VIPER319LDTR datasheet(14/40 Pages) STMICROELECTRONICS

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Figure 24. Typical deliverable output power vs. TAMB (Vin: 85-265VAC)

4.3

Primary MOSFET

The primary switch is implemented with an avalanche rugged N-channel MOSFET with 800 V minimum

breakdown voltage, VBVDSS, and 3.5 Ω maximum on-resistance, RDS(on).

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

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

mode 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 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 pin. A minor portion is sunk by the controller block.

Power on: at startup, as the voltage across the DRAIN pin exceeds the VHV_START threshold, the HV current

source is turned on, charging linearly the VCC capacitor with the current ICH2 (8.8 mA typ.). This charging current

is reduced to ICH1 (1 mA typ.) in case VCC is lower than 1 V (typical value), in order to limit IC power dissipation in

case the pin is accidentally shorted to GND.

As VCC reaches the startup threshold, VCCon, 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 VCC capacitor. The IC

can be supplied through a transformer auxiliary winding or, in case of not isolated topologies with VOUT ≥ 5 V,

directly from the converter’s output.

The supply operating range is from 4.5 V to 30.5 V. If VCC pin voltage exceeds 30.5 V (referred to VGND), the

internal clamp could be reached, which causes the VIPER31 to stop switching.

This condition is potentially dangerous for the VIPER31 and must be avoided, by means of proper transformer

design and/or external protection (zener diode between VCC and GND pins).

In normal operation the HV current source is always kept off by maintaining VCC above VCson, so its residual

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

Equation 1

(1)

PHV(VIN) = VIN2/RG

VIPER31

Primary MOSFET

DS13285 - Rev 3

page 14/40

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