A Fast-Transient Switched-Capacitor DC-DC Converter with a Current Sensing Control Technique

A low-power switched-capacitor step-down DC-DC converter using the proposed current sensing control technique to enhance the transient response is presented in this paper. By sensing the current outflow from the DC-DC converter and the load current simultaneously, the proposed technique scales the switching frequency (Fsw) accordingly from the very beginning of the load or the line transient. It leads to better transient response without depending on the output capacitance. It is suitable for sensor applications which usually use an LDO with a large output capacitor, resulting in lower efficiency. The converter is implemented in a 0.25-μm CMOS process with integrated flying capacitors and generates a fixed regulated output voltage of 1.8V from an input voltage ranging from 3.8V to 5.4V. From the measurement results, the transient response time is about 0.3μs after a step load current from 20μA to 2.5mA which is sufficient for low-power sensor applications. Its power efficiency (η) is higher than 70% over a wide range of the load current with a peak value of 72.8%.

I. Introduction

In modern mixed-signal SoCs [1], the analog and digital modules are designed to consume extremely low power (<10mW) to prolong the life of the battery. As shown in Fig. 1 [2], the SoC is powered by one or two buck/boost converters in the PMIC but each module in the SoC requires its own supply voltage. Consequently, a large number of on-chip linear regulators have conventionally been used as on-chip DC-DC converters to support the local supplies voltage from the global supplies [3]. However, as the voltage drops between the global power supplies and the local supplies increase, the collective power loss from the linear regulators becomes significant. A fully integrated switched-capacitor DC-DC converter is a promising alternative to an on-chip linear regulator due to its higher power efficiency (η) with less area/cost for low-power applications.