VIPER27 datasheet(25/31 Pages) STMICROELECTRONICS

25 / 31 page

VIPER27

Operation descriptions

Doc ID 15133 Rev 5

25/31

7.13

2nd level over current protection and hiccup mode

The VIPER27 is protected against short circuit of the secondary rectifier, short circuit on the

secondary winding or a hard-saturation of fly-back transformer. Such as anomalous

condition is invoked when the drain current exceed the threshold IDMAX, see Table 8 on

page 7.

To distinguish a real malfunction from a disturbance (e.g. induced during ESD tests) a

“warning state” is entered after the first signal trip. If in the subsequent switching cycle the

signal is not tripped, a temporary disturbance is assumed and the protection logic will be

reset in its idle state; otherwise if the IDMAX threshold is exceeded for two consecutive

switching cycles a real malfunction is assumed and the power MOSFET is turned OFF.

The shutdown condition is latched as long as the device is supplied. While it is disabled, no

energy is transferred from the auxiliary winding; hence the voltage on the VDD capacitor

decays till the VDD under voltage threshold (VDDoff), which clears the latch.

The start up HV current generator is still off, until VDD voltage goes below its restart voltage,

VDD(RESTART). After this condition the VDD capacitor is charged again by 600 µA current,

and the converter switching restarts if the VDDon occurs. If the fault condition is not removed

the device enters in auto-restart mode. This behavioral results in a low-frequency

intermittent operation (Hiccup-mode operation), with very low stress on the power circuit.

See the timing diagram of

Figure 32.

Hiccup-mode OCP: timing diagram

Vcc

VDRAIN

IDRAIN

Secondary diode is shorted here

DMAX

off

(RESTART)

Secondary diode is shorted here

VDD

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