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LM339: Question about protection against negative voltage.

Kazuya Nakai59
Guru 22295 points

Part Number: LM339
Other Parts Discussed in Thread: TL331, LM393

Tool/software:

Hello guys,

One of my customers is considering using LM339 for their new products.

At this moment, they have the following questions.
Could you please give me your reply?

Q1.
They want to measure +/-10V with LM339 as the Figure 6-7 on page 20 of the following TI document.

Application Note Application Design Guidelines for LM339, LM393, TL331 Family Comparators Including the New B-versions
https://www.ti.com/jp/lit/an/snoaa35f/snoaa35f.pdf

Are their understanding correct about LM339 negative voltage protection?

1. Vf of D1 should be 0.3V or less.
2. The current of RI should be 1mA or less.
3. Pay attention to the voltage drop caused by R1 × IR.

Q2.
Is it better to place a capacitor (about 1000pF) between INPUT+ and GND (=between D1 both side)?
If this answer is yes, is 1000pF OK for the capacitor?

Your reply would be much appreciated.

Best regards,
Kazuya.

  • 0
    Guru 322010 points

    1. Yes, that circuit protects against input voltages below that of the GND pin. (If you have ±10 V signals, why don't you use ±12 V supplies?)

    2. A capacitor in this circuit, together with the resistor, will act as a low-pass filter. I do not know what frequencies you want to filter out.

  • 0 in reply to Clemens Ladisch
    TI__Genius 13425 points

    Hi Kazuya-san,

    Q1:

    1. The overall design goal is to ensure that that input of the LM339 never sees a negative voltage less than -300mV. This could be done with your suggestion of selecting a diode with the forward voltage of 300mV or less, or you could follow the design circuit on page 21 Figure 6-9.

    2. Yes, the applications note also speaks about tradeoffs of resistance values.

    3. The voltage drop across R1 is the input voltage that the comparator "sees", so this is a value that you need to pay attention to.

    Q2:

    Like Clemens' response, an input capacitor would act as an RC low pass filter. The addition of a capacitor depends on the desired function for it in your application.