High side switch - Page 1
Forum section: electronics / projects

author: Matt74
author_rank: Newbie
timestamp: January 10, 2020, 08:03:08 am
content: Hello,I need a high side switch to power ON/OFF a sub-circuit (to save power, it runs on battery) and I was wondering what would be the advantages/drawbacks of using : an N-channel Mosfet (DMN2005LP4K-7) with a driver (LTC1981) versus a P-channel MOSFET (PMF170XP,115) connected directly to the microcontroller output (nRF52)There is only one voltage rail available (2.8V). That would be the voltage at the gate and at the source (hence the need for a driver in the N configuration)The load is very low, Id does not exceed 5mAThe switching freqency is very low (ON for 200us every 20ms)I'll post a schematics as soon as get my ECAD software to work again...Thanks !

author: mjs
author_rank: Regular Contributor
timestamp: January 10, 2020, 08:22:55 am
content: N-MOSFETs have typically better performance for high load currents or voltages. Or you might want to prevent some power sequencing faults.If you can find P-MOSFET with suitable Vgs and Idss(off), go with that.

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: January 10, 2020, 09:33:15 am
content: I use these very often (SK2301AA1SHB) driven directly by a gpio:https://lcsc.com/products/MOSFET_381.html?q=a1shbhttps://datasheet.lcsc.com/szlcsc/SK-SK2301AA1SHB_C118153.pdf

author: magic
author_rank: Super Contributor
timestamp: January 10, 2020, 10:06:35 am
content: PMOS has higher RDS(on) so the load will see lower voltage when it's switched on. For example, 2Ω RDS(on) will make your 2.8V supply 2.79V when 5mA is drawn from it, but 1Ω would make it only 2.795V.NMOS requires a complex driver which consumes power, board space and $$.With only 5mA of load current the choice is obvious

author: IgorIgor1233123
author_rank: Newbie
timestamp: January 10, 2020, 10:49:41 am
content: you dont need complicated electronics, in most cases you can just use a voltage pump thats only a view small components, also today you have everywhere microcontrollers so you can create the square wave directly. So really not difficult or expensive. But yes for 5mA there is no reason not to use a P Channel

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: January 10, 2020, 10:56:46 am
content: Quote from: Matt74 on January 10, 2020, 08:03:08 amHmmm, only 5 mA? You can power that directly with a gpio!Quote

author: Psi
author_rank: Super Contributor
timestamp: January 10, 2020, 11:06:20 am
content: I vote for P-channel option, or direct IO power if you can get away with that in your application.P-channel is simpler and the Rds difference doesn't really matter at the current you are switching. (You can still get very low Rds on in P-ch.)

author: Siwastaja
author_rank: Super Contributor
timestamp: January 10, 2020, 11:33:58 am
content: N channel being better than P channel applies to the die area used, i.e., cost inhigh-powerswitching. Also relevant in making the Vdsmax * Rds(on) * Qg(tot) product better; again, for high power. Not relevant for your case.For a 2.8V, 5mA load, justifying the extra complexity of the bootstrapped N-channel FET will be very hard. So, go for the P channel.With 5mA load, it seems likely you won't have too much of capacitive load to charge, correct? I would guess the peak current won't cause an issue, so no need for active current limit / protection. So, again, bare P channel MOSFET.Going even further, if Rds(on) varying between about 10-30 ohm as a function of everything is good enough for you, i.e., no accurate load regulation needed, directly power your load off the IO pin.This is, assuming no large capacitive load, and assuming no short circuit will ever happen. If you are concerned about short circuits or precharging a large C, use a proper high-side switch IC with current limit and thermal protection.

author: Matt74
author_rank: Newbie
timestamp: January 10, 2020, 11:48:40 am
content: Thanks all. Unanimous answer that makes total sense !cheers