AN2603 datasheet(11/23 Pages) STMICROELECTRONICS

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AN2603

Selecting components for the application

Doc ID 13778 Rev 3

11/23

2.2

Output capacitor

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

a small inductor value is useful to reduce the size of the coil, but this increases the current

ripple. Therefore, to reduce the output voltage ripple a low ESR capacitor is required. The

output voltage ripple (VOUT_RIPPLE), in continuous mode, is:

Equation 7

where

ΔI is the ripple current and F

SW is the switching frequency.

2.3

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 Iout_max, which is 0.4-0.8 A

with Iout_max = 2 A. The approximate inductor value is obtained using the following formula:

Equation 8

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

For example, with VOUT = 3.3 V, VIN = 5 V and ΔIOUT = 0.45 A, the inductor value is around

2.8 µH. The peak current thought the inductor is given by:

Equation 9

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, for fixed peak current protection, a

higher value of the inductor permits a higher value for the output current.

OUT1RIPPLE

ΔIERS

OUT

⋅⋅

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

⎝⎠

⎛⎞

⋅

OUT

–

ΔI

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

⋅

OUT

ΔI

-----

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