EVALUATION KIT AVAILABLE
Highest Efficiency Supply
for 2 to 6 Series WLEDs in a
2mm x 2mm TDFN Package
MAX8901A/MAX8901B
General Description
Benefits and Features
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
handheld devices. The MAX8901_ operate at a fixed
750kHz (typ) switching frequency, allowing for tiny external components, and are optimized for the highest possible efficiency 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 shutdown 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 lockout (UVLO) and thermal shutdown.
The MAX8901_ are available in tiny 8-pin, 2mm x 2mm
TDFN-EP packages (0.8mm max height).
• Cost-Effective and Flexible Solution for Wide Range of
Applications
Display Backlight (from 2 to 6 WLEDs)
Cellular Phones
PDAs and Smartphones
MP3 and Portable Media Players
Portable Navigation Devices
Digital Cameras
Backlighting Applications
• Supplies from 2 to 6 WLEDs with 1.3% LED Current
Accuracy
• 2.6V to 5.5V Input Voltage Range
• Fixed-Frequency Operation
• Flexible Dimming Control
• Direct PWM Dimming (MAX8901A)
• 32-Step, Single-Wire Serial Dimming (MAX8901B)
• Protection Features Improve System Reliability
• WLED Overvoltage Protection (25V typ)
• Internal Soft-Start and Thermal Shutdown
• Input Undervoltage Lockout
• Input Overvoltage Lockout
• High Efficiency Extends Battery Life
• Up to 91% Efficiency
• 0.01µA (typ) Shutdown Current
• No WLED Current in Shutdown
• 8-Pin, 2mm x 2mm TDFN Package Saves Space
Ordering Information
PART
PIN-PACKAGE
TOP MARK
MAX8901AETA+T
8 TDFN-EP*
2mm x 2mm
+ABA
MAX8901BETA+T
8 TDFN-EP*
2mm x 2mm
+ABB
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.
Pin Configuration appears at end of data sheet.
Typical Operating Circuits
INPUT
2.6V TO 5.5V
INPUT
2.6V TO 5.5V
3
LX
IN
4
OUTPUT
UP TO 25V
3
6
COMP
OVP
GND
PGND
PWM DIMMING
CONTROL
2
19-0865; Rev 4; 5/15
ON
EP
4
MAX8901B
MAX8901A
7
LX
IN
CS
1
5
8
7
SERIAL-PULSE
DIMMING
CONTROL
6
2
OVP
COMP
GND
PGND
ON
EP
CS
1
5
8
OUTPUT
UP TO 25V
Highest Efficiency Supply
for 2 to 6 Series WLEDs in a
2mm x 2mm TDFN Package
MAX8901A/MAX8901B
Absolute Maximum Ratings
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
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.
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)
PARAMETER
Input Voltage
Input Undervoltage Lockout
Threshold
SYMBOL
CONDITIONS
VIN
VIN_UVLO
VIN rising
MIN
TYP
MAX
UNITS
2.6
5.5
V
2.25
2.55
VIN falling
Input Overvoltage Lockout
Threshold
Shutdown Input Current
Quiescent Current
Output Voltage Range
OVP Overvoltage Protection
Threshold
OVP Input Current
VIN_OVLO
ISHDN
IQ
VIN rising
6.2
6.35
VIN falling
6.0
6.3
VON = 0V
VCS = 0.55V, no load
(not switching)
V
2.53
TA = +25°C
0.01
TA = +85°C
0.1
1
MAX8901A
70
135
MAX8901B
115
185
VOUT
VDIODE = external boost diode voltage drop
VOV
6 LEDs with 25V OVP option
IOV
VOVP = 20V
VCS
VIN VDIODE
V
µA
µA
VOV
V
26
V
24
25
No dimming
0.475
0.50
0.525
V
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
ON Low Voltage
VON_LO
2.6V < VIN < 5.5V
ON Input Current
ION
CS Regulation Voltage
ON Shutdown Delay
Initial ON High Pulse Width
tHI_INIT
ON = IN
20
V
0.4
TA = +25°C
0.01
TA = +85°C
0.1
First ON high pulse to enable IC
(MAX8901B) (Figure 1)
40
ON High Pulse Width
tHI
MAX8901B (Figure 1)
0.5
ON Low Pulse Width
tLO
MAX8901B (Figure 1)
0.5
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µA
1
V
µA
µs
µs
500.0
µs
Maxim Integrated | 2
Highest Efficiency Supply
for 2 to 6 Series WLEDs in a
2mm x 2mm TDFN Package
MAX8901A/MAX8901B
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)
PARAMETER
SYMBOL
Serial Dimming
Full-Scale LED Current Accuracy
CONDITIONS
ILED = 24.75mA
(MAX8901B)
PWM Frequency for PWM
Dimming Control
fPWM
MIN
TA = +25°C
MAX
UNITS
-1.3
+1.3
%
-3
+3
%
TA = -40°C to +85°C
MAX8901A
100
CS to COMP Transconductance
40
tSOFT-
Soft-Start Interval
START
TYP
CCOMP = 0.022µF (Figure 1)
Thermal Shutdown
kHz
60
80
µS
10
ms
160
°C
OSCILLATOR
Operating Frequency
fSW
Maximum Duty Cycle
DMAX
VCS = 0.4V
ILXLKG
VLX = 27V
700
750
90
92
800
kHz
%
n-CHANNEL SWITCH
LX Leakage Current
n-Channel Switch On-Resistance
RLX
n-Channel Current Limit
ILIM
TA = +25°C
0.1
TA = +85°C
0.1
0.63
5
µA
0.7
1.4
Ω
0.70
0.77
A
Note 1: Specifications to -40°C are guaranteed by design and characterization and are not production tested.
STEP
tHI_INIT
> 40μs
1
0
2
3
4
5
28
29
30
31
32
33
ON
tHI
tSOFT-START
FULL
tLO
500ns TO 500μs
32/33 31/33
30/33 29/33
ILED_
tSHDN
32/33 1.33ms (typ)
28/33
6/33
SHUTDOWN
FULL
> 500ns
5/33
4/33
3/33
2/33
1/33
SHUTDOWN
Figure 1. MAX8901B Timing Diagram
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Maxim Integrated | 3
Highest Efficiency Supply
for 2 to 6 Series WLEDs in a
2mm x 2mm TDFN Package
MAX8901A/MAX8901B
Typical Operating Characteristics
(Circuit of Figure 5, ILED = 24.75mA, VIN = VON = 3.6V, C1 = 1µF, C2 = 1µF, TA = +25°C, unless otherwise noted.)
LED EFFICIENCY vs. INPUT VOLTAGE
3 WLEDs
4 WLEDs
80
6 WLEDs
85
75
3 WLEDs
80
6 WLEDs
75
INDUCTOR: TOKO 1069AS-220M
DIODE: CENTRAL SEMI CMDSH05-4
INDUCTOR: TOKO 1098AS-100M
DIODE: NXP PMEG3005EB
70
70
3
4
5
6
4
5
6
LED EFFICIENCY vs. LED CURRENT
(6 WLEDs)
LED EFFICIENCY vs. LED CURRENT
(6 WLEDs)
VIN = 3.6V
75
VIN = 4.2V
VIN = 3V
70
65
VIN = 5V VIN = 5.5V
85
80
EFFICIENCY (%)
80
90
MAX8901A/B toc03
VIN = 5.5V
VIN = 5V
85
VIN = 4.2V
VIN = 3.6V
75
VIN = 3V
70
65
60
60
INDUCTOR: TOKO 1069AS-220M
DIODE: CENTRAL SEMI CMDSH05-4
55
55
INDUCTOR: TOKO 1098AS-100M
DIODE: NXP PMEG3005EB
50
5
10
15
20
0
25
5
10
15
20
LED CURRENT (mA)
LED CURRENT (mA)
LED EFFICIENCY vs. LED CURRENT
(4 WLEDs)
LED EFFICIENCY vs. LED CURRENT
(4 WLEDs)
VIN = 5V
VIN = 5.5V
85
VIN = 4.2V
VIN = 3.6V
80
75
VIN = 3V
70
90
VIN = 5V
65
VIN = 5.5V
85
80
EFFICIENCY (%)
90
MAX8901A/B toc05
0
VIN = 4.2V
75
25
MAX8901A/B toc06
50
EFFICIENCY (%)
3
INPUT VOLTAGE (V)
90
EFFICIENCY (%)
2
INPUT VOLTAGE (V)
MAX8901A/B toc04
2
VIN = 3.6V
VIN = 3V
70
65
60
60
INDUCTOR: TOKO 1069AS-220M
DIODE: CENTRAL SEMI CMDSH05-4
55
55
INDUCTOR: TOKO 1098AS-100M
DIODE: NXP PMEG3005EB
50
50
0
5
10
15
LED CURRENT (mA)
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MAX8901A/B toc02
4 WLEDs
EFFICIENCY (%)
85
EFFICIENCY (%)
LED EFFICIENCY vs. INPUT VOLTAGE
90
MAX8901A/B toc01
90
20
25
0
5
10
15
20
25
LED CURRENT (mA)
Maxim Integrated | 4
Highest Efficiency Supply
for 2 to 6 Series WLEDs in a
2mm x 2mm TDFN Package
MAX8901A/MAX8901B
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.)
LED AND BOOST EFFICIENCY
vs. INPUT VOLTAGE
IN CURRENT (IIN)
vs. SUPPLY VOLTAGE
90
MAX8901A/B toc08
BOOST
MAX8901B
700
600
LED
MAX8901A
500
85
IIN (μA)
EFFICIENCY (%)
800
MAX8901A/B toc07
95
80
400
300
200
75
6WLEDs,
INDUCTOR: TOKO A915AY-220M,
DIODE: CENTRAL SEMI CMDSH05-4
70
2
3
4
5
100
0
2
6
3
4
5
6
7
SUPPLY VOLTAGE (V)
INPUT VOLTAGE (V)
INPUT OVERVOLTAGE
PROTECTION RESPONSE
SWITCHING WAVEFORMS
MAX8901A/B toc9
MAX8901A/B toc10
50mV/div
AC-COUPLED
VI
6.4V
VIN
5.4V
VOUT
VLX
1V/div
(AC-COUPLED)
10V/div
10V/div
0V
100mV/div
AC-COUPLED
VOUT
0V
ILEDs
20mA/div
0mA
100mA/div
ILX
0mA
20mA/div
VLX
1μs/div
0V
20ms/div
STARTUP AND SHUTDOWN
RESPONSE (MAX8901B)
STARTUP AND SHUTDOWN
RESPONSE (MAX8901A)
MAX8901A/B toc12
MAX8901A/B toc11
C2 = 0.1μF
VON
2V/div
0V
10V/div
10V/div
VOUT
0V
VOUT
0V
100mA/div
100mA/div
ISUPPLY
0mA
ILEDs
20mA/div
0mA
10ms/div
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2V/div
0V
VON
ISUPPLY
0mA
20mA/div
ILEDs
0mA
10ms/div
Maxim Integrated | 5
Highest Efficiency Supply
for 2 to 6 Series WLEDs in a
2mm x 2mm TDFN Package
MAX8901A/MAX8901B
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.)
PWM DIMMING RESPONSE
(MAX8901A)
SERIAL PULSE DIMMING
RESPONSE (MAX8901B)
MAX8901A/B toc13
MAX8901A/B toc14
2V/div
VON
D = 90%
D = 10%
VIN
VCS
2V/div
0V
VON
0V
50mV/div
(AC-COUPLED)
VIN
50mV/div
(AC-COUPLED)
500mV/div
VCS
1V/div
0V
0V
VOUT
VOUT
20mV/div
fON_PWM = 100kHz, DUTY
CYCLE CHANGE: 90% TO
10%, C2 = 0.1μF
ILEDs
0V
ILEDs
20mA/div
20mA/div
0mA
0mA
4ms/div
10ms/div
LED CURRENT vs. PWM
DIMMING (MAX8901A)
VCS vs. LED CURRENT STEP
(MAX8901B)
25
507.0
506.5
506.0
VCS (mV)
20
15
10
505.5
505.0
504.5
504.0
503.5
5
RCS = 20Ω,
fPWM = 100kHz
0
0
20
40
60
80
503.0
502.5
100
PWM DUTY CYCLE (%)
10
15
20
25
LED CURRENT STEP
MAXIMUM LED CURRENT
vs. TEMPERATURE (MAX8901B)
MINIMUM LED CURRENT
vs. TEMPERATURE (MAX8901B)
24.8
30
745
ILED (μA)
24.6
5
750
MAX8901A/B toc17
25.0
0
35
MAX8901A/B toc18
ILED (mA)
MAX8901A/B toc16
507.5
MAX8901A/B toc15
30
ILED (mA)
20mV/div
0V
740
24.4
735
24.2
730
24.0
-40
-15
10
35
TEMPERATURE (°C)
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60
85
-40
-15
10
35
60
85
TEMPERATURE (°C)
Maxim Integrated | 6
MAX8901A/MAX8901B
Highest Efficiency Supply
for 2 to 6 Series WLEDs in a
2mm x 2mm TDFN Package
Pin Description
PIN
NAME
1
OVP
FUNCTION
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.
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.
2
ON
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.
IN
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
6
GND
3
7
COMP
Power Ground. Connect to GND and the exposed paddle (EP) with a short, wide trace.
Analog Ground. Connect GND to the exposed paddle with a short, wide trace.
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.
8
CS
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.
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Maxim Integrated | 7
Highest Efficiency Supply
for 2 to 6 Series WLEDs in a
2mm x 2mm TDFN Package
MAX8901A/MAX8901B
MAX8901A
GND
REF
THERMAL
SHUTDOWN
IC
POWER
BOOST
CONTROL
LOGIC
INTERNAL
750kHz OSC
IN
LX
SHUTDOWN
TIMER
PGND
OVP
CS
Gm
D*0.5V
PWM
COMP
ON
EP
Figure 2. MAX8901A Block Diagram
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
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2mm x 2mm devices operate at a constant 750kHz
switching frequency, and use small external components
to achieve minimal input and output ripple while occupying 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
Maxim Integrated | 8
Highest Efficiency Supply
for 2 to 6 Series WLEDs in a
2mm x 2mm TDFN Package
MAX8901A/MAX8901B
MAX8901B
GND
REF
THERMAL
SHUTDOWN
IC
POWER
BOOST
CONTROL
LOGIC
INTERNAL
750kHz OSC
SHUTDOWN
TIMER
IN
LX
PGND
OVP
CS
Gm
0.5V
PULSE
DIMMING
COMP
ON
EP
Figure 3. MAX8901B Block Diagram
reduces the WLED intensity in 32 equal steps. Full-scale
WLED current for serial-pulse dimming is 24.75mA.
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.
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Input Overvoltage Lockout
When V IN 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.
Maxim Integrated | 9
MAX8901A/MAX8901B
Highest Efficiency Supply
for 2 to 6 Series WLEDs in a
2mm x 2mm TDFN Package
Startup and Soft-Start
PWM Dimming Control (MAX8901A)
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, C COMP 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.
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 dimming 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 programs maximum ILED. See the Current-Sense Resistor
(MAX8901A Only) section for more details.
Shutdown
The MAX8901_ enter shutdown when VON is held low
for more than 1.33ms (typ), or when an output overvoltage condition is engaged. In shutdown, the supply current is reduced to 0.01µA (typ) by powering down the
entire IC except for the ON voltage-detection circuitry.
C COMP 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 forward voltage is large enough to keep leakage current
low, less than 1µA (typ). Shutdown timing characteristics 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.
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 maximum 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:
IPEAK =
VOUT(MAX) × ILED(MAX)
900 x VIN(MIN)
+
VIN(MIN) x 1.2
2xL
where ILED(MAX) is in mA and L is in µH.
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Maxim Integrated | 10
MAX8901A/MAX8901B
Highest Efficiency Supply
for 2 to 6 Series WLEDs in a
2mm x 2mm TDFN Package
Schottky Diode Selection
Current-Sense Resistor (MAX8901A Only)
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:
The MAX8901A uses a sense resistor (RCS) connected
from CS to GND to program the maximum WLED current for 100% PWM duty cycle. The MAX8901A regulates VCS to 0.5V (typ) for 100% duty cycle. Calculate
RCS (in ohms) using the following equation:
IDIODE (RMS) =
ILED × IPEAK
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.
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RCS =
500
ILED(MAX)
where I LED(MAX) is the maximum WLED current in
milliamps. Maximum WLED current is programmed to
25mA using a 20Ω resistor.
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 R CS. 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.
Maxim Integrated | 11
Highest Efficiency Supply
for 2 to 6 Series WLEDs in a
2mm x 2mm TDFN Package
MAX8901A/MAX8901B
L1
22μH
INPUT
2.6V TO 5.5V
C1
1μF
3
LX
IN
OUTPUT
UP TO 25V
D1
4
C2
0.1μF
MAX8901A
7
C3
0.022μF
6
COMP
OVP
GND
PGND
PWM DIMMING
CONTROL
2
ON
CS
EP
1
D2–D7
5
8
R1 (RCS)
20Ω
Figure 4. MAX8901A Application Circuit
L1
22μH
INPUT
2.6V TO 5.5V
C1
1μF
3
7
LX
IN
SERIAL-PULSE
DIMMING
CONTROL
OVP
6
2
PGND
GND
ON
EP
OUTPUT
UP TO 25V
D1
C2
1μF
MAX8901B
COMP
R1
0Ω
C3
0.022μF
4
CS
1
5
8
Figure 5. MAX8901B Application Circuit
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Maxim Integrated | 12
Highest Efficiency Supply
for 2 to 6 Series WLEDs in a
2mm x 2mm TDFN Package
MAX8901A/MAX8901B
Pin Configuration
Chip Information
PROCESS: BiCMOS
TOP VIEW
CS
8
COMP GND PGND
7
6
5
Package Information
MAX8901
*EP
+
1
2
3
4
OVP
ON
IN
LX
TDFN-EP
(2mm × 2mm)
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.
8 TDFN-EP
T822-1
21-0168
90-0064
*EP = EXPOSED PAD
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Maxim Integrated | 13
Highest Efficiency Supply
for 2 to 6 Series WLEDs in a
2mm x 2mm TDFN Package
MAX8901A/MAX8901B
Revision History
REVISION
NUMBER
REVISION
DATE
PAGES
CHANGED
DESCRIPTION
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.
3
8/10
Removed Input Undervoltage Lockout Threshold (VIN falling) minimum
value and added soldering temperature
2
4
4/15
Updated Benefits and Features section
1
1, 2, 3
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.
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 and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc.
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