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LM393 driving small MOSFET

Discussion in 'Electronic Components' started by ian field, Feb 19, 2007.

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  1. ian field

    ian field Guest

    My problem is "how fast can an LM393 drive the gate of a small MOSFET"? At
    the moment the exact type of MOSFET isn't finalised, and even if it was I've
    yet to see a datasheet give a straight answer on the actual value of gate
    capacitance, if I knew that I could calculate the charge/discharge time with
    the minimum allowable 393 O/P pull up resistor.

    One thing that would ease the problem, is if it is possible to wire the pair
    of comparators in parallel to increase the O/P sink capability, the
    alternative is to drive an external transistor to get more current - but the
    whole point of using the 393 was to eliminate a quantity of discrete
    transistors.

    TIA.
     
  2. Eeyore

    Eeyore Guest

    Really ?

    You might be better off with an op-amp then.

    Graham
     
  3. ian field

    ian field Guest

    Already looked at OP-AMPs - with the low Vcc headroom none of the devices I
    looked at could guarantee close enough to true "rail to rail" O/P.
     
  4. Eeyore

    Eeyore Guest

    What supply voltage is available ?

    Graham
     
  5. ian field

    ian field Guest

    5.4 - 7.2V
     
  6. John

    John Guest

    A 2N7000 or 2N7002 is a "small" MOSFET. The data sheet shows that about 1 nC
    of gate charge will get it through the Miller region. The 393 output current
    is specified at 6 mA minimum so you will be using about 1200R pull-up. That
    should get you about 270 ns if I've done my arithmetic correctly.

    Good luck.

    John
     
  7. Eeyore

    Eeyore Guest

    I see. Is that even enough to turn on the device well ?

    Graham
     
  8. Eeyore

    Eeyore Guest

    It'll pull up faster with a constant current load but then you're back to using
    discretes.

    Graham
     
  9. ian field

    ian field Guest

    Which device are you referring to? The comparator should be perfectly fine
    on TTL supply and the MOSFETs I have to hand have a Vgs-thr upper tolerance
    of about 3 or 4V.

    The Vgs-thr lower tolerance is about 1V, looking at the graph for the O/P
    transistor Vce-sat V's sink current shows it reaches 1V at just over 10mA.
    Don't forget the transistor has to discharge the gate capacitance as well as
    sinking the load resistor current, since the MOSFET is driving a flyback
    inductor the cut off needs to be as fast as possible.
     
  10. ian field

    ian field Guest

    .. The 393 output current
    Thanks for that, the data sheet I have neglected to mention that figure.

    By the look of your figures it should be plenty fast enough, I'll have to
    look if the data sheet for the MOSFET gives that parameter.

    Thanks again.
     
  11. ian field

    ian field Guest

    The main motivation for using a chip was I'd run out of E-line BC182s and
    the TO92 equivalents use more strip board because they're bulkier, however
    someone left a Sanyo TV in the bin room and it had loads of 2SC1740s which
    are even smaller than E-line and almost identical spec to the 182.

    OTOH the figures John provided suggest I shouldn't be having difficulty with
    this, if the figures translate well into the MOSFET I'm using the comparator
    circuit should switch more efficiently than the discretes with little
    difference in board size.

    Thanks all.
     
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