VIPER0PHD datasheet(17/36 Pages) STMICROELECTRONICS

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VIPer0P

General description

DocID028423 Rev 2

17/36

4.4

High voltage startup

The embedded high voltage startup includes both the 800 V startup FET, whose gate is

biased through the resistor RG, and the switchable HV current source, delivering the current

IHV. The major portion of IHV, (ICH), charges the capacitor connected to VCC. A minor

portion is sunk by the controller block.

At start up, as the voltage across the DRAIN pin exceeds the VHV_START threshold, the HV

current source is turned on, charging linearly the Cs capacitor. At the very beginning of the

start-up, when Cs is fully discharged, the charging current is low (ICH1 = 1 mA typ.) in order

to avoid IC damaging in case VCC is accidentally shorted to SGND. As VCC exceeds 1 V,

ICH is increased to ICH2 (3.2 mA, typ.) in order to speed up the charging of CS.

As VCC reaches the startup threshold VCCon (8 V typ.) the chip starts operating, the primary

MOSFET is enabled to switch, the HV current source is disabled and the device is powered

by the energy stored in the CS capacitor.

In steady-state the IC supports two different kind of supplies: self-supply and external

supply, as shown in Figure 25: "IC supply modes: self-supply and external supply".

Figure 25: IC supply modes: self-supply and external supply

In self-supply only a capacitor CS is connected to the VCC and the device is supplied by

the energy stored in CS. After the IC startup, due to its internal consumption, the VCC

decays to VCSon (4.25 V, typ.) and the HV current source is turned on delivering the current

ICH3 (7.8 mA typ.) until VCC is recharged to VCCon. The HV current source is reactivated

when VCC decays to VCSon again. The ICH3 is supplied during the switching OFF time only.

In external supply the HV current source is always kept off by maintaining the VCC above

VCSon. This can be obtained through a transformer auxiliary winding or a connection from

the output, the latter only in case of non-isolated topology. In this case the residual

consumption is given by the power dissipated on RG, calculated as follows:

GIPD160720151024MT

ICH

VCC

Self -supply

External supply

from the output

from auxiliary winding

VOUT

ICH

VCC

ICH

VCC

VAux

INDC

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