Very Low Power 32kHz Oscillator


The 32-kHz low-power
clock oscillator offers numerous advantages over conventional oscillator
circuits based on a CMOS inverter. Such
inverter circuits present problems, for example, supply currents
fluctuate widely over a 3V to 6V supply range, while current consumption
below 250 µA is difficult to attain. Also, operation can be unreliable
with wide variations in the supply voltage and the inverter’s input
characteristics are subject to wide tolerances and differences among
manufacturers. The circuit shown here solves the above problems. Drawing
just 13 µA from a 3V supply, it consists of a one-transistor
amplifier/oscillator (T1) and a low-power comparator/reference device
(IC1).

Circuit diagram:

Very Low Power 32kHz Oscillator Circuit

Very Low Power 32kHz Oscillator Circuit Diagram

The base of T1 is biased at 1.25 V using R5/R4 and the reference in
IC1. T1 may be any small-signal transistor with a decent beta of 100 or
so at 5 µA (defined here by R3, fixing the collector voltage at about 1 V
below Vcc). The amplifier’s nominal gain is approximately 2 V/V. The
quartz crystal combined with load capacitors C1 and C3 forms a feedback
path around T1, whose 180 degrees of phase shift causes the oscillation.
The bias voltage of 1.25 V for the comparator inside the MAX931 is
defined by the reference via R2. The comparator’s input swing is thus
accurately centred around the reference voltage.

Operating at 3 V and 32 kHz, IC1 draws just 7 µA. The comparator
output can source and sink 40 mA and 5 mA respectively, which is ample
for most low-power loads. However, the moderate rise/fall times of 500
ns and 100 ns respectively can cause standard, high-speed CMOS
logic to draw higher than usual switching currents. The optional 74HC14
Schmitt trigger shown at the circuit output can handle the comparator’s
rise/fall times with only a small penalty in supply current.


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