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L5985

Application informations

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5.3

Output capacitor selection

The current in the capacitor has a triangular waveform (with zero average value) 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.

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

order to have a

∆V

OUT = 0.01·VOUT, if the multi layer 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 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

CD1

15 to 22

2.9 to 3.6

UP2.8B

4.7 to 10

2.7 to 3.9

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 6.

Inductors

Manufacturer

Series

Inductor Value (

µH)

Saturation Current (A)

∆V

OUT

ESR

∆I

MAX

⋅

∆I

MAX

OUT

⋅⋅

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