A 2.5μW 0.0067mm2 automatic back-biasing compensation unit achieving 50% leakage reduction in FDSOI 28nm over 0.35-to-1V VDD range

Abstract

Worst-case design and post-silicon tuning are well established digital design practices reducing timing violations in presence of process, temperature, aging and voltage variations, but they suffer from extra power consumption due to overdesign [1]. Adaptive voltage scaling (AVS) [2] and body bias modulation [1] are well-known strategies to dynamically ensure that the digital core can operate at a targeted frequency, even in the presence of delay degradation due to variations. In a multiple voltage islands context, AVS requires many integrated supply generators, such as switched capacitor converters that need to be controlled accurately. Also, for fine-grained compensation, level shifters are required, impacting circuit performance. As FDSOI technology offers the ability to adjust transistor speed through high sensitivity (85mV/VBB) VTH tuning by acting on buried Nwell (NW) and Pwell (PW) voltages, back-biasing generators have been investigated [3-5]. However, they require an external controller to reach the optimal Back Bias (BB) voltages (no self-adjustment) ([3-4] and [5]), imposing a non-negligible area overhead for a sub-mm2 digital core having a narrow compensation range limited to 0.35–0.45V VDD. We therefore propose a variation-aware BB compensation unit (BBC), which dynamically self-adjusts the N- and PMOS transistors' BB voltages to maintain the target frequency with low-latency tuning (100μs) across a wide range of supply voltage (0.35–1V) and temperature (−40–125°C). The low reported area of 0.0067mm2 makes it affordable for a small digital core area (0.1–2mm2). Requiring only a reference frequency signal FTGT, the self-operating BBC exhibits 2.5μW quiescent current without any external components. Compared to a worst-case design strategy, the BBC unit brings up to 50% leakage reduction @0.45VDD, 120°C and reduces the energy per cycle up to 32% compared to worst-case design. By providing continuous BB voltage adjustment (continuous VTH tuning), the target frequency is maintained within ±3.5% accuracy.