Finding the perfect N-channel mosfet for 3.3V CPU - Page 1
Forum section: electronics / projects

author: tru3533
author_rank: Regular Contributor
timestamp: July 30, 2018, 08:21:41 pm
content: 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 CPUNeed 3 channelsSwitching load is 12V 100mA and 24V 100mAMosfet 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?

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: July 30, 2018, 08:47:19 pm
content: 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.

author: tru3533
author_rank: Regular Contributor
timestamp: July 30, 2018, 08:54:57 pm
content: NopeCoin 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.

author: tru3533
author_rank: Regular Contributor
timestamp: July 30, 2018, 09:04:28 pm
content: You can read about the CPU here:https://www.nordicsemi.com/eng/Products/Bluetooth-low-energy/nRF52832

author: aandrew
author_rank: Frequent Contributor
timestamp: July 30, 2018, 10:15:43 pm
content: 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.

author: tru3533
author_rank: Regular Contributor
timestamp: July 30, 2018, 10:30:38 pm
content: 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 - 3.3VCorrect or is this OK on this voltage range?

author: tru3533
author_rank: Regular Contributor
timestamp: July 30, 2018, 10:59:23 pm
content: Looking at the datasheet for NTZD3155C.It has N-channel 20V VDSS so I can use that on the 12V output channelI think it will be to low for my 24V output channelThe RDS(on) part is confusing for me. It saysRDS(on) Typ0.4 ohm @ 4.5 V0.5 ohm @ 2.5 V0.7 ohm @ 1.8 VSo how many volt do I need to turn this mosfet fully on?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 30, 2018, 11:00:11 pm
content: RUM002N05T2L ?Tim

author: tru3533
author_rank: Regular Contributor
timestamp: July 30, 2018, 11:21:07 pm
content: T3sl4co1lTim, I think you nailed itVDSS 50V (good for 12V and 24V)Id 200mA (I need max 100mA)1,2V drive (fantastic)Cheap (even more fantastic)Perfect sizeThank you so much!

author: .rpv
author_rank: Contributor
timestamp: July 31, 2018, 02:40:55 am
content: Hi, if you need a second choice, check the AO3400, has better rdson.

author: drussell
author_rank: Super Contributor
timestamp: July 31, 2018, 03:01:29 am
content: Quote from: blueskull on July 31, 2018, 02:46:31 amWow... FemtoFET... lolI'll have to remember those, they actually look pretty stout.(Not that I usually actually need anything super tiny...)

author: BrianHG
author_rank: Super Contributor
timestamp: July 31, 2018, 03:15:49 am
content: 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...)

author: tru3533
author_rank: Regular Contributor
timestamp: July 31, 2018, 12:06:51 pm
content: Thank you allI 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 AO3400Looking at the drive voltage (minimum/maximum voltage to switch the mosfet fully on)I haveRUM002N05T2L min=1,2V max=4,5VAO3400 min=2,5V max=10VSo 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

author: carljrb
author_rank: Contributor
timestamp: July 31, 2018, 12:47:25 pm
content: When it comes to mid-range N channel logic-level MOSFETs, I personally like the BSH103,215 from Nexperia.Vds max: 30VRds(on): 0.6ohm @ 1.8V - 0.4ohm @ 4.5VPlain old SOT23 packageDecently priced tooToo bad there'szerostock anywhere right now (ditto for BSH103,235), and lead time is 53 weeks...

author: tru3533
author_rank: Regular Contributor
timestamp: July 31, 2018, 01:05:50 pm
content: Thank you carljrb, nice suggestion, looks like a solid reliable oldtimer. Too bad with the lead time.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 31, 2018, 01:36:29 pm
content: 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.

author: tru3533
author_rank: Regular Contributor
timestamp: July 31, 2018, 02:00:46 pm
content: Thank you Siwastaja for your words of wisdom, I can tell its coming from many years of being in the fieldI will for sure add this to my design rules.

author: tru3533
author_rank: Regular Contributor
timestamp: July 31, 2018, 03:40:58 pm
content: 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

author: Zero999
author_rank: Super Contributor
timestamp: July 31, 2018, 04:59:04 pm
content: Quote from: tru3533 on July 30, 2018, 08:21:41 pmA MOSFET is a transistor. The last letter in the name stands for transistor.https://en.wikipedia.org/wiki/MOSFETIt 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.pdfhttps://www.diodes.com/assets/Datasheets/ds11107.pdf

author: BrianHG
author_rank: Super Contributor
timestamp: July 31, 2018, 06:17:16 pm
content: One of the cheapest available to drive 100ma clean at 1.7vgs: NTR4170NT1GAnd it still goes down to 0.110ohm at 2.5vgs, 0.07 ohm at 4.5vgs.

author: wpwrak
author_rank: Regular Contributor
timestamp: July 31, 2018, 06:45:02 pm
content: 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

author: Siwastaja
author_rank: Super Contributor
timestamp: July 31, 2018, 06:54:08 pm
content: 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.

author: .rpv
author_rank: Contributor
timestamp: July 31, 2018, 09:46:04 pm
content: Quote from: tru3533 on July 31, 2018, 03:40:58 pmMosfets 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.

author: tru3533
author_rank: Regular Contributor
timestamp: August 01, 2018, 09:32:05 pm
content: Thank you all for showing me your favorites, what a fantastic forum this is.I will add them all to my collectionHero999You are absolutely right, I'm forgetting I'm discussing with expertsFrom now on I will even use SoC instead of CPU.BC817 will be my choice..rpvReason 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 RpdThe 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 BC817Thank you Siwastaja I'm all in backing up my trails, the Rb can easily be replaced by an bodgeTom