General > General Technical Chat
Low ESR Output Capacitor for REF5010 and REF5025
magic:
--- Quote from: EPAIII on July 08, 2022, 09:47:50 am ---And magic asked, "How exactly is noise supposed to increase with too low ESR?" I have no idea. The
TI data sheet says it, but there is no explanation. And their block diagram of how it works is really very basic, no details. "Proprietary technology", their words, not mine. Perhaps they explain it somewhere else.
--- End quote ---
The block diagram is typical for that sort of IC and a few other types that have been torn down showed good match with their datasheet block diagrams. Hence, it is likely correct.
I suspect gain peaking, i.e. the scaling opamp is barely stable when driving ultra low ESR and amplifies reference noise more than normally at some high frequency (many kHz). In such case, low frequency noise wouldn't be affected. You would probably see ringing after load transients too ::)
David Hess:
--- Quote from: EPAIII on July 07, 2022, 04:22:17 am ---I guess the biggest question in my mind at the present time is just what is the ESR of a multi layer ceramic.
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All ceramic and plastic film capacitors have an ESR of essentially zero.
--- Quote ---What do they mean by "low" or "very low"? They seem to include the low ESR in the bullet list of advantages of these capacitors, but list no actual values. I find that rather strange. Perhaps I just need to check more manufacturers' spec. sheets.
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Aluminum electrolytic and solid tantalum capacitors are the standard and come in standard and low ESR versions, although many old parts would be considered low ESR by modern standards. Ceramic and film have always had essentially zero ESR.
Where this has changed is that polymer aluminum electrolytic and polymer capacitors have become available which fit between "low" ESR parts and "zero" ESR parts.
Kleinstein:
Film type capacitors usually have very low ESR. For the X7R and similar MLCCs it depends on how the ESR is actually defiened. The model with ideal capacitor and series resistor is not very accurate. So it depends on how it is measrured / relevant or the circuit. X7R is low ESR, but still relatively high loss, while the film capacitors may have slightly higher ESR, but usually lower loss. So the capacitance and ESR alone is not suficient to describe the capacitor.
An amplfier or votlage regulator usually does not like a large very low ESR capacitor at the output, as the amplifier's own output impedance is often inductive, at least for the higher frequencies. With a low loss capacitors this gives weakly damped ringing. A combination of some low ESR capacitance (e.g. 100 nF X7R) and some lossy capacitance (Al-electrolytic) may be more suitable.
Siwastaja:
--- Quote from: Kleinstein on July 09, 2022, 09:04:39 am ---So it depends on how it is measrured / relevant or the circuit. X7R is low ESR, but still relatively high loss, while the film capacitors may have slightly higher ESR, but usually lower loss. So the capacitance and ESR alone is not suficient to describe the capacitor.
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What is this "loss" you are talking about, and how do you define it?
Kleinstein:
A major loss mechinsm in capacitors is dielectric relaxation: that is the dielectric takes some time to react to the electric flield. For the X7R capacitors the mechnism is likely to a large part from obstacles in the domain movement of ferroelectrc parts. Another part is local intermediate conductivity that can bypass parts of the dielectric. This results in a frequency dependent capacity and a phase shift between voltage and current that is slightly smaller than the ideal 90 degrees.
The simple ESR model gives loss proportional to the frequency, but this is not a good description for all capacitors.
The more detailed model circuit for a real capacitor is including a sereies of RC elements in parallel to the main capacitor. As the losses are frequency dependent it is not just 1 element, but ideally many, like 1 for every decade in the frequency. X7R adds additional complications with a nonlinear capacitance.
Capacitor data-sheets may give the dissipation factor for some frequencies (depends on the use and capacitor size) to describe the loss - though in quite some cases this is only and upper limit, possibly even just the limit of the test instrument. Despite the reltively low ERS the X7R capacitors have quite high dissipation factor over a large frequency range.
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