ADP8863ACPZ-R7 datasheet(20/48 Pages) AD

20 / 48 page

ADP8863

Rev. 0 | Page 20 of 48

BACKLIGHT

C2+

C2–

C1+

C1–

GNDx

ADP8863

ADP8861

CMP_IN

0.1µF

PHOTO

SENSOR

VALS

nRST

SDA

SCL

nINT

VIN

1µF

nRST

SDA

SCL

nINT

VIN

1µF

VOUT

C1+

C1–

C2+

C2–

1µF

KEYPAD OR

FUN LIGHTING

Figure 44. 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

Gain

(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) ×

IOUT(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 VHR(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.

VDDIO

nRST

VDDIO

SDA

VDDIO

SCL

VDDIO

nINT

VIN

1µF

GND1

GND2

C2+

C2–

C1+

C1–

VOUT

1µF

ADP8863

1µF

Figure 45. Alternate Schematic for Low Vf LEDs

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