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LM239: What is the output when power voltage is lower than 2V

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Part Number: LM239

Hi team,

   My customer is using this device as below,

  • Monitor the power level of 5V, and the power supply for LM239 is +/-12V;
  • The output is pulled up to 3.3V with 4.7k resistor;
  • One thing is that the 12V will power off first than 5V, and they can detect fault signal at the output; See waveform below.

They are curious about when the power voltage of LM239 is lower than 2V, such as 1V. Vp>Vn, what is the output voltage of LM239? Thanks. 

  • Hi Charles,

    There are many things happening when operating below the minimum supply voltage. What the output does will depend on the input voltages, supply voltage and  temperature

    Please see section 2.1 of this application note:

    Application Design Guidelines for LM339/LM393/TL331 Family Comparators

    At 2V supply, at room temperature, the valid input range is 0 to 0.5V. The comparator will most likely stop operating around 1.6V (output can no longer sink). The output should go high when un-powered, as it takes supply power to drive the base of the output transistor (with legal input voltages).

    If both inputs are above the input voltage limit, the output is indeterminate since both inputs are cut-off and you are relying on internal leakages to determine the output state - and even that is suspect because the internal biasing will also be failing as supply decreases.

    I would NOT recommend relying on any 'predictable" behavior - because that can change over time due to process changes - or changes in suppliers. Temperature, particularly cold, will have a big influence on this behavior.

    They may need to have the system ignore faults at turn-on/off.

    Or they can power the comparator from a supply that does not collapse as fast as the 5V supply. This could be as simple as a large cap and a isolation diode on the comparator power supply to allow more "hang time" on the comparator supply votlage.

    It was not very clear - is the comparator powered directly from the ±12V? or from +3.3V derived from the 12V? We do have comparators that have POR circuits that have a known start-up/turn-off conditions. If the supply is under 5V, see the TLV7011 family. If the supply is over 5V, please see the TLV1805.

    Regards,

    Paul Grohe

    TI Comparators (CMPS) Applications Group

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