23.4 Dual-source single-inductor 0.18μm CMOS charger-supply with nested hysteretic and adaptive on-time PWM control

A major challenge with emerging microsensors, biomedical implants, and other portable devices is operational life, because tiny batteries exhaust quickly. And even though 1g fuel cells store 5 to 10× more energy than 1g Li-ion batteries, fuel cells supply 10 to 20× less power [1]. This means fuel cells last longer with light loads and Li-ion batteries output more power across shorter periods. Therefore, when the peak power is far greater than the average power, which, for example, is typically the case in wireless sensors, a hybrid source can occupy less space than a single source [1]. Still, managing a fuel cell and a battery to supply a load and recharge the battery, which also acts as an output, with little space is difficult. Switched-inductor circuits are appealing in this context because they draw and supply more power with higher efficiency than their linear and switched-capacitor counterparts. Inductors, however, are bulky, and thus microsystems can only rely on one inductor [2-3]. Today, most single-inductor multiple-output systems derive power from one source [3, 4, 6], and therefore, the fuel cell and the battery require considerable space. Systems that use two sources either do not manage how much power each source should supply across loading conditions [5], or if they do [2] their efficiency is low. The advantage of the prototyped 0.18μm CMOS dual-source single-inductor system presented here is its less overall volume because it incorporates the functional intelligence of [2] with much higher efficiency.