VIPER53DIP-E datasheet(22/36 Pages) STMICROELECTRONICS

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High voltage Start-up current source

VIPer53 - E

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High voltage Start-up current source

An integrated high voltage current source provides a bias current from the DRAIN pin during

the start-up phase. This current is partially absorbed by internal control circuits in standby

mode with reduced consumption, and also supplies the external capacitor connected to the

VDD pin. As soon as the voltage on this pin reaches the high voltage threshold VDDon of the

UVLO logic, the device turns into active mode and starts switching. The start-up current

generator is switched off, and the converter should normally provide the needed current on

the VDD pin through the auxiliary winding of the transformer, as shown on Figure 19 on

page 17.

The external capacitor CVDD on the VDD pin must be sized according to the time needed by

the converter to start-up, when the device starts switching. This time tss depends on many

parameters, including transformer design, output capacitors, soft start feature, and

compensation network implemented on the COMP pin and possible secondary feedback

circuit.

The following formula can be used for defining the minimum capacitor needed:

Equation 5

Figure 21 on page 23 shows a typical start-up event. VDD starts from 0V with a charging

current IDDch1 at about 9 mA. When about VDDoff is reached, the charging current is reduced

down to IDDch2 which is about 0.6mA. This lower current leads to a slope change on the VDD

rise. Device starts switching for VDD equal to VDDon, and the auxiliary winding delivers some

energy to VDD capacitor after the start-up time tss.

The charging current change at VDDoff allows a fast complete start-up time tSDU, and

maintains a low restart duty cycle. This is especially useful for short circuits and overloads

conditions, as described in the following section.

VDD

DD1

tss

⋅

DDhyst

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