Datasheet

ADP8863
Rev. A | Page 20 of 52
BACKLIGHT
D2 D3 D4 D5 D6 D7 D1
NC
NC
NC
NC
D2 D3 D4 D5 D6 D7 D1
C2+
C2–
C1+
C1–
GNDx
GNDx
ADP8863
ADP8861
08392-047
C
MP_IN
0.1µF
PHOTO
SENSOR
V
ALS
nRST
SDA
SCL
nINT
VIN
1µF
nRST
SDA
SCL
nINT
VIN
1µF
1µF
VOUT
VOUT
C1+
C1–
C2+
C2–
C1
1µF
C2
1µF
KEYPAD OR
FUN LIGHTING
Figure 45. Connecting the ADP8863 to an ADP8861 to Power More LEDs
OPERATING LEDS FROM ALTERNATIVE SUPPLIES
For some applications, it is advantageous to operate the LEDs
from a voltage source other than the ADP8863 charge pump
output. For example, it may be possible to operate a red LED
over the entire battery voltage range without any charge pump
boosting. For a charge pump,
OUT
IN
IGainI ×=
(7)
Therefore, operating the red LEDs directly from the battery
removes output current of the red LEDs from the charge pump
draw. This in turn reduces the total input current by (Gain − 1) ×
I
OUT(red)
.
However, care must be taken when selecting LEDs to operate
from a different voltage input. Specifically, the voltage source
must at least be able to support the maximum forward voltage
(Vf) of the LED plus the maximum V
HR(UP)
(276 mV, given in
Table 1). Additionally, operating an LED from an independent
voltage source may interfere with the ADP8863’s gain selection
algorithm. This algorithm selects the optimal gain for the
charge pump based on all seven diodes. By operating one or
more of the diodes from another supply, the algorithm may not
switch the gain back down to a lower state until the LEDs are
disabled or the part enters standby.
D1 D2 D3 D4 D5 D6 D7
VDDIO
nRST
VDDIO
SDA
VDDIO
SCL
VDDIO
nINT
VIN
1µF
GND1 GND2
C2+
C2–
C1+
C1–
VOUT
1µF
ADP8863
0
8392-030
C2
1µF
C1
1µF
Figure 46. Alternate Schematic for Low Vf LEDs