Operating waveforms

AN2840

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Operating waveforms

Figure 4 and Figure 5 show some VIPER17HN waveforms during normal operation at full

converter design takes advantage of both operating modes. In the illustrations below, the

VDD voltage powering the device and the feedback pin voltage are also shown.

In Figure 6 it is worth noting that once the circuit is working in DCM, the voltage on the FB

pin has a triangular shape due to frequency jittering. The loop must adjust the FB pin

voltage according to the actual switching frequency to keep the output voltage regulated.

This happens because the transferred power in DCM is directly proportional to the switching

frequency. Hence, the low frequency sawtooth superimposed on the FB pin has the same

frequency as the modulating frequency (250 Hz typ.).

On the drain waveform in Figure 7, the modulation depth of the jittering function is visible.

The waveform is captured while synchronizing the scope on the rising edge and using the

Envelope acquisition. Thus the trailing edge of the following switching cycle shows

frequency variation that is identical to the oscillator frequency with a variation depending on

the jittering of the oscillator, in this case 115 kHz and +/- 8 kHz of modulation.

Figure 4.

load, normal operation

Figure 5.

load, normal operation

CH1: VIPER17HN drain pin (black trace)

CH1: VIPER17HN drain pin (black trace)

CH2: VIPER17HN VDD pin (green trace)

CH2: VIPER17HN VDD pin (green trace)

CH3: VIPER17HN FB pin (red trace)

CH3: VIPER17HN FB pin (red trace)