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. > > ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu