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LM2903B-Q1: How to understand pull up to Vlogic

Part Number: LM2903B-Q1
Other Parts Discussed in Thread: LM2903, LM393LV, LM339

Hello Experts,

We are researching on the LM2903B-Q1. And according to the typical application, there is a pull up resistor on the output to Vlogic. I suppose this is for to make the output voltage to the MCU logic voltage. While I'm not very clear on this, if the output voltage is around 30V(close to input), the logic voltage is 3.3V, how can we drive the output voltage to 3.3V such a low logic voltage?

Many thanks!



  • Hello Frank,

    The output is an “open collector” output. It can only go “low” by itself.

     A “pull-up” resistor is required for the output to go “high”. This can be independent of the comparator supply voltage.

    The pullup voltage should be the required “high” voltage for the logic.

    So if you want the output to go to 3.3V, then tie the pull-up resistor between the output and the 3.3V logic supply. In this configuration, even if the comparator power supply is 30V, the output will only go to 3.3V.

     Please beware that at low supplies, the LM2903 family input voltage is limited to Vcc-2V, or 1.3V on a 3.3V comparator power supply (or 28V on a 30V comparator supply).

     If the supply and input voltages are below 5V, we recommend the LM393LV.

     For more information, please see the LM339 family appnote:

     Application Design Guidelines for LM339, LM393, TL331 Family Comparators