AN1723

Output capacitor

Doc ID 9553 Rev 4

9/24

4

Output capacitor

The output capacitor is very important in order to satisfy the output voltage ripple

requirement. Using a small inductor value is useful to reduce the size of the choke but

increases the current ripple. So, to reduce the output voltage ripple, a low ESR capacitor is

required. Nevertheless, the ESR of the output capacitor introduces a zero in the open loop

gain, which helps to increase the phase margin of the system. If the zero goes to every high

frequency, its effect is negligible. For this reason, ceramic capacitors and very low ESR

capacitors in general should be avoided. Tantalum and electrolytic capacitors are usually a

good choice for this purpose. A list of some tantalum capacitor manufacturers is provided in

Table 3.

4.1

Inductor

The inductor value is very important as it fixes the ripple current flowing through the output

Equation 4

The peak current through the inductor is given by:

Equation 5

and it can be observed that if the inductor value decreases, the peak current (which must be

lower than the current limit of the device) increases. So, when the peak current is fixed, a

higher inductor value allows a higher value for the output current. In Table 4, some inductor

manufacturers are listed.

Table 3.

Recommended output capacitors

Manufacturer

Series

Cap value (

μF)

Rated voltage (V)

ESR (m

Ω)

AVX

TPS

100 to 470

4 to 35

50 to 200

KEMET

T494/5

100 to 470

4 to 20

30 to 200

TPA/B/C

100 to 470

4 to 16

40 to 80

Sprague

595D

220 to 390

4 to 20

160 to 650

1.

POSCAP capacitors have characteristics very similar to tantalum capacitors.

L

V

IN

V

OUT

–

()

I

Δ

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T

ON

⋅

=

I

PK

I

O

I

Δ

2

-----

+

=