This design example senses the output current across the DCR of the inductor. When using DCR current sensing, matching the time constant of the sense network with the time constant of the inductor such that the voltage across the isense capacitor replicates the voltage across the inductor is important. Use the following steps to achieve accurate DCR sensing:

1. Use Equation 32 to calculate the sense resistor by matching the inductor time constant to the sensing time constant.
   Equation 32. ${\mathrm{R}}_{\mathrm{c}\mathrm{s}}=\frac{{\mathrm{L}}_{\mathrm{O}}}{{\mathrm{C}}_{\mathrm{C}\mathrm{S}}\times {\mathrm{R}}_{\mathrm{D}\mathrm{c}\mathrm{R}}}=\frac{0.68\mathrm{\mu }\mathrm{H}}{0.1\mathrm{\mu }\mathrm{F}\times 4\mathrm{m}\mathrm{\Omega }}=1.7\mathrm{k}\mathrm{\Omega }$
2. Use Equation 33 to calculate the current limit given the DCR of the inductor.
   Equation 33. ${\mathrm{I}}_{\mathrm{C}\mathrm{L}}=\frac{{\mathrm{V}}_{\mathrm{C}\mathrm{S}-\mathrm{T}\mathrm{H}}}{{\mathrm{R}}_{\mathrm{D}\mathrm{C}\mathrm{R}}}=\frac{60\mathrm{m}\mathrm{V}}{4\mathrm{m}\mathrm{\Omega }}=15\mathrm{A}$

   where

   • V_CS-TH is the 60mV current limit threshold.
3. Place the sense resistor and capacitor close to the inductor.
4. Use Kelvin-sense connections, and route the sense lines differentially from the sense network to the LM25139-Q1.
5. The CS-to-output propagation delay (related to the current limit comparator, internal logic, and power MOSFET gate drivers) causes the peak current to increase above the calculated current limit threshold. For a total propagation delay t_DELAY-ISNS of 70ns, use Equation 34 to calculate the worst-case peak inductor current with the output shorted.
   Equation 34. ${\mathrm{I}}_{\mathrm{L}\mathrm{O}-\mathrm{P}\mathrm{K}\left(\mathrm{S}\mathrm{C}\right)}=\frac{{\mathrm{V}}_{\mathrm{C}\mathrm{S}-\mathrm{T}\mathrm{H}}}{{\mathrm{R}}_{\mathrm{D}\mathrm{C}\mathrm{R}}}+\frac{{\mathrm{V}}_{\mathrm{I}\mathrm{N}\left(\mathrm{m}\mathrm{a}\mathrm{x}\right)}\times {\mathrm{t}}_{\mathrm{D}\mathrm{E}\mathrm{L}\mathrm{A}\mathrm{Y}-\mathrm{I}\mathrm{S}\mathrm{N}\mathrm{S}}}{{\mathrm{L}}_{\mathrm{O}}}=\frac{60\mathrm{m}\mathrm{V}}{4\mathrm{m}\mathrm{\Omega }}+\frac{18\mathrm{V}\times 70\mathrm{n}\mathrm{s}}{0.68\mathrm{\mu }\mathrm{H}}=16.8\mathrm{A}$
6. Based on the result, choose an inductor with a saturation current of 18A or greater across the full operating temperature range.

The DCR current sense configuration is dependent on the inductor DCR, therefore, choosing an inductor with DCR characteristics which align with the desired current limit of the application is important. The saturation current of the inductor also must be greater than the expected current limit.

When using a series sense resistor, use the following procedure instead:

1. Calculate the current-sense resistance based on a maximum peak current capability of at least 25% higher than the peak inductor current at full load to provide sufficient margin during start-up and load-on transients. Use Equation 35 to calculate the current sense resistances.
   Equation 35. ${\mathrm{R}}_{\mathrm{s}}=\frac{V_{CS-TH}}{1.25\times {\mathrm{I}}_{\mathrm{L}\mathrm{O}\left(\mathrm{P}\mathrm{K}\right)}}=\frac{60\mathrm{m}\mathrm{V}}{1.25\times 9.2\mathrm{A}}=5.2\mathrm{m}\mathrm{\Omega }$
2. Select a standard resistance value of 5mΩ for the shunt. An 0508 footprint component with wide aspect ratio termination design provides 1W power rating, low parasitic series inductance, and compact PCB layout. Carefully adhere to the layout guidelines in Section 7.4.1 to make sure that noise and DC errors do not corrupt the differential current-sense voltages measured at the ISNS+ and VOUT pins.
3. Place the shunt resistor close to the inductor.
4. Use Kelvin-sense connections, and route the sense lines differentially from the shunt to the LM25139-Q1.
5. The CS-to-output propagation delay (related to the current limit comparator, internal logic, and power MOSFET gate drivers) causes the peak current to increase above the calculated current limit threshold. For a total propagation delay t_DELAY-ISNS of 70ns, use Equation 36 to calculate the worst-case peak inductor current with the output shorted.
   Equation 36. ${\mathrm{I}}_{\mathrm{L}\mathrm{O}-\mathrm{P}\mathrm{K}\left(\mathrm{S}\mathrm{C}\right)}=\frac{{\mathrm{V}}_{\mathrm{C}\mathrm{S}-\mathrm{T}\mathrm{H}}}{{\mathrm{R}}_{\mathrm{S}}}+\frac{{\mathrm{V}}_{\mathrm{I}\mathrm{N}\left(\mathrm{m}\mathrm{a}\mathrm{x}\right)}\times {\mathrm{t}}_{\mathrm{D}\mathrm{E}\mathrm{L}\mathrm{A}\mathrm{Y}-\mathrm{I}\mathrm{S}\mathrm{N}\mathrm{S}}}{{\mathrm{L}}_{\mathrm{O}}}=\frac{60\mathrm{m}\mathrm{V}}{5\mathrm{m}\mathrm{\Omega }}+\frac{18\mathrm{V}\times 70\mathrm{n}\mathrm{s}}{0.68\mathrm{\mu }\mathrm{H}}=13.8\mathrm{A}$
6. Based on this result, select an inductor with saturation current greater than 16A across the full operating temperature range.