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Interfacing a PIC w/ high-side mosfet

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Here is what I am trying to figure out.

A pic uC connected to a stop/turn/tail LED light for a truck. The grounds are tied at the light and that means high-side switching only. If my searches would have resulted in anything conclusive or usable, I would not have posted this question here.

So far, what I have deduced is I need a logic-level P-channel MOSFET capable of twice the current that I need. It needs to be driven low to turn on the fet. But, some data that I have read states that I need a mosfet driver, another n-channel mosfet or bjt transistor to drive the larger p-channel mosfet. I have also found that there is a capacitive element to the p-ch mosfet that I have to compensate for to prevent latch-up. The bright and higher current "brake light" portion is 2.1A and the "tail" is 500mA, all at 12-16Vdc. As in any truck, the two sections need to be independently controlled. 

Now you may see the confusion as I have found no straight forward answer. I'm not looking for someone to do it for me, just point me to some relevant reading on the subject or let me know how you did something similar and I can spin it into something I can use. ( Then again, If you want to help that would be great!)

Thanks for some guidance....


--- Quote from: dcel on February 03, 2012, 05:47:25 pm ---Thanks for some guidance....

--- End quote ---

Find ITS724G or ITS711.

Thank you, Rufus, that PROFETĀ® ITS724G is one smart chip, well actually two smart chips in one case! Pricey at 4.02 USD  for quan. ten, but much easier and probably cheaper than trying to do this project with all discrete components.


Edit grammer.

Yeah, you need to pull the gate of the P type fet low to switch it on.
The voltage difference between the 12v rail and the gate pin has to be larger than the fets turn-on voltage.

So, for example if a p type fet has a 6V turn on voltage and your battery was 12v you would need to pull the gate down to 8V or less to turn it on.

If you connected the gate to a microcontroller output direct then you could only vary the gate voltage between 0 and 5V (micro vcc).
Which would mean the fet would stay on all the time and you couldn't switch it off, (since you need more than 8V and the micro is only 5v).
You can't even set the pin to input mode and rely on a pullup to +12V because a mcu pin can never be driven outside it's supply range or power will feed backwards onto the power rail.

So you need to add an N type transistor/fet to pull the gate to ground. Then you can control that from the microcontroller without issue.

eg (diagram here)  http://electronics-diy.com/electronic_schematic.php?id=1012

Thank you Psi. That's even more help for me in the "I understand now" department.

What I really did not understand before your post was the fact that the gate needs to be pulled low with respect to the voltage on the source, not ground. That is why the "negative" voltages with the p-mosfets. I think I get this enough now that I 'could' do it with discretes, thanks.

Could a non-logic level p-chan mosfet be used if driven by a smaller n-ch mosfet? So, replace the bjt in the link you sent me with a 2n7000 and use any 'jelly bean' p-ch fet?

Thanks again...


Edit grammer.


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