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LM293: Unused pin processing

Part Number: LM293

Hi all,

Is there something wrong with the IC being broken when connecting as shown in the figure below by mistaking the processing method of unused comparator?
What are the possible problems?
Regards,
Toshi

 

  • Toshi-san,

    The LM293 is an open-collector comparator. Refering to LM293 datasheet Fig. 6, and connecting it as shown in your diagram shows that the output stage NPN open-collector is connected back to the base of the input PNP transistor. This would be an unusual circuit connection for a comparator. The actual output state of the comparator might be somewhat unpredictable because it would be dependent on the particular comparator's voltage offset, bias current and likely any noise introduced into the circuit.

    If the LM293 common-mode input voltage (Vicr) weren't limited to Vcc - 1.5 V, a predictable output state could be achieved by simply connecting the one input to ground and the other to V+. However, because of that Vicr limitation the minimum component solution would be simply to ground both inputs and let the output pin float open. In that case, the output state will be determined by the voltage offset of the comparator. Since the output pin is open the output will simply be off if the offset is in one direction, or simply won't draw any collector current if the offset is in the other direction. If you don't like the idea of letting the output pin float, a very high resistance resistor such as 1 Megohm, could be added from the output to V+. The output will either be set high, or low, depending on the comparator's offset.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • Hi Thomas,

      I am using LM293 as a voltage windows comparator. To make input voltage stable, is it okay to add a smaller capacitor on each input of comparator?

    Thanks,

    Tony

  • Tony,

    You can add the capacitor to either input, however, remember that adding the capacitor will add a delay.

    If the comparison is speed critical, you will want to add the filter capacitor to the reference input ("stable" or slowest voltage input).

    Do not add capacitors to inputs that have added hysteresis, as that also delays the hysteresis action and can cause "bursts" during transitions.