FUJITSU MB39A104 - Manual (Page 30)

MB39A104
30
• Smoothing Capacitor
The smoothing capacitor is an indispensable element for reducing ripple voltage in output. In selecting a smooth-
ing capacitor it is essential to consider equivalent series resistance (ESR) and allowable ripple current. Higher
ESR means higher ripple voltage, so that to reduce ripple voltage it is necessary to select a capacitor with low
ESR. However, the use of a capacitor with low ESR can have substantial effects on loop phase characteristics,
and therefore requires attention to system stability. Care should also be taken to use a capacity with sufficient
margin for allowable ripple current. This application uses the (OS-CON
TM ) 6SVP82M made by SANYO. The
ESR, capacitance value, and ripple current can be calculated from the following formulas.
Equivalent Series Resistance : ESR
Capacitance value : C
L
Ripple current : IC
L
rms
Example: Using the 6SVP82M
Rated voltage
= 6.3 V, ESR = 50 mΩ, maximum allowable ripple current = 1570 mArms
Equivalent series resistance
CH1
ESR ≤ ∆
V
O −
1
∆I
L
2
πfC
L
C
L
≥
∆I
L
2 πf (∆V
O
− ∆I
L
× ESR)
IC
L
rms ≥
(V
IN
− V
O
) ton
2
√3L
ESR ≤ ∆
V
O −
1
∆I
L
2
πfC
L
≤
0.050
−
1
0.491 2
π Ч 500 Ч 10
3
Ч 82 Ч 10 −6
≤ 98.0 mΩ