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From: firstname.lastname@example.org (Jeff Walther)
Subject: How to Choose Cap to Filter Pwr for IC?
Organization: Illuminati University
NNTP-Posting-Date: Thu, 19 Sep 2002 02:15:42 CDT
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Date: Thu, 19 Sep 2002 07:15:42 GMT
I think I understand the basic principal of putting a small cap near the
power Vcc pin of digital ICs so that the cap can supply extra current when
the IC draws heavily. And it's my understanding that there's a tension
between chosing a large or small capacitance in that large gives a greater
reservoir of charge but small reacts to demand more quickly. Please
correct me if my understanding is wrong or incomplete.
So, how does one chose the value of the capacitor used? Is there a rule
of thumb? Arcane arts? Pspice simulations that will make me lose my
In my particular case I have a small DIMM about 4.6" X 1.2" with four
Flash chips on it. The flash chips may be 5V or 3.3V 8 Mb parts in the
44 pin PSOP (SOIC) package. Their speed is 90 - 100 ns. For example
Hyundai HY29F800 or Atmel AT49BV8192.
I've put spaces on the board to put a ceramic or tantalum SMD cap tied
across the power and GND planes next to each VCC pin. So, do I use 1 uF,
.1 uF, 10,000 pF? Can I calculate this? I'm guessing it's not really
that critical for these parts and it will probably work regardless of what
I install, but I'm curious how to choose.
Also, tantalum or ceramic or whichever is less expensive? The boards I've
examined seem to use ceramic for the small caps right next to the Vcc pins
and one or more larger value tantalum caps as a sort of backup reservoir
elsewhere on the board. Is this purely a cost issue or is ceramic faster
responding than tantulum or something?
Thank you for any helpful or humorous comments.
A friend will help you move. A real friend will help you move a body.
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