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Dear TI,
I implemented a design based on a WebBench tool and it quite nearly works great except the current output is 15% low.
This is my design.
I made a few modifications from the Web Bench design -- I increased the inductor to 220uH and eliminated the output capacitor due to my need to turn it on and off at 40Hz (a bit slow and allows Coff to drop too low if I remember right). I also added R5 to assist with keeping it running, added IADJ as an external input (2.4V for my test) and I added a flyback diode for the LED wiring inductance.
The current setpoint should be 2A with a 113mohm resistor, so I'd have expected it to be a bit low, but I'm measuring 1.715A instead of 2A. Do I need to install a smaller Rsense, or is there something else limiting the output current? I tried updated the parts in WebBench (setting the inductor larger and making the output capacitor 1pF) but varying the sense resistor didn't seem to change the operational value claiming a 2A average LED current. I tried forcing it to use an 80mohm resistor and the simulation operational values just didn't change at all. I suspect it isn't properly modeling the updated inductor and capacitor since I know the inductor will see more ripple current.
That said, I manually did the Rsense calculation using the Web Bench estimated inductor current ripple and it only suggested 113mohm, so I'm kind of confused. If the relationship between Rsense and Iout is linear, I'd expect to only need a 102mohm Rsense. But maybe C6 is the wrong size or something, or perhaps adding R5 shifted the math?
I have the board implemented here, so happy to try any recommended modifications.
Thanks so much for the help! I updated my Web Bench design to pretty closely resemble my design other than the 470k resistor.
I couldn't add R5, of course. And yes, ignore the Ruvlos and Cdim, I take care of that and filtering elsewhere.
Printing out the design operating points seems to be broken (downloaded a 26 byte garbage file three times in a row), and copying the data wasn't possible because the data isn't selectable, but...
I'm quite suspicious of a perhaps bad simulator. If Rsense can change from the optimal of 109mohm to 120mohm and still output 2.000A that's awfully difficult to buy.
But I definitely believe R4 or R5 or a diode there are good ideas too =)
I ran the full spice simulator on Web Bench and while it didn't let me probe much it did let me get the LED output current. It looks like the average is about 1.85A (so definitely not 2.000A) and my real measurement was 1.715A. All using the same Rsense. Hopefully the diode and resistor changes will make Rsense work correctly. I at least believe the simulation output results given the larger than desired resistance by 10% making a ~10% drop in current make sense.
Thank you!
I will purchase 110mOhm resistors and 100mOhm resistors to see what the impact is.
I should mention that my real output current did not match the simulation, it was actually 1.715A average value rather than 1.85A as suggested by the simulation.
Also for reference, this is the PCB layout I selected. It's a bit large of an inductance loop due to the physical size of the diode and inductor, but let me know if you see something peculiar!
Did you end up concluding that a diode in series with R5 is unnecessary?
In case you are wondering, C15 is a 47uF 80V electrolytic capacitor.
Thanks for the help!