I'm trying to drive several signal-level PWM controlled automotive devices with a 3.3v Arduino Due. I need a GND-VBattery-GND PWM signal.
Is this best achieved using a little P-channel (and maybe also an N-channel) MOSFET or is there a better way?
What is the load? Does it need much current? What frequency?
I have played similar games with using a logic level (5 v.) outputs to drive MOSFET gates to switch lots of amps at higher voltages (70 v.). I positioned a MOSFET N-channel power transistor so that the power supply voltage (12 v., in your case) is at the drain. To turn it off, the gate is held at the circuit ground, but the 5 v. logic level applied to the MOSFET gate may not be enough to fully saturate the N-channel MOSFET for lowest resistance, so use an intermediate NPN transistor to control the MOSFET gate. The NPN is turned off when the logic level is low, and flows current when the logic level is 5 v. You need to provide a resistor , say 220 ohms, in the base circuit to limit base current flow, and provide a resistor, say 5000 ohms, between 12 v. and the NPN collector. Then as the logic voltage swings between 0 and 5 volts, the NPN collector voltage swings between 12 v. and about 1 v. That voltage will successfully operate the drain of the MOSFET.
You should to pick a MOSFET with enough current handling capacity, and adequate breakdown voltage, and preferably a low source-drain resistance (like 0.1 ohm).
Hope you can wade through that explanation.
Thanks for the replies.
MarkT: It's signal level, if it did 350mA it would be safe for all applications. 500mA would be a bonus. Frequencies are fairly low to be honest. <=10kHz absolute max
JrDoner - Thanks. I guess I could get away with just an N-Channel MOSFET with a resistor from 12v to Drain - provided that whatever i was driving had high resistance the voltage at the MOSFET drain would go NearlyVBattery to NearlyGround. It certinally sounds the cheapest way.
My idea was to use a P-Channel high-side switch and N-Channel Low side with a gate driver IC to accept 3.3v input. I guess this would be more consistent for varied loads.
Does anyone know:
When picking a P-Channel and N-Channel MOSFET pair - what's the best spec-sheet items to match?
Typical Turn off delay + Typical Turn on delay, like these:
http://uk.rs-online.com/web/p/mosfet-transistors/6887121/
Also, I'm looking for a MOSFET driver IC to drive a P-Channel High side and an N-Channel low side. Any ideas?
I've seen the high-side N-Channel drivers that use a bootstrap - These won't do 100% duty - So I'd like to try a high side P-Channel instead because I'd only need to pull the gate down to turn it on.
MarkOne:
Thanks for the replies.MarkT: It's signal level, if it did 350mA it would be safe for all applications. 500mA would be a bonus. Frequencies are fairly low to be honest. <=10kHz absolute max
350mA isn't signal level!
You can probably get away with just a MIC4422 if the 12V rail is guaranteed to stay
below 18V. Can source / sink amps, but not continuously due to heating issues. Its a
driver for large MOSFETs and will switch 10kHz without any sweat. 5 and 3.3V compatible
logic input, make sure it has good decoupling though.
Just found this. Fig 2 is basically exactly what I have in mind - but I don't understand the high side.
Can anyone explain the purpose of the cap and which of the high side resistances are larger than others? Maybe I can have a go at setting some values for these components.
Any help much appreciated.