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FET switch

Discussion in 'Electronic Design' started by James, Mar 3, 2007.

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  1. James

    James Guest

    Hi all,
    Can anyone recommend a small (<=sot-23), low cost FET to switch a 3V
    2mA load? I would like it to be on the high side of the load and have
    a micro port pin drive the gate.

    Thanks,
    James
     
  2. A standard answer would be a p-channel MOSFET, or perhaps a PNP
    transistor. But the MOSFET would have to be a low-threshold-
    voltage type, and the PNP would require a base resistor. So we
    have excuses to keep looking.

    I think a good candidate would be a 1-gate logic buffer, a
    modern version of the old 74HC125 drivers. Consider 1G125
    gates in the LVC family. These have Rout less than 30 ohms
    at Vcc = 3V, so a 2mA load would drop less than 60mV.

    TI's 74LVC1G125DBV uses a sot-23-5 package, with 0.95mm lead
    spacing. Philips (NXP) part of the same size, 74LVC1G125GW,
    says it's in a sot-753 SC-74A package. TI's 74LVC1G125DCK
    uses a miniature sot-70 package, with 0.65mm lead spacing.
    NXP's 74LVC1G125G says it's in a sot-353 (TSSOP5) package.
    Sigh, whatever, no doubt both small 0.95mm parts will solder
    onto those pads, and ditto for the miniature 0.65mmm parts.

    The 74LVC1G125DBV and DCK are 39 cents at DigiKey qty 1.
    The NXP parts are 30 cents, six cents in a full reel.
     
  3. Think of these as a low-cost low-threshold p-channel MOSFETs.
    These are tri-state gates, so if you tie the input to Vcc you
    can use the tri-state control as if it was the MOSFET's gate.

    __ 74LVC1G125DBV
    | \
    Vcc ---| >--- OUT
    |__/
    O (Vcc and GND supply connections not shown)
    |
    control ---'

    Speaking of low-threshold voltages, offhand I don't know
    of any discrete MOSFETs that can do as well, here're the
    datasheet Ron values at low voltages, down to 1.65 volts:

    Vcc R_hi R_lo
    ---- --- ----
    1.65 112 112 ohms
    2.3 50 37.5
    2.7 42 33
    3.0 30 23
    4.5 22 17
     
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