P82B715: I2C with isolation, level translate and multiple slaves

Part Number: P82B715
Other Parts Discussed in Thread: TCA9517, TCA9517A, ISO1540, PCA9306, LSF0102

Hi Team,

We plan to build a system as showing in the picture, could you help review does this work or there is any problem, thank you very much.

PCBA board a is host using 5V for I2C, has P82B715 buffer.

board b is slave I2C is 3.3V, has TCA9517 for voltage level translate and another buffer P82B715, and ISO1540DR for isolation, ISO1540DR VCC2/GND2 are from board a, VCC1/GND1 from board b itself.

board c, d, e, are same design with board b, but different I2C address.

board a will connect to board b/c/d/e at the same time using only one interface.

Pull up resistors are pending calculation. I2C speed is100Khz.

5353.I2C connection.pptx

over 2 years ago

0 Chris Ayoub over 2 years ago

TI__Genius 15345 points

Hello,

An engineer has been assigned this question. Because of the holidays, TI E2E design support forum responses may be delayed through Jan. 3. Thank you for your patience.

Best,

Chris

0 Clemens Ladisch over 2 years ago

Guru 317350 points

Having the buffer behind the isolator does not make sense. It must be connected directly to the cable (and supplied by 5V-2).

The B side of the TCA9517A and side 1 of the ISO1540 use both a voltage offset, and are not compatible with each other. Consider replacing the TCA9517A with the LSF0102/PCA9306 (which is not a buffer).

0 AD over 2 years ago in reply to Clemens Ladisch

Intellectual 300 points

Hi Clemens, Chris,

Appreciate your support and reply.

I changed the voltage level translator to PCA9306, and swaped buffer and isolator. could you help review again.

Another question about buffer P82B715, how many slaves can support to connect to one host? currently we have 4 slaves, maybe increase in future.

Thanks again.

I2C connection--2.pptx

0 Clemens Ladisch over 2 years ago in reply to AD

Guru 317350 points

The limit is not the number of devices, but the total capacitance of the bus (all devices plus the cables). Standard I²C has a limit of 400 pF; the P82B715 extends this to 3000 pF.

The I²C pull-up resistors are chosen to balance speed and power usage. The P82B715 supports higher currents, i.e., it allows stronger pull-up resistors.

I do not know the capacitance of your cables; when in doubt, check the waveforms with an oscilloscope.

0 AD over 2 years ago in reply to Clemens Ladisch

Intellectual 300 points

Hi Clemens,

Thanks for reply, the updated structure /connection way is ok, right? we will build protype board to test it.

+1 Clemens Ladisch over 2 years ago in reply to AD

Guru 317350 points

Yes, looks OK.

0 AD over 2 years ago in reply to Clemens Ladisch

Intellectual 300 points

Thanks a lot!

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P82B715: I2C with isolation, level translate and multiple slaves