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At one time, the industry accepted that ESL was constant with frequency,=20
but we now know that is not true. We did see that the inductance varied=20
with measuring the mounted capacitor as opposed to the fixtured component, =
and for the most of the manufacturers, the measurements are on mounted=20
components as this represents the use conditions.
In the higher capacitance structures, we've been able to see that=20
inductance decreases to some asymptotic level (depending on case size)=20
that we were not able to measure with lower values. Because of this,=20
where the manufacturer measures the ESL may also introduce additional=20
variance between their ratings for similar devices. Some measure the ESL=20
at self-resonance (higher ESLs as KEMET reports) and others measure well=20
above self-resonance"lower is better". If a manufacturer specifies an ESL =
for all capacitance values in a case size as constant, then they are=20
using the ESL well above self-resonance. There are cases where this is=20
true, and in their catalogs, they'll include Impedance versus Frequency=20
responses for multiple capacitance values. Based on the capacitance=20
value, and the self-resonance of the Impedance plot, the derived ESL does=20
not match the "claimed" ESL.
The fact is that both of these are incorrect. The ESL does decay from the =
self-resonant frequency to well above this frequency. The theory is that=20
at self-resonance, the current is spread fairly evenly through the height=20
of the chip, but as frequency increases, the loop compresses until the=20
current is compressed into a minimized loop in the component. The=20
discrepancies in low frequency ESL and high frequency ESL is greatest for=20
those components utilizing the full height of the chip for the electrodes=20
(higher capacitance).
Within the next six months, we will be introducing a variable ESL model=20
in our Spice program, concentrating on the higher capacitance values at=20
first. Attached is a layout of the model which incorporates an "RL"=20
circuit in place of a single, fixed "L" component which will simulate this =
decaying ESL. In lower frequencies the ESL is based on a summation of L1=20
through L4, but in higher frequencies, L1 and L2 are the only inductive=20
elements, with L1 representing the termination and pad inductances. The=20
parallel resonance is factored by L2 and C2, and a secondary series=20
resonance by L1 and C2. Instead of replacing the existing model, this=20
model may be optional to the existing model.
=20
From:
Leonard Dieguez <ldieguez (AT) altera (DOT) com>
To:
Joel Brown <joel (AT) zmicro (DOT) com>, "si-list (AT) freelists (DOT) org"=20
<si-list (AT) freelists (DOT) org>
Cc:
John Oh <JOH (AT) altera (DOT) com>
Date:
07/21/2009 01:35 PM
Subject:
[SI-LIST] Re: Altera PDN Tool
Sent by:
si-list-bounce (AT) freelists (DOT) org
Joel,
I'm not 100 % sure what manufacturer was used when those values where=20
input to the tool but probably AVX or Murata. The values do not represent=20
the mounting inductance since the tool calculates that depending on the=20
mounting style and dimensions that are input to the tool. You can also=20
change your library or use "user values" for ESL /ESR. As with any other=20
EDA tool Garbage in equals garbage out. So check the values for the caps=20
that you are actually using and also check things like the stack up=20
heights Via placement and diameters and trace widths.
Leonard Dieguez
High Speed IO Applications,
Altera Corporation
9330 Scranton Road, Suite 400
San Diego, CA 92121
ldieguez (AT) altera (DOT) com
858.202.3511 (office)
"There are two kinds of engineers - those who have signal integrity=20
problems, and those who will." - Eric Bogatin.
-----Original Message-----
From: si-list-bounce (AT) freelists (DOT) org [mailto:si-list-bounce (AT) freelists (DOT) org]=20
On Behalf Of Joel Brown
Sent: Monday, July 20, 2009 5:17 PM
To: si-list (AT) freelists (DOT) org
Subject: [SI-LIST] Altera PDN Tool
I have been playing around with the Altera PDN tool.
When I look at the ESR and ESL values on the Library page it seems the
values match very closely with AVX SpiCap3 which makes me think that is=20
were
the values came from.
When I used Kemet's spice program I get values that are quite different.=20
One
which stood out was an ESL of 1.9nh for a .01uF 0402 capacitor.
So it makes me wonder if I use the values in the Altera library if the
results are going to be anywhere close to reality especially if we are
buying capacitors from Kemet or another vendor.
I am assuming the ESL values in the Altera library don't include the
mounting inductance since they model that separately.
Is there any information comparing ESL from different capacitor vendors=20
for
0402 or any other size capacitors?
Thanks - Joel
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