VIPER37HE datasheet(31/35 Pages) STMICROELECTRONICS

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VIPER37

Operation description

Doc ID 022218 Rev 1

31/35

In order to enable the brownout function, the BR pin voltage must be higher than the

maximum VDIS threshold (150 mV), see Table 8.

8.13

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. Such an anomalous

condition is invoked when the drain current exceeds the threshold IDMAX (see Table 8).

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 is 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 to the VDD undervoltage threshold (VDDoff), which clears the latch.

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

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

current, and the converter switching restarts if the 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.

See the timing diagram of

Figure 34.

Timing diagram: hiccup-mode OCP

VDRAIN

VDD

IDRAIN

VDDon

time

VDDoff

VDD(RESTART)

time

Hiccup-mode

Secondary diode

short circuit

Normal operation

IDMAX

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