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AN1645 - APPLICATION NOTE

7/22

Figure 8 shows the turn-off circuit when using a diode and pull-up resistor to synchronize the STSR2P,

the same configuration of Q1 and R2 can be used with a resistor divider circuit.

5.2 Discontinuous Conduction Mode

As shown in figure 4, in discontinuous mode operation there can be some problems in detecting the

primary switching transitions. The internal peak detector is only able to determine the peak value

reached by the signal at the Ck pin, neglecting all signals that have a lower value. A minimum voltage

difference V1=400mV between the switching transition waveform and the sinusoidal waveform must be

assured in order to allow the Peak Detector to work properly. If the ringing presents almost the same

value as the square waveform, it is possible to add a filter circuit to the CK pin in order to obtain a better

operation. This circuit is showed in figure 9. R1-R2 and C2 form a low pass filter, which allows a reduction

of the ringing amplitude. But R1-R2 and C2 also cause an undesired delay, so the R3 and C1 group

reduce this delay during fast switching transitions.

Figure 9: Filter to CK input

As mentioned in the previous paragraph, if the input voltage variability range is higher than 1:2, at high

voltage the signal at the CK pin will be clamped. In these conditions, both switching transition waveform

and the sinusoidal waveform are clamped and the peak detector is unable to operate correctly (see figure

10). In this case it is possible to use an external signal, which turns off completely the device in no load or

light load conditions (figure 8).

+5V

R1

R2

STSR2P

4

5

2

6

3

Ck

INHIBIT

Vcc

SGLGND

SETANT

FREE WHEEL

FORWARD

R3

C1

C2