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FETs/switches for 3V and under?

My brother is working on a microprocessor-based project that will run from a 3.7V Li-Poly cell. Since it needs to switch loads on the order of a half Amp or more, he needs some sort of driver connected to each GPIO port pin.

He has tried a BJT (2N2222), but the voltage drop across it is too high for it to be useful, so my first thought was an xxxFET of some kind. My second thought was that most of the FETs I was familiar with either wouldn't carry the load or required too high a Vgs (typically on the order of 4V).

So I checked Mouser and found that they were now carrying several new products that were a much better match:

Rds(on) Q1 Part Vds(max) Id(max) (mOhm) Vgs(th) 0.89 MTP3055VL 60V 12A 180

Reply to
Frnak McKenney
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What load rail voltage? 3.7V?

Vth or Vgs curves mean nothing. You need to have guaranteed max Rdson values in the tables for the minimum expected driver voltage.

Sure:

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Problem: They cost only 10c, not 25c :-)

Too pricey, only over my dead body.

might care to contribute.

This was just a quick shot, there are more devices and you might find some cheaper ones as well.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

Try something like the AON2400, about 25 cents in 1K, which has Rds(on) of 23m ohm max with 1.2V drive (you also have to take increase of Rds(on) with temperature into account.

In general, look for MOSFETs with very low Vds(max), they will tend to be the ones that are guaranteed to work with low voltage drive.

Reply to
Spehro Pefhany

might care to contribute.

Look at that one.. Vgs(th) ~ 0.7 and less...

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Jamie

Reply to
Jamie

How about a ZTX869 BJT - TO92 package Max Ic = 5A continuous, 20A peak Max Vceo = 25V Typical Vce(sat) at 0.5A = 25mV (50mV max)

- that's equivalent to a slope of 50mohm (100mohm max) Gain much higher than 2N2222

For Vceo 60V at a slightly higher Vce(sat) there's the ZTX851.

Don't know how much they cost or how readily available they are, though.

Reply to
Pimpom

Hi, Joerg.

Thanks for the reply.

Yes. And ( I forgot to mention ) a resistive load, not an inductive load. ( Important at times. )

The thing that impressed me was that the graphs even bothered with curves that low. The xxxFET datasheets I had been looking at before didn't bother drawing curves below 4.0V or 3.5V.

Well, from Mouser they're Q1@$0.41, but, true, Q3000@$0.102.

Thank you. I think I missed these because I had been restricting my searches for through-hole "fit into a solderless breadboard" parts; given the distinctly better performance, maybe I'll just send my brother some SMD-to-DIP adapters.

Thank you.

Frank

--
  Five senses; an incurably abstract intellect; a haphazardly 
  selective memory; a set of preconceptions and assumptions so 
  numerous that I can never examine more than minority of them - never 
  become conscious of them all.  How much of total reality can such an 
  apparatus let through?     -- C. S. Lewis
Reply to
Frnak McKenney

Hi, Spehro.

Thanks for reply> >

[...]

From Digi-Key, Q1 @ $0.67, Q3000 @ $0.1891. Nice. ( And even more fun to hand-solder than Joerg's FDV305N. )

Thank you.

Frank

--
  There is gold in the San Juans, and silver also, but the real 
  treasure is in the air, the trees, the wildflowers, and the big, 
  wide, open wonderful country.  If you don't have it, all the money 
  in the world won't buy it.  And it is there for anybody who will 
  use it kindly. 
                -- Louis L'Amour / The Sackett Companion
Reply to
Frnak McKenney

You might also look at the FDC6401N

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It's a dual in a sot23-6. Save some real estate if you are using multiple GPIO. The PSPICE model will work with LTSpice.

--
Chisolm 
Republic of Texas
Reply to
Joe Chisolm

Hi, Jamie.

Thank you for comment> Frnak McKenney wrote:

[...]

Also impressive. In SOT23-3. Mouser: Q1 @ $0.43, Q1000 @ 0.12.

And Mouser also carries the Schmartboard 204-0003-01 which has 2 ( somewhat oversized, for my purposes ) SOT23-to-DIP carrier boards.

Thanks.

Frank

--
  Gallipoli illustrates a principle which began to be apparent in 
  European wars as early as the Armada.  It is that distance lends 
  enchantment to the most crackbrained strategic ideas.  Not that the 
  high command or the home public is any less well informed about 
  operations in distant theaters.  All the necessary facts may be at 
  hand; they simply lack urgency; the imagination has freer play with 
  them.     -- Charles Fair / From the Jaws of Victory
Reply to
Frnak McKenney

[...]

[...]

Nice. A four-pack would make a nice BJT H-Bridge.

Also a nice-looking part.

Mouser list these as:

ZTX869: Q1 @ $1.25, Q1000 @ $0.532 ZTX851: Q1 @ $0.96, Q1000 @ $0.465

Both in-stock.

Thank you for pointing these out.

Frank

--
  A complex system that works is invariably found to have evolved 
  from a simple system that worked ...A complex system designed from 
  scratch never works and cannot be patched up to make it work.  You 
  have to start over, beginning with a working simple system. 
                             -- Grady Booch
Reply to
Frnak McKenney

[...]

FDC6401N 20V 3A, but a two-fer. Half an H-Bridge.

Mouser: Q1 @ $0.41, Q1000 @ $0.174

Thanks...

Frank

--
  The scientific theory I like best is that the rings of Saturn are 
  composed entirely of lost airline luggage.     --Mark Russell
Reply to
Frnak McKenney

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