Datasheet

AN2994 Test results and significant waveforms
Doc ID 15796 Rev 2 19/38
2.5 Brownout function
A dangerous event for any PFC is the operation during a mains undervoltage. This condition
may cause overheating of the power section due to an excess of RMS current. To protect
the PFC from this abnormal operation, a brownout protection is needed. It is basically a non-
latched shutdown function that has to be activated when a mains undervoltage condition is
detected.
The L6563S has a dedicated inhibit pin (RUN, #10) that embeds a comparator with
hysteresis and which stops the L6563S operation if the voltage applied is below its
threshold. Because the L6563S V
FF
pin delivers a voltage signal proportional to the input
mains, the complete brownout function can be easily realized using a simple resistor divider
connected between the V
FF
and RUN pins. In this demonstration board, the divider resistors
setting the turn-on threshold are R26 and R27 on the schematic in Figure 1. Figure 29
shows a start-up tentative below the threshold. As visible at start-up, the RUN pin does not
allow the PFC start-up.
Figure 30 and Figure 31 show the waveforms of the circuit during operation of the brownout
protection. In both cases, the mains voltages were increased or decreased slowly. As you
can see, at both turn-on and turn-off, there are no bouncing or starting attempts by the
converter.
Figure 29. EVL6563S-400W start-up attempt at 80 Vac, 60 Hz, full load
CH1: Output voltage
CH2: Pin #14 - Vcc voltage
CH3: Pin #10 - RUN
CH4: Pin #13 - Gate drive