LP8866-Q1: Boost converter noise and gnd concerns

andres_dalmati

Part Number: LP8866-Q1
Other Parts Discussed in Thread: PCA9306, TCA9406, SN65DSI84

Tool/software:

Sorry for the delay. I’ve been working on the board all this time without any success.
The first test I ran was adding tantalum capacitors to all VDD rails (12V, 5V, 3V3, and 1V8) on the board. I also added ferrite beads to the I²C lines. Visually, things seemed to improve slightly, but it still didn’t work properly.

One of the last tests we did was replacing the TCA9406 with a PCA9306 (which doesn’t have a One-Shot Accelerator). I²C communication worked fine, but when the PWM pin was set high, the LP8866 always stayed in BOOST_STARTUP mode.

On the other hand, I suspect the noise was also affecting the DSI/LVDS lines of the SN65DSI84, which distorted the display image. There was no way to get it working reliably—the image would flash for a few milliseconds and then go completely black.

Eventually, I removed the LP8866 and powered the backlight directly using a regulated power supply with the correct voltage and current. The backlight now works perfectly, and the strange thing is that the display image is now also stable.

Do you have any suggestions to solve this issue?
Would improving the GND traces to the LP8866 help?
I didn’t include vias to the ground plane in the LP8866 footprint. Should I add them?

Thanks in adavnce

Andrés

2 months ago

0 Suchen Dong 2 months ago

TI__Expert 3371 points

Hi Andres,

Our expert will reply to you later. Thanks for you patience.

0 Zhiqian Wu 2 months ago in reply to Suchen Dong

TI__Mastermind 31405 points

Hi, Andres

Could you pls kindly share your schematic first?

For the ground plane, vias is recommend.

0 andres_dalmati 2 months ago in reply to Zhiqian Wu

Expert 1110 points

These are my schematics

8156.lp8866.pdf

3771.sn65dsi84_2.pdf

0 Lucia Gao 2 months ago in reply to andres_dalmati

TI__Mastermind 30291 points

Hi Andres,

Thanks for your feedback! Our expert Zhiqian will give you reply soon.

BRs

Lucia

0 Zhiqian Wu 2 months ago in reply to andres_dalmati

TI__Mastermind 31405 points

Hi, Andres

Looks like some fault is happened in our device, is it possible for you to read back the fault device reported?

Depending on the schematic, some problem has found in below, for charge pump, 5V input is no need for the charge pump function.

0 andres_dalmati 2 months ago in reply to Zhiqian Wu

Expert 1110 points

Hi!

I ran another test: I connected a 75Ω 10W resistor to simulate the display load. EN and PWM pins are set to 3.3V. The actual display has 9 LEDs per string, 4 strings total, with Vf = 27V and If = 360 mA.

On the oscilloscope, I observed that the current waveform (CH1, the sensibility of current probe is 500mV/A) is not continuous, and the voltage at the output pin (CH2) shows a strange pattern.

Shouldn’t the current waveform be continuous when using 100% PWM duty cycle? Also, I noticed that the current never reaches 360 mA. I used a design tool (Excel file) to select the correct Riset value.

Also, when I try to disable the charge pump, why does the 5V input become unstable?

I don’t see any I²C errors. Interestingly, even if I disconnect the flat cable from the LCD panel, no error is reported. I have four boards to test. For this last test, I didn't use the board where the LP8866 was always stuck in BOOST_STARTUP mode. Instead, I used a new one.

Thanks in advance

Andrés

0 Lucia Gao 2 months ago in reply to andres_dalmati

TI__Mastermind 30291 points

Hi Andreas,

Thanks for your feedback! Zhiqian will get back to you early next week. Thank you!

BRs

Lucia

0 Zhiqian Wu 2 months ago in reply to andres_dalmati

TI__Mastermind 31405 points

Hi, Andres

Depending on your waveform, device see some fault and enter the fault recovery.

Pls check with below step:

Put FB/GD/OUT/Boost output together and share the waveform.
Change the R14 to 20mohm to have a try.
Add a electrical cap on the output to have a try.

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LP8866-Q1: Boost converter noise and gnd concerns