Datasheet

ADP8863
Rev. A | Page 19 of 52
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
8392-034
D1
C1
1µF
C2
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.
V
OUT
V
DX
I
DX
= I
1 +
I
2 +
I
3
R
BALLAST
R
BALLAST
R
BALLAST
I
1
I
3
I
2
V
f1
+
–
Vf2
+
–
Vf3
+
–
08392-033
Figure 43. 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 44. Powering Additional LEDs with Ballast Resistors
Adding Additional Parallel Sinks
I
Vf
R
BALLAST
Δ
Δ
≈
(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
RILEDVfeffVf ×+= )()(
(6)
The ADP8863 can be set up with an ADP8860 or ADP8861. An
example using the ADP8861 is shown in Figure 45.
The I
LED
× R
BALLAST
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 V
OUT(REG)
− V
HR(UP)
(see Table 1).