A Fast-transient Buck Converter with One-Cycle-Balancing Control for Single and Consecutive Load Steps

To power large-scale digital circuits such as FPGAs and microprocessors, fast transient buck converters are crucial for maintaining energy efficiency and load performance [1]–[6]. One-cycle charge balance (OCB), minimizing the undershoot/overshoot (US/OS) voltage $\text{(Vus/Vos)}$ and their recovery time $(t_{\mathrm{R}})$, is the optimal transient response of a buck converter. [7]–[9] utilized capacitor current sensor (CCS) and computational method to achieve OCB, but need complicated calibrations on 3 parameters, assisted by accurate inductor current $(l_{\mathrm{L}})$ sensors. [10] introduced an emulated-curve method that realizes a cost-effective computational OCB control. Fig. 1 explains its US OCB control: (1) After quickly detecting the transient event at $t_{o}$, the controller turns on the high-side power switch to raise $l_{L}$. (2) $l_{L}$ intersects the load current $/\mathrm{L}0$ at $t_{1}$. [10] proved that for any $/\text{LD}$ step, the emulated curve (EC, the expression for US given in eq. 1, starting from VREF) always intersects Vo's valley at $t_{1}. \Delta \mathrm{Q}=\mathrm{Q}_{1}$ is the charge loss in the output capacitor Co, represented by the valley value $\Delta V_0=Q_1 / C_0$. (3) Based on $\Delta \mathrm{O}$, the OCB controller calculates the high-side switch turn-off time instant $t_{2}$, and the OCB control end time $t_{3}$ for Co charge balance $\mathrm{Q}_{2}=\mathrm{Q}_{1}$. The OS OCB control is almost the same, except changing the EC expression for OS. This method is insensitive to the variations of Co's equivalent series inductance (ESL) and resistance (ESR), requiring only 1-parameter calibration (the $\text{Co}^{\mathrm{x}}L\mathrm{c}$ term in eq. 1).