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TLV7021: Input voltage at unpowered comparator (same question for TLV7041)

Oliver Filsch
Intellectual 390 points
Part Number: TLV7021
Other Parts Discussed in Thread: TLV7041, TL331B, TLV1861

Tool/software:

Hello,

I want to use this comparator (TLV7021/TLV7041) to monitor the input voltage of a device.

However, as the input voltage of the device can be as much as 36V, the supply voltage of the comparator is generated with an LDO that is located after some DCIN ramp up circuitry.

That means, that for some time the comparator is unpowered, but already has voltage at the input.

Another case is reverse input voltage. In this case, the comparator is unpowered (protection circuit prohibits neg. comparator supply), but there is a negative voltage at the input present.

My approach is to limit the comparator input current via appropriately dimensioned input dividers to <<10mA.

My questions:

  1. Is this sufficient/ok for long time reliability?
  2. The IC has internal ESD clamping diodes. At which voltage these diodes begin to conduct?
  3. Should I use additional external clamping diodes at the inputs to Vcc and Vee? If yes, do I need schottky diodes (see question 2)?

Many thanks in advance.

  • 0
    Guru 310880 points

    The absolute maximum ratings describe the clamping diodes; 0.3 V is below the forward voltage where a dangerous current flows.

    The absolute maximum ratings allow voltages above VCC (with or without power), so there is no clamping diode to VCC. A current limit would not help; you have to ensure that the voltage never exceeds 6 V.

  • 0 in reply to Clemens Ladisch
    Intellectual 390 points

    Ok, so I will place a diode to Vcc.

    What about the 0.3V negative voltage? Does that mean, that there is a internal diode that begins to conduct from 0.3V below GND?

  • 0
    TI__Guru 57766 points

    Hello Oliver,

    We will never say it is OK to apply a voltage more that the Absolute Maximum Ratings.

    Applying 36V to any pin will DESTROY a 6V device.

    What is your supply voltage?

    The TL331B family can accept input voltages up to 38V, even with the power is off. But the input range is Vcc-2V (not R-R input).

    You may be interested in the TLV1861, which is a 40 V device with failsafe inputs and can easily tolerate 36V on any pin, even with the power off.

    It also has reverse battery protection (on the power supply pin, the other pins cannot go below ground). But beware that this is a nano-power device and is much, much slower than the TLV7021 (45us vs 260ns).

    But we always recommend adding external clamping if an over-voltage could occur as part of normal operation. Never rely on the internal ESD diodes as they are not designed for continuous use.

  • 0 in reply to Paul Grohe
    Intellectual 390 points

    Hello Paul,

    the comparator has a resistive input voltage divider. In order to prevent overvoltage at the comparator input, i will place a clamping diode to Vcc. So that will not be an issue (Vcc=5V, but ~0V when power off; abs. max. rating of input pins: 6V)

    The other case is reverse input voltage with the comparator in unpowered state. The internal ESD diode will conduct in this case. Current will be limited to <<10mA by the input divider.

    When I add an external clamping diode for negative input voltage, what type should I use? I'd prever a standard silicon diode, because leakage of schottky diodes + input divider resistance could introduce error in comparator threshold.

    Standard silicon diode clamps at -0.6...-0.7V.

    → Will this be ok for the comparator, when input current is limited to <<10mA, for example <=1mA?

    Again, here is the description from abs. max. ratings:

    As you mentioned the TLV1861 which has failsafe inputs: The TLV7021 as well as TLV7041 has failsafe inputs too, according to product page (see TLV7021), but in the datasheet I cannot see any description of failsafe inputs.

    I don't think the TLV7021 has failsafe inputs, what do you think about this?

  • 0 in reply to Oliver Filsch
    Guru 310880 points

    The only reason to use an external diode for negative voltages is to increase the current capacity. If the input divider already limits the current, no external diode is needed.

    (If you are using an external diode, a resistor (even a small one) between the diode and the input pin will shunt most of the current through the external diode.)

  • 0 in reply to Oliver Filsch
    Guru 310880 points

    "Failsafe inputs" means that the input pins allow voltages above VCC. This is shown in the absolute maximum ratings.

  • 0 in reply to Clemens Ladisch
    TI__Mastermind 25390 points

    TLV7021 is also a fail-safe device.  Sorry for the delay.

    Chuck

  • 0
    Intellectual 390 points

    Thanks for all the suggestions!

    I will use a voltage divider that limits the current, followed by external diode. And like Clemens suggested, a resistor between the diode and the comparator input pin.