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Peak current to charge a capacitor

Discussion in 'Electronic Basics' started by Peter, Aug 2, 2004.

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  1. Peter

    Peter Guest

    How do I calculate the peak current flow when charging a capacitor?

    To be specific I want to connect a capacitor (among other things) to
    the output of a 74HC595. There will be a diode between the IC and the
    cap. What is the maximum size capacitor that I can use without blowing
    up the chip?
     
  2. John Larkin

    John Larkin Guest

    No limit; you won't blow up the chip doing this.

    John
     
  3. Peter

    Peter Guest

    Are you saying I could attach a 1 Farad capacitor to the chip and not
    overload the output???
     
  4. John Larkin

    John Larkin Guest

    Yup. Or even a short to ground.

    John
     
  5. The HC outputs have an impedance that limits the current. I believe the limit
    is something around 5-6mA, or so, with 5V. This means that for a 1F cap, you'd
    be looking at a max rate of about 5mV/second rate of change. Slow.

    Jon
     
  6. John Larkin

    John Larkin Guest

    Most HCs will actually source a bunch of current, 50 mA into a short,
    25 mA into +3 volts, roughly. But a single HC output won't be damaged
    by driving flat-out in the pullup direction. If you shorted all the
    outputs of a many-gate HC to ground, you could get it mighty hot.

    The bus driver parts are even fiercer.

    John
     
  7. Hmm. I suddenly got worried about my aging memory banks, so I just pulled down
    the SN74HC595 datasheet from TI. It says +/-6mA output drive at 5V on the front
    page! I think that's where I got my 5-6mA figure -- it stuck in my mind that
    way.

    It also gives an absolute package maximum of 70mA continuous and a continuous
    output maximum of 35mA. But, as they also say, operating at these absolute
    ratings is wrong-minded -- they are only to tell what maximum stress they can
    tolerate before being damaged, not operate.

    Did I miss something, then?

    Jon
     
  8. John Larkin

    John Larkin Guest


    My numbers were based on typical curves in the Motorola HC handbook,
    confirmed by some measurements made some time back. The smaller
    numbers you quote are probably guaranteed values for sustaining a
    legal high or low level.

    Hmmm (stands on table to reach dusty bookshelf) here's the TI 1988 HC
    databook. Voh is typically 0.2 below Vcc at 6 mA load, which gives us
    an output resistance of 33 ohms in the pullup direction. That would
    extrapolate to 150 mA into ground, except it's nonlinear so you really
    won't get that much.

    One appnote in the back shows an HC04 dumping about 42 mA into ground,
    20 mA into 3.8 volts.

    Oh, cool, page 4-50 has the output curves. Standard HC has an initial
    pullup slope (near Vcc) of 56 ohms, and short-circuit output of about
    42 mA. The bus drivers are more like 26 ohms and 75 mA, typicals of
    course.

    So these parts theoretically exceed their own max current ratings when
    shorted. I guess long-term damage is theoretically possible,
    electromigration or something. Chargin a cap once in a while should be
    no big deal. You could charge a farad in a minute roughly.

    John
     
  9. Ah, now this curve is what I couldn't find on the datasheet.

    Thanks!

    Jon
     
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