AN833 Datasheet, PDF (1/5 Pages) STMicroelectronics – FREQUENCY MODULATION ON L4981B

AN833
SUB-1 V CURRENT SENSING WITH THE TS1001,
A 0 . 8 V, 0 . 6 µA OP- A MP
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.
Rev. 1.1 1/15
Copyright © 2015 by Silicon Laboratories
AN833