TPS568231: ripple injection zero

Fred Cheng 

See section 7.3.1 PWM Operation and D-CAP3 Control Mode of the datasheet.

Fred Cheng 

They are not based on a specific equation.  They are recommendations based on empirical measurements and design evaluations from the development of the TPS568231.

Fred Cheng 

Please follow the recommended output capacitor ranges provided in the datasheet.

As a general rule with D-CAP3 control, I recommend avoiding setting the L-C resonance double pole lower than 1/2 of the internal zero frequency.  Setting the L-C resonance pole less than 1/2 of the internal zero frequency can result in low frequency oscillations in the output voltage during Soft-Start.

Setting the output capacitance lower than the recommended minimum can result in the frequency modulation operating between two frequencies, one lower than the nominal and one higher., resulting in lower efficiency and higher output ripple.

Fred Cheng  

When the LC pole frequency is too low, the loop phase margin can drop near zero before compensation zero increases the phase.  This is not classic instability because the loop still has high gain at this low phase point, and the phase rises before the loop gain falls to unity.  In this case, the loop is stable under small signal operating conditions, but can become unstable when higher gain is required of the loop at lower frequencies, such as when the output voltage is tracking a moving reference voltage during soft-start.

Fred Cheng 

It comes from experience working with a large number of D-CAP2 and D-CAP3 designs.  I am not aware of a specific document on the topic.

Fred Cheng 

The minimum output capacitance is a factor of the ramp generation time constant, which has not been provided, rather than the internal zero, the inductor value, the feedback divider attenuation, and the switching frequency.

Cout > 2 / ( π * Fsw * Vout/Vref * L / τ ) 

Without enough capacitance, the output ripple is out of phase with the switching and the switching frequency will not be stable, increasing output ripple voltage.

If a feed-forward capacitor is placed across the high-side feedback resistor, the Vout/Vref factor can be reduced, improving transient response but also increasing the minimum required capacitance.

Fred Cheng 

I do not have Tc or Acp immediately available.  For the TPS568231, TPS536211, and TPS563219.