Not sure right now what my PWM would be. I'd need to check my source code and see what freq I am currently running to see what pwm freq is possible.

Thanks for the explanation. But I am still a little confused with my NMOS, I am currently switching 12V off and on with 5V on the gate. But I didn't measure the voltage. Are you saying I would not have 12V on my source (in my right most figure)?

Sorry for being so dense.

So haul out your multimeter.
The source voltage will always be a couple of volts less than the gate voltage when the transistor is on. This is mode is called a source follower (not an emitter follower, as previously mentioned).

 

I would also like to know how to search for logic level mosfets on mouser. I asked this question before and was told search for "logic level" . Well that does not work. For example NDP6020P is a logic level P Channel FET yet does not come up on mouser when "logic Level" is searched.

Had same trouble.

Search 'Logic Mosfet', -> 'Transistors'.
Then filter for Gate-Source values from +- 8 v up to 10-12 or so.

 

Had same trouble.

Search 'Logic Mosfet', -> 'Transistors'.
Then filter for Gate-Source values from +- 8 v up to 10-12 or so.

Yeah searching on "Logic Mosfet" doesn't do it either. I wanted a PMOS logic level through hole and it only comes up with the one. There have to be more than that I would think.

Fairchild's parametric search is horrible, IR is a lot better but they still don't make it easy.

I sent mouser a note saying logic level should be a parameter on their search.

 

Yeah searching on "Logic Mosfet" doesn't do it either. I wanted a PMOS logic level through hole and it only comes up with the one. There have to be more than that I would think.

Fairchild's parametric search is horrible, IR is a lot better but they still don't make it easy.

I sent mouser a note saying logic level should be a parameter on their search.

I selected all the various TO-220 packages (PMOS only) and came up with 9 pages, which is over 200 parts.
I did discover that the Rds(on) sort (value increasing or decreasing) doesn't work.
Unfortunately, they include the Vgs spec for Rds(on) with the value of Rds(on), so you can't sort for logic level Vgs
.
Digikey has a good filter. I was thinking they have a minimum order of $25, but I couldn't find that on their web site.

 

I selected all the various TO-220 packages (PMOS only) and came up with 9 pages, which is over 200 parts.
I did discover that the Rds(on) sort (value increasing or decreasing) doesn't work.
Unfortunately, they include the Vgs spec for Rds(on) with the value of Rds(on), so you can't sort for logic level Vgs
.
Digikey has a good filter. I was thinking they have a minimum order of $25, but I couldn't find that on their web site.

Thanks. I'm glad someone that knows what he is doing has trouble with finding logic level fets on mouser. I'll give digikey a try.

 

Thanks. I'm glad someone that knows what he is doing has trouble with finding logic level fets on mouser. I'll give digikey a try.

Ah, another victim of my façade.

 

If you have access to a different power supply, you could also run the LED string to a -12 volt rail. I don't know if that meets your requirements of common ground but at least your MOSFET switch would turn on. You will have to move your current limiting resistor North of the MOSFET instead of between the LED and ground since your LED array is a 12V device. This way you can be more certain that you have the top of the LED near uController ground.

 

If you have access to a different power supply, you could also run the LED string to a -12 volt rail. I don't know if that meets your requirements of common ground but at least your MOSFET switch would turn on. You will have to move your current limiting resistor North of the MOSFET instead of between the LED and ground since your LED array is a 12V device. This way you can be more certain that you have the top of the LED near uController ground.

Thanks but all I have is +12V (battery). This is for a solar light project.

 

I use the proposed circuit all the time but if 2 MOSFETs is too big of a part count then you can use a High side driver like this one:

http://www.digikey.com/product-detail/en/VN750(012Y)/1026-VN750(012Y)-CHP/2521581

I didn't check it but in general they are faster than discharging the gate of a PFET through 10k.
They are probably not the best choice for really fast switching though.

 

I use the proposed circuit all the time but if 2 MOSFETs is too big of a part count then you can use a High side driver like this one:

http://www.digikey.com/product-detail/en/VN750(012Y)/1026-VN750(012Y)-CHP/2521581

I didn't check it but in general they are faster than discharging the gate of a PFET through 10k.
They are probably not the best choice for really fast switching though.

So this device is all I need?