MAXIM MAX8901A, MAX8901B User Manual

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 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_ features 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).

Applications

Display Backlight (from 2 to 6 WLEDs)

Cellular Phones

PDAs and Smartphones

MP3 and Portable Media Players

Portable Navigation Devices

Digital Cameras

Features

♦ High Efficiency, Up to 91%

♦ 2.6V to 5.5V Input Voltage Range

♦ Fixed-Frequency Operation

♦ Supplies from 2 to 6 WLEDs with 1% LED Current

Accuracy

♦ Flexible Dimming Control

Direct PWM Dimming (MAX8901A)
32-Step, 1-Wire Serial Dimming
(MAX8901B)

♦ Input Undervoltage Lockout

♦ Input Overvoltage Lockout

♦ WLED Overvoltage Protection (25V typ)

♦ 0.01µA (typ) Shutdown Current

♦ No WLED Current in Shutdown

♦ Internal 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 0; 8/07

Note: All devices are specified over the -40°C to +85°C operating
temperature range.

+

Denotes a lead-free package.

*

EP = Exposed paddle.

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

Pin Configuration appears at end of data sheet.

PART PIN-PACKAGE

MAX8901AETA+T

MAX8901BETA+T

8 TDFN-EP*
2mm x 2mm

8 TDFN-EP*
2mm x 2mm

PKG

CODE

T822-1 +ABA

T822-1 +ABB

TOP

MARK

INPUT

2.6V TO 5.5V

OUTPUT

3

IN

MAX8901B

7

COMP

SERIAL-PULSE

DIMMING
CONTROL

6

GND

2

ON

EP

4

LX

1

OVP

5

PGND

8

CS

UP TO 25V

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= V

OVP

= 3.6V, V

PGND

= V

GND

= 0V, COMP, CS, and LX are unconnected, TA= -40°C to +85°C, unless otherwise noted.

Typical values are at T

A

= +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 (V

IN

+ 0.3V)

OVP, LX to GND .....................................................-0.3V to +28V

PGND to GND .......................................................-0.3V to +0.3V

LX Current ..................................................................770mA

RMS

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

Input Voltage V

Input Undervoltage Lockout
Threshold

Input Overvoltage Lockout
Threshold

Shutdown Input Current I

Quiescent Current I

Output Voltage Range V

OVP Overvoltage Protection
Threshold

OVP Input Current I

CS Regulation Voltage V

ON Shutdown Delay t

ON High Voltage V

ON Low Voltage V

ON Input Current I

Initial ON High Pulse Width t

ON High Pulse Width t

ON Low Pulse Width t

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

IN

V

IN_UVLO

V

IN_OVLO

SHDN

OUT

V

OV

OV

CS

SHDN

ON_HI

ON_LO

ON

HI_INIT

HI

LO

VIN rising 2.25 2.50

VIN falling 2.20 2.53

VIN rising 6.2 6.3

VIN falling 6.0 6.3

VON = 0V

VCS = 0.55V, no
load (not

Q

switching)

V

= external boost diode voltage drop

DIODE

6 LEDs with 25V OVP option 24 25 26 V

V

= 20V 20 µA

OVP

No dimming 0.475 0.50 0.525 V

Time VON is below low threshold until
shutdown (Figure 1)

2.6V < VIN < 5.5V 1.3 V

2.6V < VIN < 5.5V 0.4 V

ON = IN

First ON high pulse to enable IC
(MAX8901B) (Figure 1)

MAX8901B (Figure 1) 0.5 µs

MAX8901B (Figure 1) 0.5 500.0 µs

2.6 5.5 V

TA = +25°C 0.01 1

= +85°C 0.1

T

A

MAX8901A 70 135

MAX8901B 115 185

V

-

IN

V

DIODE

1.18 1.33 1.50 ms

TA = +25°C 0.01 1

= +85°C 0.1

T

A

40 µs

V

OV

V

V

µA

µA

V

µA

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.

ELECTRICAL CHARACTERISTICS (continued)

(VIN= VON= V

OVP

= 3.6V, V

PGND

= V

GND

= 0V, COMP, CS, and LX are unconnected, TA= -40°C to +85°C, unless otherwise noted.

Typical values are at T

A

= +25°C.) (Note 1)

Figure 1. MAX8901B Timing Diagram

Serial Dimming
Full-Scale LED Current Accuracy

PWM Frequency for PWM
Dimming Control

CS to COMP Transconductance 40 60 80 µS

Soft-Start Interval

Thermal Shutdown 160 °C

OSCILLATOR

Operating Frequency f

Maximum Duty Cycle D

n-CHANNEL SWITCH

LX Leakage Current I

n-Channel Switch On-Resistance R

n-Channel Current Limit I

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

TA = +25°C-1 +1%

T

= -40°C to +85°C-3 +3 %

A

700 750 800 kHz

TA = +25°C 0.1 5

T

= +85°C 0.1

A

0.7 1.4 Ω

0.63 0.70 0.77 A

f

PWM

t

SOFT-

START

SW

MAX

LXLKG

LX

LIM

I

= 24.75mA

LED

(MAX8901B)

MAX8901A 100 kHz

C

= 0.022µF (Figure 1) 10 ms

COMP

VCS = 0.4V 90 92 %

VLX = 27V

µA

STEP

I

LED_

t

HI_INIT

> 40μs

t

SOFT-START

SHUTDOWN

ON

FULL

102 3 4 5 28 29 30 31 32

32/33

31/33

500ns TO 500μs

30/33

29/33

28/33

33

t

6/33

HI

> 500ns

5/33

4/33

3/33

2/33

FULL

1/33

t

LO

32/33

t

SHDN

1.33ms (typ)

SHUTDOWN

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, I

LED

= 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, I

LED

= 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)

I

IN

(μA)

MAX8901B

MAX8901A

1

μs/div

SWITCHING WAVEFORMS

50mV/div
AC-COUPLED

MAX8901A/B toc9

0mA

10V/div

0V

100mV/div
AC-COUPLED

100mA/div

V

I

V

LX

V

OUT

I

LX

20ms/div

INPUT OVERVOLTAGE

PROTECTION RESPONSE

MAX8901A/B toc10

0V

0mA

10V/div

1V/div
(AC-COUPLED)

0V

20mA/div

20mA/div

V

IN

V

OUT

I

LEDs

V

LX

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

V

ON

V

OUT

I

SUPPLY

I

LEDs

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

V

ON

V

OUT

I

SUPPLY

I

LEDs

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, I

LED

= 24.75mA, VIN= VON= 3.6V, C1 = 1µF, C2 = 1µF, TA= +25°C, unless otherwise noted.)

499.4

499.8

499.6

500.2

500.0

500.8

500.6

500.4

501.0

0105 1520253035

VCS vs. LED CURRENT STEP

(MAX8901B)

MAX8901A/B toc16

LED CURRENT STEP

V

CS

(mV)

I

LED

= 0.75mA

I

LED

= 24.75mA

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)

I

LED

(mA)

MINIMUM LED CURRENT

vs. TEMPERATURE (MAX8901B)

MAX8901A/B toc18

TEMPERATURE (°C)

I

LED

(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

V

ON

V

IN

V

CS

V

OUT

I

LEDs

D = 10%

D = 90%

50mV/div
(AC-COUPLED)

500mV/div

f

ON_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

V

ON

V

IN

V

CS

V

OUT

I

LEDs

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 (%)

I

LED

(mA)

RCS = 20Ω,
f

PWM

= 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

WLED Overvoltage Protection Input. OVP monitors voltage at the WLEDs. Connect OVP to the

1 OVP

2ON

3IN

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

8CS

—EP

positive terminal of the output capacitor. If an OVP condition is detected, the MAX8901_ latches off.
Cycle V

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 V
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.

Power Supply Input. Bypass IN to GND with a 1µF ceramic capacitor placed as close as possible to
the IC. If V
switching and no WLED current flows (if the forward voltage of the WLED string is greater than V
When V
initiated and normal operation resumes.

WLED Boost Compensation Node. Connect a 0.022µF ceramic capacitor from COMP to GND.
C
shutdown.

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 V
regulates V

Exposed Metal Paddle. Connect EP to GND. For good thermal dissipation, the exposed paddle must
be connected to a large ground plane.

or toggle VON to restart the IC.

IN

is above V

IN

exceeds the input overvoltage lockout threshold (6.5V max, VIN rising), the IC stops

IN

falls below the overvoltage lockout hysteresis level (6.0V min, VIN falling), soft-start is

IN

stabilizes the converter and sets the soft-start time. COMP discharges to GND when in

COMP

to 0.5V (typ) for maximum duty cycle only.

CS

and ON is driven high, the MAX8901A enters soft-start

UVLO

to 0.5V (typ) for all I

CS

LED.

The MAX8901A

).

IN

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 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
reduces the WLED intensity in 32 equal steps. Full-scale
WLED current for serial-pulse dimming is 24.75mA.

Figure 2. MAX8901A Block Diagram

MAX8901A

GND

IC

IN

LX

THERMAL

SHUTDOWN

INTERNAL

750kHz OSC

REF

BOOST

CONTROL

LOGIC

POWER

SHUTDOWN

TIMER

Gm

PGND

OVP

CS

D*0.5V

PWM

COMP

ON

EP

MAX8901A/MAX8901B

Highest Efficiency Supply for 2 to 6

Series WLEDs in a 2mm x 2mm TDFN Package

_______________________________________________________________________________________ 9

The MAX8901_ features:

• 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 VINexceeds 6.2V (min), input overvoltage
lockout (OVLO) is engaged to protect the MAX8901_
from high-input voltage conditions. Once input OVLO
occurs, the MAX8901_ stops switching and no WLED
current flows, provided the forward voltage of the
WLED string is greater than VIN. When VINfalls 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 VINor toggle ON to reenable the IC and enter
soft-start.

Figure 3. MAX8901B Block Diagram

MAX8901B

IC

POWER

GND

IN

LX

THERMAL

SHUTDOWN

INTERNAL

750kHz OSC

REF

BOOST

CONTROL

LOGIC

SHUTDOWN

TIMER

Gm

PGND

OVP

CS

0.5V

EP

PULSE

DIMMING

COMP

ON

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_ is first turned on, C

COMP

is
charged to 1.25V with a 60µA current source. When
V

COMP

rises above 1.25V, C

COMP

is charged with a
4µA current source, and the internal MOSFET begins
switching at a reduced duty cycle. When V

COMP

rises
above 2.25V, the duty cycle is at its maximum. The
V

COMP

where the IC exits soft-start depends on the
final duty cycle required by the load. Typical shutdown
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 VONis 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 includes 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 VINis 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 I

LED

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 I

LED

. 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 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:

where I

LED(MAX)

is in mA and L is in µH.

I

PEAK

V

OUT(MAX)

=

×+I

LED MAX()

V

x

900 2

IN(MIN)

V

IN(MIN) x .2

1

x L

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 V

OUT

. 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 cur­rent for 100% PWM duty cycle. The MAX8901A regu­lates V

CS

to 0.5V (typ) for 100% duty cycle. Calculate

RCS(in ohms) using the following equation:

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 CINand C

OUT

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.

I

DIODE RMS()

=× II

LED PEAK

=

I

LED(MAX)

500

R

CS

MAX8901A/MAX8901B

Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package

12 ______________________________________________________________________________________

Figure 4. MAX8901A Application Circuit

Figure 5. MAX8901B Application Circuit

INPUT

2.6V TO 5.5V

C1

1μF

0.022μF

PWM DIMMING

CONTROL

INPUT

2.6V TO 5.5V

C1

1μF

0Ω

3

IN

COMP

GND

ON

IN

COMP

MAX8901A

MAX8901B

7

C3

6

2

3

7

R1

L1

22μH

EP

L1

22μH

OVP

PGND

OVP

C2

0.1μF

C2

1μF

OUTPUT

UP TO 25V

R1 (R
20Ω

OUTPUT

UP TO 25V

D2–D7

)

CS

4

LX

1

5

8

CS

4

LX

1

D1

D1

SERIAL-PULSE

DIMMING
CONTROL

0.022μF

C3

6

GND

2

ON

EP

PGND

5

8

CS

MAX8901A/MAX8901B

Highest Efficiency Supply for 2 to 6

Series WLEDs in a 2mm x 2mm TDFN Package

______________________________________________________________________________________ 13

Pin Configuration

Chip Information

PROCESS: BiCMOS

TOP VIEW

*EXPOSED PAD

CS PGND

COMP

GND

865

7

MAX8901

+

1

2

OVP IN LXON

TDFN-EP

(2mm × 2mm)

*EP

34

MAX8901A/MAX8901B

Highest Efficiency Supply for 2 to 6
Series WLEDs in a 2mm x 2mm TDFN Package

14 ______________________________________________________________________________________

Package Information

(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information
go to www.maxim-ic.com/packages

.)

8L TDFN EXPOSED PADS.EPS

PACKAGE OUTLINE
6 & 8L TDFN EXPOSED PAD, 2x2x0.80mm

21-0168

1

D

2

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.

Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________

15

© 2007 Maxim Integrated Products is a registered trademark of Maxim Integrated Products, Inc.

Package Information (continued)

(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information
go to www.maxim-ic.com/packages

.)

MIN.

0.70

1.90

0.00

0.20

0.25 MIN.

0.20 REF.

MAX.

0.80

2.101.90

2.10

0.05

0.40

N

0.90±0.10

6

E2D2

1.60±0.10 1.30 REF

1.30±0.10 1.50 REF0.25±0.050.50 TYP. 0.70±0.108T822-1

1.20±0.10 1.50 REF0.25±0.050.50 TYP. 0.80±0.108T822-2 0.125

e

0.65 TYP.

b

0.30±0.05

r

0.150

0.125

[(N/2)-1] x e

COMMON DIMENSIONS

SYMBOL

A
D
E
A1
L
k
A2

PACKAGE VARIATIONS
PKG. CODE

T622-1

PACKAGE OUTLINE
6 & 8L TDFN EXPOSED PAD, 2x2x0.80mm

21-0168

2

D

2