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Low RDSon Logic CMOS Gate

Discussion in 'Electronic Design' started by Klaus Kragelund, Oct 1, 2012.

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  1. Hi

    I need a CMOS gate with low voltage drop at about 30mA current source and sink. The tiny logic NL27WZ14 has 600mV drop at 32mA which equates to 18ohm RDS on.

    I am searching for a device with less than 5-10 ohms and it would need to be ok to parallel to get lower resistance. Also, it must not have to much crossover shoot-through (so probably schmitt trigger type) and must have low dissipation at high operational frequency (1-10MHz)

    Any one have a part in mind?


  2. hamilton

    hamilton Guest

    Please describe shoot-through.
  3. Been through some posts, but did not find it. Can you point me in the rightdirection? :)


  4. Fred Bartoli

    Fred Bartoli Guest

    Klaus Kragelund a écrit :
    Mucho thanks for posting this...

    In an attempt to answer I had a look at a recent design and noticed that
    a 5V powered LVC14 escaped all the design reviews :-(

    It survived all the stress test: 27MHz switching, with PCB temp=100°C
    while driving a 250pF Ciss Mosfet...

    How did you know ?-)
  5. I found maybe a slightly better one, the NC7SZ14. The device lists CPD of 24 (Power dissipation capacitance, but no information on the unit. Icc = Cpd * V * f.

    (page 5 of

    I guess it's in pC (pico coloumb), which corresponds to 100uA/MHz.


  6. whit3rd

    whit3rd Guest

    One can use discrete complementary MOSFETs to make a CMOS-like inverter,
    of course. Ferrite beads can help with the shoot-through problem, your
    frequency of interest seems low enough not to care. It may matter what
    voltage you're powering from, as well. At 5V, there's more options than
    at 2V.
  7. Hi Jim

    Thank you for taking time to elaborate on this ;-)

    The email is:

    It is valid and hotmail spamfilter works, I hardly get spam any more...


  8. That's a secret ;-)

    Funny how some circuit work when they are not supposed to and other circuits that look clean and nice has "gotchas"....


  9. I have 3.3V, can make 5V if I need to. This is for a high efficiency device, so any powerloss is critical. Shoot through if a FET pair can easily go up to several amps, which won't be noticed if you are not looking.


  10. Fred Bartoli

    Fred Bartoli Guest

    Klaus Kragelund a écrit :
    The unit is pF.
    Multiply by 1/2 Vsupply^2 and you have the energy quantum lost at each

    Total power loss is PL = f x Cpd x Vsupply^2
  11. Nico Coesel

    Nico Coesel Guest

    Maybe look for a MOSFET driver chip.
  12. Guest

    (from John Devereux, earlier this year)


    DigiKey had competing parts, too.
  13. Jamie

    Jamie Guest

    Talking about Digikey, they getting any better at stocking more of what
    they advertise? I have been leaning towards the other parts suppliers

  14. Guest

    that was my first thought, something like fan3111, but..

    datasheet doesn't say what the dropout is, it maybe a bit slow for
    10MHz and who
    knows how much power it will use

    maybe a buffer/linedriver

  15. Guest

    HC or HCT ?

    TI <3mA per pin at 2.4Vin
    NXP <590uA at 2.9Vin

    that shouldn't be a problem driven with a 3.3V output

  16. whit3rd

    whit3rd Guest

    A dual MOSFET (P-channel/N-channel pair) like Rohm EM6M2 can easily handle the task.
    If you add a couple of 2 ohm drain resistors, it limits the shoot-through to well under one
    amp, but the output resistance only gets two ohms drop (still well below your 5 ohm
    goal for 3.3V power supplies). Note the PFET has rather high capacitance, that's
    required to get the low on-resistance.

    For best shoot-through limit, it gets complicated (you need something more than
    a logic drive, it takes turn-on delayed but turn-off prompt).
  17. ...Snip..

    The EM6M2 has 1nC of gate charge. Running that at 5V and 200kHz will consume 1mW. I was trying to reduce the gate charge losses by using a logic IC

    On that subject, anyone know the value of the gate charge for a run-of-the-mill gate inside a chip?


  18. Guest

    For sure there are higher Rds(on) parts with lower capacitances,
    right? I was just throwing out an extreme example, since I'm an
    extremist :).

    I suspect the make-before-break convenience, integration, and
    capacitances in other switches will be attractive.

    I didn't have time to screen parts--I mostly have to post and run
    these days.

  19. Guest

    I've destroyed 74AC parts that way. Smoked 'em.
  20. Guest

    HCT stuff isn't tuned for speed so has little crossover current (none,
    ideally). AC logic is a whole different kettle. Better decouple AC gates
    well. ;-)
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