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I have a device that runs on voltage between 3v and 3v3 and consumes current anywhere between 4mA and 50mA. The device is powered from a battery with a 3v3 regulator.

I need to design a circuit that can switch on/off the device and I can not decide between a BJT and a mosfet for switching.

If I use a MMBT3904 npn transistor, I will have to keep base current at approximately 1mA so that It can drive the device at its maximum current consumption of 50mA (with a gain of 60). It seems like it would be very inefficient when only 4mA of current is required. Also there is the Vce voltage drop which the datasheet says can be 300mV at Ic=50mA. Voltage drop greater than 300mV will cause my device to malfunction.

As far as mosfet is concerned I can not seem to find a mosfet that can drive 50mA with Vgs of 3.3v.

My electronics is a little rusty and It does not seem to me helping me.

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  • \$\begingroup\$ Why can't you use an on/off switch? \$\endgroup\$ – Andy aka Oct 3 '15 at 8:50
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    \$\begingroup\$ @Andyaka The device is not user accessible. \$\endgroup\$ – Hassan Oct 3 '15 at 9:28
  • \$\begingroup\$ fairchildsemi.com/datasheets/FD/FDN327N.pdf will not work? 1.8V Vgs, 2A. \$\endgroup\$ – Fizz Oct 3 '15 at 9:37
  • \$\begingroup\$ If you have to/want to use a high side switch, take a look at this \$\endgroup\$ – EM Fields Oct 3 '15 at 9:50
  • \$\begingroup\$ Also, there are MOSFETs with a lot lower Rdson than what I've indicated above (and is suggested in the answer) but those with ultra-low Rdson don't come in SOT23. \$\endgroup\$ – Fizz Oct 3 '15 at 11:08
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There are so many MOSFETs that will fit this bill. If you are sure that you want a low-side switch, then AO3414 is a reasonable choice, it will drop only a few mV at 50mA and is very inexpensive.

You might want to consider suing a p-channel MOSFET as a high-side switch, depending on the exact application.

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    \$\begingroup\$ The keyword here seems to be search for 1.8V Vgs MOSFETS; both AO3414 and FDN327N are advertised this way... Yours has better RDS(ON). \$\endgroup\$ – Fizz Oct 3 '15 at 10:03
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    \$\begingroup\$ @RespawnedFluff There does not seem to be a reliable way of searching on Digikey, say without opening a lot of data sheets. The manufacturers want to quote the lowest Rds(on) so they often use 10V Vgs. A&O makes mostly low Vgs parts. Starting with low Vgs(th) and then looking at the guaranteed numbers in the data sheets works but is a bit tedious considering the number of possible part numbers. \$\endgroup\$ – Spehro Pefhany Oct 3 '15 at 10:07
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    \$\begingroup\$ No, but it's a reliable way on google, which searches datasheets directly :-) The part you recommended seems a bit of an Asian specialty; Farnell doesn't have it (has the Fairchild one though). But if I got this aspect right from another question, hackerbro7 lives in Asia... so he'll probably be happier with that. \$\endgroup\$ – Fizz Oct 3 '15 at 10:12
  • \$\begingroup\$ @SpehroPefhany Is there any significant advantage of using a P-channel mosfet? \$\endgroup\$ – Hassan Oct 3 '15 at 12:51
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    \$\begingroup\$ It just depends on your application. Nchannel mosfets will easily switch the ground side of the device on and off, p channel will switch the positive side. If you don't care, n channel devices have better performance in general, so use them. \$\endgroup\$ – michaelyoyo Oct 3 '15 at 13:27

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