L7981 datasheet(18/44 Pages) STMICROELECTRONICS

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Application informations

L7981

18/44

Doc ID 15182 Rev 3

6.3

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 (charge or discharge of the output

capacitor) and the resistive component (due to the voltage drop across its 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 12

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 6.4, it will be illustrated how to consider its effect in the

system stability.

For example with VOUT=5 V, VIN=24 V, ΔIL=0.9 A (resulting by the inductor value), in order to

have a

ΔV

OUT=0.01·VOUT, if the multi layer ceramic capacitor are adopted, 10uF are needed

and 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 in case of 330 µF with ESR=30 m

Ω, the resistive component of the drop

dominates and the voltage ripple is 28 mV

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.

Table 8.

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

PANASONIC

ECJ

10 to 22

6.3

< 5

EEFCD

10 to 68

6.3

15 to 55

SANYO

TPA/B/C

100 to 470

4 to 16

40 to 80

TDK

C3225

22 to 100

6.3

< 5

ΔV

OUT

ESR

ΔI

MAX

⋅

ΔI

MAX

OUT

⋅⋅

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