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MB39A105
23
• 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 TEPSLA21A475M8R (NEC/TOKIN) . The ESR,
capacitance value, and ripple current can be calculated from the following formulas.
Equivalent Series Resistance : ESR
Capacitance value : C
L
Ripple current : ∆ICL
Example: Using the TEPSLA21A475M8R (Three piecies are parallel.)
Rated voltage = 10 V, ESR = 500 mΩ, maximum allowable ripple current = 1 A
p−p
Equivalent series resistance
Capacitance value : C
L
Ripple current : ∆ICL
ESR ≤
∆VO
−
1
∆IL 2πfCL
CL ≥
∆IL
2πf (∆VO − ∆IL × ESR)
∆IC
L ≥
V
IN
ton
L
ESR ≤
∆V
O
−
1
∆IL 2πfCL
≤
0.18
−
1
0.654 2π × 500 × 10
3
× 14.1 × 10
−6
≤ 252.7 mΩ
CL ≥
∆IL
2πf (∆VO − ∆IL × ESR)
≥
0.39
2π × 500 × 10
3
× (0.18 − 0.654 × 0.167)
≥ 2.94
µF
∆ICL ≥
V
IN
ton
L
≥
4 × (9 − 4)
×
1
6.8 × 10
−6
× 9 500 × 10
3
≥ 0.654 Ap−p