AN2544 datasheet(11/17 Pages) STMICROELECTRONICS

11 / 17 page

AN2544

Circuit operation

11/17

The VIPer internal 1 mA current source charges up the Vdd capacitor. When the voltage on

the Vdd pin reaches the Vdd startup threshold (Worse case is 13 V) the VIPer starts pulsing,

raising the output voltage to the point of bootstrapping. The Vdd capacitor needs to supply

the energy to supply the necessary output current and to charge up the output capacitor,

before the Vdd voltage falls below the Vdd under voltage shutdown threshold (worse case is

9 V). Figure 8 and 9 show a Vdd cap that is not large enough to start up the evaluation board

under a resistive load of 350 mA.

In Figure 8 the purple trace is the Vdd voltage rising to ~14 V. The energy with the 2.2 µF

capacitor does not store enough energy. As seen the output voltage (green trace) does not

reach high enough to bootstrap, It succeeds the second time after there is a partial charge

on the output cap. Figure 9 is using a 4.7 µF Vdd cap. With adequate energy the power

supply starts the first time.

1.7

Burst mode in no-load or very light load

At very light load, the on-time becomes so small that some pulses are skipped in order to

stay in regulation and meet energy requirements such as Blue Angel or Energy Star. This

mode is called burst mode. It skips as many cycles as needed to maintain regulation. In the

case below at no-load about 9 cycles are skipped to maintain an output.

Figure 8.

Bad start

Figure 9.

Good start

AN2544 Datasheet(PDF) 11 Page - STMicroelectronics