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I'm working on an add-on board that needs 3.3V power (~500mA) and should take power from its host board. The host can operate at 3.3V or 5V and I'd like to avoid using jumpers that would require the user to select the host power.

I was thinking of using a boost converter to generate, say, 6V from the host board and then use a buck regulator to bring it back to 3.3V. Is there a better approach?

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Maybe a buck-boost? Like this for an example: http://www.linear.com/product/LTC3536

The LTC®3536 is an extended VIN range, fixed frequency, synchronous buck-boost DC/DC converter that operates from input voltages above, below or equal to the regulated output voltage

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Here's a method that uses a low-cost coupled inductor and meets your requirements.

enter image description here

You can also look at Sepic and Cuk topologies. Here's a comparative study entitled "A COMPARATIVE STUDY OF SEPIC, CUK AND ZETA CONVERTERS", but I suspect your solution will be more of a shopping expedition for the best chip for this task.

If your user can connect this to whatever random supplies they have lying about, you should consider reverse voltage and overvoltage protection as well as undervoltage lockout and thermal or overcurrent protection against brownout conditions.

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