voltage VOUT_SKIP No load, VFB = VBIAS 4.925 5 5.15 V
Adjustable Output Voltage
Range VOUT_ADJ FB connected to external resistive divider 1 10 V
Load Regulation VFB = VBIAS, 30mA < ILOAD < 3A 0.5 %
Line Regulation VFB = VBIAS, 6V < VSUPSW < 36V 0.02 %/V
BST Input Current IBST_ON High-side on, VBST - VLX = 5V 1.5 2.5 mA
LX Current Limit ILX (Note 2) 3.4 4.1 6 A
Skip-Mode Threshold ISKIP_TH 300 mA
Power-Switch On-Resistance RON RON measured between SUPSW and LX,
ILX = 1A, VBIAS = 5V 70 150 mI
High-Side Switch Leakage
Current VSUP = 36V, VLX = 0V, TA = +25°C 1FA
TRANSCONDUCTANCE AMPLIFIER (COMP)
FB Input Current IFB 10 nA
FB Regulation Voltage VFB
FB connected to an external resistive
divider; 0°C < TA < +125°C 0.99 1.0 1.01 V
-40°C < TA < +125°C 0.985 1.0 1.015
FB Line Regulation DVLINE 6V < VSUP < 36V 0.02 %/V
Transconductance (from FB to
COMP) gmVFB = 1V, VBIAS = 5V (Note 2) 900 FS
Minimum On-Time tON_MIN (Note 2) 110 ns
Maximum Duty Cycle DCMAX
fSW = 1MHz 98 %
fSW = 220kHz 99
OSCILLATOR FREQUENCY
Oscillator Frequency RFOSC = 66.5kI360 400 444 kHz
EXTERNAL CLOCK INPUT (FSYNC)
FSYNC Input Current TA = +25°C 1FA
External Input Clock Acquisition
Time tFSYNC 1 Cycles
External Input Clock Frequency (Note 2) fOSC +
10% Hz
External Input Clock
High Threshold VFSYNC_HI VFSYNC rising 1.4 V
External Input Clock
Low Threshold VFSYNC_LO VFSYNC falling 0.4 V
Soft-Start Time tSS 8.5 ms
4Maxim Integrated
MAX16909
36V, 220kHz to 1MHz Step-Down Converter
with Low Operating Current
ELECTRICAL CHARACTERISTICS (continued)
(VSUP = VSUPSW = 14V, VEN = 14V, RFOSC = 66.5kI, TA = TJ = -40NC to +125NC, unless otherwise noted. Typical values are at
TA = +25NC.)
Note 2: Guaranteed by design; not production tested.
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
ENABLE INPUT (EN)
Enable Input-High Threshold VEN_HI 2 V
Enable Input-Low Threshold VEN_LO 0.9 V
Enable Threshold Voltage
Hysteresis VEN,HYS 0.2 V
Enable Input Current IEN TA = +25°C 1FA
RESET
Output Overvoltage Trip
Threshold VOUT_OV 105 110 115 %VFB
PGOOD Switching Level VTH_RISING VFB rising, VPGOOD = high 93 95 97 %VFB
VTH_FALLING VFB falling, VPGOOD = low 90 92.5 95
PGOOD Debounce 10 35 60 Fs
PGOOD Output Low Voltage ISINK = 5mA 0.4 V
PGOOD Leakage Current VOUT in regulation, TA = +25NC1FA
5Maxim Integrated
MAX16909
36V, 220kHz to 1MHz Step-Down Converter
with Low Operating Current
Typical Operating Characteristics
(VSUP = VSUPSW = VEN = 14V, VOUT = 1.8V, R1 = 80.6kΩ, R2 = 100kΩ, TA = +25NC (Figure 4), unless otherwise noted.)
SWITCHING FREQUENCY vs. ILOAD
(1.8V/400kHz)
MAX16909 toc06
SWITCHING FREQUENCY (kHz)
391
392
393
394
395
396
397
398
399
400
390
ILOAD (A)
2.52.01.51.00.50 3.0
VIN = 14V
SWITCHING FREQUENCY vs. RFOSC
MAX16909 toc05
RFOSC (kI)
SWITCHING FREQUENCY (MHz)
1009070 8040 50 6030
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0
20 110
VIN = 14V
0.010.0010.00010 0.1
EFFICIENCY vs. LOAD CURRENT
VIN = 14V
MAX16909 toc04
LOAD CURRENT (A)
EFFICIENCY (%)
10
20
30
40
50
60
70
80
90
100
0
1.8V
3.3V
5V
8V
D1: B360B-13-F, DIODES
L1: DRA125-150-R,
COOPER BUSSMANN
EFFICIENCY vs. LOAD CURRENT
VIN = 14V
MAX16909 toc03
LOAD CURRENT (A)
EFFICIENCY (%)
0 0.5 1.51.0 2.52.0 3.0
10
20
30
40
50
60
70
80
90
100
0
DIODE: B360B-13-F FROM DIODES
INDUCTOR: DRA125-150-R,
COOPER BUSSMANN
1.8V
3.3V 5V 8V
MAX16909 toc02
VIN
VOUT
VPGOOD
5V/div
5V/div
1V/div
0V
0V
0V
2ms/div
STARTUP INTO NO LOAD
(1.8V/400kHz)
STARTUP INTO HEAVY LOAD
(1.8V/400kHz)
MAX16909 toc01
VIN
VOUT
ILOAD
VPGOOD
5V/div
5V/div
1V/div
2A/div
0V
0V
0A
0V
2ms/div
0.6I RESISTIVE LOAD
6Maxim Integrated
MAX16909
36V, 220kHz to 1MHz Step-Down Converter
with Low Operating Current
Typical Operating Characteristics (continued)
(VSUP = VSUPSW = VEN = 14V, VOUT = 1.8V, R1 = 80.6kΩ, R2 = 100kΩ, TA = +25NC (Figure 4), unless otherwise noted.)
OUTPUT RESPONSE TO SLOW INPUT
(ILOAD = 3A)
MAX16909 toc12
VIN
VLX
VPGOOD
VOUT
5V/div
10V/div
10V/div
2V/div
0V
0V
0V
0V
2s/div
1.8V/400kHz
0.6I RESISTIVE LOAD
LOAD DUMP TEST (1.8V/400kHz)
MAX16909 toc11
VIN
VOUT
10V/div
2V/div
0V
0V
10ms/div
14V
42V
UNDERVOLTAGE PULSE (1.8V/400kHz)
MAX16909 toc10
VSUPSW
VLX
ILOAD
VOUT 2V/div
5V/div
20V/div
5A/div
0A
0V
0V
0V
20ms/div
SUP = 5V
RESISITIVE LOAD = 0.6I
FSYNC TRANSITION FROM INTERNAL
TO EXTERNAL FREQUENCY (1.8V/400kHz)
MAX16909 toc09
VFSNC
VLX
5V/div
10V/div
0V
0V
2µs/div
fFSYNC = 440kHz
SKIP-MODE LOAD-TRANSIENT
RESPONSE (1.8V/400kHz)
MAX16909 toc08
VOUT
AC-COUPLED
ILOAD
50mV/div
10mA/div
0A
100µs/div
VIN = 14V
PWM MODE LOAD-TRANSIENT
RESPONSE (1.8V/400kHz)
MAX16909 toc07
VOUT
AC-COUPLED
ILOAD
200mV/div
1A/div
0A
100µs/div
VIN = 14V
0.5A
7Maxim Integrated
MAX16909
36V, 220kHz to 1MHz Step-Down Converter
with Low Operating Current
Typical Operating Characteristics (continued)
(VSUP = VSUPSW = VEN = 14V, VOUT = 1.8V, R1 = 80.6kΩ, R2 = 100kΩ, TA = +25NC (Figure 4), unless otherwise noted.)
VOUT vs. SUPPLY VOLTAGE
(1.8V/400kHz)
MAX16909 toc17
SUPPLY VOLTAGE (V)
VOUT (V)
302418126
03
6
ILOAD = 0A
1.76
1.77
1.78
1.79
1.80
1.81
1.82
1.83
1.84
1.85
1.75
VOUT vs. SUPPLY VOLTAGE
(1.8V/400kHz)
MAX16909 toc16
SUPPLY VOLTAGE (V)
30
2418126
03
6
ILOAD = 3A
VOUT (V)
1.76
1.77
1.78
1.79
1.80
1.81
1.82
1.83
1.84
1.85
1.75
VOUT vs. TEMPERATURE
(1.8V/400kHz)
VOUT (V)
1.72
1.74
1.76
1.78
1.82
1.84
1.86
1.88
1.90
1.70
MAX16909 toc15
1.80
TEMPERATURE (°C)
1109580655035205-10-25-40 125
ILOAD = 3A
ILOAD = 0A
VIN = 14V
VOUT LOAD REGULATION (1.8V/400kHz)
MAX16909 toc14
ILOAD (A)
VOUT (V)
2.52.01.51.00.5
1.76
1.77
1.78
1.79
1.80
1.81
1.82
1.83
1.84
1.85
1.75
0 3.0
VIN = 14V
MAX16909 toc13
VOUT
ILX
VPGOOD
5V/div
1V/div
2A/div
0A
0V
0V
10ms/div
SHORT CIRCUIT TO GROUND TEST
(1.8V/400kHz)
8Maxim Integrated
MAX16909
36V, 220kHz to 1MHz Step-Down Converter
with Low Operating Current
Typical Operating Characteristics (continued)
(VSUP = VSUPSW = VEN = 14V, VOUT = 1.8V, R1 = 80.6kΩ, R2 = 100kΩ, TA = +25NC (Figure 4), unless otherwise noted.)
181612 144 6 8 102
02
0
VBIAS LOAD REGULATION
(1.8V/400kHz)
MAX16909 toc18
IBIAS (mA)
VBIAS (V)
5.20
5.18
5.16
5.14
5.12
5.10
5.08
5.06
5.04
5.02
5.00
4.98
4.96
4.94
4.92
4.90
TA = 125°C
TA = -40°C
TA = 25°C
MAX16909 toc22
VOUT
VIN
VPGOOD
VLX 10V/div
2V/div
10V/div
5V/div
0V
0V
0V
0V
10ms/div
LINE TRANSIENT TEST
(1.8V/400kHz)
1.8V/400kHz
0.6I LOAD
MAX16909 toc21
VOUT
VIN
VPGOOD
VLX
10V/div
2V/div
10V/div
5V/div
0V
0V
0V
0V
10ms/div
DIPS AND DROP TEST
(1.8V/400kHz)
0.6I RESISTIVE LOAD
ISHDN vs. TEMPERATURE
ISHDN (µA)
4
4
5
5
5
5
6
6
6
4
MAX16909 toc20
5
TEMPERATURE (°C)
1109580655035205-10-25-40 125
VEN = 0V
ISHDN vs. SUPPLY VOLTAGE
MAX16909 toc19
SUPPLY VOLTAGE (V)
ISHDN (µA)
3831241710
2
4
6
8
10
12
14
16
18
20
0
34
5
TA = -40°C
TA = 25°C
TA = 125°C
VEN = 0V
VIN = 14V
9Maxim Integrated
MAX16909
36V, 220kHz to 1MHz Step-Down Converter
with Low Operating Current
Pin Configurations
Pin Descriptions
PIN NAME FUNCTION
TSSOP TQFN
1 15 FSYNC
Synchronization Input. The device synchronizes to an external signal applied to FSYNC.
The external clock frequency must be 10% greater than the internal clock frequency for
proper operation. Connect FSYNC to GND if the internal clock is used.
2 16 FOSC Resistor-Programmable Switching-Frequency Setting Control Input. Connect a resistor
from FOSC to GND to set the switching frequency.
3 1 PGOOD Open-Drain, Active-Low Output. PGOOD asserts when VOUT is below the 92.5% regula-
tion point. PGOOD deasserts when VOUT is above the 95% regulation point.
4 2 OUT Switch Regulator Output. OUT also provides power to the internal circuitry when the out-
put voltage of the converter is set between 3V and 5V during standby mode.
5 3 FB Feedback Input. Connect an external resistive divider from OUT to FB and GND to set
the output voltage. Connect to BIAS to set the output voltage to 5V.
6 4 COMP Error-Amplifier Output. Connect an RC network from COMP to GND for stable operation.
See the Compensation Network section for more details.
7 5 BIAS Linear Regulator Output. BIAS powers up the internal circuitry. Bypass with a 1FF
capacitor to ground.
8 6 GND Ground
9 7 BST High-Side Driver Supply. Connect a 0.1FF capacitor between LX and BST for proper
operation.
++
TSSOP
13
4
SUPSW
OUT
14
3
SUPSW
PGOOD
15
2
EN
FOSC
16
1
TOP VIEW
I.C.FSYNC
10
7
SUP
BIAS
11
6
LX
COMP
9
8
BST
GND
12
5
LX
FB
EP EP
MAX16909 MAX16909
15
16
14
13
6
5
7
OUT
COMP
8
PGOOD
SUPSW
LX
SUPSW
12
I.C.
4
12 11 9
FSYNC
FOSC
SUP
BST
GND
BIAS
FB LX
3
10
EN
TQFN
(5mm × 5mm)
TOP VIEW
10Maxim Integrated
MAX16909
36V, 220kHz to 1MHz Step-Down Converter
with Low Operating Current
Pin Descriptions (continued)
Internal Block Diagram
PIN NAME FUNCTION
TSSOP TQFN
10 8 SUP Voltage Supply Input. SUP powers up the internal linear regulator. Connect a minimum
4.7FF capacitor to ground.
11, 12 9, 10 LX Inductor Switching Node. Connect a Schottky diode between LX and GND.
13, 14 11, 12 SUPSW Internal High-Side Switch-Supply Input. SUPSW provides power to the internal switch.
Connect a 0.1FF decoupling capacitor and a 4.7FF ceramic capacitor to ground.
15 13 EN SUP Voltage-Compatible Enable Input. Drive EN low to disable the device. Drive EN
high to enable the device.
16 14 I.C. Internally Connected. Connect to ground for proper operation.
— — EP
Exposed Pad. Connect EP to a large-area contiguous copper ground plane for effective
power dissipation. Do not use as the only IC ground connection. EP must be connected
to GND.
MAX16909
FBSW
OUT COMP PGOOD EN
FB
SOFT-
START
SLOPE
COMP
FBOK
EAMP
HSD
AON
LOGIC
CS
REF
HVLDO
SWITCH-
OVER
OSC
PWM
SUP BIAS
BST
SUPSW
LX
GND
FSYNC FOSC
11Maxim Integrated
MAX16909
36V, 220kHz to 1MHz Step-Down Converter
with Low Operating Current
Detailed Description
The MAX16909 is a constant-frequency, current-mode,
automotive buck converter with an integrated high-side
switch. The device operates with input voltages from 3.5V
to 36V and tolerates input transients from 3.5V up to 42V.
During undervoltage events, such as cold-crank condi-
tions, the internal pass device maintains 98% duty cycle.
The switching frequency is resistor programmable from
220kHz to 1MHz to allow optimization for efficiency, noise,
and board space. A synchronization input FSYNC allows
the device to synchronize to an external clock frequency.
During light-load conditions, the device enters skip mode
for high efficiency. The 5V fixed output voltage eliminates
the need for external resistors and reduces the supply
current to 30FA. See the Internal Block Diagram for more
information.
Wide Input Voltage Range (3.5V to 36V)
The device includes two separate supply inputs, SUP
and SUPSW, specified for a wide 3.5V to 36V input
voltage range. VSUP provides power to the device and
VSUPSW provides power to the internal switch. When
the device is operating with a 3.5V input supply, certain
conditions such as cold crank can cause the voltage at
SUPSW to drop below the programmed output voltage.
As such, the device operates in a high duty-cycle mode
to maintain output regulation.
Linear Regulator Output (BIAS)
The device includes a 5V linear regulator, BIAS, that
provides power to the internal circuitry. Connect a 1FF
ceramic capacitor from BIAS to GND.
External Clock Input (FSYNC)
The device synchronizes to an external clock signal
applied at FSYNC. The signal at FSYNC must have a
10% higher frequency than the internal clock frequency
for proper synchronization.
Soft-Start
The device includes an 8.5ms fixed soft-start time for up
to 500FF capacitive load with a 3A resistive load.
Minimum On-Time
The device features a 110ns minimum on-time that
ensures proper operation at 1MHz switching frequency
and high differential voltage between the input and the
output. This feature is extremely beneficial in automo-
tive applications where the board space is limited and
the converter needs to maintain a well-regulated output
voltage using an input voltage that varies from 9V to
18V. Additionally, the device incorporates an innovative
design for fast-loop response that further ensures good
output-voltage regulation during transients.
System Enable (EN)
An enable-control input (EN) activates the device from its
low-power shutdown mode. EN is compatible with inputs
from automotive battery level down to 3.3V. The high-
voltage compatibility allows EN to be connected to SUP,
KEY/KL30, or the INH pin of a CAN transceiver.
EN turns on the internal regulator. Once VBIAS is above
the internal lockout threshold, VUVL = 3.15V (typ), the
controller activates and the output voltage ramps up
within 8.5ms.
A logic-low at EN shuts down the device. During shut-
down, the internal linear regulator and gate drivers turn
off. Shutdown is the lowest power state and reduces the
quiescent current to 5FA (typ). Drive EN high to bring the
device out of shutdown.
Overvoltage Protection
The device includes overvoltage protection circuitry that
protects the device when there is an overvoltage condi-
tion at the output. If the output voltage increases by more
than 110% of its set voltage, the device stops switching.
The device resumes regulation once the overvoltage
condition is removed.
Fast Load-Transient Response
Current-mode buck converters include an integrator
architecture and a load-line architecture. The integra-
tor architecture has large loop gain but slow transient
response. The load-line architecture has fast transient
response but low loop gain. The device features an inte-
grator architecture with innovative designs to improve
transient response. Thus, the device delivers high output-
voltage accuracy, plus the output can recover quickly
from a transient overshoot, which could damage other
on-board components during load transients.
Overload Protection
The overload protection circuitry is triggered when the
device is in current limit and VOUT is below the reset
threshold. Under these conditions the device turns the
high-side FET off for 16ms and re-enters soft-start. If the
overload condition is still present, the device repeats the
cycle.
12Maxim Integrated
MAX16909
36V, 220kHz to 1MHz Step-Down Converter
with Low Operating Current
Skip Mode/Standby Mode
During light-load operation the device enters skip mode
operation. Skip mode turns off the majority of circuitry
and allows the output to drop below regulation voltage
before the switch is turned on again. The lower the load
current, the longer it takes for the regulator to initiate
a new cycle. Because the converter skips unneces-
sary cycles and turns off the majority of circuitry, the
converter efficiency increases. When the high-side FET
stops switching for more than 50Fs, most of the internal
circuitry, including LDO, draws power from VOUT (VOUT
= 3V to 5.5V), allowing current consumption from the bat-
tery to drop to only 30FA.
Overtemperature Protection
Thermal-overload protection limits the total power dis-
sipation in the device. When the junction temperature
exceeds +175NC (typ), an internal thermal sensor
shuts down the internal bias regulator and the step-
down converter, allowing the IC to cool. The thermal
sensor turns on the IC again after the junction tempera-
ture cools by 15NC.
Applications Information
Setting the Output Voltage
Connect FB to BIAS for a fixed 5V output voltage. To set
the output to other voltages between 1V and 10V, con-
nect a resistive divider from output (OUT) to FB to GND
(Figure 1). Calculate RFB1 (OUT to FB resistor) with the
following equation:
OUT
FB1 FB2 FB
V
RR 1
V
= −
where VFB = 1V (see the Electrical Characteristics table).
Internal Oscillator
The switching frequency, fSW, is set by a resistor (RFOSC)
connected from FOSC to GND. See Figure 2 to select the
correct RFOSC value for the desired switching frequency.
For example, a 400kHz switching frequency is set with
RFOSC = 65kI. Higher frequencies allow designs with
lower inductor values and less output capacitance.
Consequently, peak currents and I2R losses are lower
at higher switching frequencies, but core losses, gate
charge currents, and switching losses increase.
Inductor Selection
Three key inductor parameters must be specified for
operation with the device: inductance value (L), inductor
saturation current (ISAT), and DC resistance (RDCR). To
select inductance value, the ratio of inductor peak-to-
peak AC current to DC average current (LIR) must be
selected first. A good compromise between size and loss
is a 30% peak-to-peak ripple current to average-current
ratio (LIR = 0.3). The switching frequency, input voltage,
output voltage, and selected LIR then determine the
inductor value as follows:
OUT SUP OUT
SUP SW OUT
V(V V)
LV f I LIR
−
=
where VSUP, VOUT, and IOUT are typical values (so that
efficiency is optimum for typical conditions). The switch-
ing frequency is set by RFOSC (see the Internal Oscillator
section). The exact inductor value is not critical and can
be adjusted to make trade-offs among size, cost, efficien-
cy, and transient response requirements. Table 1 shows
a comparison between small and large inductor sizes.
Figure 1. Adjustable Output-Voltage Setting
Figure 2. Switching Frequency vs. RFOSC
RFB2
RFB1
FB
MAX16909
VOUT
SWITCHING FREQUENCY vs. RFOSC
MAX16909 toc05
RFOSC (kI)
SWITCHING FREQUENCY (MHz)
1009070 8040 50 6030
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0
20 110
VIN = 14V
13Maxim Integrated
MAX16909
36V, 220kHz to 1MHz Step-Down Converter
with Low Operating Current
The inductor value must be chosen so that the maximum
inductor current does not reach the device’s minimum
current limit. The optimum operating point is usually
found between 25% and 35% ripple current. When pulse
skipping (FSYNC low and light loads), the inductor value
also determines the load-current value at which PFM/
PWM switchover occurs.
Find a low-loss inductor having the lowest possible
DC resistance that fits in the allotted dimensions. Most
inductor manufacturers provide inductors in standard
values, such as 1.0FH, 1.5FH, 2.2FH, 3.3FH, etc. Also
look for nonstandard values, which can provide a bet-
ter compromise in LIR across the input voltage range. If
using a swinging inductor (where the no-load inductance
decreases linearly with increasing current), evaluate
the LIR with properly scaled inductance values. For
the selected inductance value, the actual peak-to-peak
inductor ripple current (DIINDUCTOR) is defined by:
OUT SUP OUT
INDUCTOR SUP SW
V(V V)
IV fL
−
∆=
××
where DIINDUCTOR is in A, L is in H, and fSW is in Hz.
Ferrite cores are often the best choices, although pow-
dered iron is inexpensive and can work well at 200kHz.
The core must be large enough not to saturate at the
peak inductor current (IPEAK):
INDUCTOR
PEAK LOAD(MAX)
I
II 2
∆
= +
Input Capacitor
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
ripple on the input caused by the circuit’s switching.
The input capacitor RMS current requirement (IRMS) is
defined by the following equation:
OUT SUP OUT
RMS LOAD(MAX) SUP
V(V V)
II V
−
=
IRMS has a maximum value when the input voltage
equals twice the output voltage (VSUP = 2VOUT), so
IRMS(MAX) = ILOAD(MAX)/2.
Choose an input capacitor that exhibits less than +10NC
self-heating temperature rise at the RMS input current for
optimal long-term reliability.
The input-voltage ripple is composed of DVQ (caused
by the capacitor discharge) and DVESR (caused by the
equivalent series resistance (ESR) of the capacitor). Use
low-ESR ceramic capacitors with high ripple-current
capability at the input. Assume the contribution from the
ESR and capacitor discharge equal to 50%. Calculate
the input capacitance and ESR required for a specified
input-voltage ripple using the following equations:
ESR
IN L
OUT
V
ESR I
I2
∆
=∆
+
where
SUP OUT OUT
LSUP SW
(V V ) V
IV fL
−×
∆= ××
and
OUT
IN Q SW
I D(1 D)
CVf
×−
=∆×
and
OUT
SUPSW
V
DV
=
where IOUT is the maximum output current, and D is the
duty cycle.
Output Capacitor
The output filter capacitor must have low enough ESR to
meet output ripple and load-transient requirements, yet
have high enough ESR to satisfy stability requirements.
The output capacitance must be high enough to absorb
the inductor energy while transitioning from full-load
to no-load conditions without tripping the overvoltage
fault protection. When using high-capacitance, low-ESR
capacitors, the filter capacitor’s ESR dominates the
output-voltage ripple. So the size of the output capaci-
tor depends on the maximum ESR required to meet the
output-voltage ripple (VRIPPLE(P-P)) specifications:
VRIPPLE(P-P) = ESR × ILOAD(MAX) × LIR
The actual capacitance value required relates to the
physical size needed to achieve low ESR, as well as
to the chemistry of the capacitor technology. Thus, the
capacitor is usually selected by ESR and voltage rating
rather than by capacitance value.
Table 1. Inductor Size Comparison
INDUCTOR SIZE
SMALLER LARGER
Lower price Smaller ripple
Smaller form factor Higher efficiency
Faster load response Larger fixed-frequency
range in skip mode
14Maxim Integrated
MAX16909
36V, 220kHz to 1MHz Step-Down Converter
with Low Operating Current
When using low-capacity filter capacitors, such as
ceramic capacitors, size is usually determined by the
capacity needed to prevent voltage droop and volt-
age rise from causing problems during load transients.
Generally, once enough capacitance is added to meet
the overshoot requirement, undershoot at the rising load
edge is no longer a problem. However, low-capacity filter
capacitors typically have high-ESR zeros that can affect
the overall stability.
Rectifier Selection
The device requires an external Schottky diode recti-
fier as a freewheeling diode. Connect this rectifier close
to the device using short leads and short PCB traces.
Choose a rectifier with a voltage rating greater than the
maximum expected input voltage, VSUPSW. Use a low
forward-voltage-drop Schottky rectifier to limit the nega-
tive voltage at LX. Avoid higher than necessary reverse-
voltage Schottky rectifiers that have higher forward-
voltage drops.
Compensation Network
The device uses an internal transconductance error
amplifier with its inverting input and its output available
to the user for external frequency compensation. The
output capacitor and compensation network determine
the loop stability. The inductor and the output capaci-
tor are chosen based on performance, size, and cost.
Additionally, the compensation network optimizes the
control-loop stability.
The controller uses a current-mode control scheme that
regulates the output voltage by forcing the required current
through the external inductor. The device uses the volt-
age drop across the high-side MOSFET to sense inductor
current. Current-mode control eliminates the double pole
in the feedback loop caused by the inductor and output
capacitor, resulting in a smaller phase shift and requiring
less elaborate error-amplifier compensation than voltage-
mode control. Only a simple single-series resistor (RC)
and capacitor (CC) are required to have a stable, high-
bandwidth loop in applications where ceramic capacitors
are used for output filtering (Figure 3). For other types of
capacitors, due to the higher capacitance and ESR, the
frequency of the zero created by the capacitance and
ESR is lower than the desired closed-loop crossover fre-
quency. To stabilize a nonceramic output capacitor loop,
add another compensation capacitor (CF) from COMP to
GND to cancel this ESR zero.
The basic regulator loop is modeled as a power modula-
tor, output feedback divider, and an error amplifier. The
power modulator has a DC gain set by gmc x RLOAD,
with a pole and zero pair set by RLOAD, the output
capacitor (COUT), and its ESR. The following equations
allow to approximate the value for the gain of the power
modulator (GAINMOD(DC)), neglecting the effect of the
ramp stabilization. Ramp stabilization is necessary when
the duty cycle is above 50% and is internally done for
the device.
MOD(DC) mc LOAD
GAIN g R= ×
where RLOAD = VOUT/ILOUT(MAX) in I, fSW is the switch-
ing frequency in MHz, L is the output inductance in H,
and gmc = 3S.
In a current-mode step-down converter, the output
capacitor, its ESR, and the load resistance introduce a
pole at the following frequency:
( )
pMOD OUT LOAD
1
f2 C R ESR
=π× × +
The output capacitor and its ESR also introduce a zero at:
zMOD OUT
1
f2 ESR C
=π× ×
When COUT is composed of “n” identical capacitors
in parallel, the resulting COUT = n x COUT(EACH) and
ESR = ESR(EACH)/n. Note that the capacitor zero for a
parallel combination of alike capacitors is the same as
for an individual capacitor.
Figure 3. Compensation Network
R2
R1
VREF
VOUT
RC
CC
CF
COMP
gm
15Maxim Integrated
MAX16909
36V, 220kHz to 1MHz Step-Down Converter
with Low Operating Current
The feedback voltage-divider has a gain of GAINFB =
VFB/VOUT, where VFB is 1V (typ).
The transconductance error amplifier has a DC gain of
GAINEA(DC) = gm,EA x ROUT,EA, where gm,EA is the error-
amplifier transconductance, which is 900FS (typ), and
ROUT,EA is the output resistance of the error amplifier.
A dominant pole (fdpEA) is set by the compensa-
tion capacitor (CC) and the amplifier output resistance
(ROUT,EA). A zero (fzEA) is set by the compensation
resistor (RC) and the compensation capacitor (CC).
There is an optional pole (fpEA) set by CF and RC to
cancel the output capacitor ESR zero if it occurs near
the crossover frequency (fC, where the loop gain equals
1 (0dB)). Thus:
dpEA C O U T ,E A C
1
f2 C (R R )
=π× × +
zEA CC
1
f2C R
=π× ×
pEA FC
1
f2CR
=π× ×
The loop-gain crossover frequency (fC) should be set
below 1/5th of the switching frequency and much higher
than the power-modulator pole (fpMOD):
SW
pMOD C
f
ff
5
<< ≤
The total loop gain as the product of the modulator gain,
the feedback voltage-divider gain, and the error-amplifier
gain at fC should be equal to 1. So:
FB
MOD(fC) EA(fC)
OUT
V
GAIN GAIN 1
V
×× =
For the case where fzMOD is greater than fC:
GAINEA(fC) = gm,EA × RC
pMOD
MOD(fC) MOD(dc) C
f
GAIN GAIN f
= ×
Therefore:
FB
MOD(fC) m,EA C
OUT
V
GAIN g R 1
V
× × ×=
Solving for RC:
OUT
Cm,EA FB MOD(fC)
V
Rg V GAIN
=××
Set the error-amplifier compensation zero formed by RC
and CC (fzEA) at the fpMOD. Calculate the value of CC
as follows:
CpMOD C
1
C2f R
=π× ×
If fzMOD is less than 5 x fC, add a second capacitor,
CF, from COMP to GND and set the compensation pole
formed by RC and CF (fpEA) at the fzMOD. Calculate the
value of CF as follows:
FzMOD C
1
C2f R
=π× ×
As the load current decreases, the modulator pole
also decreases; however, the modulator gain increases
accordingly and the crossover frequency remains the
same.
For the case where fzMOD is less than fC:
The power-modulator gain at fC is:
pMOD
MOD(fC) MOD(dc) zMOD
f
GAIN GAIN f
= ×
The error-amplifier gain at fC is:
zMOD
EA(fC) m,EA C C
f
GAIN g R f
= ××
Therefore:
zMOD
FB
MOD(fC) m,EA C
OUT C
f
V
GAIN g R 1
Vf
× × ×× =
Solving for RC:
OUT C
Cm,EA FB MOD(fC) zMOD
Vf
Rg V GAIN f
×
=×× ×
Set the error-amplifier compensation zero formed by RC
and CC at the fpMOD (fzEA = fpMOD) as follows:
CpMOD C
1
C2f R
=π× ×
If fzMOD is less than 5 x fC, add a second capacitor CF
from COMP to GND. Set fpEA = fzMOD and calculate CF
as follows:
FzMOD C
1
C2f R
=π× ×
16Maxim Integrated
MAX16909
36V, 220kHz to 1MHz Step-Down Converter
with Low Operating Current
PCB Layout Guidelines
Careful PCB layout is critical to achieve low switching
losses and clean, stable operation. Use a multilayer
board whenever possible for better noise immunity and
power dissipation. Follow these guidelines for good PCB
layout:
1) Use a large contiguous copper plane under the IC
package. Ensure that all heat-dissipating components
have adequate cooling. The bottom pad of the device
must be soldered down to this copper plane for effec-
tive heat dissipation and for getting the full power out
of the IC. Use multiple vias or a single large via in this
plane for heat dissipation.
2) Isolate the power components and high-current path
from the sensitive analog circuitry. This is essential to
prevent any noise coupling into the analog signals.
3) Keep the high-current paths short, especially at the
ground terminals. This practice is essential for stable,
jitter-free operation. The high-current path composed
of input capacitor, high-side FET, inductor, and output
capacitor should be as short as possible.
4) Keep the power traces and load connections short.
This practice is essential for high efficiency. Use
thick copper PCBs (2oz vs. 1oz) to enhance full-load
efficiency.
5) The analog signal lines should be routed away from
the high-frequency planes. This ensures integrity of
sensitive signals feeding back into the IC.
6) The ground connection for the analog and power
section should be close to the IC. This keeps the
ground current loops to a minimum. In cases where
only one ground is used, enough isolation between
analog return signals and high-power signals must
be maintained.
Figure 4. 1.8V/3A Configuration
D1 COUT
100µF
C2
4.7µF
C5
0.1µF
RCOMP
9.1kI
RPGOOD
10kI
RFOSC
62kI
L1
15µH VOUT
1.8V AT 3A
CBST
0.1µF
LX
BST
VOUT
OUT
VBAT
FB
VBIAS
PGOOD
FOSC
CBIAS
1µF
CCOMP2
12pF
BIAS
CCOMP1
821pF
COMP
FSYNC
EN
SUPSWSUP
GND
C1
47µF
C4
0.1µF
C3
4.7µF
POWER GOOD
MAX16909
R1
80.6kI
R2
100kI
17Maxim Integrated
MAX16909
36V, 220kHz to 1MHz Step-Down Converter
with Low Operating Current
Ordering Information
/V denotes an automotive qualified part.
+Denotes a lead(Pb)-free/RoHS-compliant package.
**EP = Exposed pad.
Package Information
For the latest package outline information and land patterns (foot-
prints), 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.
Chip Information
PROCESS: BiCMOS
PART TEMP RANGE PIN-PACKAGE
MAX16909RAUE+ -40NC to +125NC16 TSSOP-EP*
MAX16909RAUE/V+ -40NC to +125NC16 TSSOP-EP*
MAX16909RATE+ -40NC to +125NC16 TQFN-EP*
MAX16909RATE/V+ -40NC to +125NC16 TQFN-EP* PACKAGE
TYPE
PACKAGE
CODE
OUTLINE
NO.
LAND
PATTERN NO.
16 TSSOP-EP U16E+3 21-0108 90-0120
16 TQFN-EP T1655+4 21-0140 90-0121
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied.
Maxim 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 18
© 2012 Maxim Integrated The Maxim logo and Maxim Integrated are trademarks of Maxim Integrated Products, Inc.
MAX16909
36V, 220kHz to 1MHz Step-Down Converter
with Low Operating Current
Revision History
REVISION
NUMBER
REVISION
DATE DESCRIPTION PAGES
CHANGED
0 3/11 Initial release —
1 9/11
Changed RFOSC = 120kI to RFOSC =66.5kI in the Electrical Characteristics
globals and table; changed the min, typ, max values for RFOSC from 190kHz
(min), 220kHz (typ), 250kHz (max) to 360kHz (min), 400kHz (typ), 444kHz (max);
changed the minimum on-time (tON_MIN) from 80ns (typ) to 110ns (typ); updated the
GAINMOD(DC) and fpMOD equations; removed future status from the 16-pin TQFN
package in the Ordering Information table
2, 3, 4, 11, 14,
17
2 8/12 Added two new OPNs in the Ordering Information table 17
