Mail*Link(r) SMTP FWD>DES: bulk decoupling caps
Jerry, I hope the following helps
>Date: 8/2/96 11:22 AM
>From: Jerry Cupples
>Dear TN subscribers,
>In looking at some new designs for high speed comm boards, we are seeing
>competitors use 22 uF aluminum electrolytic caps in radial leaded packages
>for decoupling.
>Most of our designs have used solid tantalum parts in "D" case the past few
>years.
>Part of the reason for this may be cost, as it seems the Ta parts are about
>40 cents vs. 10 cents for the AL parts.
>Of course, it is much simpler to automatically place the SMT devices, but
>what I'm wondering is...
>Is there some electronic design superiority related to the package or
>typical circuit "physics" (i.e. trace inductance, etc.) to choose the AL
>radial leaded part?
In previous life, there were some major electrical considerations is selecting
Ta electrolytic capacitors over Al. One was tolerance, most Al capacitors had
tolerances of -20 or -50% to +100 or more %. Due to the way of manufacture,
Al capacitors had high inductances and therefore were less effective at high
frequencies --- meaning the 10's or 100's of kHz not even MHz's. The
"equivalent series resistance" as a function of frequency (which is a measure
of the alternating current resistance) was like "lousy" for the Al capacitors.
In addition the maximum storage/use temperature was about 20 degrees C less
than the Ta's. Other than that, no problem. (;-)
Things have somewhat improved over the years, and for most applications there
is not much major differences between Ta and Al electrolytic capacitors for
"bulk" brute force filter capacitors. As was mentioned in a previous posting
on "technet" about bypass capacitors (I can dig it out and send it to you if
you want/need) the key to bypass capacitors is to use "good" (relatively low
valued about 1-100 nF) RF ceramic capacitors VERY close to the power pins of
the IC. Give preference to keeping the grounded lead/connection of both the
component and bypass capacitor as short as possible (except for ECL logic
families). The purpose of the ceramic bypass capacitor is to take care of the
very high frequency spiking currents inherent with the fast pulse rise/fall
times and prevent the rounding of the edge or corners of the signal
pulse/clock. The purpose of the "bulk" Ta or Al bypass capacitors is to
stabilize the voltage of the power distribution system for the longer "flat or
table" portion of the signal pulses. Therefore, the goal is to have the Ta or
Al bypass capacitor work like a great big storage tank of energy (voltage) to
maintain a constant voltage on the power distribution system. In theory, the
"bulk" capacitor should not see any really high frequency stuff (it does see
the clock frequency) if the power and ground system "is a good one".
Jerry, the following sketch is a gross illustration of the verbal description
of the "wave forms" for rises/fall times and "flat" portion of a pulse. In
particular square and sometimes pulsed waveforms are kind of unique because if
you know the basics on how to read them they can idicate what the problem is.
xxxxxxx xxx xx
x x x x x xx
x x x x x xxxx
x x x x x x
x x x x x x
xxxxxx xxxxxxxxxxx xxxxxxxxxxxxx xxxxxx
The first is a This second This third pulse shows
square pulse, pulse has the "flat" portion has
just like what rounded corners "drooped", indicating
you want. that indicate the supply system doesn't
the bypass cap. have sufficient capacity
isn't doing the Ta or Al to sustain the
job. "constant voltage" to the load.
>Any guess, speculation, or real intelligence would be appreciated!
This let's me out---
>regards,
>Jerry Cupples
>Interphase Corporation
>Dallas, TX USA
Hope this helps,
Ralph Hersey
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Date: Fri, 2 Aug 1996 11:50:33 -0500
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From: [log in to unmask] (Jerry Cupples)
Subject: DES: bulk decoupling caps
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