ISO1640: Design review and low temperature connection issue

JH Shin

Guru 11060 points

Part Number: ISO1640
Other Parts Discussed in Thread: INA238, , INA226

Tool/software:

Hello,

My customer designed an isolated I2C interface circuit using ISO1640 to interface with INA238 as shown below.

Q1. Please review the above circuit design to see if there are any problems.

Q2. They have an issue where I2C communication works fine at room temperature during board testing, but does not work at low temperatures.

Here is the measurement of the I2C connection between the INA238 and the ISO1640:

1) Room temperature(normal)

2) Low temperature(about -20 degrees)

Please advise on what causes this issue and how to fix it.

Thank you.

5 months ago

0 Varun Kumar 5 months ago

TI__Expert 7625 points

Hi JH,

Thanks for reaching out. I see two issues with the design, below are my inputs:

Since Side1 is connected to INA238, the Logic compatibility does not match between the two device:
1. The VOL1(max) of ISO1640 is 0.71V where as for INA238 - VIL(min) is 0V and VIL(max) is 0.4V.
2. The Communication won't happen between the wo device in above scenario.
3. please refer this FAQ for more understanding on why VOL1(max) is on the higher side : https://e2e.ti.com/support/isolation-group/isolation/f/isolation-forum/757564/faq-why-is-the-logic-low-level-output-voltage-vol1-up-to-0-8v-on-side1-of-the-iso1540-iso1541-and-iso1640-iso1641-bidirectional-i2c-isolators
Even if the logic compatibility somehow matched hypothetically, the Cap loading of 33pF is close to 40pF limit for Side1 of ISO1640.

My recommendation is swapping Side1 and Side2 and then doing the test.

Regards
Varun

0 JH Shin 5 months ago in reply to Varun Kumar

Guru 11060 points

Hi Varun,

Thank you for your kind support.

The customer tested it by swapping Side 1 and Side 2 and confirmed that there is no low temperature issue.

Could you please advise if the problem can be solved by replacing INA238 with INA226(VIL:0.3*VS) without swapping ISO1640?

Regards,

0 Varun Kumar 5 months ago in reply to JH Shin

TI__Expert 7625 points

Hi JH,

The VIL for INA226 would be around 0.99V and should be compatible with ISO1640 Side1 logic levels. But Please do check for min VIL numbers of INA226 which should be higher than VOL1(max) of 0.71V.

Also, in case of not swapping ISO1640, I see 33pF Cap as a threat as the max Cap loading allowed on Side 1 is 40pF(for side2 it is 400pF) which can happen if we add Trace Caps on the PCB. I would recommend to test with lower or no cap on SDA/SCL line for Side1.

Regards
Varun

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ISO1640: Design review and low temperature connection issue