Ultra-Low Standby Power Embedded SRAM Design Techniques for Smart IoT Applications

In the IoT for multi-sensor node application [1], there are demanded both high-performance for computation in an active mode, and an ultra-low standby power in a sleep mode for longtime buttery life in the energy efficient system. Ultra-low standby MCU with embedded NVM using 90 nm CMOS technology is also reported [2], but its SRAM cache is not sufficient for applying imaging processing which requires large memory capacity with imaging sensors in edge computing. Embedded SRAMs can be achieved fast read/write access within a few nanoseconds, but consume standby leakage power for data retention, being dominant power in the sleep mode. To reduce the standby leakage power of embedded SRAMs, the source bias design technique with power-gating is introduced on a bulk CMOS device [3]. In the resume standby (RS) mode, the VSS source bias control for SRAM array is demonstrated as an effective SRAM leakage power reduction. Besides, once the large SRAM array are into the resume standby mode, it takes long time to wake-up again. It is not applicable for high-speed CPU operation. Then, a fast resume standby mode (FRS) for cache memories is proposed to wake-up as much as fast. In that FRS mode, the power supply for SRAM peripheral is cutoff but SRAM array is keeping with a VDD supply. Each standby mode and its circuit diagram are summarized in Fig. 1. Meanwhile, dopant-less channel fully-depleted (FD) silicon-on-insulator (SOI) device is proposed [4]-[6] to improve the performance, power, and area (PPA) of embedded SRAMs. FD-SOI enables to control the back-bias (BB) dynamically under the buried-oxide (BOX) to reduce leakage current and/or to improve performance by applying reverse or forward bias, respectively. Fig. 2 presents the block diagram of BB control scheme for voltage scaling in IoT application with FD-SOI structure [7]-[9]. BBs for NMOS (VBN) and PMOS (VBP) can be applied individually by VBB generator (VBB Gen.) circuit. Four operation modes for smart IoT applications are considered. High-performance, normal, low-power operating modes are introduced, along with the required loads in the system. The sleep mode with SRAM data retention for long-term waiting in the sensor nodes is also proposed. Each supply voltage and BB is also shown in Fig. 2. The comparison of embedded SRAM leakage power is shown in Fig. 3.