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TPS65218D0: DDR3 Dynamic On-Die Termination

Flavio Simoncini1
Prodigy 130 points
Part Number: TPS65218D0
Other Parts Discussed in Thread: AM3358, , TPS51206

Hi, 

In my design I'm powering AM3358 Sitara processor with TPS65218D0 PMIC, and I'm wondering if it is possible to use one of its load switches as a sort of DDT termination for the DDR3L RAM. If so, can you suggest me how I can provide VTT and VREF to the memory?

Thanks

Flavio

  • 0
    TI__Mastermind 30280 points

    Flavio,

    For simple systems with a single DDR chip and point-to-point routing, the VREF voltage can be generated by a resistor divider, like the one shown below:

    The strength of the resistors may vary (1kOhm to 10kOhm) for sinking or sourcing a couple hundred uA up to about a mA of current, and the capacitor C114 can be modified or marked as DNP depending on the response time needed. Regardless, the goal is to keep a stable voltage of 1.35V/2 = 0.675V at the VREF pins (this rail is also referred to as VTT).

    I do not see any motivation to use the load switches to provide 0.675V to VREF, because it could introduce a voltage drop where it is not ideal and does not regulate the output voltage.

    For more complex systems where sinking and sourcing a lot is current is required to keep this voltage within the limits, a DDR terminator like the TPS51206 can be used.

  • 0 in reply to Brian Berner
    Prodigy 130 points

    Hi Brian, 

    very clear answer. 

    I was just evaluating the proposed DDR3 termination, but I'm really space limited in my design and, as you mentioned I have 1GB DDR3L and a direct connection to the Sitara processor. So can I be confident just providing the VREF (VTT) voltage with a resistor divider instead of adding an extra component (TPS51206)?

    Thank you very much!

  • 0 in reply to Flavio Simoncini1
    TI__Mastermind 30280 points

    Flavio,

    Yes, I am using a MT41K256M16HA-125M:E-ND memory IC on my design where I copied that circuit from. I just ran some scripts to test the DDR memory and they all passed. I am unable to take any precise analog measurements at the moment.

    Looks like I have the 4Gb (512MB) version, so half the memory of your 1GB version, but I expect this resistor divider VREF implementation will still work on your board.