The current sense resistor is selected to avoid triggering peak current limit protection when the minimum supply voltage is present, V_{SUPPLY_min}, and supplying the maximum output power, P_{OUT_total}. Due to component tolerances and inefficiencies of the regulator, the peak current limit is set some margin above the calculated peak current of the transformer primary winding. A margin of 20% to 30% (M_{I_LIMIT}=0.2-0.3) is a good starting point. Equation 9 is used to calculate the desired peak switch current limit value. In this design example, M_{I_LIMIT} is selected to be 30%.

Equation 9.

Selecting the correct current sense resistor is an iterative process. The first step is calculating the maximum current sense resistor value, assuming that no external slope compensation is required (R_SL = 0Ω). The maximum current sense resistor value is selected using Equation 10.

Equation 10.

where

• V_SL is the internal fixed internal slope compensation of the LM5155

Assuming that no external slope compensation is required, the current sense resistor value is calculated using Equation 11.

Equation 11.

where

• V_CLTH is the current limit threshold of the LM5155

If the calculated R_{S_wo_sl} resistance value is less than the R_{S_MAX} resistance value, then R_{S_wo_sl} is selected for the current sense resistor value (R_S). If the calculated R_{S_wo_sl} resistance value is greater than the calculated R_{S_MAX} resistance value, there are two approaches to take; decrease the current sense resistor value or add external slope compensation.

• Decreasing the current sense resistor value increases the effectiveness of the internal slope compensation. With no external slope compensation the peak inductor current limit will be constant regardless of the duty cycle when using the LM5155. A lower current sense resistor value results in a larger switch peak current limit value, which increases the required saturation current rating of the primary winding.
• Adding external slope compensation. The peak inductor current limit varies with supply voltage when external slope compensation is added to the current sense network.

External slope compensation is added by setting R_SL to a non-zero value, but less than 1kΩ. In applications where external slope compensation is required, R_S is calculated using Equation 12.

Equation 12.

R_SL is calculated using Equation 13.

Equation 13.

where

• I_SLOPE is the slope compensation current source of the LM5155
• D is the duty cycle at the minimum supply voltage

If the calculated R_SL value is negative, the internal slope compensation is adequate and the additional slope compensation is not required. If the calculated R_SL value exceeds the maximum value of the 1kΩ, the down slope of the sensed current needs to be reduce. To reduce the down slope of the primary winding current, the primary winding inductance value of L_M must be increased. If the primary winding inductance value is changed the current sense resistor calculations must be redone.

Following the design procedure, a current sense resistor value is selected to be 20mΩ (R_S), which is the nearest standard resistor value to the calculated value in Equation 14. No external slope compensation is required and R_SL is selected to be 0Ω. The peak current limit of the transformer primary winding is calculated using Equation 14.

Equation 14.

where

• D is they duty cycle at the minimum supply voltage

The peak current limit of the transformer primary winding is constant, regardless of the supply voltage, because there is no added external slope compensation. The saturation current rating of the transformer primary winding is 6A, and is adequate for the selected R_S value of 20mΩ.