The Page 15 Section title "Loop Compensation" discusses the generic modulator gain (Amod) value as the change in output voltage (dVout) per change in control voltage (dVc) As VIN / Vs
In this equation Vs is the "full period ramp amplitude" - That is the voltage amplitude the ramp would have if it continued for the full switching period D = 1. It is normally just called the Ramp Voltage or Vramp, but in the case of the TPS40055 and TPS40057 controller, there is a difference.
While the amplitude of the ramp voltage is clamped to 2V, it does not always take a full switching cycle to achieve a 2V amplitude due to the voltage feedforward cycle.
For basic estimates, the ramp voltage is 2V at programmable UVLO (VIN min from the RKFF equation) and incerases with increased input voltage for a Vs = 2.0V * (VIN / VIN(min))
For a more accurate calculation, you'll need to calculate the IKFF current at UVLO and the IKFF current at your operating point and the equation becomes Vs = 2.0V * (IKFF / IKFF(uvlo))
