Most modern mobile, wearable, and Internet of Things (IoT) devices utilize Li-ion batteries as power supplies. Since the 2.8-4.2V Li-ion output voltage range is not compatible with the 0.6-1.0V voltage requirements of most system-on-chips (SoCs) implemented in scaled CMOS, a DC-DC converter, typically implemented as a discrete power management integrated circuit (PMIC), is placed between the battery and the load.
Researchers at UC San Diego have an invention that enables a pragmatic solution for high efficiency fully integrated Li-ion-compatible (2.8V<Vin<4.5V) DC-DC conversion in scaled CMOS (<=28nm CMOS technology), using the 1.5V transistors available in scaled technology, for powering wearable and IoT devices.
To achieve high efficiency, PMICs tend to use inductive switching converter topologies, and to support the Li-ion voltage range, they are typically implemented in larger-geometry CMOS nodes (e.g., 180- 350nm). Both decisions yield larger than desired implementation area and board complexity.
In place of an off-chip PMIC, integrating all DC-DC conversion functionality onto the load SoC itself can help minimize area and complexity, and the small on-chip passive elements enable high-frequency switching that enables fast transient voltage tracking and reduced voltage droop. However, there are two principal challenges that, taken together, have prevented prior-art from becoming a pragmatic replacement of discrete PMICs:
To enable operation at Li-ion-compatible voltages in scaled CMOS, it is necessary to stack low-voltage transistors (or flying capacitors) such that each transistor (or capacitor) experiences only a fraction of the battery voltage across any of its terminals. Unfortunately, stacking devices incurs large conduction and switching losses, and increases driver complexity due to the need to integrate level shifters. Similarly, the large series resistance and low inductance of on-chip inductors yield large conduction losses and thus requires higher-than-desired switching frequencies which incur high switching losses. While switched-capacitor (SC) converters can achieve higher on-chip efficiency, this occurs only at limited ratios of input-to-output voltages, with efficiency degrading significantly at large (i.e., Li-ion to SoC voltage) ratios.
This invention enables a pragmatic solution for high efficiency fully integrated Li-ion-compatible (2.8V<Vin<4.5V) DC-DC conversion in scaled CMOS (<=28nm CMOS technology), using the 1.5V transistors available in scaled technology, for powering wearable and IoT devices.
This work introduces a modified 4-level hybrid converter that achieves high efficiency with 1.5V transistors and on-chip flying passives by:
A working prototype in silicon has been demonstrated
Patent pending with worldwide right available.