ISO1640: Placing master on side 2 rather than side 1

Flynn Stilwell

Part Number: ISO1640

Hi Support,

My design's master is to be connected to the ISO1640 via a ~1m cable and the isolated side is to be connected to a TCA9546 I2C switch and in turn to 3 buses with 2 nodes each - all on the same board. My assumption is that there will be a higher capacitance from the cabling on the master side than the isolated slave side.

Because of this, I thought of connecting the high capacitance master to side 2 rather than side 1 of the ISO1640 since the former is much more powerful.

Would the ISO1640 operate as normal in this configuration?

Many thanks,

Flynn

over 1 year ago

0 Andrew J over 1 year ago

TI__Mastermind 29595 points

Hello Flynn,

Thank you for reaching out. The load capacitance on side 1 and side 2 must be less than the C1 and C2 capacitive load spec in the datasheet. In general, I2C is only recommended for communications between IC's on the same board, for off-board communications more robust protocols are recommended (like CAN or RS485).

Side 1 of ISO1640 is designed for a direct connection to an MCU. As a result, it is not designed for high capacitive loads. See the following FAQ for more details [FAQ] ISO1640: Why are the maximum load capacitance and load current ratings for Side 1 of the ISO1640/ISO1641 less than Side 2?

Best,
Andrew

0 Flynn Stilwell over 1 year ago in reply to Andrew J

Prodigy 10 points

Hi Andrew,

Thanks for getting back to me. I understand this configuration is less than ideal, but I would like to investigate it as I've had success with off-board I2C in the past.

So if I understand correctly, it doesn't matter where the master is located so long as the load capacitances are within spec?

Thanks,

Flynn

0 Aaditya V. over 1 year ago in reply to Flynn Stilwell

TI__Genius 17825 points

Hi Flynn,

Thank you for your question.

Please give us until after the weekend as we have a holiday. we will review this information and get back to you after this weekend.

Regards,
Aaditya Vittal

+1 Andrew J over 1 year ago in reply to Aaditya V.

TI__Mastermind 29595 points

Hello Flynn,

Flynn Stilwell said:
So if I understand correctly, it doesn't matter where the master is located so long as the load capacitances are within spec?

Yes, your understanding is correct the device should operate as long as the load capacitances are within spec.

However, the long cable lengths will likely cause load capacitances outside of the listed Recommended Operating Conditions. As a result the device may no behave exactly as spec'd in the datasheet.

Best,
Andrew

Isolation

Isolation forum

ISO1640: Placing master on side 2 rather than side 1