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TPS92518: changing LED IF

KANJI SHIBATANI
Mastermind 6205 points
Part Number: TPS92518


 

Hello

My customer had chosen TPS92518 for 2019 LED light, and the key feature is 0.1% dimming ratio.

So, customer think [LEDx_PKTH_DAC] = 255 (the maximum value) for accuracy because

we see description *  in the data sheet p44.

*

" For the highest current set-point, set the peak threshold register to be as high as possible. Use [LEDx_PKTH_DAC] = 255 (the maximum value) if possible to increase the converter accuracy."

On the other hand , they want change LED current 3 case in the same PCB.

Attached ppt is block digram .

Q1

We don't know the relatonship between [LEDx_PKTH_DAC] = 255 (the maximum value) and accuacy for 0.1% dimming ration well.

Do you suggest to use [LEDx_PKTH_DAC] = 255 (fixed ) for 0.1% ratio ? ( Offcourse it should depends on PWM pulse resolution)

 

Q2

If we fix [LEDx_PKTH_DAC] = 255, then we should change IF( Rsense resistor)  using switch ..( Pls refer to ppt)

Do you think this is okay in practical? 

Would you please  have any idea to change IF value with 0.1% dimming ratio? ( including Q1 )

 

Q3

Vin varies 48V and 24V in case by case .

External components and resitor setting may be keep same even for different Vin ?

 

If Vin were changed, switching frequency should be changed for same TOFF, but if it was okay,

not necessary to change inductor and resgister setting..... Is it right?

But UVLO setting should be changed in d/s Fig 31.

 

Q4

Would you please have your opinion about Over Current protection for LED short, Over voltage protection for LED Open

and also IF( Rsense) changing about attached ppt kindly ?

Best Regards

Block_Diagram_.pptx

  • 0
    TI__Mastermind 28655 points
    Hello Kanji,

    Q1) Using 255 for the peak current threshold will make the overall accuracy the highest (analog). If you are PWM dimming, shunt or enable?, the accuracy will depend on other things. From the datasheet:
    LEDx_PKTH_DAC = 255 245 255 265 mV means 265/255 gives output 3.92% higher than set point
    LEDx_PKTH_DAC = 127 118.5 127 135.5 mV means 135.5/127 gives output 6.7% higher than set point
    This has nothing to do with PWM dimming, just the accuracy related to where the peak current threshold trip point is. So it if is set at 255 it will be the most accurate. Since it appears this is begin used at 350 mA, 500 mA, and 525 mA you may find it accurate enough to use the PKTH DAC to set the peak current threshold, the 350 mA setting would be the least accurate but it may be close enough for your design.

    Q2) You would have to calculate the resistor values and see if the RDSon of the MOSFETs affects the resistance. I would try using the PKTH to see if it is accurate enough for your design.

    Q3) The TPS92518 uses Toff to keep the inductor current ripple at a constant amplitude. This means if the input or output voltage change the switching frequency will change, this is how it works. The components do not need to change but they need to be designed for both operating points. UVLO does not need to change unless it has to disable above 24V for 48V operation. No reason to change inductor and register setting unless it is desired.

    Q4) TPS92518 regulates peak current so it will protect from LED short as long as the Toffmax is calculated correct. Overvoltage protection can be done by reading the LED stack voltage from the TPS92518. The Current sense resistor switches will need more circuitry to operate correct, they also need to be P-channel to work as drawn.

    Regards,
  • 0 in reply to Irwin Nederbragt
    Mastermind 6205 points
    Dear Irwin-san,
    Thank you for your answer! I will talk with customer, but thank you for your advice.