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LM5576 _how to select the Fs

Sunney Kumar
Intellectual 300 points
Other Parts Discussed in Thread: LM5576, LM5088, LM5116

Hello Sir/Madam,

I have one design, in which LM5576 is being used

Vin-16V to 72V

Vout 13V

Cramp 1.2nF

but i am not understanding that how we are calculating the value of Fs, which is almost base for other variables.

  • 0
    TI__Expert 3950 points

    Hello Sunney,

    choosing the switching frequency really depends on the efficiency that you want to achieve and the size of your regulator.

    The higher the switching frequency the higher the switching losses but the smaller the required size of your components for a particular current ripple and/or voltage ripple.

    Furthermore with a higher switching frequency it is possible to achieve faster load transient response therefore it really depends on the purpose of your application.

    I invite you to use our WEBENCH tool in order to generate customized design with our parts.

    Please visit:

    http://www.ti.com/product/lm5576

    and you will find on the right column a WEBENCH window.

    Regards,

    Giuseppe

  • 0 in reply to Giuseppe Pinto
    Intellectual 300 points

    Hi ,

    thanks for ur reply

    but i am not able to simulate it in webbench

    Vin 16 to 72V

    Vout 13V

    ILoad 3A

    that is correct that if i increase the switching frequency inductor size will reduce but losses increase.

    switching losses means more ripple at output voltage or what and also for the above mentioned values what how i can use webbench please suggest

  • 0 in reply to Sunney Kumar
    TI__Expert 3950 points

    Hello,

    this is a high duty cycle design which requires more slope compensation to avoid oscillations. Theincreased slope compensation effectively lowers the current limit threshold for the part.

    Do you really need 3A?

  • 0 in reply to Giuseppe Pinto
    Prodigy 10 points

    hi,

    yes the current requirement is 3A and slope compensation can be done by pulling the Vcc pin if i am not wrong.

  • 0 in reply to Anupam Bittu
    TI__Expert 3950 points

    Hello,

    the LM5576 is not suitable for your application unless you reduce the current to 2A because by adding slope compensation in order to avoid subharmonic oscillation (hence instability) you also hit the current limit earlier.

    For your application i would suggest you to use the LM5088 or the LM5116.

    You can use the LM5576 but you have to reduce the current to 2A.

    Regards,

    Giuseppe