7.3.10 DCR Current Sensing

For high-power applications which do not require high accuracy current-limit protection, DCR sensing may be preferable. This technique provides lossless and continuous monitoring of the output current using an RC sense network in parallel with the inductor. Using an inductor with a low DCR tolerance, the user can achieve a typical current limit accuracy within the range of ±10% to ±15% at room temperature.

Components R_CS and C_CS in Figure 25 create a low-pass filter across the inductor to enable differential sensing of the voltage drop across inductor DCR. When R_CS × C_CS is equal to L_OUT/R_DCR, the voltage developed across the sense capacitor, C_CS, is a replica of the inductor DCR voltage waveform. Choose the capacitance of C_CS to be greater than 0.1 μF to maintain a low impedance sensing network, thus reducing the susceptibility of noise pickup from the switch node. Carefully observe the PCB layout guidelines to ensure the noise and DC errors do not corrupt the differential current-sense signals applied across the CS and VOUT pins.

The voltage drop across C_CS:

Equation 12.

Figure 25. DCR Current Sensing

R_CSC_CS = L_OUT/R_DCR → accurate DC and AC current sensing

If the RC time constant is not equal to the L_OUT/L_DRC time constant there will be an error

R_CSC_CS> L_OUT/R_DCR → DC level still correct, the AC amplitude will be attenuated

R_CSC_CS< L_OUT/R_DCR→ DC level still correct, the AC amplitude will be amplified