8.2.2.3 Current Sense Resistor

When calculating the current sense resistor, the maximum output current capability (I_OUT(MAX)) should be at least 20% higher than the required full load current to account for tolerances, ripple current, and load transients. For this example, 120% of the 6.41-A peak inductor current calculated in the previous section (Ipk) is 7.69 A. The current sense resistor value can be calculated using Equation 23:

Equation 23.

Equation 24.

where

• V_(CS) is the 75 mV current limit threshold

The R_SENSE value selected is 9 mΩ

Carefully observe the PCB layout guidelines to ensure that noise and DC errors do not corrupt the differential current sense signals between the CS and VOUT pins. Place the sense resistor close to the devices with short, direct traces, creating Kelvin-sense connections between the current-sense resistor and the LM25141-Q1.

The propagation delays through the current limit comparator, logic, and external MOSFET gate drivers allow the peak current to increase above the calculated current limit threshold. For a propagation delay of t_dly, the worst case peak current through the inductor with the output shorted can be calculated from Equation 25:

Equation 25.

From the Electrical Characteristics, t_dly is typically 40 ns.

Equation 26.

Once the peak current and the inductance parameters are known, the inductor can be chosen. An inductor with a saturation current greater than Ipk_SCKT (8.81 Apk) should be selected.