Inductors Allow Loss - Less Current Sensing in Multiphase DC - DC Converters

igh performance microprocessors require a lowvoltage, high-current power supply with fast transient response. Therefore, interleaved multiphase synchronous buck converters have gained popularity as the VRM for these microprocessors. Multiphase converters must share load current to balance thermal stress on each phase, so it’s important to address current sensing and sharing. You can use the current signal in adaptive voltage positioning to reduce the output voltage deviation during transients, which requires accurate current information. Current sense resistors have high accuracy, less than ±5%, and their temperature coeffi cient (TC) is small. However, the sense resistor connection is in the power path, which generates additional power loss and lowers effi ciency—especially in high output current applications . Another disadvantage of the sense resistor is its relatively high cost. Current sensing through MOSFET R DS(on) is a loss-less method, but this on-resistance has a large tolerance from lot to lot even for a given manufacturer and changes with the gate-drive voltage of MOSFET . The TC tolerance of R DS(on) isn’t well defi ned, and is not always available. You can use a dc resistance of an inductor to sense current. The inductor dc resistance changes with temperature, but the TC of copper wire is well defi ned, and independent of the inductor design of the manufacturer. A resistor-capacitor circuit in parallel with the inductor supplies the current information, which is then amplifi ed by a current sense amplifi er. The use of the amplifi ed current signal is not only for the PWM ramp and adaptive voltage positioning, but also for current protection. Fig. 1, on page 60, shows a buck converter with the inductor current sensing circuit, where L is the inductance and R L is the inductor’s dc resistance. In parallel with the inductor is a fi lter consisting of low-power resistor R S and small ceramic capacitor C S . Current sense amplifi er, CSA, amplifi es the voltage across C S , which represents dc and ac components of inductor current. One reference cited a similar scheme [4] to obtain dc current information, mainly for overcurrent protection. It received a patent in 1995, despite a prior art in 1981 [5] and a relevant paper in 1994 . There are other current sensing schemes that utilize inductor dc resistance. Instead of the simple resistor-capacitor fi lter across the inductor, you can use the differential active fi lter [7] or inductor current estimator circuit . As shown in Fig. 1, if the current through the inductor is i L , the voltage across the capacitor C S is: