L6599
Application information
The timing diagram of Figure 31 shows this operation.
Note that if during T the supply voltage of the L6599 (Vcc) falls below the UVLO
STOP
threshold the IC keeps memory of the event and will not restart immediately after V
CC
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
overload lasting less than T , the value of T for the next overload will be lower if they are
SH
SH
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.
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STMICROELECTRONICS [ ST ]
