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Oscillations on VDDA/CY8C5868AXI | Cypress Semiconductor

Oscillations on VDDA/CY8C5868AXI

Summary: 2 Replies, Latest post by hli on 31 Mar 2016 12:54 PM PDT
Verified Answers: 0
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lopezconteras's picture
9 posts

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,



user_242978793's picture
959 posts

Where is your 10nf and if needed 1nf. You never said what kind of signal you where applying to the PSOC chip. Also not all capacitor are created equal. Some are better for high frequency reduction that others. Also a star ground will reduce ground loop oscillating issues.

user_78878863's picture
2551 posts

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.

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