Components Selection; Select The Inductance (L1) For The Requirement Of Current Ripple; Select The Capacitance (C2) For The Requirement Of Voltage Ripple - Samsung S3F84A5 Application Note

S3F84P4_AN_REV0.00 (PRELIMINARY SPEC)

2.2 COMPONENTS SELECTION

Assume the following two conditions:
  
V ( V I 1
O FW FW

V 5 V
I
If non-divided system clock is selected as the clock source of the 6+6 PWM, its base frequency
f

OSC
125 KHz
6
2
is .

2.2.1 SELECT THE INDUCTANCE (L1) FOR THE REQUIREMENT OF CURRENT RIPPLE


( V V
I O
V
 
L

1
I
I

T
  
L I (5 3.85)
1 ripple
Therefore, 20% current ripple means

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,

I
f

PWM
C
2
V
ripple
0.35*0.2
 
C V
2 ripple
125 K
Therefore, 1% voltage ripple means
The freewheeling diode should be a Schottky diode, as the system requires low turn-on voltage and fast switching.
  
ohm ) 3.5 0.35 3.85
D

)
f
PWM
ripple
3.85
 
7.084

5 125 K

L 100
1


0.56

C 47 u
2
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V

u H
.
F
.
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