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TPS795: power up not linear

fangyi jiang
Prodigy 150 points
Part Number: TPS795
Other Parts Discussed in Thread: TPS793

hi TI expert

      now，used tps79501 convert 1.2V，at the power up,found the curve was not monotonou,has much step ,  could you give some support  ,

    the same issue was found at tps79318,refer the second data 

the like the picture

  • 0 in reply to fangyi jiang
    TI__Expert 4350 points
    Hi Fangyi,

    Could you include a trace on your scope shots showing Vin? What is your output load current? Can you share your schematic for the TPS79501?
    Is the startup with 1 step from the TPS79501 and one with 4 steps form the TPS79318?

    TPS795, TPS793
  • 0 in reply to Eric.Preiss
    Prodigy 150 points

    1V2's circuit for your information,1V2 and 1V8 have the same input 3v3 source.to test 3v3,the signa of power up is perfect,have no step, 1v2 and 1v8's load was ov2718,TPS79501 has 4 steps and TPS79318 has 1 step from the capture data,the below data is from PMP3782 Test Results down load from ti website

    from this, can be found some step at timebase 100us.what my capture data was at 40us, can be seem exactly 

  • 0 in reply to fangyi jiang
    TI__Expert 4350 points
    Hi Fangyi,

    Thanks for answering my questions. I have a few more questions:
    1. Could you capture a scope shot showing 3.3 V, 1.8 V and 1.2 V together on one screen?
    2. Do you have any other regulated voltages on your board?
    3. Do you have any ESD diodes on the 3.3 V, 1.8 V or 1.2 V? Do you have any diodes connecting any of these nets?
    4. Are you powering both the DOVDD and AVDD inputs to the OV2718 with the one TPS79318 1.8 V output?
    5. Can you provide any details from the OV2718 datasheet on expected load current or power draw for each power rail? The product brochure does not provide details.
  • 0 in reply to Eric.Preiss
    Prodigy 150 points

    1. Could you capture a scope shot showing 3.3 V, 1.8 V and 1.2 V together on one screen? 

        oscilloscope was  borrowed by other guys, so i will test later 


    2. Do you have any other regulated voltages on your board? 

       sensor board has only two regulated voltage

    3. Do you have any ESD diodes on the 3.3 V, 1.8 V or 1.2 V? Do you have any diodes connecting any of these nets?

       none


    4. Are you powering both the DOVDD and AVDD inputs to the OV2718 with the one TPS79318 1.8 V output? 

      yes


    5. Can you provide any details from the OV2718 datasheet on expected load current or power draw for each power rail? The product brochure does not provide details.

     

  • 0 in reply to fangyi jiang
    TI__Expert 4350 points
    Thanks Fangyi.

    I looked at the plots again:
    TPS79501 (1.2 V) 3 intermediate steps on startup: 0.5 V, 0.8 V, 1.1 V
    The PMP3782 scope shot of TPS795xx regulator startup shows similar overshoot to what you see on each step and the final regulated voltage.

    TPS79318 (1.8 V) 1 intermediate step on startup: 0.5 V

    The 0.5 V is close to a diode voltage drop and it occurs on both regulators.

    Each of the steps look like the LDO is in dropout during the step before startup ramp resumes.

    Hopefully we will learn something from a scope shot of 3.3 V, 1.8 V and 1.2 V on the same screen.