Electronics > Projects, Designs, and Technical Stuff
DC/DC Converter Noise Troubles
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MosFett:

--- Quote ---Is there... actually no input filter cap?  How much wire length is there to the next capacitor on +12V? ???
--- End quote ---

There is a 220uF cap between VDD and ground less than a centimeter away from the MC33063 - sorry I forgot to show that in the schematic segment.


--- Quote ---This is abusing the MC33063's output transistor driver as a negative resistance oscillator.
--- End quote ---

I don't understand how the transistor driver is acting as a negative resistance device. It seems like a higher voltage at the base would result in a higher current. Maybe I'm looking at it the wrong way - could you elaborate on why it's behaving like a negative resistance oscillator?


--- Quote ---Also, why is 1.5nF from CS to GND?  Isn't CS measured relative to VDD?  Wouldn't you filter it there..?
--- End quote ---

I added the 1.5nF cap from CS to GND after I saw the oscillations because I didn't see much noise on VDD but I did at CS, so I wanted to try filtering CS. It helped reduce the amplitude of the oscillations seen at CS by about a factor of 5, although it didn't affect the amplitude of oscillations noticeably at any other node.

magic:
Dunno, try a 100nF THT ceramic bodged right onto GND/VCC pins.

Regarding the current sense cap, the point was that this node is referenced to VCC, so it should be filtered to VCC rather than to GND, if at all.
OldEE:
How much power do you need?  Can you use a charge pump for the negative voltage?

Does the amplifier need frequency response down to DC?  If not how about a single rail supply and and capacitors on the input and output?

Larry
MosFett:

--- Quote ---Can you use a charge pump for the negative voltage?
--- End quote ---

I was going to use a charge pump originally, but later realized it wouldn't work; I'm powering the device w/ a 9V battery and 12V DC jack (one at a time ofc), so I need the device to work all the way down to about 5V in order to use most of the battery's energy. I built a dynamic range compressor into my amp which needs a minimum of 12 volts to operate. To get -10V out with 5V in (so, 15V Vdd-Vss) I'd need two charge pumps. The problem here is when 12V is input, Vss would be -24V, giving 36V total out which is the absolute maximum rated voltage for my op amps. A 12.1V input could fry my circuit then. The MC33063 takes care of these issues.
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