So I built up a buck regulator circuit with the mc34063 on a breadboard and was hoping to get a second opinion on my calculations. My hand calculations are attached and these are the values i'm using in the circuit. Could someone give me some feedback on my math here as to whether it looks correct or not. Ive tested the circuit and it kinda works which leads me into my next question/s.
How do you figure out the min. Vin voltage. Testing this with no load on the ouput, I see 3.5V until I get down to 5 V and then my output drops to 3.0V. Why is this? The only thing i could come up with is that my frequency is increasing the lower my input voltage (because Ton is fixed) and its getting above 100Khz, but this doesn't make sense with my math.
When I remove Cout my voltage jumps up to 5V or so. Is Cout required for stability of the regulator? Whats going on there?
When I have no load on the output it looks like it regulates fine. When I add a 320 ohm resistor on the output with the input set to 10V the output voltage looks ok. When I swap the 320 for a 47 ohm my output voltage gets dragged down to 3.3V. Why would my output voltage get dragged down like this? I'm pulling very little current with the 47 ohm. If I leave the 47 ohm in and increase my input voltage to 30V my Vout gets to 3.5V like it should and the output increases to 3.5 as i increase the input voltage to 30V.
One note, my inductor has a max DC current limit of 0.53A. I know this will screw up my math on paper for I_outmax, but for the problems above im only pulling a few milliamps. Im a little confused because it seems like my regulator should be having more problems at 30V because my ripple current is roughly 0.5A which is the limit of my inductor, but for some reason this regulator seems to work better at 30V than 5V, 8V, or event 10V on Vin.
Any feedback will be a great help as I'm obviously still learning. What I'm trying to do is allow an input range from 5V to 30V by designing for the worst case.
Thanks,
Rob
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Forum Post: RE: Quick question about the MC34063
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