STMicroelectronics EVAL BOARD FOR L6563 EVL6563H-250W EVL6563H-250W Data Sheet

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Doc ID 16047 Rev 3

As the Vcc voltage reaches the start-up threshold (12 V typ.) the low-voltage chip starts 
operating and the HV generator is cut off by the Vcc_OK signal asserted high. The device is 
powered by the energy stored in the Vcc capacitor until the self-supply circuit (we assume 
that it is made with an auxiliary winding in the transformer of the cascaded dc-dc converter 
and a steering diode) develops a voltage high enough to sustain the operation. The residual 
consumption of this circuit is just the one on the 15 M

Ω resistor (10 mW at 400 Vdc), 

typically 50-70 times lower, under the same conditions, as compared to a standard start-up 
circuit made with external dropping resistors.

At converter power-down the dc-dc converter loses regulation as soon as the input voltage 
is so low that either peak current or maximum duty cycle limitation is tripped. Vcc then drops 
and stops IC activity as it falls below the UVLO threshold (9.5 V typ.). The Vcc_OK signal is 
de-asserted as the Vcc voltage goes below a threshold VCCrestart located at about 6 V. 
The HV generator can now restart. However, if Vin < VHVstart, HV_EN is de-asserted too 
and the HV generator is disabled. This prevents converter's restart attempts and ensures 
monotonic output voltage decay at power-down in systems where brownout protection (see 
the relevant section) is not used.

If the device detects a fault due to feedback failure the pin PWM_LATCH is asserted high 
(see “Feedback failure protection” section for more details) and, in order to maintain alive 
this signal to be provided to the DC-DC converter, the internal VCCrestart is brought up to 
over the VccOff (Turn-off threshold). As a result, shown in 

, the voltage at pin Vcc, 

oscillates between its turn-on and turn-off thresholds until the HV bus is recycled and drops 
below the start up threshold of the HV generator. 

The High Voltage Start-up circuitry is capable to guarantee a safe behavior in case of short 
circuit present on the dc-dc output when the Vcc of both controllers are generated by the 
same auxiliary winding. The 

 shows how the PFC manages the Vcc cycling and 

the associated power transfer. At short circuit the auxiliary circuit is no longer able to sustain 
the Vcc which start dropping; reaching its VccOFF threshold the IC stops switching, reduces 

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