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

Background capacitor-current-sensor (CCS) calibration (CAL) is proposed for dc–dc converters to achieve fast load-transient response for small undershoot
$V_{\mathrm {US}}$
, overshoot
$V_{\mathrm {OS}}$
, and short settling time
$t_{\mathrm {S}}$
in the output voltage
$V_{\mathrm {O}}$
. The CCS’ impedance
$Z_{\mathrm {Cs}}$
is calibrated to a scaled replica of the output capacitor’s impedance
$Z_{\mathrm {Co}}$
, which varies with printed-circuit-board parasitics, process-voltage-temperature variations, and aging. Thus, the CCS shunts a current
$I_{\mathrm {Cs}}$
equal to a scaled output-capacitor current
$I_{\mathrm {Co}}$
, which instantly and accurately reflects load-current
$I_{\mathrm {load}}$
transients. The CAL operates at a fixed switching frequency
$f_{\mathrm {SW}}$
. At this
$f_{\mathrm {SW}}$
, although
$Z_{\mathrm {Cs}}$
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-
$f_{\mathrm {SW}}$
CAL overcomes the bottleneck of the prior state-of-the-art CCS CAL, which must hop
$f_{\mathrm {SW}}$
to lower values to calibrate
$Z_{\mathrm {Cs}}$
capacitive and resistive parts, thereby enlarging
$V_{\mathrm {O}}$
steady-state ripples and transient fluctuations during CAL. Thus, only this fixed-
$f_{\mathrm {SW}}$
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
$\mu \text{s}$
at
$f_{\mathrm {SW}} = 30$
MHz without interruption. A buck converter with this CCS CAL is fabricated in 0.18-
$\mu \text{m}$
CMOS process and occupies 2.25 mm
²
. Under a 2-A/4-ns step-up (step-down) load transient, the measured
$V_{\mathrm {US}}$
(
$V_{\mathrm {OS}}$
) and
$t_{\mathrm {S}}$
are 36 mV (29 mV) and 56 ns (45 ns), respectively. Compared with prior state of the arts, this work achieves much smaller
$V_{\mathrm {O}}$
ripples and fluctuations during CAL, shorter CAL period, and faster load-transient response.