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Original question:

LM339B: Possible to connect 2 Out ports together (use as OR function)?

LM339B: 2 Out ports tied together (use as OR function), get only 1.5V on output when 1 input is high and 5V when 2 inputs are high

Kenneth Andersen
Prodigy 30 points
Part Number: LM339B
Other Parts Discussed in Thread: LM339

Hi,

Chen tieing the 2 output ports together, I get only 1.5V on the output if only one of the inputs are high. When high on both inputs the output is 5V. Previously I was told I could just tie the 2 output ports together to achieve an OR function. Could you please advice on what I have done wrong in this case (see attached screnshot from design). I guess I am missing some diodes, but would like your opinion.Design with 2 output ports tied

Best regards, Kenneth

  • 0
    Guru 311870 points

    The output signal is low if the first input is low OR the second input is low.

    You can invert the polarity of the input signals by exchanging the + and  − pins, then the output signal is low if the first input is high OR the second input is high. But an open-collector output can only pull down, so you cannot invert the polarity of the output.

    (And the low-level output voltage should be lower than 1.5 V. Is there another load?)

  • 0
    TI__Guru 57986 points

    Hello Kenneth,

    As Clemens mentioned, what else is the output connected to? Either there is another load, or, something is clamping.

    With a 5.6k resistor, and 5V pullup votlage, that is 5V / 5.6k=0.9mA. According to figure 7-20, the output low should be about 30mV.

    Do note that the input voltage range is Vcc-2V, or +3V. Make sure that all your input signals are below 3V.

    For some fun reading, please see the LM339 family application note, sections 3.4 and 4.1.

     Application Design Guidelines for LM339, LM393, TL331 Family Comparators