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ISO7240C: About pull-up resistor for EN pins

Masanori Tachibana
Genius 5350 points
Part Number: ISO7240C

Hello support team,

I have some questions about ISO724x series.

I think that the EN pin of them should be fixed at 'H' level when using them in the always enabled state.
In Figure 19 on page 27 of the data sheet, Figure 20 on page 28 and , Figure 22 and 23 on page 28, the ENx pins are pulled up directly to Vccx.
On the other hand, in Figure 26 on page 30, the ENx pins are pulled up to Vccx via 4.7 kΩ of series resistors.
In the former datasheet (eg. Rev. M), the pull-up circuit via 4.7 kΩ was not described.

[Question]
1. Which is recommended, whether to be pulled up directly or via 4.7 kΩ of series resistor?
2. Why 4.7 kΩ of series resistor is needed?
What kind of case requires a 4.7 kΩ pull-up resistor?

Sincerely,
M. Tachibana

  • 0
    TI__Mastermind 49415 points
    Hi Masanori-san,

    Thank you for reaching out to us with the above question and seeking clarification. Apologies that the pull-up resistor usage is varying across schematics in datasheet.

    For typical operating conditions, the pull-up resistor doesn't make much of difference. As long as the datasheet specs are complied, it doesn't matter if a pull-up resistor is used or not. But if an application expects voltage transients or over-voltage levels on power supply then the pull-up resistor helps limit current into the EN pin. The pull-up resistor primary intent is to make sure there is no current flowing into EN pin even under some fault conditions.

    A value between 1kΩ and 10kΩ is considered good to limit the current and 4.7kΩ is chosen as a mid-point value. I hope that answers your question, thank you.


    Regards,
    Koteshwar Rao
  • 0 in reply to Koteshwar Rao
    Genius 5350 points
    Hi Rao-san,

    Thank you very much for your prompt reply.
    I have been understanding that logical operation is not problem regardless of the presence or absence of a pull-up resistor in normal conditions.
    This time, I understood that pull-up resistor is necessary for the purpose of protecting from power supply overvoltage. Your advice is very helpful.

    I will recommend the customers to put a pull-up resistor if possible.

    Sincerely,
    M. Tachibana