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Charge Pump Driver for LCD

White LED Backlights

ADM8845

Rev. C

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However, no responsibility is assumed by Analog Devices for its use, nor for any

infringements of patents or other rights of third parties that may result from its use.

Specifications subject to change without notice. No license is granted by implication

or otherwise under any patent or patent rights of Analog Devices. Trademarks and

registered trademarks are the property of their respective owners.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.

Tel: 781.329.4700

www.analog.com

FEATURES

Drives 6 LEDs from 2.6 V to 5.5 V (Li-Ion) input supply

1×/1.5×/2× fractional charge pump to maximize power

efficiency

1% max LED current matching

Up to 88% power efficiency over Li-Ion range

Powers main and sub display LEDs with individual shutdown

Package footprint only 9 mm

(3 mm × 3 mm)

Package height only 0.9 mm

Low power shutdown mode

Shutdown function

Soft-start limiting in-rush current

APPLICATIONS

Cellular phones with main and sub displays

White LED backlighting

Camera flash/strobes and movie lights

Micro TFT color displays

DSC

PDAs

GENERAL DESCRIPTION

The ADM8845 uses charge pump technology to provide the

power required to drive up to six LEDs. The LEDs are used for

backlighting a color LCD display, having regulated constant

current for uniform brightness intensity. The main display can

have up to four LEDs, and the sub display can have one or two

LEDs. The digital CTRL1 and CTRL2 input control pins control

the shutdown operation and the brightness of the main and sub

displays.

To maximize power efficiency, the charge pump can operate in

a 1×, 1.5×, or 2× mode. The charge pump automatically

switches between 1×/1.5×/2× modes, based on the input voltage,

to maintain sufficient drive for the LED anodes at the highest

power efficiency.

Improved brightness matching of the LEDs is achieved by a

feedback pin to sense individual LED current with a maximum

matching accuracy of 1%.

FUNCTIONAL BLOCK DIAGRAM

CURRENT

CONTROL 1

CURRENT

CONTROL 2

CURRENT

CONTROL 3

CURRENT

CONTROL 4

CURRENT

CONTROL 5

CURRENT

CONTROL 6

CONTROL

LOGIC

REF

CHARGE PUMP

1×/1.5×/2× MODE

OSC

LED

CURRENT

CONTROL

CIRCUIT

FB1

CURRENT CONTROLLED SINKS

FB2

FB3

FB4

FB5

FB6

OUT

4.7µF

1µF

CTRL1

CTRL2

1µF

SET

GND

2.2µF

ADM8845

04867-0-001

MAIN SUB

Figure 1.

Summary of content (20 pages)

PAGE 1
Charge Pump Driver for LCD White LED Backlights ADM8845 FEATURES GENERAL DESCRIPTION Drives 6 LEDs from 2.6 V to 5.5 V (Li-Ion) input supply 1×/1.5×/2× fractional charge pump to maximize power efficiency 1% max LED current matching Up to 88% power efficiency over Li-Ion range Powers main and sub display LEDs with individual shutdown Package footprint only 9 mm2 (3 mm × 3 mm) Package height only 0.
PAGE 2
ADM8845 TABLE OF CONTENTS Features .............................................................................................. 1 Brightness Control with a Digital PWM Signal ..................... 11 Applications ....................................................................................... 1 LED Brightness Control Using a PWM Signal Applied to VPWM ............................................................................................. 13 General Description ............................
PAGE 3
ADM8845 SPECIFICATIONS VCC = 2.6 V to 5.5 V; TA = −40°C to +85°C, unless otherwise noted; C1, C2 = 1.0 µF; C3 = 2.2 µF; C4 = 4.7 µF. Table 1. Parameter INPUT VOLTAGE, VCC SUPPLY CURRENT, ICC SHUTDOWN CURRENT CHARGE PUMP FREQUENCY CHARGE PUMP MODE THRESHOLDS 1.5× to 2× Accuracy 2× to 1.5× Accuracy Hysteresis 1× to 1.5× Accuracy 1.
PAGE 4
ADM8845 ABSOLUTE MAXIMUM RATINGS TA = 25°C, unless otherwise noted. Table 2. Parameter Supply Voltage, VCC ISET CTRL1, CTRL2 VOUT Shorted 1 Feedback Pins FB1 to FB6 Operating Temperature Range Six LEDs Enabled with 30 mA/LED 2 Six LEDs Enabled with 20 mA/LED2 VOUT 3 Storage Temperature Range Power Dissipation ESD Class Rating –0.3 V to +6.0 V –0.3 V to +2.0 V –0.3 V to +6.0 V Indefinite –0.3 V to +6.
PAGE 5
ADM8845 11 C2– 10 GND 9 FB6 NOTES 1. CONNECT EXPOSED PADDLE TO GND. 04867-0-003 FB4 7 12 CTRL2 FB5 8 TOP VIEW (Not to Scale) FB2 5 FB1 4 ADM8845 FB3 6 ISET 3 13 CTRL1 16 C1+ PIN 1 INDICATOR VOUT 1 C2+ 2 15 VCC 14 C1– PIN CONFIGURATION AND FUNCTION DESCRIPTIONS Figure 2. Pin Configuration Table 3. Pin Function Descriptions Pin No. 1 Mnemonic VOUT 2 3 C2+ ISET 4 to 9 FB1 to FB6 10 11 12 13 14 15 16 - GND C2− CTRL2 CTRL1 C1− VCC C1+ EP Function Charge Pump Output. A 2.
PAGE 6
ADM8845 TYPICAL PERFORMANCE CHARACTERISTICS 0.4 35 0.3 MAX POSITIVE MATCHING ERROR 30 25 20 15 0.1 0 –0.1 –0.2 10 MAX NEGATIVE MATCHING ERROR 5 4.75 10.75 8.75 RSET (k) 6.75 12.75 04867-0-004 –0.3 14.75 –0.4 2.6 Figure 3. ILED (mA) Current vs. RSET 3.4 3.8 4.2 4.6 SUPPLY VOLTAGE (V) 5.0 5.4 Figure 6. ILED (mA) Matching Error (%) vs. Supply Voltage (V), TA = 25°C and ILED = 20 mA 20.35 20.24 20.22 20.30 –40C 20.20 20.25 LED CURRENT (mA) LED CURRENT (mA) 3.
PAGE 7
ADM8845 95 20 90 85 EFFICIENCY (%) LED CURRENT (mA) 16 12 8 80 75 70 4 0 20 40 60 DUTY CYCLE (%) 80 100 60 0 Figure 9. ILED (mA) vs. PWM Dimming (Varying Duty Cycle), Six LEDs Enabled, Frequency = 1 kHz 10 20 30 40 50 60 DUTY CYCLE (%) 70 80 90 100 04867-0-013 0 04867-0-010 65 Figure 12. LED Efficiency vs. Varying Duty Cycle of 1 kHz PWM Signal, Six LEDs Enabled, 20 mA/LED 300 CTRL1/2 200 1 20mA/LED CURRENT 150 15mA/LED 2 100 VOUT 50 3.0 3.4 3.8 4.2 4.
PAGE 8
ADM8845 90 VF = 3.8V 85 VF = 4.0V 1 2 70 65 60 55 VF = 3.6V 04867-0-016 50 CH1 20.0mV B W CH2 20.0mV B W M 400ns CH1 VF = 3.2V 45 40 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4.0 4.1 4.2 220mV VCC (V) Figure 18. Power Efficiency vs. Supply Voltage over Li-Ion Range, Six LEDS @ 20 mA/LED Figure 15. 1× Mode Operating Waveforms 90 90 VF = 3.8V VF = 4.0V VF = 4.0V 80 80 VF = 4.3V POWER EFFICIENCY (%) POWER EFFICIENCY (%) VF = 3.8V 85 85 75 70 65 60 55 VF = 3.
PAGE 9
ADM8845 30mA 65°C 85°C 04867-0-022 20mA Figure 21. Maximum ILED (mA) vs. Ambient Temperature, Six LEDs Connected Rev.
PAGE 10
ADM8845 THEORY OF OPERATION The ADM8845 charge pump driver for LCD white LED backlights implements a multiple gain charge pump (1×, 1.5×, 2×) to maintain the correct voltage on the anodes of the LEDs over a 2.6 V to 5.5 V (Li-Ion) input supply voltage. The charge pump automatically switches between 1×/1.5×/2× modes, based on the input voltage, to maintain sufficient drive for the LED anodes, with VCC input voltages as low as 2.6 V.
PAGE 11
ADM8845 OUTPUT CURRENT CAPABILITY CURRENT MATCHING The ADM8845 can drive up to 30 mA of current to each of the six LEDs given an input voltage of 2.6 V to 5.5 V. The LED currents have a maximum current matching of 1% between any two LED currents. An external resistor, RSET, sets the output current, approximated by the following equation: The 1% maximum current matching performance is defined by the following equations: RSET = 120 × (1.18 V/ILED) where VF is the LED forward voltage.
PAGE 12
ADM8845 The main and sub display brightness of the ADM8845 can be controlled together or separately by applying a digital PWM signal to both CTRL1 and CTRL2 pins. The duty cycle of the applied digital PWM signal determines the brightness of the main and sub displays together. Varying the duty cycle of the applied PWM signal also varies the brightness of the main and sub displays from 0% to 100%.
PAGE 13
ADM8845 LED CONFIG. FULL ON SUB AND MAIN 50% DUTY CYCLE MAIN AND SUB OFF MAIN 80% DUTY CYCLE, SUB OFF tCP CTRL1 CTRL2 VOUT ILED (SUB) ILED (MAIN) 100% SUB DISPLAY BRIGHTNESS 50% SHDN 100% MAIN DISPLAY BRIGHTNESS 80% 50% 04867-0-025 SHDN 37ms > tCP > 15ms Figure 25.
PAGE 14
ADM8845 APPLICATIONS LAYOUT CONSIDERATIONS AND NOISE DRIVING FEWER THAN SIX LEDs Because of the s switching behavior of the ADM8845, PCB trace layout is an important consideration. To ensure optimum performance, a ground plane should be used, and all capacitors (C1, C2, C3, C4) must be located with minimal track lengths to the pins of the ADM8845. The ADM8845 can be operated with fewer than six LEDs in parallel by simply leaving the unused FBx pins floating or connected to GND.
PAGE 15
ADM8845 DRIVING FLASH LEDs DRIVING CAMERA LIGHT, MAIN, AND SUB LEDs The ADM8845 can be operated with any two FBx pins used in parallel to double the combined LED current supplied by the ADM8845. For example, if three flash LEDs need to be driven with 60 mA/LED, the ADM8845 can be configured as in Figure 31 (see also Figure 21). The ADM8845 can be configured to power a camera light that is composed of four white LEDs in parallel, along with the main and sub display bundled into one package.
PAGE 16
ADM8845 DRIVING FOUR BACKLIGHT WHITE LEDs AND FLASH LEDs CTRL1 controls the flash on/off, and CTRL2 controls the backlight on/off and brightness control. Because the RSET resistor sets the current that each of the six current control blocks can sink, a PWM signal is used to change the current in the backlight from 20 mA to 5 mA/LED. The CTRL2 duty cycle is 15/20 to give 15 mA/backlight LED. The ADM8845 can be configured to power four backlight white LEDs and a camera flash, bundled into one package.
PAGE 17
ADM8845 POWER EFFICIENCY The ADM8845 power efficiency (η) equations are Example 1 η = POUT/PIN PIN = ((VCC × ILOAD × Gain) + (IQ × VCC)) POUT = 6 × (VF × ILED) The ADM8845 driving six white LED with 20 mA/LED at VCC = 3.4 V (1.5× mode), LED VF = 4.5 V. PIN = ((VCC × ILOAD × Gain) + (VCC × IQ)) PIN = ((3.4 × 120 mA × 1.5) + (3.4 × 2.6 mA)) PIN = ((0.612) + (0.00884)) PIN = 0.62084 where: IQ is the quiescent current of the ADM8845, 2.6 mA. VF is the LED forward voltage.
PAGE 18
ADM8845 OUTLINE DIMENSIONS 3.00 BSC SQ 0.60 MAX 0.45 TOP VIEW 13 16 12 (BOTTOM VIEW) 1 2.75 BSC SQ SEATING PLANE 0.05 MAX 0.02 NOM 0.30 0.23 0.18 9 8 5 4 0.25 MIN 1.50 REF 0.80 MAX 0.65 TYP 12° MAX *1.45 1.30 SQ 1.15 EXPOSED PAD 0.50 BSC 1.00 0.85 0.80 PIN 1 INDICATOR FOR PROPER CONNECTION OF THE EXPOSED PAD, REFER TO THE PIN CONFIGURATION AND FUNCTION DESCRIPTIONS SECTION OF THIS DATA SHEET. 0.20 REF 072208-A PIN 1 INDICATOR 0.50 0.40 0.
PAGE 19
ADM8845 NOTES Rev.