General Description
The MAX8901A/MAX8901B step-up converters drive from
two to six series-connected white LEDs (WLEDs) with
constant current to provide uniform WLED intensity for
LCD backlighting in cell phones, PDAs, and other hand-
held devices. The MAX8901_ operate at a fixed 750kHz
(typ) switching frequency, allowing for tiny external com-
ponents, and are optimized for the highest possible effi-
ciency over the full 1-cell Li+/Li-Poly battery range.
These converters use a single input (ON) to enable the
IC and to control WLED intensity. The MAX8901A
requires a direct PWM input to regulate WLED intensity
with the WLED current proportional to the PWM duty
cycle. The MAX8901B uses single-wire, serial-pulse
dimming that reduces the WLED intensity in 32 linear
steps. Full-scale WLED current for serial-pulse
dimming is 24.75mA (MAX8901B, 0.75mA/step).
The MAX8901_ feature an internal soft-start to eliminate
inrush currents during startup, input overvoltage
protection, WLED overvoltage protection, and a shut-
down mode with 0.01µA (typ) shutdown current. No
WLED current is present in shutdown provided the
WLED forward voltage is greater than the input supply
voltage. Additional features include undervoltage lock-
out (UVLO) and thermal shutdown.
The MAX8901_ are available in tiny 8-pin, 2mm x 2mm
TDFN-EP packages (0.8mm max height).
Applications
Display Backlight (from 2 to 6 WLEDs)
Cellular Phones
PDAs and Smartphones
MP3 and Portable Media Players
Portable Navigation Devices
Digital Cameras
Features
oHigh Efficiency, Up to 91%
o2.6V to 5.5V Input Voltage Range
oFixed-Frequency Operation
oSupplies from 2 to 6 WLEDs with 1.3% LED Current
Accuracy
oFlexible Dimming Control
Direct PWM Dimming (MAX8901A)
32-Step, Single-Wire Serial Dimming
(MAX8901B)
oInput Undervoltage Lockout
oInput Overvoltage Lockout
oWLED Overvoltage Protection (25V typ)
o0.01µA (typ) Shutdown Current
oNo WLED Current in Shutdown
oInternal Soft-Start and Thermal Shutdown
MAX8901A/MAX8901B
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
________________________________________________________________
Maxim Integrated Products
1
Ordering Information
MAX8901A
IN
COMP
GND
ON
LX
OVP
PGND
CS
OUTPUT
UP TO 25V
4
1
5
8
2
EP
3
7
6
INPUT
2.6V TO 5.5V
PWM DIMMING
CONTROL
Typical Operating Circuits
19-0865; Rev 3; 8/10
Note: All devices are specified over the -40°C to +85°C operating
temperature range.
+
Denotes a lead(Pb)-free/RoHS-compliant package.
*
EP = Exposed pad.
T = Tape and reel.
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim's website at www.maxim-ic.com.
EVALUATION KIT
AVAILABLE
PART PIN-PACKAGE TOP MARK
MAX8901AETA+T 8 TDFN-EP*
2mm x 2mm +ABA
MAX8901BETA+T 8 TDFN-EP*
2mm x 2mm +ABB
MAX8901B
IN
COMP
GND
ON
LX
OVP
PGND
CS
OUTPUT
UP TO 25V
4
1
5
8
2
EP
3
7
6
INPUT
2.6V TO 5.5V
SERIAL-PULSE
DIMMING
CONTROL
Pin Configuration appears at end of data sheet.
MAX8901A/MAX8901B
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VIN = VON = VOVP = 3.6V, VPGND = VGND = 0V, COMP, CS, and LX are unconnected, TA= -40°C to +85°C, unless otherwise noted.
Typical values are at TA= +25°C.) (Note 1)
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.
IN to GND .................................................................-0.3V to +7V
CS, COMP, ON to GND ...............................-0.3V to (VIN + 0.3V)
OVP, LX to GND .....................................................-0.3V to +28V
PGND to GND .......................................................-0.3V to +0.3V
LX Current ..................................................................770mARMS
Continuous Power Dissipation (multilayer board at +70°C)
8-pin, 2mm x 2mm TDFN
(derate above +70°C by 11.9mW/°C)..........................953mW
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) .......................................+260°C
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Input Voltage VIN 2.6 5.5 V
VIN rising 2.25 2.55
Input Undervoltage Lockout
Threshold VIN_UVLO
VIN falling 2.53
V
VIN rising 6.2 6.35
Input Overvoltage Lockout
Threshold VIN_OVLO VIN falling 6.0 6.3 V
TA = +25°C 0.01 1
Shutdown Input Current ISHDN VON = 0V TA = +85°C 0.1 µA
MAX8901A 70 135
Quiescent Current IQVCS = 0.55V, no load
(not switching) MAX8901B 115 185 µA
Output Voltage Range VOUT VDIODE = external boost diode voltage drop VIN -
VDIODE VOV V
OVP Overvoltage Protection
Threshold VOV 6 LEDs with 25V OVP option 24 25 26 V
OVP Input Current IOV VOVP = 20V 20 µA
CS Regulation Voltage VCS No dimming 0.475 0.50 0.525 V
ON Shutdown Delay tSHDN Time VON is below low threshold until
shutdown (Figure 1) 1.18 1.33 1.50 ms
ON High Voltage VON_HI 2.6V < VIN < 5.5V 1.3 V
ON Low Voltage VON_LO 2.6V < VIN < 5.5V 0.4 V
TA = +25°C 0.01 1
ON Input Current ION ON = IN TA = +85°C 0.1 µA
Initial ON High Pulse Width tHI_INIT First ON high pulse to enable IC
(MAX8901B) (Figure 1) 40 µs
ON High Pulse Width tHI MAX8901B (Figure 1) 0.5 µs
ON Low Pulse Width tLO MAX8901B (Figure 1) 0.5 500.0 µs
MAX8901A/MAX8901B
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
_______________________________________________________________________________________ 3
Note 1: Specifications to -40°C are guaranteed by design and characterization and are not production tested.
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
TA = +25°C -1.3 +1.3 %
Serial Dimming
Full-Scale LED Current Accuracy
ILED = 24.75mA
(MAX8901B) TA = -40°C to +85°C-3 +3 %
PWM Frequency for PWM
Dimming Control fPWM MAX8901A 100 kHz
CS to COMP Transconductance 40 60 80 µS
Soft-Start Interval tSOFT-
START CCOMP = 0.022µF (Figure 1) 10 ms
Thermal Shutdown 160 °C
OSCILLATOR
Operating Frequency fSW 700 750 800 kHz
Maximum Duty Cycle DMAX VCS = 0.4V 90 92 %
n-CHANNEL SWITCH
TA = +25°C 0.1 5
LX Leakage Current ILXLKG VLX = 27V TA = +85°C 0.1 µA
n-Channel Switch On-Resistance RLX 0.7 1.4 Ω
n-Channel Current Limit ILIM 0.63 0.70 0.77 A
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VON = VOVP = 3.6V, VPGND = VGND = 0V, COMP, CS, and LX are unconnected, TA= -40°C to +85°C, unless otherwise noted.
Typical values are at TA= +25°C.) (Note 1)
1
02 3 4 5 28 29 30 31 32
SHUTDOWN
tSHDN
1.33ms (typ)
2/33 1/33
3/33
4/33
5/33
6/33
28/33
29/33
30/33
31/33
32/33 32/33
SHUTDOWN
FULL FULL
ILED_
tSOFT-START
tHI_INIT
tLO
tHI
> 40μs
> 500ns
500ns TO 500μs
STEP 33
ON
Figure 1. MAX8901B Timing Diagram
MAX8901A/MAX8901B
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
4 _______________________________________________________________________________________
Typical Operating Characteristics
(Circuit of Figure 5, ILED = 24.75mA, VIN = VON = 3.6V, C1 = 1µF, C2 = 1µF, TA= +25°C, unless otherwise noted.)
90
85
80
75
70
24356
LED EFFICIENCY vs. INPUT VOLTAGE
MAX8901A/B toc01
INPUT VOLTAGE (V)
EFFICIENCY (%)
4 WLEDs
6 WLEDs
3 WLEDs
INDUCTOR: TOKO 1069AS-220M
DIODE: CENTRAL SEMI CMDSH05-4
90
85
80
75
70
24356
LED EFFICIENCY vs. INPUT VOLTAGE
MAX8901A/B toc02
INPUT VOLTAGE (V)
EFFICIENCY (%)
4 WLEDs
6 WLEDs
3 WLEDs
INDUCTOR: TOKO 1098AS-100M
DIODE: NXP PMEG3005EB
50
55
60
65
70
75
80
85
90
0 5 10 15 20 25
LED EFFICIENCY vs. LED CURRENT
(6 WLEDs)
MAX8901A/B toc03
LED CURRENT (mA)
EFFICIENCY (%)
VIN = 5V VIN = 5.5V
VIN = 3.6V
VIN = 4.2V VIN = 3V
INDUCTOR: TOKO 1069AS-220M
DIODE: CENTRAL SEMI CMDSH05-4
50
55
60
65
70
75
80
85
90
0 5 10 15 20 25
LED EFFICIENCY vs. LED CURRENT
(6 WLEDs)
MAX8901A/B toc04
LED CURRENT (mA)
EFFICIENCY (%)
VIN = 5V VIN = 5.5V
VIN = 3.6V
VIN = 4.2V
VIN = 3V
INDUCTOR: TOKO 1098AS-100M
DIODE: NXP PMEG3005EB
50
55
60
65
70
75
80
85
90
0 5 10 15 20 25
LED EFFICIENCY vs. LED CURRENT
(4 WLEDs)
MAX8901A/B toc05
LED CURRENT (mA)
EFFICIENCY (%)
VIN = 5V VIN = 5.5V
VIN = 3.6V
VIN = 4.2V
VIN = 3V
INDUCTOR: TOKO 1069AS-220M
DIODE: CENTRAL SEMI CMDSH05-4
50
55
60
65
70
75
80
85
90
0 5 10 15 20 25
LED EFFICIENCY vs. LED CURRENT
(4 WLEDs)
MAX8901A/B toc06
LED CURRENT (mA)
EFFICIENCY (%)
VIN = 5V VIN = 5.5V
VIN = 3.6V
VIN = 4.2V
VIN = 3V
INDUCTOR: TOKO 1098AS-100M
DIODE: NXP PMEG3005EB
MAX8901A/MAX8901B
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
_______________________________________________________________________________________
5
Typical Operating Characteristics (continued)
(Circuit of Figure 5, ILED = 24.75mA, VIN = VON = 3.6V, C1 = 1µF, C2 = 1µF, TA= +25°C, unless otherwise noted.)
70
75
85
80
90
95
LED AND BOOST EFFICIENCY
vs. INPUT VOLTAGE
MAX8901A/B toc07
INPUT VOLTAGE (V)
EFFICIENCY (%)
24356
BOOST
LED
6WLEDs,
INDUCTOR: TOKO A915AY-220M,
DIODE: CENTRAL SEMI CMDSH05-4
0
100
200
300
400
500
600
700
800
234567
IN CURRENT (IIN)
vs. SUPPLY VOLTAGE
MAX8901A/B toc08
SUPPLY VOLTAGE (V)
IIN (μA)
MAX8901B
MAX8901A
20ms/div
INPUT OVERVOLTAGE
PROTECTION RESPONSE
MAX8901A/B toc10
0V
0mA
10V/div
1V/div
(AC-COUPLED)
0V
20mA/div
20mA/div
VIN
VOUT
ILEDs
VLX
6.4V
5.4V
10ms/div
STARTUP AND SHUTDOWN
RESPONSE (MAX8901A)
MAX8901A/B toc11
0mA
0mA
10V/div
2V/div
0V
0V
20mA/div
100mA/div
VON
VOUT
ISUPPLY
ILEDs
C2 = 0.1μF
10ms/div
STARTUP AND SHUTDOWN
RESPONSE (MAX8901B)
MAX8901A/B toc12
0mA
0mA
10V/div
2V/div
0V
0V
20mA/div
100mA/div
VON
VOUT
ISUPPLY
ILEDs
MAX8901A/MAX8901B
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
6 _______________________________________________________________________________________
Typical Operating Characteristics (continued)
(Circuit of Figure 5, ILED = 24.75mA, VIN = VON = 3.6V, C1 = 1µF, C2 = 1µF, TA= +25°C, unless otherwise noted.)
VCS vs. LED CURRENT STEP
(MAX8901B)
LED CURRENT STEP
VCS (mV)
MAX8901A/B toc16
0 5 10 15 20 25 30 35
502.5
503.0
503.5
504.0
504.5
505.0
505.5
506.0
506.5
507.0
507.5
24.0
24.2
24.6
24.4
24.8
25.0
-40 10-15 35 60 85
MAXIMUM LED CURRENT
vs. TEMPERATURE (MAX8901B)
MAX8901A/B toc17
TEMPERATURE (°C)
ILED (mA)
MINIMUM LED CURRENT
vs. TEMPERATURE (MAX8901B)
MAX8901A/B toc18
TEMPERATURE (°C)
ILED (μA)
603510-15
735
740
745
750
730
-40 85
4ms/div
PWM DIMMING RESPONSE
(MAX8901A)
MAX8901A/B toc13
0V
0V
0mA
2V/div
0V
20mA/div
20mV/div
VON
VIN
VCS
VOUT
ILEDs
D = 10%
D = 90%
50mV/div
(AC-COUPLED)
500mV/div
fON_PWM = 100kHz, DUTY
CYCLE CHANGE: 90% TO
10%, C2 = 0.1μF
10ms/div
SERIAL PULSE DIMMING
RESPONSE (MAX8901B)
MAX8901A/B toc14
0V
0V
0mA
2V/div
0V
20mA/div
20mV/div
VON
VIN
VCS
VOUT
ILEDs
50mV/div
(AC-COUPLED)
1V/div
0
10
5
20
15
25
30
04020 60 80 100
LED CURRENT vs. PWM
DIMMING (MAX8901A)
MAX8901A/B toc15
PWM DUTY CYCLE (%)
ILED (mA)
RCS = 20Ω,
fPWM = 100kHz
MAX8901A/MAX8901B
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
_______________________________________________________________________________________ 7
Pin Description
PIN NAME FUNCTION
1 OVP
WLED Overvoltage Protection Input. OVP monitors voltage at the WLEDs. Connect OVP to the
positive terminal of the output capacitor. If an OVP condition is detected, the MAX8901_ latch off.
Cycle VIN or toggle VON to restart the IC.
2ON
Enable and Dimming Control Input. Drive ON high to enable the IC. Drive ON low for greater than
1.33ms (typ) to shutdown the WLED current regulator.
For the MAX8901A: After VIN is above VUVLO and ON is driven high, the MAX8901A enters soft-start
and increases the WLED current to full brightness. Apply a minimum 30kHz (500kHz max) PWM
signal to ON to adjust the WLED brightness from 100% to off, proportional to the duty cycle of the
PWM signal. See the PWM Dimming Control (MAX8901A) section for more details.
For the MAX8901B: After ON is driven high for 40µs (min), the MAX8901B enters soft-start and
increases the WLED current to full brightness. Subsequent pulses on ON cause the MAX8901B
WLED current to decrease in 32 equal steps. See the Serial-Pulse Dimming Control (MAX8901B)
section for more details.
3IN
Power Supply Input. Bypass IN to GND with a 1µF ceramic capacitor placed as close as possible to
the IC. If VIN exceeds the input overvoltage lockout threshold (6.5V max, VIN rising), the IC stops
switching and no WLED current flows (if the forward voltage of the WLED string is greater than VIN).
When VIN falls below the overvoltage lockout hysteresis level (6.0V min, VIN falling), soft-start is
initiated and normal operation resumes.
4 LX Boost Inductor Node. Connect an inductor between IN and LX. LX is high impedance in shutdown.
5 PGND Power Ground. Connect to GND and the exposed paddle (EP) with a short, wide trace.
6 GND Analog Ground. Connect GND to the exposed paddle with a short, wide trace.
7 COMP
WLED Boost Compensation Node. Connect a 0.022µF ceramic capacitor from COMP to GND.
CCOMP stabilizes the converter and sets the soft-start time. COMP discharges to GND when in
shutdown.
8CS
WLED Current Sense Input. For the MAX8901A, connect a current-sense resistor from CS to GND.
Voltage sensed at CS regulates the WLED current. For the MAX8901B, do not connect a sense
resistor from CS to GND. The MAX8901B provides an internal current source from CS to GND to
program the WLED current. The MAX8901B regulates VCS to 0.5V (typ) for all ILED. The MAX8901A
regulates VCS to 0.5V (typ) for maximum duty cycle only.
—EP
Exposed Metal Paddle. Connect EP to GND. For good thermal dissipation, the exposed paddle must
be connected to a large ground plane.
MAX8901A/MAX8901B
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
8 _______________________________________________________________________________________
Detailed Description
The high efficiency and tiny size of the MAX8901A/
MAX8901B WLED step-up converters make them ideally
suited for driving LCD backlighting in cell phones, PDAs,
and other portable and handheld devices. The
MAX8901_ drive from 2 to 6 series-connected WLEDs
with constant current for uniform WLED intensity and are
optimized for the highest possible efficiency (up to 91%)
over the full 1-cell Li+/Li-Poly battery range. These tiny
2mm x 2mm devices operate at a constant 750kHz
switching frequency, and use small external components
to achieve minimal input and output ripple while occupy-
ing the smallest possible footprint.
These converters use a single input (ON) to enable the IC
and to control WLED intensity. The MAX8901A requires a
direct PWM input to modulate WLED intensity, with WLED
current proportional to the PWM duty cycle. The
MAX8901B uses single-wire, serial-pulse dimming that
reduces the WLED intensity in 32 equal steps. Full-scale
WLED current for serial-pulse dimming is 24.75mA.
THERMAL
SHUTDOWN
BOOST
CONTROL
LOGIC
IC
POWER
REF
PWM
GND
INTERNAL
750kHz OSC
SHUTDOWN
TIMER
MAX8901A
IN
LX
PGND
D*0.5V
Gm
OVP
CS
COMP
ON
EP
Figure 2. MAX8901A Block Diagram
MAX8901A/MAX8901B
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
_______________________________________________________________________________________ 9
The MAX8901_ feature:
• Internal soft-start that gradually illuminates the
WLEDs to eliminate inrush current during startup.
• Input overvoltage protection (6.5V max) that
prevents IC switching when exceeded.
• WLED overvoltage protection (25V typ) that latches
off the IC.
• Shutdown mode that reduces current to 0.01µA (typ).
No WLED current is present in shutdown or during an
overvoltage condition if the WLED forward voltage is
greater than the input supply voltage. Figures 2 and 3
show the block diagrams for the MAX8901A and
MAX8901B, respectively.
Input Overvoltage Lockout
When VIN exceeds 6.2V (min), input overvoltage
lockout (OVLO) is engaged to protect the MAX8901_
from high-input voltage conditions. Once input OVLO
occurs, the MAX8901_ stop switching and no WLED
current flows, provided the forward voltage of the
WLED string is greater than VIN. When VIN falls below
6V (min), the input OVLO condition is cleared, and the
IC is enabled and enters soft-start.
WLED Overvoltage Protection
WLED overvoltage protection (OVP) occurs when the
WLED output voltage rises above the WLED OVP
threshold. The WLED OVP protection circuitry latches
off the IC and the IC enters shutdown when the WLED
OVP threshold is exceeded. After OVP is engaged,
cycle VIN or toggle ON to reenable the IC and enter
soft-start.
THERMAL
SHUTDOWN
BOOST
CONTROL
LOGIC
IC
POWER
REF GND
INTERNAL
750kHz OSC
SHUTDOWN
TIMER
MAX8901B
IN
LX
PGND
0.5V
Gm
OVP
CS
COMP
ON
PULSE
DIMMING
EP
Figure 3. MAX8901B Block Diagram
MAX8901A/MAX8901B
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
10 ______________________________________________________________________________________
Startup and Soft-Start
The MAX8901_ use a single input (ON) to enable the IC
and to control WLED intensity. After ON is driven high,
the MAX8901_ enter soft-start and gradually increase
the WLED current to full brightness.
When the MAX8901_ are first turned on, CCOMP is
charged to 1.25V with a 60µA current source. When
VCOMP rises above 1.25V, CCOMP is charged with a
4µA current source, and the internal MOSFET begins
switching at a reduced duty cycle. When VCOMP rises
above 2.25V, the duty cycle is at its maximum. The
VCOMP where the IC exits soft-start depends on the
final duty cycle required by the load. Typical startup
timing characteristics are shown in the
Typical
Operating Characteristics
section.
A PWM signal (MAX8901A) or serial pulses (MAX8901B)
are applied to dim the WLEDs. See the
PWM Dimming
Control (MAX8901A)
and
Serial-Pulse Dimming Control
(MAX8901B)
sections for more details.
Shutdown
The MAX8901_ enter shutdown when VON is held low
for more than 1.33ms (typ), or when an output overvolt-
age condition is engaged. In shutdown, the supply cur-
rent is reduced to 0.01µA (typ) by powering down the
entire IC except for the ON voltage-detection circuitry.
CCOMP is discharged to GND during shutdown,
allowing the device to reinitiate soft-start when it is
re-enabled. Although the internal n-channel MOSFET
does not switch in shutdown, there is still a DC current
path between the input and the WLEDs through the
inductor and Schottky diode. In shutdown, the minimum
forward voltage of the WLED string must exceed the
maximum input voltage to ensure that the WLEDs
remain off. Typically, with two or more WLEDs, the for-
ward voltage is large enough to keep leakage current
low, less than 1µA (typ). Shutdown timing characteris-
tics are shown in the
Typical Operating Characteristics
.
Thermal Shutdown
The MAX8901_ include a thermal shutdown feature that
protects the IC by turning it off when the die temperature
reaches +160°C (typ). After thermal shutdown occurs,
the MAX8901_ must be restarted by toggling the ON pin
low, then high, or by cycling the input voltage.
PWM Dimming Control (MAX8901A)
After VIN is above UVLO, apply a PWM signal to ON for
a WLED current that is proportional to the PWM signal’s
duty cycle (0% duty cycle corresponds to zero LED
current and 100% duty cycle corresponds to full LED
current). Restrict PWM frequency to between 30kHz to
500kHz for a maximum ILED current accuracy. If dim-
ming control is not required, ON works as a simple on/off
control. Drive ON high to enable the IC, or drive ON low
for shutdown. A resistor connected from CS to GND pro-
grams maximum ILED. See the
Current-Sense Resistor
(MAX8901A Only)
section for more details.
Serial-Pulse Dimming Control (MAX8901B)
After the MAX8901B is enabled by driving ON high for
the minimum initial ON high pulse width (40µs, min),
soft-start is engaged and brings WLED current to maxi-
mum brightness. After soft-start is completed, dim the
MAX8901B by pulsing ON low (500ns to 500µs pulse
width). Each pulse reduces the WLED current by
0.75mA. The maximum WLED current is 24.75mA for
the MAX8901B (0.75mA/step). The overall dimming
range for serial-pulse dimming control is from maximum
WLED current down to 1/33 of maximum WLED current.
The WLEDs are turned off by holding ON low for at
least 1.33ms (typ). See Figure 1 for the serial-pulse
dimming control timing diagram.
Applications Information
Inductor Selection
Recommended inductor values range from 10µH to
47µH. A 22µH inductor optimizes the efficiency for most
applications while maintaining low input voltage ripple.
With input voltages near 5V, a larger value of
inductance may be more efficient. To prevent core
saturation, ensure that the inductor saturation current
rating exceeds the peak inductor current for the
application. Calculate the peak inductor current with
the following formula:
where ILED(MAX) is in mA and L is in µH.
IV
V
V
x L
PEAK OUT(MAX)
IN(MIN)
IN(MIN) x .2
=×+
I
x
LED MAX()
900 2
1
MAX8901A/MAX8901B
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
______________________________________________________________________________________ 11
Schottky Diode Selection
The high switching frequency of the MAX8901_
demands a high-speed rectification diode for optimum
efficiency. A Schottky diode is recommended due to its
fast recovery time and low forward voltage drop.
Ensure that the diode’s average and peak current
rating exceeds the average output current and peak
inductor current. In addition, the diode’s reverse
breakdown voltage must exceed VOUT. The RMS diode
current can be approximated from the following
equation:
Capacitor Selection
Ceramic capacitors with X5R, X7R, or better dielectric
are recommended for stable operation over the entire
operating temperature range. The exact values of input
(C1) and output (C2) capacitors are not critical (see
Figures 4 and 5). The typical value for the input
capacitor is 1µF, and the typical value for the output
capacitor is 0.1µF (MAX8901A) or 1µF (MAX8901B).
Higher value capacitors can reduce input and output
ripple, but at the expense of size and higher cost. The
compensation capacitor (C3) stabilizes the converter
and controls soft-start. The compensation capacitor is
typically chosen to be 0.022µF for most applications.
Current-Sense Resistor (MAX8901A Only)
The MAX8901A uses a sense resistor (RCS) connected
from CS to GND to program the maximum WLED cur-
rent for 100% PWM duty cycle. The MAX8901A regu-
lates VCS to 0.5V (typ) for 100% duty cycle. Calculate
RCS (in ohms) using the following equation:
where ILED(MAX) is the maximum WLED current in
milliamps. Maximum WLED current is programmed to
25mA using a 20Ωresistor.
PCB Layout
Due to fast switching waveforms and high-current
paths, careful attention is required when the PCB layout
is designed. Minimize trace lengths between the IC and
the inductor, the diode, the input capacitor, and the
output capacitor. For the MAX8901A, minimize the
trace length between the IC and RCS. Keep traces
short, direct, and wide. Keep noisy traces, such as the
LX node trace, away from CS. The input bypass
capacitor (CIN) should be placed as close as possible
to the IC. The ground connections of CIN and COUT
should be as close together as possible. PGND and
GND should be connected together at the input
capacitor ground terminal. Refer to the MAX8901 EV Kit
for an example layout.
RCS LED(MAX)
500
I
=
IDIODE RMS()
=× II
LED PEAK
MAX8901A/MAX8901B
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
12 ______________________________________________________________________________________
MAX8901A
IN
COMP
GND
ON
LX
OVP
PGND
CS
OUTPUT
UP TO 25V
D2–D7
R1 (RCS)
20Ω
C2
0.1μF
D1
4
1
5
8
2
EP
L1
22μH
3
7
6
C3
0.022μF
C1
1μF
INPUT
2.6V TO 5.5V
PWM DIMMING
CONTROL
Figure 4. MAX8901A Application Circuit
MAX8901B
IN
COMP
GND
ON
LX
OVP
PGND
CS
OUTPUT
UP TO 25V
C2
1μF
D1
4
1
5
8
2
L1
22μH
3
7
6
C3
0.022μF
INPUT
2.6V TO 5.5V
SERIAL-PULSE
DIMMING
CONTROL
R1
0Ω
C1
1μF
EP
Figure 5. MAX8901B Application Circuit
MAX8901A/MAX8901B
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
______________________________________________________________________________________ 13
1
+
34
865
CS PGND
MAX8901
2
7
COMP
OVP IN LXON
TDFN-EP
(2mm × 2mm)
GND
*EP
*EP = EXPOSED PAD
TOP VIEW
Pin Configuration Chip Information
PROCESS: BiCMOS
Package Information
For the latest package outline information and land patterns, go
to www.maxim-ic.com/packages. Note that a “+”, “#”, or “-” in
the package code indicates RoHS status only. Package draw-
ings may show a different suffix character, but the drawing per-
tains to the package regardless of RoHS status.
PACKAGE
TYPE
PACKAGE
CODE
OUTLINE
NO.
LAND
PATTERN NO.
8 TDFN-EP T822-1 21-0168 90-0064
MAX8901A/MAX8901B
Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package
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.
14
____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2010 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.
Revision History
REVISION
NUMBER
REVISION
DATE DESCRIPTION PAGES
CHANGED
0 8/07 Initial release —
1 5/08 Replaced TOC 16 6
2 6/08 Updated LED current accuracy, input undervoltage lockout threshold, and
input overvoltage lockout threshold. 1, 2, 3
3 8/10 Removed Input Undervoltage Lockout Threshold (VIN falling) minimum value
and added soldering temperature 2