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| 型号： | ADP8863ACPZ-R7 |
| 厂商： | Analog Devices Inc |
| 文件页数： | 19/52页 |
| 文件大小： | 0K |
| 描述： | IC LED DRVR WHIT BCKLGT 20-WLCSP |
| 标准包装： | 1 |
| 拓扑： | 切换式电容器（充电泵） |
| 输出数： | 7 |
| 内部驱动器： | 是 |
| 类型 - 主要： | 背光，LED 闪烁器 |
| 类型 - 次要： | RGB，白色 LED |
| 频率： | 800kHz ~ 1.32MHz |
| 电源电压： | 2.5 V ~ 5.5 V |
| 输出电压： | 4.3 V ~ 5.5 V |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 20-UFBGA，WLCSP |
| 供应商设备封装： | 20-WLCSP |
| 包装： | 标准包装 |
| 工作温度： | -40°C ~ 85°C |
| 其它名称： | ADP8863ACPZ-R7DKR |
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ADP8863
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
D1
D2
D3
D4
D5
D6
D7
In the first method, multiple LEDs can be attached to any one
LED driver with the use of ballast resistors.
VIN
1μF
I 1
VOUT
I 3
VDDIO
VOUT
1μF
I 2
+
Vf1
–
+
Vf2
–
R BALLAST
+
Vf3
–
R BALLAST
R BALLAST
nRST
SDA
VDDIO
ADP8863
C1+
C1–
C1
1μF
V DX
VDDIO
C2+
I DX = I 1 + I 2 + I 3
SCL
C2–
C2
1μF
VDDIO
Figure 43. Ballast Resistor Arrangement
Ballast resistors attempt to compensate for the forward voltage
nINT
(Vf) mismatch inherent in any parallel combination of LEDs.
GND1
GND2
R BALLAST ≈
Δ Vf
Δ I
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:
(5)
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
Vf ( eff ) = Vf ( LED ) + I LED × R BALLAST (6)
Figure 44. Powering Additional LEDs with Ballast Resistors
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 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).
Rev. A | Page 19 of 52
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