This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LP2998 VDDQ = 1.35V

Other Parts Discussed in Thread: LP2998

Hi,


I would like to use the LP2998 in a DDR3L application. Therefore, VDDQ = PVIN = 1.35V and AVIN = 2.5V.


I'm looking for the max IVTT current that this regulator will supply. Am I reading the datasheet correctly that it will only supply 200 mA under these conditions?


Thanks!

-Dan

  • Hi Dan,

    At PVIN=1.35V, I would treat 200mA as the maximum current, since any higher current has not yet been characterized in the datasheet for this PVIN value.

    Maximum output current is highly dependant on the value of PVIN due to RDSon limitations of the internal FETs. Unfortunately, we do not have a characteristic curve available that shows the maximum output current down to PVIN=1.35V. If higher current is needed, you may raise the PVIN voltage, but your power dissipation will increase.

    Please see the Typical Performance Characteristics graphs to see the PVIN effect on maximum output current.

    Tim

  • Hi Dan,

    Please also make sure that your values for VDDQ, PVIN, AVIN, and current, are acceptable for your thermal requirements. You can read the thermal dissipation section for details on calculating power and ensure that the thermal performance is within your requirements.

    Tim

  • Hi Tim,


    Thanks for your response! Let me explain the problem that I'm debugging better...hopefully you can help.


    I'm seeing instances where the LP2998 regulator isn't properly regulating the VTT output. As I mentioned, we're using a VDDQ = 1.35V, so the VTT output should be ~ 0.675V. However, on occasion the part starts up with a VTT output of ~1.35V (not VDDQ / 2 as it should be). When the memory interface starts the output then drops down to an oscillating 0.4-0.5V.

    Power cycling our device will usually cause the LP2998 to begin regulating properly again.

    I was thinking that perhaps we were drawing too much current (hence yesterday's question), but we measured that to be ~150mA. Regardless, I did try to increase PVIN to 2.5V (from 1.35V), but this did not keep this problem from occurring.


    Do you know of any reason why the LP2998 would randomly behave like this?

    Thanks,
    Dan

  • Hi Dan,

    Can you send a schematic with labelled operating voltages? Let me double check this as a first step.

    Also note that PVIN should not be higher than AVIN.

    Tim

  • Hi Tim,

    Here's the schematic you asked for:

    All of the shown caps are located near the DDR3.


    The PVIN mod that I attempted was to tie it to AVIN (2.5V), so they were equal.


    Thanks for your help!
    -Dan

  • The oscillations lead me to believe this to be a stability issue. The datasheet recommends a ceramic capacitor in the range of 0.1uF to 0.01uF at VREF. You are exceeding this by >40X. Can you try reducing to the recommended values to see if it solves the issue?

  • Hi Tim,


    I removed the 2x 2.2uF capacitors on VREF and replaced them with a single 100nF capacitor....this didn't seem to make a difference. I still do have occasions where VTT starts off as 1.35V and drops low to ~0.5V.


    To point out though, even when VTT isn't regulating properly, VREF is still correct and stable.


    Thanks,
    Dan

  • Dan,

    Can you attach a scope shot that shows VTT starting off at 1.35V, then dropping down to 0.5V?

    Can you show what the oscillation looks like on VTT?

    Do you see the issues above only when driving 150mA? What about when you are not driving any current?

    Tim

  • Hi Tim,

    Here is a shot of VTT dropping from ~1.35 to ~400mV:

    VTT rising to 0.675v normally:

    VTT rising to 1.35v (incorrectly - not the difference in time scale)

    As for the oscillation, that was caused by a grounding issue I was having. I removed that issue to no effect on the VTT problem.

    I'll see if I can recreate the problem without a load attached and get back to you.

    Thanks,
    Dan

  • Are there any power sequencing requirements for PVIN and AVIN?

  • Thanks for showing. This is not normal behavior. It may also be possible that the part has been damaged. Correct me if I am wrong, but my understanding is that sometimes on power up, you get the correct VTT, and on other power ups, you have VTT ramp up to 1.35V, then down to ~0.4V as shown in your shots.

    As for sequencing, AVIN should be powered before applying voltage to VDDQ since AVIN supplies the Vcc to the internal comparator. PVIN and AVIN can be powered up at the same time.

  • Hi Tim,


    Your understanding is correct. All of the above captures are from the same device. Most of the time it outputs VTT properly. It only has the ramp to 1.35V and then drop to ~0.4v occasionally.

    Our 1.35v (VDDQ and PVIN) and 2.5v (AVIN) supplies are enabled with the same control signal come on within about 100us of each other. Do you think we need to delay the 1.35v supply more?

    Thanks,
    Dan

  • Hi Dan,

    If the 1.35V (VDDQ and PVIN) and the 2.5V (AVIN) are within 100us of each other, and the ramp time for AVIN is >100us, then you may have a sequencing issue.

    Make sure that AVIN is high first, then bring VDDQ and PVIN high and see if that resolves the issue.

    Tim

  • Hi Tim,


    I don't have the ability to easily sequence 2.5v and 1.35v on my board. However, I could tie the shutdown signal of the LP2998 to the powergood signal of my 2.5v and 1.35v regulators. This will ensure that both supplies are up before the LP2998 is enabled, but it doesn't necessarily ensure the proper sequence. Is this still acceptable?


    Thanks,
    Dan

  • Judging by its description, I believe the SD pin opens/closes the connection between VDDQ and the comparator that outputs to VTT, so this should be fine to power sequence the part.

    Please give this a try to see if it solves your startup issue.

  • Hi Tim,


    So far so good with the sequencing! I'll let you know if anything changes.


    Thanks for the help!

    -Dan