STMicroelectronics HVLED805 Evaluation Board EVALHVLED805 EVALHVLED805 Data Sheet

Doc ID 18077 Rev 1

23/29

The soft start feature is automatically implemented by the constant current block, as the 
primary peak current will be limited from the voltage on the C

LED

 capacitor.

During start-up, as the output voltage is zero, the IC will start in CC mode with no high peak 
current operations. In this way the voltage on the output capacitor will increase slowly and 
the soft-start feature will be ensured. 

Actually the C

LED

 value is not important to define the soft-start time, as its duration depends 

on others circuit parameters, like transformer ratio, sense resistor, output capacitors and 
load. The user will define the best appropriate value by experiments.

The device is also protected against short circuit of the secondary rectifier, short circuit on 
the secondary winding or a hard-saturated flyback transformer. A comparator monitors 
continuously the voltage on the R

SENSE

 and activates a protection circuitry if this voltage 

exceeds 1 V.

To distinguish an actual malfunction from a disturbance (e.g. induced during ESD tests), the 
first time the comparator is tripped the protection circuit enters a “warning state”. If in the 
subsequent switching cycle the comparator is not tripped, a temporary disturbance is 
assumed and the protection logic will be reset in its idle state; if the comparator will be 
tripped again a real malfunction is assumed and the device will be stopped.

This condition is latched as long as the device is supplied. While it is disabled, however, no 
energy is coming from the self-supply circuit; hence the voltage on the V

CC

 capacitor will 

decay and cross the UVLO threshold after some time, which clears the latch. The internal 
start-up generator is still off, then the V

CC

 voltage still needs to go below its restart voltage 

         

65 mV

hyster.

Normal-mode

Burst-mode

Normal-mode

T

START

T

START

T

START

T

START

V

COMPL