ALTAIR05T-800 Datasheet, PDF(21/28 Page) STMicroelectronics – Off-line all-primary-sensing switching regulator

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ALTAIR05T-800 Datasheet, PDF (21/28 Pages) STMicroelectronics – Off-line all-primary-sensing switching regulator

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ALTAIR05T-800

Application information

5.10

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 RSENSE 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 âwarning stateâ. If in the

subsequent 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 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 VCC capacitor decays

and cross 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. 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 19.

Figure 19. Hiccup-mode OCP: timing diagram

VCC

Secondary diode is shorted here

VccON

VccOFF

Vccrest

VS OURC E

Vcsdis

VDS

Two switching cycles

5.11

Layout recommendations

A proper printed circuit board layout is essential for correct operation of any switch-mode

converter and this is true for the ALTAIR05T-800 as well. Careful component placing, correct

traces routing, appropriate traces widths and compliance with isolation distances are the

major issues. In particular:

â The compensation network should be connected as close as possible to the COMP

pin, maintaining the trace for the GND as short as possible

â Signal Ground should be routed separately from power ground, as well from the sense

resistor trace.

Doc ID 17957 Rev 1

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