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LMV7235: Output pulses on power-up

Mark Pearson
Genius 12705 points
Part Number: LMV7235
Other Parts Discussed in Thread: LMV7219, TLV1805, TLV7011, TLV7021, TLV7031, TLV7041

What are good ways to keep the output of the LMV7235 (Open Drain) 'low' during power-up?

Same question for Push-Pull outputs like the LMV7219.

Here is an example:

Thanks!

  • 0
    TI__Guru 53826 points

    Hello Mark,

    This is common for most comparators. When the supply votlage is well below the minimum operating votlage, the device is just not "awake" yet, as internal biases have not been established  and the output is not fully under control (like me getting up in the morning...).

    A common "fix" is to add a FET or depletion mode MOSFET across the output, with the gate tied to V+, to hold the output low until the supply comes up. This should only be done for a open-drain/collector output and NOT push-pull (such as the LMV7219).

    We now have a family of comparators with power-on reset, that hold the output in a known condition until the supply voltage approaches the minimum voltage.

    The following contain the power-on reset (POR) that will avoid the power-on glitch.

    TLV1805 - R-R Out, 250ns, 40V

    TLV7011 - RR Out, 300ns, 5.5V

    TLV7021 - Open Drain, 300ns, 5.5V

    TLV7031 - RR Out, Micropower, 3us, 6.5V

    TLV7041 - Open Drain, Micropower, 3us, 6.5V

    However, they are all slower than the LMV7235 and LMV7235.

    Also - the scope photo seems to show a supply voltage over 5.5V, which exceeds the maximum supply voltage for the existing devices. Is that correct?

  • 0 in reply to Paul Grohe
    TI__Genius 12705 points
    That's what I thought. Thanks for confirming - and the POR comparator list. I assume a series/blocking analog switch (NC) would also be an option on the output as well (good for PP or OD types).

    By the way - The TLV7011/21 datasheet indicates that a SOT23-5 package is a possible future option: page 3 of the datasheet shows both an SC70 and a SOT23 package... Thoughts on that?
  • 0 in reply to Mark Pearson
    TI__Guru 53826 points
    Hi Mark,

    The SOT-23 version is scheduled to release at the end of the month. Contact me internally if you wish to get samples.
  • 0 in reply to Paul Grohe
    TI__Genius 12705 points
    An interesting observation: The POR circuit holds the OD output in a high-Z mode and the output is therefore pulled high by the pull-up resistor. Not what the customer wants. The POR holds the Push-pull output Low (good/desired output), but the open drain is what they are after because the output can be pulled up to an arbitrary level - independent of the VCC of the comparator.
    I'll close the post, as I think we got as far as we could here.
    Thanks!
  • 0 in reply to Mark Pearson
    TI__Guru 53826 points
    Hi Mark,

    They can turn the Push-Pull output into a open-drain with:

    a) A diode in series with the output or
    b) A MOSFET or transistor on the output.

    "a" sacrifices the output "low" voltage. "b" requires one or two more devices.
  • 0 in reply to Paul Grohe
    TI__Genius 12705 points
    Update - The TLV7031 seems to be working well in the customer circuit.