Finding the perfect N-channel mosfet for 3.3V CPU

[tru3533]

I'm working on a project who need drivers on the output channels from a CPU.
CPU power operates between 1,7V - 3,3V and is powered by a coin cell.

Each output channel can handle max 5mA from the CPU
Need 3 channels

Switching load is 12V 100mA and 24V 100mA

Mosfet needs to be small in size: SOT23?

So the question is what mosfet can I use for switching the load on and off? or should I use a transistor instead?

[joseph nicholas]

Pretty sure the cpu is going to need more that a coin cell to power it up.

I think you need a logic level n channel mosfet .  If its connected to a computer (the cpu)motherboard, I think the input to the cpu is 12 volts and gets regulated down to 3.3volts. 

Hope this helps.

[tru3533]

Nope 
Coin cell it is. CPU is designed for IoT and not larger than the coin cell in size complete.

Problem is the low operating voltage (1,7V) hard to find a device who fully switches on at that level.

[tru3533]

You can read about the CPU here:
https://www.nordicsemi.com/eng/Products/Bluetooth-low-energy/nRF52832

[aandrew]

I'm a HUGE fan of the NTZD3155C (SOT563) and it's physically bigger cousin, the NTGD4167 (SOT23-6) -- they'll both handle your application and have both a P and N channel in the package.

I see the 3155 is out of stock though, bummer. I use that for damn near everything, it's a popcorn part for me. I particularly like using it to drive LEDs from 1.8V logic.

There are hundreds of these kinds of part on digikey; you should spend some time hunting around their page and using their parametric search.  I always click "in stock" and "cut tape" to help reduce the search space.

[tru3533]

Thank you Aandrew,

If I'm not mistaken it seems to be difficult to use them here.
The Rds On (Max) @ Id, Vgs seems to be 4.5 volt (TTL level)
I understand this is the voltage the mosfet needs to be fully on.
and the voltage range I have is 1,7V - 3.3V
Correct or is this OK on this voltage range?

[tru3533]

Looking at the datasheet for NTZD3155C.
It has N-channel 20V VDSS so I can use that on the 12V output channel
I think it will be to low for my 24V output channel

The RDS(on) part is confusing for me. It says
RDS(on) Typ
0.4 ohm @ 4.5 V
0.5 ohm @ 2.5 V
0.7 ohm @ 1.8 V

So how many volt do I need to turn this mosfet fully on?

[T3sl4co1l]

RUM002N05T2L ?

Tim

[tru3533]

T3sl4co1l
Tim, I think you nailed it 

VDSS 50V (good for 12V and 24V)
Id 200mA (I need max 100mA)
1,2V drive (fantastic)
Cheap (even more fantastic)
Perfect size

Thank you so much!

[.rpv]

Hi, if you need a second choice, check the AO3400, has better rdson.

[drussell]

You want it to be small? CSD17483F4 .

Wow...  FemtoFET...  lol 

I'll have to remember those, they actually look pretty stout. 
(Not that I usually actually need anything super tiny...)

[BrianHG]

Need to guarantee no heat dissipation down at a 1.8v Vgs, (0.055ohm) , in sot-23, use this guy: BSS806N
(This is way overkill for 100ma.... but it is sot-23 and I use it in many projects...)

[tru3533]

Thank you all
I think trying to use the same mosfet for all outputs is a good choice.
So VDSS > 24V (drain to source voltage rating, maximum a mosfet can withstand with the gate shorted to the source)

Comparing the suggestions I have the following:
NTZD3155C VDSS= 20V (so not ideal)
RUM002N05T2L VDSS=50V (in the ballpark)
AO3400 VDSS=30V (in the ballpark)
CSD17483F4 (Blueskull I have balls, but not that size  )
BSS806N VDSS= 20V (Love this overkill, will try it on other projects)

That leaves me with RUM002N05T2L and AO3400
Looking at the drive voltage (minimum/maximum voltage to switch the mosfet fully on)
I have
RUM002N05T2L min=1,2V max=4,5V
AO3400 min=2,5V max=10V

So when the CPU operates within the voltage range 1,7V the AO3400 will not turn fully on.
RUM002N05T2L is still standing I think.

Let me know if you have a better suggestion or if I missed something.

Tom

[carljrb]

When it comes to mid-range N channel logic-level MOSFETs, I personally like the BSH103,215 from Nexperia.

Vds max: 30V
Rds(on): 0.6ohm @ 1.8V - 0.4ohm @ 4.5V
Plain old SOT23 package
Decently priced too

Too bad there's zero stock anywhere right now (ditto for BSH103,235), and lead time is 53 weeks...

[tru3533]

Thank you carljrb, nice suggestion, looks like a solid reliable oldtimer. Too bad with the lead time.

[Siwastaja]

Doesn't matter much with those specs.

Use the parametric search of your favorite distributor.

Sort by price.

Note that some at Vds(max)=60V (which is preferred for your 24V load switching, especially if there is any inductance) may not be completely on at Vgs=3V, but they may still be suitable when the current is only 100mA (limited by external load resistance) - look at the actual curves (the Id vs. Vds curve set with different Vgs).

With today's difficult situation, design in one with pin-compatible, same-package replacements from as many different manufacturers as possible. Not difficult with these specs.

SOT-23-3 is great because it's ubiquitous, you find similar replacements very easily, it's easy to add bodges to and rework because it's big enough. Even though it's not actually the package of choice for ultimate miniatyrization, you often find room for it between top layer routing, being able to route a bunch of traces between the pads, even in tightly integrated designs.

Just a FET or a bipolar transistor may not be enough if your load is inductive, or you have a long wiring to the load.

[tru3533]

Thank you Siwastaja for your words of wisdom, I can tell its coming from many years of being in the field 
I will for sure add this to my design rules.

[tru3533]

The next step for finding the perfect n-channel mosfet is to design the perfect circuit around it.
How would you calculate the load resistor?
Would you add anything to the circuit?
If inductive load I will add a diode to the load.

Tom

[Zero999]

I'm working on a project who need drivers on the output channels from a CPU.
CPU power operates between 1,7V - 3,3V and is powered by a coin cell.

Each output channel can handle max 5mA from the CPU
Need 3 channels

Switching load is 12V 100mA and 24V 100mA

Mosfet needs to be small in size: SOT23?

So the question is what mosfet can I use for switching the load on and off? or should I use a transistor instead?

A MOSFET is a transistor. The last letter in the name stands for transistor.
https://en.wikipedia.org/wiki/MOSFET

It think what you mean is a BJT and yes, that's a perfectly sensible suggestion.

The good old BC337, or BC817 which is SMT, will do. It will drop under 1V, at 100mA, with a base current of just 1mA, but I'd aim for a higher base current than that, just to be sure.
http://www.mouser.com/ds/2/149/BC337-193546.pdf
https://www.diodes.com/assets/Datasheets/ds11107.pdf

[BrianHG]

One of the cheapest available to drive 100ma clean at 1.7vgs: NTR4170NT1G
And it still goes down to 0.110ohm at 2.5vgs, 0.07 ohm at 4.5vgs.

[wpwrak]

Everyone has their favourite :-) I like the Diodes DMT1012T (n-channel) and DMT1013T (p-channel). SOT-523, inexpensive, Vgsth(max) = 1.0 V, should be coming out of the resistive region around 1.7 V, good for up to about 0.5 A (n) or 0.3 A (p) continuous, 20 Vds limit.

- Werner

[Siwastaja]

You may notice that if you are optimizing for cost, a BJT may be slightly cheaper than a MOSFET - and while 20-30Vds(max) are almost as cheap, if you really need to 60V or even 80V, then a BJT will usually be considerably cheaper (such as 2 cents instead of 6 cents!)

Adding the base resistor (or using a more expensive "pre-biased" BJT, which will also be slighly less replaceable with fewer alternatives) partially offsets this advantage, though.

[.rpv]

The next step for finding the perfect n-channel mosfet is to design the perfect circuit around it.
How would you calculate the load resistor?
Would you add anything to the circuit?
If inductive load I will add a diode to the load.

Tom

Mosfets doesn't require a MCU-Gate resistor (contrary to transistors), but they do require a pull-down resistor (or pull-up for P-Channel), because they have some capacitance on the gate that makes unable to turn them down once opened. I use a 10k resistor with the AO3400 on 3.3v at 39khz, works fine.

The AO3400 has 1.45v as max threshold, the datasheet says that at 2v you should be able to drain 14A at 5V as vds (theoretical), so at 1.7 should be ok to drain 200mA with 10v vds.

[tru3533]

Thank you all for showing me your favorites, what a fantastic forum this is.
I will add them all to my collection

Hero999
You are absolutely right, I'm forgetting I'm discussing with experts  From now on I will even use SoC instead of CPU. 
BC817 will be my choice.

.rpv
Reason for not using pull down resistor was that the SoC has a internal programmable pull down resistor if needed.
Anyhow the SoC will be considered floating during reset so I include it. I will use 20K for Rpd

The resistor Rb's function is to suppress ringing and reduce emitted EMC for long wire runs, I have now reduced the distance to 5mm so no longer a problem.
I will leave Rb there as I will use BC817 and SOC23. If I change my mind later I have lots of possibilities to play around. For now I will use 470 Ohm for the BC817

Thank you Siwastaja I'm all in backing up my trails, the Rb can easily be replaced by an bodge 

Tom