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LM3410: Inductor Current

tateo yamashiro
Guru 10890 points
Part Number: LM3410

Hi,

We got a question from the customer about LM3410.
Could you help us?

[Question]
They evaluated this IC by EVM. So they have three question. Please refer to the attached file for details.

LM3410EVM Inductor Current_Aug2118.pptx

1. The inductor current overshoot occurs at the rising edge of DIM signal. It seems to be big as increasing the output capacitor. Therefore I assume the inrush current causes this phenomenon. Is this my understanding correct? If there are other solution except for decreasing the output capacitor, please give us the solution.

2. When the input voltage is 4V, the inductor current reaches the current limit and the big ringing occurs. Are there any solution for improving the stability?

3. Could you provide an information of the best IC under the following condition?

   Vin=4~7.2V
   Vf=11V typ
   ILED=350mA
   Dinimmg=Analog or PWM

Best Regards,
tateo

  • 0
    TI__Mastermind 28655 points

    Hello,

    1)  Is this during the first turn-on or when dimming, every cycle?  You should be looking at the LED current not the output current, which is the inductor current.  A boost will have an inrush when first powered on, if the inductor is boosting the current because of this a bypass diode can be added across the inductor to limit how much energy the inductor stores during turn on.  If you increase the output capacitor the LED current overshoot will be reduced, look at both the inductor current AND the LED current.

    2)  The inductor value of 10 uH and output capacitor value of 2.2 uF puts the resonance close to what is seen on the oscilloscope picture though I cannot read it that clear.  You can try increasing the output capacitor (maybe try the same value as the input, 22 uF or 10 uF) it may be interfering with the internal compensation since the resonance is a fairly high frequency.

    3)  This is probably the best choice due to being operated down to 4V input.

    Regards,

  • 0 in reply to Irwin Nederbragt
    Guru 10890 points

    Hi, Thank you for your reply. I changed the inductance from 10uH to 6.8uH. Then, the big ringing of inductor current is solved. I assume this problem is related to loop response.

    And I got the plots of inductor and LED current. Regardless of the output capacitor, LED current is stable. But inductor current overshoot becomes bigger as increasing the output capacitor. Please refer to the attached file for details. I assume the rush current into the output capacitor causes this phenomenon. Is this my understanding correct?

    LM3410EVM Inductor and LED Current_Aug2318.pptx

    Best Regards,
    tateo

  • 0 in reply to tateo yamashiro
    TI__Intellectual 2590 points

    The increased overshoot at the DIM rising edge is due to the control loop bandwidth being too low.  This effect is clearly the response of the error amplifier in the control loop.  When you change the output capacitor, you change the system and therefore need to change the compensation capacitor if you want to improve the transient response.

    -Jim

  • 0 in reply to James Patterson
    TI__Mastermind 28655 points
    I have not heard back from you so I believe you may have solved the issue.
    The thread will close now but will reopen if you reply.