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Original question:

INT-TPS61193-Q1 FB resister value

TPS61193: My LED string does not start

Paolo Colombara
Intellectual 260 points

Part Number: TPS61193

Hello,

I am using a TPS61193 to turn on a 15-LED string, with a total Vf of 42V. The problem is the initial voltage (k=0.88), which is 40.59V with R1=910k and R2=130k, so the string sometimes does not start due to insufficient voltage and the component goes into error state.

I have two options:

1- raise R1 to 1Mohm: initial voltage will be 44,48, so the led string starts (tested), however the theoretical output voltage will be 49V, which will never be reached because the LED string will limit it.at 42V.

2- set R1 at 470k and R2 at 22k, thereforeobtaining initial voltage of 42,84 (tested and working) and maximum voltage at 45V (good). The thing is I am violating the recommendation to keep R2 at 130 kohm. Considering that I cannot modify the PCB because it has already been built (so a T-divider is not an option), what are the consequences of setting R2 at 22kohm?

Given the choice, what solution would you recommend in terms of reliability?

Thanks,

Paol.

  • 0
    TI__Mastermind 22745 points

    Hi Paolo,

        We basically recommend with the option 1. Since R2 will enter into small signal control loop, we usually don't want to disturb the control loop. But we don't recommend to use such high output voltage.  we recommend replacing the four or six channel device that may be more suitable.

    Regards

    Sean

  • 0 in reply to Sean Zhou
    Intellectual 260 points

    Hi Sean, 

    thanks for your reply. Replacing the device with a new one is basically out of the question, because we have already submitted the board for certification, we have conducted thermal tests, and so on. We could find a more suitable value for R1, like 953kohm or 976 kohm, this would give 46.8V and 48V respectively, which I believe are more prudent. Nevertheless, the output would never go that high anyway, due to the presence of the LED string, therefore that output voltage is only theoretical, don't you agree?

    Another possibility I had thought about would be placing a capacitor in parallel to R2, so that, until it is charged, the control loop would see a lower voltage and therefore would boost the output thus helping turning on the string... I have not tried this solution yet, but, if you are worried about changing the R2 value, I believe that you would agree even less to add a capacitor in parallel to it... What do you think?

     Finally, what T- divider could be used in our case, if we decided to modify the PCB in the future?

    Thanks,

    Paolo.

  • 0 in reply to Paolo Colombara
    TI__Mastermind 22745 points

    Hi Paolo,

        I agree with you, I would like keep R2=130k, and adjust the R1 may around 950k. This may help you.

    Thanks.

    Regards

    Sean