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TCA39416: Signal boosting and bus capacitance redriving

Ivica Matovic
Prodigy 10 points

Part Number: TCA39416
Other Parts Discussed in Thread: TCA9617B

Dear, 

In my board, I will have I3C lines of up to ~60cm, which estimates bus capacitance of ~100pF, 

Intention was to use multiple TCA39416 along the bus to redrive the signals. 

I found cnflicting information in TCA3914 documentation: 

  1. EVM user guide SCPU047 explicitly says "this device does not redrive or separate the capacitive loading. This means enabling both sides violates the max loading allowed by I3C standard "
  2. I3C Voltage Translator and Multiplexer reference SCPA072 explicitly says "TCA39416 provides low voltage level shifting and signal boosting". 

Which one should I follow? 

Do you recommend using 2 or 3 TCA39416 in series in order to keep each bus segment below 50pF capacitance? 

Thank you. 

  • 0
    TI__Guru 61818 points

    Hi Ivica,

    Ivica Matovic said:
    • EVM user guide SCPU047 explicitly says "this device does not redrive or separate the capacitive loading. This means enabling both sides violates the max loading allowed by I3C standard "
    • I3C Voltage Translator and Multiplexer reference SCPA072 explicitly says "TCA39416 provides low voltage level shifting and signal boosting". 

    This information is not to be conflicting. 

    We used the term "signal boosting" to show there is some edge rate acceleration through the use of a rise-time accelerator and fall-time accelerator. Even with these circuits in the chip, the TCA39416 is technically not a buffer because it doesn't have a high impedance path that separates side A and side B.  

    See that a passFET is used to separate side A and side B

    The capacitance seen at "A" side will appear in parallel to side "B". 

    A true buffer would have the following structure (TCA9617B): 

    However, TCA9617B is only for I2C FM+ capable. 

    TCA39416 can still be used to "re-drive / boost a cap load" but it does not separate the cap loading. 

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    Prodigy 10 points

    Thanks Tyler, this now makes perfect sense. 

    Do you have some proposal for this: 

    We have a host, then ~60cm long lines, then connector to another board, and 10cm more on that another board to the peripheral. 

    Does using multiple TCA39416 on the bus makes sense, or we just need to slow down the speed? 

  • 0 in reply to Ivica Matovic
    TI__Guru 61818 points

    Hi Ivica,

    The I3C spec allows up to 50pF. 

    If you consider ~ 6pF from the host and 6pF from the downstream target, +connector cap, +trace cap, you would likely exceed 50pF if you assume 0.8pF/cm of trace. This estimate for trace capacitance is dependent on the type of trace routing, whether microstrip or stripline. 

    A TCA39416 would help boost the rising and falling edges but does not help with capacitance total. You might be able to leverage the boosting capabilities of the TCA39416, but you will still be outside spec since cap exceeds 50pF, especially since TCA39416 adds about 12.5 pF of cap by itself. 

    12.5 MHz clock is a maximum spec. I3C can always be ran at reduced clock rates, so this is an option. 

    Regards,

    Tyler

  • +1 in reply to Tyler Townsend
    Prodigy 10 points

    Agree. That's exactly how I estimate. 

    Thank you, Tyler.