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UCD90120A PMBus Addressing

StuartT
Prodigy 250 points
Other Parts Discussed in Thread: UCD90120, UCD90120A

Hi,

My original design had a UCD90120 and I had the PMBUS_ADDR(1:0) resistors both as 47K5. According to Table 5 of the UCD90120 datasheet (SLVS966, Sept 09) this would give address bins of 6. Using the PMBus Address Equation, this would give an address of 78 dec.

However, it was noted that the UCD90120 wasn't recommended for new designs so I switched to the UCD90120A, and this is what I have fitted on my board.

The PMBus address resistor values change between these devices but I didn't notice this right away. As such, I still have the 47K5 resistors fitted. Table 9 in the UCD90120A datasheet (SLVSAN9, April '11) doesn't have 47K5 resistors as an option.

When I use the Digital Fusion tool, it correctly detects the device and identifies it as a UCD90120A, with address 78 dec. This address is what would be expected using the original UCD90120 version, but it's not clear what I should really expect with the non-preferred values when using the UCD90120A part.

Can you advise how this might still be working despite having incorrect resistances? All other functions appear to be working.

Thanks

Stuart

  • 0
    TI__Intellectual 1595 points

    Hi Stuart,

    The PMBus address resistance table has been updated for the second generation devices to allow more margining and flexibility for the customer in the choice of a resistor accommodating the tolerances of resistors over temperature and the process variations of the part itself. The updated table reports a much more centered resistor value for each bin and also excludes reporting the PMBus minimum and maximum voltages to avoid confusion for the customer in calculating the resistor based on the voltages. If you see in the UCD90120A table the resistor for bin6 is reported as the mid value from the min and max of the old table and your value of 47.5 still points to bin6 only. Errors with the UCD90120 table were reported with bin4 where the 27.4K resistance might not point to bin4 if there is any slight variation in the bias current or with tolerance of the resistor.

    So you wouldn't be seeing any problem with your old resistor value but we recommend to follow the new table which maintains a sufficient width between the bins.

    Thanks, Spandana