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AN458 APPLICATION NOTE

In overload or in short circuit conditions, the switching frequency decreases below the minimum limit fixed in

inside the audible range in worst case. Too low inductance values are not suitable because the increased ripple

current in the core may generate too high ripple voltage in the output.

Example:

We want to use the L4963 in an application in the following conditions:

The right inductance for this application is calculated as follows:

Suggested value for L is in this case 40

µH that corresponds to about a 15% less the max. allowed inductance.

OUTPUT CAPACITOR SELECTION

All the considerations for the choice of the filter capacitor in a system working in continuous mode are still valid

in a discontinuous mode operation (Ref. L296 Appl.note). Let summarize the results with some useful sugges-

tion for this specific system.

(6)

Where:

(7)

(8)

Once fixed the amount of ripple voltage desired for the application, with the first term we determine the minimum

suitable capacitor value and with the second one we determine the maximum ESR acceptable.

Normally, for frequencies above 20KHz, the maximum ESR defines the choice of the filter capacitor value. In

general, lower capacitor values have higher ESR ratings, so higher output capacitors than is calculated in eq.

(7) should be used.

To guarantee a proper operation of the internal Error Amplifier, the minimum ripple voltage in the output must

exceeds 15mV, to ensure a minimum voltage difference across its input terminals.

D

max

51

+

15

1.5

–

1

+

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

0.41

==

L

15

1.5

–

5

–

() 0.41

()

⋅

2 1.5 25 10

3

–

⋅⋅

⋅

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

46

µH

=

≤

V

c

I

Lp eak

8C

out

f

⋅⋅

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

2I

Lpea k

8C

out

f

⋅⋅

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

I

out

4C

out

f

⋅⋅

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

==

=

V

ESR

I

Lp eack

ESR

2 I

out

ESR

⋅⋅

=

⋅

=