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OPT8241-CDK-EVM IB : Why FET and Laser connected in parallel?

Other Parts Discussed in Thread: TPS2559

In OPT8241-CDK-EVM Illumination board. MosFET (seems Illumination driver) and Laser connected in parallel. I can't understand why. From my understands Illumination driver must be like below.

  • Hi Mr. Suzuka,

    Both are possible ways of switching laser diode. In the parallel switch method as used in the EVM, laser turns on when the mosfet gate is low and turns off when the mosfet gate is high. The series method is preferred when we use voltage mode driver circuit. We have used current mode driving for the laser and hence preferred parallel switch. The driver circuitry used in the EVM helps to bring down the ringing in the optical waveform.

    Thanks and regards

    Nithin

  • Hi Nithin,

    "We have used current mode driving for the laser and hence preferred parallel switch."

    for LED driver circuit,it is current mode ,I Can understand,but for Laser driver circuit,it seem like not current mode but voltage mode.

    because the TPS2559 is just load switch. is there right?

    thanks and regards
  • Dear None,


    TPS2559 is actually used as a current source. Actual design of TPS2559 is as a current limiting load switch. We use the current limiting feature of TPS2559, and make sure the voltage supply to TPS2559 is greater than the necessary voltage. Then, TPS2559 goes into current limiting and works like a current source with a limit on the higher voltage.

    Regards,
    Subhash

  • Hi Subhash,

        Thanks for reply.

        If you want to make TPS2559 work under the mode of what you said, it means to the load current should greater than the tps2559 current limit setting.

        I check the OPT8241-CDK-EVM IB sch file,I foud tps2599 current limit to 2.4A. so the load current must be greater than 2.4A. but 22045498 Datasheet the laser 

    max operating current is 1.4A. So I still think it work on voltage mode. maybe there is something wrong.

        

     thanks&regards

     None

  • Dear None,

    The laser peak power is 2.4W for 50% duty cycle. We operate the laser in that mode.

    For a peak power of 2.4W, the current has to be of values greater than 1.4A. The 1.4A value is for continuous waveform operation.


    Regards,
    Subhash

  • Hi Subhash,

         May be the datasheet about laser 22045498 I have is brief version.

        because,  for a peak of power of 2.4W,if want the current is greater than 1.4A,it should be the min operating voltage is V=2.4W/1.4A=1.71V.

        may be operating in pluse mode this parameter is different. because there is something wrong.

       thanks & regards

       None

  • None,

    I think the pulse currents can be higher than the CW currents mentioned int he datasheet. Please contact Lumentum for details as it is not a TI part.

    Regards,
    Subhash
  • Hi Subhash,

    Thanks for you information.

    the laser part as used in OPT8241-CDK-EVM, I think TI can have the laser Datasheet,If you feel convenient,whether can give it for me.

    it may be more troublesome for me to contact Lumentum.

    thanks&regards

    None
  • None,

    I am not sure if I can give our datasheet. However, our datasheet shows the same data as yours. So you wouldn't get any extra information from our datasheet.
    It would be best if you contact Lumentum directly to be able to get the exact information.

    Regards,
    Subhash
  • Hi Subhash,

    Thanks for you reply.

    I will try to contact Lumentum for the datasheet. if i get it, i will share the pluse current for you.

    Thanks & Regards

    None
  • Hi Subhash,

         From Laser Datasheet,it laser peak optical power is 2.4W(Max),it's test condition is at 50% duty cycle, so i think this is the pulse current.

         but it slope efficiency is 1.1W/A, so we can get the max pulse current is 2.4/1.1=2.18A. according this, i think the laser driver circut is operating in voltage mode  not current mode.

      Thanks & Regards

      None