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Charge Pump Driver for LCD White LED Backlights ADM8845
FEATURES
Drives 6 LEDs from 2.6 V to 5.5 V (Li-Ion) input supply 1x/1.5x/2x fractional charge pump to maximize power efficiency 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 x 3 mm) Package height only 0.9 mm Low power shutdown mode Shutdown function Soft-start limiting in-rush current
GENERAL DESCRIPTION
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. To maximize power efficiency, the charge pump can operate in either a 1x, 1.5x, or 2x mode. The charge pump automatically switches between 1x/1.5x/2x 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 to sense individual LED current with a maximum matching accuracy of 1%.
APPLICATIONS
Cellular phones with main and sub displays White LED backlighting Camera flash/strobes and movie lights Micro TFT color displays DSC PDAs
FUNCTIONAL BLOCK DIAGRAM
C1 1F C2 1F
VCC C4 4.7F
ADM8845
CHARGE PUMP 1x/1.5x/2x MODE
VOUT C3 2.2F MAIN SUB
OSC CTRL1 CTRL2 CONTROL LOGIC
VREF FB1 FB2 FB3 FB4 FB5 FB6
CURRENT CONTROL 1
CURRENT CONTROL 2
CURRENT CONTROL 3
CURRENT CONTROL 4
CURRENT CONTROL 5
ISET RSET
LED CURRENT CONTROL CIRCUIT
CURRENT CONTROL 6
CURRENT CONTROLLED SINKS GND
Figure 1. Rev. 0
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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 (c) 2004 Analog Devices, Inc. All rights reserved.
04867-0-001
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 PWM Signal Applied to VPWM ............................................................................................. 13 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
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 = -40C to +85C, unless otherwise noted; C1,C2 = 1.0 F; C3 = 2.2 F; C4 = 4.7 F
Table 1.
Parameter INPUT VOLTAGE, VCC SUPPLY CURRENT, ICC SHUTDOWN CURRENT CHARGE PUMP FREQUENCY CHARGE PUMP MODE THRESHOLDS 1.5x to 2x Accuracy 2x to 1.5x Accuracy Hysteresis 1x to 1.5x Accuracy 1.5x to 1x Accuracy Hysteresis ISET PIN LED : LED Matching LED : ISET Accuracy ISET Pin Voltage ILED to ISET Ratio MIN COMPLIANCE ON FBx PIN Charge Pump Output Resistance Min 2.6 Typ 2.6 Max 5.5 5 5 1.5 3.33 4 3.36 4 40 4.77 4 4.81 4 40 -1 -1 1.18 120 0.2 1.2 3.5 8.0 0.1 0.5 VCC 0.3 VCC 1 88 30 +1 +1 Unit V mA A MHz V % V % mV V % V % mV % % V 0.3 1.8 5.1 14 30 200 V mA kHz V V A % mV ISET = 20 mA 1x mode 1.5x mode 2x mode See Note 1 and Figure 21 ILED = 20 mA, VFB =0.4 V ILED = 20 mA, RSET = 7.08 k, VFB = 0.4 V, VCC = 3.6 V, TA = 25C Test Conditions All six LEDs disabled, VCC = 3.3 V, RSET = 7.08 k CTRL1 = 1, CRTL2 = 1
LED Current PWM DIGITAL INPUTS Input High Input Low Input Leakage Current CHARGE PUMP POWER EFFICIENCY VOUT RIPPLE
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
_______________________
1
Guaranteed by design. Not 100% production tested.
Rev. 0 | Page 3 of 20
ADM8845 ABSOLUTE MAXIMUM RATINGS
TA = 25C, unless otherwise noted. Table 2.
Parameter Supply Voltage, VCC ISET CTRL1, CTRL2 VOUT Shorted1 Feedback pins FB1 to FB6 Operating Temperature Range Six LEDs Enabled with 30 mA/LED2 Six LEDs Enabled with 20 mA/LED2 VOUT3 Storage Temperature Range Power Dissipation ESD Class 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 -40C to +65C -40C to +85C 180 mA -65C to +125C 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: JA = 50C/W
1 2 3
Short through LED. LED current should be derated above TA > 65C, refer to Figure 21. 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
16 C1+ 15 VCC 13 CTRL1 14 C1-
VOUT 1 C2+ 2 ISET 3 FB1 4
PIN 1 INDICATOR
12 CTRL2 11 C2- 10 GND 9 FB6
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ADM8845
TOP VIEW (Not to Scale)
FB2 5
Figure 2. Pin Configuration
Table 3. Pin Function Descriptions
Pin No. 1 2 3 4-9 Mnemonic VOUT C2+ ISET FB1-FB6 Function Charge Pump Output. A 2.2 F 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 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. 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. 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 F decoupling capacitor. Flying Capacitor 1 Positive Connection. Expose Paddle. Connect the exposed paddle to GND.
10 11 12 13 14 15 16 -
GND C2- CTRL2 CTRL1 C1- VCC C1+ EP
Rev. 0 | Page 5 of 20
FB3 6
FB4 7
FB5 8
ADM8845 TYPICAL PERFORMANCE CHARACTERISTICS
35
0.4 0.3
30
MAX POSITIVE MATCHING ERROR 0.2
25
MATCHING ERROR (%)
LED CURRENT (mA)
0.1 0 -0.1 -0.2 -0.3 MAX NEGATIVE MATCHING ERROR
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04867-0-009 04867-0-008
20
15
10
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5 4.75
6.75
10.75 8.75 RSET (k)
12.75
14.75
-0.4 2.6
3.0
3.4
3.8 4.2 4.6 SUPPLY VOLTAGE (V)
5.0
5.4
Figure 3. ILED (mA) Current vs. RSET
Figure 6. ILED (mA) Matching Error (%) vs. Supply Voltage (V), TA = 25C and ILED = 20 mA
20.35 20.30 -40C 20.25
20.24 20.22 20.20
LED CURRENT (mA)
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LED CURRENT (mA)
20.20 25C 20.15 85C 20.10 20.05
20.18 20.16 20.14 20.12 20.10
20.00 2.6
3.1
4.1 3.6 4.6 SUPPLY VOLTAGE (k)
5.1
5.6
20.08 -40
0
40 TEMPERATURE (C)
80
Figure 4. ILED (mA) vs. Temperature (C), Six LEDs Enabled
Figure 7. ILED (mA) Variation over Temperature (C), VCC = 3.6 V
0.3
35
0.2
30
0.1
% ERROR
LED CURRENT (mA)
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25
0
20
-0.1
15
-0.2
10
-0.3 -40 -20 25 0 45 TEMPERATURE (C) 65 85
5 2.6
3.0
3.4
3.8 4.2 4.6 SUPPLY VOLTAGE (V)
5.0
5.4
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
20
90
16
LED CURRENT (mA)
85
EFFICIENCY (%)
04867-0-010
12
80 75 70
8
4
65 60 0 10 20 30 40 50 60 DUTY CYCLE (%) 70 80 90 100
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04867-0-015
0 0 20 40 60 DUTY CYCLE (%) 80 100
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
250
CTRL1/2
SUPPLY CURRENT ICC (mA)
200
20mA/LED
1
150 15mA/LED
CURRENT
2
100
0 2.6
3.0
3.4
3.8 4.2 4.6 SUPPLY VOLTAGE (V)
5.0
5.4
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3
CH1 2.00V CH3 1.00V
B
W
CH2 160mA
B
W
M 5.00s CH2
180mV
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
VCC 1
VCC
1
VOUT
2
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VOUT
2
CH1 20.0mV
B
W
CH2 20.0mV
B
W
M 400ns
CH1
220mV
CH1 20.0mV
B
W
CH2 20.0mV
B
W
M 400ns CH1
220mV
Figure 11. 1.5x Mode Operating Waveforms
Figure 14 .2x Mode Operating Waveform
Rev. 0 | Page 7 of 20
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50
VOUT
ADM8845
90 85 80 VF = 3.6V VF = 4.0V VF = 4.3V
VCC
1
POWER EFFICIENCY
75 70 65 60 55 50 VF = 3.8V VF = 3.2V
VOUT
2
04867-0-016
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
04867-0-019
CH1 20.0mV
B
W
CH2 20.0mV
B
W
M 400ns CH1
220mV
Figure 15. 1x Mode Operating Waveforms
Figure 18. Power Efficiency vs. Supply Voltage over Li-Ion Range, Six LEDS @ 20 mA/LED
90 VF = 3.8V 85 VF = 4.0V 80 VF = 4.3V
90 85 80 VF = 4.3V VF = 4.0V VF = 3.8V
POWER EFFICIENCY
75 70 65 60 55 50 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
04867-0-017
POWER EFFICIENCY
75 70 65 60 55 50 45 VF = 3.6V VF = 3.2V
VF = 3.6V VF = 3.2V
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
Figure 16. Power Efficiency vs. Supply Voltage over Li-ion Range, Six LEDS @ 15 mA/LED
90 85 80 VF = 3.8V
Figure 19. Power Efficiency vs. Supply Voltage over Li-Ion Range Four LEDS @ 20 mA/LED
CTRL1/2
VF = 4.0V VF = 4.3V
1
C2 FALL : 44.0ms @: -44.4ms 200s LOW SIGNAL AMPLITUDE
POWER EFFICIENCY
75 70 65 60 55 VF = 3.6V 50 45
VOUT
2
04867-0-018
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
CH1 2.00V
CH2 2.00V
M10.0ms
CH2
4.36V
Figure 17. Power Efficiency vs. Supply Voltage over Li-Ion Range, Four LEDS @ 15 mA/LED
Figure 20. TPC Delay
Rev. 0 | Page 8 of 20
04867-0-021
VF = 3.2V
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ADM8845
30mA
20mA
65C
85C
Figure 21. Maximum ILED (mA) vs. Ambient Temperature, Six LEDs Connected
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Rev. 0 | Page 9 of 20
ADM8845 THEORY OF OPERATION
The ADM8845 charge pump driver for LCD white LED backlights implements a multiple gain charge pump (1x, 1.5x, 2x) 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 1x/1.5x/2x 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 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). Table 4. Shutdown Truth Table
CTRL1 0 0 1 1 CTRL2 0 1 0 1 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
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 x 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 performance 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). Softstart 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.
ADM8845
ISET VBRIGHT 0V-2.5V R = 15k RSET = 13.4k
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Figure 22. PWM Brightness Control Using a DC Voltage Applied to VBRIGHT
C1 1F C2 1F
VCC C4 4.7F
ADM8845
CHARGE PUMP 1x/1.5x/2x MODE
VOUT C3 2.2F MAIN SUB
OSC CTRL1 CTRL2 CONTROL LOGIC
VREF FB1 FB2 FB3 FB4 FB5 FB6
CURRENT CONTROL 1
CURRENT CONTROL 2
CURRENT CONTROL 3
CURRENT CONTROL 4
CURRENT CONTROL 5
ISET RSET
LED CURRENT CONTROL CIRCUIT
CURRENT CONTROLLED SINKS GND
Figure 23. Functional Block Diagram
Rev. 0 | Page 10 of 20
CURRENT CONTROL 6
ADM8845
OUTPUT CURRENT CAPABILITY
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: RSET = 120 x (1.18 V/ILED) 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: VOUT - 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 15 mA 20 mA 30 mA RSET 9.44 k 7.08 k 4.72 k Typ. Compliance 0.17 V 0.20 V 0.34 V
CURRENT MATCHING
The 1% maximum current matching performance is defined by the following equations: IAVG = (IMAX + IMIN)/2 Max Matching Error = [(IMAX - IAVG)/IAVG] x 100 or Min Matching Error = [(IMIN - IAVG)/IAVG] x 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). 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 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 1F C2 1F VOUT
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. Table 6. Charge Pump Switching Thresholds
Gain 1.5x to 2x 2x to 1.5x 1x to 1.5x 1.5x to 1x Threshold 3.33 V 3.36 V 4.77 V 4.81 V
ADM8845
C3 2.2F
PWM INPUT OR HIGH/LOW PWM INPUT OR HIGH/LOW
CTRL1 CTRL2 FB1 FB2 FB3 FB4 FB5 FB6
ISET RSET
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
04867-0-024
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 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 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 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). 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.
Table 7. 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 2
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. 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
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 80%
100% MAIN DISPLAY BRIGHTNESS 50%
SHDN
04867-0-025
37ms > tCP > 15ms
Figure 25. Application Timing
LED BRIGHTNESS CONTROL USING A PWM SIGNAL APPLIED TO VPWM
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 configuration, 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.
I LED (1 - Duty Cycle) I _ Voltage = SET x 120 x R SET x 2R 100 R SET + 2R
LED BRIGHTNESS CONTROL USING A DC VOLTAGE APPLIED TO VBRIGHT
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 ISET = [(1/RSET + 1/R)(VSET)] - [(1/R)(VBRIGHT)] ILED = 120 x ISET where R = 15 k and VSET = voltage at ISET pin (1.18 V).
2.5V VBRIGHT 1.6V 0.8V 0V
20mA
04867-0-028
13.6mA ILED 100% = ILED = 0mA 0% = ILED = 20mA 7.2mA 0mA
ADM8845
ISET
Figure 27. PWM Brightness Control Application Diagram Using a DC Voltage Applied to VBRIGHT
04867-0-026
VPWM0V-2.5V
R = 7.5k R = 7.5k C5 = 1F RSET = 13.4k
Figure 26. PWM Brightness Control Using Filtered-PWM Signal
Rev. 0 | Page 13 of 20
ADM8845 APPLICATIONS
LAYOUT CONSIDERATIONS AND NOISE
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.
DRIVING FEWER THAN SIX LEDS
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 x 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 x 120 mA) as programmed by RSET through the shorted LED.
LCD
MAIN DISPLAY
SUB DISPLAY
VCC 2.6V-5.5V VOUT
VCC 2.6V-5.5V VOUT
ADM8845
CTRL1 CTRL2 FB1 FB2 FB3 FB4 FB5 FB6
ADM8845
CTRL1 CTRL2 FB1 FB2 FB3 FB4 FB5 FB6
GND
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RSET
Figure 28. Driving Five White LEDs
MAIN DISPLAY
Figure 29. Driving Three Main LEDs and One Sub LED
SUB DISPLAY
VCC 2.6V-5.5V VOUT
ADM8845
CTRL1 CTRL2 FB1 FB2 FB3 FB4 FB5 FB6
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ISET RSET
GND
Figure 30. Typical Application Diagram
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04867-0-030
ISET
GND
ADM8845
DRIVING FLASH 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).
VCC 2.6V-5.5V VOUT
DRIVING CAMERA LIGHT, MAIN, AND SUB LEDS
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 maximum load on the ADM8845 charge pump is 180 mA (see Figure 21).
ADM8845
CTRL1 CTRL2 FB1 FB2 FB3 FB4 FB5 FB6
60mA
60mA
60mA
GND RSET 4.27k
Figure 31. Driving Three Flash LEDs
C1 1F
C2 1F
VCC
ADM8845
CHARGE PUMP 1x/1.5x/2x MODE
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VOUT C3 2.2F CAMERA 15mA/LED VREF FB1 FB2 FB3 FB4 FB5 FB6 CURRENT CONTROL7 R MAIN 15mA/LED SUB 15mA/LED
C4 4.7F
OSC CTRL1 CTRL2 CONTROL LOGIC
CURRENT CONTROL 1
CURRENT CONTROL 2
CURRENT CONTROL 3
CURRENT CONTROL 4
CURRENT CONTROL 5
ISET RSET 9.44k
LED CURRENT CONTROL CIRCUIT
CURRENT CONTROL 6
CURRENT CONTROLLED SINKS
GND
Figure 32. Driving Camera Light, Two Main LEDs, and One Sub LED
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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 backlight 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 1F
C2 1F
VCC
ADM8845
CHARGE PUMP 1x/1.5x/2x MODE VOUT C3 2.2F BACKLIGHT 15mA/LED VREF FB1 FB2 FB3 FB4 FB5 FB6 CURRENT CONTROL 7 R FLASH 20mA/LED
C4 4.7F CTRL1 CONTROL LOGIC CTRL2
OSC
CURRENT CONTROL 1
CURRENT CONTROL 2
CURRENT CONTROL 3
CURRENT CONTROL 4
CURRENT CONTROL 5
ISET RSET 7.32k
LED CURRENT CONTROL CIRCUIT
CURRENT CONTROL 6
CURRENT CONTROLLED SINKS GND
Figure 33. Driving Four Backlight LEDs and Flash LED
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ADM8845
POWER EFFICIENCY
The ADM8845 power efficiency () equations are = POUT/PIN PIN = ((VCC x ILOAD x Gain) + (IQ x VCC)) POUT = 6x(VF x ILED) where: IQ is the quiescent current of the ADM8845, 2.6 mA. VF is the LED forward voltage. Gain is equal to charge pump mode (1x, 1.5x, 2x).
3.4V I IN VCC VOUT ILOAD
Example 1 The ADM8845 driving six white LED with 20 mA/LED at VCC = 3.4 V (1.5x mode), LED VF = 4.5 V. PIN = ((VCC x ILOAD x Gain) + (VCC x IQ)) PIN = ((3.4 x 120 mA x 1.5) + (3.4 x 2.6 mA)) PIN = ((0.612) + (0.00884)) PIN = 0.62084 POUT = 6(VF x ILED) POUT = 6(4.5V x 20 mA) POUT = 0.54 = POUT/PIN = 0.54/0.62084 = 87 % Example 2
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ADM8845
VCC CTRL1 CTRL2 FB1 FB2 FB3 FB4 FB5 FB6
ISET RSET
GND
The ADM8845 driving six white LED with 20 mA/LED at VCC = 3.4 (1.5x mode), LED VF = 3.6 V. PIN = ((VCC x ILOAD x Gain) + (VCC x IQ)) PIN = ((3.4 x 120 mA x 1.5) + (3.4 x 2.6 mA)) PIN = ((0.612) + (0.00884)) PIN = 0.62084 POUT = 6(VF x ILED) POUT = 6(3.6 V x 20 mA) POUT = 0.432 = POUT/PIN = 0.432/0.62084 = 70 %
Figure 34. Charge Pump Power Efficiency Diagram
Examples 1 and 2 show calculations of the ADM8845 power efficiency; also see Figure 34.
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ADM8845 OUTLINE DIMENSIONS
0.50 0.40 0.30 PIN 1 INDICATOR
16 1
EXPOSED PAD
3.00 BSC SQ 0.45 PIN 1 INDICATOR TOP VIEW 2.75 BSC SQ 0.50 BSC 12 MAX 0.90 0.85 0.80 SEATING PLANE 0.30 0.23 0.18 0.80 MAX 0.65 TYP 0.05 MAX 0.02 NOM 0.20 REF
0.60 MAX
13 12
1.45 1.30 SQ* 1.15
9
(BOTTOM VIEW)
8
5
4
0.25 MIN
1.50 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 ADM8845ACP-REEL ADM8845ACP-REEL7 ADM8845ACPZ-REEL1 ADM8845ACPZ-REEL71 ADM8845ACPZ-WP1, 2 EVAL-ADM8845EB Temperature Range -40C to + 85C -40C to + 85C -40C to + 85C -40C to + 85C -40C to + 85C Package Description 16-Lead LFCSP 16-Lead LFCSP 16-Lead LFCSP 16-Lead LFCSP 16-Lead LFCSP Evaluation Board Package Option CP-16 CP-16 CP-16 CP-16 CP-16 Branding M0P M0P M0P M0P M0P
1 2
Z = Pb-free part. WP = waffle pack.
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ADM8845 NOTES
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ADM8845 NOTES
(c) 2004 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D04867-0-10/04(0)
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