VIPER012LS datasheet(17/36 Pages) STMICROELECTRONICS

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VIPer01

General description

DocID028751 Rev 1

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4.3

Primary MOSFET

The primary switch is implemented with an avalanche-rugged N-channel MOSFET with

minimum breakdown voltage 800 V, VBVDSS, and maximum on-

resistance of 30 Ω, RDS(on).

The sense-FET is embedded and it allows a virtually lossless current sensing. The

MOSFET is embedded and it allows the HV voltage start-up operation.

The MOSFET gate driver controls the gate current during both turn-on and turn-off in order

to minimize EMI. Under UVLO conditions the embedded pull-down circuit holds the gate

low in order to ensure that the MOSFET cannot be turned on accidentally.

4.4

High voltage startup

The embedded high voltage startup includes both the 800 V start-up 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 startup, 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

startup, when Cs is fully discharged, the charging current is low, ICH1, in order to avoid IC

damaging in case VCC is accidentally shorted to GND. As VCC exceeds 1 V, ICH is increased

to ICH2 in order to speed up the charging of CS.

As VCC reaches the start-up 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 one 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 VCCson (4.25 V, typ.) and the HV current source is turned on delivering the current

ICH3 until VCC is recharged to VCCon. The HV current source is reactivated when VCC decays

to VCCson 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

GIPD160720151024MT

ICH

VCC

Self -supply

External supply

from the output

from auxiliary winding

VOUT

ICH

VCC

ICH

VCC

VAux

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