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LM3404 - this application uses sub-1 kHz frequency PWM dimming on the datasheet P28

Hirotaka Matsumoto
Guru 27525 points
Other Parts Discussed in Thread: LM3404

Hi all

Would you mind if we ask LM3404?

The datasheet P28 shows that "this application uses sub-1kHz frequency PWM dimming."
TI's LED driver frequently recomends to use 1kHz frequency PWM dimming, if it has some reasons, could you let us know?

Kind regards,

Hirotaka Matsumoto

  • 0
    TI__Mastermind 25500 points

    The chosen dimming frequency depends on how much range you need, how much linearity you need, and LED ripple current requirements. The linearity is affected by the rise/fall time of the LED current.

    In the case of the application you are referring to it looks like the intent was to reduce ripple current by using an output capacitor. That will slow the rise/fall times so if you need good range and linearity you want to go with a lower PWM frequency. With no output capacitor the rise/fall times are simply v=Ldi/dt, with an output capacitor it is limited by the charging/discharging of the capacitor as well.

    In any case you can use as low a PWM frequency as you would like if you need increased range and linearity.

  • 0 in reply to Clinton Jensen
    Guru 27525 points

    Clinton san

    Thank you for your surpport always.

    We would like to confirm one point.
    Is LM3404's PWM frequency avilable over 100kHz?
    We guess that it is possiblw because the datasheet shows that " In general, fDIM must be at least one order of magnitude lower than the steady state switching frequency to prevent aliasing."
    So, if Fsw is 1MHz, fDIM will be maximum 100kHz.
    (Of course, we recognize that LED can respond to this high fDIM.)

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    TI__Mastermind 25500 points

    Yes, you can. But the dimming will not be quite linear due to the first turn on being a full ramp up from 0A to the set output current. You also need to be careful with the thermals if you plan on switching at 1MHz. If your input voltage is high the part could get very hot due to AC losses. You may also want to consider shunt FET dimming if you really need to do 100kHz.