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AN458 APPLICATION NOTE

value or, better, put in parallel several capacitors in order to reduce the total ESR. The capacitors’ voltage rating

should be at least 1.25 times greater than the given output voltage.

The big advantage of this system is to greatly reduce number of external components compared to the contin-

uous mode solution. The only drawback is a higher ripple voltage on the output that can be up three times larger

than in continuous mode.

A proper choice of low ESR filtering capacitors can solve this problem greatly reducing the output ripple.

4. CATCH DIODE SELECTION:

The catch diode must comply with several requirements and its choice requires special care. The current rating

must be at least 1.2 times greater respect the maximum load current, but this is not enough because in short

circuit conditions, the maximum current limiter threshold is 6A to which correspond an average output current

The reverse voltage rating of the diode should be at least 1.25 times the maximum input voltage. The diode

recovery speed is not so important because the power stage is turned-on only when the inductor is fully dis-

charged and the diode is definitely off. So there is not simultaneous conduction between them.

This allows a reduction of the disturbances with respect to the continuous mode, because they are mainly radi-

ated during the transistor switch on, for the steep slopes during the simultaneous conduction of transistor and

reverse conductiondiode.

LOW COST APPLICATION

If the remote inhibit, the reset and the power fail functions are not used we can reduce further the xternal com-

ponent count. It is possible in this case, to have a very efficientswitch mode power supply for very low cost ap-

plications.

Two examples of minimal component count regulators are shown in fig.21 and fig.22.

Figure 21. A Minimal 5.1 Fixed Regulator – Very Few Components are Required