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LM3404: 3σ(MIN and MAX ) of the switching frequency in case of Fsw =400kHz

Hirotaka Matsumoto
Guru 27525 points
Part Number: LM3404

Hi all

Would you mind if we ask LM3404?
How much is 3σ(MIN and MAX ) of the switching frequency in case of Fsw =400kHz?
If you have some expected value, could you give us these?

Kind regards,

Hirotaka Matsumoto

  • 0
    TI__Mastermind 25500 points

    Hello Hirotaka,

    Unfortunately it isn't possible to give a switching frequency spread for a hysteretic device such as the LM3404. It is a constant on-time, peak detect control device. So the switching frequency depends on and can vary with input voltage, Ron tolerance, inductor tolerance, LED dynamic resistance, and LED Vf variation as they heat/cool. There are too many variables unlike with a constant switching frequency current mode control device.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Guru 27525 points

    Hi Clinton san

    Thank you for your reply always!

    When the device operation is CCM, is it impossible to give us a switching frequency spread for a hysteretic device?
    In case of CCM mode, we guess that the switching frequency is fixed value.
    How do the device change the mode from COT to CCM?(depending on the load?) 

    And in case of CCM mode, does the switching frequency depend on and can vary with input voltage, Ron tolerance, inductor tolerance, LED dynamic resistance, and LED Vf variation
    the same as COT mode?

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    TI__Mastermind 25500 points

    Hello Hirotaka-san,

    CCM just means that the inductor current is continuously conducting, meaning it never drops to zero. The LM3404 will still always operate in COT mode as that is the type of control circuitry it uses. So the switching frequency variation will still depends on all of those factors at all times. So you can calculate what it should be at any given operating point and it will be pretty close, but there is no way to specify a spread with the LM3404 alone. As I said it will even change as the LEDs heat up and the forward voltage drops.

    Regards,

    Clint