STMicroelectronics Double output SMPS for power line application using ALTAIR04-900 primary controller EVLALTAIR900-M1 EVLALTAIR900-M1 Hoja De Datos

20/29

Doc ID 18211 Rev 2

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 the R

SENSE

 and activates a protection circuitry if this voltage 

exceeds 1 V.

To distinguish an actual malfunction from a disturbance (e.g. induced during ESD tests), the 
first time the comparator is tripped the protection circuit enters a “warning state”. If in the 
subsequent switching cycle the comparator is not tripped, a temporary disturbance is 
assumed and the protection logic will be reset in its idle state; if the comparator is tripped 
again a real malfunction is assumed and the device is stopped.

This condition is latched as long as the device is supplied. While it is disabled, however, no 
energy is coming from the self-supply circuit; hence the voltage on the V

CC

 capacitor decays 

and cross the UVLO threshold after some time, which clears the latch. The internal start-up 
generator is still off, then the V

CC

 voltage still needs to go below its restart voltage before the 

V

CC

 capacitor is charged again and the device restarted. Ultimately, 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 

 

Vcc

ON

Vcc

OF F

Vcc

rest

Secondary diode is shorted here

t

t

t

1 V

Two switching cycles

Vcs

dis