1/18

AN1537

APPLICATION NOTE

May 2003

Introduction

L5991's Standby function is a valuable help in reducing light-load input consumption of offline converters and

making them compliant with energy saving standards such as EnergyStar, Energy2000 and others. This func-

tion, optimized for flyback topology, is the ability of automatically - and abruptly - reducing the oscillator frequen-

cy (i.e. converter's switching frequency) as the converter's load falls below a defined threshold and restoring the

normal oscillator frequency as the load increases and exceeds a second threshold.

The frequency shift allows minimizing power losses related to switching frequency, which represent most of

losses at light or no load, without giving up the advantages of a higher switching frequency at full load.

except for an offset, is proportional to the peak primary current and then to the energy handled by the transform-

123

4

Pin

Normal operation

Standby

Normal operation

Standby

Undershoot

during transition

Overshoot

during transition

1

234

fsw

by C. Adragna and G. Gattavari

A SIMPLE TRICK ENHANCES

L5991’s STANDBY FUNCTION

This application notes describes a simple technique that allows improving the Standby function of the

advanced PWM controller L5991. The price to pay for that is the addition of just two resistors and two

diodes, but the benefit brought in terms of no-load consumption in mains-operated converters is worth

this small fee. The effectiveness of the improved Standby function will be proved and assessed on a

couple of existing designs.