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Designing ultralow power crystal-oscillator

19 Oct 2012  | Thomas Mathews

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For battery-powered circuits, it is easy to create an ultralow-current crystal oscillator designed around a 32.768kHz crystal. This crystal is common for real-time-clock circuits. Because these circuits must operate at all times, achieving the lowest current draw possible is mandatory. Traditional gate-oscillator circuits—the 74HC04, for example—can draw several milliamps; the circuit shown in figure 1 normally draws only about 5µA. This circuit uses the Texas Instruments LPV7215MF comparator, which is housed in a five-pin SOT-23 package. The operating current for this comparator is 580 nA; the entire circuit shown in figure 1 draws only 5µA when running from a 3.3V supply.

Multiple copies of this circuit have been built and tested to confirm the 5µA current draw. The largest portion of the 5µA goes to the largely unavoidable operations of charging and discharging the output load capacitance. The circuit was tested using a standard 10-MΩ oscilloscope probe with about 10 pF of shunt capacitance; operating current into more capacitive loads will be higher. Table 1 breaks down the power consumption piece by piece.

Capacitive loads must be charged by the upper transistor in the active output stage of the LPV7215. To charge a capacitor to 3.3V, note the capacitor equation Q=C×V. This charge is transferred into the capacitance during the first half of each cycle of the 32.768kHz oscillation. During the second half, the charge is transferred to ground. As a result, the output-stage current, i, will be i=f×Q=f×C×V. For 20 pF (a 10-pF scope probe plus a PCB parasitic), i=(32.768kHz) (20 pF)(3.3V)=2.163µA.

From the equation above, it can be seen that additional output loading or higher operating frequencies will draw more output current. Anything that can be done to reduce the capacitive load will reduce the total current draw.

Figure 2 shows an example of the test boards used to create the crystal-oscillator circuit.

Typical LR44 alkaline button-cell batteries (figure 3) have a capacity of 150 mAhr. With 5µA of current draw, this clock circuit could run for about 30,000 hours, or 3.4 years.

About the author
Thomas Mathews is from Texas Instruments.

To download the PDF version of this article, click here.

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