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LM3406: Minimum PWM on period

Kaji@PAN
Mastermind 7350 points
Part Number: LM3406

Hi,

I am creating a circuit using the LM3406 device.
I input an external PWM signal and adjust the brightness of the LED, but I would like to ask you about that PWM.
I will attach the waveform.
Yellow is the waveform of LED current.
Green is the waveform of the DIM pin, and the PWM signal is applied externally.
The cycle of external PWM signal is 20us.
The operating frequency of the LM3406 is 500kHz(2us).
When the PWM H period(ON period) is 13us, the LED current is stable.
However, I want to set the H period of PWM to 5us at the minimum.

(question 1)
The LED current gradually increases at the beginning of the H period of PWM, and the LED current becomes stable at the third pulse of the operating frequency of the LM3406.
What is proper us for the minimum H period of PWM?
Is it possible to set the H period of PWM to 5us?

(Question 2)
The pulse width of the 1st pulse and the 2nd pulse in the H period of PWM is longer than the steady 500kHz.
Is this normal?

Best regards,

  • 0
    TI__Mastermind 28655 points

    Hello,

    A couple of questions;  Are you looking at LED current?  Do you have an output capacitor?

    1)  There are limitations.  One being the inductor value with Vin and Vled.  Current rise is V = L * (di/dt), solve for di.  V for on time is (Vin - Vled), for off-time it is Vled.  If these voltages are different the rise and fall slopes will be different.  As I can see in Vout current waveform (Vin - Vled) is smaller than Vled, or Vled is higher than 1/2 of Vin.

    2)  The LM3406 is determine its on-time uses both input voltage and output voltage to determine on-time,  on-time will change if one of them is changing.  See section 8.1 of the datasheet.

    Can you add Vout to the oscilloscope picture above?

    5 us pulses dimming in this fashion is difficult due to rise and fall time of the inductor current.  Shunt FET dimming will is better for high frequency dimming.

    Best Regards,