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TPS629210EVM: Switch node ringing during off time of converter

Ethan Frommer
Intellectual 1900 points
Part Number: TPS629210EVM

Hello,

I am testing the EVM at 3.3V and a 250 mA load.

I noticed ringing on the switch node that is not at centralized around the settling voltage of the high duty cycle point. See scope capture below:

The expected duty cycle here would be around 25% so the ringing is showing up during the off time. What is the purpose/root of the ringing?

  • 0
    TI__Mastermind 47760 points

    Hi Ethan,

    I think your test condition is 12Vin, 3.3Vout, 0.25A, inductor is 3.3uH, switching frequency sets to 1MHz with PFM/PWM mode. If I am correct, the switching node waveform is expected. At this configuration, the converter works in power save mode due to the output current 0.25A below 1/2 ripple current. In power save mode, the switching frequency decreases linearly with the load current maintaining high efficiency. Regarding the ringing on SW node, this is basic of buck converter. That is when both high-side and low-side FETs are turned off. SW node value will be close to Vout on average. The ringing is due to the decay of the remaining non-zero inductor current via the FET parasitic capacitance. Eventually the current decays to zero and ringing decays with it.

    Hope this helps,

    Thanks,

    Nancy