Digital low-dropout voltage regulators (DLDOs) have drawn increasing attention for the easy implementation within nanoscale devices. Despite their various benefits over analog LDOs, disadvantages may arise in the form of bias temperature instability (BTI) induced performance degradation. In this Chapter, conventional DLDO operation and BTI effects are explained. Reliability enhanced DLDO topologies with performance improvement for both steady-state and transient operations are discussed. DLDOs with adaptive gain scaling (AGS) technique, where the number of power transistors that are turned on/off per clock cycle changes dynamically according to load current conditions, have not been explored in view of reliability concerns. As the benefits of AGS technique can be promising regarding DLDO transient performance improvement, a simple and effective reliability aware AGS technique with a steady-state capture feature is proposed in this work. AGS senses the steady-state output of a DLDO and reduces the gain to the minimum value to obtain a stable output voltage. Moreover, a novel unidirectional barrel shifter is proposed to reduce the aging effect of the DLDO. This unidirectional barrel shifter evenly distributes the load among DLDO output stages to obtain a longer lifetime. The benefits of the proposed techniques are explored and highlighted through extensive simulations. The proposed techniques also have negligible power and area overhead. NBTI-aware design with AGS can reduce the transient response time by 59.5% as compared to aging unaware conventional DLDO and mitigate the aging effect by up to 33%.
A design space exploration of the countermeasures for hardware masking is proposed in this paper. The assumption of independence among shares used in hardware masking can be violated in practical designs. Recently, the security impact of noise coupling among multiple masking shares has been demonstrated both in practical FPGA implementations and with extensive transistor level simulations. Due to the highly sophisticated interactions in modern VLSI circuits, the interactions among multiple masking shares are quite challenging to model and thus information leakage from one share to another through noise coupling is difficult to mitigate. In this paper, the implications of utilizing on-chip voltage regulators to minimize the coupling among multiple masking shares through a shared power delivery network (PDN) are investigated. Specifically, different voltage regulator configurations where the power is delivered to different shares through various configurations are investigated. The placement of a voltage regulator relative to the masking shares is demonstrated to a have a significant impact on the coupling between masking shares. A PDN consisting of two shares is simulated with an ideal voltage regulator, strong DLDO, normal DLDO, weak DLDO, two DLDOs, and two DLDOs with 180∘ phase shift. An 18 × 18 grid PDN with a normal DLDO is simulated to demonstrate the effect of PDN impedance on security. The security analysis is performed using correlation and t-test analyses where a low correlation between shares can be inferred as security improvement and a t-test value below 4.5 means that the shares have negligible coupling, and thus the proposed method is secure. In certain cases, the proposed techniques achieve up to an 80% reduction in the correlation between masking shares. The PDN with two DLDOs and two-phase DLDO with 180∘ phase shift achieve satisfactory security levels since t-test values remain under 4.5 with 100,000 traces of simulations. The security of the PDN improves if DLDO is placed closer to any one of the masking shares.
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