AN458 APPLICATION NOTE

16/19

For a Quick calculation of some standard output voltages, the following table is useful:

2. INDUCTOR SELECTION:

The max. duty cycle is determined by the following formula:

The maximum inductor value is then calculated:

Where: f(min) 20KHz to be out of the audible range.

to verify that the coil will not saturate in overload or short circuit conditions damaging the output power stage

due to the high dI/dt ratio.

Therefore, a correct dimensioning requires a saturation current above the maximum current limit threshold

3. Output Capacitor Selection:

The output voltage ripple depends on the current ripple in the inductor and on the performance of the output

capacitor at the switching frequency .The minimum value of the output filter capacitor is obtained from:

Clearly this formula doesn’t take care of the capacitor Equivalent Series Resistance (ESR) value, that is the

dominant factor to define the output ripple voltage at switching frequencies greater than 20KHz.

So we suggest to use also the following formula:

Where the ESR(max) requirement is not satisfied by the capacitor value given by the first formula, use an higher

Resistor Values for Standard Output Voltages

R6

R5

12

4.7K

Ω

6.2K

Ω

15

4.7K

Ω

9.1K

Ω

18

4.7K

Ω

12K

Ω

24

4.7K

Ω

18K

Ω

D

max

V

out

V

F

+

V

in min

()

V

ce sa t

()

–

V

F

+

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

=

Lmax

V

in m i n

()

V

ce sat

()

–

V

out

–

2I

out m ax

()

⋅⋅

----------------------------------------------------------------- Dmax

⋅

=

C

out

I

out m ax

()

4V

ri ppl e p

p

–

⋅⋅

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

=

ESR max

()

v

ri ppl e p

p

–

()

2I

out m ax

()

⋅

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

=