[SI-LIST] Re: Switch Mode PS Noise
- From: steve weir <weirsi@xxxxxxxxxx>
- To: "Declan Hegarty" <declan.hegarty@xxxxxxxxx>
- Date: Mon, 23 Oct 2006 07:14:24 -0700
Declan, if you fashion a probe with minimal loop area between the
shield and the probe tip, you will minimize your noise pick-up. If
you are using a probe with a ground clip attached to an inch or two
of wire, you will have lots of pick-up.
When the switcher runs it could be imparting noise on the input in a
frequency range that the linear doesn't suppress. An additional item
to check is comparison of the noise at the input with the output.
Good luck.
Steve.
At 06:35 AM 10/23/2006, Declan Hegarty wrote:
>Thanks very much for your input guys. I think I've confused things
>with a poor description of my circuit. The switch-mode output here is
>not the input to the linear regulator. The IC I'm using, although
>designed as a dual-ouput switch mode controller, also happens to have
>a small linear regulator controller built in, comprising a feedback
>pin (to a fixed internal reference of 1V), and a base drive pin. So
>electrically, the switch-mode and linear supplies should be
>independent. The input bias for the pnp comes from a different
>source,
>
>However (possibly due to poor component placement ) I believe the
>linear regulator external transistor is picking up switching noise.
>When I shut the switch-mode output down, the linear regulator works
>just fine. When the switch-mode output is running, I can see voltage
>spikes on the base of the pnp, synchronous with the switching
>frequency. I'm just trying to establish how much of this noise is
>"real" and how much is spurious pickup by the probe.
>
>Regards,
>Declan.
>
>On 10/23/06, steve weir <weirsi@xxxxxxxxxx> wrote:
>>Declan, just to add a hopefully simple test to the suggestions Istvan
>>offered: break the connection from the switcher to the LDO, and
>>source the LDO from a suitable bench supply. If the problem is gross
>>instability in the LDO loop, oscillations will still
>>develop. Compensation is your friend.
>>
>>Good hunting.
>>
>>
>>Steve.
>>
>>
>>At 06:07 AM 10/23/2006, Istvan Novak wrote:
>> >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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