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TPS61199: Controller does not handover from soft-start to OVP control

Josh Deng1
Prodigy 50 points
Part Number: TPS61199

Hi there,

I've run into another issue with TPS61199 where one of my ICs regulates at too low of a OUT voltage. The OUT voltage (LED rail) is at 38V instead of an expected ~41V.

I probed the COMP pin and compared to another IC on the same board, and the COMP pin behaves identically for both, saturating high and asking for full duty cycle on the boost converter. However, on the "bad" driver, the GDRV ramps up from 12% to 20%, and maxes out at 20%. On a "good" driver, the the GDRV ramps from 12% to 70%. The bad driver's OUT ramp-up time is much slower than a good driver's.


(yellow = bad OUT rail, blue = good OUT rail)

Here is a plot of GDRV on the bad driver never going beyond ~16% duty cycle:

Here is a plot of expected behavior, from a good driver, with GDRV ramping up to 70% duty cycle:

I have noticed in testing that the IC uses a static soft-start stage to bring up the OUT rail to a certain proportion of final rail voltage, before handing over control of the feedback loop to the OVP pin and its associated feedback circuitry. My theory is that this particular IC is getting stuck in the soft-start stage, and never exits the soft-start to go into normal control. This was corroborated for me when I accidentally made R3 = 200ohm. I would have expected the controller to react and the OUT rail to shoot up and hit OVP; instead, the rail remained at the same voltage with no change at all.

If this fault is true, could you tell me whether I just need to replace this IC, or if there is something I can change in my circuit to better force the handover from soft-start to regular control?

Here are my schematic values:
R2 = 237k
R3 = 10k
R4 = 20k
C4 = 0.015uF
C5 = 100pF
R5 = 681k
R6 = 169k
R7 = 160k (500kHz)

Thanks!

  • 0
    TI__Genius 9630 points

    hi Josh

    how many pieces of device have this issue so far?  For the results above, have you kept other conditions totally the same and only changed the device? 

    The comp is charged by the output current of EA, which should be determined by input differential signals (Vovp-Vifb) shown in the DS block. So Would you mind further check where the comp difference is coming from by monitor OVP pin and IFB pin?

  • 0 in reply to Mason Wang
    Prodigy 50 points

    Hi Mason,

    This has only happened on 1 IC so far. You're correct, I have 7 ICs total on my board, and I am comparing this bad driver with a good driver on the same board that has an identical circuit.

    I expect that on any switching controller, if COMP is high, that GDRV will be at a high duty cycle.

    I have checked OVP, but I don't have a plot of it during startup. OVP is, correctly, a scaled down version of the LED rail as shown above. 

    Here is a plot of an IFB. You can see that there is no headroom on the IFB pin and it basically regulates at 0V. This is because the rail voltage on the bad driver is too low.

    On a good driver, the IFBs regulate at ~2V.

    Thanks, Josh

  • 0 in reply to Josh Deng1
    TI__Genius 9630 points

    hi Josh

    Yes, I agree that if COMP is high, that GDRV should be increased to a quite large duty cycle. So it is reasonable to assume that device is stuck in soft start mode and does not enter normal mode.

    For TPS61199, I just got something that the start up time can be roughly estimated by t=K*(Vtarget*R3/(R2+R3)-Vcons). So when you decrease R3 to only 200ohm, I guess the proportion is too small to get a valid time. To decrease the start up time, please try to increase R2 to 390k and check if any difference.

  • 0 in reply to Mason Wang
    Prodigy 50 points

    Hi Mason,

    Regardless of start up time, the IC should eventually enter normal mode. It sounds like I have a defective device.

    Since we have 7 ICs per unit, the risk of rework is high for us. I have just found another failure of this kind today. Could you find out what is the probability of this defect happening?

    Thanks, Josh

  • +1 in reply to Josh Deng1
    TI__Genius 9630 points

    hi Josh

    Did you purchase TPS61199 directly from TI or its agent? 

    If you have already done ABA swap test, I suggest to contact your local FAE for FA. 

  • 0 in reply to Mason Wang
    TI__Genius 9630 points

    hi josh

    As this has been moved to mail. I would like to close this thread. thanks