ALTAIR04-900TR Ver la hoja de datos (PDF) - STMicroelectronics

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Fabricante

ALTAIR04-900TR

Off-line all-primary-sensing switching regulator

STMicroelectronics 

Application information

ALTAIR04-900

5.9

Hiccup-mode OCP

The device is also protected against short-circuit of the secondary rectifier, short-circuit on

the secondary winding or a hard-saturated flyback transformer. A comparator monitors

continuously the voltage on RSENSE and activates a protection circuitry if this voltage

exceeds 1 V.

To distinguish a malfunction from a disturbance (induced during ESD tests), the first time the

comparator is tripped, the protection circuit enters a “warning state”. If in the following

switching cycle the comparator is not tripped, a temporary disturbance is assumed and the

protection logic is reset in its idle state; if the comparator is tripped again a real malfunction

is assumed and the device stops.

This condition is latched as long as the device is supplied. Any energy comes from the self-

supply circuit; hence the voltage on the Vcc capacitor decays and crosses the UVLO

threshold after some time, which clears the latch. The internal start-up generator is still off,

then the Vcc voltage still needs to go below its restart voltage before the Vcc capacitor is

charged again and the device restarted. Finally, this results in a low-frequency intermittent

operation (hiccup-mode operation), with very low stress on the power circuit. This special

condition is illustrated in the timing diagram of Figure 18.

Figure 18. Hiccup-mode OCP: timing diagram

Secondary diode is shorted here

VCC

VccON

VccOF F

Vc cr e st

VSOU RCE

t

Vcs d is

1V

t

VDS

Two switching cycles

t

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