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'[EE] A question on ESR'
2010\03\23@190507 by Vitaliy

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Hi List,

Once we started using the 3V LDO regulators, we ran into the issue of output
cap ESR. Some regulators specify 1.5 ohms or less, others <5 ohms.

How critical is the ESR, and what does it affect? Do I understand correctly
that higher ESRs with heavy loads will cause the regulator to start
oscillating?

Here is an article I read on measuring ESR:

http://www.emcesd.com/tt020100.htm

So let's say you have two caps with ESR <10 ohms, can you put them in
parallel to get <5 ohms resistance? And by the same token, would the total
inductance be lower?

Vitaliy

2010\03\23@194805 by Marcel Duchamp

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On 3/23/2010 4:04 PM, Vitaliy wrote:
{Quote hidden}

The esr of the cap affects the stability of the feedback loop of the
regulator.  And although you mention "x ohms or less", some parts
specify "x ohms or greater" and still others will give a range of
acceptable esr.

In one sense, esr isolates the capacitance from the regulator.  If it is
very small (such as on many ceramic caps) it can cause the regulator to
have sufficient phase shift that it will oscillate. The phase shift is
due to the output impedance of the regulator driving the capacitor.

2010\03\23@204409 by M.L.

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On Tue, Mar 23, 2010 at 7:04 PM, Vitaliy <spam_OUTpiclistTakeThisOuTspammaksimov.org> wrote:
> Hi List,
>
> Once we started using the 3V LDO regulators, we ran into the issue of output
> cap ESR. Some regulators specify 1.5 ohms or less, others <5 ohms.
>
> How critical is the ESR, and what does it affect? Do I understand correctly
> that higher ESRs with heavy loads will cause the regulator to start
> oscillating?
>
> Here is an article I read on measuring ESR:
>
> http://www.emcesd.com/tt020100.htm
>
> So let's say you have two caps with ESR <10 ohms, can you put them in
> parallel to get <5 ohms resistance? And by the same token, would the total
> inductance be lower?
>
> Vitaliy


The datasheet should have a pretty good chart of the stable region for
ESR. Can you tell us which part you're referring to?
I haven't used any <5v regulators lately but the 5v ones I use tend to
be zero ESR stable.

--
Martin K.

2010\03\23@210009 by Vitaliy

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M.L. wrote:
> The datasheet should have a pretty good chart of the stable region for
> ESR. Can you tell us which part you're referring to?
> I haven't used any <5v regulators lately but the 5v ones I use tend to
> be zero ESR stable.

For example, the NCP699.

Vitaliy

2010\03\23@211333 by Russell McMahon

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> > For example, the NCP699.

That looks like a bad example to use - or it claims to be
See datasheet and at end

      http://www.onsemi.com/pub_link/Collateral/NCP699-D.PDF

However, prior comments should be useful.

- Certain capacitance / ESR combinations cause phase shift such that
positive feedback and instability occurs.

- Data sheets of reputable parts usually specify conditons.

- Normal R & L laws apply BUT PCB is part of that (usually small for
good layout).

>From datasheet:


The NCP699 is a stable regulator and does not require any
specific Equivalent Series Resistance (ESR) or a minimum
output current. Capacitors exhibiting ESRs ranging from a
few m  up to 5.0   can thus safely be used. The minimum
decoupling value is 1.0  F and can be augmented to fulfill
stringent load transient requirements. The regulator accepts
ceramic chip capacitors as well as tantalum capacitors.
Larger values improve noise rejection and load regulation
transient response.
TDK capacitor: C2012X5R1C105K, C1608X5R1A105K,
or C3216X7R1C105K

2010\03\23@220801 by Xiaofan Chen

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On Wed, Mar 24, 2010 at 7:04 AM, Vitaliy <.....piclistKILLspamspam@spam@maksimov.org> wrote:
> Hi List,
>
> Once we started using the 3V LDO regulators, we ran into the issue of output
> cap ESR. Some regulators specify 1.5 ohms or less, others <5 ohms.

Very few capacitors will have ESR of more than 2 Ohms, so actually you
can normally ignore the high limit (but may not be true if you use wrong
type of capacitors). The lower limit is the key here, especially
for older LDOs. Newer LDOs work with ceramic capacitors with very
low ESR (mili Ohm range).

> How critical is the ESR, and what does it affect? Do I understand correctly
> that higher ESRs with heavy loads will cause the regulator to start
> oscillating?
>

Often for older LDOs,  lower ESR will often be the culprit of
causing instability. Very high ESR can cause problem as
well but usually you will not hit that high limit unless you
add resistor in series by yourself.

Just use Google and it finds some good reference.

Here is another good article from National Semi.
http://www.national.com/nationaledge/jul02/article.html

Some TI articles for reference:
focus.ti.com.cn/cn/lit/an/slyt187/slyt187.pdf
http://focus.ti.com/lit/an/slva115/slva115.pdf
http://focus.ti.com/lit/an/slyt201/slyt201.pdf
http://focus.tij.co.jp/jp/lit/an/slyt151/slyt151.pdf



--
Xiaofan http://mcuee.blogspot.com

2010\03\23@233805 by Richard Prosser

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On 24 March 2010 15:08, Xiaofan Chen <xiaofancspamKILLspamgmail.com> wrote:
> On Wed, Mar 24, 2010 at 7:04 AM, Vitaliy <.....piclistKILLspamspam.....maksimov.org> wrote:
>> Hi List,
>>
>> Once we started using the 3V LDO regulators, we ran into the issue of output
>> cap ESR. Some regulators specify 1.5 ohms or less, others <5 ohms.
>
> Very few capacitors will have ESR of more than 2 Ohms, so actually you
> can normally ignore the high limit (but may not be true if you use wrong
> type of capacitors). The lower limit is the key here, especially
> for older LDOs. Newer LDOs work with ceramic capacitors with very
> low ESR (mili Ohm range).
>

Just be a bit careful here as I got caught by it. The small cheap,
electrolytic caps can have marginal ESR. And if they get cold they can
get very much worse. And iIf they get hot they dry out and the ESR
increases also. So if you are going to be working over an extended
temperature range it would pay to have a close look at the specs or
use an alternative type.

Or if you are happy to pay top dollar for good quality components,
then it's less of an issue.

Richard P

2010\03\24@005307 by William \Chops\ Westfield

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On Mar 23, 2010, at 7:08 PM, Xiaofan Chen wrote:

> Very few capacitors will have ESR of more than 2 Ohms

That's what I would have thought, but apparently it's not as true as  
all that.
Almost half of digikey's 10uF tantalum caps have an ESR over 2 ohms.  
The percentage is lower for Aluminum (surprisingly?) and not listed in  
the parametric search for ceramics (although most ceramic caps have  
very low ESRs.)  It seems that these moderately low-value  
electrolytics are the sore point for ESR.  I found this tables:

http://www.your-book.co.uk/design/esrchart.htm

The regulators that have both min and max ESR requirements make me  
particularly nervous.  You can always change to ceramic and add  
parallel caps to lower ESR, but ensuring a minimum ESR in an actual  
circuit with who knows what additional bypass caps "nearby" seems  
fraught with peril...

BillW

2010\03\24@011509 by Xiaofan Chen

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On Wed, Mar 24, 2010 at 11:38 AM, Richard Prosser <EraseMErhprosserspam_OUTspamTakeThisOuTgmail.com> wrote:
>> Very few capacitors will have ESR of more than 2 Ohms, so actually you
>> can normally ignore the high limit (but may not be true if you use wrong
>> type of capacitors). The lower limit is the key here, especially
>> for older LDOs. Newer LDOs work with ceramic capacitors with very
>> low ESR (mili Ohm range).
>>
>
> Just be a bit careful here as I got caught by it. The small cheap,
> electrolytic caps can have marginal ESR. And if they get cold they can
> get very much worse. And iIf they get hot they dry out and the ESR
> increases also. So if you are going to be working over an extended
> temperature range it would pay to have a close look at the specs or
> use an alternative type.

As per the article from National Semi, do not use Electrolytic for LDO
and I have never used Electrolytic for LDO. I used Tantalum capacitor
last time. For new designs, typically I use ceramic but occasionally
also Tantalum.
http://www.national.com/nationaledge/jul02/article.html

"The biggest reasons many LDO's oscillate are:

a) Using an aluminum electrolytic output capacitor in a design
that operates at cold temperatures. Aluminum capacitors may
have an ESR in the “stable” range at room temperature, but their
ESR increases exponentially as the temperature goes below about
10 degrees Centigrade. These capacitors must never be used with
LDO's if cold temperatures can occur in the application.

b) Using a ceramic output capacitor on an LDO not designed for it.
The typical 2.2 - 4.7 uF ceramic capacitor will have an ESR of about
5 milli Ohms. This puts the ESR zero somewhere around 6 MHz
where it clearly won't help compensate the loop. Using ceramics
on the output of LDO's which are not designed to work with them
is presently the #1 reason for unstable LDO operation.

...

Most LDO's designed in the late 1980's and early 1990's were
made assuming a Tantalum capacitor would be used for the
output capacitor, and so they don't tolerate ceramics very well."


> Or if you are happy to pay top dollar for good quality components,
> then it's less of an issue.
>
I am in this market segment where quality is more important
in most cases than cost. ;-)


--
Xiaofan http://mcuee.blogspot.com

2010\03\24@012136 by Xiaofan Chen

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On Wed, Mar 24, 2010 at 12:53 PM, William "Chops" Westfield
<westfwspamspam_OUTmac.com> wrote:

> The regulators that have both min and max ESR requirements make me
> particularly nervous.  You can always change to ceramic and add
> parallel caps to lower ESR, but ensuring a minimum ESR in an actual
> circuit with who knows what additional bypass caps "nearby" seems
> fraught with peril...

As per the theoretical analysis, there will be a lower limit and a
upper limit.

With the newer ceramic stable LDO, the lower limit will be
zero ohms (to use ceramic), but there will still be a upper limit. If you
use ceramic, you will not hit the upper limit anyway.
http://www.national.com/nationaledge/jul02/article2.html

And typically I do not use LDO for very high current applications.
So I am fine with ceramic capacitors for LDOs in my current
designs.


--
Xiaofan http://mcuee.blogspot.com

2010\03\24@081809 by Olin Lathrop

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Vitaliy wrote:
> Once we started using the 3V LDO regulators, we ran into the issue of
> output cap ESR. Some regulators specify 1.5 ohms or less, others <5
> ohms.

Be careful, some also specify a *minimum* ESR, particularly the cheap
knockoffs, although some mainstream LDOs do too.  This is one reason we use
the Microchip MCP1700 a lot.  It's relatively cheap, has decently low
dropout voltage, comes in SOT23 and SOT89 packages, and is specified to work
with a output cap down to 0 ESR.

Nowadays you want to use a SMD ceramic capacitor on the output.  These have
very low ESR, so you'd actually have to add a resistor in series to use them
with some LDOs.  Note that your 1.5 ohm and 5 ohm figures are much much
higher than the ESR of a 1 or 10uF 10V SMD 0805 or 0603 cap.

> How critical is the ESR, and what does it affect?

ESR is as critical as the datasheet says it is.

> Do I understand
> correctly that higher ESRs with heavy loads will cause the regulator
> to start oscillating?

That could happen.  The ESR of the output cap, the impedence of the input
voltage, and the load current all have effects on stability.  Different
designs have different tradeoffs, so as always, you have to pay attention to
the datasheet for the particular LDO you are using.

> So let's say you have two caps with ESR <10 ohms, can you put them in
> parallel to get <5 ohms resistance?

Yes.

> And by the same token, would the
> total inductance be lower?

Yes, for that part of the inductance that is unique to each cap and its feed
lines.  Inductance really shouldn't be a issue with good layout.  Put the
input and output caps right next to the LDO.


By the way, I've lately used a neat trick a lot to get a clean supply
voltage.  The MCP1700 series works very well with the input voltage one
silicon diode drop above the output.  I use a PNP transistor accross the LDO
such that it comes on when the input is the B-E drop above the output.  This
is used as feedback to a switcher to indicate when the output is above the
regulation threshold.  You can go further and put a resistor divider on the
output of this sense transistor.  The output of this divider will be above
some threshold when the transistor is on (LDO input > output + 600mV) but
also when the input is high enough.  This keeps the switcher from being
fooled as the supply is coming up and the output is lagging the input by
more than the B-E drop.

You can see a example of this on page 1 of
http://www.embedinc.com/ioext/hcan2.pdf.  The output of the R3/R4 divider
has to be above the internal 600mV reference of the 10F204 for the switcher
output to be considered high.  This particular power supply runs from USB
power, and produces a nice clean regulated 5V, even though USB power can
range from something like 4.3V to 5.5V.

The B-E drop of a silicon transistor happens to be almost perfect for this.
There is several 100mV room above before the 6V input limit is reached, and
room below before the dropout limit is reached.  The switcher ripple is
easily well below that, so everything hums along nicely at just about the
optimal point.  With a nominal 600mV drop on the LDO, it only disspates 60mW
at 100mA out, which a SOT23 case can handle.  Even at the maximum 250mA out,
the dissipation is only 150mW.  A SOT89 can handle that fine.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

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