Per the title, we have a 7" TFT with a Goodix GT911 touchscreen controller which connects on a small FPC (power, ground, I2C and RST/INT lines), however when the touch controller is operational we see hefty bursts of noise on the 3v3 supply rail despite it being fairly well decoupled right on the FPC connector with 100nF + 10uF MLCC's, a more than adequate power supply rail (AP7366 rated 600mA continuous - GT911 claims ~8mA consumption) with a thick PCB trace, and good solid grounding right around the connector with multiple vias down to the ground plane.
The noise appears to have a 50 kHz main component and happens in bursts which I assume correspond to scans of the touch area(s).
I can find nothing in the datasheet / design guide or general internet searching about this sort of thing being a problem, so was wondering if anyone here had experienced similar issues or had any useful info or app notes or suchlike?
Scope trace showing AC component of 3.3 V rail measured right on FPC connector next to decoupling/smoothing caps:
Before you say it, yes I know the Rigol scope is fairly noisy by default.
- Scope probe is standard X10 probe, probing on the decoupling capacitors which are only millimetres from the FPC connector, ground lead to PCB ground pour which is all round the connector.
- FPC is 10-way 0.5mm pitch, wired GND, VDD, SCL, GND, SDA, GND, NRST, NC, INT, GND. GT911 controller plus associated passives are on this FPC.
- PCB is 4-layer, FPC grounds come out of connector into ground pour, with multiple vias down to ground plane below.
- Snippet of PCB below showing connector, cap, grounding, and scope probe points.
- R157 + R158 are the I2C pullups, 4.7 k.
- C94 was 100nF MLCC, then I added a 10uF MLCC on top of it (stacked 0603's). Adding a 47 uF electrolytic across the pads of C94 had no appreciable effect. I can't say for sure this is a problem but the touchscreens are sometimes causing problems (I2C lockups) which is what led me here, and the spikes feel unwelcome at the very least. GT911 specifies <=50 mVpp supply ripple so we're borderline.
- I added 2x 100R in parallel (to increase the load) but no difference - the AP7366 is stable with zero load and it's a linear LDO which doesn't have different modes of operation.
- I made a pigtail and re-measured it right across the ends of C94 and it made no difference at all to the trace.
I added a 47uF electrolytic on the rail, it reduced the amplitude by a few millivolts and that's about it.
Here's a zoomed-in trace of noise burst: