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LM3435: About ILED

Junya Kato
Expert 1635 points
Part Number: LM3435

Hello Team,

We are considering turning on/off(PWM control) between LED_cathode and G/B/RLED_terminal via MOSFET.
When checking the operation, the shorter the ON time, the larger the ILED becomes than the register set value.
Therefore, even if changing the duty ratio of the FET, the brightness that can be seen does not change.
Does the LM3435 control the LED current by ON time so that the brightness looks the same?

Best regards,
Kato

  • 0
    TI__Mastermind 25500 points

    Hello Kato,

    Yes, the LED control is fully on or off when controlled with PWM. This is so that there are no wavelength shifts in the light output when they are on. Is that what you are asking?

    Thanks,

    Clint

  • 0 in reply to Clinton Jensen
    Expert 1635 points

    Hello Clint-san,

    We consider PWM control by inserting MOSFET as below.


    Usually I think that it controls by inputting the PWM signal to the G/B/R_CTRL pin, but we want to control at 42kHz, so we are considering this control.
    In the control of this configuration, the following measurement result was obtained.
    Is the LM3435 controlling to make the average current constant?

    ・Register setting value：35mA
    ・Measured value：
    　　Duty 50% -> 70mA (35mA/50% ?)
    　　Duty 30% -> 115mA (35mA/30% ?)
    　　Duty 10% -> 400mA (35mA/10% ?)

    Best regards,
    Kato

  • 0 in reply to Junya Kato
    TI__Mastermind 25500 points

    Hello Kato,

    I believe the xLED pins are already FETs to ground and that is how the regulation and PWM dimming is done. But I am not sure of the PWM frequency capabilities. I'm going to forward this to someone who knows much more about this device than I do and hopefully he can help.

    Regards,

    Clint

  • 0 in reply to Junya Kato
    TI__Expert 3920 points
    Hi Kato-san,

    It is NOT recommended to add MOSFET in-series to the LEDs' connections to LM3435.

    The current is mirrored internally according to the register contents with an assumption of the load is connected.

    LM3435 might not work correctly with a series-MOSFET connection between LED and the device as the MOSFET in-series affected the control loop.

    For a 42-kHz dimming, your customer can try set the DELAY register content to minimum (~ 5 us) and drive the xCTRL pins directly with the PWM control signal.

    Thank you!