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LM339: Schematic double check

Wesley Huang
Expert 1030 points
Part Number: LM339
Other Parts Discussed in Thread: TLV7034, ,
Hi team，

Could you please double check the schematic? Is the input common mode voltage within the normal operating range? Is there a better suggestion for the selection of input resistance?

1.  USB_TX, USB-RX are both high-Z status or pulled to GND.

2. USB_DP is less than 2.1V or 2.7V.

3. NTC_ 3 is a 10k NTC resistor (- 10 ℃ = 43K Ω, 100 ℃ = 1K Ω) connected to GND.

Best Regards

Wesley Huang

  • 0
    Guru 283370 points

    The guaranteed common-mode input-voltage range goes only to 2 V below VCC, so you cannot compare voltages above 1.3 V.

    Use a comparator with rail-to-rail inputs, e.g., LM339LV. To get rid of those expensive pull-up resistors, use a comparator with push/pull outputs, e.g., TLV7034.

  • 0 in reply to Clemens Ladisch
    TI__Mastermind 24520 points

    All valid points.  It appears that you have met the VCM requirements of the LM339 but as Clemens mentioned, there are better devices to use since you are operating at 3.3V.  We created the devices Clemens mentioned for this use case so the VCM is not such a burden on the design.  Likewise, his comment to consider a device with a push-pull output stage is warranted because you are not level shifting at the output.

    Chuck