Can I use a higher inductor and capacitor for this CC design? - Page 1
Forum section: electronics / projects

author: Mutad0r
author_rank: Contributor
timestamp: January 08, 2024, 08:08:29 am
content: Hi,I'm making a CC driver using the LT3474 (https://www.analog.com/media/en/technical-documentation/data-sheets/3474fd.pdf)I've gone through the datasheet and it seems to me that by using a higher inductor and capacitor value than what the datasheet recommends, I can improve the ripple of the output.The datasheet seems to go out of its way to describe using smaller value components, but it also seems that the only reason for that is to save space on the PCB.The inductor and capacitor are next to each other, so I'm worried what kind of effect changing these values would have.I hope attaching the image worked out correctly.In the output the datasheet shows a L1 = 10uH and C2 = 2.2uF.Based on the formulas in the datasheet, increasing the L1 and C2 would affect the output ripple. (L1 affects other things as well, but those are not an issue).So the question is, would increasing these values have any unintended effects?Considering all other components I've chosen, and all other formulas in the datasheet, Here's how much the ripple would be improved:(Ripple Min and Max are based on potential minimum and maximum output voltage the system would be used in)L1 = 10uH ; C2 = 2.2uFRipple Min = 12,5mV ; Max = 16,9mVL1 = 15uH ; C2 = 15uF (These are the values I would use)Ripple Min = 1,2mV ; Max = 1,7mVL1 = 150uH ; C2 = 150uF (A brave man would add a 0)Ripple Min = 0,012uV ; Max = 0,0165uVShould I just Design for the second solution, and try with other value components as well, and see if the magic smoke comes out or not?

author: jonpaul
author_rank: Super Contributor
timestamp: January 09, 2024, 01:10:20 am
content: Due to indr shunt capacitance and other parasitics you will never get to uV of noise.See Jim Williams app notesat LT for low noise SMPS design.Jon

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 09, 2024, 07:47:29 am
content: Ripple why? LEDs don't care about ripple, at least until it increases heat dissipation notably.If you're looking for a low-noise light source, I would just consider alternate methods entirely. The given device uses slope compensation, meaning its control loop depends upon nominal-value inductance to function normally. Excessive inductance causes subharmonic oscillation and chaotic behavior, worsening ripple and increasing switching loss a bit.C2 can most likely be increased arbitrarily, as long as startup conditions are honored and Vc compensation is changed to suit.If you need a quiet (low modulation) optical source, I would suggest any ordinary clean DC source, a resistor, and the LEDs. Or a halogen lamp even, which is also far broader (uniform spectrum).Tim

author: Mutad0r
author_rank: Contributor
timestamp: January 10, 2024, 08:12:26 pm
content: Thanks for the comment. I think I got what I was looking for from your reply.The resason for reducing ripple is because at my work there's a tendency to want things to be as close to perfection as possible, even if it doesn't make a difference, just so that they could use impressing low numbers when talking about how good we are.I was afraid that changing the values might have unintended consequences, so I'll leave the inductance alone. I'll try a bit bigger capacitor, but perhaps I'll get various caps, to try out.We need to drive RGB LED's at specific wavelengths, and turn the channels off at will, so a halogen wouldn't work in this situation.Thanks again for the comments.