PCM2707 voltage problem - diyAudio
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Old 9th March 2009, 12:50 PM   #1
Aiace is offline Aiace  Italy
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Join Date: Feb 2003
Location: Florence
Default PCM2707 voltage problem

I made a circuit with PCM2707 using an extarnal regulator to generate +3,3V. It works but I measure +4V on the regulator output. I changed the regulator but the +4V always is there. So I think that it's due to the PCM2707: in any way the internal regulator is working olso if I disabled it.
I post my schematic.
Any help will be appreciate

Thank you
Aiace
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File Type: gif schematic.gif (40.3 KB, 175 views)
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Old 14th March 2009, 03:10 AM   #2
rco3 is offline rco3  United States
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Your image's resolution isn't very good, so I can't see what's going on, but when I find a Low Dropout Regulator that isn't regulating, I always look to see if the output filter capacitance is suitable. Some parts are happy with ceramics on the outputs, some need the ESR of an electrolytic for stability, but nearly all of them require some sort of capacitor on their output to maintain stability. The LT1963, for example, requires a 10 µF capacitor. Again, I can't really see your schematic very clearly, but I don't see such an output capacitance.

Did you measure the 4V with a DMM or observe the output waveform with an oscilloscope?
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Old 14th March 2009, 11:44 AM   #3
Aiace is offline Aiace  Italy
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Join Date: Feb 2003
Location: Florence
thank yoo RCO3.

I measured it with the oscilloscope.
I am not able to attach a bigger picture here, so a try to write.
The LT1963 has 1uF ceramic cap soldered between output-gnd pins followed by one 150uF Os-con electrolitic.
What I see with the oscilloscope is that the Vbus pin of the PCM2707 is 4V and very noisy so I had soldered 1uF between Vbus pin anf gnd ( if I soldered it few mils away it was worthless). Without this 1uF cap the sound sucks.
I 'm becoming to think that the PCM2707 is dameged, maybe the internal regulator is working even if I desabled it.
Infact I'm using another LT1963 for the analog part of PCM2707 and it works very well.

Any clue?
Thank Aiace
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Old 14th March 2009, 08:54 PM   #4
rco3 is offline rco3  United States
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Hmmm... no, nothing else is coming to mind.

I'll PM you with an email address, perhaps you could send me a larger copy of the schematic or else the schematic file (I use EAGLE myself).
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