I'm working on a project where i need to charge multiple li-ion cells. I'm thinking of buck converter circuit powered by ac-dc 24V 1A wall adapter and controlled by Atmega328P MC PWM. I've covered theory behind li-ion battery charging, buck converters, PWM. I believe constant voltage mode implementation is quite straight forward. A simple voltage divider to get feedback voltage to ADC and Rsense resistor with current sense amp to ADC adjusting PWM duty cycle to keep stable voltage while checking current drop to 10% to cut off power(correct me if i'm wrong about this). But i'm unsure how to approach constant current mode.

 

I'm working on a project where i need to charge multiple li-ion cells. I'm thinking of buck converter circuit powered by ac-dc 24V 1A wall adapter and controlled by Atmega328P MC PWM. I've covered theory behind li-ion battery charging, buck converters, PWM. I believe constant voltage mode implementation is quite straight forward. A simple voltage divider to get feedback voltage to ADC and Rsense resistor with current sense amp to ADC adjusting PWM duty cycle to keep stable voltage while checking current drop to 10% to cut off power(correct me if i'm wrong about this). But i'm unsure how to approach constant current mode.

You can use the same Rsense resistor to limit the current during the initial charge

 

You can use the same Rsense resistor to limit the current during the initial charge

How can i limit current with Rsense resistor?

 

How can i limit current with Rsense resistor?

You limit it with your PWM. You just read the Rsense with the A-D and change the duty cycle to keep the current constant at less than 1C and less than 4.2 volts. Once the voltage rises to 4.2 volts you can then monitor Rsense until the current drops to about .07C.
Here is a pretty good write up.
http://powerelectronics.com/site-fi...archive/powerelectronics.com/mag/504PET23.pdf

 

How can i limit current with Rsense resistor?

You know the resistance, then if you measure the voltage drop across the resistor with your ADC, you can calculate the current = V/R. Then use PWM to control current.

 

You limit the current the same way you limit the voltage, only you get your sense from a resistor. "OR" the current control to the voltage control.