For battery-powered circuits, it is easy to build an
ultralow-current crystal oscillator designed around a
32.768-kHz 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 5 μA. 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 5 μA when running from a 3.3V supply.
Multiple copies of this circuit have been built and tested
to confirm the 5-μA current draw. The largest portion of the
5 μA 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 32.768-kHz 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=(32.768 kHz)
(20 pF)(3.3V)=2.163 μA.
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 5 μA of current draw,
this clock circuit could run for about 30,000 hours, or