Q: On page 19 of the datasheet it is not clear how to select alpha (offset introduced at the Vout). Secondly if we select alpha=0.1% as said in the datasheet then the resultant R1 should be less than or equal to 50kohm. However in the EVKIT of MAX17559 for a 5V output we have R1 (R23 on the board) is 86.6kohm. What’s the reason of this and how appropriate alpha selected?
Ans: The alpha selection will cause the percentage tolerance of output voltage to increase. Basically, the output voltage tolerance is the sum of the offset of the leakage current, the Vref voltage tolerance, and the feedback resistor tolerance. It depends on the specification design from customer, so we can select the alpha to calculate the feedback resistor value correctly.
The alpha=0.1% on page 19 of MAX17559 is only an example. The Vref tolerance is 1.68%, and the feedback resistor tolerance is 1%. Thus, the total output voltage tolerance is 2.78% (0.1% + 1.68%+1%). If customer can accept +/-3% tolerance, then you can select the alpha up to 0.22% or even more if you select 0.1% tolerance for feedback resistor.