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Original question:

LM5575: Enable circuit not working

LM5575: feedback circuit behaves when Vin is low

Srinivasu V
Intellectual 360 points
Part Number: LM5575
Other Parts Discussed in Thread: LM5164

Hello,

We have used LM5575 for our new project.

I have calculated the feedback resistors for 15V output and Vin minimum is 17.5V.

My question is, How LM5575 will behave as feedback resistor is for 15V ?

in the simulation, I am finding only Vout is dropping to 14.5V instead of 15V. ( I could not able to see the ripple in the simulations)

and,

As per LM5575 datasheet, to adjust the Vin min( 17.5V). we have to  select the frequency range between 200Khz to 250Khz range.

but, due to EMC requirements, we have decide to 450Khz frequency range. So, for 450Khz minimum Vin should be 20V.

Here my question is, as Vout is 15V and Vin will be 17.5V, How LM5575 will behave? 

Thank you,

Srinivasu

  • 0
    TI__Mastermind 49557 points

    Hi Srinivasu,

    It appears you are operating close to the dropout condition. If the minimum off-time condition is reached as Vin is reduced (or Fsw is increased), then the output voltage operates at a lower voltage relative to its nominal setpoint.

    Another option here is the LM5164 100V/1A synchronous buck converter as it provides very low dropout performance.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Intellectual 360 points

    Hello Tim,

    Thank you for quick response!.

    output voltage operates at a lower voltage relative to its nominal setpoint - not a problem for our circuit. because our other input range is 12.5V to and above.even if I get 14V also, no problem.

    because of Vout is less then set voltage, LM will not go to shunt down mode or any condition  is it? 

    and suggested alternate component : actuallly, we have to consider only release components to Bosch. So, I do not have rights to consider another compoents.

    Thank you,

    Srinivasu

  • 0 in reply to Srinivasu V
    TI__Mastermind 49557 points

    My understanding is that Vout will just operate below its nominal setpoint once the toff-min of 500ns is reached. Take a look at the relevant section in the datasheet - you can also review operation of the EVM.

    Regards,

    Tim