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L6599AF

Application information

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higher than 0.3 V. Also the PFC_STOP pin stays low as long as V(DELAY) is greater than

the next overload is lower if they are close to one another.

6.5

Latched shutdown

The L6599AF is equipped with a comparator having the non-inverting input externally

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

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

consumption reduced to a low value. The information is latched and it is necessary to let the

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

the upper resistor is an NTC physically located close to a heating element like the MOSFET,

or the secondary diode or transformer.

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

overvoltage condition via an optocoupler.

6.6

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 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.

L6599AF shutdown upon input undervoltage is accomplished by means of an internal

comparator, as shown in the block diagram of Figure 13, whose non-inverting input is

available at pin 7 (LINE). The comparator is internally referenced to 1.24 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, instead, fixing one threshold

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