APPLICATION NOTE 1949

Bi-directional Current-Sense with Single Output

By:  Franco Contadini
© Mar 25, 2003, Maxim Integrated Products, Inc.

Abstract: Battery operated devices often need to monitor both charge and discharge currents. A dual current-sense amplifier and differential amplifier are combined to produce a single output voltage that indicates magnitude and direction of battery current.

Systems such as laptop computers and other devices that have internal charge circuitry require a precise bi-directional current-sense amplifier to monitor accurately the battery's current regardless of polarity. The MAX4377 (a dual low-cost current-sense) can be used to produce a bi-directional current monitor.

Figure 1.
Figure 1.

Output voltage OUT_ is proportional to the magnitude of the sense voltage (VRS+ - VRS-).

OUT_ is approximately zero when VRS- > VRS+.

When VRS+ > VRS-, VOUT = (GAIN)(RSENSE)(ILOAD)

where GAIN = 20 for MAX4377T.

For example, RSENSE = 100mΩ and ILOAD = 1A produce, in the case of the MAX4377T, a full-scale output of 2V. However this circuit needs a two channel ADC in order to evaluate the charge and discharge currents. Simply adding a differential amplifier such as the MAX4198 produces a circuit with a single output able to provide the information of charge or discharge current.

Figure 2.
Figure 2.

The output VOUT will be (OUT2 - OUT1) + REF. Using a REF voltage of 2.5V we obtain an output swing from 0.5V to 4.5V (from 2.5V to 4.5V for discharge current and from 2.5V to 0.5V for charge current).

New bi-directional current-sense amplifiers such as the MAX4070, include the differential amplifier and reference on-chip.

A similar version of this article appeared in the September 2, 2002 issue of Mundo Electronico magazine.

Related Parts
MAX4070 Bidirectional, High-Side, Current-Sense Amplifiers with Reference Free Samples  
MAX4198 Micropower, Single-Supply, Rail-to-Rail Precision Differential Amplifiers Free Samples  
MAX4377 Single/Dual/Quad High-Side Current-Sense Amplifiers with Internal Gain Free Samples  


Next Steps
EE-Mail Subscribe to EE-Mail and receive automatic notice of new documents in your areas of interest.
Download Download, PDF Format (37kB)  
Share
Other Channels  Email this page to an associate or friend. 


The content on this webpage is protected by copyright laws of the United States and of foreign countries. For requests to copy this content, contact us.

APP 1949: Mar 25, 2003
APPLICATION NOTE 1949, AN1949, AN 1949, APP1949, Appnote1949, Appnote 1949