A 65 nm Sub- Microcontroller With Integrated SRAM and Switched Capacitor DC-DC Converter

Aggressive supply voltage scaling to below the device threshold voltage provides significant energy and leakage power reduction in logic and SRAM circuits. Consequently, it is a compelling strategy for energy-constrained systems with relaxed performance requirements. However, effects of process variation become more prominent at low voltages, particularly in deeply scaled technologies. This paper presents a 65 nm system-on-a-chip which demonstrates techniques to mitigate variation, enabling sub-threshold operation down to 300 mV. A 16-bit microcontroller core is designed with a custom sub-threshold cell library and timing methodology to address output voltage failures and propagation delays in logic gates. A 128 kb SRAM employs an 8 T bit-cell to ensure read stability, and peripheral assist circuitry to allow subreading and writing. The logic and SRAM function in the range of 300 mV to 600 mV, consume 27.2 pJ/cycle at the optimal of 500 mV, and 1 W standby power at 300 mV. To supply variable voltages at these low power levels, a switched capacitor DC-DC converter is integrated on-chip and achieves above 75% efficiency while delivering between 10 W to 250 W of load power.