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AM3358: Trace impedance for DDR3L

Daisuke Maeda
Guru 20465 points

Part Number: AM3358

Our customer want to use DDR3L (MT41K512M16VRP-107 IT:P) with AM3358BZCZA.

According to the datasheet, for DDR3 and DDR3L, the Zo (single-ended impedance) range is specified as 50 ohm (TYP) to 75 ohm (MAX).

They have found that achieving 50 ohm (TYP) is difficult for their PCB design.

For the Zo (single-ended impedance), is any value within the range of 50 ohm (TYP) to 75 ohm (MAX) acceptable?

If so, is impedance control required within ±5 ohm of the selected target impedance value (Zo)?

Other than 50 ohm (TYP), what value would you recommend?

If a value other than 50 ohm (TYP) is used, are there any precautions or considerations that should be taken into account?

Best regards,

Daisuke

  • +1
    TI__Guru* 96541 points

    yes, any value between 50 and 75 ohms is acceptable.  Even below 50ohm is acceptable (50ohm is typical), most designs are around 50ohm.  Regardless of which trace impedance you choose, ensure you follow good board layout practices as outlined in the datasheet, keeping impedances matched, trace to trace widths as specified, solid ground reference planes, etc.

    Regards,

    James

  • 0 in reply to JJD
    Guru 20465 points

    Hi James-san,

    Thank you for your reply.

    Does the trace impedance (Zo) value affect the calibration resistor values?

    • For AM335x, DDR_VTP (VTP Compensation Resistor) is required to be tied to ground via a 49.9 Ω (±1%, 20 mW) resistor.

    • For DDR3L memory, ZQ (External Reference Ball for Output Drive Calibration) is required to be tied to VSSQ (DQ ground) via a 240 Ω resistor.

    For trace impedances (Zo) other than 50 Ω (TYP), are different calibration resistor values used?

    Best regards,

    Daisuke

  • 0 in reply to JJD
    Guru 20465 points

    Hi James-san,

    Please give me an answer as soon as possible. Your prompt reply would be appreciated.

    Best regards,

    Daisuke

  • +1 in reply to Daisuke Maeda
    TI__Guru* 96541 points

    No, the VTP Compensation Resistor value and the External Reference resistor value remain the same.

    Regards,

    James