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Wimpy Comparator Open Collector/Drain Current

Discussion in 'Electronic Design' started by D from BC, May 20, 2007.

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  1. D from BC

    D from BC Guest

    I want to discharge a mosfet gate really fast!
    Cg~2nF at worst.
    Vg=12V

    I'd like to have a open collector/open drain output comparator do the
    job.

    For example.

    The LMV393 open collector sink output acts like a 12 ohm resistor..
    The graph stops at 50mA sink..

    The LMV7235 open drain levels off at 60mA @ a Vlow of 2V!!!
    Built in current limit?
    Wtf? There are mosfets with milliohm conduction...Why not put'm in the
    comparator..

    Is it safe to assume all common comparators have wimpy internal open
    collector/drain output transistors?

    Can I get a 12 amp spike!! :) Arrrffff Arrrfff...

    I suspect I'll have to resort an external transistor to short out the
    charge.
    D from BC
     
  2. MooseFET

    MooseFET Guest

    Take a look at the LM111.
    It will do about 100mA. It is also easy to add a power NPN to its
    output. The emitter connection will go up a diode drop very quickly.
     
  3. Joerg

    Joerg Guest


    That's how I do those things. A BSS123 or 2N7002 can drain the gate
    capacitance really fast. But mind its max peak current. You can always
    use more than one. Sure beats finding that expensive boutique comparator
    and then the purchasing department can't get them anymore some day.
     
  4. MooseFET

    MooseFET Guest

    On a normal open collector comparitor, a PNP is often a better way to
    go if fast pull down is needed. You don't have to wait while the pull
    up charges the gate of a MOSFET.
     
  5. D from BC

    D from BC Guest

    Slow charge up is ok in my app.. It's for that AC breaker circuit
    where fast MOS shut down is needed.

    Do you mean use the PNP like this for fast turn off?

    +12V
    |
    R
    1
    K
    R
    |
    |---|2nF|PowerMosGate---GGGGG
    |
    E
    (PNP)B------sink---->Output open collector comp
    C
    |
    |
    GGGGGGGGGGGGGGGGGGGGGG

    A combination of base and collector current shorts out the gate. ??

    D from BC
     
  6. MooseFET

    MooseFET Guest

    Yes but don't forget the E-B resistor on the PNP. You need to provide
    a path for the leakage current of the open collector circuit. The
    resistor is also what pulls the gate of the MOSFET that last 0.7V
     
  7. D from BC

    D from BC Guest

    I dunno about that 2N7002..
    For shorting out a power mos gate fast the
    NTR4501N mosfet looks super...
    70mohm Rdson
    Ipulse 10Amps!
    Qgtot 2.4nC
    SOT 23 package
    D from BC
     
  8. Joerg

    Joerg Guest


    Yeah but it's input capacitance is 200pF and something needs to swing
    that around. 10:1 is not a lot of a step-down from the 2000pF you've
    already got there.
     
  9. D from BC

    D from BC Guest

    True..
    The NTR4501 has a 200pf Ciss @Vds10V&1Mhz..
    It's tougher to push around compared to the 2N7002 Ciss @ around 20pF.
    The 2N7002 might be quick to turn on but it's higher on resistance at
    7.5ohms discharges a power mosfet gate slower.
    That's a 107:1 ratio (2N7001:NTR4501).

    The drive current (~50mA) has to charge a small mos fast..
    This means a low gate charge (or low gate capacitance) is required..
    Then it's gotta conduct alot.
    This means a low Rdson for the most discharge current to short out the
    charged load(power mos gate) fast.
    D from BC
     
  10. Winfield

    Winfield Guest

    Kudos for you D, for looking up the datasheets, etc. But
    you're barking up the wrong tree. First, comparators are
    meant for general purpose, of course, and not for driving
    MOSFETs. Second, choosing an ntr4501 instead of a 2n7000
    isn't a solution, with its higher capacitance -- whereas
    a low-cost honest MOSFET driver IC would be. If you're
    adverse to such an engineered approach, go with a single
    2n4403 PNP follower, that'll do a respectable job on fast
    turnoff -0.5A drive for small MOSFETs, and beyond that you
    should really should be using a MOSFET driver.

    The truly serious folks use a fast 3A driver, or etc., and
    then they use that to drive a set of 20 to 30A NMOS + PMOS
    transistors, which can drive an IGBT module's gates, etc.
     
  11. Winfield

    Winfield Guest

    I should add, some power MOSFET drivers have internal
    comparator inputs. These are often parts meant for
    PWM applications, etc., and may include half-bridge
    or H-bridge drivers. For example, the HIP4080A types.
     
  12. MooseFET

    MooseFET Guest

    I think you'll find a 2N4403 does more like 700mA when driven by the
    LM111.

    There are transistors like ZXTP25040 from Zetex that do better.
     
  13. Joerg

    Joerg Guest

    Don't know D's application but those things cost several Dollars.
    Sometimes a nice (cheap) push pull pair of BJTs after the comparator can
    be enough.
     
  14. Let's see, an LM311 costs $0.45 plus a ZXTP2504 pnp costs $0.79
    = $1.24, whereas a TC4427CPA dual driver costs $1.05 (single
    pieces from Digikey). A TC4420CPA driver, with 6A capability,
    costs $1.35, so we see that while playing with discrete driver
    circuits is fun, it no longer saves the big bucks.
     
  15. D from BC

    D from BC Guest

    You might upset the discreterotoes.. :)

    Besides...
    I tend to squish my circuits down as far as possible because I make my
    own PCB's..
    So I gravitate toward IC solutions and minimal parts.

    The component reduction caused me to look for general comparators with
    open drains capable of a tiny 5A to 12A pulse.
    But.. looks like a bust..
    D from BC
     
  16. D from BC

    D from BC Guest


    Thanks..
    I'll look for that..
    D from BC
     
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