Need to increase duty cycle of a varying frequency pulse train

I am taking an output from a TSL 230R-LF ( light to frequency converter IC) and trying to convert frequency into a proportional voltage using LM331N IC(V/F or F/V converter). LM331N it seems, is not able to detect the duty cycle of the pulse train from tsl-230r-lf.
Now i need to increase the duty cycle of the pulse train to at least 2% for the lm to detect it.
And also the frequency is constantly changing.
please help