Electronics > Beginners

Low voltage drop transistor (or other device)

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electrodacus:
Yes an N-channel like MCH3484  (overkill) will work great and you will have almost no voltage drop.

permal:

--- Quote from: electrodacus on December 02, 2018, 07:48:37 pm ---Yes an N-channel like MCH3484  (overkill) will work great and you will have almost no voltage drop.

--- End quote ---
Thanks, but sadly the gate and source are switched compared to the SOT-23 so it won't fit the already existing PCB.

Zero999:

--- Quote from: permal on December 02, 2018, 07:03:15 pm ---3.3V, updated OP.

I've never used a Mostfet before :)

I'm currently using a NPN transistor, ZXTN25050DFHTA. Changing that to a Mosfet, it'd have to be a P-channel Mosfet, right?

--- End quote ---
That's an NPN BJT, so would need to be replaced with an N-channel MOSFET.

What's wrong with the ZXTN25050DFHTA? It looks perfectly good to me.

--- Quote from: permal on December 02, 2018, 07:45:38 pm ---
--- Quote from: oPossum on December 02, 2018, 07:37:24 pm ---
--- Quote from: permal on December 02, 2018, 07:30:08 pm ---Why is it a problem to put an NPN transistor on the high side?
--- End quote ---

The base voltage would have to exceed the supply voltage to get the lowest possible Vce. That is not a problem with PNP/P-channel because the base/gate is pulled towards ground.

You can easily get less than 100 mV Vce at 100 mA with ordinary PNP transistors like PN2907 and 2N4403.

--- End quote ---
Oh, of course.

Thanks for the suggestions, those are however THT mounted, I need a SOT23 to match the existing footprint. CMPDM7002AHC seems like a candidate, but what voltage is required on the gate to open it, I can't see that in the datasheet?

--- End quote ---
That looks reasonable, also try the IRLML2402TR.

T3sl4co1l:
Yeah, high side. You need a PNP or P-ch.  Hack that in there (use pigtails if you have to fudge a footprint), and the enable signal will be inverted (active low).  Vce(sat) is typically much lower than Vbe.

Tim

oPossum:
If you replace the NPN with a N channel MOSFET, you will have greater voltage drop. You have to bodge in a P channel device as  T3sl4co1l says.

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