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TPS54340B-Q1: Regarding DCM waveform of SW node.

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Part Number: TPS54340B-Q1

Hi TI support,

I recently noticed some strange pulse waveforms at SW node of TPS54340B-Q1 at 50mA load. Design is developed for 5V/1.5A. It shows some sinusoidal content along with pulse switching. While at 0.7A or 1.5A load it shows good pulse waveforms (no sinusoidal content) corresponding to designed switching frequency. I have attached both normal & DCM waveforms. Kindly check & advise if DCM pulses at SW node are OK or there is something in design.

Thanks,

jagdish

Waveform with 50mA load current, Input is 12VDC                                                       Waveform with 0.7A load current. Input is 12VDC

  • Jagdish,

    This is expected behavior.

    At lighter loads the inductor ripple current amplitude will be higher than the average current so the inductor current will want to go negative. This part will enter DCM, as you noted, when the current reaches (near) zero, shutting off the low-side FET to reduce power consumption from the unnecessary circulating current. When the low-side FET (and high-side FETs) is off, the inductor will ring with the parasitic capacitance of the FETs. This is the sinusoid you are seeing.

    -Sam

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  • In reply to Samuel Jaffe:

    Hi Sam,

    Got it. Thanks for your response.

    Regards,
    jagdish

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