An auxiliary winding could be used also on the choke of the +5.1V section, either to power the +12V lin-

ear regulator or to derive a further supplemental output, however the 3.3 V section has been provided

with some features aimed at ensuring a proper operation under all circumstances.

For a correct operation of the regulator, the voltage at pin V13IN must not be too low. The flyback con-

nection of the two windings ensures a well regulated voltage, provided there is good magnetic coupling.

The coupled inductors configuration, however, is not able to sustain the auxiliary voltage if the main out-

put is lightly loaded: the secondary voltage drops and the system goes out of regulation.

To overcome this problem, when the V13IN voltage falls below a certain threshold (13.7 V +/- 5%) be-

cause of too light a load on the 3.3V section, the relevant synchronous rectifier is turned on for 1.5

µs

max. during the interval in which the inductor current is zero ("one-shot" feature, see fig. 6). In this way,

the inductor current reverses and draws from the output capacitor energy which is forward transferred to

the auxiliary output.

In case the 3.3V section is working at full load and the linear regulator is lightly loaded, the voltage at pin

V13IN can exceed the expected value. In fact, Ds and Cs act as a peak-holding circuit and V13IN is in-

fluenced by the voltage spikes at switching transients. An internal clamp limits the voltage but, in case of

intervention, the chip power dissipation will rise.

When the 3.3V regulator is disabled, the linear regulator is disabled as well and is placed in a low-power

mode to reduce device consumption.

DETAILED FUNCTIONAL DESCRIPTION (continued)

1

n

to V13IN

3.3V

D97IN575

Figure 5: 12V regulator supply with auxiliary winding.

t

t

t

H3GATE

L3GATE

IL

D97IN576A

1.5

µs

t

V13IN

13.7V

Figure 6: "One shot" feature to sustain V13IN voltage.

L4992

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