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S3F84P4_AN_REV0.00 (PRELIMINARY SPEC) 2 HARDWARE IMPLEMENTATION
2.2 COMPONENTS SELECTION
Assume the following two conditions:
( 1 ) 3.5 0.35 3.85
OFWFW
V V I ohm V
5
I
VV
If non-divided system clock is selected as the clock source of the 6+6 PWM, its base frequency
is
6
125
2
OSC
f
KHz
.
2.2.1 SELECT THE INDUCTANCE (L1) FOR THE RE
QUIREMENT OF CURRENT RIPPLE
1
()
IO
P
WM
ripple
D
VV
f
V
L
I
I
T
1
3.85
(5 3.85) 7.084
5 125
ripple
LI
K
Therefore, 20% current ripple means .
1
100Lu H
2.2.2 SELECT THE CAPACITANCE (C2) FOR THE RE
QUIREMENT OF VOLTAGE RIPPLE
To reduce the voltage ripple and power loss, a capacitor with small ESR like Tantalum Capacitor should be
chosen as C2. When ESL and ESR are negligible, then,
2
P
WM
ripple
I
f
C
V
2
0.35*0.2
0.56
125
ripple
CV
K
Therefore, 1% voltage ripple means .
2
47Cu F
The free
wheeling diode should be a Schottky diode, as the system requires low turn-on voltage and fast switching.
S
amsun
g
Con
f
idential
12