Could someone check out my calculations I've attached and let me know if I've messed up somewhere? I built up this circuit and it kind of works. The problem I'm having is that loads (i.e. 47 ohm resistor, 100 ohm resistor etc...) are dragging down my output voltage. I am trying to support a range of input voltages from the lowest ~5V to 30V. When I run the regulator at 25 or 30 volts it works better than at 10V. My voltage drops as I move down towards 10V.
A couple things to note before i get to the scope traces. The inductor I'm using is a 470uH that supports 0.53A continuous current. PN: 12LRS474C. The output cap is a 470uF PN: EEU-FR0J471B. I thought (assuming my calculations are correct) that the ripple current through the inductor would go down as my input voltage goes down therefore making it easier to regulate at a lower input voltage than a higher input voltage. Basically at lower input voltages I am further away from the saturation point of the inductor since my ripple current goes down and I'm not pulling mucgh through my resistor anyways.
Any help would be greatly appreciated. I'm getting an understanding for how to design buck regulators, but I'm not there yet I guess.
Below are my scope traces. Yellow is my output voltage with this channel set in AC coupling mode so you can see the voltage ripple without the DC component. Basically yellow is the output ripple riding on 3.5V without showing the 3.5V. Blue is clipped on to pin 2 looking at the output switch. I also have large noise spikes when the switch turns on. These are much worse at lower input voltages than at high input voltages and I'm not sure why.
Vin = 10V, Rload = 47ohm
Same as above just zoomed in on edges
Same as first one just not zoomed in quite as much
Vin = 25V, Rload = 47ohm
Same as above
At 25 volts I feel like that output voltage ripple doesn't look too bad. The output cap's datasheet says it has a ESR value of 0.13 ohms which makes me think this matches up with my calculations of my ripple current at 0.5A when multiplied by .13 gives me, roughly, 50mV. Also, if someone could give me feedback on whether I calculated the frequencies correctly or not that would be a great help.
Thanks a lot,
Rob