AD ADM8843

4 White LED Backlight Driver
ADM8843
FEATURES
GENERAL DESCRIPTION
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 0.9 mm
Low power shutdown mode
Shutdown function
Soft-start limiting in-rush current
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.
APPLICATIONS
Cellular phones with main and sub displays
White LED backlighting
Camera flash/strobes and movie light applications
Micro TFT color displays
DSC
PDAs
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
C1
1µF
VCC
C2
1µF
ADM8843
VOUT
CHARGE PUMP
1×/1.5×/2× MODE
C4
4.7µF
C3
2.2µF
MAIN
SUB
OSC
CTRL1
CTRL2
CONTROL
LOGIC
VREF
CURRENT CONTROLLED SINKS
GND
05050-001
CURRENT
CONTROL 4
CURRENT
CONTROL 3
RSET
LED
CURRENT
CONTROL
CIRCUIT
CURRENT
CONTROL 2
ISET
CURRENT
CONTROL 1
FB1
FB2
FB3
FB4
Figure 1.
Rev. 0
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: 781.326.8703
© 2004 Analog Devices, Inc. All rights reserved.
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
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
10/04—Revision 0: Initial Version
Rev. 0 | Page 2 of 16
ADM8843
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
Min
2.6
1
Max
5.5
5
Unit
V
mA
5
1.5
µA
MHz
3.33
3.36
40
4.77
4.81
40
V
V
mV
V
V
mV
0.3
1.18
120
±5%
0.15
1.2
3.5
8.0
%
V
ILED = 20 mA, VFB = 0.4 V
V
Ω
Ω
Ω
mA
kHz
ISET = 15 mA
1× mode
1.5× mode
2× mode
2.6
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
ISET PIN
LED : LED Matching
ISET Pin Voltage
ILED to ISET Ratio
ILED to ISET Ratio Accuracy
MIN COMPLIANCE ON FBx PIN
CHARGE PUMP OUTPUT RESISTANCE
LED CURRENT
PWM
DIGITAL INPUTS
Input High
Input Low
Input Leakage Current
CHARGE PUMP POWER EFFICIENCY
VOUT RIPPLE
______________________
Typ
20
200
0.1
0.7 VCC
0.3 VCC
1
88
30
V
V
µA
%
mV
Guaranteed by design. Not 100% production tested.
Rev. 0 | Page 3 of 16
Test Conditions
All four LEDs disabled, VCC = 3.3 V, RSET = 7.08 kΩ,
CTRL1 = 1, CRTL2 = 1
Note 1
CTRL1 = 1, CRTL2 = 1, VCC = 3.4V, VFB = 0.2 V, IFB = 20 mA
VCC = 3.6 V, ILED = 20 mA, all four LEDs enabled
ADM8843
ABSOLUTE MAXIMUM RATINGS
TA = 25°C, unless otherwise noted.
Table 2.
Parameter
Supply Voltage VCC
ISET
CTRL1, CTRL2
VOUT Shorted1
Feedback Pins FB1 to FB4
Operating Temperature Range
VOUT2
Storage Temperature Range
Power Dissipation
ESD Class
___________________________
1
2
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 Package:
θJA = 50°C/W
Short through LED.
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.
Rev. 0 | Page 4 of 16
ADM8843
12 CTRL2
11 C2–
10 GND
05050-003
9 GND
FB4 8
14 C1–
FB3 7
TOP VIEW
(Not to Scale)
FB1 5
GND 4
ADM8843
FB2 6
ISET 3
13 CTRL1
PIN 1
INDICATOR
VOUT 1
C2+ 2
15 VCC
16 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, 9, 10
5–8
GND
FB1–FB4
11
12
C2−
CTRL2
13
CTRL1
14
15
C1−
VCC
16
-
C1+
EP
Description
Charge Pump Output. A 2.2 µF capacitor to ground is required on this pin. Connect VOUT to the anodes of
all the LEDs.
Flying Capacitor 2 Positive Connection.
Bias Current Set Input. The current flowing through the RSET resistor, ISET, is gained up by 120 to give the
ILED current. Connect a resistor RSET to GND to set the bias current as VSET/RSET. Note that VSET = 1.18 V.
Device Ground Pins.
LED1–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.
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.
Rev. 0 | Page 5 of 16
ADM8843
TYPICAL PERFORMANCE CHARACTERISTICS
35
20.35
30
20.30
LED CURRENT (mA)
20
15
10
20.25
20.20
+25°C
20.15
+85°C
20.10
5
4.75
10.75
8.75
RSET (kΩ)
6.75
12.75
14.75
05050-004
20.05
20.00
2.6
Figure 3. ILED (mA) vs. RSET
4.1
3.6
4.6
SUPPLY VOLTAGE (kΩ)
3.1
5.1
05050-007
LED CURRENT (mA)
–40°C
25
5.6
Figure 6. ILED (mA) vs. Temperature (°C), Four LEDs Enabled
20.24
35
20.22
30
LED CURRENT (mA)
LED CURRENT (mA)
20.20
20.18
20.16
20.14
25
20
15
20.12
0
40
TEMPERATURE (°C)
80
05050-005
20.08
–40
5
2.6
3.0
Figure 4. ILED (mA) Variation over Temperature (°C), VCC = 3.6 V
3.4
3.8
4.2
4.6
SUPPLY VOLTAGE (V)
5.0
5.4
05050-008
10
20.10
Figure 7. ILED (mA) vs. Supply Voltage (V)
95
20
90
EFFICIENCY (%)
85
12
8
80
75
70
4
0
0
20
40
60
DUTY CYCLE (%)
80
100
Figure 5. ILED (mA) vs. PWM Dimming (Varying Duty Cycle),
Four LEDs Enabled, Frequency = 1 kHz
60
0
10
20
30
40
50
60
DUTY CYCLE (%)
70
80
90
100
05050-009
65
05050-006
LED CURRENT (mA)
16
Figure 8. LED Efficiency vs. Varying Duty Cycle of 1 kHz PWM Signal,
Four LEDs Enabled, 20 mA/LED
Rev. 0 | Page 6 of 16
ADM8843
180
–40°C
CTRL1/2
+25°C
+85°C
1
140
CURRENT
120
2
100
VOUT
80
3
3.0
3.4
3.8
4.2
4.6
SUPPLY VOLTAGE (V)
5.0
5.4
05050-010
60
2.6
Figure 9. Supply Current vs. Supply Voltage over Temperature,
Four LEDs Enabled @ 20 mA/LED
2
VOUT
1
CH1 2.00V
CH3 1.00V
B
CH2 160mA
W
B
W
M 10.0µs CH2
–2.44mV
Figure 12. Soft Start Showing the Initial In-Rush Current and VOUT Variation,
Four LEDs @ 20 mA/LED, VCC = 3.6 V
VCC
1
VOUT
2
CH1 20.0mV
B
W
CH2 20.0mV
B
W
M 10.0µs CH1
05050-014
05050-011
VCC
5050-013
SUPPLY CURRENT (mA)
160
–12.8mV
CH1 20.0mV
Figure 10. 1.5× Mode Operating Waveforms
B
W
CH2 20.0mV
B
W
M 400ns
CH2
2.4mV
Figure 13. 2× Mode Operating Waveforms
90
VF = 3.6V
85
VF = 4.0V
75
70
65
60
55
2
50
CH1 20.0mV
B
W
CH2 20.0mV
B
W
M 400ns CH2
VF = 4.3V
VF = 3.8V
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
2.4mV
VCC
Figure 11. 1× Mode Operating Waveforms
Figure 14. Power Efficiency vs. Supply Voltage over Li-Ion Range,
Four LEDs @ 20 mA/LED
Rev. 0 | Page 7 of 16
5050-015
VOUT
1
5050-012
VCC
POWER EFFICIENCY
80
ADM8843
85
VF = 3.8V
80
VF = 4.0V
75
VF = 4.3V
C2 FALL
∆: 44.0ms
@: –44.4ms 200µs
LOW SIGNAL
AMPLITUDE
CTRL1/2
1
70
65
VOUT
60
55
VF = 3.6V
50
VF = 3.2V
05050-017
2
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
VCC
05050-016
POWER EFFICIENCY
90
CH1 2.00V
CH2 2.00mV
M 10.0ms CH2
Figure 16. TPC Delay
Figure 15. Power Efficiency vs. Supply Voltage over Li-Ion Range,
Four LEDs @ 15 mA/LED
Rev. 0 | Page 8 of 16
4.36mV
ADM8843
THEORY OF OPERATION
The ADM8843 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. 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).
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 individual current sinks
to individually sense each LED current 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). Softstart 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.
Table 4. Shutdown Truth Table
LED Shutdown Operation
Sub Display Off / Main Display Off
Sub Display Off / Main Display On
Sub Display On / Main Display Off
Sub Display On / Main Display On
ADM8843
ISET
R = 15kΩ
RSET = 13.4kΩ
05050-022
VBRIGHT
0V–2.5V
Figure 17. PWM Brightness Control Using a DC Voltage Applied to VBRIGHT
C1
1µF
VCC
C2
1µF
ADM8843
VOUT
CHARGE PUMP
1×/1.5×/2× MODE
C4
4.7µF
C3
2.2µF
MAIN
SUB
OSC
CTRL1
CTRL2
CONTROL
LOGIC
VREF
CURRENT CONTROLLED SINKS
GND
Figure 18. Functional Block Diagram
Rev. 0 | Page 9 of 16
05050-001
RSET
LED
CURRENT
CONTROL
CIRCUIT
CURRENT
CONTROL 4
ISET
CURRENT
CONTROL 3
FB1
FB2
FB3
FB4
CURRENT
CONTROL 2
CTRL2
0
1
0
1
CURRENT
CONTROL 1
CTRL1
0
0
1
1
ADM8843
By applying a digital PWM signal to the digital input control
pins, CTRL1 and/or CTRL2 adjust 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.
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.
The ADM8843’s main and sub display brightness can be
controlled together or separately. It does this 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%.
Table 5. Charge Pump Switching Thresholds
Gain
1.5× to 2×
2× to 1.5×
1× to 1.5×
1.5× to 1×
Threshold
3.33 V
3.36 V
4.77 V
4.81 V
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.
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 the digital
input control pins, CTRL1 and/or 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 could 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 being half the programmed 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, ILED being half the programmed LED current.
C1
1µF
C2
1µF
3.4V I
IN
C3
2.2µF
ADM8845
PWM INPUT
OR
HIGH/LOW
CTRL1
PWM INPUT
OR
HIGH/LOW
CTRL2
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 brightness 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 display is turned on 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. 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).
VOUT
VCC
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.
FB1
FB2
FB3
FB4
If digital PWM brightness control of the LEDs is not required, a
constant Logic Level 1 (VCC) or 0 (GND) must be applied.
Figure 19. Digital PWM Brightness Control Application Diagram
05050-018
ISET
RSET
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.
Rev. 0 | Page 10 of 16
ADM8843
Table 6. Digital Inputs Truth Table
CTRL1
0
0
1
1
0
PWM
1
PWM
PWM
CTRL2
0
1
0
1
PWM
0
PWM
1
PWM
LED Operation
Sub Display Off / Main Display Off (Full Shutdown)1, 2
Sub Display Off / Main Display On1, 3
Sub Display On / Main Display Off1, 2
Sub Display On / Main Display On (Full On) 1, 3
Sub Display Off/ Digital PWM Brightness Control on Main Display4, 5
Digital PWM Brightness Control on Sub Display / Main Display Off2, 4
Sub Display On/ Digital PWM Brightness Control on Main Display1, 5
Digital PWM Brightness Control on Sub Display / Main Display On5
Digital PWM Brightness Control on Sub and Main Display
5
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.
Main display off means the main display only is off. CTRL2 = 0 means a constant logic level (GND) is applied to CTRL2.
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 the sub display LEDs only 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.
2
3
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%
80%
50%
SHDN
37ms > tCP > 15ms
Figure 20. Application Timing
Rev. 0 | Page 11 of 16
05050-020
MAIN DISPLAY
BRIGHTNESS
ADM8843
LED BRIGHTNESS CONTROL USING A
PWM SIGNAL APPLIED TO VPWM
LED BRIGHTNESS CONTROL USING A DC
VOLTAGE APPLIED TO VBRIGHT
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 configuration, CTRL1 and CTRL2 digital logic pins can control
shutdown of the white LEDs, while VPWM can control the brightness 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, while 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 be 0 V and 2.5 V only in order to have 20 mA
flowing in each LED.
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 a 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)
2.5V
VBRIGHT
1.6V
0.8V
0V
20mA
100% = ILED = 0mA
0% = ILED = 20mA
0mA
ISET
VPWM
0V–2.5V
Figure 22. PWM Brightness Control Application Diagram Using a
DC Voltage Applied to VBRIGHT
R = 7.5kΩ R = 7.5kΩ
C5 = 1µF
RSET = 13.4kΩ
7.2mA
ILED
ADM8843
Figure 21. PWM Brightness Control Using
Filter -PWM Signal
Rev. 0 | Page 12 of 16
05050-023
13.6mA
05050-021
I LED
(1 − Duty Cycle)
I
_ Voltage
= SET
× 120 ×
RSET × 2R
100
RSET + 2R
ADM8843
APPLICATIONS
LAYOUT CONSIDERATIONS AND NOISE
DRIVING FOUR LEDS IN THE MAIN DISPLAY ONLY
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.
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.
WHITE LED SHORTING
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.
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 currents sinks to
produce the LED current. If an LED is shorted, the ADM8843
continues to sink (ISET × 120
DRIVING FEWER THAN FOUR LEDS
LCD
MAIN DISPLAY
VCC
2.6V–5.5V
VCC
2.6V–5.5V
VOUT
VOUT
ADM8843
ADM8843
FB1
FB2
FB3
FB4
GND
ISET
05050-024
RSET
GND
RSET
Figure 23. Driving Four White LEDs
Figure 24. Driving Three White LEDs
MAIN DISPLAY
SUB DISPLAY
VCC
2.6V–5.5V
VOUT
ADM8843
FB1
FB2
FB3
FB4
CTRL1
CTRL2
ISET
GND
05050-002
ISET
FB1
FB2
FB3
FB4
CTRL1
CTRL2
05050-025
CTRL1
CTRL2
RSET
Figure 25. Typical Application Diagram
Rev. 0 | Page 13 of 16
ADM8843
USING SMALLER CAPACITOR VALUES
The ADM8843 can be operated with the smaller capacitor values described here to reduce capacitor footprint sizes.
Option 1
Option 2
Input and output ripple plots for 1× and 1.5× mode operation
are shown with C1,C2 = 0.22 µF; C3 = 0.47 µF and C4 = 1 µF.
Input and output ripple plots for 1× and 1.5× mode operation
are shown with C1,C2 = 0.22 µF; C3 = 0.47 µF and C4 = 4.7 µF.
2
VOUT
3
2
VOUT
3
CH2 10.0mV
B
B
W
M 400ns CH3
–33mV
CH2 10.0mV
CH3 10.0mV
W
Figure 26. 1× Mode Operation with Four LEDs with
20 mA/LED at VCC = 5.0 V, with a 1 µF VCC Decoupling Capacitor
2
VOUT
3
B
W
M 1.00µs CH3
–33mV
W
Figure 28. 1× Mode Operation with Four LEDs with
20 mA/LED at VCC = 5.0 V, with a 4.7 µF VCC Decoupling Capacitor
VCC
2
VOUT
3
05050-027
VCC
B
05050-029
CH3 10.0mV
05050-028
05050-026
VCC
VCC
CH2 20.0mV
CH3 50.0mV
B
B
W
M 400ns CH3
CH2 20.0mV
–33mV
CH3 50.0mV
W
B
B
W
M 400ns CH3
–33mV
W
Figure 29. 1.5× Mode Operation with Four LEDs with
20 mA/LED at VCC = 3.6 V, with a 4.7 µF VCC Decoupling Capacitor
Figure 27. 1.5× Mode Operation with Four LEDs with
20 mA/LED at VCC = 3.6 V, with a 1 µF VCC Decoupling Capacitor
Rev. 0 | Page 14 of 16
ADM8843
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
Example 2
The ADM8843 driving four white LED with 20 mA/LED at
VCC = 3.4 V (1.5× mode), LED VF = 4.5 V.
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
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
POUT = 4(VF × ILED)
POUT = 4(3.6 V × 20 mA)
POUT = 0.288
η = POUT/PIN
η = 0.36/0.41684
η = 87 %
η = POUT/PIN
η = 0.288/0.41684
η = 70 %
VCC
= 3.4V
ILOAD
IIN
VOUT
VCC
ADM8843
FB1
FB2
FB3
FB4
CTRL1
CTRL2
ISET
GND
05050-019
RSET
7.32kΩ
Figure 30. Charge Pump Power Efficiency Diagram, Example 1
Rev. 0 | Page 15 of 16
ADM8843
OUTLINE DIMENSIONS
3.00
BSC SQ
0.60 MAX
13
12
0.45
PIN 1
INDICATOR
2.75
BSC SQ
TOP
VIEW
0.80 MAX
0.65 TYP
12° MAX
1.00
0.85
0.80
PIN 1 INDICATOR
16
1
1.65
1.50 SQ*
1.35
EXPOSED
PAD
0.50
BSC
SEATING
PLANE
0.50
0.40
0.30
9 (BOTTOM VIEW) 4
8
5
0.25 MIN
1.50 REF
0.05 MAX
0.02 NOM
0.30
0.23
0.18
0.20 REF
*COMPLIANT TO JEDEC STANDARDS MO-220-VEED-2
EXCEPT FOR EXPOSED PAD DIMENSION
Figure 31. 16-Lead Lead Frame Chip Scale Package [LFCSP]
3 mm × 3 mm Body
(CP-16-3)
Dimensions shown in millimeters
ORDERING GUIDE
Model
ADM8843ACPZ-REEL71
1
Temperature Range
−40ºC to + 85ºC
Package Description
16-Lead Lead Frame Chip Scale Package
Z = Pb-free part.
© 2004 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D05050–0–10/04(0)
Rev. 0 | Page 16 of 16
Package Option
CP-16-3
Branding
M2U