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LMC567: How to estimate Finput @>50kHz ?

Kotaro Yamashita
Intellectual 2560 points
Part Number: LMC567

Hi Team,

I would like to set Finput = 250kHz, however equation(5) has no accurate result. So, I would like to know how to estimate this.

Could you support?

Regards,

Kotaro Yamashita

  • 0
    TI__Expert 6295 points

    hello Kotaro,

        LMC567 (> 50 kHz (Fosc = 100 kHz), internal delays cause the actual frequency to be lower than predicted.

    The choice of Rt and Ct is a tradeoff between supply current and practical capacitor values. if you have EVM in hand, you can play around of Rt, Ct value according

    to datasheet page8 guidance. I will let Sam to comments you more ideas.

    Regards

    Linda

  • 0 in reply to Xiaolin Qin
    TI__Intellectual 2560 points

    Linda,

    Thanks. Is there approximation formula?

    Regards,

    Kotaro Yamashita

  • 0
    TI__Mastermind 19080 points

    Hi Kotaro,

    As the effect of internal propagation delay will be more significant for higher input frequency, the actual result can be a little bit lower than expected by Eq.8.

    In your case, if we set Ct=300pF (nominal), you will need a ~4.7kOhm Rt according to the Eq. As the capacitance is usually less accurate, you may keep the Ct and adjust the Rt little by little. It's most likely that Rt value should be increased a little to detect your 250kHz input.

    Any further question, please let us know. Thanks!

    Regards,

    Hao