10.2.2.1 Current Sensing Network

The current sensing network consists of R_CS, R_CSF, C_CSF, and optional R_P. Typically, the direct current sense signal contains a large amplitude leading edge spike associated with the turnon of the main power MOSFET, reverse recovery of the output rectifier, and other factors including charging and discharging of parasitic capacitances. Therefore, C_CSF and R_CSF form a low-pass filter that provides additional immunity beyond the internal blanking time to suppress the leading edge spike. For this converter, C_CSF is chosen to be 270 pF to provide enough filtering.

Without R_P, R_CS 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 R_CS.

The high current sense threshold help to provide better noise immunity but the current sense loss is increased. The current sense loss can be minimized by injecting offset voltage into the current sense signal. R_P and R_CS 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 19.

Equation 19.