VIPER17LDTR datasheet(16/33 Pages) STMICROELECTRONICS

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Operation descriptions

VIPER17

16/33

7.3

Power-up and soft-start up

If the input voltage rises up till the device start level (VDRAIN_START), the VDD voltage begins

to grow due to the IDD_ch current (see Table 7 on page 7) coming from the internal high

voltage start up circuit. If the VDD voltage reaches VDDon threshold (~14 V) the power

MOSFET starts switching and the HV current generator is turned OFF. See Figure 23 on

page 17.

The IC is powered by the energy stored in the capacitor on the VDD pin, CVDD, until when

the self-supply circuit (typically an auxiliary winding of the transformer and a steering diode)

develops a voltage high enough to sustain the operation.

CVDD capacitor must be sized enough to avoid fast discharge and keep the needed voltage

value higher than VDDoff threshold. In fact, a too low capacitance value could terminate the

switching operation before the controller receives any energy from the auxiliary winding.

The following formula can be used for the VDD capacitor calculation:

Equation 1

The tSSaux is the time needed for the steady state of the auxiliary voltage. This time is

estimated by applicator according to the output stage configurations (transformer, output

capacitances, etc.).

During the converter start up time, the drain current limitation is progressively increased to

the maximum value. In this way the stress on the secondary diode is considerably reduced.

It also helps to prevent transformer saturation. The soft-start time lasts 8.5 ms and the

feature is implemented for every attempt of start up converter or after a fault.

Figure 22. Start up IDD current

VDD

DDch

SSaux

DDon

DDoff

–

----------------------------------------

-4 mA

-1 mA

-3 mA

-2 mA

1 mA

2 mA

VDDoff

VDDon

IDD0

VDS = 120V

FSW = 0 kHz

VDD

IDD

VDDrestart

IDD_OFF

IDD_FAULT

IDS_CH_FAULT

IDS_CH

AFTER FAULT

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