Electronic Design

Low-Power Wide-Supply-Range Clock

Many times, 32-kHz oscillators are used to generate a system clock or an auxiliary sleep clock in low-power instruments and microcontrollers. The typical implementation employs a CMOS inverter (74HC04 or CD4049UB type) biased as a linear amplifier by connecting a large-valued resistor from input to output.

Inverter circuits present problems, though. Supply currents fluctuate widely over a 3-V to 6-V supply range, and currents below 250 µA are difficult to attain. Operation can be unreliable for wide variations in supply voltage. Furthermore, the inverter's input characteristics can vary dramatically (particularly among different manufacturers) and aren't guaranteed.

A very low-power crystal oscillator solves these problems (see the figure). Drawing only 13 µA from a 3-V supply, it consists of a single-transistor amplifier/oscillator (Q1) and a low-power comparator/reference device (IC1). Q1's base is biased at 1.25 V via R5, R4, and the reference in IC1. VBE is about 0.7 V, placing the emitter at approximately 0.5 V.

This constant voltage across R3 sets the transistor's quiescent current at 5 µA, which fixes the collector voltage at about 1 V below VCC. The amplifier's nominal gain (R1/R2) is approximately 2 V/V.

The crystal and the load capacitors (C1 and C3) form a feedback path around Q1, whose 180° of phase shift causes the oscillation. C4 couples this signal to the comparator input; the input's quiescent voltage (1.25 V) is set by the reference via R2. The comparator's input swing is thus centered around the reference voltage. Operating at 3 V and 32 kHz, IC1 draws about 7 µA.

The comparator output can source 40 mA and sink 5 mA—more than enough for most low-power loads. However, the moderate-speed rise/fall times (500 ns and 100 ns, respectively) may cause standard high-speed CMOS logic to draw higher-than-normal switching currents.

In that case, the optional Schmitt trigger shown (IC2) can handle the comparator's rise/fall times with only a small penalty in supply current (see the table in the figure). If the oscillator drives a microcontroller's crystal-input terminal, the Schmitt trigger can be eliminated.

Unlike inverter-based oscillators (which exhibit startup difficulties, finicky operation, and a decade of change in supply current over a 3-V to 6-V range), this circuit starts quickly and reliably at any supply voltage. Component values are generally not critical. Any small-signal transistor with a decent beta of 100 or so at 5 µA can be substituted for Q1. Supply currents are nearly flat over the supply range of 2.5 V to 11 V, which is the maximum voltage allowed for IC1.

Update by John Wettroth
I did the original circuit for this design idea in 1995, and it appeared in print a little less than a year later. After being notified that it had been selected as a classic design idea and getting over the fun of this, I was asked to update it and improve it. Since Maxim has made more than a dozen new low-power comparators since 1995, I thought it would be simple to use a newer part to significantly reduce the supply current and get on with my work.

My first attempt at "improvement" was to use a MAX917. It's a comparator/reference combination similar to the MAX931 used in the circuit, except it draws an incredibly low 750 nA of supply current. (The MAX919 comparator-only version draws just 380 nA!) These parts also are offered in a SOT package, which would be an improvement too.

Unfortunately, though the MAX917 comparator is micropower, it's a little bit too slow to reliably operate at 32 kHz. It has a propagation delay between 30 and 95 µs (typical). Its reference source current also is limited. It won't allow the topology that has the reference stabilizing the transistor oscillator's bias.

I also looked at the 300-ns propagation-delay MAX9040, but its supply current is 38 µA. Though this might make a good 1-MHz oscillator, it's a bit too fast and power hungry for 32 kHz.

Finally, I looked at the MAX9075 540-ns comparator. It has the same supply current as the MAX931—only 3 µA. But it's about 20 times faster. Unfortunately, it's missing the reference and was only rated to 5.5-V supplies. That lack of a reference would necessitate the use of high-value resistors, which can make these circuits finicky and susceptible to supply change.

I returned to Maxim's internal editor and said, "I guess it really is a classic. It can't be improved." This gnawed at me, as I know that anything can be improved—especially in five years of electronic development. I spent some additional time and arrived at the improved circuit shown in the updated figure.

The enhanced circuit still uses the MAX931 comparator/reference. This part draws about 3 µA of supply current at dc, and a bit less than 10 µA when running at 32 kHz. The enhancement was made by eliminating the transistor oscillator preceding the comparator. The comparator functions as the gain element for the oscillator. This approach somewhat reduces the supply current (especially at higher voltages). It also simplifies the circuit significantly. The improved circuit retains the major benefits of the original, while eliminating a lot of complexity.

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