We present the first known energy management IC to allow low-power systems, such as biomedical implants, to optimally use ultracapacitors instead of batteries as their chief energy storage elements. The IC, fabricated in a 0.18 μm CMOS process, consists of a switched-capacitor DC-DC converter, a 4 nW bandgap voltage reference, a high-efficiency rectifier to allow wireless recharging of the capacitor bank, and a switch matrix and digital control circuitry to govern the stacking and unstacking of the ultracapacitors. The stacking procedure allows for more than 98% of the initial energy stored in the capacitors to be removed before the output voltage drops unsuitably low. The DC-DC converter achieves a peak efficiency of 51% for loads between 10 and 100 μW, operates for input voltages between 1.25 and 2.5 V.
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