[SI-LIST] Re: Switch Mode PS Noise

Declan,

I have some questions:

1. Are the FETs of the switch mode supply in the chip or external to the
chip?=20

2. What is the current rating of the switch mode supplies?

3. Where does the base of the PNP connect? Is it to the chip?

4. Where does the emitter of the PNP connect? Is it to the chip?

5. What pin is the regulated voltage on the PNP?

Don't spend a lot of time debugging until you can determine that you are
seeing the real thing on your scope. Measure things that you are sure of
and see if they are what you think they should be. For example, connect
the ground clip of your single ended scope probe to the probe tip and
use it like a loop antenna around the circuit. Do you see any noise? Now
connect the probe tip and ground clip to the ground of the chip. You
should see nothing because you are looking at ground, right, maybe not.
Do you see any noise? I think you will.

Most likely you will need a differential probe to see the real base
voltage. BTW you can make a nice differential probe using a 180 degree
combiner. I keep one in my tool kit for such issues. It does not go down
to DC, but it can help you understand the circuit. See the paper by Doug
Smith.=20

http://emcesd.com/pdf/cd94scr.pdf

Best regards,

John

John Hill
Design Engineer
TK  HOLDINGS INC  =20
Electronics=20
27200 Haggerty Road  Suite B-12 =20
Farmington Hills, MI 48331
Office Phone: 248 699 2915
Mobile Phone 248 765 3599



=20

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-----Original Message-----

From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Declan Hegarty
Sent: Monday, October 23, 2006 5:22 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Switch Mode PS Noise

Hi folks,

I'm currently investigating a power supply circuit based on an IC with
two switch-mode outputs and a small linear regulator.  The linear
regulator uses an external pnp transistor in a simple low dropout,
common emitter configuration, and is currently running out of
regulation, with an average DC level of 2.39V instead of the required
1.8V.

I believe that noise from one of the switcher outputs is coupling into
the base of this transistor, causing current pulses at the output
which are being averaged across the output capacitor to increase the
nominal DC level.  When I shut down the offending switch mode output,
the linear regulator works properly.  So to my question.......

I'm using a fairly cheap and cheerful oscilloscope and probe.  When I
probe the base of the pnp transistor, I see 600mV spikes
time-correlated with the switch-mode transitions from 0-12V.  I'd like
to understand how much of this noise is just pickup by the probe
leads, and how much is actually present on the base pin of the pnp.  I
wondered if the following was a valid approach:

1.  Place the ground lead of the probe at a fixed ground point on the
PCB, near the area of interest.
2.  Probe a ground via as close to the pnp base as I can find.
3.  Observe the noise on this measurement.
4.  Now probe the pnp base.
5.  Observe the noise on this measurement.  Subtract the noise
observed in 3 to approximate the "real" coupled noise at the pnp base.

Clearly this isn't going to be high precision, I know.  Any other
suggestions for an improved understanding of what's going on here
would be much appreciated.

Thanks. D.
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