Fixed-Switching-Frequency Background Capacitor-Current-Sensor Calibration for DC–DC Converters

Abstract

Background capacitor-current-sensor (CCS) calibration (CAL) is proposed for dc–dc converters to achieve fast load-transient response for small undershoot V US, overshoot VOS, and short settling time tS in the output voltage VO. The CCS’ impedance ZCs is calibrated to a scaled replica of the output capacitor’s impedance ZCo, which varies with printed-circuit-board parasitics, process-voltage-temperature variations, and aging. Thus, the CCS shunts a current I Cs equal to a scaled output-capacitor current ICo,w h i c hi n s t a n t l y and accurately reflects load-current Iload transients. The CAL operates at a fixed switching frequency fSW.A tt h i s fSW, although ZCs is dominated by its inductive part, this work can still calibrate its capacitive and resistive parts based on the time derivatives dV O/dt and dICs/dt, respectively. This fixed- fSW CAL overcomes the bottleneck of th e prior state-of-the-art CCS CAL, which must hop fSW to lower values to calibrate ZCs capacitive and resistive parts, thereby enlarging VO steady-state ripples and transient fluctuations during CAL. Thus, only this fixed- f SW CAL is suitable for background operation. Moreover, this CAL can be interrupted by a load-transient or dynamic- voltage-scaling event and be reactivated in quasi-steady state. The shortest CAL period is 8.8 µsa t f SW = 30 MHz without interruption. A buck converter with this CCS CAL is fabricated in 0.18- µm CMOS process and occupies 2.25 mm 2. Under a 2-A/4-ns step-up (step-down) lo