[SI-LIST] Re: Switch Mode PS Noise

Hi Declan,

If the LDO output is higher than the set point, the likely problem is 
self-oscillation,
especially that you use a PNP transistor in the LDO.  The signal you see 
at the
base of the PNP transistor may appear to be related to the switching 
frequency
of the upstream converter, but unless you can see both waveforms on the 
same
scope screen, synchronized, I would first suspect the self-oscillation 
of the LDO.

Suggestions:
- check the ripple at the output of DC-DC converter, which is the input 
of the
LDO.  Supposedly you have buffer capacitors at that point, and the switching
ripple should be below 100mVpp.  If bigger than 100mVpp, you need more
capacitance or better capacitors at the point.  If below 100mVpp:
- change the LDO, by adding a small series resistance (10-30 ohms) to the
base of the PNP transistor.  This may already kill the self-oscillation, 
if in
fact that causes your problem
- if this is not enough, you can overcompensate the LDO regulation loop. 
If there is access to the feedback loop, try adding capacitors to reduce the
loop bandwidth (where and how much, depends on the LDO).   You can
also try adding capacitance (nF range) in parallel to the base-collector of
the PNP transistor
- note also that LDO stability may be affected by the output load and
output capacitance and any extra phase shift in the sense and feedback
looop

Once you know where the problem is coming from, we can think about 
fixing it.

Regards,

Istvan Novak
SUN Microsystems

Declan Hegarty wrote:

>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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