A Replica Technique for Wordline and Sense Control in Low-Power SRAM’s

With the migration toward low supply voltages in low-power SRAM designs, threshold and supply voltage fluctuations will begin to have larger impacts on the speed and power specifications of SRAM’s. We present techniques based on replica circuits which minimize the effect of operating conditions’ variability on the speed and power. Replica memory cells and bitlines are used to create a reference signal whose delay tracks that of the bitlines. This signal is used to generate the sense clock with minimal slack time and control wordline pulsewidths to limit bitline swings. We implemented the circuits for two variants of the technique, one using bitline capacitance ratioing in a 1.2m 8-kbyte SRAM, and the other using cell current ratioing in a 0.35m 2-kbyte SRAM. Both the RAM’s were measured to operate over a wide range of supply voltages, with the latter dissipating 3.6 mW at 150 MHz at 1 V and 5.2 W at 980 kHz at 0.4 V.