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LM3405: use of less than 200mA

Takao Yamamoto5
Expert 1690 points
Part Number: LM3405

Dear all,

At 'Setting the LED Current' section, you recommend to use above 200 mA for stable operation and to use PWM dimming less than 200mA.

Why should we use this device above 200mA with resistor setting? and Why does this device be unstable less than 200mA?

Must we use PWM at 100mA?

Best regards,

  • 0
    TI__Mastermind 25500 points

    Hello Takao,

    I am not 100% sure and it would take some time to find out. The original product development folks are no longer around. But I can try if absolutely necessary.

    However, I'm guessing it is a balance between the inductor/switch current ripple and the continuous/discontinuous conduction mode boundary. It is a current mode control device so it will require some minimum amount of slope on the internal Rsense resistor in series with the FET. But ideally you would like to operate in continuous conduction mode as that keeps the BOOST capacitor fully charge each cycle.

    You can do testing at 100mA with no PWM and if there are no issues I am not sure why it can't be done. But to be on the safe side I would recommend following the datasheet and setting for 200mA and PWM dim at 50% duty cycle for 100mA average output current.

    Regards,

    Clint