Four White LED Backlight Driver
ADM8843
Rev. C
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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 mm2 (3 mm × 3 mm)
Package height only 0.55 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
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%.
FUNCTIONAL BLOCK DIAGRAM
CURRENT
CONTROL 1
CURRENT
CONTROL 2
CURRENT
CONTROL 3
CURRENT
CONTROL 4
CONTROL
LOGIC V
REF
CHARGE PUMP
1×/1.5×/2× MODE
OSC
LED
CURRENT
CONTROL
CIRCUIT
FB1
CURRENT CONTROLLED SINKS
FB2
FB3
FB4
V
OUT
C4
4.7μF
V
CC
C1
1μF
CTRL1
CTRL2
C2
1μF
R
SET
I
SET
GND
C3
2.2μF
ADM8843
05050-001
MAIN SUB
Figure 1.
ADM8843
Rev. C | Page 2 of 16
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 VPWM .................................... 12
LED Brightness Control
Using a DC Voltage Applied to VBRIGHT ................................... 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
ADM8843
Rev. C | Page 3 of 16
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 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, VCC = 3.3 V, RSET = 7.08 kΩ,
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
ISET PIN
LED : LED Matching 0.3 % ILED = 20 mA, VFB = 0.4 V
ISET Pin Voltage 1.18 V
ILED-to-ISET Ratio 120
ILED-to-ISET Ratio Accuracy ±5%
MIN COMPLIANCE ON FBx PIN 0.15 V ISET = 15 mA
CHARGE PUMP OUTPUT RESISTANCE 1.2 Ω 1× mode
3.5 Ω 1.5× mode
8.0 Ω 2× mode
LED CURRENT 20 mA
PWM1 0.1 200 kHz
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 = 3.4 V, VFB = 0.2 V, IFB = 20 mA
VOUT RIPPLE 30 mV VCC = 3.6 V, ILED = 20 mA, all four LEDs enabled
1 Guaranteed by design. Not 100% production tested.
ADM8843
Rev. C | Page 4 of 16
ABSOLUTE MAXIMUM RATINGS
TA = 25°C, unless otherwise noted.
Table 2.
Parameter Rating
Supply Voltage, VCC –0.3 V to +6.0 V
ISET –0.3 V to +2.0 V
CTRL1, CTRL2 –0.3 V to +6.0 V
VOUT Shorted1 Indefinite
Feedback Pins FB1 to FB4 –0.3 V to +6.0 V
Operating Temperature Range –40°C to +85°C
VOUT2 180 mA
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.
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:
θJA =50°C/W
ESD CAUTION
ADM8843
Rev. C | Page 5 of 16
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
PIN 1
INDICATOR
1V
OUT
2C2+ 3I
SET
4GND
NOTES
1. CO NNECT THE EX P OSED PADDL E TO G ND.
11 C2–
12 CTRL2
10 GND
9GND
5
FB1
6
FB2 7
FB3 8
FB4
15 V
CC
16 C1+
14 C1–
13 CTRL1
TOP VI EW
(Not to Scale)
ADM8843
05050-003
Figure 2. Pin Configuration
Table 3. Pin Function Descriptions
Pin No. Mnemonic Description
1 VOUT Charge Pump Output. A 2.2 μF capacitor to ground is required on this pin. Connect VOUT to the anodes of all the LEDs.
2 C2+ Flying Capacitor 2 Positive Connection.
3 ISET Bias Current Set Input. The current flowing through the RSET resistor, ISET, is gained up by 120 to provide the ILED
current. Connect a resistor, RSET, to GND to set the bias current as VSET/RSET. Note that VSET = 1.18 V.
4, 9, 10 GND Device Ground Pins.
5 to 8 FB1 to FB4 LED1 to LED4 Cathode Connection and Charge Pump Feedback. The current flowing in these LEDs is 120 times
the current flowing through RSET, ISET. When using fewer than four LEDs, this pin can be left 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 VCC 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.
ADM8843
Rev. C | Page 6 of 16
TYPICAL PERFORMANCE CHARACTERISTICS
5
10
15
20
25
30
35
10.75
05050-004
R
SET
(kΩ)
LED CURRENT (mA)
4.75 6.75 12.75 14.758.75
Figure 3. ILED (mA) vs. RSET
05050-005
TEMPERATURE (°C)
LED CURRENT (mA)
–40 0 8040
20.08
20.12
20.14
20.16
20.18
20.20
20.24
20.10
20.22
Figure 4. ILED (mA) Variation over Temperature (°C), VCC = 3.6 V
0
4
8
12
16
20
05050-006
DUTY CYCLE (%)
LED CURRENT (mA)
0 20 60 1008040
Figure 5. ILED (mA) vs. PWM Dimming (Varying Duty Cycle),
Four LEDs Enabled, Frequency = 1 kHz
20.00
20.10
20.15
20.20
20.25
20.30
20.35
4.1
05050-007
SUPPLY VOLTAGE (V )
LED CURRENT (mA)
2.6 3.1 4.6 5.63.6 5.1
–40°C
+25°C
+85°C
20.05
Figure 6. ILED (mA) vs. Temperature (°C), Four LEDs Enabled
5
10
15
20
25
30
35
3.8
05050-008
SUPPLY VOLTAGE (V)
LED CURRENT (mA)
2.6 3.0 4.6 5.43.4 5.04.2
Figure 7. ILED (mA) vs. Supply Voltage (V)
60
65
70
80
90
95
05050-009
DUTY CYCLE (%)
EFFICIENCY (%)
04060807050
85
90 100
10 20 30
75
Figure 8. LED Efficiency vs. Varying Duty Cycle of 1 kHz PWM Signal,
Four LEDs Enabled, 20 mA/LED
ADM8843
Rev. C | Page 7 of 16
60
80
100
120
160
180
05050-010
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (mA)
2.6 3.0 3.8 4.64.23.4
140
5.0 5.4
–40°C
+25°C
+85°C
Figure 9. Supply Current vs. Supply Voltage over Temperature,
Four LEDs Enabled @ 20 mA/LED
05050-011
CH2 20.0mVCH1 20.0mV M 10.0μs CH1 –12.8mV
2
1
BWBW
V
OUT
V
CC
Figure 10. 1.5× Mode Operating Waveforms
5050-012
CH2 20.0mVCH1 20.0mV M 400ns CH2 2.4mV
2
1
BWBW
V
OUT
V
CC
Figure 11. 1× Mode Operating Waveforms
5050-013
CH2 160mACH1 2.00V M 10.0μs CH2 –2.44mV
2
1
BWBW
C
URRENT
3
V
OUT
CTRL1/2
CH3 1.00V
Figure 12. Soft Start Showing the Initial In-Rush Current and VOUT Variation,
Four LEDs @ 20 mA/LED, VCC = 3.6 V
05050-014
CH2 20.0mVCH1 20.0mV M 400ns CH2 2.4mV
2
1
BWBW
V
OUT
V
CC
Figure 13. 2× Mode Operating Waveforms
40
45
50
60
80
90
5050-015
V
CC
(V)
POWER EFFICIEN
C
Y (%)
70
55
65
75
85 V
F
= 4. 0V
V
F
= 3. 8V
V
F
= 4. 3V
V
F
= 3. 2V
V
F
= 3. 6V
2.8 3.0 3.4 3.83.63.2 4.0 4.22.93.13.33.53.73.94.1
Figure 14. Power Efficiency vs. Supply Voltage over Li-Ion Range,
Four LEDs @ 20 mA/LED
ADM8843
Rev. C | Page 8 of 16
40
45
50
60
80
90
05050-016
V
CC
(V)
POWER EFFICIEN
C
Y (%)
70
55
65
75
85
V
F
= 4.0V
V
F
= 3.8V
V
F
= 4.3V
V
F
= 3. 2V
V
F
= 3. 6V
2.8 3.0 3.4 3.83.63.2 4.0 4.22.9 3.1 3.3 3.5 3.7 3.9 4.1
Figure 15. Power Efficiency vs. Supply Voltage over Li-Ion Range,
Four LEDs @ 15 mA/LED
05050-017
CH2 2.00mVCH1 2.00V M 10.0ms CH2 4.36mV
2
1
V
OUT
CTRL1/2
Δ: 44.0ms
@: –44.4ms C2 FALL
200μs
LOW SIGNAL
AMPLITUDE
Figure 16. TPC Delay
ADM8843
Rev. C | Page 9 of 16
THEORY OF OPERATION
The ADM8843 charge pump driver for LCD white LED back-
lights 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 among 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. It also includes regulation
of the charge pump output voltage for supply voltages up to 5.5 V.
The ADM8843’s four LEDs are arranged into two groups, main
and sub. The main display can have up to three LEDs (FB1 to
FB3), and the sub display has one LED (FB4) (see Figure 18).
The CTRL1 and CTRL2 digital input control pins control the
shutdown operation and the brightness of the main and sub
displays (see Table 4).
Table 4. Shutdown Truth Table
CTRL1 CTRL2 LED Shutdown Operation
0 0 Sub display off/main display off
0 1 Sub display off/main display on
1 0 Sub display on/main display off
1 1 Sub display on/main display on
An external resistor, RSET, is connected between the ISET pin and
GND. This resistor sets up a reference current, ISET, which is
internally gained up by 120 within the ADM8843 to produce
ILED currents of up to 20 mA/LED (ILED = ISET × 120 and ISET =
1.18 V/RSET). The ADM8843 uses four separate current sinks to
sense each LED current individually with a typical matching
performance of 0.3%. This current matching performance
ensures uniform brightness across a color display.
The ADM8843 lets the user control the brightness of the white
LEDs with a digital PWM signal applied to CTRL1 and/or
CTRL2. The duty cycle of the applied PWM signal determines
the brightness of the main and/or sub display backlight white
LEDs. The ADM8843 also allows the brightness of the white
LEDs to be controlled using a dc voltage (see Figure 17). Soft
start circuitry limits the in-rush current flow at power-up. The
ADM8843 is fabricated using CMOS technology for minimal
power consumption and is packaged in a 16-lead lead frame
chip scale package.
I
SET
R
SET
= 13.4kΩ
R = 15kΩ
V
BRIGHT
0V–2.5V
ADM8843
05050-022
Figure 17. PWM Brightness Control Using a DC Voltage Applied to VBRIGHT
CURRENT
CONTROL 1
CURRENT
CONTROL 2
CURRENT
CONTROL 3
CURRENT
CONTROL 4
CONTROL
LOGIC V
REF
CHARGE PUMP
1×/1.5×/2× MODE
OSC
LED
CURRENT
CONTROL
CIRCUIT
FB1
CURRENT CONTROLLED SINKS
FB2
FB3
FB4
V
OUT
C4
4.7μF
V
CC
C1
1μF
CTRL1
CTRL2
C2
1μF
R
SET
I
SET
GND
C3
2.2μF
ADM8843
05050-001
MAIN SUB
Figure 18. Functional Block Diagram
ADM8843
Rev. C | Page 10 of 16
AUTOMATIC GAIN CONTROL
The automatic gain control block controls the operation of the
charge pump by selecting the appropriate gain for the charge
pump. Doing so maintains sufficient drive for the LED anodes
at the highest power efficiency over a 2.6 V to 5.5 V input
supply range. The charge pump switching thresholds are
described in Table 5.
Table 5. Charge Pump Switching Thresholds
Gain Threshold (V)
1.5× to 2× 3.33
2× to 1.5× 3.36
1× to 1.5× 4.77
1.5× to 1× 4.81
BRIGHTNESS CONTROL WITH A
DIGITAL PWM SIGNAL
PWM brightness control provides the widest brightness control
method by pulsing the white LEDs on and off using one or both
digital input control pins, CTRL1 and CTRL2. PWM brightness
control also removes any chromaticity shifts associated with
changing the white LED current, because the LEDs operate at
either zero current or full current (set by RSET).
The digital PWM signal applied with a frequency of 100 Hz to
200 kHz turns the current control sinks on and off using CTRL1
and/or CTRL2. The average current through the LEDs changes
with the PWM signal duty cycle. If the PWM frequency is much
less than 100 Hz, flicker may be seen in the LEDs. For the
ADM8843, zero duty cycle turns off the LEDs, and a 50% duty
cycle results in an average LED current, ILED, that is half the pro-
grammed LED current. For example, if RSET is set to program
20 mA/LED, a 50% duty cycle results in an average ILED of
10 mA/LED, which is half the programmed LED current.
FB1
FB2
FB3
FB4
CTRL1
CTRL2
R
SET
I
SET
ADM8845
05050-018
3.4V
V
CC
V
OUT
I
IN
C3
2.2μF
C1
1μFC2
1μF
PWM INPUT
OR
HIGH/LOW
PWM INPUT
OR
HIGH/LOW
Figure 19. Digital PWM Brightness Control Application Diagram
Applying a digital PWM signal to one or both digital input
control pins, CTRL1 and CTRL2, adjusts the brightness of the
sub and/or main displays. The ADM8843’s four white LEDs are
organized into two groups, main display (FB1 to FB3) and sub
display (FB4); refer to the Theory of Operation section.
The brightness of the ADM8843’s main and sub displays can be
controlled together or separately. This is achieved by applying a
digital PWM signal to both the 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 varies the brightness of the main and
sub displays from 0% to 100%.
By holding CTRL1 low and applying a digital PWM signal to
CTRL2, the sub display is turned off and the main display is
turned on. Then the brightness of the main display is determined
by the duty cycle of the applied digital PWM signal.
By applying a digital PWM signal to CTRL1 and holding CTRL2
low, the sub display is turned on and the main display is turned
off. Then the brightness of the sub display is determined by the
duty cycle of the applied digital PWM signal.
By applying a digital PWM signal to CTRL1 and holding
CTRL2 high, the sub display is turned on and the main display
is turned on. Then the brightness of the sub display is determined
by the duty cycle of the applied digital PWM signal. The bright-
ness of the main display is set to the maximum (maximum is set
by RSET).
By holding CTRL1 high and applying a digital PWM signal to
CTRL2, the sub and main displays are turned on. Then the
brightness of the main display is determined by the duty cycle of
the applied digital PWM signal. The brightness of the sub
display is set to the maximum (maximum is set by RSET).
When CTRL1 and CTRL2 are low, the LED current control
sinks shutdown. Shutdown of the charge pump is delayed by
15 ms. This timeout period, tCP, allows the ADM8843 to
determine if a digital PWM signal is present on CTRL1 and
CTRL2, or if the user has selected a full chip shutdown (see
Figure 20).
If digital PWM brightness control of the LEDs is not required, a
constant Logic Level 1 (VCC) or Logic Level 0 (GND) must be
applied.
The four white LED in the ADM8843 are arranged into two
groups, sub and main. It is possible to configure the four LEDs
as in Table 6 . Refer also to Figure 20.
ADM8843
Rev. C | Page 11 of 16
Table 6. Digital Inputs Truth Table
CTRL1 CTRL2 LED Operation
0 0 Sub display off/main display off (full shutdown)1, 2
0 1 Sub display off/main display on1, 3
1 0 Sub display on/main display off1, 2
1 1 Sub display on/main display on (full on)1, 3
0 PWM Sub display off/digital PWM brightness control on main display4, 5
PWM 0 Digital PWM brightness control on sub display/main display off2, 4
1 PWM Sub display on/digital PWM brightness control on main display1, 5
PWM 1 Digital PWM brightness control on sub display/main display on5
PWM PWM Digital PWM brightness control on sub and main display5
1 Sub display on means the display is on with the maximum brightness set by the RSET resistor. CTRL1 = 1 means a constant logic level (VCC) is applied to CTRL1.
2 Main display off means only the main display is off. CTRL2 = 0 means a constant logic level (GND) is applied to CTRL2.
3 Main display on means the display is on with the maximum brightness set by the RSET resistor. CTRL2 = 1 means a constant logic level (VCC) is applied to CTRL2.
4 Sub display off means only the sub display LEDs is off. CTRL1 = 0 means a constant logic level (GND) is applied to CTRL1.
5 PWM means a digital PWM signal is applied to the CTRL1 and/or the CTRL2 pin with a frequency from 100 Hz to 200 kHz.
LED CONFIG. FULL ON SUB AND MAIN 50%
DUTY CYCLE MAIN AND SUB OFF
CTRL1
CTRL2
V
OUT
I
LED
(SUB)
I
LED
(MAIN)
SUB DISPLAY
BRIGHTNESS
MAIN DISPLAY
BRIGHTNESS
37ms >
t
CP
> 15ms
SHDN
SHDN
50%
50%
100%
100%
80%
t
CP
05050-020
MAIN 80% DUTY CYCLE,
SUB OFF
Figure 20. Application Timing
ADM8843
Rev. C | Page 12 of 16
LED BRIGHTNESS CONTROL USING A
PWM SIGNAL APPLIED TO VPWM
Adding two external resistors and a capacitor, as shown in
Figure 21, can also be used for PWM brightness control. This
PWM brightness control method can be used instead of CTRL1
and/or CTRL2 digital PWM brightness control. With this con-
figuration, CTRL1 and CTRL2 digital logic pins can control
shutdown of the white LEDs, and VPWM can control the bright-
ness of all the white LEDs. This is done by applying a high
frequency PWM signal (amplitude 0 V to 2.5 V) to drive an
R-C-R filter on the ISET pin of the ADM8843. A 0% PWM duty
cycle corresponds to 20 mA/LED, and a 100% PWM duty cycle
corresponds to a 0 mA/LED. At PWM frequencies above 5 kHz,
C5 may be reduced (see Figure 21). The amplitude of the PWM
signal must only be 0 V and 2.5 V to result in 20 mA flowing in
each LED.
100
)1(
120
2
2
_CycleDuty
RR
RR
VoltageI
I
SET
SET
SET
LED
−
××
+
×
=
I
SET
R
SET
= 13.4kΩ
R = 7.5kΩ
V
PWM
0V–2.5V
ADM8843
05050-021
100% = I
LED
= 0mA
0% = I
LED
= 20mA
R = 7.5kΩ
C5 = 1μF
Figure 21. PWM Brightness Control Using
Filter-PWM Signal
LED BRIGHTNESS CONTROL USING A DC
VOLTAGE APPLIED TO VBRIGHT
By adding one resistor, as shown in Figure 17, this configuration
can also be used for brightness control of the white LEDs by
using a dc voltage applied to the VBRIGHT node. Figure 22 shows
an application example of LED brightness control using a dc
voltage with an amplitude of 0 V to 2.5 V applied to VBRIGHT.
The equation for ILED is
ISET = [(1/RSET + 1/R)(VSET)] – [(1/R)(VBRIGHT)]
ILED = 120 × ISET
where:
R = 15 kΩ
VSET = voltage at ISET pin (1.18 V)
VBRIGHT
05050-023
ILED
2.5V
0mA
1.6V
7.2mA
0.8V
13.6mA
0V
20mA
Figure 22. PWM Brightness Control Application Diagram Using a
DC Voltage Applied to VBRIGHT
ADM8843
Rev. C | Page 13 of 16
APPLICATIONS
LAYOUT CONSIDERATIONS AND NOISE
Because of the ADM8843’s switching behavior, 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 ADM8843.
WHITE LED SHORTING
If an LED is shorted, the ADM8843 continues to drive the
remaining LEDs with ILED per LED (ILED = ISET × 120 mA). This
is because the ADM8843 uses four internal current sinks to
produce the LED current. If an LED is shorted, the ADM8843
continues to sink (ISET × 120 mA).
DRIVING FOUR LEDS IN THE MAIN DISPLAY ONLY
The ADM8843 can be operated with four LEDs in the main
display only (see Figure 23). With this configuration, CTRL1
and CTRL2 are used together to control the main display
shutdown operation and brightness control.
DRIVING FEWER THAN FOUR LEDS
The ADM8843 can be operated with fewer than four LEDs in
parallel by simply leaving the unused FBx pins floating or by
connecting them to GND. For example, Figure 24 shows three
LEDs being powered by the ADM8843.
V
OUT
FB1
FB2
FB3
FB4
V
CC
2.6V–5.5V
CTRL1
CTRL2
GND
ADM8843
05050-024
LCD
R
SET
I
SET
Figure 23. Driving Four White LEDs
V
OUT
FB1
FB2
FB3
FB4
V
CC
2.6V–5.5V
CTRL1
CTRL2
GND
ADM8843
05050-025
MAIN DISPLAY
R
SET
I
SET
Figure 24. Driving Three White LEDs
VOUT
FB1
FB2
FB3
FB4
VCC
2.6V–5.5V
CTRL1
CTRL2
GND
ADM8843
RSET
ISET
05050-002
MAIN DISPLAY
SUB DISPLAY
Figure 25. Typical Application Diagram
ADM8843
Rev. C | Page 14 of 16
USING SMALLER CAPACITOR VALUES
The ADM8843 can be operated with the smaller capacitor values described here to reduce capacitor footprint sizes.
Option 1
Input and output ripple plots for 1× and 1.5× modes of operation
are shown with C1, C2 = 0.22 μF; C3 = 0.47 μF; and C4 = 1 μF.
05050-026
CH2 10.0mV
CH3 10.0mV M 400ns CH3 –33mV
3
2
BW
BW
VOUT
VCC
Figure 26. 1× Mode Operation, Four LEDs with 20 mA/LED at VCC = 5.0 V,
with a 1 μF VCC Decoupling Capacitor
05050-027
CH2 20.0mV M 400ns CH3 –33mV
3
2
BW
V
OUT
V
CC
CH3 50.0mV
BW
Figure 27. 1.5× Mode Operation, Four LEDs with 20 mA/LED at VCC = 3.6 V,
with a 1 μF VCC Decoupling Capacitor
Option 2
Input and output ripple plots for 1× and 1.5× modes of operation
are shown with C1, C2 = 0.22 μF; C3 = 0.47 μF; and C4 = 4.7 μF.
05050-028
CH2 10.0mV
CH3 10.0mV M 1.00
μ
s CH3 –33mV
3
2
BW
BW
V
OUT
V
CC
Figure 28. 1× Mode Operation, Four LEDs with 20 mA/LED at VCC = 5.0 V,
with a 4.7 μF VCC Decoupling Capacitor
05050-029
CH2 20.0mV
CH3 50.0mV M 400ns CH3 –33mV
3
2
BW
BW
V
OUT
V
CC
Figure 29. 1.5× Mode Operation, Four LEDs with 20 mA/LED at VCC = 3.6 V,
with a 4.7 μF VCC Decoupling Capacitor
ADM8843
Rev. C | Page 15 of 16
POWER EFFICIENCY
The ADM8843 power efficiency (η) equations are as follows:
η = POUT/PIN
PIN = ((VCC × ILOAD × Gain) + (IQ × VCC))
POUT = 4 × (VF × ILED)
where:
IQ is the quiescent current of the ADM8843, 2.6 mA.
VF is the LED forward voltage.
Gain is the charge pump mode (1×, 1.5×, 2×).
Example 1
The ADM8843 driving four 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 × 80 mA × 1.5) + (3.4 × 2.6 mA))
PIN = ((0.408) + (0.00884))
PIN = 0.41684
POUT = 4(VF × ILED)
POUT = 4(4.5 V × 20 mA)
POUT = 0.36
η = POUT/PIN
η = 0.36/0.41684
η = 87 %
V
OUT
FB1
FB2
FB3
FB4
V
CC
= 3.4V
CTRL1
CTRL2
GND
ADM8843
R
SET
7.32kΩ
I
SET
05050-019
V
CC
I
IN
I
LOAD
Figure 30. Charge Pump Power Efficiency Diagram, Example 1
Example 2
The ADM8843 driving four white LED with 20 mA/LED at
VCC = 3.4 (1.5× mode), LED VF = 3.6 V.
PIN = ((VCC × ILOAD × Gain) + (VCC × IQ))
PIN = ((3.4 × 80 mA × 1.5) + (3.4 × 2.6 mA))
PIN = ((0.408) + (0.00884))
PIN = 0.41684
POUT = 4(VF × ILED)
POUT = 4(3.6 V × 20 mA)
POUT = 0.288
η = POUT/PIN
η = 0.288/0.41684
η = 70 %
ADM8843
Rev. C | Page 16 of 16
OUTLINE DIMENSIONS
*COMPLIANT
TO
JEDEC STANDARDS MO-220-VEED-2
EXCEPT FOR EXPOSED PAD DIMENSION.
1
0.50
BSC
0.60 MAX
PIN1
INDICATOR
1.50 REF
0.50
0.40
0.30
0.25 MIN
0.45
2.75
BSC SQ
TOP
VIEW
12° MAX 0.80 MAX
0.65 TYP
SEATING
PLANE
PIN 1
INDICATOR
0.90
0.85
0.80
0.30
0.23
0.18
0.05 MAX
0.02 NOM
0.20 REF
3.00
BSC SQ
*1.65
1.50 SQ
1.35
16
5
13
8
9
12
4
EXPOSED
PAD
BOTTOM VIEW
FOR PROPER CONNECTION OF
THE EXPOSED PAD, REFER TO
THE PIN CONFIGURATION AND
FUNCTION DESCRIPTIONS
SECTION OF THIS DATA SHEET.
07-17-2008-A
Figure 31. 16-Lead Lead Frame Chip Scale Package [LFCSP_VQ]
3 mm × 3 mm Body, Very Thin Quad
(CP-16-3)
Dimensions shown in millimeters
ORDERING GUIDE
Model1 Temperature Range Package Description Package Option Branding
ADM8843ACPZ-REEL7 −40°C to +85°C 16-Lead Lead Frame Chip Scale Package [LFCSP_VQ] CP-16-3 M2U
ADM8843EB-EVALZ Evaluation Board
1 Z = RoHS Compliant Part.
©2004–2011 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D05050-0-1/11(C)
