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  1. #1
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    High speed MOSFET DRIVER?

    I have this problem,

    Have CMOS output 3.3V , V0<0.4V V1>2.9V Rise/Fal<4ns , speed is responding 12MHz clock max load capacitance 15pF

    and need drive MOSFET , It is charge 200pF, 9nC ,on min 5.5V, frequency 12MHz.

    Can you recommend a MOSFET driver that would do it?

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  2. #2
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    Re: High speed MOSFET DRIVER?

    Hi,

    focus on the mosfet.

    you say 9nC. This is the charge you need to push into the gate.
    Now you need to know what max. time you allow for this. (this is not the 4ns CMOS output rise/fall time)
    Then divide the charge by the time and get the average gate current (during charge).

    Then you need to know if you want to drive a high side or a low side MOSFET, and whether you need isolation or not...

    Power side voltage is interesting, but it should be already included in your charge calculation (9nC).

    Klaus
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  3. #3
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    Re: High speed MOSFET DRIVER?

    I need simple Low Side non isolated driver.
    In terms of current, anything above 1.2A will pass.
    I did a practical exam with what I had at hand.
    As clock source i use any fluke generator clock level 0/5V 12MHz rise/fall 4ns
    This clock drive CMOS gate 74HCxx, TI say one gate on 6V drive 50pF load on 6ns , I use 4pcs ||,
    it worked quite reliably on 6V.
    Unfortunately, in the design I only have a 5.6V, 25V and more.
    For good working on 5.6V was need 5 gate ||.

    I look on MIC4416
    may be 7.5Ohm N and P MOSFET on output output stage current is good for me, input stage voltage logic level too. Where do I see the problem?
    Delay Time, VCTL Rising 42ns in my opinion, it will omit pulses.



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    Re: High speed MOSFET DRIVER?

    Hi,

    I´m confused.
    --> Draw a picture/schematic of how you want your circuit.

    in my opinion, it will omit pulses.
    Why? Please explain.

    Klaus
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    Re: High speed MOSFET DRIVER?

    MOSFET drivers' risetime will be spec'd with a specific load.
    Your 200pF (Cgg?) is way lower than what most power
    MOSFET drivers are spec'd against. You would look for
    some risetime vs load, or maybe get into LTSpice with
    vendor models for driver and FET off the Web (mfr or
    LTSpice group on Yahoo!).

    Just because delay is large doesn't mean pulses will be
    skipped either. The drive "taper chain" is like a pipeline
    to great extent. What you might worry more about is
    how much of the time the FET is well driven and how
    much, it and the driver are in higher-than-rated power
    dissipation and might cook themselves and/or the load.



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  6. #6
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    Re: High speed MOSFET DRIVER?

    The 4416 may work, but at 12 MHz it will get warm - so plenty of copper trace for heatsinking ...

    - - - Updated - - -

    I note that at 10MHz and zero load capacitance it draws ~ 40mA so at 3v3 this is 132mW, bu there is another issue, the rise and fall times are in the order of 30-40nS, given that at 12MHz the half period is ~40nS - this may be a problem, you will end up with sinusoidal gate drive and possibly Vdrain will bee sinusiodal too ...

    - - - Updated - - -

    Try Renesas: EL7202, EL7212, EL7222 High Speed, Dual Channel Power, 4v5 min operating tho'



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