EVAL-ADM8845EB [ADI]
Charge Pump Driver for LCD White LED Backlights; 电荷泵驱动器，用于LCD白光LED背光
| 型号： | EVAL-ADM8845EB |
| 厂家： | 
ADI |
| 描述： | Charge Pump Driver for LCD White LED Backlights |
| 文件： | 总20页 (文件大小：372K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
Charge Pump Driver for LCD
White LED Backlights
ADM8845
GENERAL DESCRIPTION
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
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.
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 0.9 mm
Low power shutdown mode
To maximize power efficiency, the charge pump can operate in
either 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.
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
Improved brightness matching of the LEDs is achieved by a
feedback pin to sense individual LED current with a maximum
matching accuracy of 1%.
PDAs
FUNCTIONAL BLOCK DIAGRAM
C1
C2
1µF
1µF
V
ADM8845
CC
V
OUT
CHARGE PUMP
1×/1.5×/2× MODE
C3
2.2µF
C4
4.7µF
MAIN
SUB
OSC
CTRL1
CTRL2
CONTROL
LOGIC
V
REF
FB1
FB2
FB3
FB4
FB5
FB6
LED
I
SET
CURRENT
CONTROL
CIRCUIT
R
SET
CURRENT CONTROLLED SINKS
GND
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
Fax: 781.326.8703
www.analog.com
© 2004 Analog Devices, Inc. All rights reserved.
ADM8845
TABLE OF CONTENTS
Specifications......................................................................................3
Absolute Maximum Ratings.............................................................4
Thermal Characteristics .............................................................. 4
ESD Caution.................................................................................. 4
Pin Configuration and Function Description ...............................5
Typical Performance Characteristics ..............................................6
Theory of Operation ...................................................................... 10
Output Current Capability........................................................ 11
Automatic Gain Control............................................................ 11
Current Matching....................................................................... 11
Brightness Control with a Digital PWM Signal ..................... 11
LED Brightness Control Using a DC Voltage Applied to
VBRIGHT .......................................................................................... 13
Applications......................................................................................14
Layout Considerations and Noise ............................................ 14
White LED Shorting .................................................................. 14
Driving Fewer than Six LEDs ................................................... 14
Driving Flash LEDs.................................................................... 15
Driving Camera Light, Main, and Sub LEDs.......................... 15
Driving Four Backlight White LEDs and Flash LEDs........... 16
Power Efficiency......................................................................... 17
Outline Dimensions........................................................................18
Ordering Guide .......................................................................... 18
LED Brightness Control Using a PWM Signal Applied to
VPWM ............................................................................................. 13
REVISION HISTORY
10/04—Revision 0: Initial Version
Rev. 0 | Page 2 of 20
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
Min
Typ
Max
5.5
5
Unit
V
Test Conditions
INPUT VOLTAGE, VCC
SUPPLY CURRENT, ICC
2.6
2.6
mA
All six LEDs disabled, VCC = 3.3 V, RSET = 7.08 kΩ
CTRL1 = 1, CRTL2 = 1
SHUTDOWN CURRENT
CHARGE PUMP FREQUENCY
CHARGE PUMP MODE THRESHOLDS
1.5× to 2×
Accuracy
2× to 1.5×
Accuracy
Hysteresis
1× to 1.5×
5
µA
1.5
MHz
3.33
3.36
V
%
V
%
mV
V
4
4
40
4.77
Accuracy
1.5× to 1×
Accuracy
Hysteresis
4
4
%
V
%
mV
4.81
40
ISET PIN
LED : LED Matching
LED : ISET Accuracy
−1
−1
+1
+1
%
%
ILED = 20 mA, VFB =0.4 V
ILED = 20 mA, RSET = 7.08 kΩ, VFB = 0.4 V, VCC = 3.6 V,
TA = 25°C
ISET Pin Voltage
ILED to ISET Ratio
1.18
120
0.2
1.2
3.5
V
MIN COMPLIANCE ON FBx PIN
Charge Pump Output Resistance
0.3
1.8
5.1
14
V
Ω
Ω
Ω
ISET = 20 mA
1× mode
1.5× mode
2× mode
8.0
LED Current
PWM
30
mA
kHz
See Note 1 and Figure 21
0.1
200
DIGITAL INPUTS
Input High
0.5 VCC
V
Input Low
0.3 VCC
1
V
µA
%
Input Leakage Current
CHARGE PUMP POWER EFFICIENCY
VOUT RIPPLE
88
30
CTRL1 = 1, CRTL2 = 1, VCC = 3.4 V, VFB = 0.2 V, IFB = 20 mA
VCC = 3.6 V, ILED = 20 mA, all six LEDs enabled
m V
_______________________
1 Guaranteed by design. Not 100% production tested.
Rev. 0 | Page 3 of 20
ADM8845
ABSOLUTE MAXIMUM RATINGS
TA = 25°C, unless otherwise noted.
Table 2.
Parameter
Rating
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.
Supply Voltage, VCC
ISET
CTRL1, CTRL2
VOUT Shorted1
–0.3 V to +6.0 V
–0.3 V to +2.0 V
–0.3 V to +6.0 V
Indefinite
Feedback pins FB1 to FB6
Operating Temperature Range
Six LEDs Enabled with 30 mA/LED2
Six LEDs Enabled with 20 mA/LED2
–0.3 V to +6.0 V
–40°C to +65°C
–40°C to +85°C
180 mA
–65°C to +125°C
2 mW
THERMAL CHARACTERISTICS
16-Lead LFCSP Package:
3
VOUT
Storage Temperature Range
Power Dissipation
ESD Class
θJA = 50°C/W
1
1 Short through LED.
2 LED current should be derated above TA > 65°C, refer to Figure 21.
3 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 20
ADM8845
PIN CONFIGURATION AND FUNCTION DESCRIPTION
PIN 1
INDICATOR
12 CTRL2
11 C2–
V
1
2
3
4
OUT
C2+
ADM8845
TOP VIEW
(Not to Scale)
I
10 GND
SET
FB1
9 FB6
Figure 2. Pin Configuration
Table 3. Pin Function Descriptions
Pin No. Mnemonic Function
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
3
C2+
ISET
Flying Capacitor 2 Positive Connection.
Bias Current Set Input. The current flowing through RSET, ISET, is gained up by 120 to give the ILED current.
Connect RSET to GND to set the bias current as VSET/RSET. Note that VSET = 1.18 V.
4–9
FB1–FB6
LED1–LED6 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 six LEDs, this pin can be left unconnected or
connected to GND.
10
11
12
13
14
15
16
-
GND
C2−
CTRL2
CTRL1
C1−
VCC
Device Ground Pin.
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.
C1+
EP
Expose Paddle. Connect the exposed paddle to GND.
Rev. 0 | Page 5 of 20
ADM8845
TYPICAL PERFORMANCE CHARACTERISTICS
0.4
0.3
35
MAX POSITIVE MATCHING ERROR
30
25
20
15
10
5
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
MAX NEGATIVE MATCHING ERROR
4.75
6.75
8.75
10.75
(kΩ)
12.75
14.75
2.6
3.0
3.4
3.8
4.2
4.6
5.0
5.4
R
SUPPLY VOLTAGE (V)
SET
Figure 6. ILED (mA) Matching Error (%) vs. Supply Voltage (V),
A = 25°C and ILED = 20 mA
Figure 3. ILED (mA) Current vs. RSET
T
20.35
20.30
20.25
20.20
20.15
20.10
20.05
20.00
20.24
20.22
20.20
20.18
20.16
20.14
20.12
20.10
20.08
–40°C
25°C
85°C
2.6
3.1
3.6
4.1
4.6
5.1
5.6
–40
0
40
80
SUPPLY VOLTAGE (kΩ)
TEMPERATURE (°C)
Figure 4. ILED (mA) vs. Temperature (°C), Six LEDs Enabled
Figure 7. ILED (mA) Variation over Temperature (°C), VCC = 3.6 V
0.3
0.2
0.1
0
35
30
25
20
15
10
5
–0.1
–0.2
–0.3
–40
–20
0
25
45
65
85
2.6
3.0
3.4
3.8
4.2
4.6
5.0
5.4
TEMPERATURE (°C)
SUPPLY VOLTAGE (V)
Figure 5. ILED Matching (%) over Temperature (°C), VCC = 3.6 V, ILED = 20 mA,
Six LEDs Enabled
Figure 8. ILED (mA) vs. Supply Voltage (V)
Rev. 0 | Page 6 of 20
ADM8845
95
90
85
80
75
70
65
60
20
16
12
8
4
0
0
20
40
60
80
100
10
20
0
30
40
50
60
70
80
90
100
DUTY CYCLE (%)
DUTY CYCLE (%)
Figure 9. ILED (mA) vs. PWM Dimming (Varying Duty Cycle),
Six LEDs Enabled, Frequency = 1 kHz
Figure 12. LED Efficiency vs. Varying Duty Cycle of 1 kHz PWM Signal,
Six LEDs Enabled, 20 mA/LED
300
CTRL1/2
1
250
200
150
100
50
20mA/LED
15mA/LED
CURRENT
2
V
OUT
3
0
2.6
B
B
W
3.0
3.4
3.8
4.2
4.6
5.0
5.4
CH1 2.00V
CH3 1.00V
CH2 160mA
M 5.00µs CH2
180mV
W
SUPPLY VOLTAGE (V)
Figure 10. Input Current vs. Supply Voltage,
Six LEDs Enabled
Figure 13. Soft Start Showing the Initial In-Rush Current and VOUT Variation,
Six LEDs @ 20 mA/LED, VCC = 3.6 V
V
1
CC
V
1
CC
2
V
OUT
2
V
OUT
B
B
W
CH1 20.0mV
CH2 20.0mV
M 400ns CH1
220mV
W
B
B
W
CH1 20.0mV
CH2 20.0mV
M 400ns CH1
220mV
W
Figure 11. 1.5× Mode Operating Waveforms
Figure 14 .2× Mode Operating Waveform
Rev. 0 | Page 7 of 20
ADM8845
90
85
80
75
70
65
60
55
50
45
40
V
V
V
= 3.6V
= 4.0V
= 4.3V
F
F
F
1
V
CC
2
V
V
V
= 3.8V
= 3.2V
OUT
F
F
B
B
W
CH1 20.0mV
CH2 20.0mV
M 400ns CH1
220mV
W
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
V
CC
Figure 15. 1× Mode Operating Waveforms
Figure 18. Power Efficiency vs. Supply Voltage over Li-Ion Range,
Six LEDS @ 20 mA/LED
90
90
V
V
V
= 4.3V
= 4.0V
= 3.8V
V
V
V
= 3.8V
F
F
F
F
F
F
85
80
75
70
65
60
55
50
45
40
85
80
75
70
65
60
55
50
45
40
= 4.0V
= 4.3V
V
V
= 3.6V
= 3.2V
F
V
V
= 3.6V
= 3.2V
F
F
F
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
2.8 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
V
CC
V
CC
Figure 19. Power Efficiency vs. Supply Voltage over Li-Ion Range
Four LEDS @ 20 mA/LED
Figure 16. Power Efficiency vs. Supply Voltage over Li-ion Range,
Six LEDS @ 15 mA/LED
90
V
V
V
= 3.8V
= 4.0V
= 4.3V
F
F
F
C2 FALL
200µs
LOW SIGNAL
AMPLITUDE
85
80
75
70
65
60
55
50
45
40
∆: 44.0ms
@: –44.4ms
CTRL1/2
1
V
OUT
V
V
= 3.6V
= 3.2V
F
F
2
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
CH1 2.00V CH2 2.00V
M10.0ms
CH2
4.36V
V
CC
Figure 20. TPC Delay
Figure 17. Power Efficiency vs. Supply Voltage over Li-Ion Range,
Four LEDS @ 15 mA/LED
Rev. 0 | Page 8 of 20
ADM8845
30mA
20mA
65°C
85°C
Figure 21. Maximum ILED (mA) vs. Ambient Temperature,
Six LEDs Connected
Rev. 0 | Page 9 of 20
ADM8845
THEORY OF OPERATION
The ADM8845 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 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 regula-
tion of the charge pump output voltage for supply voltages up to
5.5 V. The ADM8845’s six LEDs are arranged into two groups,
main and sub. The main display can have up to four LEDs (FB1
to FB4), and the sub display can have one or two LEDs (FB5
and FB6) (see Figure 23). Two digital input control pins, CTRL1
and CTRL2, 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 ADM8845 to produce
the ILED currents of up to 30 mA/LED (ILED = ISET × 120 and ISET
= 1.18 V/RSET). The ADM8845 uses six individual current sinks
to individually sense each LED current with a maximum
matching performance of 1%. This current matching perform-
ance ensures uniform brightness across a color display.
The ADM8845 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 ADM8845 also allows the brightness of the white
LEDs to be controlled using a dc voltage (see Figure 22). Soft-
start circuitry limits the in-rush current flow at power-up. The
ADM8845 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
CTRL1
CTRL2
LED Shutdown Operation
0
0
1
1
0
1
0
1
Sub Display Off / Main Display Off
Sub Display Off / Main Display On
Sub Display On / Main Display Off
Sub Display On / Main Display On
ADM8845
I
SET
R = 15kΩ
V
BRIGHT
0V–2.5V
R
= 13.4kΩ
SET
Figure 22. PWM Brightness Control Using a DC Voltage Applied to VBRIGHT
C1
C2
1µF
1µF
V
ADM8845
CC
V
OUT
CHARGE PUMP
/1.5 /2 MODE
C3
2.2
C4
4.7µF
1
×
× ×
MAIN
SUB
µ
F
OSC
CTRL1
CTRL2
CONTROL
LOGIC
V
REF
FB1
FB2
FB3
FB4
FB5
FB6
LED
I
SET
CURRENT
CONTROL
CIRCUIT
R
SET
CURRENT CONTROLLED SINKS
GND
Figure 23. Functional Block Diagram
Rev. 0 | Page 10 of 20
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:
I
AVG = (IMAX + IMIN)/2
Max Matching Error = [(IMAX – IAVG)/IAVG] × 100
R
SET = 120 × (1.18 V/ILED)
or
To regulate the LED currents properly, sufficient headroom
voltage (compliance) must be present. The compliance refers to
the minimum amount of voltage that must be present across the
internal current sinks to ensure that the desired current and
matching performance is realizable. To ensure that the desired
current is obtained, use the following equation to find the
minimum input voltage required:
Min Matching Error = [(IMIN – IAVG)/IAVG] × 100
where IMAX is the largest ILED current, and IMIN is the smallest ILED
current.
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
either at zero current or full current (set by RSET).
V
OUT – VF ≥ Compliance
where VF is the LED forward voltage. For 20 mA/LED, the
compliance is 0.20 V typ and 0.30 V max (see Table 5).
Table 5. ILED, RSET, and Compliance Table
ILED
RSET
Typ. Compliance
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
ADM8845, zero duty cycle turns off the LEDs, and a 50% duty
cycle results in an average LED current ILED being 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, ILED being half the programmed LED current.
9.44 kΩ
7.08 kΩ
4.72 kΩ
15 mA
20 mA
30 mA
0.17 V
0.20 V
0.34 V
When the ADM8845 charge pump is loaded with 180 mA (six
LEDs at 30 mA/LED), the ambient operating temperature is
reduced (see Figure 21).
AUTOMATIC GAIN CONTROL
The automatic gain control block controls the operation of the
charge pump by selecting the appropriate gain for the charge
pump. This 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 6.
C1
1µF
C2
1µF
V
OUT
ADM8845
C3
2.2µF
PWM INPUT
OR HIGH/LOW
CTRL1
CTRL2
PWM INPUT
OR HIGH/LOW
FB1
FB2
FB3
FB4
FB5
FB6
Table 6. Charge Pump Switching Thresholds
Gain
Threshold
I
1.5× to 2×
2× to 1.5×
1× to 1.5×
1.5× to 1×
3.33 V
3.36 V
4.77 V
4.81 V
SET
R
SET
Figure 24. Digital PWM Brightness Control Application Diagram
By applying a digital PWM signal to the digital input control
pins, CTRL1 and/or CTRL2 can adjust the brightness of the sub
and/or main displays. The ADM8845’s six white LEDs are
organized into two groups, main display (FB1 to FB4) and sub
display (FB4 to FB6); refer to the Theory of Operation section.
Rev. 0 | Page 11 of 20
ADM8845
The ADM8845’s main and sub display brightness 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%.
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 bright-
ness of the sub display is set to the maximum (maximum is set
by RSET).
When CTRL1 and CTRL2 go low, the LED current control
sinks shutdown. Shutdown of the charge pump is delayed by
15 ms. This timeout period (tCP) allows the ADM8845 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 25).
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.
If digital PWM brightness control of the LEDs is not required, a
constant Logic Level 1 (VCC) or 0 (GND) must be applied.
The six white LED in the ADM8845 are arranged in two groups,
sub and main. It is possible to configure the six LEDs as in
Table 7; refer also to Figure 25.
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).
Table 7. Digital Inputs Truth Table
CTRL1
CTRL2
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
0
0
1
1
0
1
0
1
0
PWM
0
PWM
1
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
PWM
1
PWM
PWM
5
PWM
Digital PWM Brightness Control on Sub and Main Display
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 the main display only 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 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.
Rev. 0 | Page 12 of 20
ADM8845
SUB AND MAIN 50%
DUTY CYCLE
MAIN 80% DUTY CYCLE,
SUB OFF
FULL ON
MAIN AND SUB OFF
LED CONFIG.
tCP
CTRL1
CTRL2
V
OUT
I
LED
(SUB)
I
LED
(MAIN)
100%
SUB DISPLAY
BRIGHTNESS
50%
50%
SHDN
SHDN
100%
MAIN DISPLAY
BRIGHTNESS
80%
37ms > tCP > 15ms
Figure 25. Application Timing
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 on
Figure 26, also can be used to control PWM brightness. This
PWM brightness control method can be used instead of CTRL1
and/or CTRL2 digital PWM brightness control. With this con-
figuration, The CTRL1 and CTRL2 digital logic pins can be
used to control shutdown of the white LEDs, while VPWM can be
used to control the brightness of all the white LEDs 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 ADM8845. 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 26). To have 20 mA
flowing in each LED, the amplitude of the PWM signal must be
0 V and 2.5 V only.
Adding one resistor, as shown in Figure 22, this configuration
can also be used to control brightness the white LEDs by using
a dc voltage applied to the VBRIGHT node. Figure 27 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
I
I
SET = [(1/RSET + 1/R)(VSET)] – [(1/R)(VBRIGHT)]
LED = 120 × ISET
where R = 15 kΩ and VSET = voltage at ISET pin (1.18 V).
2.5V
V
1.6V
BRIGHT
0.8V
(1− Duty Cycle)
ISET _Voltage
RSET ×2R
0V
ILED
=
×120×
100
RSET + 2R
20mA
13.6mA
7.2mA
I
LED
100% = I
= 0mA
= 20mA
LED
0mA
ADM8845
0% = I
LED
Figure 27. PWM Brightness Control Application Diagram Using a
DC Voltage Applied to VBRIGHT
I
SET
R = 7.5kΩ R = 7.5kΩ
C5 = 1µF
V
0V–2.5V
PWM
R
= 13.4kΩ
SET
Figure 26. PWM Brightness Control Using Filtered-PWM Signal
Rev. 0 | Page 13 of 20
ADM8845
APPLICATIONS
LAYOUT CONSIDERATIONS AND NOISE
DRIVING FEWER THAN SIX LEDS
Because of the ADM8845’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 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. For example, Figure 28 shows five LEDs
being powered by the ADM8845, and Figure 29 shows three
main LEDs and one sub LED.
WHITE LED SHORTING
If an LED is shorted, the ADM8845 continues to drive the
remaining LEDs with ILED per LED (ILED = ISET × 120 mA). This
is because the ADM8845 uses six internal currents sinks to
produce the LED current. If an LED is shorted, the ADM8845
continues to sink (ISET × 120 mA) as programmed by RSET
through the shorted LED.
LCD
MAIN DISPLAY
SUB DISPLAY
V
CC
V
CC
2.6V–5.5V
2.6V–5.5V
V
V
OUT
OUT
ADM8845
CTRL1
CTRL2
ADM8845
FB1
FB2
FB3
FB4
FB5
FB6
FB1
FB2
FB3
FB4
FB5
FB6
CTRL1
CTRL2
GND
GND
I
SET
R
SET
Figure 28. Driving Five White LEDs
Figure 29. Driving Three Main LEDs and One Sub LED
MAIN DISPLAY
SUB DISPLAY
V
CC
2.6V–5.5V
V
OUT
ADM8845
CTRL1
CTRL2
FB1
FB2
FB3
FB4
FB5
FB6
GND
I
SET
R
SET
Figure 30. Typical Application Diagram
Rev. 0 | Page 14 of 20
ADM8845
DRIVING FLASH LEDS
DRIVING CAMERA LIGHT, MAIN, AND SUB LEDS
The ADM8845 can be operated with any two FBx pins operated
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 also be configured to power a camera light
that is composed of four white LEDs in parallel, packaged into
one package. FB1 to FB4 now power the camera light, and FB5
and FB6 power the main display. The sub display LED is powered
from the ADM8845 by using an external current mirror to
control the current flowing through the sub white LED (see
Figure 32). All white LEDs have 15 mA/LED, therefore total
load on the ADM8845 charge pump is 105 mA, and the maxi-
mum load on the ADM8845 charge pump is 180 mA (see
Figure 21).
V
CC
2.6V–5.5V
V
OUT
ADM8845
60mA 60mA 60mA
FB1
FB2
FB3
FB4
FB5
FB6
CTRL1
CTRL2
GND
R
SET
4.27kΩ
Figure 31. Driving Three Flash LEDs
C1
C2
1µF
1µF
VCC ADM8845
V
OUT
CHARGE PUMP
1×/1.5×/2× MODE
C3
2.2µF
C4
4.7µF
CAMERA
MAIN
SUB
OSC
CTRL1
CTRL2
15mA/LED
15mA/LED
15mA/LED
CONTROL
LOGIC
V
REF
CURRENT
CONTROL7
FB1
FB2
FB3
FB4
FB5
FB6
R
LED
I
SET
CURRENT
CONTROL
CIRCUIT
R
SET
9.44kΩ
CURRENT CONTROLLED SINKS
ꢀ
GND
Figure 32. Driving Camera Light, Two Main LEDs, and One Sub LED
Rev. 0 | Page 15 of 20
ADM8845
DRIVING FOUR BACKLIGHT WHITE LEDS AND FLASH LEDS
The ADM8845 also can be configured to power four backlight
white LEDs and a camera flash, packaged into one package. FB1
to FB4 power the backlight light, FB5 and FB6 powers the two
of the flash LEDs, and the third is powered an external current
mirror to control the current flowing through the third flash LED
(see Figure 33). All the backlight white LEDs have 15 mA/LED,
and the flash current is 20 mA/LED. The total load on the
ADM8845 charge pump is 120 mA; the maximum load on
the ADM8845 charge pump is 180 mA (see Figure 21).
CTRL1 controls the flash on/off, and CTRL2 controls the back-
light 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.
C1
C2
1µF
1µF
ADM8845
V
CC
V
OUT
CHARGE PUMP
1×/1.5×/2× MODE
C4
4.7µF
C3
2.2µF
BACK-
LIGHT
FLASH
CTRL1
OSC
15mA/LED
20mA/LED
CONTROL
LOGIC
V
REF
CTRL2
CURRENT
CONTROL 7
FB1
FB2
FB3
FB4
FB5
FB6
R
LED
I
SET
CURRENT
CONTROL
CIRCUIT
R
SET
7.32kΩ
CURRENT CONTROLLED SINKS
GND
Figure 33. Driving Four Backlight LEDs and Flash LED
Rev. 0 | Page 16 of 20
ADM8845
POWER EFFICIENCY
The ADM8845 power efficiency (η) equations are
Example 1
The ADM8845 driving six white LED with 20 mA/LED at
η = POUT/PIN
P
IN = ((VCC × ILOAD × Gain) + (IQ × VCC))
VCC = 3.4 V (1.5× mode), LED VF = 4.5 V.
P
OUT = 6×(VF × ILED
)
P
P
P
P
IN = ((VCC × ILOAD × Gain) + (VCC × IQ))
IN = ((3.4 × 120 mA × 1.5) + (3.4 × 2.6 mA))
IN = ((0.612) + (0.00884))
where:
IQ is the quiescent current of the ADM8845, 2.6 mA.
VF is the LED forward voltage.
IN = 0.62084
Gain is equal to charge pump mode (1×, 1.5×, 2×).
3.4V
P
P
OUT = 6(VF × ILED
)
I
I
LOAD
IN
OUT = 6(4.5V × 20 mA)
V
V
OUT
CC
POUT = 0.54
ADM8845
V
η = POUT/PIN
η = 0.54/0.62084
η = 87 %
CC
CTRL1
CTRL2
FB1
FB2
FB3
FB4
FB5
FB6
Example 2
I
SET
GND
The ADM8845 driving six white LED with 20 mA/LED at
VCC = 3.4 (1.5× mode), LED VF = 3.6 V.
R
SET
P
P
P
P
IN = ((VCC × ILOAD × Gain) + (VCC × IQ))
IN = ((3.4 × 120 mA × 1.5) + (3.4 × 2.6 mA))
IN = ((0.612) + (0.00884))
Figure 34. Charge Pump Power Efficiency Diagram
Examples 1 and 2 show calculations of the ADM8845 power
efficiency; also see Figure 34.
IN = 0.62084
P
P
P
OUT = 6(VF × ILED)
OUT = 6(3.6 V × 20 mA)
OUT = 0.432
η = POUT/PIN
η = 0.432/0.62084
η = 70 %
Rev. 0 | Page 17 of 20
ADM8845
OUTLINE DIMENSIONS
0.50
0.40
0.30
3.00
0.60 MAX
BSC SQ
PIN 1 INDICATOR
1.45
0.45
13
12
16
1
4
*
1.30 SQ
1.15
PIN 1
INDICATOR
2.75
BSC SQ
TOP
VIEW
EXPOSED
PA D
(BOTTOMVIEW)
8
9
0.50
BSC
5
0.25 MIN
1.50 REF
0.80 MAX
12° MAX
0.65 TYP
0.90
0.85
0.80
0.05 MAX
0.02 NOM
SEATING
PLANE
0.30
0.23
0.18
0.20 REF
*
COMPLIANT TO JEDEC STANDARDS MO-220-VEED-2
EXCEPT FOR EXPOSED PAD DIMENSION
Figure 35. 16-Lead Lead Frame Chip Scale Package [LFCSP]
(CP-16)
Dimensions shown in millimeters
ORDERING GUIDE
Model
Temperature Range
−40ºC to + 85ºC
−40ºC to + 85ºC
−40ºC to + 85ºC
−40ºC to + 85ºC
−40ºC to + 85ºC
Package Description
16-Lead LFCSP
16-Lead LFCSP
16-Lead LFCSP
16-Lead LFCSP
16-Lead LFCSP
Evaluation Board
Package Option
CP-16
Branding
M0P
ADM8845ACP-REEL
ADM8845ACP-REEL7
ADM8845ACPZ-REEL1
ADM8845ACPZ-REEL71
ADM8845ACPZ-WP1, 2
EVAL-ADM8845EB
CP-16
M0P
CP-16
M0P
CP-16
M0P
CP-16
M0P
1 Z = Pb-free part.
2 WP = waffle pack.
Rev. 0 | Page 18 of 20
ADM8845
NOTES
Rev. 0 | Page 19 of 20
ADM8845
NOTES
©
2004 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D04867–0–10/04(0)
Rev. 0 | Page 20 of 20
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