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It started with a typical simple high-side P-MOS battery supply switch with inverting logic. Vs = 3.3-4.2V. Vg = 0-2.8V. 0V on, Imax = 2.5A. A P-MOS with SOT-23-3 package was chosen and works fine. Suddenly people want to change to positive logic without change the PCB, sourcing issue, you know. So I am locked with SOT-23-3. I can't find a non-inverting P-MOS or high side N-MOS. Edit : wondering if there is a drop-in solution that can perform power switch with positive logic (2.5-2.8V on instead of 0V on).

Sink
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    What do you mean by non-inverting pMOS? A pMOS turns on when its gate is driven low - this is device physics. – nanofarad Jan 27 '22 at 18:31
  • If they want absolutely no change to the PCB, you need a drop-in replacement, however I don't know how you'd do it. It is possible to use a high-side nMOS, but you'd need a gate driver. If they want to switch to positive logic, you could use an inverting gate just for the pMOS. I think people dropped an impossible task on your lap: change everything without changing anything – Victor Souza Jan 27 '22 at 18:46
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    Transistors don't work that way. ICs are possible but almost certainly not as drop-in . I don't understand why a 'sourcing issue' would affect the logic, or is that about sourcing a revised PCB? In any case, a high-side switch needs a ground reference to compare to the control signal, so 4 pins minimum. – Spehro Pefhany Jan 27 '22 at 18:49
  • No way to turn on a high-side NMOS without a higher voltage than your load. There are plenty of PMOS out there that will work as a drop in replacement. Whoever is telling you that they're having "supply chain issues" is not looking hard enough (or at all). – vir Jan 27 '22 at 18:50
  • I apologize if I didn't make my question clear. I would like to find if it's possible to have a drop-in solution that perform power switch with none-inverting logic input (2.5V on instead of 0V on). – Sink Jan 27 '22 at 19:03
  • Don't apologize for not making your question clear - EDIT your question to make it clear! And while you're at it, some formatting would make it more easily readable too. – brhans Jan 27 '22 at 19:09
  • Seems like what you're looking for could be described as a N-Channel depletion mode logic-level MOSFET. I don't know if such a device exists in the current range you're looking at though. – brhans Jan 27 '22 at 19:43
  • The best Digikey has to offer has 500m-ohm Rds-on, and there's nothing smaller than a DPak. – brhans Jan 27 '22 at 19:48

1 Answers1

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There is no drop-in replacement.

Depletion-mode MOSFETs would behave like you want (switch off when the gate becomes more negative than the source). But they are a niche product, and there are none that would be efficient enough to allow switching 2.5 A with a reasonable voltage drop, especially not in a small SOT-23 package.

Load switches would behave like you want.

TPS27081A pinout (TPS27081A)

They can be constructed with an additional N-channel MOSFET, or with much more internal logic to handle things like strict switching thresholds, thermal protection, or output discharge. But in all cases, they need at least a fourth pin for ground.

CL.
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