Boosting the output voltage of common alkaline button-cells to at least 1.8V, as needed by microcontrollers, provides an “always on” standby power source sufficient for low-power oscillator interrupt/sleep state operation.
In this design, two ultra-low-power op amps are used in a charge pump configuration to double an input voltage, creating an output voltage of approximately 2× the input voltage. Output currents up to 100μA are available at 90% efficiency; even load currents as low as 10μA achieve 80% efficiency, beating commercially available charge-pump ICs and inductor-based boost regulators.
When the microcontroller wakes, primary power may be fed in at diode-OR point at C4 from a separate power supply capable of providing the full on-state power. In a typical scenario, an interrupt causing the microcontroller to wake also enables the primary supply, which may be an inductor-based boost regulator. This primary supply remains on as long as the microcontroller requires full power, and shuts down when the microcontroller goes to sleep, allowing the micropower charge pump to take over providing low power at high efficiency.
In very-low duty-cycle systems, where the microcontroller spends the majority of its life sleeping, while waking only rarely to make measurements or respond to a stimulus, the low-power sleep-state current draw largely defines the battery life. Thus, the efficiency of the micropower boost regulator becomes critically important.
Referring to Figure 1 , op amp U1 is configured as a relaxation oscillator, serving as the master charge-pump clock. Capacitor C2 charges and discharges primarily through resistor R4 to set the frequency, and U1’s output directly drives the bottom of flying capacitor C3 between IN and GND voltages.
Figure 1: A microwatt charge-pump boost converter
from two ultra-low-power op amps
(click here to see enlarged image).
Amplifier U1, the 0.8V/0.6 µA TS1001 op amp from Touchstone Semiconductor, is particularly well-suited to the task, as it has the unusual combination of sub-microamp supply current, sub-1V operation,