AN1330 datasheet(18/31 Pages) STMICROELECTRONICS

18 / 31 page

Application information

AN1330

18/31

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.

Table 3 below provides a list of some tantalum capacitor manufacturers.

●

Inductor

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

output capacitor.

The ripple current is usually fixed at 20-40% of IOmax, which is 0.2 - 0.4 A with IOmax = 1 A.

The approximate inductor value is obtained using the following formula:

Equation 19

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 ∆IO = 0.3 A, the inductor value is about

35 µH.

The peak current through the inductor is given by:

Equation 20

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: Inductor selection, some inductor manufacturers are listed.

Table 3.

Output capacitor selection

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

Sanyo POSCAP (1)

POSCAP capacitors have characteristic very similar to tantalum capacitors.

TPA/B/C

100 to 470

4 to 16

40 to 80

Sprague

595D

220 to 390

4 to 20

160 to 650

Table 4.

Inductor selection

Manufacturer

Series

Inductor value (

µH)

Saturation current (A)

Coilcraft

DO1813HC

22 to 33

1 to 1.2

DO3316

33 to 47

1.6 to 2

OUT

–

()

∆

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

•

∆

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