Application informations

L5980

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5.3

Output capacitor selection

The current in the capacitor has a triangular waveform which generates a voltage ripple

across it. This ripple is due to the capacitive component and the resistive component (ESR).

So the output capacitor has to be selected in order to have a voltage ripple compliant with

the application requirements.

The amount of the voltage ripple can be calculated starting from the current ripple obtained

by the inductor selection.

Equation 9

Usually the resistive component of the ripple is much higher than the capacitive one, if the

output capacitor adopted is not a multi layer ceramic capacitor (MLCC) with very low ESR

value.

The output capacitor is important also for loop stability: it fixes the double LC filter pole and

the zero due to its ESR. In Chapter 5.4, it will be illustrated how to consider its effect in the

system stability.

to have a

∆V

the ESR effect on the output voltage ripple can be neglected. In case of not negligible ESR

(electrolytic or tantalum capacitors), the capacitor is chosen taking into account its ESR

value. So 100uF with ESR=40m

Ω is compliant with the requested output voltage ripple.

The output capacitor is also important to sustain the output voltage when a load transient

with high slew rate is required by the load. When the load transient slew rate exceeds the

system bandwidth the output capacitor provides the current to the load. So if the high slew

rate load transient is required by the application the output capacitor and system bandwidth

have to be chosen in order to sustain the load transient .

In the table below some capacitor series are listed.

Coiltronics

DRQ73

10 to 22

1.67 to 2.47

LD2

27 to 47

1.64 to 2.1

SUMIDA

CDR6D28MN

10 to 22

1.65 to 2.5

CDRH105RNP

27 to 56

1.9 to 2.7

Table 6.

Inductors (continued)

Manufacturer

Series

Inductor value (

µH)

Saturation current (A)

Table 7.

Output capacitors

Manufacturer

Series

Cap value (

µF)

Rated voltage (V)

ESR (m

Ω)

MURATA

GRM32

22 to 100

6.3 to 25

< 5

GRM31

10 to 47

6.3 to 25

< 5

∆V

OUT

ESR

∆I

MAX

⋅

∆I

MAX

8C

OUT

f

SW

⋅⋅

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+

=