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external Clock

T Kishi
Genius 4425 points
Part Number: TPS54240

Hi

my customer has question.

Because there is a possibility of Lost, I would like an answer as soon as possible!

Customer would like to use the external synchronization clock function of the TPS54240.
(Clk=360kHz RT=330kΩ)

It is stated that it is necessary to terminate 50 Ω in the data sheet.
However, with the clock driver currently in use, the Drive capability is insufficient and the clock Level decreases.
Is this 50 Ω mandatory? Is it no problem changing the circuit?
For example, change the part of 330 kΩ + 50 Ω to 165 kΩ + 165 kΩ.

Best Regard

T Kishi

  • 0
    TI__Genius 11085 points
    The concern I think is how your clock driver would fail. If your clock driver would fail low impedance to ground, then it is ok to not using the 50 Ohm. In case the clock driver fails, the switcher will operate at the frequency set by Rfset, in Figure 41 of the datasheet, page 24.

    But if the external clock source fails open circuit to ground, then without the 50 Ohm, the RT/CLK pin is high impedance to ground. The oscillator cannot work properly and the device will operate at a very high frequency.

    I think is ok to use a larger termination resistor that can be driven by the clock source. Then reduce the Rfset value, so that the total of the two sets the correct frequency in case of ext clock failure,

    Regards,

    Yang
  • 0 in reply to YangZhang
    Genius 4425 points
    Hi Yang

    Figure 41 on the "TPS54240" data sheet has circuits and constants for using the external clock.
    In this circuit's RC series circuit there are 10 pF and 4 kΩ. Is it OK to change this 10pF ⇒100pF?

    As background of the question, the customer inputs an external clock 380 kHz.
    A problem occurred in which the TPS 54240 stopped when applying an external clock when load＠2.4A. And, when the external clock circuit was set to 10pF ⇒100pF, the problem was solved.
    Do you have problems if you change to 100pF?

    Best Regard
    T Kishi
  • 0 in reply to T Kishi
    TI__Genius 11085 points
    I think you are just changing the time constant of the clock rising edge from 0.04us to 0.4us. For 380kHz clock frequency, I think it should be fine.
  • 0 in reply to YangZhang
    Genius 4425 points
    Hi Yang

    Thank you for reply.

    I think can close with your answer.
    There is something I would like to ask for your reference for future support.
    Depending on the frequency of the external clock, is it necessary to change the constant of the RC series circuit?
    If you need to change it, I want to know what kind of thinking should be changed.

    Best Regard
    T Kishi
  • 0 in reply to T Kishi
    TI__Genius 11085 points
    I don't think you have to change the RC based on frequency. RC only affects how fast the edges change. If the RC time constant is long and the frequency is high, it may not have enough time for the edge to rise to certain level and you might have issue with incorrect frequency.