R7986A datasheet(18/37 Pages) STMICROELECTRONICS

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Application information

R7986A

18/37

DocID026997 Rev 1

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 must 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 15

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. Section 6.4 illustrates how to consider its effect in the system

stability.

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

order to have a

VOUT = 0.01·VOUT, if the multi-layer ceramic capacitor is adopted, 10 µF

are needed and the ESR effect on the output voltage ripple can be neglected. In the case of

not negligible ESR (electrolytic or tantalum capacitors), the capacitor is chosen taking into

account its ESR value. So, in the case of 330 µF with ESR = 730 m

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

must be chosen in order to sustain the load transient.

In Table 8 some capacitor series are listed.

Table 8. Output capacitors

Manufacturer

Series

Cap value (

F)

Rated voltage (V)

ESR (m

)

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

V

OUT

ESR

I

MAX



I

MAX

8COUT fSW



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