This work presents an ultra-low-power oscillator designed for wake-up timers in compact wireless sensors. In a conventional relaxation oscillator, a capacitor periodically resets to a fixed voltage using a continuous comparator, thereby generating an output clock. The reset is triggered by a continuous comparator and thus the clock period is dependent on the delay of the continuous comparator which therefore needs to be fast compared to the period, making this approach power hungry. To avoid the power penalty of a fast continuous comparator, a constant charge subtraction scheme is proposed in this paper. As a constant amount of charge is subtracted for each cycle, rather than discharging/charging the capacitor to a fixed voltage, the clock period becomes independent of comparator delay. Therefore, the high power continuous comparator can be replaced with a coarse clocked comparator, facilitating low-power time tracking. For precise wake-up signal generation, an accurate continuous comparator is only enabled for one clock period at the end of the specified wakeup time. A wake-up timer using the proposed scheme is fabricated in a 0.18 mu m CMOS process. The timer consumes 5.8 nW at room temperature with temperature stability of 45 ppm/degrees C (-10 degrees C to 90 degrees C) and line sensitivity of 1/V (1.2 V to 2.2 V).
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