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lm5576 DC-12V to DC-N50V

kuda wu
Prodigy 190 points
Other Parts Discussed in Thread: LM5576

Hi!

         our application is class D amplifier.

         We want  to use lm5576 as our negative power source and follow TI webench reference design. please find the attached file.

          DC In : DC-12V

          DC out:  DC-N50V

          when power up,  DC-12V  buck to DC-N50V, it work fine without load.

          But  when add 50ohm load, it only buck to N30V and the switch pin (pin17.18) not stable.

          Could someone help us to check the reference design is correct?

          We can't find the formula and information between COMP(pin6) and FB(pin5) in the negative mode from datasheet.

                                                                                                                            kuda 

https://webench.ti.com/webench5/power/webench5.cgi?origin=ti_panel&lang_chosen=en_US&VinMin=45&VinMax=45&O1V=-48&O1I=0.5&op_TA=30

         

  • 0
    TI__Mastermind 23815 points
    Yes,
    i believe you exceed the current limit of the part.
    Based on your input, your duty cycle is roughly = 50/62 = 0.806
    Therefore the maximum current that you can do is (1-D) x 4.2A which is the current limit of the part.
    The maximum current is reduced because the configuration is become an inverting buck boost instead the regular buck, therefore the maximum current change.
    You can check this app note
    www.ti.com/.../snva722b.pdf

    If you only loaded by 100 ohm or 150 ohm, the converter should be ok

    Thanks
    -Arief
  • 0 in reply to Arief Hernadi
    Prodigy 190 points
    Hi! Arief,


    Base on the maximum current formula, change the input voltage from DC-12V to DC-24V and the output current would be increase.
    duty cycle 50 / 74 = 0.675
    maximum current (1-D) x 4.2A = 1.3A
    About these information, I can't simulate from webench.
    Is it right?
    Thank you.

    kuda
  • 0 in reply to kuda wu
    TI__Mastermind 23815 points

    Yes, that is correct.

    If you have higher DC input voltage. But at 12V you might not be able to support the current. 

    If you have a current probe you can check whether the inductor current ever hits the current limit of 4.2A

    Or you can try to reduce the load current or higher VIN and see if it behaves ok

    Thanks

    -Arief