ISO7761-Q1: Circuit inquiry when there are 5 SPI slaves

HYUNJUN KIM

Prodigy 50 points

Part Number: ISO7761-Q1
Other Parts Discussed in Thread: ISO7761, , SN74LVC125A

Tool/software:

Hello, I designed an SPI circuit using ISO7761 BUFFER as shown in the attached picture.

There are 5 SLAVEs connected to one SPI line. Are there any problems if designed like this?

If you have any recommended designs, please let me know.

1 month ago

0 Clemens Ladisch 1 month ago

Guru 290240 points

The ISO7761-Q1 outputs have a drive strength of 2 mA (with a 3.3 V supply). This is similar to the outputs of an MCU.

The circuit should work, unless the traces are very long and you're using very high speeds. What is the total trace length? If you want to increase the drive strength, add a buffer like the SN74LVC125A.

+1 Aaditya V. 1 month ago

TI__Genius 12715 points

Hello,

As Clemens stated, the setup you currently have should work. The main consideration for this system is the board capacitance and also the output current capability of the device. Longer traces means the impedance/capacitance will be increased, requiring more driving current. Since the digital isolator has a specific output current per data rate, trace length will need to be considered.

Regards,
Aaditya Vittal

0 HYUNJUN KIM 1 month ago in reply to Aaditya V.

Prodigy 50 points

0 HYUNJUN KIM 1 month ago in reply to HYUNJUN KIM

Prodigy 50 points

Thank you for your kind response.The length of the trace is as shown in the picture above.

1. Could the length of the trace be a problem?
2. Currently, the iso7761 buffer has a 0402 size resistor. Can this resistor have an effect?

+1 Aaditya V. 30 days ago in reply to HYUNJUN KIM

TI__Genius 12715 points

Hi Hyunjun,

Thanks for your response and the additional information.

The length of the traces are not long, you should be good to use the ISO7761 device in your system. This would also depend on your data rate, but if the data rate is not really high, you should be fine with your setup. The below link addresses a similar question:

ISO7342FC: ISO7342FC SPI Isolation with 4 slaves - Isolation forum - Isolation - TI E2E support forums

Regards,
Aaditya Vittal