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SN74LVC1G123: Is it possible to use ceramic capacitor for CEXT?

Shinichi Inoue
Guru 21696 points
Part Number: SN74LVC1G123

Dear Specialists,

My customer is considering SN74LVC1G123, and has a question.

Could you please advise?

Is it possible to use ceramic capacitor for CEXT?

According to the datasheet, it is not described.

He'd like to use ceramic cap.

I appreciate your great help.

Best regards,

Shinichi

  • 0
    Guru 21696 points
    Dear All

    This is an additional information.

    A ceramic capacitor has extremely low ESR (less than tens of milliohm) and bug capacitance(more than 22uF), it may cause a big rush current and power dissipation.

    In this case, could the device (SN74LVC1G123) withstand such stress?

    I appreciate your great help.

    Best regards,
    Shinichi
  • 0 in reply to Shinichi Inoue
    Guru 317060 points
    I don't know if the '123 charges the capacitor directly through the resistor or with a constant-current source controlled by the resistor, but in any case, the current is limited, so large capacitance values are not a problem.

    The datasheet does not mention any particular capacitor type because all types work.
  • 0 in reply to Clemens Ladisch
    Guru 21696 points

    Dear Clemens

    Thank you for your reply.

    I  understand there is no limitation about capacitor type.

    The customer would like to know how is the current limit value of CEXT.

    Could you please advise?

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    Guru 317060 points

    As shown in the application report Designing with the SN74LVC1G123 Monostable Multivibrator (SLVA720), no current goes through the chip itself while the capacitor is charging. The discharge current is limited by the MOSFET's internal resistance; I'd assume this is designed to be safe in all cases.

  • 0 in reply to Clemens Ladisch
    Guru 21696 points
    Dear Clemens

    Thank you for your reply.

    I understand the current through CEXT is anticipated less than 1mA.

    I'll feedback to the customer.

    If he has more question, I'll talk with you.

    I appreciate your great help and cooperation.

    Best regards,
    Shinichi
  • 0 in reply to Shinichi Inoue
    Guru 21696 points
    Dear Clemens,

    Thank you for your reply again.

    The charge current is not depended on the device and discharge current is depend on the MOSFET inside.

    In this case, how much is the resistance of MOSFET?
    I think generally tens of ohms, is it right?

    If so, there is no concern to use any capacitance for CEXT.

    I appreciate your great help and cooperation.

    Best regards,
    Shinichi
  • 0 in reply to Shinichi Inoue
    Guru 317060 points
    This is not specified. You'd heave to measure it.

    But again, I assume this is designed to be safe; anything else would not make sense.
  • 0 in reply to Clemens Ladisch
    Guru 21696 points
    Dear Clemens

    Thank you for your reply.

    All right, there is no problem to use even large amount of ceramic capacitor.

    I appreciate your great help and cooperation.

    Best regards,
    Shinichi