hi
I originally posted this in amplifier forum but it probably should be here in PS section. I have a puzzling problem where I have two different switcher supplies, one for my 5V analog circuitry and a totally seperate supply for the speaker circuit. They both are fed from the same 3V battery source. The analog switcher is folloed by an LDO (LM3982). Whenever the PWM audio is turned on ny a PWM from a MSP430, it causes noise in my analog circuit output(gain of 400). The speaker circuit is comprised of a 1" 8 ohm speaker that gets pulled to ground by a N ch Fet controlled by the PWM signal. On the positive side of the speaker I have a huge storage cap(0.33F Supercap) to store the energy. The 5.8V switcher output is connected to the cap through a 50 ohm resister. such that the cap charges when the speaker is off which is 93% of the time.
What is puzzling is that even when I disconnect the 51 ohm resistor and manual charge the cap, I see the noise.
I thought it might be coupled through the air but thats not the case, since I get the same results using an 8 ohm resister instead. the only common connection between the two circuits is the ground which is tied right at the negative battery terminal to the board. Although the micro digital ground is referenced to the FET I dont thik there is any feedback through that path. The board is approximately 3" wide by 6" long. Cost constraints doesn't allow for more then a 2 layer board, however, I have a very large groaund trace(1 cm) goind across the board to the analog circuitry which only draws 2 ma. I also have a short fat speaker ground trace from the speaker circuit again all tied at the battery minus input. Any ideas or suggestions are helpful. See schematic attachments
Thanks
