Any sot-23 controlled loadswitch? - Page 1
Forum section: electronics / beginners

author: implor
author_rank: Regular Contributor
timestamp: October 10, 2025, 04:53:28 am
content: Long story short. I f**k up.I put in a high-side p-ch to act as load switch for a 5V rail. Gate is pulled up with 100k to 5V and then 1k to an 3.3V MCU.As you might see here I now can't close the gate.So i'm looking for a solution that's not to re-spin the boards.1. Is there any P-ch with built in n-ch control in sot-23-3?2. Or Load switched with EN in sot-23?3. Maybe someone know a terrible p-ch that will not open at 3.3V?I have ofc already looked without finding much but I may just be looking in the wrong category on digikey and mouser.

author: fzabkar
author_rank: Super Contributor
timestamp: October 10, 2025, 05:10:03 am
content: Is the MCU pin a GPIO pin? If so, can you program the pin as an input until such time as you need it, and then reprogram it as an output and pull it to ground? This might work if the GPIOs aren't diode-clamped to Vcc.

author: Whales
author_rank: Super Contributor
timestamp: October 10, 2025, 05:16:50 am
content: Does it switch very often? You can convert the microcontroller output into an open-collector output by adding a diode. You might even be able to just replace your 1K resistor with a diode and not have to respin the board.

author: implor
author_rank: Regular Contributor
timestamp: October 10, 2025, 06:11:28 am
content: Quote from: Whales on October 10, 2025, 05:16:50 amQuote from: fzabkar on October 10, 2025, 05:10:03 amI tried this. Gate will still be at ~3,6V. Maybe if works if I lower the gate pull up from 100k to 10k.Main problem on this board is it's sometimes pulling 250mA (polyfuse triggers). It may have something with this p-chan and connection to MCU, or it's something else. I have checked it with a thermal camera and I can only see the polyfuse getting hot. I have many boards so temting to just sacrafice one and see what burns up (by removing the polyfuse).

author: Psi
author_rank: Super Contributor
timestamp: October 10, 2025, 06:56:19 am
content: If you can change the 100k to a lower value without causing issues due to higher current draw that is what i would look at first.

author: Psi
author_rank: Super Contributor
timestamp: October 10, 2025, 07:11:05 am
content: The highest min gate threshold voltage i could find with a quick look was these.I dunno what current you need though, and how bad the Rds can be while still usable for you.Si2325DS 2.5V minimum gate threshold voltage (150V 660mA 1.4ohms@6V sot32-3 pfet)FDN86265P 2.0V minimum gate threshold voltage (150V 800A 1.4ohms@6V sot32-3 pfet)

author: Berni
author_rank: Super Contributor
timestamp: October 10, 2025, 07:15:38 am
content: Most small 3.3V MCUs have so called "5V compatible" pins (often not on all of them! Read the docs).This means you can feed a 5V signal into such a pin without any harm or excess current draw. This can be abused to create a 5V output too, set the pin for open collector output mode and add a 10k pullup to 5V. Now you can write to the pin and it will toggle between 0V and 5V. This could be used to drive your P FET gate.

author: Psi
author_rank: Super Contributor
timestamp: October 10, 2025, 07:19:01 am
content: The only high side load switch i found in sot32-3 was RT9742 (nfet with built in charge pump).https://www.digikey.co.nz/en/products/detail/richtek-usa-inc/RT9742SNGV/65686872.7V ~ 6V VCC and 500mA load. 70mR onPinout isPin1=VINPin2=VOUTPin3 (other side center) = GND/control

author: gamalot
author_rank: Super Contributor
timestamp: October 10, 2025, 07:28:12 am
content: Looks like Si2337 has a higer gate threshold voltage then other SOT23 P-channel MOSFETs.https://www.vishay.com/docs/73533/si2337ds.pdf

author: voltsandjolts
author_rank: Unknown
timestamp: October 10, 2025, 08:02:37 am
content: If you're just making a few dozen of these, bodge a sot23 nfet onto the 1k footprint, plus a gnd wire (or gnd pour available?).If you already have hundreds of populated boards, hmm.

author: Whales
author_rank: Super Contributor
timestamp: October 11, 2025, 12:17:39 am
content: Quote from: Whales on October 10, 2025, 05:16:50 amRetract that, I was wrong. This will still pull down the voltage to about 3.9V when the microcontroller is outputting 3.3V (high); it's not truly open collector.You would have to add an open-collector or open-drain transistor to your design for what I am thinking to work, which requires changing the PCB.

author: Ian.M
author_rank: Super Contributor
timestamp: October 11, 2025, 01:55:13 am
content: Quote from: voltsandjolts on October 10, 2025, 08:02:37 am... or if you don't want to invert your output logic, cascode it with a N-MOSFET bodged on the 1K footprint, drain to P-MOSFET gate, source to output pin, gate jumpered to +3.3V.Cascoding with a BJT + base resistor to logic Vcc (so the base can swing down to 1 Vbe drop above output_low) is an old trick to increase the voltage rating of an open collector output without inversion. Cascoding with a MOSFET pretty much eliminates the extra Vce_sat drop of the external transistor, but degrades rise and fall times due to the gate capacitance.However, if you can choose which logic level is ON, and you've got adjacent ground plane, I'd vastly prefer Voltsandjolts suggestion.

author: fzabkar
author_rank: Super Contributor
timestamp: October 11, 2025, 03:59:02 am
content: Quote from: Whales on October 11, 2025, 12:17:39 amReplace the 1K resistor with a 1.8V Zener, and maybe pick a lower value for the 100K resistor.

author: Psi
author_rank: Super Contributor
timestamp: October 11, 2025, 05:22:33 am
content: Quote from: gamalot on October 10, 2025, 07:28:12 amSi2325DS is higher, 2.5 vs 2.0 on SI2337DS

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 13, 2025, 12:53:04 pm
content: SOT-23 load switches do exist. Just did a simple search and found plenty results, although they tend to have more pins (such as SOT23-6 packages).Compared to simple FET's the load switches have extra functions such (adjustable) current limit or sensing and protection against over current, over voltage, overheating etc. And I assume they have better defined switching threshold compared to a P-channel fet (which is referenced from the high voltage, and not from GND such as the logic). But if you want a GND referened input signal, then your switch would at least need a 4th pin to sense the GND level (And for it's own power supply).https://html.duckduckgo.com/html?q=sot23+high+side+switchIf you want to stay with a SOT23-3 your options are far more limited. Selecting a P-channel FET with a threshold voltage that works for your application seems doable. If your logic switches between GND and 3V3 (so 1.65 is halfway),then a threshold voltage of: 5 - 1.65 = 3.35V would be your goal. But how much ripple is there on your 5V supply?

author: Nominal Animal
author_rank: Super Contributor
timestamp: October 13, 2025, 05:21:45 pm
content: Bodge an74LVC1G45orTXU0101voltage level translator between the pin and the P-MOS. A flying bug construction will work, and you can hide the mess (the five wires needed) using heat shrink tubing.

author: wraper
author_rank: Unknown
timestamp: October 13, 2025, 05:34:14 pm
content: What is the MCU used? If it has 5V tolerant GPIO (while being powered from lower voltage), you just need to set it as an input. If it's ATTINY1616 you mentioned in other thread, then it won't work but will give another ~0.6V above the VDD though. You could use AP2331, AP2337 (depending on current required) and use their ~2.5V ULVO feature (voltage between input and GND pins) to switch it off.