SNVSAP9A March 2017 – February 2018 LM25141-Q1
PRODUCTION DATA.
When calculating the current sense resistor, the maximum output current capability (IOUT(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:
where
The RSENSE 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 tdly, the worst case peak current through the inductor with the output shorted can be calculated from Equation 25:
From the Electrical Characteristics, tdly is typically 40 ns.
Once the peak current and the inductance parameters are known, the inductor can be chosen. An inductor with a saturation current greater than IpkSCKT (8.81 Apk) should be selected.