ADP8863 Analog Devices, ADP8863 Datasheet - Page 19

• Manufacturer Part Number: ADP8863
• Description: Charge Pump, 7-Channel Fun Lighting LED Driver
• Manufacturer: Analog Devices

Specifications of ADP8863

• Vin Range: 2.5 to 5.5V
• Vout (v): 4.3 to 5.5
• Synchronous: No
• Package: 20-Lead LFCSP,20-Lead WLCSP
• Led Configuration: Parallel
• Topology: Capacitive
• I2c Support: Yes
• Max Iout (ma): 60mA
• Brightness Control: I2C
• Peak Efficiency (%): 89%
• Switching Frequency: 1.32MHz
• Over Volt Protection (v): 5V

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.
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:
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.
The addition of the ballast resistor brings the effective Vf of the
LED to
The I
harder for this additional voltage drop. Furthermore, for
guaranteed operation with the ADP8863, the total Vf(eff)
should never exceed V
Vf
R
LED
BALLAST
(
eff
× R
)
BALLAST
=
≈
Vf
Figure 43. Ballast Resistor Arrangement
Vf1
Δ
I
(
1
Δ
+
–
Vf
LED
term forces the charge pump to work a little
I
R
BALLAST
OUT(REG)
)
+
Vf2
I
VOUT
+
–
LED
R
V
− V
I
DX
BALLAST
I
×
2
DX
R
= I
HR(UP)
BALLAST
Vf3
1 +
+
–
I
2 +
R
I
(see Table 1).
3
BALLAST
I
3
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Adding Additional Parallel Sinks
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.
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 ADP8863 can be set up with an
example using the ADP8861 is shown in Figure 45.
nRST
nINT
SDA
SCL
VIN
1µF
VDDIO
VDDIO
VDDIO
VDDIO
Figure 44. Powering Additional LEDs with Ballast Resistors
D1
D2
GND1
D3
ADP8863
D4
D5
ADP8860
GND2
D6
D7
or ADP8861. An
ADP8863
C1+
C1–
C2+
C2–
1µF
VOUT
C1
1µF
C2
1µF