Tool/software: WEBENCH® Design Tools
I'm having trouble getting the maximum designed output current at the lower end of Vin. Actually, only a little less than max. Used WebBench to design it.
Design parameters:
VinMin: 4.5V
VinMax: 12.5V
Source: DC
Vout: 18V
Iout: 1.35A
Load: 13.33 Ohms
So, with a 13.4 Ohm load, if Vin drops below 10.5V, it goes into current limiting. The design called for a 6 mOhm current sense, but for some reason, it got built with a 8 mOhm. My first thought was to change it to a lower value, but with the 6 mOhm it actually went into current limiting at a higher, 10,8V. I check the compensation network and tweaked it a little but with no change in the low Vin fold-back situation.
I think the layout is pretty good. Large copper areas along high current loops with all components in close proximity. Looks like a good Thevenin connection to Rsense. Good grounding with Agnd tied to Pgnd at one point.
I'm no guru at this, but have done dozens of switcher designs with good success, but never had such a radical departure from the design goals.
Can you suggest some things to look at?
Should I try changing the compensation network? I'm nowhere close to low Vin so that seems futile.
Any help would be appreciated. Thanks.
