input voltage

Discussion in 'General Electronics Chat' started by shortbus, Nov 26, 2009.

  1. shortbus

    Thread Starter AAC Fanatic!

    Sep 30, 2009
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    On another group that I belong to, we are having a disagreement. So I'm asking the question here because this forum has the most knowledgeable people.

    They are having problems blowing a motor driver chip. The chip, SN754410NE ( http://focus.ti.com/docs/prod/folders/print/sn754410.html )
    The data sheet says that the logic input is TTL or low voltage CMOS, in the chart of electrical values it says that the "max" input is 5VDC.

    In their circuit they are using a 12VDC input voltage.

    I say that is the reason they're blowing the chip. The gurus over there say all modern chips can take the 12VDC and the 5 Vdc means nothing.

    What does the knowledge of this site have to say?

    Thank you for your answers, Cary
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Well, the absolute maximum for Vcc1 (logic supply) is 36v, minimum is -0.5v. However, the recommended range is 4.5v to 5.5v with respect to ground.

    If they're operating them outside of the recommended range, they're begging for problems.

    After all, would you expect the logic to function properly if Vcc1 were -0.5v?
     
  3. steveb

    Senior Member

    Jul 3, 2008
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    I agree with you and SgtWookie. Exceeding 5.5 V on the logic input is likely to damage the chip. Use TTL chips or 5 V cmos such as the 74HCxx series, or CD4000 series powered by 5 V. Why take a chance operating in an uncertain regime?

    There is still the possibility that your issue is something else entirely. But, when you have a problem like this, you must remove all uncertainty before you can be sure of the problem.
     
  4. t06afre

    AAC Fanatic!

    May 11, 2009
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    If 12 volt is used as VCC1 and as control voltage it should work. And not harm the chip. But if VCC1 is 5 volt and the control voltage is 12. That may harm the chip.
    What did they mean by that. All modern chips may certainly not tolerate 12 volt as input. This is pure nonsense
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    If they want to prevent damage to the IC, they will need to ensure that the inputs are clamped to the power rails.

    I suggest using a pair of Schottky diodes (something like 1N5817's) with an input current limiting resistor, like this:

    [​IMG]

    Standard rectifier diodes (like 1N4007, etc) will have too high of a Vf to protect the inputs, which is why I suggest Schottky diodes.

    Automotive environments are brutal, both temperature-wise and electrical-noise-wise.
     
    Last edited: Nov 27, 2009
  6. mik3

    Senior Member

    Feb 4, 2008
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    The best way to solve this is to post the circuit on this forum. The problem may be something not straight forward.
     
  7. shortbus

    Thread Starter AAC Fanatic!

    Sep 30, 2009
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    Thank you ALL for your answers. You guys are the best.

    Cary
     
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