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Dear All:
I have a small oscillation on signal VDDA of PSOC chip CY8C5868AXI (see attached oscilloscope image, Fig_1).
After some work, I have realized that if I disconnect this signal from the chip it dissapears. Please find attached also the power schematic of the CY8C5868 (Fig_2) and the the power generation of my board (Fig_3) As you can see, there are basically two 5 volts power signal: VCC for digital stuff and +5V_A for analog stuff.
In Fig.3, 5V_IN is generated in a external power supply, then get into the board through CN1, after a ferrite bead filter, becomes VCC.
As I wanted to have a clear analog power supply, +5V_A is generated using U20 and U21, see also Fig.3.
I have measured with a multimeter VCC and +5V_A and get: VCC= 5.049V and +5V_A= 5.025.
On datasheet for this chip, pag 12/139, says "VDDA must be the highest voltage present on the device. All other supply pins must be less than or equal to VDDA." In my board, as I have described above, this is not true. Do you think that small difference of voltages between VCC and +5V_A is the cause of this oscillation?
Thanks in advance,
Joaquín.
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PSoC 5LP
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The statement from the datasheet should be observed, because otherwise your PSoC might get damaged. Whether this oscillation comes from the Vdda voltage being slightly higher that Vddd is difficult to answer. It might be that you LDO starts oscillating when under load (the outout cap for the LP2981 must NOT be ceramic, as stated in its DS).
What you can check: test your power supply circuit with some load. Add a diode inbetween the Vddd wire so its lower than Vdda.