Part Number: TCA9406
For a new design I using TI TCA9406 I2C Level Shifter – to translate from 1.8V to 3.3V voltage levels.
Up to now I have used TI PCA9306, but since the new design needs to support 800KHz mode, I changed to TCA9406
However I facing difficulties with the new part, mainly signal integrity issues.
For the same PCB and same I2C master and slave the ne part does not work – and the old one does (at 100KHz/400KHz).
I ask for your advice on how to remedy this issue.
How much capacitance is on your bus? Did you measure it? What are your pull up resistor values. I would like to see your schematic to verify connections and component selection.
Can you take scope shots of both SCL_A, SDA_A, SCL_B, and SCL_B. Ideally you will need a 4 channel o-scope.
Depending on parasitics, this parts can sometime have issues due to one-shot accelerators. NOTE, this is a pass fet architecture and doesn't isolate bus capacitance between the A and B side.
If you are having problems with this device we have the TCA9617B that is designed to buffer and translate for fast mode plus applications.
In reply to fhoude:
I advanced on solving the issue myself, I did try adding capacitance on the SCL/SDA lines and it seems to improve the signal integrity.
I am out of office today so I cannot attach scope shots of the signals, but what I saw are spikes on both SCL and SCL when the signal what at "0" – that happened when the other signal what rising from "0" to "1". That is if whenever that was a clock rise – SDA had a significant spike.
This is original schema:
There are 2Kohm pull up resistors to 1.8V on the I2CS_SCL/SDA_S_H_2 lines that are not visible here.
What I did is to add a 400pF capacitor from both SCL and SDA to GND (close to J36).
Do you have more input?
In reply to Shlomi Yehezkia:
Please see waveforms attached.
"I2C_A2" is the with added 400pF capacitors.
No cross coupling on the PCB - the signal is perfect with TCA9306 wired on the board.
I’m considering using the TCA9406 device for new design (starting layout) and I’m very warry about the below SI issue.
I talked with the guy that use the TCA9406 before and It looks that adding capacitors on the lines improved the performance but not completely solved the problem.
Can you please advise (that suspect that the problem is cross coupling – see below) if you can provide detailed layout guiding and if you can recommend for decoupling capacitor or other thinks that can prevent the problem because I still in the Schematic/Layout phase of the new design
In reply to Bobby Nguyen:
Thanks, I’m already familiar with those requirements.
I used 100nF decupling capacitors is (for each of the two power rails) it ok if we take in count the fast switching time?
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