(

VMUL voltage is connected to the output voltage, in this way providing foldback current limiting. This

function provides a short circuit current lower then the normal load current, thus greatly reducing the

stress on the power components in case of failure.

The diode D3 avoids latchoff, that is prevents the output current from dropping to zero during short cir-

cuits and staying at zero even when the short circuit is removed. Furthermore, at start-up, there is a soft-

start action because of the limitation of the current inflow.

The regulator will exhibit a better efficiency over its mag amp counterpart: the dissipation on the MOS-

FET is quite limited (about 1W with the parts suggested in Tab.1), while the mag amp dissipates on the

copper and the core of the reactor, as well as some hundreds mW on the reset PNP transistor.

The "peak" current mode control performed by the circuit offers many more benefits: excellent load regu-

lation from very low output current up to the maximum load, fast response to step-load changes, good

line regulation due to the inherent input voltage feedforward and one-pole transfer function of the output

stage, which simplifies frequency compensation.

Additionally, since the L6561 can be disabled by grounding the ZCD pin, the entire regulator can be

either enabled or shut down with a logic signal. Besides, being the L6561 on the secondary side, the in-

terface needed to feed the enable/disable signal will be extremely simple, like the one shown in the

schematic of fig. 4.

This disable feature achieves a real zero-power shutdown. In fact, when the regulator is disabled, Q1 is

kept off and the secondary winding is an open circuit . The only power consumption is a couple of mA

from the supply pin of the L6561.

This functionality turns out to be extremely useful in those systems were power management is required,

as stated in Intel’s Power Supply ’98 specifications, for example.

Conclusions

The replacement of a mag amp-based post regulator with an L6561-based one for the generation of a

given auxiliary power rail allows to save numerous external parts among which the bulky and expensive

mag amp core. Such a regulator will benefit a dramatic cost reduction from the lower number and cost of

the parts used, from the shrinkage of the PCB area needed to accommodate the circuit and from a cut of

the design time.

In spite of that, the result is a much more performant system which takes advantage of all the benefits

inherent in switching regulation with current mode control as well as of the functionalities available from

the versatile L6561.

REFERENCE

[1] "L6561, Enhanced Transition Mode Power Factor Corrector" (AN966)

AN1007 APPLICATION NOTE

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