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LM239: Do I need to add input current limit resistor at the input of LM239?

Howard Zou
Genius 12365 points
Part Number: LM239

Hi,

some comparators need to add input current limit resistor at the input. How about LM239?

  • 0
    Guru 140200 points
    Hi Howard,

    yes, the input current must be limited when the input voltage becomes negative. Read section 7.1 of datasheet and the footnote (4).

    Kai
  • 0 in reply to kai klaas69
    TI__Genius 12365 points

    Thank you. But why you emphasize negative voltage?

    According to footnote (4):

    Input current flows through parasitic diode to ground and will turn on parasitic transistors (my understanding is that this should happen when IN+ voltage is higher than GND shown in the picture below)  that will increase ICC and may cause output to

    be incorrect. Normal operation resumes when input is removed.

    And what's the value of resistor do you recommend?

    Kai,

  • 0 in reply to Howard Zou
    TI__Mastermind 49836 points

    Hello Howard,

    We recommend limiting the current to 1mA or less (less is better). A "rule of thumb" is 1k ohm per volt of over-voltage, so if 5V max, use a 5k minimum.

    The resistance can be part of the input divider or other network.

  • 0 in reply to Howard Zou
    Guru 140200 points

    Hi Howard,

    this scheme shows one path where an insane input current can flow, if the input voltage is negative: From the collector to the base of input transistor. Remember, the input transistor is a PNP.

    But there are also some other parasitic pn-junctions on the die of LM239.

    Kai