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banjo said:Wow, 400V to 7.5V is a lot of voltage drop. Even at just 100mA load current you will have to dissipate 39W as heat. Your main difficulties are dumping that much heat and the semiconductors withstanding the large voltage differential. The voltage differential will most likely require some external transistors outside of the regulator IC. The heat issue is a much bigger problem.
I would suggest a switching regulator in a buck configuration. It would much more efficient. At 400V input, less than 2% of the power is delivered to the load. The remainder is waste heat.