L6566B datasheet(38/51 Pages) STMICROELECTRONICS

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Application information

L6566B

38/51

Figure 25.

Brownout protection: internal block diagram and timing diagram

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, the following relationships can be established for the ON

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

Equation 16

which, solved for RH and RL, yield:

Equation 17

Sensed voltage

Vcc

(pin 5)

VsenON

VsenOFF

(pin 4)

Vout

VAC_OK

(pin 16)

IHYS

15 µA

AC_FAIL

0.45V

0.485V

L6566B

AC_FAIL

AC_OK

Vcc

0.485V

0.45V

15 µA

Sensed

voltage

OFF

Vsen

485

Vsen

−

⋅

−

Vsen

;

Vsen

078

Vsen

OFF

−

⋅

−

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