Samsung S3F84A5 - Manual (Page 12)

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
O FW FW
V V I ohm V
     
5
I
V V

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 REQUIREMENT OF CURRENT RIPPLE
1
( )
I O
PWM
ripple
D
V V
f V
L
I I
T
 
 


1
3.85
(5 3.85) 7.084
5 125
ripple
L I
K
     

Therefore, 20% current ripple means . 1
100 L u

H
2.2.2 SELECT THE CAPACITANCE (C2) FOR THE REQUIREMENT 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
PWM
ripple
I
f
C
V


2
0.35* 0.2
0.56
125
ripple
C V
K
   
Therefore, 1% voltage ripple means . 2
47 C u

F
The freewheeling diode should be a Schottky diode, as the system requires low turn-on voltage and fast switching.
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