VIPER31SP datasheet(13/16 Pages) STMICROELECTRONICS

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this value to provide a soft start feature, of which

the duration depends on some circuit parameters,

like transformer ratio, sense resistor, output

capacitors and load. The user will define the best

appropriate value by experiments.

SHORT CIRCUIT OPERATION

In case of abnormal condition where the auxiliary

winding is unable to provide the low voltage

supply current to the VCC pin (i.e. short circuit on

the

output

the

converter), the

external

capacitors discharge themselves down to the low

threshold voltage VDDoff of the UVLO logic, and

the device get back to the inactive state where the

internal circuits are in standby mode and the start

up current source is activated. The converter

enters a endless start up cycle, with a start-up

duty cycle defined by the ratio of charging current

towards discharging when the VIPer31 tries to

start. This ratio is fixed by design to 1.5 to 12,

which gives a 11% start up duty cycle, while the

power dissipation at start up is approximately 0.6

W, for a 230 Vrms input voltage.

The average output short circuit current is the

product of the start up duty cycle by the output

current flowing during the active phase of the

device (See figure 14). This output current is

limited by either the CREF pin voltage, or the

internal current limitation of 1.3 A. These values

together with the low value of start-up duty cycle

prevents the stress of the output rectifiers and of

the transformer when in short circuit.

Figure 13: Start Up Circuit and Sequence

AUXILIARY

WINDING

VCC

VDD

VCC

VDD

DRAIN

SOURCE

GND

VIPer31

UVLO LOGIC

Ref

ON/OFF

LDO

Reg.

2mA

VDD

VCC

(V)

VDDreg

VDDon

VDDoff

tss

SC12150

VDD

Iout

Isc

VDDoff

VDDon

Average

output current

SC12160

Figure 14 : Short circuit operation

VIPer31SP

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