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'[OT] Re: Adjustable Power Supply HELP'
1999\11\09@165532 by

<x-flowed>At 08:56 AM 11/9/99 +0000, Michael wrote:
>Note that a 2200uF capacitor on the output is really way too much.  This
>amount of capacitance does not help the regulator work better, it slugs
>the transient response.  A single 100nF will suffice in most cases with
>maybe a 10uF cap if you are expecting big load spikes.

Although I'm not an EE, it's my understanding that the criteria for
post-regulator capacitors are as follows:

1. The 78xx (and presumably LM317) design specs require a minimum 0.1 uF
bypass capacitor for compensation of the internal feedback loop.

2. Too high a capacitor will result in overloading of the regulator at
lower frequencies (i.e., too low an impedance load), resulting in too long
a rise time for voltage to reach spec.

Since C V = i t, and the PIC requires power up within 10 ms, the maximum
capacitor on the output of, say, the 7805 regulator would be

C = (1 A) (0.01 s)/(5 V) = 2000 uF

With a low-current device, this could be reduced even further to 200 uF.

So the output capacitor should be no smaller than 0.1 uF and no larger than
200 uF. Usually the breakpoint is in the cost of the capacitor. Since you
can buy 0.47-1.0 uF tantalum caps for the same price as 0.1 uF, using a
0.47 uF or 1.0 uF tantalum is the best choice.

Larger capacitors provide no advantage at low frequency, since the output
impedance of the regulator is very low (e.g., 1 mOhm). At high frequencies,
distributed bypassing works better in any even.

There is apparently never a reason to use higher than 100 uF or lower than
0.1 uF on the regulator output. And with lower values, you don't need a
discharge resistor or diode to protect the regulator.

================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: rallcfltd.com
Least Cost Formulations, Ltd.                   URL: http://lcfltd.com/
824 Timberlake Drive                            Tel: 757-467-0954
Virginia Beach, VA 23464-3239                   Fax: 757-467-2947

"Vere scire est per causae scire"
================================================================

</x-flowed>
> There is apparently never a reason to use higher than 100 uF or lower than
> 0.1 uF on the regulator output. And with lower values, you don't need a
> discharge resistor or diode to protect the regulator.

Important point, the 100 or whatever is IN ADDITION to the 0.1uF.
The larger caps don't work so well at 1-2 MHz, which is what the 0.1 is
there for.

At 16:53 9/11/99 -0500, you wrote:
{Quote hidden}

Been a long time, I would guess that Paul or Sean will correct me, but I
think that the formulee is wrong.
Q = IT
=> Q = 1(Assume max current) * 0.01 = 0.01 Joules

EK = 1/2C(V)2 => 0.01 = 1/2C*25 = 800uF

Now this assumes that everything is hunky dory.

>So the output capacitor should be no smaller than 0.1 uF and no larger than
>200 uF. Usually the breakpoint is in the cost of the capacitor. Since you
>can buy 0.47-1.0 uF tantalum caps for the same price as 0.1 uF, using a
>0.47 uF or 1.0 uF tantalum is the best choice.

You have made suumptions here in that the maximum current from the REG is
1A, also if the circuit peaks at 1A for a period of 10mS, the coapacitor
would be effectily discharged (2000uF), and the regulator would make up the
other bit to hold the votage up, and rough rule of thumb the output will be
down by 63% och! This is due to the CAP taking the available current from
the reg to hold const voltage on its terminals (OK so not quite correct but
should give the purpose) This does not take into account other influences
like ESR etc.

>
>Larger capacitors provide no advantage at low frequency, since the output
>impedance of the regulator is very low (e.g., 1 mOhm). At high frequencies,
>distributed bypassing works better in any even.

Only if the CAPS are suitable for high frequency stuff. There is no need to
use a large cap (Say 1uF) that has a high impedance at RF. In this case a
small cap say 100pF ceramic would be better.

>
>There is apparently never a reason to use higher than 100 uF or lower than
>0.1 uF on the regulator output. And with lower values, you don't need a
>discharge resistor or diode to protect the regulator.

You have never had to meet Tempest ratings have you.

Dennis

{Quote hidden}

<x-flowed>At 04:19 PM 11/9/99 -0500, Dave wrote:
> > There is apparently never a reason to use higher than 100 uF or lower than
> > 0.1 uF on the regulator output. And with lower values, you don't need a
> > discharge resistor or diode to protect the regulator.
>
>Important point, the 100 or whatever is IN ADDITION to the 0.1uF.
>The larger caps don't work so well at 1-2 MHz, which is what the 0.1 is
>there for.

Again, I'm not an EE. However, I'm not sure I agree with this 'conventional
wisdom'. The point of concern is the impedance of the capacitor at the
frequency of interest. Although smaller capacitors may have less lead
inductance, the higher capacitance of the larger ones may result in a lower
impedance at that frequency, giving better performance.

Any analysis I've seen has always indicated that higher capacitance is
better, even at reasonably high frequencies (e.g., 20 MHz). E.g., a 10 uF
tantalum is superior to a 0.1 uF tantalum or ceramic.

Perhaps Paul or Michael have something to contribute on this issue.

================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: rallcfltd.com
Least Cost Formulations, Ltd.                   URL: http://lcfltd.com/
824 Timberlake Drive                            Tel: 757-467-0954
Virginia Beach, VA 23464-3239                   Fax: 757-467-2947

"Vere scire est per causae scire"
================================================================

</x-flowed>
<x-flowed>At 09:35 AM 11/10/99 +1100, Dennis wrote:
> >Since C V = i t, and the PIC requires power up within 10 ms, the maximum
> >capacitor on the output of, say, the 7805 regulator would be
> >
> >         C = (1 A) (0.01 s)/(5 V) = 2000 uF
> >
> >With a low-current device, this could be reduced even further to 200 uF.
> >
>
>Been a long time, I would guess that Paul or Sean will correct me, but I
>think that the formulee is wrong.
>Q = IT
>=> Q = 1(Assume max current) * 0.01 = 0.01 Joules

I=amperes = coulombs/sec; t = sec; Q = coulombs

So Q = I t gives coulombs, not joules.

>EK = 1/2C(V)2 => 0.01 = 1/2C*25 = 800uF

This would be correct if Q was in joules. However C V = Q in coulombs.

>You have made suumptions here in that the maximum current from the REG is
>1A, also if the circuit peaks at 1A for a period of 10mS, the coapacitor
>would be effectily discharged (2000uF), and the regulator would make up the
>other bit to hold the votage up, and rough rule of thumb the output will be
>down by 63% och! This is due to the CAP taking the available current from
>the reg to hold const voltage on its terminals (OK so not quite correct but
>should give the purpose) This does not take into account other influences
>like ESR etc.

I was talking about the power-up time. ESR would only delay the power-up
further, since the regulator would not supply the capacitor at full tilt.

> >Larger capacitors provide no advantage at low frequency, since the output
> >impedance of the regulator is very low (e.g., 1 mOhm). At high frequencies,
> >distributed bypassing works better in any event.
>
>Only if the CAPS are suitable for high frequency stuff. There is no need to
>use a large cap (Say 1uF) that has a high impedance at RF. In this case a
>small cap say 100pF ceramic would be better.

This might be true at very high frequencies, where mica caps are required,
but my belief is that it's not true for frequencies ~ 40 MHz and below. The
bigger the capacitor, the lower the impedance. (E.g., 10 uF vs. 0.1 uF)

================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: rallcfltd.com
Least Cost Formulations, Ltd.                   URL: http://lcfltd.com/
824 Timberlake Drive                            Tel: 757-467-0954
Virginia Beach, VA 23464-3239                   Fax: 757-467-2947

"Vere scire est per causae scire"
================================================================

</x-flowed>
In practice, I have found small electrolytic capacitors to have quite a low
impedance to HF range RF signals (just by trying them for bypassing,not a
scientific study ;-) SO, I would suspect that you are right, Robert.

It all depends on the self-resonant frequency of the cap. Below that freq.,
it will act like a cap. Above it, it will act like an inductor. I don't
know what typical values are for a small electrolytic.

ESR might actually make a MUCH bigger differnce than inductance,come to
think of it. The ESR of a small electrolytic can easily be 20 ohms. So, if
I'm figuring correctly, it would never appear to have a lower impedance
than 20 ohms. I also think that ESR may increase with frequency (due to
dielectric losses).

Considering all of this, I would say that using only an electrolytic is
probably OK,BUT make sure it has a low ESR. One would wonder,though,what
the point of placing large capacitances on the output of a regulator would
be? For bypassing,you want to place the caps as close to the components
being bypassed as possible. The cap on the regulator output normally does
nothing except stabilize the feedback loop,as you said,and must be right
next to the regulator. In certain circumstances, a large cap would be
necessary to imoprove transient response,but that probably only happens
with LDO or micropower regs (can't remember which usually have the problem).

Sean

At 05:53 PM 11/9/99 -0500, you wrote:
{Quote hidden}

| Sean Breheny
| Electrical Engineering Student
\--------------=----------------
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
shb7cornell.edu ICQ #: 3329174

> Again, I'm not an EE. However, I'm not sure I agree with this 'conventional
> wisdom'. The point of concern is the impedance of the capacitor at the
> frequency of interest. Although smaller capacitors may have less lead
> inductance, the higher capacitance of the larger ones may result in a lower
> impedance at that frequency, giving better performance.

A large capacitor (low esr) at any regulator output is not to work as a
filter, since a good regulator will do this job, but instead, the large
capacitor is just working as a very low impedance supply for high energy
surges. This is most related to power circuits that needs steady low
impedances supply.

Of course, one can say; "but if a regulator has a negative impedance how
a capacitor can works better?"

Sean Breheny wrote:
>
> In practice, I have found small electrolytic capacitors to have quite a low
> impedance to HF range RF signals (just by trying them for bypassing,not a
> scientific study ;-) SO, I would suspect that you are right, Robert.
>
> It all depends on the self-resonant frequency of the cap. Below that freq.,
> it will act like a cap. Above it, it will act like an inductor. I don't
> know what typical values are for a small electrolytic.
>
> ESR might actually make a MUCH bigger differnce than inductance,come to
> think of it. The ESR of a small electrolytic can easily be 20 ohms. So, if
> I'm figuring correctly, it would never appear to have a lower impedance
> than 20 ohms. I also think that ESR may increase with frequency (due to
> dielectric losses).

Actually, the Panasonic HFS series 105¡, 16V 100µF (p/n ECE-A1CFS101)
has only 1 Ohm of impedance at 100Hz and 0.1 Ohm at 1kHz.

> Again, I'm not an EE. However, I'm not sure I agree with this
'conventional
> wisdom'. The point of concern is the impedance of the capacitor at the
> frequency of interest. Although smaller capacitors may have less lead
> inductance, the higher capacitance of the larger ones may result in a
lower
> impedance at that frequency, giving better performance.

My spectrum analyzer and the part datasheets beg to differ.

>The point of concern is the impedance of the capacitor at the
>frequency of interest.

Yes

>Although smaller capacitors may have less lead
>inductance, the higher capacitance of the larger ones may result in a lower
>impedance at that frequency, giving better performance.
>
>Any analysis I've seen has always indicated that higher capacitance is
>better, even at reasonably high frequencies (e.g., 20 MHz). E.g., a 10 uF
>tantalum is superior to a 0.1 uF tantalum or ceramic.

Unfortunately, it's not just LEAD inductance.
Electrolytic (and other) capacitors can have a significant inductive
component internally.
Using a small capacitor optimised for high frequency performance as well as
the requisite ecap is often a good idea.

A 10uF tantalum AND a 0.1 uF tantalum in parallel would probably work better
than either alone.

That said, - I know you just mentioned tantalum capacitors as an example but
I would recommend NEVER using a tantalum in a  power supply decoupling
application unless I had taken extraordinary steps to ensure that the
voltage never EVER exceeded the tantalum's voltage rating. Tantalums become
very effective non-fusible shorting links when exposed to voltages only very
slightly above their rated voltage  - even for VERY short periods of time.
Use of solid Aluminium (OK solid Aluminum for the USA ers) will meet most of
the problems that Tantalums are specd for without the extra problems.

Tantalum caps make very amusing toys - you can get any mix of  smoke, flame,
smell, noise (from whistle and shriek through to explosion) by injudicious
application of voltage. I've seen / heard / smelled all of these from a
single failure.

Russell McMahon
_____________________________

>From another world - http://www.easttimor.com

What can one man* do?
Help the hungry at no cost to yourself!
at  http://www.thehungersite.com/

(* - or woman, child or internet enabled intelligent entity :-))

-----Original Message-----
From: Robert A. LaBudde <ralLCFLTD.COM>
To: PICLISTMITVMA.MIT.EDU <PICLISTMITVMA.MIT.EDU>
Date: Wednesday, 10 November 1999 11:42
Subject: Re: [OT] Re: Adjustable Power Supply HELP

> Actually, the Panasonic HFS series 105¡, 16V 100µF (p/n ECE-A1CFS101)
> has only 1 Ohm of impedance at 100Hz and 0.1 Ohm at 1kHz.

A truly remarkable performance for a component whose intended reactive
component (capacitive) alone has an impedance of 1.6 ohms at 1 KHz :-)
Now, if it had been 100 mF .... :-)

RM

part 0 2775 bytes
<P><FONT SIZE=2 FACE="Arial">Again, I'm not an EE. However, I'm not sure I agree with this 'conventional</FONT>
<BR><FONT SIZE=2 FACE="Arial">wisdom'. The point of concern is the impedance of the capacitor at the</FONT>
<BR><FONT SIZE=2 FACE="Arial">frequency of interest. Although smaller capacitors may have less lead</FONT>
<BR><FONT SIZE=2 FACE="Arial">inductance, the higher capacitance of the larger ones may result in a lower</FONT>
<BR><FONT SIZE=2 FACE="Arial">impedance at that frequency, giving better performance.</FONT>
</P>

<P><FONT SIZE=2 FACE="Arial">Any analysis I've seen has always indicated that higher capacitance is</FONT>
<BR><FONT SIZE=2 FACE="Arial">better, even at reasonably high frequencies (e.g., 20 MHz). E.g., a 10 uF</FONT>
<BR><FONT SIZE=2 FACE="Arial">tantalum is superior to a 0.1 uF tantalum or ceramic.</FONT>
</P>

<P><FONT SIZE=2 FACE="Arial">Perhaps Paul or Michael have something to contribute on this issue.</FONT>
</P>

<P><FONT COLOR="#0000FF" SIZE=2 FACE="Arial">R</FONT><FONT SIZE=2 FACE="Arial">obert A. LaBudde, PhD, PAS, Dpl. ACAFS&nbsp; e-mail: rallcfltd.com</FONT>
</P>
</UL>
<P><FONT COLOR="#0000FF" SIZE=2 FACE="Arial">Well, at work I play with laser drivers working at 2.5 GBits/s, and electrolytics certainly don't do very much at that frequency.&nbsp; I've never personaly measured the losses in an electroylitic, but if you take a look at almost any RF circuitry, you won't find many high value decoupling caps.&nbsp; Maybe one or two for low frequency bypass, the rest will be small ceramic devices.&nbsp; As you say, larger caps will have a higher inductance and ESR than smaller ones, and this will become the dominant factor at high frequencies, swamping the very small capacitive reactance.</FONT></P>

<P><FONT COLOR="#0000FF" SIZE=2 FACE="Arial">As further evidence that the 2200uF output cap in the original design was superfluous, this is straight from the Nat Semi datasheet:</FONT></P>

<P><FONT FACE="Times New Roman">&quot;Normally, no capacitors are needed unless the device is situated more than 6 inches from the input filter capacitors in which case an input bypass is needed. An optional output capacitor can be added to improve transient response&quot;</FONT></P>

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