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LP2998-Q1: DDR3L Schematic Design

Furkan Fehmi Sefiloglu
Expert 3118 points
Part Number: LP2998-Q1
Other Parts Discussed in Thread: LP2998, TPS51200A-Q1

Hi Team,

We are working on new design with DDR3L, can you please check and advise with our schematic if you have any suggestions?

Thanks in advance

Best Regards

Furkan Sefiloglu

  • 0
    TI__Genius 15680 points

    Furkan,

    I have the following observations:

    1. I recommend adding a 0-ohm resistor between VSENSE and VTT to break the net, in the event that there is some sort of issue with the output/output sensing that needs to be debugged in the future.

    2. What is the current requirement for the output? If a higher current is required, it may be best to power AVIN and PVIN with the same source (this also eliminates the need to bypass both sources separately). The tradeoff will be a larger amount of power usage internally.

    3. What is the transient requirement that this part has to meet? Depending on the transient/maximum output current spec, there may need to be more input/output capacitance in the design.

    Thanks,

    Kadeem

  • 0 in reply to Kadeem Samuel
    TI__Expert 3118 points

    Hi Kadeem,

    Thanks for feedback, we have following updates, can you please check and advise?

    "

    You can find my answers to the questions below:
    1-2) I changed the design as follows. I think the recommended connection for AVIN and PVIN is 2V5 supply.


    3) There will be only one DDR3L memory with the following connections in the design. I don't know exactly what is meant by the transient requirement, but a single memory will draw current.

    "

    Thanks in advance

    Best Regards

    Furkan

  • 0 in reply to Furkan Fehmi Sefiloglu
    TI__Genius 15680 points

    Furkan,

    What I mean is the following:

    • What is the peak current requirement for the DIMM?
    • What is the maximum slew rate of the current the LP2998 must supply?
    • What is the maximum output voltage ripple supported by the DIMM input?

    Thanks,

    Kadeem

  • 0 in reply to Kadeem Samuel
    TI__Expert 3118 points

    Hi Kadeem

    The feedback is:

    "There is no DIMM module in the middle. There is only one DDR3L memory chip.

    The maximum current it draws on the datasheet appears to be 200mA.

    2% ripple is allowed in VREF."

    Thanks in advance

    Best Regards

    Furkan

  • 0 in reply to Furkan Fehmi Sefiloglu
    TI__Genius 15680 points

    Furkhan,

    Per the datasheet, there is +/- 2.8% ripple with no output load and PVIN = 1.35V, and +/- 5.2% ripple with 0.2A output load and PVIN = 1.35V; see the table below:

    With this specific part, the typical output voltage does not sit at 0.675V at 0.2A output load, but rather at 0.688V, which is contributing to this issue.

    The ripple can be mitigated with larger output capacitance (see the below example from the datasheet), but it may not be enough to meet your requirements:

    If this is not suitable, would you like me to recommend alternative parts that you can use for your system?

    Thanks,

    Kadeem

  • 0 in reply to Kadeem Samuel
    TI__Expert 3118 points

    Hi Kadeem,

    Can you please see feedback and advise?

    "

    I changed the design as below. AVIN=PVIN=2.5V.

    Does this solve the mentioned issue with VTT? Can you ask?

    "

    Thanks in advance

    Best Regards

    Furkan Sefiloglu

  • 0 in reply to Furkan Fehmi Sefiloglu
    TI__Genius 15680 points

    Furkan,

    Unfortunately, this will still not solve the ripple issue. I apologize, this is the table for VREF, the above one is for VTT:

    I believe that you may be best off considering a different part - the TPS51200A-Q1 is far newer, and REFOUT has a +/- 15mV tolerance to REFIN, with VO having a +/-25mV tolerance to REFOUT.

    Thanks,

    Kadeem