The current sensing network consists of the primary-side current sensing resistor (R_CS), filtering components R_CSF and 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 immunity to suppress the leading edge spike. For this converter, C_CSF is chosen to be 100 pF.

Without R_P, R_CS sets the maximum peak current in the transformer primary based on the maximum amplitude of the CS pin, which is specified to be 1 V. To achieve 1.36-A primary side peak current, a 0.75-Ω resistor is chosen for R_CS.

The high current sense threshold of CS helps to provide better noise immunity to the system but also results in higher losses in the current sense resistor. These current sense losses can be minimized by injecting an offset voltage into the current sense signal using R_P. R_P and R_CSF form a resistor divider network from the current sense signal to the device’s reference voltage (V_VREF) which adds an offset to the current sense voltage. This technique still achieves current mode control with cycle-by-cycle over-current protection. To calculate required offset value (V_OFFSET), use Equation 16.

Equation 16.

Once R_P is added, adjust the R_CS accordingly.