The current sensing network consists of RCS, RCSF, CCSF, and optional RP. Typically, the direct current sense signal contains a large-amplitude leading-edge spike associated with the turn-on of the main power MOSFET, reverse recovery of the output rectifier, and other factors including charging and discharging of parasitic capacitances. Therefore, CCSF and RCSF form a low-pass filter that provides additional immunity beyond the internal blanking time to suppress the leading edge spike. For this converter, CCSF is chosen to be 270 pF to provide enough filtering.
Without RP, RCS sets the maximum peak current in the transformer primary based on the maximum amplitude of CS pin, 1 V. To achieve 1.425-A primary side peak current, a 0.75-Ω resistor is chosen for RCS.
The high current-sense threshold helps to provide better noise immunity but the current-sense loss is increased. The current-sense loss can be minimized by injecting an offset voltage into the current-sense signal. RP and RCSF form a resistor-divider network from the current-sense signal to the device’s reference voltage to offset the current-sense voltage. This technique still achieves current-mode control with cycle-by-cycle overcurrent protection. To calculate required offset value (Voffset), use Equation 16.