ADP8863ACPZ-R7 datasheet(19/48 Pages) AD

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ADP8863

Rev. 0 | Page 19 of 48

VDDIO

nRST

VDDIO

SDA

VDDIO

SCL

VDDIO

nINT

VIN

1µF

GND1

GND2

C2+

C2–

C1+

C1–

VOUT

1µF

ADP8863

1µF

USING THE ADP8863 TO DRIVE ADDITIONAL LEDS

In some situations, it may be desirable to drive more than seven

LEDs. This can be done in one of two ways: paralleling LEDs

using ballast resistors, or using the ADP8863 to power

additional LED drivers.

Ballast Resistors

In the first method, multiple LEDs can be attached to any one

LED driver with the use of ballast resistors.

VOUT

VDX

IDX = I1 + I2 + I3

RBALLAST RBALLAST

RBALLAST

Vf1

–

Vf2

–

Vf3

–

Figure 42. Ballast Resistor Arrangement

Ballast resistors attempt to compensate for the forward voltage

(Vf) mismatch inherent in any parallel combination of LEDs.

The choice of a ballasting resistor is a trade-off between the

efficiency and the current matching of the LEDs in parallel.

Smaller ballast resistors give better efficiency. Larger ballast

resistors gives better current matching, because the resistor

balances out the current differences for a voltage drop. The

relationship is summarized with the following:

Figure 43. Powering Additional LEDs with Ballast Resistors

Adding Additional Parallel Sinks

BALLAST

≈

(5)

The ballast resistor’s compromises of efficiency and matching

are not suitable for many applications. Therefore, another

option is to use the ADP8863 charge pump to power additional

current sinks. First, the charge pump must be optimized for this

option by setting the GDWN_DIS bit in Register 0x01, which

prevents the charge pump from switching back down in gain,

and thus stabilizes it against the unknown loads that the

additional current sinks present.

where:

ΔVf

is the difference between the maximum Vf and the

minimum Vf of the LEDs in parallel.

ΔI

is the difference between the parallel LED currents.

To use the sinks, turn the ADP8863 charge pump on before

activating the additional sinks. If the additional sinks are

activated first, the ADP8863 soft start may not complete.

The addition of the ballast resistor brings the effective Vf of the

LED to

BALLAST

LED

eff

)

(

)

(

(6)

The ADP8863 can be set up with an ADP8860 or ADP8861. An

example using the ADP8861 is shown in Figure 44.

The ILED × RBALLAST term forces the charge pump to work a little

harder for this additional voltage drop. Furthermore, for

guaranteed operation with the ADP8863, the total Vf(eff)

should never exceed VOUT(REG) − VHR(UP) (see Table 1).

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ADP8863ACPZ-R7 数据表(PDF) 19 Page - Analog Devices

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