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LMG1020: Question for LMG1020 drain current waveform when driving laser diode

Jacky Wang(QD)
Genius 9920 points
Part Number: LMG1020

Hi Experts,

Our customer is using LMG1020 to drive a GaN FET with 50MHz switching signal to drive a laser diode, schematic is as below. Now the problem is when testing the circuit, we found that the laser diode current can not go to zero with a relative constant DC current as below waveform showing. Can you help to analysis what's the possible reasons? We are suspecting it could be parasitic parameters influence, but having no idea how to optimize it. Looking forward to getting some comments from your side. Thanks.

  • 0
    TI__Genius 13445 points

    Hello Jacky,

    Thanks for reaching out.

    From the waveform, it looks like ~-2V undershoot during the falling edge of the gate, can you please confirm your measurement technique use tip & barrel to limit inductance from scope ground leads?

    In addition to the waveform above, can you please share input signal?

    From your description of the issue, the constant current through the laser diode seems to indicate that the FET might not be turning-off fully, what does the VDS voltage looks like across VDS to rule this theory out. Parasitic could certainly be a factor specifically stray inductance in gate drive path and more likely traces around the laser diode and FET, if available kindly share PCB layout.

    Regards,

    -Mamadou