AN832 Datasheet, PDF (1/3 Pages) STMicroelectronics – L4981A SYNCHRONIZATION

AN832
A Microwatt Charge Pump Boosts 1 V to 1.8 V at 90% Efficiency, Providing
“Always On” Standby Power for Microcontrollers
1. Introduction
IN
(1V-2.5V)
1 cell battery-
e.g. LR44
R1
5.1M
R2
5.1M
C1
22uF
R3
1M
+
‐
R4
2.2M
U1
TS1001
C2
1nF
R6
2.2M
R5
820K
PRIMARY POWER
T1
BSH205
P‐ch
R10
510K
T2
2N3904
D1
1N5817
C3
47uF
OUT
approx 2 x IN - VD1
@ ˜ 100uA
C4
22uF
R11
2M
U2
TS1001
Figure 1. MicroWatt Charge Pump
Boosting the output voltage of common alkaline button-cells to at least 1.8 V needed by microcontrollers provides
an “always on” standby power source sufficient for low-power oscillator interrupt/sleep state operation. Two ultra-
low power op amps are used in a charge pump configuration to double an input voltage, creating an output voltage
of approximately 2x the input voltage. Output currents up to 100 µA are available at 90% efficiency; even load
currents as low as 10 µA achieve 80% efficiency, beating commercially available charge pump ICs and inductor-
based boost regulators.
2. Overview
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, and 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 the figure above, 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. Amplifier U1, the 0.8 V/
0.6 µA TS1001 op amp, is particularly well-suited to the task, as it has the unusual combination of sub-microamp
supply current, sub-1V operation, and reasonable output drive capability to charge C3.
Rev. 1.0 1/15
Copyright © 2015 by Silicon Laboratories
AN832