AN458 APPLICATION NOTE

4/19

The system is working in a variable frequency mode, with a switching frequency that is depending on the current

delivered to the load. Bigger is the load current higher is the stored energy and longer is the time during which

the comparator will block the discharge of the timing capacitor Ct, decreasing the system operating frequency.

In fig.4 are shown the waveforms associated with this mode of operation.

CALCULATION OF THE INDUCTANCE VALUE, L

To calculate the inductance, that is a critical element in the circuit, we have to consider that:

s

s

So to calculate the inductor value we have to

Figure 4. Self-oscillating Operation

specify the minimum operation frequency (higher than 20KHz to avoid audible noise), for the minimum input

where:

L

V

in min

()

V

ces at

–

V

o

–

() D

⋅

2I

out m ax

⋅⋅

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

≤

D

V

o

V

F

+

V

in m in

()

V

ce sa

V

F

+

–

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

=