Output range for switch mode regulator
I need voltage to vary between 0v to 10v


The problem faced with conventional adjustable voltage regulators is that they cannot simply produce output voltages lower than their internal reference voltage. 

This is because the feedback pin compares the output voltage (potted down via resistors in a lot of cases) to that voltage reference and sets the operating output voltage level accordingly.

However, there are techniques that "falsely" raise the voltage fed-back that can convince the internal workings of the regulator to output a voltage lower than Vref and, in some cases, a voltage close to zero volts.

You could, with appropriate bias resistors, "add" 1.23V (Vref) to the fed-back voltage - this would naturally force the output to be zero volts. You could do it more accurately using an op-amp but care must be taken to avoid the op-amp causing extra phase shift in the feedback path because this could turn the regulator into an oscillator.