L5985_08 datasheet(16/37 Pages) STMICROELECTRONICS

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L5985

Application information

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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 (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 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.

For example with VOUT = 3.3 V, VIN = 12 V, ΔIL = 0.6 A (resulting by the inductor value), in

order to have a

ΔV

OUT = 0.01·VOUT, if the multi layer capacitor are adopted, 10 uF 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 100 uF with ESR = 40 m

Ω, the resistive component of

the drop dominates and the voltage ripple is 25 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.

HM76-3

15 to 33

2.5 to 3.7

SUMIDA

CDRH8D28

4.7 to 10

2.5 to 3.4

CDRH8D28/HP

15 to 22

2.5 to 2.8

Table 7.

Inductors (continued)

Manufacturer

Series

Inductor value (

μH)

Saturation current (A)

ΔV

OUT

ESR

ΔI

MAX

⋅

ΔI

MAX

OUT

⋅⋅

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