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