Four White LED Backlight Driver
ADM8843
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
2
(3 mm × 3 mm)
Package height only 0.55 mm
Low power shutdown mode
Shutdown function
Soft start limiting in-rush current
GENERAL DESCRIPTION
The ADM8843 uses charge pump technology to provide the
power to drive up to four LEDs. The LEDs are used for back-
lighting a color LCD display that has 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 opera-
tion and the brightness of the main and sub displays.
To maximize power efficiency, the charge pump can operate in
1×, 1.5×, or 2× mode. The charge pump automatically switches
among 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%.
APPLICATIONS
Cellular phones with main and sub displays
White LED backlighting
Camera flash/strobes and movie light applications
Micro TFT color displays
DSC
PDAs
FUNCTIONAL BLOCK DIAGRAM
C1
1μF
C2
1μF
V
CC
C4
4.7μF
ADM8843
CHARGE PUMP
1×/1.5×/2× MODE
V
OUT
C3
2.2μF
MAIN
SUB
OSC
CTRL1
CTRL2
CONTROL
LOGIC
V
REF
FB1
FB2
FB3
FB4
CURRENT
CONTROL 1
CURRENT
CONTROL 2
CURRENT
CONTROL 3
I
SET
R
SET
LED
CURRENT
CONTROL
CIRCUIT
GND
Figure 1.
Rev. C
Information furnished by Analog Devices is believed to be accurate and reliable. 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
Fax: 781.461.3113 ©2004–201 Analog Devices, Inc. All rights reserved.
05050-001
CURRENT CONTROLLED SINKS
CURRENT
CONTROL 4
ADM8843
TABLE OF CONTENTS
Features .............................................................................................. 1
Applications....................................................................................... 1
General Description ......................................................................... 1
Functional Block Diagram .............................................................. 1
Revision History ............................................................................... 2
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 V
PWM
.................................... 12
LED Brightness Control
Using a DC Voltage Applied to V
BRIGHT
................................... 12
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
REVISION HISTORY
1/11—Rev. B to Rev. C
Changes to Figure 14........................................................................ 7
4/10—Rev. A to Rev. B
Removed CP-16-12 Package .............................................Universal
Change to Figure 2 ........................................................................... 5
Change to Figure 6 ........................................................................... 6
Changes to Figure 14........................................................................ 7
Changes to Figure 15........................................................................ 8
Updated Outline Dimensions ....................................................... 16
Changes to Ordering Guide .......................................................... 16
12/06—Rev. 0 to Rev. A
Change to Thermal Characteristics Section ................................. 4
Changes to Outline Dimensions................................................... 16
Changes to Ordering Guide .......................................................... 16
10/04—Revision 0: Initial Version
Rev. C | Page 2 of 16
ADM8843
SPECIFICATIONS
V
CC
= 2.6 V to 5.5 V; T
A
= −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, V
CC
SUPPLY CURRENT, I
CC
SHUTDOWN CURRENT
CHARGE PUMP FREQUENCY
CHARGE PUMP MODE THRESHOLDS
1.5× to 2×
2× to 1.5×
Hysteresis
1× to 1.5×
1.5× to 1×
Hysteresis
I
SET
PIN
LED : LED Matching
I
SET
Pin Voltage
I
LED
-to-I
SET
Ratio
I
LED
-to-I
SET
Ratio Accuracy
MIN COMPLIANCE ON FBx PIN
CHARGE PUMP OUTPUT RESISTANCE
Min
2.6
Typ
2.6
Max
5.5
5
5
1.5
3.33
3.36
40
4.77
4.81
40
0.3
1.18
120
±5%
0.15
1.2
3.5
8.0
0.1
0.7 V
CC
20
200
V
0.3 V
CC
1
88
30
Unit
V
mA
μA
MHz
V
V
mV
V
V
mV
%
V
I
LED
= 20 mA, V
FB
= 0.4 V
Test Conditions
All four LEDs disabled, V
CC
= 3.3 V, R
SET
= 7.08 kΩ,
CTRL1 = 1, CRTL2 = 1
LED CURRENT
PWM
1
DIGITAL INPUTS
Input High
Input Low
Input Leakage Current
CHARGE PUMP POWER EFFICIENCY
V
OUT
RIPPLE
1
V
Ω
Ω
Ω
mA
kHz
I
SET
= 15 mA
1× mode
1.5× mode
2× mode
V
μA
%
mV
CTRL1 = 1, CRTL2 = 1, V
CC
= 3.4 V, V
FB
= 0.2 V, I
FB
= 20 mA
V
CC
= 3.6 V, I
LED
= 20 mA, all four LEDs enabled
Guaranteed by design. Not 100% production tested.
Rev. C | Page 3 of 16
ADM8843
ABSOLUTE MAXIMUM RATINGS
T
A
= 25°C, unless otherwise noted.
Table 2.
Parameter
Supply Voltage, V
CC
I
SET
CTRL1, CTRL2
V
OUT
Shorted
1
Feedback Pins FB1 to FB4
Operating Temperature Range
V
OUT2
Storage Temperature Range
Power Dissipation
ESD Class
1
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 +6.0 V
–40°C to +85°C
180 mA
–65°C to +125°C
2 mW
1
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 (CP-16-3) package:
θ
JA
=50°C/W
Short through LED.
2
Based on long-term current density limitations.
ESD CAUTION
Rev. C | Page 4 of 16
ADM8843
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
15 V
CC
16 C1+
13 CTRL1
14 C1–
V
OUT
1
C2+ 2
I
SET
3
GND 4
PIN 1
INDICATOR
12 CTRL2
11 C2–
10 GND
9 GND
ADM8843
TOP VIEW
(Not to Scale)
FB4 8
FB3 7
FB1 5
FB2 6
NOTES
1. CONNECT THE EXPOSED PADDLE TO GND.
Figure 2. Pin Configuration
Table 3. Pin Function Descriptions
Pin No.
1
2
3
4, 9, 10
5 to 8
Mnemonic
V
OUT
C2+
I
SET
GND
FB1 to FB4
Description
Charge Pump Output. A 2.2 μF capacitor to ground is required on this pin. Connect V
OUT
to the anodes of all the LEDs.
Flying Capacitor 2 Positive Connection.
Bias Current Set Input. The current flowing through the R
SET
resistor, I
SET
, is gained up by 120 to provide the I
LED
current. Connect a resistor, R
SET
, to GND to set the bias current as V
SET
/R
SET
. Note that V
SET
= 1.18 V.
Device Ground Pins.
LED1 to LED4 Cathode Connection and Charge Pump Feedback. The current flowing in these LEDs is 120 times
the current flowing through R
SET
, I
SET
. When using fewer than four LEDs, this pin can be left unconnected or
connected to GND.
Flying Capacitor 2 Negative Connection.
Digital Input. 3 V CMOS Logic. Used with CTRL1 to control the shutdown operation of the main and sub LEDs.
Digital Input. 3 V CMOS Logic. Used with CTRL2 to control the shutdown operation of the main and sub LEDs.
Flying Capacitor 1 Negative Connection.
Positive Supply Voltage Input. Connect this pin to a 2.6 V to 5.5 V supply with a 4.7 μF decoupling capacitor.
Flying Capacitor 1 Positive Connection.
Expose Paddle. Connect the exposed paddle to GND.
11
12
13
14
15
16
–
C2−
CTRL2
CTRL1
C1−
V
CC
C1+
EP
Rev. C | Page 5 of 16
05050-003
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