The key point of this strategy is a low switching frequency. It is well-known that many of the power loss

sources in a lightly loaded flyback waste energy proportionally to the switching frequency, hence this

should be reduced as much as possible. On the other hand, it is equally well-known that a low switching

frequency leads to bigger and heavier magnetics and makes filtering more troublesome.

It is then desirable to make the system operate at high frequency under nominal load condition and to

reduce the frequency when the system works in a low-consumption mode. This requires a special func-

tionality of the controller: it should be able to recognize automatically the condition of light or heavy load

and should adequate its operating frequency accordingly.

The L5991 PWM controller, with its "Standby function", meets exactly this requirement. The function is

optimized for flyback topology: in fact, power supply of office equipment lies most often in the medium-

low power range, where flyback topology features the lowest cost/performance ratio and is, therefore,

the favorite one.

However, the goal of power losses minimization cannot be achieved with only a simple reduction of the

switching frequency. Although the most important, this is only one of the numerous points of a wide-

ranging strategy that must be looked into on the whole.

This application note is composed of two distinct parts. The first part deals with the L5991, describes the

operation of the "Standby function" in detail and states several relationships useful for the design. The

second one provides an overview of the points to be considered in the above mentioned strategy, as

well as a number of tips that can be helpful.

1) DESIGNING WITH THE L5991 PWM CONTROLLER

The L5991

The device, whose internal block diagram is shown in fig. 1, is based on a standard "peak" current

mode PWM controller, such as the UC384x family, with the addition of numerous ancillary features

among which Standby function is the most noticeable.

The L5991, which is available in DIP16 and SO16N packages, features the following characteristics:

Very low start-up current (75

µA typ. - 120 µA max.);

low quiescent current (7 mA typ. - 10 mA max.);

internal reference with 1% precision guaranteed (@ Tj=25°C);

high current capability, large bandwidth, high slew-rate error amplifier;

high-speed current loop (< 100 ns delay to output);

high current capability totem-pole output for MOSFET or IGBT drive;

Standby function;

IN/OUT synchronization;

precise maximum duty cycle control;

programmable soft-start

100 ns Leading Edge Blanking on current sense for increased noise immunity;

overcurrent protection with soft-start intervention;

latched disable function;

All these characteristics are described in detail in the datasheet of the device. In this context, however, it

is worth emphasizing the low current consumption of the device, both before start-up and when running.

Along with the standby function, the low consumption turns out to be particularly useful for minimizing

losses.

Table 1 compares these characteristics with the UC384XA/B family.

Table 1. L5991 vs. UC384XA/B family

CONTROLLER

START-UP CURRENT

QUIESCENT CURRENT

STANDBY FUNCTION

L5991

75

µA typ.

120

µA max.

7.0mA typ.

10.0mA max.

yes

UC384XA/B

300

µA typ.

500

µA max.

12mA typ.

17mA max.

no

AN1049 APPLICATION NOTE

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