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LM3409-Q1: If the input voltage of the device changes, will it affect the output voltage or current?

? ?
Prodigy 10 points
Part Number: LM3409-Q1
Other Parts Discussed in Thread: LM3409HV-Q1

Tool/software:

Question: LM3409HV-Q1 is used to drive three RGB lights. The front-end of this circuit is connected to a 9V~15V power supply, and the output of the chip is connected to the RGB lights. When the VADJ is fixed, adjusting the input voltage (for example, when it is currently 12V and adjusted to 10V) will change the color of the lights. Will the input voltage change of this device affect the output voltage or current change? The chip is driven by a constant current, and if the voltage of VIN is adjusted normally, VOUT should be stable and unchanged. Is there a design error on my part?

https://www.ti.com/cn/lit/ds/symlink/lm3409-q1.pdf?ts=1729065404188&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252Fzh-cn%252FLM3409-Q1%253FkeyMatch%253Dlm3409%2526tisearch%253Duniversal_search%2526usecase%253Dpartmatches

The above diagram is a link to the specifications, which were drawn according to page 12 of the specification sheet. However, the RGB three lights are connected in parallel, and the common anode of the three LEDs is connected to VOUT. The cathodes of the LEDs are controlled by MOS transistors and grounded.

Many Thanks for your support.

  • 0
    TI__Genius 17240 points

    Can you help share the schematic, and capture the waveforms of inductor current, loader current etc.? Thanks. 

  • 0 in reply to Xiaoguang
    Prodigy 10 points

    Thanks for your response

    The schematic diagram is as follows

    The pin 3  (EN)  receiving PWM signal (18 Khz) . Change the output voltage when adjusting the duty cycle.

    I don't have the test relevant current testing equipment , Can you roughly help analyze the schematic diagram of the theory, if this problem occurs, which aspects of the problem may occur?

  • 0 in reply to ? ?
    TI__Genius 17240 points

    I'm not sure about your question. What is waveform in BLUE? 

    The waveform seems ok for me. 

  • 0 in reply to Xiaoguang
    Prodigy 10 points

    The blue waveform is EN, the network name is BL_PWM, when the BL_PWM duty cycle is very low, and then adjust the +12V voltage to 9V, the output light will have a brightness change, may I ask this phenomenon is normal?

  • +1 in reply to ? ?
    TI__Genius 17240 points

    If you can share me exactly the waveforms at the moment, I can tell if it is normal or something wrong. 

    Can you help capture Vin,  inductor current, loader current and SW waveform in one oscilloscope waveform? Thanks. 

    I also don't understand why there is SW waveform when EN is low while no SW when EN is high. 

  • 0 in reply to Xiaoguang
    Prodigy 10 points

    OK，Thanks for your respons, Since I don't have a current probe, I can't measure the current of the inductor,  so that's the end of the problem. Wait until the relevant waveform diagram is later, I will consult you. Thanks.