Quote:
Originally Posted by Al Skierkiewicz
Austin,
The difference in capacitor values stems from Tantalums being very close to labeled value and electrolytics being a little less precise and varying over their life.
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Actually, this is due to their differing ESR (
effective series resistance) and ESL (effective series inductance). Ormore generally, their frequency responses. In general, an electrolytic will have a higher ESR than a tant of the nominal capacitance and tech level.
Different capacitor technologies have different applications. Electrolytics are typically for big, low frequency capacitance. Ceramics are typically for small, high frequency capacitance. Tants are somewhere between the two. However, technology has improved to the point where the three are bleeding into each other. Ceramics are getting to have higher capacitance densities, tants are getting more robust, etc.
A painfully large portion of my day job is to make sure the buggers still work right at frequency. I spent a year in the low-GHz range, where the parasitic elements are really beginning to dominate. An electrolytic registers as an inductor at those frequencies, and is therefore completely ineffective!
It is still important at lower frequencies, such as those found in class-D amplifiers (i.e. motor drivers). At your frequencies, electrolytics are beginning to really feel the effects of their ESR, but they aren't dead yet. You therefore get to make the price/size trade-off.
As an example, the 24V boost supply uses ceramics, but the 5V buck supply uses electrolytics. ESR is much more important in a boost supply, because of the current surge in each cycle. Bulk capacitance is more important in a buck supply, because the inductor smoothes things out. Even though they are switching at roughly the same frequency, the boost supply's current wave form has much richer harmonics and therefore needs capacitors capable of handling those higher frequency components.
In short, a capacitor is really defined by its total impedence at the frequency of interest. This is roughly proportional to: C/f + ESR + ESL*f
Knowing C alone only tells 1/4 of the story, though an EE more experienced than I can usually ballpark the rest.
Edit: Al just pointed out to me that you are using a linear, not a switcher. I thought this was for the 33887. If you look at page 8/23 of the regulators datasheet, you will find the following:
Quote:
Figure 1 shows the basic hookup diagram for fixed-voltage
models. All models require an output capacitor for proper
operation, and for improving high-frequency load
regulation; a 10μF tantalum capacitor is recommended.
Aluminum electrolytic types of 50μF or greater can also be
used. A high-quality capacitor should be used to assure
that the ESR (Effective Series Resistance) is less than
0.5Ω.
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This clearly shows that the discrepency in capacitance is because Tants typically have lower (better) ESR. If you can find a 10uF electroylitic with >.5Ω ESR, go for it.