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TPS65910: Passive crystals for TP65910 have the slice waveform

James Zhan
Expert 4365 points
Part Number: TPS65910

Hi team,

My customer is using TPS65910 and they have found that the waveform on the passive crystals. Could you please help to explain why?

The waveform in below are: OUT of crystals, IN of crystals,CLK32KOUT(PIN38 of TPS65910)

  • 0
    TI__Mastermind 49151 points
    Mr. Zhan,
    Thanks a for your interest in TI device; your question has been assigned to an expert; you can expect to get answer in 24 hours.
    Thanks!
    Phil
  • 0
    TI__Mastermind 30310 points

    James Zhan,

    What is your question?

    These all look like similar waveforms with the same frequency: ~32.5kHz, with the last of the waveforms looking like a clean, buffered clock signal.

    Thanks,

    Brian

  • 0 in reply to Brian Berner
    TI__Expert 4365 points

    Hi Brian,

    As you can see the first waveform has the slice on the bottom, could you help to explain why?

    Two cap is 25pF on the Passive crystals as the EVM setting.

  • 0 in reply to James Zhan
    TI__Mastermind 30310 points

    James,

    The maximum Coscin, Coscout value is 25pF.

    The minimum  Coscin, Coscout value is 12pF, as shown on page 13 of the TPS65910 datasheet.

    Coscout = Coscin = CL, or the load capacitance on the crystal. You can modify the load capacitance as long as you keep the two values equal, but you have a nice clean waveform so I do not see any reason to change the capacitance.

    The Output (at OSC32KOUT) waveform you are seeing is generated by the PMIC, as you can see in Figure 6-3 on page 52 of the TPS65910 datasheet. The OSC32KOUT pin voltage is generated by a current source and a switch (FET). Since the switch is referenced to GND, I see no way that the OSC32KOUT pin can generate a voltage below GND potential. This is why it is clipping at the bottom.

    But the OSC32KIN pin is the input to the comparator, which generates the "32 kHz to digital block" signal. This signal is buffered and becomes CLK32KOUT, which is used by the system.

    Since OSC32KIN is a very clean sine wave without clipping, and CLK32KOUT is a square wave with perfect frequency, it doesn't not matter how non-ideal OSC32KOUT is. This pin is simply used as the stimuli to start the crystal oscillator and keep it running.

    Thanks,

    Brian