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LM3405: LED PWM Dimming Ratio

NY
Expert 3040 points
Part Number: LM3405

Hi 

My customer would like to know the mean in the description of datasheet 7.3.5 of LM3405. 

The last sentence says "The dimming ratio reduces drastically when the applied PWM dimming frequency is greater than 5kHz."

Does this mean when PWM is faster than 5kHz, LEDs will lit 100% so no longer dimming?

Regards, NY

  • 0
    TI__Mastermind 28655 points
    Hello,

    No, it will still dim but the accuracy and resolution will be worse:
    "As explained in the previous section, there is approximately a 100-μs delay from the EN/DIM
    signal going high to fully establishing the LED current as shown in Figure 19. This 100-μs delay sets a maximum
    frequency limit for the driving signal that can be applied to the EN/DIM pin for PWM dimming. Figure 20 shows
    the average LED current versus duty cycle of PWM dimming signal for various frequencies. The applicable
    frequency range to drive LM3405 for PWM dimming is from 100 Hz to 5 kHz. The dimming ratio reduces
    drastically when the applied PWM dimming frequency is greater than 5 kHz."

    Even dimming at 5 KHz will be less accurate/resolution then dimming at 100 Hz.

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    TI__Expert 3040 points

    Hi, Thanks you very much. I understand.regards, ny