Hello,
I am trying to design a regulator using the LM25085 with the following design requirements:
Vin(min) = 10V
Vin(nom) = 12V
Vin(max) = 14V
Vout = 4.2V
Max load current = 4A
Min load current = 0A
Ambient temp = 30 degrees C
I am having trouble determining, with confidence, the current rating required for the PFET, inductor, and schottky diode. I have calculated values based on the design example and equations in the datasheet and reviewed the recommended circuits / components produced by the Webench tool on the website. I even created a spreadsheet of my own, which, I think, correctly implements the calculations from the design example in the datasheet (attached for your reference).
If my calculations and understanding are correct, then my Icl(max) = 8.82A (see attached spreadsheet). And the PFET, Inductor, and Schottky diode must then be rated for a minimum of 8.82A.
However, I can’t seem to reconcile this against some of the parts Webench recommends. Specifically, I'd like to replace the Webench-recommended default components with physically smaller parts listed in the Webench alternate options. However, many of these mechanically smaller, recommended alternates don't appear to meet the current requirement. For example:
1. Webench-recommended alternate for L1 = Coilcraft p/n XAL5050-562MEB with Isat = 6.3A; 2.5A less than my calculated requirement.
2. Webench-recommended altenate for M1 = Vishay p/n SI3867DV-E3, with Id = 3.9A; 4.9A less than my calculated requirement.
Is Webench recommending parts that don't meet the current required? Are my calculations all wrong? Can you help me better understand the current rating requirements / calculations for these external components? Below is a screengrab of the "default" recommended Webench design for reference.
Thanks in advance,
Paul
