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AN1645 - APPLICATION NOTE

4/22

Figure 4: DCM waveform

5.1 Continuous Conduction Mode

When the converter is working in continuous mode the voltage across the source and drain of the free

wheeling mosfet has a square shape. This voltage can be applied to the Ck pin using two different

configurations: with a resistor divider (figure 6) or with a diode and pull-up resistor (figure 7). In most

cases a spike is present during turn-off of the synchronous mosfet; this spike must be eliminated at the

Ck pin in order to avoid false synchronization.

Using the resistor divider, the spike is eliminated by adding a small capacitor (C1) as shown in figure 6.

Off Time

Dead Time

V

1

Peak

Detector

Input

Peak

Detector

Output

On

Time

Off Time

Dead Time

V

1

Peak

Detector

Input

Peak

Detector

Output

On

Time