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| A8521 Datasheet, PDF (33/35 Pages) Allegro MicroSystems – The A8521 is a multi-output white LED driver for small-size LCD backlighting. | |||
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A8521
Wide Input Voltage Range, High Efficiency
Fault Tolerant LED Driver
Step 9 Selecting coupling capacitor CSW. The minimum capaci-
tance of CSW is related to the maximum voltage ripple allowed
across it:
CSW =
IOUT
âVSW
DMAX
fSW
(50)
= 0.24 (A) 0.765 = 0.92 μF
0.1 (V) 2 (MHz)
The rms current requirement of the coupling capacitor is given
by:
ICSWrms = IIN(max) 1 â D(max)
(51)
D(max)
= 0.848 (A)
1 â 0.765
0.765
= 0.47 A
The voltage rating of the coupling capacitor must be greater than
VIN(max), or 16 V in this case. A ceramic capacitor rated for
2.2 μF 25 V will suffice for this application.
VIN
9 to 16 V
RSC
0.033 Ω
CIN
2.2 μF
RADJ
249 Ω
RC
20 Ω
VC
100 kΩ
RISET
11 kΩ
Q1
CC
22 nF
CVDD
0.1 μF
RFSET
10 kΩ
L1
10 μH
CSW
2.2 μF
L2
10 μH
D1
2 A / 60 V
VOUT
GATE
SW
VSENSE
VIN A8521
VDD
PAD
FAULT
PWM/EN
APWM
ISET
FSET/SYNC
OVP
LED1
LED2
LED3
LED4
COMP
AGND PGND
ROVP
39.2 kΩ
COUT
4.7 μF
CP
120 pF
RZ 150 Ω
CZ 0.47 μF
Figure 34. Typical application showing SEPIC configuration, with accurate input current sense, and VSENSE
to ground protection.
Allegro MicroSystems, Inc.
33
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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