ALTAIR04-900 Datasheet, PDF (20/29 Pages) STMicroelectronics – Off-line all-primary-sensing switching regulator

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 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 Figure 18.
Figure 18. Hiccup-mode OCP: timing diagram
Secondary diode is shorted here
VCC
Vc cON
VccOF F
Vc cr e st
VSOU RCE
t
Vcsdis
1V
t
VDS
Two switching cycles
t
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Doc ID 18211 Rev 2