AN1537 datasheet(2/18 Pages) STMICROELECTRONICS

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AN1537 APPLICATION NOTE

2/18

it is possible to adjust the thresholds in terms of input power level (PNO, PSB) by adding a DC offset on its current

sense input (pin 13, ISEN). Reference [2] provides plenty of details on this function and its usage.

There is a maximum abrupt frequency shift allowed, which is related to the amount of hysteresis: the theoretical

maximum ratio of fosc to fSB is 5.59, however this value does not account for the dynamic changes of VCOMP

during the transients resulting from the frequency shift. As a matter of fact, depending on the closed-loop char-

acteristics of the voltage control loop and on the amplitude of the load change that causes the frequency shift,

VCOMP may overshoot or undershoot before reaching its new steady-state value (see figure 1). If during a tran-

sient the other threshold is crossed, VCOMP may bounce from one threshold to the other and the switching fre-

quency be unstable, going back and forth from one value to the other. As a result, the practical limit is less than

the theoretical value, probably less than 4 and, at any rate, the control loop dynamics needs to be kept relatively

slow to limit the aptitude of VCOMP to under- or overshooting.

In [2] it is explained also that the addition of a DC offset on the current sense pin increases the maximum fosc

to fSB ratio allowed. However, this technique is suitable for allowing a higher fosc with a given fSB. The lower limit

on fSB is determined by other considerations: if it is in the audible range (< 16kHz), the transformer will very likely

generate audible noise, especially at power levels where the frequency is about to shift back to fosc, because of

the high peak current involved.

Often, instead, for a given fosc an fSB as low as possible would be required to meet the latest design targets

aimed at complying even with the most severe energy saving standards. In this case it would be desirable to

have a very low frequency under no-load conditions, where the peak current is too small to be able to generate

audible noise, and a frequency above the audible range at power levels where audible noise issues may arise.

This is exactly the purpose of the modification to the oscillator proposed in the following section.

Standby function improvement

To realize the aforementioned function, the oscillator frequency needs to be dependent on converter's load con-

ditions - the lower the load, the lower the frequency and vice versa - and only when this is useful, that is at light

load. This can be done by adding few parts to the oscillator of the L5991, as shown in figure 2.

Assuming a perfect matching of the two diodes (with a common-cathode dual diode like the BAV70 this is closer

to reality), when VCOMP falls below 3V (oscillator's peak voltage) some of the current that charges CT is diverted

to ground through RC, D1 an R'. In this way the rate of rise of the voltage across CT is slowed down and the

oscillator frequency decreased, the lower VCOMP the lower the frequency. Instead, when VCOMP is greater than

3V D1 isolates RC and the oscillator frequency will be either fosc or fSB, like in the standard L5991 oscillator cir-

cuit. RA, RB and CT can be then calculated as usual with the formulae given in [1]; as to the determination of RC

and R' please refer to the appendix.

Figure 2. Oscillator modification to improve Standby function

D2 compensates for the temperature shift of the forward voltage drop VF of D1. Considering that the current

flowing through the diodes is in the hundred µA or less, D1 and D2 dissipate negligible power and only ambient

temperature affects their VF. Assuming D1 and D2 match perfectly, neither oscillator frequency nor the point

where RC comes into play will depend on ambient temperature. In real-world operation, considering also that

D1 and D2 do not usually carry the same current, a minimum temperature effect can be observed.

L5991

Vref

16 S_BY

6COMP

RCT

additional parts

D1, D2

2 x 1N4148

1 x BAV70

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