General description

VIPER26K

14/28

DS12978 Rev 1

4.4

High voltage startup

The high voltage current generator is supplied by the DRAIN pin. At the first startup of the

converter, it is enabled when the voltage across the input bulk capacitor reaches the

VDRAIN_START threshold, sourcing the IDDch1 current; as the VDD voltage reaches the VDDon

start-up threshold, the power section starts switching and the high voltage current generator

is turned OFF. The VIPer26K is powered by the external source. After the startup, the

auxiliary winding or the diode connected to the output voltage have to power the VDD

capacitor with voltage higher than the VDDCSon threshold. During the switching, the internal

current source is disabled and the consumptions are minimized.

In case of fault, the switching is stopped and the device is self-biased by the internal high

voltage current source; it is activated between the levels VDDCSon and VDDon, delivering the

current IDDch2 to the VDD capacitor during the MOSFET OFF time.

If a very low main input voltage is applied to the converter, it is strongly recommended to

choose the VDD capacitor value by the following formula:

Equation 1

At converter power-down, the VDD voltage drops and the converter activity stops as it falls

below the VDDoff threshold.

Figure 20. Timing diagram: normal power-up and power-down sequences

CVDD =

IDDCH1 tOVL



V DD - VDDoff

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