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LMG1210: Regarding reducing HO signal pulse width

Hoonil Jeong
Prodigy 20 points
Part Number: LMG1210
Other Parts Discussed in Thread: LMG1020, LMG1025-Q1

To the engineers this may concern,

HI! This is Hoonil Jeong in SOS Lab, a LiDAR company in South Korea.

Currently, I am working on developing a laser driver circuit system with your FET driver LMG1210.
As you may know, for LiDAR applications, the shorter laser pulse width is highly preferred. In this context, we are also conducting a series of experiments to reduce the pulse width.
There, I've found an interesting point that the high-side driver output (HO) signal suddenly disappeared when the PWM pulse width (50 %) decreased down to ~3.5 ns.


Regarding this, I have a couple of questions as below.
0. Is it normal that the HO signal is to suppressed drastically when the PWM input pulse width was decreased to a certain level? 
1. Are there any methods we can reduce the HO pulse width further?
2. Are there any 200 V-Half bridge FET drivers which can produce narrower output than LMG1210?

These are the test conditions we are using for your reference
1. We are using PWM input mode
2. PWM input signal: ~4 ns of 'on' pulse & ~1 MHz of repetition frequency
3. Both DHL & DLH were minimized with R_ext~1.8 MOhm

Thank you in advance for sparing your time.

Sincerely,

Hoonil Jeong

  • 0
    TI__Genius 9301 points

    Hi Hoonil,

    Welcome to the forum! Could you please share a schematic of your driver stage so we can review it? 

    It would also be helpful if you can share a scope shot of your PWM input signal as well as the output signals.

    LMG1210 is able to handle down to 1.8ns typ minimum pulse width, so let's review your schematic and input/output signals to make sure nothing is exceeding recommended operating conditions and that the PWM signal on the input is clean. 

    We have a couple of devices targeted for LiDAR applications but these are low side gate drivers, not half bridge drivers, so I'm not sure these would meet your needs. These devices are LMG1020 (1ns typ min pulse width) and LMG1025-Q1 for automotive (1.25ns typ min pulse width).

    Best regards,

    Leslie

  • 0 in reply to Leslie Marquez
    Prodigy 20 points

    HI Leslie,

    First of all, thank you very much for the kind and informative reply.

    Below, I attached the schematic as well as the PWM input/output. Please check it. In addition, please let me know if anything is needed more.

    In the previous reply, you recommended low-side drivers. Do you have any high-side drivers also which might be suitable for the LiDAR applications? Using the high-side and the low-side drivers separately would widen the flexibility of our new driver design.

    Thank you very much.

    Hoonil Jeong

    SOS_Lab_DriverSchematic_210409.pdf

  • 0 in reply to Hoonil Jeong
    TI__Genius 9301 points

    Hi Hoonil,

    Thank you. I'll review the schematic and provide feedback by Monday. A couple of questions:

    Is the input signal probed right at the PWM/LI pin? Just want to make sure that the PWM signal plot provided is what the LMG1210 is actually seeing at its input. 

    Could you please also probe the LO-VSS so we can observed both HO-HS and LO-VSS?

    Best regards,

    Leslie

  • +1 in reply to Leslie Marquez
    TI__Genius 9301 points

    Hi Hoonil, 

    I reviewed the schematic you provided and overall it looks OK, I just have a couple of suggestions:

    1. You can reduce your bootstrap capacitor to be 0.1uF and this way the ratio between bootstrap cap and VDD cap will be higher (more than 10 times higher). 

    2. For your dead time resistors, I recommend to start with a nominal resistance value to run these debug tests (around 25Kohm) and once you have the desired signal then you can start increasing the resistance. 

    If you are still having issues after this, please provide the signals and information requested on my previous post. 

    Best regards,

    Leslie

  • 0 in reply to Leslie Marquez
    Prodigy 20 points

    SOS_Lab_210413.pdf

    Dear Leslie,

    1. The input signal was probed right at the PWM/LI pin.

    2. I attach the file with LO-VSS

    Many thanks!

  • 0 in reply to Leslie Marquez
    Prodigy 20 points

    Thank you Leslie,

    We will try that!!

  • 0 in reply to Hoonil Jeong
    TI__Genius 9301 points

    Hi Hoonil,

    Thanks for sharing the LO-VSS waveform. I see the LO-VSS signal does switch even with 3.5ns pulse width. Let us know your findings once you try our suggestions.

    Best regards,

    Leslie

  • 0 in reply to Leslie Marquez
    Prodigy 20 points

    Dear Leslie,
    I am still struggling to resolve the issue based on your suggestions.
    Meanwhile, I have a simple question.
    Can UVLO affect the sudden disappearance of HO-HS signal when PWM input or the HO-HS output reached certain values? 

    Many thanks

    Hoonil