L6699 datasheet(30/38 Pages) STMICROELECTRONICS

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Line sensing function

L6699

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Doc ID 022835 Rev 2

Line sensing function

This function basically stops the IC as the input voltage to the converter falls below the

specified range and lets it restart as the voltage goes back within the range. The sensed

voltage can be either the rectified and filtered mains voltage, in which case the function acts

as a brownout protection, or, in systems with a PFC pre-regulator front-end, the output

voltage of the PFC stage, in which case the function serves as a power-on and power-off

sequencing.

L6699 shutdown upon input undervoltage is accomplished by means of an internal

comparator, as shown in the block diagram of

Figure 22, whose non-inverting input is

available at pin 7 (LINE). The comparator is internally referenced to 1.22 V and disables the

IC if the voltage applied at the LINE pin is below the internal reference. Under these

conditions the soft-start is discharged, the PFC_STOP pin is open and the consumption of

the IC is reduced. PWM operation is re-enabled as the voltage on the pin is above the

reference. The comparator is provided with current hysteresis instead of a more usual

voltage hysteresis: an internal 13 µA current sink is ON as long as the voltage applied at the

LINE pin is below the reference and is OFF if the voltage is above the reference.

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 voltage hysteresis, on the other hand, fixing one threshold

automatically fixes the other one depending on the built-in hysteresis of the comparator.

With reference to

Figure 22, the following relationships can be established for the ON

(VinON) and OFF (VinOFF) thresholds of the input voltage:

Equation 12

which, solved for RH and RL, yields:

Equation 13

OFF

Vin

;

Vin

;

Vin

OFF

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