A5975AD datasheet(25/50 Pages) STMICROELECTRONICS

25 / 50 page

A5975AD

Application information

Doc ID 018761 Rev 1

25/50

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 7.

Inductor

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

capacitor. The ripple current is usually fixed at 20 - 40% of IOmax, which is 0.6 - 1.2 A with

IOmax = 3 A. The approximate inductor value is obtained using the following formula:

Equation 20

where TON is the ON time of the internal switch, given by D · T. For example, with

VOUT = 3.3 V, VIN = 12 V and ΔIO = 0.9 A, the inductor value is about 12 µH. The peak

current through the inductor is given by:

Table 6.

List of ceramic capacitors for the A5975AD

Manufacturer

Series

Capacitor value (µ)

Rated voltage (V)

TAIYO YUDEN

UMK325BJ106MM-T

MURATA

GRM42-2 X7R 475K 50

4.7

Table 7.

Output capacitor selection

Manufacturer

Series

Cap value (µF)

Rated voltage (V)

ESR (m

Ω)

Sanyo POSCAP(1)

POSCAP capacitors have some characteristics which are very similar to tantalum.

TAE

47 to 680

2.5 to 10

25 to 35

68 to 330

4 to 6.3

25 to 40

AVX

TPS

100 to 470

4 to 35

50 to 200

KEMET

T494/5

100 to 470

4 to 20

30 to 200

Sprague

595D

220 to 390

4 to 20

160 to 650

OUT

–

()

ΔI

---------------------------------- T

⋅

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