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TIC10024-Q1: Is the RC circuit required on each input?

Wayne Meretsky
Prodigy 240 points
Part Number: TIC10024-Q1

I presently monitor ground closure switches using a 10kΩ pull up to 5V and a Schmitt trigger input such as a 74LVX132 and aggregated using 74HC165s. Primarily to reduce board complexity, I am considering replacing this logic with a TIC10024-Q1.

I expect to operate in polling mode with 2mA wetting as more doesn't seem necessary. The "cleaning" capability is unlikely to be used. I am not constrained by "automotive" standards.

Given all of this, I'm struggling to understand the benefits of an external series resistor and/or capacitor to ground on a per-channel basis. 

Questions:

  1. Given the circuit described works well today, is there, strictly speaking, any requirement for the RC network on each input as shown in the data sheet?
    1. If so, why?
    2. If not, it would seem that a higher threshold voltage (say, 4V vs 2V) is a better choice on first principles. Do you agree?
  2. If the inputs are instead closures to Vs, is there, strictly speaking, any requirement for the RC network?

  • 0
    TI__Genius 9132 points

    Hi Wayne,

    The external resistors on the input (INx) pins are needed so the pin voltage VINX remains below the max comparator threshold of 4.7 V until the current sink is saturated by the pin voltage. This ensures a low will always be considered a low as the external system voltage VIN continues to increase. Lower comparator voltages can be used if the external components are sized to ensure that until the current sink is saturated, the voltage remains below the comparator threshold.

    You can find more details on this in Steps to Configure TIC12400-Q1 Multiple Switch Detection Interface (MSDI) Section 5.

    Regards

    Saminah

  • 0 in reply to Saminah C
    Prodigy 240 points

    Hello,

    Section 5 of the aforementioned Application Note makes no reference whatsoever to the external RC circuit shown on Page 1 and discussed in the ESD section of the Data Sheet. In fact, Figure 4 on page 5 of the Application Note does not show the external RC network shown on page 1 of the Data Sheet of the TIC 10024.

    By my reading, Section 5 only discusses stray resistance associated with the switch, and presumably, its connecting wires. The application note talks about how to adjust threshold levels to compensate for external resistance, not how to size external resistors.

    I can find no mention other than the ESD section that motivates the requirement for the external RC network.

    Am I missing something?

  • 0 in reply to Wayne Meretsky
    TI__Genius 9132 points

    HI Wayne,

    I see what you are referring to - sorry for mis-understanding your question.

    The external cap is to protect the device for any IEC / ISO level pulses. If your application does not have any ESD constraint and does not have any potential risk of damage due to exposure to high transients, then you do not need the external RC on input pins.

    Regards

    Saminah 

  • 0 in reply to Saminah C
    Prodigy 240 points

    Can you confirm that a switch directly to ground or one to Vs is a suitable application for the device?

  • 0 in reply to Wayne Meretsky
    TI__Genius 9132 points

    Yes but only if external ESD is not a concern. Since these inputs are monitoring switches, can external IEC come into play?

    Regards

    Saminah

  • 0 in reply to Saminah C
    Prodigy 240 points

    I have no idea to what the acronym IEC refers.

    If your question is about ESD, then of course, ESD can always "come into play". Let's face it, there's no way to ever know how much ESD protection is sufficient. Compliance with various standards is required by some but not all industries. It would seem that the external devices shown are recommended to jump through the ISO-10605 hoop, but I've no need (or desire) to jump through that hoop.