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LP5569: PWM output percentage using exponential scale

Bruno Lau Costa
Prodigy 150 points
Part Number: LP5569

Hello,

I intend to use the exponential scale for dimming the LEDs in LP5569. In page 13, the datasheet mentions that "... the PWM output percent can be approximated by the following two equations:"

y = 0.0125x – 0.0066  [%]       (equation 1, for code <= 64)

y = 0.7835e^(0.0217x) [%]        (equation 2, for code > 64)

I computed some values to analyze the PWM output when using those two equations:

y (1) =  0.0059% (equation 1)

y (64) = 0.7934% (equation 1)

y (65) = 3.21% (equation 2)

y (223) = 99% (equation 2)

y (255) = 198% (equation 2)

Question: are my calculations right? I found strange that:

a) y (255) returns 198% of duty cycle

b)  the gap between y (64) and y (65) is equal to (3.21 % - 0.7934 %) = 2.42 %  -> seems a huge discontinuity 

c) such results seems not to match with Figure 14 (shown below)

I'd like to better understand the power consumption of LP5569 to define light patterns that are both attractive and power efficient. For this, knowing the output duty cycle given a PWM input register value (0...255) in exponential mode is of major importance.

Thanks for helping,
Bruno

  • 0
    TI__Expert 6695 points

    Hi Bruno,

    The two equations just provides the approximative formula for PWM output percent. So there will be some gaps between the real data and calculated data. We would suggest to get the real duty percent by test and I will also check with our designer about the designed duty.

    Best Regard

    Monet Xu

  • 0 in reply to Monet Xu
    Prodigy 150 points

    Thank you Monet,

    It would be really great to have a better estimate of the duty cycle. We'll also gather real data in our lab.

    Hoping to hear from you soon,

    Bruno

  • +1 in reply to Bruno Lau Costa
    TI__Expert 6695 points

    Hi Bruno,

    Here is the validation test result for reference.

    LP5569 real brightness percent-PWM result.xlsx

    Best Regard

    Monet Xu

  • 0 in reply to Monet Xu
    Prodigy 150 points

    Hi Monet, thanks for this document, it was really helpful. 

    We measured the duty cycles in the EVM and the results match the data on the sheet you sent. 

    We also could adapt the two equations for exponential dimming, so that their results better match the real data. Below are the original equations as well as the equations we used to get more accurate calculated data:

    Equation 1, for code <= 64

    y = 0.0125x – 0.0066  [%]      (original)

    y = 0.025x – 0.0066  [%]      (seems more accurate - the angular coefficient is multiplied by 2)

    Equation 2, for code > 64

    y = 0.7835e^(0.0217x) [%]        (original)

    y = 0.39175e^(0.0217x) [%]        (seems more accurate - the base coefficient is divided by 2)

    Thanks again, we really appreciate your support,

    Bruno