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LM5176-Q1: Capacitive Load

Bill Adams1
Prodigy 190 points
Part Number: LM5176-Q1
Other Parts Discussed in Thread: LM5176

I have an LM5176-Q1 design which creates 40V from a 24V input.  With no load, the voltage properly comes up to 40V.  This design also uses a shunt regulator to prevent back emf from a DC motor.  The shun regulator has 4 2200 uF aluminum electrolytic capacitors.  When the shunt regulator is attached, the voltage only get to 15V.  I tried slowing down the ramp by changing Css to 0.33 uF to give a soft start time of 50ms.  This seemed to help on one board but it is inconsistent.  When I show the capacitance on the output in webench, it says that the phase margin is too low.  Changing the compensation capacitor helps with the error but not the functionality.  What can be done to slow the ramp so that the charging of these large capacitors does not prevent the supply from coming up?2818.MotorVoltage.pdf

  • 0
    TI__Mastermind 21720 points

    Hi Bill,

    Thanks for reaching out with your questions and for considering the LM5176 in your design.

    Based on your description it sounds like the LM5176 might be running into hiccup mode current limiting. A quick check to see if this is the issue is going to be, removing R533 and connecting the MODE pin to VCC. This will allow for the current limit to operating in standard cycle by cycle peak current limiting.

    The compensation components might not be correct for the load capacitance either. The LM5176 quick-start calculator can be used to double check the loop compensation values with the added output capacitance.

    Please let me know if you have any questions.

    Thanks,

    Garrett