Application information

AN1518

16/30

and

Equation 19

IRMS going through the input capacitor. Capacitors that can be considered are:

Electrolytic capacitors:

These are widely used due to their low price and their availability in a wide range of

RMS current ratings.

The only drawback is that, considering ripple current rating requirements, they are

physically larger than other capacitors.

Ceramic capacitors:

If available for the required value and voltage rating, these capacitors usually have a

higher RMS current rating for a given physical dimension (due to very low ESR).

The drawback is the considerably high cost.

Tantalum capacitors:

Very good, small tantalum capacitors with very low ESR are becoming more available.

However, they can occasionally burn if subjected to very high current during charge.

Therefore, it is better to avoid this type of capacitor for the input filter of the device. They

can, however, be subjected to high surge current when connected to the power supply.

●

Output capacitor

The output capacitor is very important to meet the output voltage ripple requirement.

Using a small inductor value is useful to reduce the size of the choke but it 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 a very 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 4.: Output capacitor selection.

Table 3.

List of ceramic capacitors for the L597xD

Manufacturer

Series

Capacitor value (µ)

Rated voltage (V)

TDK

C3225

10

25

MURATA

GRM32

10

25

GRM55

10

50

D

MIN

V

OUT

V

F

+

V

INMAX

V

SW

–

--------------------------------------

=