loop frequency compensation

Discussion in 'The Projects Forum' started by Shaggy, Jul 24, 2007.

  1. Shaggy

    Thread Starter New Member

    Jul 24, 2007
    3
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    I'm working on a project that needs a 3.3V power rail and a 1.2V power rail. I want to use a dual switching regulator to provide these voltages, specifically the LT3508 from Linear Technology but I'm stumped on how to design the RC networks for the Vc pins. I understand that this circuit is supposed to provide "loop frequency compensation" and stabilize the regulator somehow but I don't know anything about loop frequency compensation. Is this similar to feedback on Op-Amps? Can anybody point me to some good reference material on this?
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,163
    1,796
    I would start with the datasheet and any application notes from Linear Technology. Have you looked at these?
     
  3. Shaggy

    Thread Starter New Member

    Jul 24, 2007
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    0
    I have the datasheet right here but I'm having trouble understanding it. It gives a few examples of things like "Gain rolls off smoothly above the 100Hz pole frequency set by the 100uF output capacitor" but it doesn't provide any formulae and I don't understand how the capacitor makes a pole at that frequency.
     
  4. a_kent

    Active Member

    Jun 12, 2007
    30
    0
    Usually with more esoteric switchers, especially LT, I stick as closely as I can to the parts they recommend, as well as the layout they recommend. Especially the layout.

    Bottom line is that I believe they don't want you knowing what's in the chip!

    I have been bitten in the butt from time to time trying to reverse engineer what these things do, trying to make it 'better'.

    I have on occasion beat the efficiency specs with some by subbing parts here and there, but not often with the more modern switchers, and quite honestly never with LT's stuff...

    For instance, with many switchers, you can almost always sub a lower ESR cap and get better results, but NOT ALWAYS!! You can almost always sub a lower DC inductor and get better results, but NOT ALWAYS!! Same with the schottky diodes, etc...
    I think sometimes the silicon compensates for funky parts and then doesn't work as well with good parts...

    Best,
    Kent
     
  5. a_kent

    Active Member

    Jun 12, 2007
    30
    0
    I just looked at the datasheet and you use the voltage divider inputs to the FBx pins to set the output voltage.

    This part is very nice! They give you a wide range of values for the discreets. I have saved this datasheet. I had not looked at it before.

    So, look at the pages pertaining to the output voltage and the FBx pin of interest.

    Look at page 9 of the datasheet. There is the formula for setting the output voltage.

    Best,
    Kent
     
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