L6566ATR datasheet(39/51 Pages) STMICROELECTRONICS

39 / 51 page

L6566A

Application information

39/51

While the brownout protection is active the start-up generator keeps on working but, being

there no PWM activity, the Vcc voltage continuously oscillates between the start-up and the

HV generator restart thresholds, as shown in the timing diagram of Figure 25 on page 38.

The brownout comparator is provided with current hysteresis in addition to voltage

hysteresis: an internal 15 µA current sink is ON as long as the voltage applied on the

AC_OK pin is such that the AC_FAIL signal is high. This approach provides an additional

degree of freedom: it is possible to set the ON threshold and the OFF threshold separately

by properly choosing the resistors of the external divider (see below). With just voltage

hysteresis, instead, fixing one threshold automatically fixes the other one depending on the

built-in hysteresis of the comparator.

With reference to Figure 25 on page 38, the following relationships can be established for

the ON (VsenON) and OFF (VsenOFF) thresholds of the sensed voltage:

Equation 13

which, solved for RH and RL, yield:

Equation 14

It is typically convenient not to use additional dividers connected to high-voltage rails

because this could make it difficult to meet no-load consumption targets envisaged by

energy-saving regulations. Figure 26 shows a simple voltage sensing technique that makes

use of the divider already used by the PFC control chip to sense the ac mains voltage with

just the addition of an extra tap.

The small-signal NPN Q and the capacitor CF make a peak detector, so that the information

of the rms mains voltage can be found across CF. Tap’s position determines the dc voltage

to be sensed by the AC_OK pin. It is convenient to use a level as high as possible to

minimize the effect of VBE changes with temperature. However, it could be necessary to limit

the maximum sensed voltage below 7V to prevent Q’s emitter reverse breakdown; it would

not be destructive because the reverse current would be quite small (the resistors seen by

Figure 26. Ac voltage sensing with the L6566A

OFF

Vsen

485

Vsen

−

⋅

−

Vsen

;

Vsen

078

Vsen

OFF

−

⋅

−

L6566A

Rectified

input voltage

Sensed

voltage:

Vsen < 7V

MULT

L6561

L6562/A

L6563

Vcc

AC_OK

For minimum

temperature drift

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