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TPS61081: feedforward capacitor

Part Number: TPS61081

the figure 1 is from PDF-SLVA289, a zero and a polar are included when a feedforward capacitor C1 is added across R1.For AC small signal mode,FB is shorted,R2 is not included  in the loop,why the polar in the PDF-SLVA289 has R2?

the figure 2 is from TPS61081 DATASHEET which is an example of PDF-SLVA289,feedforward capacitor C3 is included 

  • Dear Sir,

    R3 can be shorted. R1 and R2 forms a dividing network for the small signal variation. So R2 must be included in the loop.

  • HI,feedforward capacitor and the zero-polar calculation is  usable for every powe IC with different control mode, or just usable for IC that the AC small mode that FB is not virtual gnd

    for example,IC with commom OP AMP  ,FB is  virtual gnd,the down side divide resistant is ignore in AC small mode, I think that the feedforward capacitor and the zero-polar calculation is not usable

  • for example,IC with gm  OP AMP  ,FB is not  virtual gnd,the down side divide resistant  can not be  ignored  in AC small mode, I think that the feedforward capacitor and the zero-polar calculation is usable

  • for example,IC with commom OP AMP  ,FB is  virtual gnd,the down side divide resistant is ignore in AC small mode, I think that the feedforward capacitor and the zero-polar calculation is not usable  

     for example,IC with gm  OP AMP  ,FB is not  virtual gnd,the down side divide resistant  can not be  ignored  in AC small mode, I think that the feedforward capacitor and the zero-polar calculation is usable,

    in my opinion TPS61081‘s error AMP maybe  gm  OP AMP,although the datasheet is unspecified。

    I doudt that feedforward capacitor and the zero-polar calculation is also usable to IC with commom OP AMP  with FB is  virtual gnd in AC small mode

  • Dear Zeng,

    No matter what kind of AMP they are , the zero and the pole need to be accounted. it has nothing to do with the FB. it is a dividing network.

    FB short to GND is for the EA stage deduce, not for the dividing network.

  • Thanks for your answer。but in TI document slva662,the lower resistor R4 is ignored with FB is vitual gnd。IN your opinoin,the R4 is the dividing network ,why the R4 can ignore? IN the figure upstraight,If I add a forwordfeed capacitor aside R1, the R4 is also ignored。This just changes the compensate type ,the polar and zero do not include R4。The information are all from your document,the calculation is different。

  • Dear Sir,

    You're right, for the gm amp, the dividing network should be accounted, and for the traditional op-amp in your configuration, low side res doesn't need to be accounted. But for the traditional op-amp, if there's a resistor between the amp's '-' and the connection point of R1 and R4, then R4 also need to be considered.

    For the TPS61081, it use  a gm op-amp inside.