EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: anvoice on March 28, 2018, 12:35:43 am
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I have a chip with an embedded LMR16006 buck regulator and selecting the necessary components to get 3.3V for my MCU and another component. TI has a helpful simulation which determines the values of the resistors, capacitor and inductor needed for the circuit. The capacitor turns out to be 33uF: the part selector has an electrolytic capacitor in the schematic, and I'm wondering if I can get away with using a ceramic one with the same value? That would allow me to minimize the footprint and reduce cost. From what I understand the limitation of a ceramic capacitor is that its voltage rating is relatively low, but if I only need one rated for 3.3V that doesn't seem like a problem. A bigger issue I see is drift in the value in capacitance with temperature, but not sure if that will cause a problem either.
Basically, should I use an electrolytic cap or will the ceramic work fine?
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Some other things to consider. The capacitance of many types of ceramic capacitors can drop dramatically when there is DC across it, which is what you will have in a power supply. Another potential issue is that the ESR is extremely low, this is often desirable, but not always, some regulators rely on the ESR of the capacitor and will be unstable without it.
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I see. Looking at the recommended schematic closer, it has a resistance on Cout that is 0.070 Ohms. Should I look for the exact same cap, with the exact same series resistance? Or would any electrolytic cap have a similar ESR and work fine?
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Normally anything similar will work, I've never seen one that required a very specific capacitor.
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Have you considered a tantalum.
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Have you considered a tantalum.
It looks from the datasheet like both ceramic and tantalum capacitors are an option. Moreover, from what I've read a smaller ESR might be better. Specifically, they say for ceramic caps, the ESR is small enough to be ignored in the calculations for selecting the capacitor. So it doesn't seem like this regulator needs a specific ESR to operate properly. I think I'll look into 0805 ceramic caps, maybe go with a slightly higher value than the 33uF to make sure its capacitance drop with temperature doesn't cause as big a problem.
Let me know if there's an error in my judgment here.