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ULPI interface

Amit Benjamin
Expert 6890 points
Other Parts Discussed in Thread: TPS65921, TPS65920

Hi Team,

see my customer question below regarding the ulpi matching impedance to the OMAP. see attached drawing.

pleasekeep this correspondence internally. I don't understand why it moved to external discussion in my previouse question.

thanks

Amit

Hi Amit,

 

There's still something weird about the issue.

The sketch below shows the transmission line between OMAP and TPS65921:

 

As you know, TI requires this transmission line to be a Z0=50 Ohm line. From this I assume that in order for TPS to be able to drive the line at high frequency without distortions, its own internal impedance must be 50 Ohm. Again, this is an assumption based on TI's requirement for 50 Ohm trace impedance.

This transmission line, once defined as a 50 Ohm transmission line, has an impedance of 50 Ohm from either side. This means that if we look at the OMAP side of the line, for it to drive the line without distortions, its internal impedance should also be 50 Ohms.

 

 

 

 

Now let's take a look at the OMAP-TPS65920 pair:

 

Here you can see that on the previous TPS (920), since TI required us to use a 90 Ohm impedance transmission line, we must assume that TPS65920's internal impedance was 90 Ohm.

This is fine when we look at the TPS side. Again, if we look at the transmission line from the OMAP side, we see 90 Ohm impedance, same as from the TPS side of the line. This means that in order for the OMAP to drive the line without distortions, its internal impedance should be 90 Ohm. Here lies the contradiction – it's the same OMAP. Once we conclude that its internal impedance is 50 Ohm, and another time we conclude its 90 Ohm.

 

Have I misunderstood? This issue has significant implication on my design. Please clarify.

 

Thanks,

Matan

 

  • TI__Genius 15100 points

    Hi,

    Please see some clarifications below:

    TPS65920 was designed with 90-ohm ULPI buffers. Therefore it requires 90 ohm ULPI traces on the board.

    TPS65921 was designed with 50-ohm ULPI buffers. Therefore it requires 50 ohm ULPI traces on the board.

    ULPI interface is not terminated with a low impedance. Its termination is the high impedance input gate of opposing buffer.

    It is recommended that ULPI trace impedance match PMIC ULPI buffer output impedance, as described above.

     

    Neither of TPS6592x, nor OMAP3 have terminations, the ULPI interface works with un-terminated lines.

    - OMAP3 and TPS65921 are optimized for 50-ohm.

    - TPS65920 is optimized for 90-ohm.

    - This creates a mismatch between OMAP3 and TPS65920 target impedances. BUT the interface was extensively simulated and analyzed to work with OMAP3 (==50-ohm), 90-ohm traces and TPS65920 (==90-ohm).