AN557 datasheet(39/52 Pages) STMICROELECTRONICS

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AN557 APPLICATION NOTE

Figure 50. Waveforms for L = 50

µH

Figure 51. Waveforms for L = 230

µH

Figure 52. Waveforms in case of core satura-

tion

Anyway some general rules should be observed in

order to avoid any "poor functioning". These rules

include:

a) the catch diode, that further to be the suggested

type in the test circuit, it has to be assembled on

the printed circuit very close to the output of the

regulator, in order to minimize the leakage in-

ductance and avoid over voltage due to the long

connection:

b) The inductor, avoiding the saturation at the

maximum current guaranted by the current

limitation of the device.

If oscillations on the output voltage at low ambient

temperature (i.e.: below 0°C) are originated an

output low ESR capacitor has to be used. Oscilla-

tions on the output al low frequency indicate insta-

bility of the control loop; in this case a changing of

the network compensation is suggested (see Error

Amplifier section).

LOW COST APPLICATION

The fig. 53 shows the low cost application of a

power supply of 10A and 5.1V.

In comparison of the complete application (and

this is valid for all the devices of the family) the ex-

ternal components relative to the reset and power

fail functions can be missed.

When a lower output voltage ripple is not required

it is possible to eliminate the capacitors connected

at the reference voltage pin of 5.1V (i.e.: pin 14 for

Multiwatt package. pin 13 for plastic package) The

reset input pin is suggested to connect it to

ground.

The soft start capacitor value can be reduced to

100nF for 5V output voltage.

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