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TPS7A49: Poor load regulation

Intellectual 895 points

Replies: 10

Views: 62

Part Number: TPS7A49

Hi,

Input is 16.5V from the output of a switcher which has good load regulation.  For some reason I'm getting a 400mV drop out of this LDO from 15.2V down to 14.8V when I load the output with 100mA.  I can't see anything obvious.  The FB is about 0.75ohms at DC so it will drop 75mV, but not enough to hit the drop out.  My -15V LDO works fine, but its just the +15V that has this problem.  Layouts for both supplies are similar and optimal (short distances to bypass).

Any ideas?  Did I miss a detail in the data

Thanks,

Erik

  • In reply to Erik Anderson41:

    Hi Erik,

    I'm not seeing anything obvious either. Just to be clear, Vout at no load is 15.2V measured and at 100mA load it drops to 14.8V? The overall accuracy spec in the datasheet suggests that you may see up to +/-380mV from the 15.2V, but if you are measuring 15.2V at no load then it should not exceed the .04%Vout load regulation spec. 

    You mentioned that the FB is about 0.75ohm. What exactly do you mean by that? 

    Best regards,

    Nick

  • In reply to Nick Butts:

    Yep, no load is 15.2V.  Then at 100mA load it drops to 14.8V.  With my -15V supply I only see a few mV of change which is what I expect.

    Sorry FB = ferrite bead.  At DC it has 0.75ohms of resistance.

  • In reply to Erik Anderson41:

    Hi Erik,

    How much ESR does the output cap have? The datasheet recommends less than 200mohm.

    Do you mind providing a scope shot showing Vin, Vout, and Iout from Iout = 0mA to Iout = 100mA?

    Best regards,

    Nick

  • In reply to Nick Butts:

    Output cap is GRM32ER71H106KA12L, which I'll need to look up on muratas simsurfing.

    here is a scope shot showing the output during ~150mA load transient.  It doesn't happen on all boards, so there is that clue as well...

    Erik

  • In reply to Erik Anderson41:

    Hi Erik,

    Can you please include at least Vin and ideally also Iout in the scope shot? I know you said the switcher has good load regulation, but this is not expected behavior so I would like to be sure that that is not the cause. 

    Is there maybe extra flux that has not been removed from the board that may be causing leakage between pins?

    Best regards,

    Nick

  • In reply to Nick Butts:

    Hi Nick,

    I don't have a current probe handy, I'll try to get one tomorrow.  But here is a new scope shot:

    CH1: 16.8V switcher output (pre ferrite)

    CH2: VIN of TPS7A49 (post ferrite)

    CH3: VOUT of TPS7A49

    I'm thinking maybe the TPS7A49 is damaged as I've been fairly rough on the boards bringing them up and probing different points.  And not all boards are exhibiting this issue.  The boards have been cleaned by the assembly house, there isn't any significant flux.  The solder joints look good to me.

    Erik

  • In reply to Erik Anderson41:

    Hi Erik,

    Don't worry about getting the current probe; I can see that the voltages in the scope shot are consistent with what you have described and there is plenty of headroom to avoid dropout. 

    My best guess is that you may be correct in that for some reason some of the parts are getting damaged. Have you measured/do you have a way to measure the inrush current when the board is powered up? Can you share what the TPS7A49 is powering?

    Best regards,

    Nick

  • In reply to Erik Anderson41:

    Yep, replacing the part solves the issue.  So how do you think I damaged it?  ESD?  It has current limit and thermal shutdown, so I don't think shorting the output to ground accidentally would damage it, right?

  • In reply to Erik Anderson41:

    I'm on a roll answering my own questions today.... thanks for listening Nick (-:

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