Re[2]: [SI-LIST] : Power Supply Noise Filters

Michael T Zhang ([email protected])
Mon, 30 Mar 98 16:07:00 PST

Text item:


You are welcome.

A power supply typically has a low-pass filter. If you intend to have an
additional external filter to further attenuate the input voltage noise,
you would need to use a low-pass filter. The simplest decoupling input
capacitor may not be very effective, depending on noise frequency and
the type of capacitors used. A more expensive L-C filter would be more
effective, because it is second order. But you need to consider
saturation for the inductor (or ferrite bead). The more subtle thing is
the power supply output/ regulator input stability. By adding a filter,
you changes the either the power supply output impedance or regulator
input impedance depending on where you observe. The system stability
needs to be considered. Most of the time, it is not a issue.

Of course, you do multiple-stage pi- or t- type filtering.

Hope this helps.

-Michael Zhang, Intel Corp.

______________________________ Reply Separator _________________________________
Subject: Re: [SI-LIST] : Power Supply Noise Filters
Author: [email protected] at SMTPGATE
Date: 3/30/98 2:56 PM

Hi Michael,

Thanks for your response on the Power supply email that I had sent
earlier. Actually, I didn't make myself very clear, I apologize. I am
looking for a noise filter that I can layout on my board to dissipate
any noise generated by an external +5V raw, PC, power supply coming into
a regulator that I am using to generate a +2.5V output. Your response
regarding use of regulators in this case is helpful, but I will really
appreciate if you could suggest a noise filter I could use.


Michael T Zhang wrote:
> Text item:
> You basically have two choices: linear regulator or switching regulator.
> Linear regulator would have less than 50% efficiency based on your
> input/output voltage ratio. So power consumption on the regulator might
> be an issue if the current is high. However, it would filter out much of
> the input noise (high audiosusceptibility). Switching regulator on the
> other hand has good efficiency and high audiosusceptibility, but
> introduces noise at the output due to its own switching. If noise is
> critical, you can use multiple-stage low-pass filters. Common practices
> are using linear for low-power applications; and switchers for
> high-power ones.
> -Michael Zhang

> Hello everybody,
> I have a regulator on my board that requires a +5 V input which in
> turn provides +2.5 V to my circuitry. Would you guys have any recommendation
> on any power supply noise filters that I can use to minimize any noise
> generated by my +5V external power supply. I would appreciate any
> inputs on what kind of power supply I should use.
> Thanks
> Khalid
> ____________________________________________________________________
> Khalid Ansari LSI Logic Corporation
> Applications Engineering 1551 McCarthy Blvd
> Tel: 408.954.4965 MS G-750
> Fax: 408.433.2840 Milpitas CA-95035
> ____________________________________________________________________

Khalid Ansari LSI Logic Corporation
Applications Engineering 1551 McCarthy Blvd
Tel: 408.954.4965 MS G-750
Fax: 408.433.2840 Milpitas CA-95035

Text item: External Message Header

The following mail header is for administrative use
and may be ignored unless there are problems.


Content-Transfer-Encoding: 7bit
Content-Type: text/plain; charset=us-ascii
References: <Fri, 27 Mar 98 13:40:10 [email protected]>
Subject: Re: [SI-LIST] : Power Supply Noise Filters
CC: [email protected]
To: Michael T Zhang <[email protected]>
MIME-Version: 1.0
X-Mailer: Mozilla 3.0 (X11; I; SunOS 5.5.1 sun4m)
Organization: lsi
From: Khalid Ansari <[email protected]>
Date: Mon, 30 Mar 1998 14:56:23 -0800
Message-ID: <[email protected]>
Sender: [email protected]
Received: from vp13w73 by vpcalpop (SMI-8.6/SMI-SVR4)
id OAA19020; Mon, 30 Mar 1998 14:50:15 -0800
Received: from by; Mon, 30 Mar 1998 14:50:15 -08
Received: from ([]) by with ESMTP id OAA
(8.6.12/IDA-1.6 for <[email protected]>); Mon, 30 Mar 1998 14:5
0:18 -0800
Received: from ( [])
by (8.8.6/8.8.5) with ESMTP id OAA02038
for <[email protected]>; Mon, 30 Mar 1998 14:59:04 -0800 (PS
Received: from ( []) by relay.jf (8.7.6/8.7.3) with ESMTP id OAA25437 for <[email protected]>; Mon, 30 Mar 1998 14:52:50 -0800 (PST)
Return-Path: [email protected]