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LM3405A: In PWM mode, hear to high-frequency noise

Pil Joo Huh
Prodigy 30 points
Part Number: LM3405A
Other Parts Discussed in Thread: TPS922052, TPS92200

Tool/software:

It is being developed by applying LM3405A LED Driver IC.
Currently, high-frequency noise has occurred during the testing
and has been improved in various ways, but it is not improved.
Previously, E2E applied improvement cases, but it did not improve.
(Capacitor C3, C4 : 1uF -> 4.7uF)

I am send to you as the attached file(schemetics and waveform for each port)
so please review and adivse to me.

Thanks and BR
PJLM3405A_CKT_Waveform.xlsx

  • 0
    TI__Mastermind 22830 points

    Hi PJ,

    The oscilloscope captures you sent to me are not clear and are hard to read. Could you send me clearer ones? Thanks.

    Also, please provide your LED spec (LED forward current and forward voltage).

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Prodigy 30 points

    Thanks for yout response.

    What do you want to waveform, please more detailed explain?

    And send to datasheet about LED spec
    Thanks and BR
    PJSPMWH1228FD5WAPUVJ_6500K.pdf

  • 0 in reply to Pil Joo Huh
    TI__Mastermind 22830 points

    Hi PJ,

    Steven Li said:
    The oscilloscope captures you sent to me are not clear and are hard to read. Could you send me clearer ones? Thanks.

    I need clearer version of the oscilloscope captures you sent to me above.

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Prodigy 30 points

    I send to you about more clearer waveform.

    Please checking and advise to me.

    Thanks and BR

    PJ 

    LM3405A_CKT_Waveform_V2.xlsx

  • 0 in reply to Pil Joo Huh
    TI__Mastermind 22830 points

    Hi PJ,

    Thanks. The waveform looks clearer now.

    Could you determine the frequency of the audible noise? Is it the frequency of PWM dimming (500Hz)?

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Prodigy 30 points

    Dear Steven

    From what I can hear, it's recognized as a high frequency sound. It's at least 5KHz.
    I am already change to PWM Frequency
    (100Hz ~ 1KHz). But Noise is very similar.

    If it's pwm noise, is there any way to improve it?

    Thanks and BR
    PJ

  • 0 in reply to Pil Joo Huh
    TI__Mastermind 22830 points

    Hi PJ,

    If changing the PWM frequency will not change the frequency of the audible noise, then it is less likely that this is cause by the PWM dimming. The audible noise caused by low-frequency PWM dimming is a frequently seen problem. Please double-check.

    Have you observed any periodic waveform that in the same frequency as the noise you hear?

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Prodigy 30 points

    Dear Steven

    1. High frequency noise is only generated during LED dimming operation not PWM dimming.
    (I do not hear only pwm dimming operation).

    Therefore, it is assumed that it is noise generated during LED dimming operation rather than PWM dimming noise.
    Most of the high freq. noise is come from the inductor, ours schemetics have problem about inductor cap. or bender?
    It is there a problem with the LM3405A application schemetics?
    Please check to our schemetics.

    2. I do not ever hear to periodic waveform.

    Currently our customer want to use four LM3405A for LED lighting system.
    (high freq. noise is better loud)

    Please more checking and advise to me the improvement.

    Thanks and BR

    PJ

  • 0 in reply to Pil Joo Huh
    TI__Mastermind 22830 points

    Hi PJ,

    Pil Joo Huh said:
    1. High frequency noise is only generated during LED dimming operation not PWM dimming.

    Sorry but I am not sure what do you mean by "LED dimming operation".

    I have checked your schematic. I take input voltage = 12V, Vout = 2.9V * 3 + 0.2V = 8.9V for calculation (Please correct me if I misunderstand anything). Let's assume your efficiency is 85%, then the duty cycle will be = Vin / Vout / efficiency = 8.9V / 12V / 0.85 = 87%, which is over the maximum duty cycle specified in Section 6.5 Electrical Characteristics from the datasheet. So actually LM3405A is not a suitable solution for your input / output condition. I would recommend you use TPS92200 or TPS922052/3 here. One thing to highlight is that TPS922052/3 support >20kHz PWM dimming which is out of audible range. 

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Prodigy 30 points

    Dear Steven

    Thanks for your cooperation.

    I wonder that maximum duty cycle specified in Section 6.5.
    Could you explain that caculated value duty cycle is 87%, why spec over about Dmax(85% ~ 94%) ?

    And thanks for your recommend IC.
    I have check to TPS92200D2.
    Is dimming iuput type only digital signal ?
    (Freq. : 20khz ~ 200khz)
    I need to control dimming with only mcu pwm. Is mcu pwm dimming control possible on tps92200D2 ?

    Thanks and BR

    PJ

  • 0 in reply to Pil Joo Huh
    TI__Mastermind 22830 points

    Hi PJ,

    Pil Joo Huh said:
    I wonder that maximum duty cycle specified in Section 6.5.
    Could you explain that caculated value duty cycle is 87%, why spec over about Dmax(85% ~ 94%) ?

    The MIN value of D_MAX is 85%. You should make sure that the actual duty cycle is smaller than 85%.

    Pil Joo Huh said:
    I have check to TPS92200D2.
    Is dimming iuput type only digital signal ?
    (Freq. : 20khz ~ 200khz)
    I need to control dimming with only mcu pwm. Is mcu pwm dimming control possible on tps92200D2 ?

    If you want to input PWM signal to do PWM dimming on TPS92200, you should use TPS92200D1.

    Best Regards,

    Steven