What you're asking for is impossible moctod. If you want to adjust the current you have to change the voltage, you can't have a static voltage and a variable current the load itself determines where that curve is. You can either have a constant voltage, which by the sounds of your device won't work unless the voltage regulation is absurdly high. Or what you really want is just a variable current source that's controlled linearly by a voltage source. What exactly is the load? Couldn't you just design the power circuit to handle the maximum possible load all the time and the PWM it using a voltage to duty cycle converter or micro controller?