A Quasi-Digital Ultra-Fast Capacitor-Less Low-Dropout Regulator Based on Comparator Control for x8 Current Spike of PCRAM Systems

This paper presents a comparator-based Quasi-Digital LDO (QDLDO) for the negative supply which has an ultra-fast transient response for the PCRAM systems. Even under the sharp spike of a load current having a peak value of 80 mA with a short rising time of 2.5 ns, the QDLDO effectively minimized a voltage spike on the output voltage, reducing it to 150 mV within only 40 ns. With I/O transistors to withstand I/O voltage stress, this LDO achieved the bestFOM of 0.042 V/um², among the other state-of-the-art LDOs fabricated in more advanced technologies.

Introduction

Recently, phase change random access memory (PCRAM) has become the most promising candidate to replace flash memory. For a selector, which is part of PCRAM cell, an ovonic threshold switch (OTS) is used, as it has many benefits for increasing the area efficiency and the ratio. However, as shown in Fig. 1, when a PCRAM cell is selected for a read or write operation, a sharp current spike occurs from each selected cells, due to the unique I-V characteristic of OTS [1]. Therefore, the load current of the negative power supply in PCRAM system, where the current of every selected cells are combined, contains a severely sharp current spike that can be around peak of 100 mA with a short rising time of a few ns. Accordingly, a large voltage spike occurs abruptly on the negative power line with a long settling time . Therefore, an ultra-fast capacitor-less low-dropout regulator (LDO) is necessary for a PCRAM system to guarantee its stable operation, which output voltage satisfies and ns.