Calculating of the maximum output current of a 12 V to 3.3 V step-down converter

Question

I am planning to use the AP3211 step-down converter to go from 12 V to 3.3 V. The maximum current at 12 V that I can supply to the input of the step-down converter is 0.3 A. Assuming the efficiency of the AP311 is 90%, does it mean that I will be able to get a maximum current of 0.97A at the output?

Here is how I've calculated it:

Pi = 12 V · 0.3 A = 3.6 W
Po = 3.6 W · 0.9 = 3.2 W
Io = 3.2 W / 3.3 V = 1 A (0.97 A)

Answer 1

The data sheet tells you the power conversion efficiency (they usually do provide graphs): -

So, apart from the efficiency being more like 85%, your calculations are accurate and your power out to the load will be 3.06 watts. Also consider that the inductor you choose may cause a percent or two drop in efficiency. With care, 3.00 watts out; with a really good inductor maybe 3.03 watts out. A careless choice and you might only get 2.5 watts out.

Answer 2

Your calculations are correct, but 90% efficiency is a maximum (92% from the datasheet). The chart on page 5 indicates that you'll be closer to 86% efficiency at that current and input voltage. And if your implementation is not good, you might lose a few more percent.