FETs/switches for 3V and under?

Question

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

Joerg · Accepted Answer

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.

Spehro Pefhany · Answer

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.

Jamie · Answer

might care to contribute. Look at that one.. Vgs(th) ~ 0.7 and less...  Jamie

Pimpom · Answer

"Frnak McKenney"   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.

Frnak McKenney · Answer

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 --  Frank McKenney, McKenney Associates Richmond,...

Frnak McKenney · Answer

Hi, Spehro. Thanks for replying.    [...] 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 --  Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney aatt mindspring ddoott com

Joe Chisolm · Answer

You might also look at the FDC6401N  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

Frnak McKenney · Answer

Hi, Jamie. Thank you for commenting.    [...] 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 --  Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney aatt mindspring ddoott com

Frnak McKenney · Answer

[...]    [...] 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 --  Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney aatt mindspring ddoott com

Frnak McKenney · Answer

[...]   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 --  Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney aatt mindspring ddoott com

FETs/switches for 3V and under?

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