• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 16 subscribers
  • Views 590 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ISO1541: i2c issue

Kuo Bill
Prodigy 20 points
Part Number: ISO1541
Other Parts Discussed in Thread: MSP430FR2433, TPS2388

We used a MCU(MSP430FR2433) which went through an isolator(ISO1541) to four TPS2388s and the waveform showed that some of the rising time and falling time didn't meet the spec of TPS2388.

We found that on the path from MCU to isolator, which are shown in CH1,CH2, both SDA and SCL rising time met the spec of  TPS2388, but the falling time didn't.

Figures are shown as follows:

On the path from iso1541 to TPS2388, neither SDA nor SCL falling time met the spec of TPS2388. SDA met the rising time, but SCL didn't.

Figures are shown as follows:

  • 0
    TI__Expert 6195 points

    Hi Kuo, 

    Are you able to provide us your schematic? I want to see exactly how the ISO1541 is being connected in your system. What is the supply voltage for Vcc1, Vcc2? 

    Also, sorry if I missed it in your post, but it wasn't clear to me what each channel of the oscilloscope represented. 

    CH1, CH2 - input of ISO1541? CH3, CH4 - output of ISO1541? 

    Thanks for the additional information. 

    Best regards, 

    Dan

  • 0 in reply to Dan Kisling
    Prodigy 20 points

    Hi Dan,

    Thanks for your help!

    Vcc1, GND1 and Vcc2, GND2 come from different PWM converters output.

    CH1, CH2  are input of ISO1541 and  CH3, CH4 are output of ISO1541.

    We try  other vendor's isolator(SI8602AB-B-IS) and the waveforms meet the spec of 2388.

    Best Regards,

    Bill Kuo 

  • 0 in reply to Kuo Bill
    TI__Mastermind 49415 points
    Hi Bill,

    Thank you very much for sharing schematic showing connections between MCU, ISO1541 and TPS2388.
    I am sorry to hear about the issue, we haven't heard from any other customers facing issues in using ISO1541 with TPS2388.

    I see that you are currently observing fall times between 1.6ns to 4.4ns on SDA1, SCL1, SDA2 & SCL2 and your concern is that the fall times of SDA2 & SCL2 are too fast and are less than the minimum fall time spec of TPS2388.

    From ISO1541 datasheet and under full capacitive load, you can see that the fall time of SDA2 is between 8 and 29ns while the fall time for SCL2 is between 14 and 47ns. The max capacitive load for ISO1541 Side2 is going to be 400pF while TPS2388 only has an I/O capacitance between 6-10pF. Hence, it is expected to see faster rise and fall times with this particular interface.

    One easy way to increase fall times is to increase the the bus capacitance. I see it is currently about 15-20pF when ISO1541 Side & TPS2388 capacitance are added together. To increase bus capacitance, you may be required to add external capacitance on the bus. You can add upto a maximum of 400pF on Side2 of ISO1541 and 40pF on Side1.

    To increase the rise times, you will be required to increase the pull-up resistance value. I see it is currently about 1k Ohm, you may increase this to higher values like 4.7k or 10k. You should see an increase in the rise times. To increase from 2ns rise time to >20ns, a pull-up of >10k should be used.

    Please do implement the above suggestions and let us know if you were able to fix this issue. Thanks.


    Regards,
    Koteshwar Rao