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TPS92518: linearity

KANJI SHIBATANI
Mastermind 6205 points
Part Number: TPS92518

Hello

Thank you for your support.

My Customer is evaluating TPS92518 EVM, but did not have linear curve like as

TPS92518EVM users guide(Fig4).

I think this is not correct characteristics for TPS92518 EVM right?

 

Would you please advise what should be checked by themselves?

Customer’s condition:

Vin=48V,Vled=10V,

Peak threshold=244, Dimming 1KHz,

Off time=43 And Rshunt=0.307ohm

Using TPS92518EVM ( L, C, FET is not changed)

PWMx were from customer's MCU. 

Thank you for your support.

Best Regards

TPS92518_linearity.pptx

  • 0
    TI__Mastermind 28655 points
    Hello Kanji,

    This is shunt FET dimming, what is probably happening is Max off is not set correct and the current is rising when the LEDs are shunted. Try setting LEDx_MAXOFF_DAC to 255 and see if it changes, if so then you need to calculate what TOFFMAX needs to be. The datasheet shows information for Toffmax in section 8.3.1.3.1.

    8.3.3 shows how to calculate for Shunt FET dimming.

    Regards,
  • 0 in reply to Irwin Nederbragt
    Mastermind 6205 points

    Dear Irwin-san,

    Thank you for your quick answer.

    Is  Figure 4 on TPS92518 EVM user's guide use FET shunt dimming ?? ( I thought it is PWM dimming)

    If yes, from where to gate of Qshunt ( TP16 "shunt PWM1") on Fig12 connected  ?

    Customer does NOT use FET shunt dimming , so customer does not add

    additional cicuitry on TPS92518 user's guide Fig12.

    Please adivse

    Thank you and with my best regards

  • 0 in reply to KANJI SHIBATANI
    TI__Mastermind 28655 points
    Hello Kanji,

    I assumed this was shunt fet dimming because of the Rshunt = 0.307 ohms, sorry. It does not look correct.

    What are they driving PWM with? There is as 0.1 uF capacitor on PWM1 and PMW2, C25 adn C26, if there is a lot of impedance driving this it will not work correct. What voltage level are they using for PWM, 0 - 3.3V, 0 - 5V?

    Have them look at the current in the inductor or output current to the LEDs when they are PWM dimming (both the pwm signal and the current).

    Regards,
  • 0 in reply to Irwin Nederbragt
    Mastermind 6205 points

    Dear Irwin-san

    Thank you for your answer quickly!

    Please let me confirm again.

    My understanding is " sluubm0.pdf TPS92518EVM user's guide Fig4 is based on PWM dimming only, NOT shunt FET dimming..

    Is it correct?

    Sorry to bother you again.

    Best Regards

  • 0 in reply to KANJI SHIBATANI
    Mastermind 6205 points

    Dear Irwin-san

    Please allow me to have additional question. ( I am so sorry many times and basic question)

    We see TPS92518EVM users guide page5 Fig 4  is  linear, but   Output current(Y-axis) is measurement peak current ( or average current ) on Fig4?

     

    Sorry, I hear from customer today  that their PWM dimming  test result is based on r.m.s.  using digital multimeter, and

    Irms is propotinal to SQRT( Duty(%)), so not linear to Duty(%) , it is make sense.

     

    Please give your comment

    Best Regards

     

     

     

  • 0 in reply to KANJI SHIBATANI
    TI__Mastermind 28655 points

    Hello Kanji,

    Yes, it is PWM dimming.

    Yes, they need to measure average current, RMS does not work for an LED load.

    Regards,

    Irwin

  • 0 in reply to KANJI SHIBATANI
    TI__Mastermind 28655 points
    Hello Kanji,

    It is average current. Though it doesn't really matter since it's pwm it should be fairly linear measuring both ways if the PWM frequency is much less than the switching frequency.

    Regards,
  • 0 in reply to Irwin Nederbragt
    Mastermind 6205 points

    Dear Irwin-san

    Thank you for your answer quickly!!

    Customer tested again using Average type current meter, and linearity was fine.

    Thank you again