8.5 A Fully Integrated Voltage Regulator in 14nm CMOS with Package-Embedded Air-Core Inductor Featuring Self-Trimmed, Digitally Controlled Variable On-Time Discontinuous Conduction Mode Operation

Abstract

Fully Integrated Voltage Regulators (FIVR) with package-embedded air-core inductors [1] or on-die solenoid inductors with planar magnetic core [2] promise efficient power delivery and fine-grain wide-range DVFS in complex SoCs while providing fast transient response. The FIVR must provide high conversion efficiency across a wide operating range of output voltages and load currents, including light to medium loads, to maximize the overall energy efficiency of the SoC across different power states. Phase shedding and switch scaling have been used for high-frequency FIVR designs with pulse-width modulation (PWM) control in continuous conduction mode (CCM) to maintain high efficiency for large load currents [1–5], and pulse-frequency modulation (PFM) and hysteretic control have been used to achieve high efficiency across light to medium loads [3–5]. In this paper, we present an FIVR in 14nm CMOS with a 2.5nH air-core inductor embedded in an ultrathin coreless package $( 200 \mu m$ thick) (Fig. 8.5.7), featuring self-trimmed, soft-switched and digitally controlled variable ON-time DCM operation up to 70MHz to achieve high conversion efficiencies across light to medium load currents ranging from 5mA to 500mA and wide 0.7-1.2V output voltage range. The FIVR uses a cascoded thin-gate powertrain (Fig. 8.5.1) to support input voltages up to 2Vmax with the cascode bias rail set at $V_{in} /2$ which consumes $\lt/p\gt\lt1$ uA at light load. A small thick-gate device is connected across the inductor to dampen oscillations when the power stage is in a high-impedance state. The output voltage is monitored by a comparator with sub-ns response time which triggers an inductor current pulse when the output drops below the reference voltage. A resistor divider with a feedforward capacitor is used to achieve fast response time.