Do you know what voltage you need to get the maximum current?
The only way on an arduino is to use PWM with a filter, see:- http://www.thebox.myzen.co.uk/Tutorial/PWM.html
for a discussion on the filter.
If 5V is not enough then you can feed the PWM signal through a transistor who's collector resistor is wired to a higher voltage like 12V. However 270mA is a fair amount of current, this will lower the impedance and cause you to use very large capacitors. So maybe you are better off applying the output of the filter to the gate of an FET in a source follower mode to deliver the final current.