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Part Number: LM139
I'm looking for replacing the quad-comparator LM139AD with a component with the same features but more tolerant to negative voltage.
Indeed, the power supply is +15/GND, and I would like a quad differential comparator which is not changing state even if the input voltage Vin- goes below -0.3V (with a min input voltage of -0.8 for instance).
Is such a component available ?
Thanks in advance.
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These devices are pretty rugged, but negative voltages are their Achilles heel.
The problem is that you will end up forward-biasing either the die substrate diode (for these older JI processes) or the ESD protection structures (in "modern" CMOS and Bipolar devices).
Most of the comparators will allow up to -0.3V, since this is before the diodes conduct. The diodes start to conduct around 400mV and would be in full conduction at -600mV to -800mV - so that limits the input range.
I am not aware of any comparators that can handle negative voltages beyond -0.3V.
Your best solution is to add series resistance (1k min per volt) to the input and a Shottky clamp diode to keep the clamp close to -350mV. Clamp current should be limited to 1mA or less.
Depending on the circuit and the tolerances, you may also be able to attenuate the input enough to bring the negative voltage above -0.3V (and shift the threshold accordingly).
How the comparator behaves when the input is taken below the limit varies by the device family. You may have to try different devices, over temperature, to see if they misbehave. Still...it is not a good idea to rely on a particular unspecified behavior, since that can change over time.
Do you need the Military temp range? Mil temp range and 15V quad really narrows the available selections. Not all Quads have the same pinout (usually either the LM139 pin-out or the standard quad op-amp pin-out).
Try the TLC374M. It is a drop-in replacement CMOS version (16V max) and see if it is more tolerant, but I still recommend the clamp or attenuation.
Low Power Amplifiers and Comparators (LPAC) Applications
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