Analog Devices ADM8843 Datasheet

4 White LED Backlight Driver

FEATURES

Drives 4 LEDs from a 2.6 V to 5.5 V (Li-Ion) input supply
1×/1.5×/2× fractional charge pump to maximize

power efficiency

0.3% typical 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
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 light applications
Micro TFT color displays
DSC
PDAs

2

(3 mm × 3 mm)

ADM8843

GENERAL DESCRIPTION

The ADM8843 uses charge pump technology to provide the
power to drive up to four LEDs. The LEDs are used for back­lighting a color LCD display, having regulated constant current
for uniform brightness intensity. The main display can use up to
three LEDs, and the sub display uses one LED. The CTRL1 and
CTRL2 digital 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
either 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 that senses individual LED current with a typical
matching accuracy of 0.3%.

4.7µF

CTRL1

CTRL2

R

C4

SET

V

I

CC

SET

ADM8843

CONTROL

LOGIC

LED
CURRENT
CONTROL

CIRCUIT

FUNCTIONAL BLOCK DIAGRAM

C1

C2

1µF

1µF

CHARGE PUMP

1×/1.5×/2× MODE

OSC

V

REF

CURRENT

CURRENT

CURRENT

CONTROL 1

CONTROL 2

CURRENT CONTROLLED SINKS

GND

CURRENT

CONTROL 3

CONTROL 4

Figure 1.

V

OUT

FB1
FB2
FB3
FB4

C3

2.2µF

MAIN SUB

05050-001

Rev. 0

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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: [phone]
Fax: [phone] © 2004 Analog Devices, Inc. All rights reserved.

www.analog.com

ADM8843

TABLE OF CONTENTS

Specifications......................................................................................3

Absolute Maximum Ratings.............................................................4

Thermal Characteristics .............................................................. 4

ESD Caution.................................................................................. 4

Pin Configuration and Function Descriptions..............................5

Typical Performance Characteristics ..............................................6

Theory of Operation .........................................................................9

Automatic Gain Control ............................................................ 10

Brightness Control with a Digital PWM Signal..................... 10

LED Brightness Control Using a PWM Signal Applied to

............................................................................................. 12

V

PWM

REVISION HISTORY

10/04—Revision 0: Initial Version

LED Brightness Control Using a DC Voltage Applied to

.......................................................................................... 12

V

BRIGHT

Applications......................................................................................13

Layout Considerations and Noise ............................................ 13

White LED Shorting .................................................................. 13

Driving Four LEDs in the Main Display Only ....................... 13

Driving Fewer than Four LEDs ................................................ 13

Using Smaller Capacitor Values ............................................... 14

Power Efficiency......................................................................... 15

Outline Dimensions ........................................................................16

Ordering Guide .......................................................................... 16

Rev. 0 | Page 2 of 16

ADM8843

SPECIFICATIONS

VCC = 2.6 V to 5.5 V; T

= −40°C to +85°C, unless otherwise noted; C1, C2 = 1.0 µF; C3 = 2.2 µF; C4 = 4.7 µF

A

Table 1.

PARAMETER Min Typ Max Unit Test Conditions

INPUT VOLTAGE, VCC 2.6 5.5 V
SUPPLY CURRENT, ICC 2.6 5 mA

All four LEDs disabled, V

= 3.3 V, R

CC

= 7.08 kΩ,

SET

CTRL1 = 1, CRTL2 = 1
SHUTDOWN CURRENT 5 µA
CHARGE PUMP FREQUENCY 1.5 MHz
CHARGE PUMP MODE THRESHOLDS

1.5× to 2× 3.33 V
2× to 1.5× 3.36 V

Hysteresis 40 mV

1× to 1.5× 4.77 V

1.5× to 1× 4.81 V
Hysteresis 40 mV

I

PIN

SET

LED : LED Matching 0.3 % I
I

Pin Voltage 1.18 V

SET

I

to I

LED

I

LED

Ratio 120

SET

to I

Ratio Accuracy ±5%

SET

MIN COMPLIANCE ON FBx PIN 0.15 V I

= 20 mA, VFB = 0.4 V

LED

= 15 mA

SET

CHARGE PUMP OUTPUT RESISTANCE 1.2 Ω 1× mode

3.5 Ω 1.5× mode

8.0 Ω 2× mode

LED CURRENT 20 mA
PWM 0.1 200 kHz Note 1
DIGITAL INPUTS

Input High 0.7 VCC V
Input Low 0.3 VCC V

Input Leakage Current 1 µA
CHARGE PUMP POWER EFFICIENCY 88 % CTRL1 = 1, CRTL2 = 1, VCC = 3.4V, VFB = 0.2 V, IFB = 20 mA
V

RIPPLE 30 mV VCC = 3.6 V, I

OUT

______________________

1

Guaranteed by design. Not 100% production tested.

= 20 mA, all four LEDs enabled

LED

Rev. 0 | Page 3 of 16

ADM8843

ABSOLUTE MAXIMUM RATINGS

TA = 25°C, unless otherwise noted.

Table 2.

Parameter Rating

Supply Voltage V
I

–0.3 V to +2.0 V

SET

CC

–0.3 V to +6.0 V

CTRL1, CTRL2 –0.3 V to +6.0 V
V

OUT

Shorted

1

Indefinite
Feedback Pins FB1 to FB4 –0.3 V to +6.0 V
Operating Temperature Range –40°C to +85°C

2

V

180 mA

OUT

Storage Temperature Range –65°C to +125°C
Power Dissipation 2 mW
ESD Class 1
___________________________

1

Short through LED.

2

Based on long-term current density limitations.

ESD CAUTION

ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on
the human body and test equipment and can discharge without detection. Although this product features
proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy
electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance
degradation or loss of functionality.

Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.

THERMAL CHARACTERISTICS

16-Lead LFCSP Package:

= 50°C/W

θ

JA

Rev. 0 | Page 4 of 16

ADM8843

V

PIN CONFIGURATION AND FUNCTION DESCRIPTIONS

1
L
R

+

C

–

1

1

T

C

V

C

C

C

4

3

5

6

1

1

1

1

PIN 1
INDICATOR

1

OUT

2C2+

ADM8843

3I
4GND

TOP VIEW

(Not to Scale)

5

6

1

2

B

B

F

F

SET

Figure 2. Pin Configuration

Table 3. Pin Function Descriptions

Pin No. Mnemonic Description

1 V

OUT

Charge Pump Output. A 2.2 µF capacitor to ground is required on this pin. Connect V

all the LEDs.
2 C2+ Flying Capacitor 2 Positive Connection.
3 I

SET

Bias Current Set Input. The current flowing through the R

current. Connect a resistor R

I

LED

to GND to set the bias current as V

SET

4, 9, 10 GND Device Ground Pins.
5–8 FB1–FB4

LED1–LED4 Cathode Connection and Charge Pump Feedback. The current flowing in these LEDs is

120 times the current flowing through R

unconnected or connected to GND.
11 C2− Flying Capacitor 2 Negative Connection.
12 CTRL2

Digital Input. 3 V CMOS Logic. Used with CTRL1 to control the shutdown operation of the main and

sub LEDs.
13 CTRL1

Digital Input. 3 V CMOS Logic. Used with CTRL2 to control the shutdown operation of the main and

sub LEDs.
14 C1− Flying Capacitor 1 Negative Connection.
15 V

CC

Positive Supply Voltage Input. Connect this pin to a 2.6 V to 5.5 V supply with a 4.7 µF decoupling

capacitor.
16 C1+ Flying Capacitor 1 Positive Connection.

- EP Expose Paddle. Connect the exposed paddle to GND.

12 CTRL2
11 C2–
10 GND
9 GND

8

7

4

3

B

B

F

F

05050-003

OUT

resistor, I

SET

, I

. When using fewer than four LEDs, this pin can be left

SET

SET

, is gained up by 120 to give the

SET

. Note that V

SET/RSET

to the anodes of

= 1.18 V.

SET

Rev. 0 | Page 5 of 16