Maintain constant PWM voltage with changing Vcc

Question

I am making an arduino project with a single lithium ion battery, and want to be able to maintain a constant PWM voltage, even when the battery goes from 4.2-3.2v. I have been able to read the Vcc using the internal 1.1v reference, so I am wondering if I can make some sort of feedback system to increase the duty cycle when the battery discharges, in order to keep the same average voltage on the PWM. I want to be able to make it 3.2v constant, which should be attainable on Vcc from 4.2-3.2, with 100% duty cycle when the battery is at 3.2v.

• Delta_G: That's just a math problem, and a simple one at that. Read your voltage, calculate the PWM to get the average you want, then apply that PWM. What part is sticking you?
• Gerben: That would work. Just note that the 1.1V reference can differ slightly, from chip to chip. So depending on your margin of error, you might have to do a bit of calibration. PS it sound like you already know, but just to reiterate; PWM output doesn't give out an analog voltage, despite the misleading analogWrite function name. You'd have to do some filtering to convert the PWM to a analog voltage.
• DataFiddler: Wouldn't a voltage regulator do that better than an Arduino?

Answer 1

Simple: duty = (3.2 / vcc) * 255;