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LM139AQML: ESD mitigation above 600V

David DuVal
Prodigy 40 points
Part Number: LM139AQML
Other Parts Discussed in Thread: LM139,

I need to protect the comparator inputs to meet 2kV HBM ESD. Can external Schottky diodes be used to clamp the inputs to the supply rail and ground to provide greater ESD immunity over the 600V specified. Do you have any data on internal clamp diode voltage versus current?

  • 0
    TI__Genius 17125 points

    Hello David,

    TI offers many versions of the LM139 and I want to be sure we are talking about the same version.   Your post lists the LM139AQML which is the military version of the part which came to TI when it purchased National Semiconductor.   The datasheet is LM139AQML.

    As shown in the datasheet, the input is the base of a pnp transistor and the bias current is as shown in the parametric tables.  There is the following note about the input bias current:

    (3) The direction of the input current is out of the IC due to the PNP input stage. This current is essentially constant, independent of the

    state of the output so no loading change exists on the reference or input lines.

  • 0 in reply to Kirby Kruckmeyer96848
    Prodigy 40 points
    Hi Kirby,
    Yes, this part is used in an aerospace application, procurement is to 5962R9673802VDA. Many space cabling harness and dielectric discharge events are now being modeled as ESD events and I need to plan mitigation to meet 2kV HBM. The Ti datasheet describes "built-in ESD protection" and the National Semi datasheet lists 600V HBM ESD rating. Can you tell me what this ESD protection consists of? I was assuming small clamp diodes to the voltage rails not shown in the schematic, but maybe this more of a rating of the ESD capability of the PNP input transistor? In any case, I need some confidence that adding external Schottky diodes will take the burden off of the IC.
  • 0 in reply to David DuVal
    TI__Genius 17125 points
    The "built-in ESD protection" note is a generic statement warning that the product has limited internal ESD protection, not that it has internal ESD cells.
    This product was developed before the advent of ESD protection cells and does not have ESD clamp diodes. The input design is basically as shown in the schematic of the datasheet.
  • 0 in reply to Kirby Kruckmeyer96848
    Prodigy 40 points
    So, it would seem to me that addition of external Schottky diodes to the supply rails would likely protect the part because there is not a risk of sharing with any internal ESD clamping. Would you agree with that?
    That said, is the negative transient a special case because of the PNP input structure? The datasheet describes addition of an input clamp to prevent the input from going negative by more than -0.3V "as shown in the application section" - I don't really see any clamping in the application notes that would achieve that low of a voltage drop.
  • 0 in reply to David DuVal
    TI__Genius 17125 points
    Basically, when you take the input negative you are turning on a Vbe (or forward biasing a pn junction). At room temp it will be around -0.7V. The -0.3V limit is to cover the full temperature range. The sizing of the metal and the transistors is large enough to handle up to 50 mA without damaging the part.
    I agree with your Schottky diodes comments and there won't be an internal ESD clamp diode.
  • 0 in reply to Kirby Kruckmeyer96848
    Prodigy 40 points
    Thanks for the information. Because I cannot guarantee the Schottky clamp to ground would always completely bypass the PNP junction under worst case conditions, I plan to increase the filtering to the comparator inputs so that I guarantee the inputs stays above the comparator ground reference.
  • 0 in reply to David DuVal
    Guru 140200 points
    Hi David,

    can you show a schematic of the circuit you want to protect?

    Kai