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LM2731 not switching

Andy Mitham
Prodigy 40 points
Other Parts Discussed in Thread: LM2731

Hi,

Has anyone had any experience of problems with the LM2731? I've used two in a new board that I'm working on. I'm feeding in 3.3V and have it configured for 5V output. I initially tried these unloaded, and then with a very light load (5mA). Both circuits (which are identical) just pass the input through to the output with no signs of switching occurring. I used the Webench designer to generate the circuit.

I did notice the following post, but other than requesting sending the IC's for investigation there is no outcome given.

http://e2e.ti.com/support/power_management/simple_switcher/f/858/p/183742/665559.aspx#665559

Thanks for any help.

Andy

  • 0
    TI__Guru 57555 points

    Are you saying that there is no switching, regardless of load?

    Also, check the obvious such as having shutdown tied to Vin and correct feedback divider resistors.

    Send you schematic for us to look at.

    FD

  • 0 in reply to Frank De Stasi
    Prodigy 40 points

    Hi Frank,

    Thanks for your reply.

    I'm not able to see any signs of switching occurring (circuit unloaded and also with a 120 Ohm load). I'm currently feeding a regulated 3V3 supply into this circuit. If I measure the SHDN pin I am getting very slightly less than this, so I assume the device should not be in shutdown. I've double-checked the feedback resistors fitted and these are the same as my circuit.

    The input voltage measures 3.301V and the output is 3.276V (circuit unloaded). I did notice that the feedback voltage is 2.606V, which makes me think that something strange is happening at this pin.

    I have attached a copy of our circuit.

    Many thanks for your help.

    Kind regards

    Andy

    LM2731 Problem.pdf
  • 0 in reply to Andy Mitham
    TI__Guru 57555 points

    Yes that sounds tlike were the problem is.  Make sure that there is no leakage to the FB pin from flux, etc.

    Also, the values of you resistors are a little large.  You could try reducing the magnitude of resistance with the same ratio.  Be sure that your feedforward cap is ok and not leaking.

    FD

  • 0 in reply to Frank De Stasi
    Prodigy 40 points

    Thanks for the information, Frank.

    I'm out of the office today, but shall look at the points mentioned on Monday.

    Many thanks

    Andy

  • 0 in reply to Andy Mitham
    Prodigy 40 points

    Hi Frank,

    It looks like the problem was due to the size of the feedback resistors.

    I changed them to a value which was an order of magnitude smaller and the part is now switching and also regulating well.

    A problem I had also noticed (before changing the resistors) is that I could get the part regulating when I fed in a lower voltage (in the order of 3.5V - trying to regulate the output to 5V). After changing the resistors, I was able to feed in approaching 4.9V and the output still successfully regulated to 5V.

    Thanks again for all your help.

    Kind regards

    Andy