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ADM8845ACP-REEL7-ADM8845ACPZ-REEL7
Charge Pump Driver for LCD White LED Backlights
Charge Pump Driver for LCD
White LED Backlights
Rev. PrJ_06/04
FEATURES
ADM8845 drives 6 LEDs from a 2.6 V to 5.5 V (li-ion) input
supply
1x/1.5x/2x Fractional Charge Pump to maximise 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 9mm2 (3mm x 3mm)
Package height only 0.9mm
Low power shutdown mode
Shutdown Function
Soft-start limiting inrush current
APPLICATIONS
Mobile phones with Main and Sub Displays
White LED Backlighting
Camera Flash/Strobes and Movie Light Applications
Micro TFT color displays
DSC
PDA’s
GENERAL DESCRIPTION The ADM8845 provides the power required to drive up to six
LEDs, using charge pump technology. The LEDs are used for
backlighting a color LCD display, with up to four LEDs in the
main display and up to two LEDs in the sub display, with
regulated constant current for uniform brightness intensity.
Two digital input control pins, CTRL1 and CTRL2 control the
shutdown operation and the brightness of the main and sub
displays.
To maximize power efficiency, a charge pump that can operate
in either of a 1x, 1.5x or 2x mode is used. 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 the
use of a feedback pin to sense individual LED current with a
maximum matching accuracy of 1%.
FUNCTIONAL BLOCK DIAGRAM
VCC
Iset
GND
Vout
FB6C2
Rset
CTRL1
CTRL2Figure 1. ADM8845 FUNCTIONAL BLOCK DIAGRAM
TABLE OF CONTENTS General Description.....................................................................1
ADM8845—Specifications...............................................................3
Thermal Characteristics..............................................................3
Absolute Maximum Ratings.............................................................4
ESD Caution..................................................................................4
Pin Configuration and Function Description...............................5
Typical Performance Characteristics..............................................6
ADM8845 Operation.....................................................................10
Output Current Capability........................................................11
Automatic Gain Control............................................................11
Current Matching.......................................................................11
ADM8845 Brightness Control With a Digital PWM Signal.....12
ADM8845 LED Brightness Control Using a PWM Signal
Applied to VPWM..........................................................................14
ADM8845 LED Brightness Control Using a DC Voltage
Applied to VBRIGHT.......................................................................14
ADM8845 Applications..................................................................15
Layout considerations and noise..............................................15
White LED Shorting..................................................................15
Driving Fewer than six LEDs....................................................15
Driving Flash LEDs....................................................................15
Driving Camera Light, Main and sub leds..............................16
ADM8845 Power Efficiency...........................................................17
Outline Dimensions........................................................................18
Ordering Guide..........................................................................18
REVISION HISTORY REV. PrJ 06/04
ADM8845—SPECIFICATIONS
(VCC = +2.6V TO 5.5V; TA = -40°C to 85°C unless otherwise noted; C1,C2 = 1.0µF; C3 = 2.2µF; C4 = 4.7µF)
Table 1.
THERMAL CHARACTERISTICS 16-Lead LFCSP Package:
θJA = 50°C/Watt
ABSOLUTE MAXIMUM RATINGS
Table 2. (TA = 25°C unless otherwise noted) Note 1: LED Current should be derated above TA > 65C, refer to
Figure 22.
Note 2: Guaranteed by design. Not 100% production tested.
Note 3: Short through LED.
Note 4: 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.
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.
MainDisplay
SubDisplay
Vcc
2.6V-5.5V
CTRL1
CTRL2
RSET
Figure 2. ADM8845 Typical Application Diagram
PIN CONFIGURATION AND FUNCTION DESCRIPTION
Table 3.
Pin
ADM8845 Mnemonic Function 1 VOUT Charge Pump Output. A 2.2µF 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 give the
ILED curent. Connect a resistor RSET to GND to set the bias current as VSET/RSET. (Note: Vset = 1.18V)
4 FB1 LED1 Cathode connection and Charge Pump Feedback. The current flowing in LED1 is 120 times the
current flowing through RSET, ISET.
5 FB2 LED2 Cathode connection and Charge Pump Feedback. The current flowing in LED2 is 120 times the
current flowing through RSET, ISET. When using fewer than six LEDs this pin can be left unconnected.
6 FB3 LED3 Cathode connection and Charge Pump Feedback. The current flowing in LED3 is 120 times the
current flowing through RSET, ISET. When using fewer than six LEDs this pin can be left unconnected.
7 FB4 LED4 Cathode connection and Charge Pump Feedback. The current flowing in LED4 is 120 times the
current flowing through RSET, ISET. When using fewer than six LEDs this pin can be left unconnected.
8 FB5 LED5 Cathode connection and Charge Pump Feedback. The current flowing in LED5 is 120 times the
current flowing through RSET, ISET. When using fewer than six LEDs this pin can be left unconnected.
9 FB6 LED6 Cathode connection and Charge Pump Feedback. The current flowing in LED6 is 120 times the
current flowing through RSET, ISET. When using fewer than six LEDs this pin can be left unconnected.
10 G N D Device Ground Pin.
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 4.7µF decoupling
capacitor.
16 C1+ Flying Capacitor 1 Positive Connection. EP Expose Paddle. Connect the exposed paddle to GND.
15
ADM8845
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Figure 3. ADM8845 Pin Configuration
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
TYPICAL PERFORMANCE CHARACTERISTICS Figure 4. LED Current vs. RSET Resistor
Figure 6. LED Current (mA) vs. Temperature (°C), 6 LEDs enabled.
Figure 8. ILED Matching (%) over Temperature (°C), with Vcc = 3.6,
ILED = 20mA , 6 LEDs enabled.
Figure 5. LED Current Matching Error (%) vs. Supply Voltage (V),
TA = 25°C and ILED = 20mA
Figure 7. ILED (mA) Variation over Temperature (°C), with Vcc = 3.6V
Figure 9. LED Current (mA) vs. Supply Voltage (V)
Figure 10. LED Current (mA) vs. PWM Dimming (varying Duty Cycle),
6 LEDs enabled, Freq = 1kHz.
Figure 12. Input Current vs. Supply Voltage, with 6 LEDs enabled
Figure 14. 1.5x Mode Operating Waveforms
Figure 11. LED Efficiency vs. Varying Duty Cycle of 1kHz PWM signal,
with 6 LEDs enabled, 20mA/LED.
Figure 13. Softstart showing the initial in-rush current and Vout
variation with 6 LEDs @ 20mA/LED, VCC = 3.6V
Figure 15. 2x Mode Operating Waveforms
Figure 16. 1x Mode Operating Waveforms
Figure 18. Power Efficiency vs. Supply Voltage over Li-ion Range
(6 LEDS @ 15mA/LED)
Figure 20. Power Efficiency vs. Supply Voltage over Li-ion Range
(4 LEDS @ 15mA/LED)
Figure 17. Power Efficiency vs. Supply Voltage over Li-ion Range
(6 LEDS @ 20mA/LED)
Figure 19. Power Efficiency vs. Supply Voltage over Li-ion Range
(4 LEDS @ 20mA/LED)
Figure 21. TPC Delay
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Figure 22. Max LED Current vs. Ambient Temperature (6 LEDs
Connected)
