I'm trying to design a buck converter to get 3.3v from a 6v solar panel. I am thinking of using this as my p channel mosfet:
I was thinking of the IRF6218PbF because it has a low Rds and needs only -3 to -5 volts to turn on according to the datasheet. The pwm to control the mosfet in my circuit comes from an attiny85 run at 3.3v with an fsw of 32kHz. I wanted to make sure of two things:
As the mosfet requires a negative voltage to turn on this means that my mosfet would turn on when my pwm wave falls to 0v and turn off when it goes up to 3.3v correct? So whatever I set the duty cycle in my pwm to the actual switch duty cycle be 1 minus that right?
If I'm right about part 1, how likely is it that this switching circuit will be too unreliable with a variable output from the solar panel, requiring me to find a mosfet with a very tight tolerance to avoid the switch not turning on in low light conditions.
If I left out any necessary information or my question sounds just plain stupid please let me know and I'll change or add what is needed, thanks for any help.
The buck will be used to charge a 3.7v lithium ion battery which is running some microcontrollers, the overall power draw from the load is small, about 40mA. I believe the attiny85 will be fast enough, it has an 8 MHz clock and looking at the pwm output on an oscilloscope it is very clean at 32kHz. The source V2 is in place of the attiny85 in the schematic.