Run mcu from high voltage range

[NeedCoffee]

Hi,

I would like to be able to run an mcu from 0.8v to 20v input voltage. I am wondering what the best way to do this is. I have looked at using an LDO before a boost regulator. So for example a 3v ldo then a boost regulator like the mcp1640 to step up the voltage to the mcu minimum. I am unsure of what ldo's will work at such low voltages. The mcu will need about 20ma current at 3.3v. Is there a better way to clamp the input voltage to the mcp1640? Ideally i am looking for a output voltage that will stay the same as VIN until VIN hits 3.0v then be clamped at that voltage. Any suggestions on parts or regulators that can accept up to 20v input but can still pass voltage at 1v?
Thanks

[mariush]

The 3v ldo won't work with a voltage lower than 3v, exactly how low depends on what voltage reference it uses and other factors. With less than 3v + dropout voltage, the output of the 3v ldo will be more or less input voltage - dropout voltage

Your best option is a buck-boost regulator, but such buck-boost regulators that accept high voltages  (like more than 6v) typically need higher minimum voltages

for example TPS62175 does 4.75 - 28v in, output up to 6v and 500mA

Chips like TPIC74100  or TPS55065 will work from 1.5v
https://www.digikey.com/en/products/detail/texas-instruments/TPIC74100QPWPRQ1/1531097
https://www.digikey.com/en/products/detail/texas-instruments/TPS55065QPWPRQ1/2092500

and if you look hard you'll find other chips ... here's a starting point : https://www.digikey.com/short/vbzp9tz9

[radiolistener]

if your input voltage is AC, you can use step-up transformer to transform 0.8V to something like 6-7V and then use rectifier and voltage regulator to get 3.3V or 5V

If voltage is DC, you can use one step-up DC/DC and one step-down DC/DC and switch them depends on input voltage.

[NeedCoffee]

Thanks for the replies !, The tpic74100 looks interesting but slightly over the threshold i need.  The mcp1640 will work at to boost the voltage nicely from 0.8v to 3.3v. Is there a simple way to limit or clamp this the voltage at 3v supplying the 1640 ? It does not have to be precise.

I guess an LDO or buck-boost mentioned is the closest thing to a "clamp" circuit. Is there a way that I can switch from direct supply if below 1.5v for example and when the threshold is reached switch the input to the mcp1640 over to regulated output of the tpic74100?