Shutting down inactive circuitry can save substantial power; however, this kind of power management assumes there is an actively managing "brain"¯ (typically a microcontroller) that knows when to turn things on and off. In extremely low-power systems running at sub-microamp levels, it may be desirable to let the microcontroller remain in a deep sleep mode and, instead, let a simple, ultra-low-power clock wake circuitry at periodic intervals.
While many microcontrollers have such timers, a simple analog clock built from an extremely low power comparator can run at lower power, removing the need for powering up from the interrupt timer and in some cases eliminating the need for a microcontroller altogether. As well, the analog clock may operate from low voltages -- as low as 1V from a single cell -- and provide a periodic boosted voltage without need for a separate regulator.
The circuits shown here are based on a simple relaxation oscillator utilizing a very low power comparator. Running at around 500 nanoamps, the oscillator is configured as a very low duty cycle clock, used to duty-cycle power to circuitry in small bursts. Periodically the clock goes "high," circuitry is enabled, and power is delivered; while most of the time the clock stays low, the circuitry is not powered, leaving only the oscillator running as an "always on" duty-cycling clock.
The basic oscillator design is shown in Figure 1.
A very low-power analog comparator (TSM9119) provides a very low-power clock
for applying duty-cycling control.
The timing is set up as follows: First, the upper (VUPTHR) and lower (VLWTHR) hysteretic trip thresholds are set with R1, R2 and R3:
Then, R4 can be chosen according to the desired off-time:
And R5 is chosen according to the desired on-time:
Note that since VUPTHR and VLWTHR are just a scaling of VBATT, neither TON nor TOFF are dependent on VBATT.
The TSM9119's input bias currents of less than 2nA (across temperature) enable the use of high-value resistors. Use of 10MΩ resistors yielded less than 30mV net offset referred to the comparator inputs. Many off-the-shelf bipolar transistors can be substituted for Q1 and Q2: However, gold-doped discrete transistors (most 2N3904 and 2N3906 transistors are not gold-doped) should be avoided because gold doping increases leakage currents.1
All capacitors should be ceramic for lowest leakage, generally limited by the case resistance. The circuit performs well even at hot temperatures, where leakages typically increase. Using a capacitor with an NP0 (C0G) dielectric improves frequency stability, and further reduces dielectric absorption issues. (Dielectric absorption can cause the capacitor to "remember"¯ its charge when charged and discharged in the circuit; however, this is not meant to be a precision timing circuit, so the use of a C0G should be considered optional.)
Table 1 shows currents measured for the duty-cycling clock shown:
What are some uses for this clock? Perhaps most obvious, the oscillator could serve periodically as a clock to wake a microcontroller. While most microcontrollers have built-in interrupt timers, not all have supply currents as low as this. The microcontroller could be set to a deep-sleep mode, woken up by the duty-cycling clock to check periodically system status.
This "duty-cycler"¯ might further enable current savings by periodically powering a simple measurement, the results of which then in turn wake a microcontroller. Thus, the microcontroller only wakes upon a result where it needs to take action -- not simply whenever the timer interrupts it.
A nanopower solar detector implementing duty-cycle control to keep power consumption low.
Figure 2 shows such a scenario -- in this case, a solar monitor checks the availability of sunlight, perhaps for a low power solar-powered system that should not turn until enough light power is available. The duty-cycling clock circuit provides power to the solar detector, comprising U2 and U3 and associated passives, at intervals of about 1 second. In the solar detector, photodiode D1 senses available light, and a TS1001 op amp sinks the resulting photodetected current through its VSS power supply pin, providing a positive polarity signal.
In the op amp loop, the op amp must consume power supply current according to the photodetected current (effectively canceling it); this current can be higher than 30ĀµA. However, since the solar detector does not need to be continuously on, the duty cycling clock circuit reduces this current by the factor of the duty cycle (approximately 500), resulting in a net average current consumption of the detector to less than 100nA. The TSM9117 is a voltage detector IC, where its output goes high when the input rises above 1.25V, indicating enough light is present and interrupting the processor. The desired sensitivity trip threshold voltage can be set by choosing R9.
The "duty-cycler"¯can be further configured to provide burst power at a boosted output voltage to the load. Here, the idea is to generate short bursts of higher voltage, employing a simple voltage doubling boost capacitor integrated into the clock design. The boost cap provides the higher voltage only for the short duration for which it is actually needed, eliminating the need for a separate boost converter and its associated overhead quiescent currents and delays (in fact some converters' shutdown current exceeds the currents used by this duty-cycling clock).
One technique to boost voltage using duty-cycle control.
Figure 3 shows the previous idea in a real-world example. Here, the circuit periodically transmits an identifying code, utilizing a 433MHz SAW resonator transmitter. The duty-cycling clock provides the timing for the bursts by enabling a logic pattern generator (could be a fed-back, shift-register-based code generator) to feed the transmitter with an OOK (on-off keyed) signal for transmission. As a bonus, the clock provides the transmitter with approximately doubled voltage (3.3V) during the clock's "on time," enabling the SAW resonator to transmit at a reasonable transmit power level.
1. Troubleshooting Analog Circuits by Bob Pease, page 66.
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