I'm designing a PCB which requires 3.3V and 9V power regulation. Eventually I plan on building a fair amount of these boards, so I want to plan my design so I'm able to buy components in bulk if possible. It appears the TPS54308 will fit my needs, and I used Webench Power Designer to generate the following reports with BOMs:
https://filebin.ca/4bTkfT8zP59a/TPS54308DDCR_3.3V_output.pdf
https://filebin.ca/4bTkNflWAyyc/TPS54308DDCR_9V_output.pdf
The major differences between the two circuits are the inductor which costs $1.77 more on the 9V design, and the output capacitors which cost $2.22 more each ($4.44 total) on the 9V design.
I don't understand why the recommended output capacitor changed on the 9V design since the recommended output capacitor on the 3.3V design seems to have the same specs except for voltage rating (25V output capacitor for 3.3V design, 63V output capacitor for 9V design). Could I get away with using the output capacitor recommended for the 3.3V design on the 9V design?
Similar question for the inductor. I don't have much experience with inductors, but spec wise, the inductors recommended for the 3.3V design and the 9V design seem very similar. Could I get away with using the inductor recommended for the 3.3V design on the 9V design?
Also, how can I determine "what I can get away with" in the future as it pertains to inductor and capacitor selection for switching regulators? I.e., if I'm not using the inductor recommended by TI, would I switch it out for one with a larger or smaller Henry value, larger or smaller resistance value, etc.
Best Answer
TI's Web Bench selects components (capacitors and inductors in particular) from parts offered from various manufacturers that have agreed to support TI's design tools. Generally they will select components based upon both a set of "rules" and best choice of specifications.
As I recall the Web Bench tool used to let you select alternate components and re-run the simulation to see how the component change affects the circuit efficiency, ripple voltage on output and various other parameters.
In general you do not want to be just freely substituting components in switching regulator designs unless you are well versed in the technology and the nuances involved. Many of TI's switching regulator data sheets include a detailed design steps and calculations section to help with the component selections. It can be a good idea to follow through this process even though you have used the Web Bench designer to double check the components the tool selected for you.
I must say that the tools that TI offers and the high quality switching regulators that they sell these days make the design of a decent and well behaved switching regulator circuit a lot simpler than the state of the art some 10 to 15 years ago. But with any circuit like these a huge percentage of success comes with understanding how to make the circuit board layout in the correct manner. This is a skill that will come from experience and a failure or two.