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LM3424: about Analog-dimming and PWM-dimming

ACGUY
Guru 11170 points
Part Number: LM3424

Hello E2E,

When LED current limiting to low with Analog-dimming, the LM3424 swichting start timing is to be slow response to PWM dimming signal. On same circuit, when the LED current limiting to high with Analog-dimming, response time is short.
Please let me know the reason about it.
I using this device with buck mode.

Regards,
ACGUY

  • 0
    TI__Mastermind 18890 points

    Hello ,

    I have a hard time understanding your questions here.  Please give us a schematic and what to are control signal or components you are changing to get your fast or slow response...Here I do not understand what you mean by slow response or fast response.  Please have scope capture and a little more explanation of what you are talking about.

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    Guru 11170 points

    Hi Tuan,

    Thank you for reply. I attached captured waveforms and a schematic.
    When set ILED=13mA, this device start with 3-rd pulse of DIMM, start with 1-st pulse when set ILED=170mA.
    Please ley me know the reason.

    Regards,
    ACGUY


    LM3424.pdf

  • 0 in reply to ACGUY
    TI__Mastermind 18890 points

    Hi Acguy,

    The reason that it takes longer for the current to reach the control level with the lower current setting has to due with the loop control and error amplifier.  With lower current command, the error is lower so the loop command lower charging current to the output caps to begin with...Thus, it take longer for the output voltage to ramp to the right level for the current in the LED to start.  It's the loop response of the error amplifier (COMP).

    Thanks Tuan