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Original question:

LM5176: Step up working well, but step down is not working.

LM5176: EVM modify to 12V/25A

Kami Huang60
Genius 3830 points
Part Number: LM5176
Other Parts Discussed in Thread: PMP21278, , CSD19502Q5B, PMP

Dear Sir/Ms.

My customer wants to use LM5176 Wide-VIN Buck-Boost Controller EVM Board to change to PMP21278 300W Buck-Boost 12V / 25A output.

  After modifying its parts according to the above documents
  It is found that the load current does not rise but decreases, about 10A
  When the input voltage is adjusted to fall to 12V, the output voltage is immediately pulled down to 0V.

First, the changes are as follows

1. LM5176 EVM originally changed PM21278


2. LM5176 EVM originally changed PM21278
Q1, Q2, Q3, Q4 changed to CSD19502Q5B


3. LM5176 EVM originally changed PM21278


4. LM5176 EVM originally changed PM21278

Second, EVM unchanged

LM5176 EVM SS, COMP, SLOPE set capacitor resistance has not changed to the value of PM21278

LM5176 SS, COMP, SLOPE setting

Not change.

[Condition 1]
   When the output 10A input voltage gradually drops to 11.9V  The output voltage 12V becomes 0V, DC-DC Turn OFF
   When the output 10A input voltage gradually rises to 12.1V  The output voltage 12V becomes 12V, 10A DC-DC Turn ON

   [Condition 2]
   When the output 25A input voltage gradually drops to 16.9V  12V output voltage becomes 0V, DC-DC Turn OFF
   When the output 25A input voltage gradually rises to 17.1V  The output voltage 12V becomes 12V, 25A DC-DC Turn ON

Do customer need SS, COMP, SLOPE to modify the settings?

Can you guide us how to modify the setting output to 12V / 25A?

Best Regards,

Kami Huang

  • 0
    Genius 3830 points

    BTW, Input voltage 7.5V~24V.

  • 0 in reply to Kami Huang60
    TI__Guru* 78270 points

    Hi Kami,

    Thank you for using the LM5176.  It sounds like you tripped the peak current limit.  I don't see you mention the change of inductor.  The original EVM inductor cannot support higher current. Please use the correct inductor.

    Thanks,

    Youhao Xi, Applications Engineering

  • 0 in reply to Youhao Xi
    TI__Genius 11635 points

    Hi Youhao,

    Would you help check if this inductor ok for this case ? 

  • 0 in reply to PYTsai
    TI__Guru* 78270 points

    Hi PY,

    I ran the LM5176 design calculator, and found the peak inductor current can be 48A (typical) owing to the 2.5mOhm sense resistor, which exceeds the inductor's 33A rating.  The TI reference design chose an inductor of Isat=51A, so it worked.  Please change to an inductor of higher rating. 

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    TI__Genius 11635 points

    Hi Youhao,

    We tested again using the same inductor as that was used in this reference design PMP21278  

    XAL1580-202ME spec as below 

    Below is our test result, at 25A the circuit still not work correctly, however if load is 1A it work OK.

    May we have your further suggestion ?  

  • 0 in reply to PYTsai
    TI__Guru* 78270 points

    Hi PY,

    It is obvious that your circuit hit the peak current limit. Did you really use 2.5mOhm (two 2.5mOhm in parallel)? Did you use the same type of resistor as used in the PMP design? 

    I am afraid your sense resistor may have larger parasitic inductance that causes the sensed signal greater than actual value, owing to the amount of Lr * di/dt superimposed on to the i * Rsn, where Lr is the sense resistor parasitic inductance, and di/dt is the power inductor current slope?  Can you increase C15 to 2.2nF and re-do the test, and let me know any improvements?

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    TI__Genius 11635 points

    Hi Youhao,

    Yes, you are right. The problem is on the current limit circuit. if we short directly the current sense resistor. the circuit can work correctly. it can output 25A no problem. 

    However If keep the current sense resistor and increase C15 to 2.2nF, we can only get 7A output. (only 1 A increase) . 

    We will check the current sense resistor's actual value. 

    Any other idea (besides the current sense resistor value not correct ) that might cause this symptom ? 

    Thanks!

     

  • 0 in reply to PYTsai
    TI__Guru* 78270 points

    Hi PY,

    2.2nF at lease show the improvement, aligning with our expectation. Please change the current sense resistors to a different vendor, and see if there is any improvements.  

    Thanks,

    Youhao