Low ESR caps

Go To Last Post
14 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I'm working on a new Xmega design I'll be running off a 5V supply, so I figured I'd put a 3.3V regulator by the CPU to provide its Vcc. All the parts I'm finding, though, require ceramic Cout caps with ESRs within a specified range, and I'm finding it very difficult to find caps that even specify their ESR, let alone have it in the proper range. This isn't a parameter that Digikey or Mouser let you filter by. Can anyone save me some digging and tell me what regulators and caps they've used in similar situations? Thanks.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I was looking for the same thing the other day, and couldn't find any 0805 ceramics with ESR listed on the data sheet (I need 10 uF with ESR < 5R, preferably <3R). I think that they generally are suitable, though, if you use ones that are stated to be low ESR. It is difficult to measure the ESR of ceramic capacitors, which is probably why it isn't specified.

Leon Heller G1HSM

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

leon_heller wrote:
I think that they generally are suitable, though, if you use ones that are stated to be low ESR.

DocJC's post at 7:11 p.m. in this thread makes me a little nervous making that assumption.

http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=106550&start=0&postdays=0&postorder=asc&highlight=esr

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I think that ceramics are inherently low ESR, anyway.

Leon Heller G1HSM

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

ESR can vary with temperature too, so your gadget might work at room temperature, but fail at higher or lower temperatures. And I bet it's hard to track down then.

You could 'force' a certain ESR by inserting a small resistor between the regulator output and the capacitor.

If you have a scope, a function generator and a MOSFET you can test the regulator's stability by observing how many rings the voltage shows when load current is turned on and off; the current must change quickly, e.g. must have fast edges.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

For use with LDOs, the common problem is that the ESR needs to be low, but not too low. Only a few are designed to work with "any ceramic cap".

ESR for ceramic caps is, indeed, challenging to find. You DO have to go to the spec sheet; it is rarely (never?) in the distributor catalog. It is often in a difficult to read table in the early part of the spec sheet, where it talks about temperature and mechanical stress and such. If its not there, then go to a different manufacturer.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Lautman,

You mentioned ceramic Cout's, can you use tantalums?

These two Mouser options might at least give you a starting point:

Mouser: T491T106K010AT 10uF, 10V, 10%, 5 ohm ESR, solid tant, 3528-12m package. Was $2.18 each, (USD), when I spec'd it, now it is listed at ~ 1/2 that price...

Mouser: T491B106K010AT 10uF, 10V, 10%, 3.5 ohms ESR, tant, 1210 package. $0.52 ea.

I used two of the second cap, in parallel, for a Cout of ~ 20 uF, ESR > 1 ohm, which was my goal.

Note that in the "old days" I often had huge Cin's, and 100's or 1000's of uF Cout, on 7805's.

Major paradigm shift letting the regulator do its job, and having rather small Couts.

JC

Last Edited: Mon. Jun 6, 2011 - 03:56 PM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Also, be sure to look at the Cout ESR range as a function of current load graph for your regulator.

Depending on your design, the board's current draw may vary over several orders of magnitude between an idle / power down state, and a state driving some LEDs, sensors, RF modules, etc.

You need to make sure the regulator will be stable, with the Cout you've selected, at both ends of the current draw spectrum.

JC

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

DocJ:

Most of the data sheets I've looked at specifically suggest ceramic caps. They don't say why, or what might happen if you use anything else.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

The regulators have been designed to use them because of their low cost and small size.

Leon Heller G1HSM

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

So, what happens if you use a tantalum?

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

It might oscillate.

Leon Heller G1HSM

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Docj:

When ESR is given as a graph, it seems to be expressed as a function of frequency, not current. How do you use these graphs for a linear regulator that isn't switching?

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

It's about how fast the current demand changes. Fast change = high frequency.

Even with the ESR out of spec it will be stable during steady-state, but not during transients.

The best way to test this is to use a MOSFET in series with a low value resistor, switched by a function/pulse generator to alternate between high and low load currents quickly and observe the voltage output with a scope. It should not ring excessively after each edge.

Some regulators require a compensation network; this technique is useful to tune the network to optimal response. Usually it's a compromise between fast settling, ringing and under/overshoot.