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 ${V_{{OS}}}$ , and short settling time ${t_{{S}}}$ in the output voltage ${V_{{O}}}$ . The CCS' impedance ${Z_{{Cs}}}$ is calibrated to a scaled replica of the output capacitor's impedance ${Z_{{Co}}}$ , 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 ${I_{{Co}}}$ , which instantly and accurately reflects load-current ${I_{{load}}}$ transients. The CAL operates at a fixed switching frequency ${f_{{SW}}}.$ At this ${f_{{SW}}}$ , although ${Z_{{Cs}}}$ 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 dI ${_{{Cs}}}$ /dt, respectively. This fixed- ${f_{{SW}}}$ CAL overcomes the bottleneck of the prior state-of-the-art CCS CAL, which must hop ${f_{{SW}}}$ to lower values to calibrate ${Z_{{Cs}}}$ capacitive and resistive parts, thereby enlarging ${V_{{O}}}$ 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 μ s at ${f_{{SW}}$ = 30 MHz without interruption. A buck converter with this CCS CAL is fabricated in 0.18-μ m CMOS process and occupies 2.25 mm². Under a 2-A/4-ns step-up (step-down) load transient, the measured ${V_{{US}}}$ ( ${V_{{OS}})}$ and ${t_{{S}}}$ are 36 mV (29 mV) and 56 ns (45 ns), respectively. Compared with prior state of the arts, this work achieves much smaller ${V_{{O}}}$ ripples and fluctuations during CAL, shorter CAL period, and faster load-transient response.