12.1 A 0.518mm2 quasi-current-mode hysteretic buck DC-DC converter with 3μs load transient response in 0.35μm BCDMOS

As more features have been added to mobile devices, it has become necessary to integrate more DC-DC converters into the power-management IC. Consequently, there is a growing need for an area-efficient and simple controller design for DC-DC converters. A simple hysteretic control without any additional component for compensation is a very attractive solution because it is not only cost-effective, but also immediately responds to load change. However, a conventional current-mode hysteretic controller with low-ESR output capacitor, shown in Fig. 12.1.1, has an inherent trade-off between transient response and RC network time constant for emulating inductor current [1]. For instance, at a switching frequency less than several MHz, which is widely used in industry, the RC network occupies a relatively large die area because the capacitance C_SEN of the network can be up to 100pF at a switching frequency of 1 MHz. Another issue with the hysteretic converter is its variable switching frequency, which leads to difficulty in designing an EMI filter [2]. To overcome these limitations, several state-of-the-art hysteretic converters have been reported that provide fast transient response and fixed switching frequency [2-5]. However, they suffer from noise due to the differentiator for amplifying ripple voltage [2], or need a large external or internal capacitor [3, 4], or generate high switching loss in the converter [5]. In this paper, we present an area-efficient quasi-current-mode hysteretic buck converter with fixed switching frequency. By employing a quasi inductor current emulator (QICE) with reset operation, the controller only uses a total internal capacitance of 3pF and provides fast transient response.