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LM5148-Q1: Max Rated Output Current Confirmation

Mark Long
Prodigy 25 points
Part Number: LM5148-Q1
Other Parts Discussed in Thread: LM5148, LM5146, LM5145

Tool/software:

Hi Team, 

I am planning to use the LM5148-Q1 controller for an automotive buck DC-DC converter and wanted to confirm the meaning of the listed maximum load current rating of 20A.

For context on our design: 

  • Synchronous buck DC-DC converter, 60V input to 14V output
  • Switching frequency 500 kHz
  • Max DC load 25A (20% of lifetime), Continuous load 10A (80% of lifetime)
  • Designing for 10A peak-to-peak inductor current ripple (so absolute max current around 30A)
  • FETs top-cooled with heat sink

Most of the TI LM514x controllers have programmable overcurrent protection limit, and I want to program the LM5148-Q1 for a limit above 30A. However, the listed maximum load current rating is 20A, which is rather odd since the load current flows through the power stage components and not the controller itself. We have scrutinized the choice of FETs and thermal management in our design to comfortably handle 30A, even with power derating at elevated temperature. So we should be okay as long as the controller does not trip at lower current limit. 

Looking around the forum, this thread and this thread more or less confirm my thoughts that the max current rating is just an arbitrary marketing spec in the product and is rather determined by the power stage and thermal management. But I wanted to make this post anyway to obtain a greenlight from TI that the LM5148-Q1 could indeed be programmed for current rating well above the listed 20A, provided that the power stage is designed appropriately. We just want to be very certain about this in order to not waste time and resources on designing for a controller that wouldn't work in the first place. 

Thank you so much for your help!

  • +1
    TI__Guru 53312 points

    Hi Mark,

    20A is an arbitrary limit and the device is capable of driving FETs with much higher current rating. Of course, the max output current of a particular power stage depends on the input voltage (FET voltage rating), inductor, switching frequency, component power losses, ambient temperature, airflow, etc.

    I recommend using the LM5148-Q1 quickstart file attached below for your design. Use 80V logic-level FETs, e.g. from Onsemi, and make sure to derate the ceramic output cap for applied voltage (as the specified value for Cout affects loop compensation).

    Note, however, that 500kHz is quite high for a 60V input at 25A, and really gives excessive high-side FET loss. The quickstart below is set at 300kHz and uses Onsemi NVMFS6H836NL and NVMFS6H818NL FETs, but the high-side loss is > 6W at 25A. Consider using a 2-phase setup instead at 12.5A/phase. Let me know if you need more detail.

    LM5148-Q1 quickstart calculator - 60Vin 14Vout 25A 300kHz.xlsm

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 25 points

    Hi Tim,

    Thank you for the helpful calculator and suggestions.

    The 500 kHz came from an optimization from our side mainly concerning the selection of inductor with appropriate size and high current rating. We would mostly be operating around 10A actually, the 25A is only occasional and come in short periods. In general, we are taking a slightly different approach to a common system. I'd appreciate a chance to chat with you more about our design since we are likely selecting the LM5148 controller. Hope that we can chat via private message instead, I already sent you a friend request.

    Best,
    Mark.

  • 0 in reply to Mark Long
    TI__Guru 53312 points

    Sure, Mark, no problem.

    You might also take a look at the LM5145 (75V) or LM5146 (100V) voltage-mode controllers, as they have 7.5V gate drive and thus switch faster, helping to reduce the high-side FET loss.