Application information

STCH02

18/24

DocID028766 Rev 1

3.9

Adaptive UVLO

A major problem when optimizing a converter for minimum no load consumption is that the

voltage generated by the auxiliary winding under these conditions falls considerably as

compared even to a few mA load. This very often causes the supply voltage VDD of the

control IC to drop and, as the self-supply is disabled during the burst mode, it can go below

the UVLO threshold so that the operation becomes intermittent, which is undesired.

Furthermore, this must be traded off against the need of generating a voltage not exceeding

the maximum allowed by the control IC at the full load but low enough to reduce the bias

losses as much as possible.

To help the designer to overcome this problem, the device besides reducing its own

consumption during the burst mode operation, also features a proprietary adaptive UVLO

function.

more headroom.

To prevent any malfunction the normal threshold (9.5 V typ.) is re-established when the

The normal UVLO threshold ensures that at full medium-heavy loads the MOSFET will be

driven with a proper gate to source voltage.

The mode of operation is reported in Figure 10.

3.10

Overvoltage protection

The overvoltage function of the STCH02 device monitors the voltage on the ZCD pin during

MOSFET's OFF-time, where the voltage generated by the auxiliary winding tracks

converter's output voltage. If the voltage applied to the pin exceeds an internal 2.5 V

reference, a comparator is triggered, an overvoltage condition is assumed and the device is

shut down.

Equation 3

turn's number respectively.

To reduce sensitivity to noise and prevent the latch from being erroneously activated, the

OVP comparator must be triggered for four consecutive oscillator cycles before the STCH02

device is stopped. A counter, which is reset every time the OVP comparator is not triggered

in one oscillator cycle, is provided to this purpose.

Figure 11 illustrates the timing of the function.