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Given that my 33uF Cout (C4) cap is already ceramic, is it really necessary that I use a 0.1uF ceramic (C7) for HF rejection? These designs assume that an electrolytic with high ESR is on the output so it is recommended to have a ceramic as well. Therefore, with price/size not being an issue, does it make sense to have a 33uF ceramic in parallel with a 0.1uF ceramic?
A large ceramic cap (any cap) has a self resonate frequency, above which it does not work as well. " It turns inductive." A 0.1u"F cap will have a much higher resonant frequency. It is typical in this case to parallel a large cap with a small cap. I don't have time to find the curves for you today, but if you graphed impedance and frequency you will see the point where the 33uF cap resonates and then it's impedance heads back up.
I also think the inductor may be the problem. How does it work at 1/2 power?
Well, throwing some parallel R's together I was able to load it at 90mA and 140mA. Oddly enough, the circuit as quiet as can be at 140mA, but whistling away at 90mA.
The load that the DC-DC delivers for its application is anywhere from 80mA to 120mA for reasons I can't get into. Seems like I might be in the sweet spot where it decides to hiss like crazy.
The IC turns on and off depending on the output voltage, so this frequency may be the source of the audible noise.
Have you tried implementing the I lim resistor? The lower current may help and would change the cycle time.
The IC turns on and off depending on the output voltage, so this frequency may be the source of the audible noise.
Have you tried implementing the I lim resistor? The lower current may help and would change the cycle time.
I have not, but I'm thinking it's worth a try. Just gotta find a way to slice that trace ;p
Though, if it limits the switch current to 400mA, that means I have a peak current of 400mA. This peak current yields only 3.76uJ of energy with a 47uH inductor. If I recall correctly, I calculated that a 120mA load at 12V will require about 8uJ.
Adding a 120 ohm resistor to Ilim reduces the inductor current a lot and keeps it from running in bang bang mode.
Here are the 2 sims.
I am guessing you might be using a 9 volt battery for power so I also added a cap on it.
Wow! Sweet model. Big difference on the switching. Looks like the only caveat is start-up time. 80 Ohm load should be a pretty decent model. Very cool, I'll have to try it. What is the value on C2 there? I have to budget my capacitance because of energy storage limitations, so I wouldn't even have an 88uF output cap, it's just 33uF at present.
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