The bqTesla100lp receiver (HPA687) will change the input voltage to the bq25046 depending on the output current. In the EVM's user guide in figure 5, the load will increase from 0mA to 500mA and the voltage at TP4 will change from 8V to 5.1V. Why not just set the voltage to 5.1V at all times?
The operating point is set high to better support a load step without having output voltage drop below 5V. In UG figure 5, the input voltage does drop during the load step but stays above 5V. This is independent of control loop and capacitor bank, related to power stage impedance. Control loop for the system is very slow and will not help with a transient, the voltage deceases more as the control loop adjusts voltage down.
Best Regards,
Bill Johns, Applications Engineer
bqTESLA Wireless Power Products
Texas Instruments Inc
Dallas, Texas