Omnivision patent application US20150015757 "Image sensor pixel cell readout architecture" by Trygve Willassen says "The sum of the floating diffusion voltage swing and pinning voltage typically limits the supply voltage for the active pixel sensor to a minimum of 2.5-3 Volts. However, there is a continuing demand for active pixel sensors with a supply voltage of less than 2.5-3 Volts as the demands for further miniaturization of active pixel sensors increase." So, the proposal is to reduce the FD swing by a capacitive feedback through Cfb 116:
