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LMG1020EVM-006: Resistor Selection

Mark Ackerson
Genius 10190 points
Part Number: LMG1020EVM-006

Customer is reviewing External Gate Resistor Design Guide for Gate Drivers app note to figure out the resistors they would need on the LMG1020EVM-006 and they are not getting a good answer.  In looking at the spec for the EPC2019 data sheet, it notes a Ciss of 200pF.

Attached are scope traces, blue is the K testpoint, green is enable and the yellow is gate voltage.  You can see when turning the FET off it has huge swing, I measure this to be 426 MHz.  So from the app note and equation 1, I get an inductance of 0.7nH.  If I compare that to their example on the second page, Ciss of 9250pF and frequency of 3.57 MHz, they get 2149nH of inductance.  This seems to me to be too large of a difference between the 2 examples.

Any ideas?

Regards,
Mark

report.pdf

  • 0
    TI__Genius 12990 points

    Hi Mark,

    This amount of ringing seems almost normal for this board with a high voltage applied due to the high peak laser current and resonant nature of the laser drive topology. The gate does not ring however the drain sees ringing. Turning off a GaN FET really fast that already has peak current flowing through it will make ringing happen. Check out figure 9 on page 11 of the EVM users guide. Figure 9 has a similar ring happen for 1ns pulses.

    The gate resistor selection method from the app note relies on the gate loop ring frequency (which is not really ringing) not the power loop one, which could be seen on the drain waveform due to probe capacitance.

    Make sure the laser diode leads attached to the EVM are very short. as well as the probing pig tails used for collecting waveforms. Oscillations can happen just from long laser leads due to adding to the power loop inductance.

    Once this is done and you still have excessive ring, you can try using 2-5ohm gate resistor on OUTL to see if that helps with the turn off ring seen at the drain.

    Let me know if this helps answer your question or you have any other questions.

    Thanks,

  • 0 in reply to Jeffrey Mueller
    TI__Genius 10190 points

    Hi Jeff,

    Thanks for the reply, that makes sense.

    I'm assuming when you mention the "EVM users guide", you mean the Nanosecond Laser Driver Reference Design for LiDAR document?  There's no EVM user guide on the LMG1020EVM-006 product page.

    Either way, we should have the document on the LMG1020EVM-006 product page.

    Appreciate the support.

    Regards,
    Mark

  • 0 in reply to Mark Ackerson
    TI__Genius 12990 points

    Hi Mark,

    Thanks for letting me know.

    The link for the UG is below.

    I'll see if I can figure out why the link doesn't show up in the product folder updated to show the UG.

    Thanks,

  • 0 in reply to Jeffrey Mueller
    TI__Genius 10190 points

    Hi Jeff,

    The link you sent is for the product page:

    Regards,
    Mark

  • 0 in reply to Mark Ackerson
    TI__Genius 10190 points

    Hi Jeff,

    Included below is an update from the customer:

    I found that going to a 7 ohm series resistor knocks the ringing down to less than 0.5V so what they are noting makes sense.  We are using the EVM as it is shipped in regards to the load, which is the 4 4.02 ohm resistors in parallel.  I was able to run the EVM without issues at 10V on J2 with a gate pulse waveform of 10ns pulse every 1.5ms.  I then switched the voltage at J2 to 15V.  After enabling the enable signal the output at TP6 which shows the pulse went to a slow rising turn off, the turn on was still just over 2ns but the off time is now 2.5us.  Any ideas on that?  This is actually the same thing that happened with the first EVM I had which I though I damaged because of the ringing on the output but now the ringing was less than 0.5 V on the output.

    We could schedule a call to talk through this if it's easier.

    Regards,
    Mark

    LMG1020EVM_Scope_Plot_1.pdf

  • 0 in reply to Mark Ackerson
    TI__Genius 12990 points

    Hi Mark,

    Thanks for the update, let me figure out what is going on with the UG link.

    Thanks for explaining your issue using 1020 EVM. It sounds to me like the 4 4ohm resistor load is damaged.

    Too large of a pulse on this EVM (with too high of VBUS voltage) can damage the load and a new one needs to be soldered on.

    You can use a larger load if needed to simulate the laser in the application in order to achieve the required pulses.

    If the load is damaged then the rise time is short because the drain is being pulled to ground. The fall time is long because the large impedance 'open' load is preventing the drain capacitance from recharging the output cap quickly.

    Let me know if this makes sense or you want to have a call, feel free to reach out offline.

    Thanks,

  • 0 in reply to Jeffrey Mueller
    TI__Genius 10190 points

    Hi Jeff,

    That answers all the questions.

    Thanks for the support!

    Regards,
    Mark