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galapogos

Help me choose a mosfet

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Hi,
I have an application where I have to switch on/off 2 3.3V low power
devices(~50mA max combined) with a 3.3V MCU signal. I am currently
using a TP0610K that I have lying around with the following
connections.
Source - 3.3V
Gate - 50K pullup to 3.3V, and MCU control signal
Drain - 1M pulldown and device Vcc

This seems to work, but according to the TP0610K datasheet (http://
www.vishay.com/docs/71411/71411.pdf), 3.3V doesn't even begin to bring
the fet into saturation mode. Here are the relevant specs of the
TP0610K (at least those that I think is relevant):
Vgs(th) = -1.0/-3.0V min/max
Id(on) = -50mA (Vgs = -4.5V, Vds = -10V)
Rds(on) = 10ohm (Vgs = -4.5V, Id = -25mA)
*According to the Rds(on)-Vgs graph, Rds(on) rises sharply at about
Vgs = 5V

Now, while the switch seems to work now, there are a couple of things
that worry me.

1) The Id(on) of 50mA is probably lower when Vgs, Vds = -3.3V, and
this may be a problem since my devices consume about 50mA max
2) Rds(on) rises almost to an asymptote when Vgs < 5V

With that said, I looked at a few other mosfets that I could drop into
my existing design, and came up with the following:
Fairchild FDV304P - Vth -0.86V, Id -460mA, Rds(on) 1...@-3.3V Vgs
Fairchild FDV302P - Vth 1V, Id -120mAm Rds(on) ~...@-3.3V Vgs
ON NTR1P02LT1 - Vth -1V, Id -1.3A, Rds(on) curve bends at -4V Vgs,
0.19ohm-2.5V Vgs
ON NTR2101P Vth -1V, Id -3.7A, Rds(on) 0.05...@-3.3V Vgs
ON NTR4101P - Vth -0.72V, Id -3.2A, Rds(on) 0.09...@-2.5V Vgs
Vishay Si2323DS - Vth -1V, Id -4.1A, Rds(on) 0.035...@-3.3V Vgs
Vishay Si2301BDS - Vth -0.95V, Id -2A?, Rds(on) 0.1...@-3.3V Vgs

There are a few more, but I just want to make sure if I have the right
idea. Basically what I'm thinking is that I need a mosfet that has the
following characteristics:
1) low threshold voltage(all the above have Vgs(th) >= -1V
2) Sufficient Id(-120mA and less should be sufficient?)
3) Low Rds(on)(single digit, or even <1ohm shouldn't cause any
noticable voltage dip at device Vcc)

Some of the above mosfets are for logic rather than power switching,
but I'm guessing that as long as the current capacity(Id) is
sufficient, it shouldn't matter?

Is there anything else I should worry about?

Thanks!

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Take a look here

http://www.vishay.com/mosfets/lteq-20-v/micro-foot-package/

Note the Rds on with a Vgs of only 1.5V!! Even at 2.5V they are all below .05 ohms

In general, you'll find that N channel MOSFET's have lower Rds on numbers than P channel.

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Thanks, but due to the existing design I have to use a P channel transistor. Is a low RDS and Vth all that I should be looking at? I guess I just need to know some guidelines on what parameters are important when choosing a transistor. I might even go for a PNP BJT since they seem to be more recommended.

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