VIPER17_0810 datasheet(27/33 Pages) STMICROELECTRONICS

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VIPER17

Operation descriptions

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7.13

2nd level over current protection and hiccup mode

The VIPER17 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 0.6 A typical.

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 2nd OCP 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,

VDDrest. After this condition the VDD capacitor is charged again by 600 mA current, and the

converter switching restart 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.

Figure 32. Hiccup-mode OCP: timing diagram

Vcc

VDS

IDRAIN

Vccrest

Secondary diode is shorted here

IDmax

OFF

Vccrest

Secondary diode is shorted here

IDmax

VDD

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