i am searching for some kind of pwm ic with which i can be able to control the on and off timing(the duty cycle ) of mosfet using I2C interface..

 

Have a look at this:
http://www.mouldy.org/projects/I2C-H-Bridge-Controller-with-PWM

 

The ancient 16F84 is a bad choice since 10yrs ago. The 16F628A is cheaper and has hardware PWM.
Better yet the 16F88 which has HW PWM & I2C

 

I agree. It was just a concept for our O.P. to explore.

There are a number of LED PWM drivers with I2C controls.
Some examples on Digikey: http://parts.digikey.ca/1/3/led-driver-pwm-i2c
But, we really don't know what our OP wants to do - just PWM controlled by I2C. They really need to furnish more details.

 

we are working on the design of a buck boost converter which will be using two n channel mosfet switches and two p channel mosfets. we want the output of the converter to be regulated by a micro controller according to the necessity .
so, we need to control the duty cycle of the mosfets in turn.we will feed the output voltage data from the converter to the micro controller and will be using a hit and trial algorithm to get the desired output.so we need to control the on and off of the mosfets using pwm ic.we are very much looking
forward to get some support from you....

 

Why don't you use a microcontroller that has a built-in PWM, or simply use a built-in comparator to trigger an interrupt when the output is too high or too low?

 

Sounds like a school assignment or yet another UPS project.
The 16F88 also has comparators.