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LM5109B: Half-Bridge Switching Issue

Biju Mohan K C
Prodigy 20 points
Part Number: LM5109B

Tool/software:

Hi,

We are using LM5109B for our motor driving application for driving the H-Bridge circuit given below.





We see a strange issue, We've provided a dead time of 2.5uS between the HI and LI PWM Inputs, that is the maximum supported by the Motor driver chip that we are using. But when we look at the HO and LO waveforms, the HO signal stays high even after the HI pin is low and it stays high until the LO starts to rise. See the below waveform,



Yellow- HO w.r.t. GND
Pink- LO 
Cyan- HI
Green-LI

We are using 48V for powering the H-Bridge.

Because of this we see a ringing on the rising edge of LO and similar ringing is observed on the other voltages and digital signals on the board.

Could you please help us find out the root cause of this problem?

  • 0
    TI__Intellectual 830 points

    Hi Biju,

    Thank you for the question! Based on the waveform you've provided, it seems as though the device is working as intended. When HI is pulled low, HO is also pulled low with respect to HS. As LO rises, the low-side FET turns on and pulls HS down to 0V.

    If LO is being measured at the pins of the device, the spike may be due to the parasitic inductance between LO and the low-side FET. The observed ringing on the other voltages and digital signals on the board are likely due to noise that occurs as HS goes from 48-0V, either on the PCB or the probe method. How are the signals being probed? Do the probes have a tight inductance loop? Would you be able to share the layout of the system?

    Thank you,

    Vincent Liew

  • 0 in reply to Vincent Liew
    Prodigy 20 points

    Hi Vincent,

    Thanks for your answer.

    Oops, I missed that HO is going low. 
    Regarding the probes, yes, they have a tight inductance loop. The interesting thing is we see the same ringing while simulating the circuit in LT Spice too.



    We can't share the PCB layout in an open forum due to our company policies. I can share it with you over email.

    Best Regards,
    Biju

  • +1 in reply to Biju Mohan K C
    TI__Intellectual 830 points

    Hi Biju,

    Thank you for the response! Is the ringing in the LT Spice sims only affecting LO, or is it affecting other voltages and signals on the circuit as you see in the real system? If the sim only shows this behavior on LO, this is likely due to the miller plateau of the FET, and the ringing observed on other voltages and digital signals in the physical system would likely be noise.

    I've sent you a friend request to discuss the layout and/or any further discussions about this issue and will close this thread.

    Best regards,

    Vincent Liew