Q: TPS62050 - Cff Capacitor, how to optimize for transient response. by 35669

 How do we select the optimum feed forward cap to minimize output ripple when the device is operating in SKIP mode? How much does the feed forward cap help the transient behavior of this converter when the load state changes from a sleep to full power state?

 


A: Re: TSP62050 - Cff capacitor by 10449

The Cff  (feedforward) capacitor has several purposes.

 1.  It helps reduce the effects of parasitic capacitance from the FB pin to the PWB.  With very high feedback resistor values, stray capacitance to the board becomes a problem.  With lower resistor values, there is not a problem.  The stray capacitance from FB to ground creates a pole in the feedback loop. The pole frequency is 1/(Rbottom*Cstray*2*pi). This reduces phase margin.  The Cff cap creates a zero to offset the pole.  Choosing a Cff value equal to Rbottom/Rtop*Cstray sets the pole frequency equal to the zero frequency and effectively cancels the stray capacitance with not effect on gain or phase.

2.  A larger Cff cap creates a zero in the feedback loop which increase gain and phase. This can provide slightly better tranisent response.  This is the recommended configuration shown with most of the TPS62xxx converters.  A typical value is given in the datasheet.

3.  A Cff cap plus a cap from FB to ground can be used to shunt noise away from the FB pin.  This provides better IC performance.  With large feedback resistor values, noise is more of a concern.  With  smaller values, the two capacitors can be omitted.  The TPS62040 provides details on setting the capacitor values for noise reduction. 

4.  The Cff capacitor is effective in reducing output voltage ripple in PFM mode.  The Cff cap injects the output voltage ripple into the FB pin which allows the converter to quickly see changes in output voltage.

 I don't have a specific equation for selecting the optimum Cff cap in PFM mode. Larger is better, but above a certain value, there is no additional benefit.  When Cff capacitor is large enough that the pole created by the two resistors and two capacitors is below the frequencies of the tranient, then no additional benefit is achieved.

The following app note shows how to minimize ripple with the TPS61070.  http://focus.ti.com/lit/an/slva213/slva213.pdf

The following app note shows how to optimize transient response with the Cff cap and the TPS61070.  http://focus.ti.com/lit/an/slva289/slva289.pdf