I've got a power supply design, designed almost entirely using Webbench. The DC input voltage is between 15 and 20V, and I need to generate a variety of voltages for my circuit. Webbench proposed stepping the incoming voltage down to 6V using an LM27403 and then stepping that down further to generate the required voltages for my board. This all works, except for a few issues, and it's slightly board dependent, which is worrying me.
1) We need at least 18V to make the system start reliably.
2) In some cases, the 6V works to start with, then shuts down after 5-30s. In a few cases the board always starts up reliably, even with the same firmware loaded.
3) I've now changed the main inductor to 6.8uH (from 4.7) the main reason being that the 6.8uH inductor has 23mOhm DCR, whereas the 4.7 only had 13mOhm. Since I've read a few other threads on here suggesting that you need at least 20mOhm on the inductor I've changed to that (though webbench disagrees with this). Now the 6V doesn't start at all - it produces 1.8V and nothing more.
4) I've tried using the LM27403 Regulator Design Tool, and can make sense of most of that, but I don't understand the compensation design. What sort of values do I want for Pole and Zero frequencies? What do I want to get out of the Bode Plot?
I've attached my Excel spreadsheet with various values plugged in to make things match. I need some way of getting the 6V to start up and then STAY running.LM27403 PWM Regulator Design Tool.xlsx
Many thanks,
Simon