VIPER38HD datasheet(24/31 Pages) STMICROELECTRONICS

24 / 31 page

Operation

VIPER38

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DocID025703 Rev 2

to be connected to GND if the function is not used.

Figure 26. EPT timing diagram

7.13

2nd level overcurrent protection and hiccup mode

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

secondary winding or a hard-saturation of the flyback transformer. This type of anomalous

condition is invoked when the drain current exceeds the threshold IDMAX, see Table 8:

Controller section.

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

“warning state” is entered after the first signal is tripped. If, in the subsequent switching

cycles, 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 CVDD capacitor

decays until the VDD undervoltage threshold (VDDoff), which clears the latch.

The startup HV current generator is still off, until the VDD voltage falls below its restart

voltage, VDD(RESTART). After this condition the CVDD capacitor is charged again by the

IDDch2 current, and the converter switching restarts if VDDon occurs. If the fault condition is

not removed, the device enters auto-restart mode. This behavior results in a low-frequency

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

VIPER38HD Datasheet(PDF) 24 Page - STMicroelectronics