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LM139A: Output surge current rating for capacitive load.

Part Number: LM139A
Other Parts Discussed in Thread: LM339

We currently have this comparator in one of our designs. The output is connected to a 6.8nF capacitor through 10 ohm resistor and the assumption is that, with 12V supply the output current will exceed 20mA rating for a short duration I=12/(10 + ESR). This applies to sinking current.
Please confirm whether this is acceptable for ~3us.

  • Kiran

    The device will naturally current limit at 20mA as shown in figure 3 on page 10. So you don’t need to worry about exceeding the 20mA current level. My only concern is how frequently does this occur? If this is something that occurs regularly, I would increase the resistor value but if this occurs infrequently, I do not anticipate any issues. We just don’t recommend continuous exercise of abs max conditions on our devices because this can impact a device over the life of the product.

    Chuck

  • Hi Kiran,

    caps at the output of LM339 (referenced to the ground pin of chip) with and without current limiting resistor can often be seen in circuits. But in a high reliablity application I would assure that the absolute maximum rating, here 20mA, is never exceeded, even not briefly. So, either you increase the current limiting resistor, as alread suggested by Chuck, or you could add a simple additional transistor stage to the output of LM339 to discharge the 6.8nF cap. Then you can even allow much higher discharge currents.

    Take care, there's another issue when having high discharge currents: The spiky discharge current will flow over signal ground and can cause considerable voltage drops across all involved ground connections with their non-vanishing resistances and inductances. So, connect the ground terminal of 6.8nF cap directly to the ground terminal of LM339 or discharching transistor. Provide here the shortest connection you can make.

    But even within in the LM339, the discharge current can lift the ground level of the die itself. This ground shift within the chip can result in oscillations, especially if no hysteresis is provided.

    Kai

  • All good points from Kai.

    Kai,

    Nice to see you back.  The forum has missed seeing your excellent responses.

    Chuck

  • Thank you so much, Chuck!

    Kai