HVLED807PF

Device description

Doc ID 023464 Rev 5

17/36

4.3

High voltage startup generator

Figure 12 shows the internal schematic of the high-voltage start-up generator (HV

generator). It includes an 800 V-rated N-channel MOSFET, whose gate is biased through

the series of a 12 M

Ω resistor and a 14 V Zener diode, with a controlled, temperature

compensated current generator connected to its source.

The HV generator input is in common with the DRAIN pins, while its output is the supply pin

of the device (VCC pin). A mains "UVLO" circuit (separated from the UVLO of the device

that sense VCC) keeps the HV generator off if the drain voltage is below VSTART (50 V

typical value).

Figure 12.

High-voltage start-up generator: internal schematic

With reference to the timing diagram of

Figure 13, when power is applied to the circuit and

the voltage on the input bulk capacitor is high enough, the HV generator is sufficiently

Most of this current will charge the bypass capacitor connected between the VCC pin and

ground and make its voltage rise linearly. As soon as the VCC pin voltage reaches the

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

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 constant current (CC) regulation, when

the flyback voltage generated by the auxiliary winding alone may not be able to keep VCC

AM13562v1

VCC

Icharge

CONTROL

Mains UVLO

IHV

DRAIN

12M

HV_EN

Vcc_OK

14V

GND