L6599

Application information

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The timing diagram of Figure 31 shows this operation.

exceeds the start-up threshold if V(DELAY) is still higher than 0.3V. Also the PFC_STOP pin

will stay low as long as V(DELAY) is greater than 0.3V. Note also that in case there is an

close to one another.

7.5

Latched shutdown

The device is equipped with a comparator having the non-inverting input externally available

at pin 8 (DIS) and with the inverting input internally referenced to 1.85V. As the voltage on

the pin exceeds the internal threshold, the IC is immediately shut down and its consumption

reduced at a low value. The information is latched and it is necessary to let the voltage on

the Vcc pin go below the UVLO threshold to reset the latch and restart the IC.

This function is useful to implement a latched overtemperature protection very easily by

biasing the pin with a divider from an external reference voltage, where the upper resistor is

an NTC physically located close to a heating element like the MOSFET, or the secondary

diode or the transformer.

An OVP can be implemented as well, e.g. by sensing the output voltage and transferring an

overvoltage condition via an optocoupler.

7.6

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 power-on and power-off

sequencing.

L6599 shutdown upon input undervoltage is accomplished by means of an internal

comparator, as shown in the block diagram of Figure 32, whose non-inverting input is

available at pin 7 (LINE). The comparator is internally referenced to 1.25V and disables the

IC if the voltage applied on 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 1 µA current sink is ON as long as the voltage on 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, instead, fixing one threshold

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