A high-efficiency and fast-transient digital-low-dropout regulator with the burst mode corresponding to the power-saving modes of DC-DC switching converters

摘要

Integrated power management (PM) in a system-on-a-chip (SoC) includes a high-efficiency DC-DC switching regulator (SWR) for a high conversion ratio and multiple cascaded digital-low-dropout (DLDO) regulators for post regulation to different functional blocks. Efficient power-saving modes in the SWRs improve the light-load efficiency effectively, e.g. burst mode, skip mode, pulse frequency mode (PFM), and diode emulation mode (DEM) in the constant on-time (COT). Unfortunately, a cascaded DLDO consumes significant power to suppress a large voltage ripple DV_SWR from the SWR, even using recent DLDO techniques [1-6], illustrated on the left of Fig. 18.8.1. Overall, the light-load efficiency of the PM seriously decreases. A DLDo with barrel-shifter-based control [1] induces large voltage ripple due to the limiting cycle oscillation (LCO), and the DLDO with freeze mode [2] for power reduction is exposed to large voltage ripples from the SWRs (in power saving modes). The large voltage ripples result in the DLDO frequently switching between the normal and freeze modes and consuming power. The recursive all-digital LDO (RLDO) [3] abruptly changes its control code Q[6:0] due to oscillations between hybrid proportional-derivative successive-approximation recursive (PD-SAR) and pulse width modulation (PWM) duty control, while aV_SWR is larger than the pre-defined hysteretic window. In power saving modes, the obvious disadvantage is that state-of-the-art DLDO designs cause extra switching loss and induce large output voltage ripple aV_OUT due to large aV_SWR. Thus, this paper proposes a DLDO employing a burst mode technique (BMT) to reduce the DV_OUT, thereby enhancing the overall light-load efficiency corresponding to the power saving modes in SWRs. The proposed non-linear switch control (NLSC) technique reduces both the number of on/off power switches and varies the switching frequency corresponding to the aV_SWR. Moreover, the proposed transient enhance (TE) technique improves transient performance when the DLDO leaves burst mode.