Application information

L6591

22/32

5.8

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 will

act 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 will serve as a power-on and power-off

sequencing.

L6591 shutdown upon input under voltage is accomplished by means of an internal

comparator, as shown in the block diagram of Figure 17, whose non-inverting input is

available at the LINE pin (#1). The comparator is internally referenced to 1.25 V and

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

these conditions the soft-start discharged, the pin PFC_STOP 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 15 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.

Figure 15.

Line sensing function: internal block diagram and timing diagram

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, instead, fixing one threshold

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

With reference to Figure 17, the following relationships can be established for the ON

+

-

L6591

Vin OK

HV Input bus

3V

LINE

Vcc

1

9

1.25V

15µA

HV Input bus

Vin OK

LVG, HVG

Vout

LINE

t

t

t

t

t

t

t

1.25V

15µA

Vcc