Texas Instruments has a couple of 2A step-down DC/DC converters which run at 3MHz.
With specified power efficiency of 95%, the TPS6206x devices operate from a 2.7V to 6V input voltage.
This will allow them to be used with lithium-based batteries with extended voltage ranges or a 3.3V or 5V system supply rail.
There is a power save mode with involves a switch from PWM to PFM to allow the converters to operate at light load currents and maintain efficiency over the entire load range.
Operating frequency of 3MHz allows for the use of small inductors and capacitors to achieve a 22 mm2 solution with a maximum height of 1mm.
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