STMicroelectronics HVLED805 Evaluation Board EVALHVLED805 EVALHVLED805 Fiche De Données

14/29

Doc ID 18077 Rev 1

Most of this current will charge the bypass capacitor connected between the Vcc pin and 
ground and make its voltage rise linearly.

As the Vcc voltage reaches the start-up threshold (13 V typ.) the chip starts operating, the 
internal power MOSFET is enabled to switch and the HV generator is cut off by the Vcc_OK 
signal asserted high. The IC is powered by the energy stored in the Vcc capacitor.

The chip is able to power itself directly from the rectified mains: when the voltage on the V

CC

 

pin falls below Vcc

restart

 (10.5V typ.)

, during each MOSFET’s off-time the HV current 

generator is turned on and charges the supply capacitor until it reaches the V

CCOn

 

threshold.

In this way, the self-supply circuit develops a voltage high enough to sustain the operation of 
the device. This feature is useful especially during CC regulation, when the flyback voltage 
generated by the auxiliary winding alone may not be able to keep Vcc above V

CCrestart

.

At converter power-down the system will lose regulation as soon as the input voltage falls 
below V

Start

. This prevents converter’s restart attempts and ensures monotonic output 

voltage decay at system power-down.

Vcc

ON

Vcc

restart

t

tt

t

V

Start

5.5 mA

t

t

Power-on 

Power-off 

Normal operation

CV mode

CC mode

Normal operation

Vcc

ON

Vcc

restart

t

tt

t

V

Start

5.5 mA

t

t

Power-on 

Power-off 

Normal operation

CV mode

CC mode

Normal operation