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LM3406HV PWM dimming problem

Other Parts Discussed in Thread: LM3406HV

Hi.

I am designing a LED driver module based on LM3406HV. I have used online tool for calculation of circuit parameters and, based on the obtained results, I tested the circuit on a breadboard and then manufactured prototypes.

The circuit I am using is shown below:

BOM:

I am using PWM dimming. Scope screenshots are shown below:

PWM pulse duration

PWM period (repetition rate)

The driver displays unstable behavior when dimming.

The following screenshots are taken with a scope probe attached to point A (marked with an arrow in the circuit diagram above), using the 1:10 divider on ch. Y

  1. Stable work of the circuit

2. Unstable operation mode 

I request your help in understanding the problem. Can you please explain what is happening? What am I doing wrong?

  • Hi,

    I noticed that the switch frequency in 3rd screenshot(~214kHz) is quite different from the one calculated(~122kHz).

    You may try to adjust the value of R1 or change the frequency of PWM signal.

    Information about the switch frequency and on-time is on pg.9 of data sheet.

    Thanks.

     

  • Hi, Victor!

    Glad to see your attention to my problem.

    You right, pointed on scheme R1=820k, will give different screenshot with frequency ~122kHz. I was seen this when first time was encountered with this problem.

    I was trying to solve this problem by add to R1 resistor 1M in parallel. Frequency was changed, but problem still persist. Screenshots 3 and 4 was made with R1 ~450k. Excuse me for my mistake, I should was explain this in my first message.

    I would like to add. This problem happens with different length of LED chain and different value of PWM. Also this problem persist on different voltage of PSU. For naked eye it's look as flashing light of LEDs on some value of PWM, especially frequently on low value PWM. This flashing is unacceptable. What I should do to eliminate this problem?

  • Hi, All.

    Problem has been solved by our engineers. I'm very surprized, but, datasheet for this chip from TI have a critical mistake :( it's a pity.

    I would like to say to TI's employees - if you would like to fix this bug in next revision of your LED driver, let me know by e-mail, I'll explain to you what should you do for this.

    With best regards, Dmitry.

  • Hi Dmity,

    I am glad to hear the news and congratulation to you.

    I definitely appreciate it if you could explain the slotion and mistake to us.

    Could you please send an email to victor.chen@ti.com or johnp@ti.com to explain about that? I will forward your message to our engineers and have them a update.

    Thank you so much for your time.

    Regards,

    Victor

  • Hello,

    May I know what is the mistake?

    Best regards