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LM2738XMY transient response and no-load behavior

Emil Eriksson70652
Prodigy 130 points
Other Parts Discussed in Thread: LM2738

Hi,

I'm using the LM2738XMY (and I'm assuming it's okay to ask this here now since the National acquisition is complete) in a design and I'm wondering if there is a simple way to estimate the voltage dip/peak for a given load transient? I've seen some documents that use the impedance at the crossover frequency but since I don't know the crossover frequency I cannot use this.

Secondly, I'm using this regulator in another design and when no load is applied, the frequency seems to be reduced to a very low level which causes excessive voltage ripple. Any reason why this would happen? This doesn't happen always and when it happens it disappears as soon as I apply the slightest of loads and it then stays proper after removing the load.

  • 0
    TI__Genius 10095 points

    Hi Emil,

    I haven't worked with this part but here is my guess to what is happening for the no load situation.

     

    The LM2738 is an asynchronous buck converter (uses a catch diode D1,  instead of a low side fet). With this type of converter the internal high-side Mosfet is powered by capacitor C3,often referred to as a Boot strap or Boost capacitor.  This capacitor is charged when the internal mosfet turns off and the  load current forward biases diode D1 forcing the SW pin below ground.

    So what I think is happening is Capacitor C3 is not being charged at no load, then when you apply a load this charges the capacitor enough for it to operate properly after that.

    There are a couple solutions. 

    1)  You can use lower value feedback resistors R1 and R2 to privide a small load.  Determine the required loading, and size them to provide the necessary load. The drawback is an increased static current draw.

    2) Use a lower value of inductor.  The extra current ripple in the inductor is sometimes enough to get rid of this condition in asynchronous buck converters. The drawback is slightly higher output voltage ripple.

     

     

    About the transient response.

    For a first pass I usually assume that the crossover frequency is 10 times less than the switching frequency.  The switching frequency is 550Khz for the LM2738 so the crossover will probably be around 50Khz.

    If you want to measure the crossover and have a transformer with a flat frequency response.  Here is a link to the general idea.

    http://www.national.com/AU/design/courses/190/smp02/08smp02.htm

    .

  • 0
    TI__Intellectual 1160 points

    Hello Emil,

    To estimate the load transient response, I recommend using Webench for the LM2734X. The part has the same control and switching frequency, the only difference is smaller power FET (1A rated). Webench will allow you to simulate a load step (only up to 1A) and run a bode plot to get your crossover frequency. I hope that helps get you close.

    Regards,

    Allan