Application information
L6599A
The timing diagram of Figure 29 shows this operation. Note that, if, during TSTOP, the supply
voltage of the L6599A (Vcc) falls below the UVLO threshold, the IC records the event and
does not restart immediately after Vcc exceeds the startup threshold if V(DELAY) is still
higher than 0.3 V. Also the PFC_STOP pin stays low as long as V(DELAY) is greater than
0.3 V. Note also that, in the case of an overload lasting less than TSH, the value of TSH for
the next overload is lower if they are close to one another.
7.5
Latched shutdown
The L6599A 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
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 (e.g. pin 4, RFmin), where
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.
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 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.
L6599A shutdown upon input undervoltage is accomplished by means of an internal
comparator, as shown in the block diagram of Figure 30, 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.
26/35
Doc ID 15308 Rev 7
STMICROELECTRONICS [ ST ]
