Undervoltage Monitor

Discussion in 'General Electronics Chat' started by iONic, Nov 15, 2010.

  1. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
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    Can someone explain how this circuit is functioning...when does the LED turn on?

    [​IMG]
     
  2. bertus

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    Apr 5, 2008
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    Hello,

    Did you read the datasheet?

    This is at the introduction:

    Bertus
     
  3. iONic

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    Yes, the curcuit I posted was from the datasheet, but there was no example on using an external resistor. Am I to gather that the LED will turn on when the Supply voltage is 3.8V?
    Is it designed for a 5V source only and can not be modified?
     
  4. bertus

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    Hello,

    The circuit is designed to reset micrprocessors at a to low voltage input.
    There is no possibility to change the voltage with external resistors.

    What voltage would you like to have a low voltage detection for?

    Bertus
     
  5. iONic

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    Nov 16, 2007
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    It offers the designer an economical solution for low voltage detection with a single external resistor. Apparently the resistor they are refering to is the current limiting resistor for the LED??
    In the case of the circuit I provided, it has nothing to do with resetting anything and does not nessesarily have to do anything with a microprocessor.
     
  6. bertus

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    Hello,

    The single resistor is refering to the pull-up resistor at the output, also to be used in case of driving a led.

    Bertus
     
  7. tom66

    Senior Member

    May 9, 2009
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    I suppose a resistor in line with Vcc may work, but it could cause erratic operation due to the Vcc supplying the comparator as well.

    If you are ordering many IC's, you can request a custom threshold voltage from Microchip.
     
  8. iONic

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    Still no real answer...
    Let's assume that the Power Supply is 5V. The LED will turn on when the Power Supply drops 1.2V to 3.8V, Yes? This is what is expected to be on Pin 2 for the LED to turn on. Actually probably a hair less than 3.8V. Are these assumptions correct?

    Thanks
     
  9. eblc1388

    Senior Member

    Nov 28, 2008
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    The trip voltage is clearly stated in the data sheet. It is fixed at 4.6V, with hysteresis on both sides.

    [​IMG]
     
  10. iONic

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    Nov 16, 2007
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    Guess I got stuck on reading the text and didn't see too much there.

    OK, so when the input source voltage is 5V the LED will come on when the source is ~4.6V and lower.

    The real query here is this:

    What would the trip voltage be when the input source is 9.0V?
    What would the trip voltage be when the input source is 4.2V?
    What would the trip voltage be when the input source is 3.0V?

    Is the trip voltage fixed regardless of the source? I am assuming Yes... but am not sure.

    Thanks
     
  11. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Trip voltage is 4.6V with a supply up to 10V.
    It doesn't change depending on the input voltage.
    If input is 3.0V the chip is in tripped mode.
    If input is 4.2V the chip is in tripped mode.
    If input is 9.0V the chip is in not tripped mode.

    What are you trying to do? Seems like this is the wrong chip for whatever you are thinking about.
     
  12. bertus

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    Hello,

    Did you see this graph in the datasheet?

    [​IMG]

    Bertus
     
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