VIPER22ADIP-E datasheet(10/20 Pages) STMICROELECTRONICS

10 / 20 page

Operations

VIPer22ADIP/ VIPer22AS - E

10/20

Rev1

4.4

Startup sequence

Figure 6.

Startup sequence

This device includes a high voltage start up current source connected on the drain of the

device. As soon as a voltage is applied on the input of the converter, this start up current

source is activated as long as VDD is lower than VDDon. When reaching VDDon, the start up

current source is switched OFF and the device begins to operate by turning on and off its

main power MOSFET. As the FB pin does not receive any current from the optocoupler, the

device operates at full current capacity and the output voltage rises until reaching the

regulation point where the secondary loop begins to send a current in the optocoupler. At

this point, the converter enters a regulated operation where the FB pin receives the amount

of current needed to deliver the right power on secondary side.

This sequence is shown in Figure 6. Note that during the real starting phase tss, the device

consumes some energy from the VDD capacitor, waiting for the auxiliary winding to provide

a continuous supply. If the value of this capacitor is too low, the start up phase is terminated

before receiving any energy from the auxiliary winding and the converter never starts up.

This is illustrated also in the same figure in dashed lines.

VIPER22ADIP-E 数据表(PDF) 10 Page - STMicroelectronics