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TLV1872: Current limiting resistor at inputs

Frank Song
Expert 3985 points
Part Number: TLV1872
Other Parts Discussed in Thread: DAC61416

Tool/software:

Hi team,

My customer uses DAC61416 to set the reference voltage of DUT and feed into inverting input (IN-) of TLV1872. The supply condition as follow:

  • VCCI=24.5V.
  • VEEI=-13V.
  • The positive & negative supply voltages are shared between DAC and comparator.

Because DAC61416 has a short-circuit current of 40mA, could you help to evaluate:

  • Do we need to place series current limiting resistors (RINs) at the inputs of TLV1872? If so, what is the recommended resistor value?
  • Do we need to place same RINs at both IN+ & IN- ports?

Thanks,

Frank

  • 0
    Guru 317480 points

    The TLV1872 has an input bias current of at most 0.000005 mA. You do not need to limit this current.

  • 0 in reply to Clemens Ladisch
    TI__Expert 3985 points

    Hi Clemens,

    Thanks for the feedback.

    In the datasheet I see that the maximum input current at the inputs should not exceed 10mA, and we have concern that the output current from DAC may damage the ESD diodes.

    Also, I see below comment that series resistors may be needed in section 6.4.5 ESD protection.

    Could you help to confirm under what circumstances is the current limiting resistors required?

    Thanks,

    Frank

  • 0 in reply to Frank Song
    Guru 317480 points

    You need current limiting only for voltages that exceed the absolute maximum ratings. Can this happen, when the inputs are connected only to the DAC?

  • 0 in reply to Frank Song
    TI__Guru 61206 points

    Hi Frank,

    If the source impedance is low (power supply, battery, large capacitor), that could source more than 10mA continuously, the resistance is to protect the ESD diodes should they need to clamp the input voltage.

    The idea is to protect the peak current should there be a fault, or start-up/shut-down conditions where supplies are not synchronized (i.e. ADC output applied before comparator is powered).

    If the ADC is on the same supply - they most likely be synchronous. But if they were on separate supplies, there could be a condition where one stage could overdrive the other during power up & down.

    Our rule of thumb is to add 1k of resistance for every volt of possible over-voltage (1mA max). So your worst voltage is 24V, then we would recommend at least 24k of series resistance.

  • 0 in reply to Paul Grohe
    TI__Expert 3985 points

    Hi Clemens, Paul,

    Thank you for all the comments.

    It confuses me a little because you are implying different series resistor placement. Could you help to confirm if the series resistors are needed, and should we place same resistor values for inverting & non-inverting inputs?

    Thanks,

    Frank

  • +1 in reply to Frank Song
    Guru 317480 points

    Current-limiting resistors are needed only if there can be overvoltage. In your application, the DAC and the comparator share a supply, so this is not possible.

    For CMOS inputs, the input bias currents are very small and not necessarily symmetric, so you should add resistors only if necessary for that input signal.

  • +1 in reply to Frank Song
    TI__Guru 61206 points

    Hi Frank,

    As Clemens said - because of CMOS inputs, the two bias-current balancing resistors are not needed. Only the resistance in series with the input that may be over-voltaged is needed.

    In your case, the ADC and comparator are on the same supplies, so theoretically the ADC output could not exceed the supplies and stress the input. So the series resistance is not needed.

    *IF* the comparator and ADC were on separate supplies, then I would recommend the resistor because it would take just one bad power supply sequence to damage the input.