A 4A 12-to-1 Flying Capacitor Cross-Connected DC-DC Converter with Inserted D>0.5 Control Achieving >2x Transient Inductor Current Slew Rate and 0.73× Theoretical Minimum Output Undershoot of DSD

Automotive and industrial applications require a high-efficiency DC-DC converter to directly convert power from the 12V intermediate bus to a low-voltage point-of-load (PoL). The double step-down (DSD) buck converter [1]–[4] (shown in Fig. 18.3.1) is suitable for such applications, where a flying capacitor $C_{\mathrm{F}}$ sustains a half-input-voltage $(V_{\text{IN}}/2)$ stress. Therefore, all the power switches only experience $V_{\text{IN}}/2$ stress except $M_{\mathrm{A}2}$, allowing for exploiting the benefits of low-voltage devices. Two pulse-width modulation (PWM) signals $\phi_{1}$ and $\phi_{2}$ with an equal duty cycle $D$ drive the DSD. A $\text{PoL}$ supply should have a small output capacitor $C_{0}$ if a fast dynamic voltage scaling (DVS) is needed. However, a small $C_{0}$ in the conventional DSD may cause a large output undershoot $V_{\text{US}}$ during a transient event. This comes from the low inductor current slew rate $I_{\mathrm{L}_{-}\text{SR}}=(V_{\text{IN}}/2-2V_{0})/L$, due to the amplitude of the inductor switching nodes $V_{\text{XA}1}$ and $V_{\text{XB}1}$ being reduced to $V_{\text{IN}}/2$ by $C_{\mathrm{F}}$, and the non-overlapping $\phi_{1}$ and $\phi_{2}$ in a conventional $D\leq 0.5$ control. Furthermore, the $D$ should cover a wide range to respond to an integral transient error in the control loop compensator. With $D\leq 0.5$, the DSD converter may fail to cancel the error in time, and the accumulation and release of the error result in overshoot/ringing. This would be more severe at a higher output voltage $V_{0}$ because the steady-state $D$ is closer to 0.5. A possible solution can be to have a DSD converter that works with $D>0.5$. Nevertheless, this leads to an over-sterss on $M_{\mathrm{A}1}$, and imbalance in inductor currents $I_{\text{LA}}$ and $I_{\text{LB}}$, which should be eliminated [3].