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PDF ADM8845 Data sheet ( Hoja de datos )
| Número de pieza | ADM8845 | |
| Descripción | Charge Pump Driver for LCD White LED Backlights | |
| Fabricantes | Analog Devices | |
| Logotipo |  |
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Preliminary Technical Data
Charge Pump Driver for LCD
White LED Backlights
ADM8845
FEATURES
GENERAL DESCRIPTION
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
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.
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
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
C1 C2
VCC
C4
ADM8845
Charge Pump
1x/1.5x/2x mode
Vout
C3
Osc
+
-
CTRL1
CTRL2
Control
Logic
Vref
FB1
FB2
FB3
FB4
FB5
FB6
LED Current
Iset Control
Rset
Circuit
Current
Control
1
Current
Control
2
Current
Control
3
Current
Control
4
Current
Control
5
Current
Control
6
MAIN
SUB
Current Controlled Sinks
GND
Figure 1. ADM8845 FUNCTIONAL BLOCK DIAGRAM
Rev. PrJ_06/04
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 companies.
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.
1 page  
Preliminary Technical Data
ADM8845
PIN CONFIGURATION AND FUNCTION DESCRIPTION
Table 3.
Pin
ADM8845
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
-
Mnemonic
VOUT
C2+
ISET
FB1
FB2
FB3
FB4
FB5
FB6
GND
C2−
CTRL2
CTRL1
C1−
Vcc
C1+
EP
Function
Charge Pump Output. A 2.2µF 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 curent. Connect a resistor RSET to GND to set the bias current as VSET/RSET. (Note: Vset = 1.18V)
LED1 Cathode connection and Charge Pump Feedback. The current flowing in LED1 is 120 times the
current flowing through RSET, ISET.
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.
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.
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.
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.
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.
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 4.7µF decoupling
capacitor.
Flying Capacitor 1 Positive Connection.
Expose Paddle. Connect the exposed paddle to GND.
16 15 14 13
VOUT 1
C2+ 2
ISET 3
FB1 4
ADM8845
TOP VIEW
(NOT TO SCALE)
12 CTRL2
11 C2-
10 GND
9 FB6
5 6 78
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
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. PrJ 06/04| Page 5 of 18
5 Page 
Preliminary Technical Data
ADM8845
OUTPUT CURRENT CAPABILITY
The ADM8845 is capable of driving up to 30mA of current to each of the six LEDs given an input voltage of 2.6V to 5.5V. The LED
currents have a max 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 (1.18V / ILED).
In order for the LED currents to be regulated 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 in order to ensure the desired current and
matching performance is realizable. To ensure the desired current is obtained, apply the following equations to find the minimum input
voltage required:
VOUT – VF ≥ Compliance
VF is the LED forward voltage. For 20mA/LED the compliance is 0.2V typ and 0.3V max, see Table 5.
ILED
15mA
20mA
30mA
RSET
9.44k Ohms
7.08k Ohms
4.72k Ohms
Typ. Compliance
0.17V
0.20V
0.34V
Table 5. ILED, RSET and Compliance Table
When the ADM8845 charge pump is loaded with 180mA (six LEDs at 30mA/LED) the ambient operating temperature is reduced, see
Figure 22.
AUTOMATIC GAIN CONTROL
The Automatic Gain Control block controls the operation of the charge pump by selecting the appropriate gain for the charge pump, to
maintain sufficient drive for the LED anodes at the highest power efficiency over a 2.6V to 5.5V input supply range. The charge pump
switching thresholds are:
Gain
1.5x to 2x
2x to 1.5x
1x to 1.5x
1.5x to 1x
Threshold
3.33V
3.36V
4.77V
4.81V
Table 6. Charge Pump Switching Thresholds
CURRENT MATCHING
The 1% max current matching performance is defined by the following two equations:
IAVG = (IMAX + IMIN)/2
Max Matching Error = [(IMAX – IAVG)/IAVG] * 100
or
Min Matching Error = [(IMIN – IAVG)/IAVG] * 100
Where IMAX is the largest ILED current and IMIN is the smallest ILED current.
Rev. PrJ 06/04| Page 11 of 18
11 Page | ||
| Páginas | Total 18 Páginas | |
| PDF Descargar | [ Datasheet ADM8845.PDF ] | |
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