12.6 Capacitor-current-sensor calibration technique and application in a 4-phase buck converter with load-transient optimization

For switching DC-DC converters, a large and rapid load-current transient ΔI
_load
causes a large output voltage undershoot ΔV
_US
and long settling time ts if the transient responses are slow [1]. Since the output capacitor current I
_Co
instantly reflects ΔI
_load
, transient response optimization for minimizing ΔV
_US
and ts with given charging and discharging slopes of inductor current ILo can be achieved by obtaining an accurate I
_Co
and well-controlling the I
_Lo
charging time tch and discharging time tdch [1]. However, ICo sensing accuracy is degraded if the output capacitor's impedance Z
_Co
, comprising the capacitance CO, equivalent series resistance RESR and inductance LESL, varies with different output voltage V
_O
, process variation, and/or PCB parasitics, leading to non-minimized ΔV
_US
and ts. An off-chip capacitor-current-sensor (CCS) calibration technique is reported in [2]; however, LESL is not considered, so the high-frequency information of ΔI
_load
is inaccurately sensed when large and rapid ΔI
_load
occurs. On the other hand, to well-control the tch and tdch of I
_Lo
, the transient-optimized feedback circuit reported in [1] minimizes ΔV
_US
and t
_s
. However, optimization can be achieved only under fixed C
_O
, output inductor L
_O
, input voltage V
_IN
, and V
_O
.