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LMG1025-Q1EVM: The laser diode output waveform is not good

Part Number: LMG1025-Q1EVM

Hi team,

I use TI EVM LMG1025-Q1EVM, I replaced the resistance with a laser diode which is PLT5 450B, PLT5 520B and HL63603TG.

The wave form is not good, you can have a look at figure below.

How to output a normal waveform?

Could you help?

Thanks & Regards,

Vivian

  • Hi, Vivian,

    Our office is closed for the US Memorial Day holiday on Monday. We will get back to you once we are back in the office. 

    Best regards,

    Don

  • Hi Don,

    Thanks for your quick response.

    Ok. We'll wait.

    Thanks & Best regards,

    Vivian

  • Hi Vivian,

    The attached images are very low resolution; I cannot really tell which waveform is which or what diode the correspond to. The yellow waveform seems to follow the green, is it the driver input and output? Have you measured the drain of the FET to see the waveform across the diode? 

    For these short pulses, the parasitics will have a large influence on the waveforms, and they will often look "bad" even with very good design. For example, this is from the LMG1020EVM

    So some ringing is inevitable. Another thing that could help, is to measure the light emitted from the diode. This is ultimately the most important thing, and it should be used to judge the performance of the circuit.

    Thanks,

    Alex M.

  • Hi Alex,

    Thanks for your reply.

    Sorry for the mess of the last uploaded image, we rearranged our question, you take a look at this pdf

    ╨┬╜¿ DOCX ╬─╡╡ (2).pdf

    Hope you can help adjust the situation this time.

    Thanks & Regards,

    Vivian

  • Hi Alex,

    Could you help?

    Thanks & Regards,

    Vivian

  • Hi Vivian

    yes, sorry about the delay; for some reason I didn't get a notification that you replied. 

    It looks like the gate waveform and KA waveforms are okay; I don't see any strange behavior from them. So we know the gate driver and GaNFET are behaving okay at least. Unfortunately, those are what I am most familiar with, and I am not an expert on laser diodes or optics, which seem to be the issue. I see these are CW diodes, so I believe they should support low frequency. I am most used to seeing 1-50ns LiDAR diodes used with this this device.

    My only guess right now, is that this is related to the supply on TP5. The oscillations seem to follow TP5 (which makes sense from a KVL perspective). The capacitance on this net is configured for the shorter pulses, and it looks like the VBUS is collapsing as it sources current. I would try populating C5 to see if that helps. 

    Thanks,

    Alex M.