AN2544 Circuit operation
11/17
The VIPer internal 1 mA current source charges up the V
dd
capacitor. When the voltage on
the V
dd
pin reaches the V
dd
startup threshold (Worse case is 13 V) the VIPer starts pulsing,
raising the output voltage to the point of bootstrapping. The V
dd
capacitor needs to supply
the energy to supply the necessary output current and to charge up the output capacitor,
before the V
dd
voltage falls below the V
dd
under voltage shutdown threshold (worse case is
9 V). Figure 8 and 9 show a V
dd
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 V
dd
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 V
dd
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
To Next Page
Loading...