So I was looking at an Actel A3P250 FPGA for a future project. It has a core voltage of 1.5V and can use 3.3V for the IOs.
So I'm thinking, how do I provide the 1.5V core voltage. My board has 3.3V, 5V and 8-12V variable input as well as an a bulk battery input of 6V - 30V. National Semiconductor has a neat Webench tool. I use the FPGA option, select the exact FPGA and press "Go". It tells me for a core voltage my FPGA needs 1.5V @ 2A. That's much higher than I expected (having no prior experience with FPGAs), but then it also says I need TWO separate 1.5V @ 150mA power supplies (buck converters) for the 1.5V IO and 1.5V PLL each about $3...! This bumps the cost up to a not insignificant $12 for just the supply - the FPGA only costs about $20!
I want to run the FPGA at 250 MHz, to match my SRAM clock. But the IO will be at 3.3V which is already available on my board. So all I need to sort out is the PLL supply. What's stopping me using a 1.5V LDO here? Is there some need for a buck converter?
Does anyone have any idea on how I should architect this power design?