• State
  • Locked Locked
  • Replies 5 replies
  • Subscribers 62 subscribers
  • Views 156 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM74910-Q1: Quickly removing the VBAT main power, the DGATE of 74910 failed to turn off the MOSFET, DGATE ouputs sawtooth waveform.

wang nick
Prodigy 80 points
Part Number: LM74910-Q1


Tool/software:

Hi Texas members:

     I have a question about LM74910's reverse current protection, as the picture shows below, 

    

this test environment is:  Main power VBAT_B=28V,      backup power VBAT_BACKUP_B=13.5V, 

Under normal VBAT main power adjustment, a slow decrease and increase proceed without issues. At this point, the three voltages of the MOSFET are as follows: the red waveform (DGATE) is approximately 10V higher than the Source terminal, enabling the MOSFET to be turned off.

   

When switching to 28V hot-plugging, you can see that the DGATE signal changes to an oscillating waveform. Additionally, the main power 28V voltage doesn't drop to 0V but is keep at 13.5V due to feedback from the VBAT_BACKUP backup power source which is 13.5V.

    

it looks like something is wrong with LM74910 controller, reserve current protection doesn't work normally.

Could you please help me?   thanks in advance.

  • 0
    TI__Guru 53015 points

    Hi Wang,

    Is there a load at output?

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Prodigy 80 points

    Do you mean LM74910's output? yes indeed, the soc is running and totally about 3.7A~5A@13.5V,  when I pull out 28V main power supply and it switches to 13.5V backup power supply at once, but LM74910's reverse protection doesn't work,  I can measure the main power cable and it's 13.5Volt.  It should drop to 0Volt.   

    Here I have an information to share with you.  If I add a electronic loading at 28V main power cable,  only 100mA CCL,  it can drop to 0Volt.

    But if nothing connect just pull out 28V cable, it has 13.5Volt issue again. 

    here is the waveform with 100mA CCL electronic loading.

  • 0 in reply to wang nick
    TI__Guru 53015 points

    Hi Wang,

    Can we find out if any reverse current is even flowing? Maybe probe IN on 28V rail during switchover.

    Also, can you add a 100k sort of resistor from INPUT to GROUND on 28V rail to tie low and not keep floating?

    28V source doesnt seem to be battery and hence this leakage won't degrade it.

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Prodigy 80 points

    Hi Shiven:

        oh no, 28V is main battery voltage, and 13.5V is backup battery voltage,  this leakage may cause serious problem I think.  Assume 28V cable drops out from battery and this cable has 13.5Volt, it would cause short current to GND easily on the vehicle. 

    yes, I can do this test to add a 100K resistor and won't keep VBAT floating.   But I don't understand what's wrong with LM74910 internal and DGate output such strange sawtooth wave.  

  • 0 in reply to wang nick
    TI__Guru 53015 points

    Hi Wang,

    Lets see if 100k is helping first.

    Regards,

    Shiven Dhir