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LM25576: Webench simulation provided -15V, 1.5A output but the actual PCB only maintains -15V at 1.1A

Jason Gura
Prodigy 10 points
Part Number: LM25576

Tool/software:

I have a LM25576 Inverting Buck-Boost circuit.  Vin =+14.5 - 15.5V.  Vout = -15V.  Iout = 1.5V.  The design was copied from Webench simulation.
All 5 prototype boards only generate -15V at 1.1A maximum.  At 1.5A, the output voltage is approximately -12.5V.

Could you please suggest which components should I investigate or adjust to improve the output current?

Thanks,

Jason

LM25576M_14_5V-15_5Vto-15V_1_5A.pdf

  • 0
    TI__Mastermind 21950 points

    Hi Jason,

    It sounds like the device is entering current limit.

    Try increasing the size of the inductor, which will reduce the peak to peak current ripple. You may also need to adjust the compensation and output capacitance to ensure stability. 

    You may also need to use a higher current rated device than 3A since your effective duty cycle in IBB operation is very close to 50%. Since this is a 3A device, your maximum operating current is basically at the limit of what the device can output in this configuration. 

    If the current ripple is too high, you may be reaching the peak current limit.

    Best regards,

    Ridge

  • 0
    Prodigy 10 points

    Ridge,

    Thank you for your response. 

    1. Increasing size of the inductor:  Are you referring to higher inductance or physical size for lower ESR?  Webench simulation suggests 22uH for 1A load, 18uH for 1.25A load, and 15uH for 1.5A.

    2. Do you have any recommendations for adjusting the compensation?  It seems likes lowering Rcomp would be the first step.  

    Thanks,

    Jason

  • 0 in reply to Jason Gura
    TI__Mastermind 21950 points

    Hi Jason,

    I am referring to the inductance value. If you increase the inductance value, the current ripple will be less. 

    You can use Webench to estimate your phase margin with the new inductor first, then decide if the compensation needs to be changed for this test.

    However, if you are running into current limiting issues, it will be better to use a 4A rated device to increase the possible output current in IBB configuration.

    Best regards,

    Ridge

  • 0 in reply to Ridge Lahti
    Prodigy 10 points

    Unfortunately, Webench is not allowing me to change the inductor value for the LM25576.  It only lists 15uH options for 1.5A.  I tried creating a custom inductor and webench just spins it wheels forever.  I might try to simulate in Spice later.  I will  experiment with larger inductors on the board.

    I agreed that for final design that higher current device may be the better option for 1.5A.  In the meanwhile, we have a 5 mostly working prototype PCBs.  I am just trying to get the most current I can out of LM25576.  If we can get 1.25A instead of 1.1A, then that may be enough.    

  • 0 in reply to Jason Gura
    TI__Mastermind 21950 points

    Hi Jason,

    If possible, you should try replacing the inductor quickly on the bench.

    Otherwise a higher current device would be the best option.

    Best regards,

    Ridge