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TPS51200: About DDR3 design and VTT accuracy

Tsukui Yusuke
Guru 21930 points
Part Number: TPS51200

Hi,

Let's me talk about VTT accuracy for DDR3 design.
I got two questions from customers.

Q1.
VVTT = VVDDQXR1/（R1+R2）
Is this correct in the calculation method?

Q2. 
In the case of the calculation method of Q1,
When considering the recommended value of the voltage tolerance of Vtt of DDR to be ± 2%,
(0.75V±0.2％：0.735V～0.765V)

In  R1,R2：±1％ accuracy , VVDDQ ： ±0.2V accuracy condition.

◇R1=10.1KΩ,R2=9.9KΩ, VVDDQ=1.52V
    Vtt=0.7676V

◇R1=9.9KΩ,R2=10.1KΩ, VVDDQ=1.48V
    Vtt=0.7326V

However, it can be seen that ± 2% can not be realized from the calculation result.
・Are R1 and R2 recommended 0.5% ?
・Please tell me how to calculate the expected variation of VTT at full temperature and full load range.
・Please tell me the cause of VTT variation of this device itself.

Best Regards,
Yusuke/Japan Disty

  • 0
    TI__Mastermind 24075 points
    Hi Yusuke,

    I think VTT voltage should follow VDDQ/2 with tolerance.
    For example, if VDDQ=1.52V, VTT should be 1.52V/2 with +/-2% tolerange. That means VTT should be between 0.76V*0.98 and 0.76V*1.02

    So with +/-1% accuracy for R1 and R2, VTT tolerance should meet +/-2% of VDDQ/2.

    Thanks
    Qian
  • 0 in reply to Qian Chen
    Guru 21930 points

    Qian-san,

    Thank you for your support.
    I understood your explanation about concept of VTT accuracy.

    Let me ask you one more thing.
    How should the accuracy of the TPS51200 itself be considered?
    Could you tell me about accuracy variation information and calculation method only for TPS51200?
    Customer want to know "how to calculate the expected variation of VTT at full temperature and full load range".

    Best Regards,
    Yusuke/Japan Disty

  • 0 in reply to Tsukui Yusuke
    TI__Mastermind 24075 points

    Yusuke-san,

    On TPS51200 side, REFOUT (Vref) follows REFIN, VO (VTT) follows REFOUT.

    The Vref tolerance with respect to REFIN is within +/-12mV.

    At +/-2A load condition, the VTT tolerance with respect to VREF is within +/-25mV.

    So the total VTT tolerance with respect to REFIN could be +/-37mV. It should meet VTT AC+DC tolerance spec for DDR3 application.

    Btw, TPS51200 is being used by many other customers in DDR3 application.

    Thanks

    Qian

  • 0 in reply to Qian Chen
    Guru 21930 points

    Qian-san,

    Thank you for your kind support.
    I understood your explanation.

    I’m sorry, I have another question. 
    Could you tell me about input capacitor.
    Please tell me why 1uF and 4.7uF is recommended.
    Customers are going to use 10uF.



    Best Regards,
    Yusuke/Japan Disty

  • 0 in reply to Tsukui Yusuke
    TI__Mastermind 24075 points

    Yusuke-san,

    10uF cap for VIN pin is fine.

    Thanks

    Qian