Rev. 1.1 1/15

Copyright © 2015 by Silicon Laboratories

AN833

AN833

A

1. Introduction

Current-sense amplifiers can monitor battery or solar cell currents, and are useful to estimate power capacity and

remaining life. However, if the battery or solar source is a single cell, it’s difficult to find a low voltage solution that

works below 1V and draws just microamps. A new class of nanopower analog ICs, namely the TS1001 0.8 V/

0.6 µA op amp, makes a sub-1 V supply current sense amplifier shown in Figure 1 possible. This discrete circuit

operates from as low as 0.8 V and draws 860 nA at no load while providing a 0–500 mV output for measured

currents of 0–100 mA, though the scale can be adjusted by changing the values of a few resistors. With its

extremely low power, the circuit can simply remain “always on,” providing a continuously monitored, averaged

indication of current which can subsequently be read periodically by a microcontroller, without causing too much

current drain in the battery.

2. Overview

The circuit can be used to estimate the impedance of a small coin-cell type alkaline battery to determine its

condition, its ability to source power, and estimate its remaining life. Measuring the battery voltage (without

knowing the load) generally provides only a crude estimate, and performing “pulse tests” as recommended by

some battery manufacturers can be disruptive to the circuit the battery is powering.

Figure 1. Low Voltage Current Sense Amplifier Utilizing Nanopower Op-Amp and Low-Threshold

P-Channel MOSFET

The TS1001 is configured to servo P-channel MOSFET Q1 in a current source configuration, drawing current

through R1 to compensate for the voltage drop across R2 caused by the current flow from IN to the load. R3

converts Q1’s drain current to a voltage and C1 provides filtering. The filtering is critical in allowing the current

sense amplifier to continuously provide an averaged current output, enabling the microcontroller to sleep for long

periods and save power, waking only periodically to read this mean current level.