7.3.9 Current Sensing

As mentioned, the LM27402 implements a lossless inductor DCR lossless current sense scheme designed to provide both accurate overload (current limit) and short-circuit protection. Figure 22 shows the popular inductor DCR current sense method. Figure 23 shows an implementation with current shunt resistor, R_ISNS.

Components R_S and C_S in Figure 22 create a low-pass filter across the inductor to enable differential sensing of the inductor DCR voltage drop. When R_SC_S is equal to L/R_DCR, the voltage developed across the sense capacitor, C_S, is a replica of the voltage waveform of the inductor DCR. Choose the capacitance of C_S greater than 0.1 µF to maintain low impedance of the sense network, thus reducing the susceptibility of noise pickup from the switch node.

Figure 22. Current Sensing Using Inductor DCR

Figure 23. Current Sensing Using Shunt Resistor

Note that the inductor DCR is shown schematically as a discrete element in Figure 22. With power inductors selected to provide lowest possible DCR to minimize power losses, the typical DCR ranges from 0.4 mΩ to
4 mΩ. Then, given a load current of 25 A, the voltage presented across the CS+ and CS– pins ranges between 10 mV and 100 mV.

A current sense (or current shunt) resistor in series with the inductor can also be implemented at lower output current levels to provide accurate overcurrent protection, see Figure 23. Burdened by the unavoidable efficiency penalty and/or additional cost implications, this configuration is not usually implemented in high-current applications (except where OCP setpoint accuracy and stability over the operating temperature range are critical specifications).