AN3254| Application Note
Maxim/Dallas > App Notes > MEMORY
MICROPROCESSOR SUPERVISOR CIRCUITS
Keywords: NV SRAM, module, lithium, battery monitor
Jun 03, 2004
APPLICATION NOTE 3254
Battery Monitoring in NV SRAM Modules
Dallas Semiconductor offers a variety of products that incorporate lithium coin-cell batteries. In some applications, the ability to monitor the battery status is desired to prevent any accidental loss of data when the battery is approaching its useful end-of-life. As a prerequisite to this article, it is recommended that Application Note 505 be reviewed to gain a better understanding of the general behavior of lithium coin-cell batteries. Dallas Semiconductor offers a variety of products that incorporate lithium coin-cell batteries. In some applications, the ability to monitor the battery status is desired to prevent any accidental loss of data when the battery is approaching its useful end-of-life. Battery monitoring is an integral function of the DS1330, DS1345, DS1350, DS3816C-512 and DS3832C-311 NV SRAM PowerCap Modules, as well as the DS38464 NV SRAM SIMM. This battery monitoring feature is also featured for sorting in our Parametric Search of memory products at http://para.maxim-ic.com/compare.asp?Fam
=Memory&Tree=Memory&HP=Memory.cfm
Which Applications Might Require Battery Monitoring?
In field installations requiring very long service lifetimes or those where the environment may introduce some uncertainty in the overall reliability of the battery back-up system, some ability to periodically check the remaining battery capacity is beneficial. Installations such as servers or RAID applications that are designed to be 'always-on' rely on the NV SRAM to preserve mission-critical data should a power outage occur. Reliable and expedient system recovery after power restoration is therefore a fundamental necessity, and may warrant some additional overhead in the system architecture.
How Does the Battery Monitoring Circuit Work?
As outlined in AN505, there are several characteristics of lithium coin-cell batteries that must be understood to accurately predict the service lifetime of the battery. With the flat voltage characteristics of lithium batteries over time, and faced with the environmental variables that an NV SRAM module might be exposed to during the system's life cycle, use of simple open-circuit voltage (OCV) monitoring is not a sufficient gauge of the remaining battery capacity. Figure 1 shows the typical voltage characteristics observed with a lithium coin-cell battery under a fixed 100k load. Dallas has designed a battery monitoring circuit that utilizes the characteristics of lithium batteries under exposure to a known load to determine if the cell remains a viable power source for battery back-up. As this load exposure also requires consumption of some small amount of the battery capacity, the frequency of this exposure was also of concern during the design definition.
AN3254 Application Note maxim Download PDF
Add this permalink to your bookmarks for future download of AN3254 ApplicationNote
Permalink: http://application.emcelettronica.com/maxim/AN3254
