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LM3643: Using a FET to create a modulation of the output current

Jeff Coletti
Expert 8695 points
Part Number: LM3643

Hi Support team

My customer needs to modulate the drive of the LED. In their application, the led must be driven with 100mA or more, however that produces too much brightness. What they want to do is add a FET between the cathode and ground and then drive this LED at 50 or 100KHz, with a duty cycle between 50%~100% to control the brightness. This is not a visible LED and the illumination is used for a video type application.  Our question is, will this modulation mess up the LM3643 control loop, will it cause any issues/problems?

Second since they are driving a laser diode, safety is very important,m they do not want a single point of failure to allow the device to output unregulated energy. What are the failure modes of this part, can the switch fail shorted and allow the input to directly drive the diode? Is there any protection if the 86mohm switch fails  shorted that the current drivers will be disabled? Looking for secondary protection. 

Thanks

Jeff Coletti

  • 0
    TI__Mastermind 22745 points

    Hi Jeff,

        For the first question, I don't think your customer needs a FET, since LM3643 has a "STROBE" pin which can be triggered by the PWM signal, you can control the PWM duty cycle to control the brightness.

        For the second question, as you can see in the block diagram, there is a FET between Vin and "Vout" but there is another current sink between FET and Vout, so even the FET short, it will not affect the current sink, so in this situation, it is still under control. thanks.

    Regards

    Sean

  • 0 in reply to Sean Zhou
    TI__Expert 8695 points

    Hi Sean

    Can I strobe at 100Khz? and with a 50%~90% duty cycle? This is a video application. 

    Regards

  • 0 in reply to Jeff Coletti
    TI__Mastermind 22745 points

    Hi Jeff,

        No, the maximum strobe frequency is about 20kHz. It is ok to use a FET with 100kHz dimming frequency, since it may trigger Short fault with low frequency, for 100khz, it is ok.

    Regards

    Sean