AN4324
Application note
STC3115 system integration
Aurelien Mazard
Introduction
Mobile application users demand accurate battery capacity monitoring. In particular, they are interested
to know if the battery state of charge (SOC) in stressful conditions is logical and accurate. To this end,
STMicroelectronics has developed a device (STC3115) which uses its internal algorithm to provide the
user with accurate and reliable information about the battery’s SOC. This tiny chip can be used in any
handheld electronic mobile phone or tablet. The purpose of this application note is to provide the basics
of how to build an accurate fuel gauge system using the STC3115.
A complete fuel gauge system is a combination of hardware and software functions. The STC3115 has
been designed to convert the physical nonlinear measurements of lithium-ion battery models to stable
and compensated information, which can easily be reported to the final user of a mobile application.
Chipset manages the real-time aspects of the battery gauging. It combines the voltage method and
coulomb counter to provide the best accuracy in every application state. The application software is
concerned with the reporting and calculation of application-dependent information.
November 2014
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Contents
AN4324
Contents
1
2
Architectural overview .................................................................... 4
STC3115 selection guide ................................................................ 5
2.1
2.2
STC3115 version selection ............................................................... 5
STC3115 version detection ............................................................... 6
Battery internal impedance identification........................................... 8
3
Battery parameters .......................................................................... 7
3.1
3.2
Battery open circuit voltage (OCV) curve measurement and
calculation ...................................................................................................... 9
4
Schematic guidelines .................................................................... 10
4.1
Sense resistor selection .................................................................. 10
4.1.1
4.1.2
4.1.3
4.1.4
4.1.5
Sense resistor impedance value ...................................................... 10
Sense resistance impedance accuracy ............................................ 10
Sense resistance power dissipation capability ................................. 10
Sense resistor deviation over temperature range (TCR) ................. 11
Sense resistor selection proposal .................................................... 11
4.2
4.3
BatD pin system integration ............................................................ 11
RSTIO pin system integration ......................................................... 14
Vin pin connection ........................................................................... 16
External sense resistor.................................................................... 17
Temperature considerations............................................................ 18
STMicroelectronics PCB design recommendations ........................ 19
STC3115 RAM memory organization proposal ............................... 21
Chip initialization ............................................................................. 22
STC3115 running configuration....................................................... 27
Periodic device access .................................................................... 27
6.4.1
First STC3115 access after initialization .......................................... 27
5
Place and route guidelines ........................................................... 16
5.1
5.2
5.3
5.4
6
Software ......................................................................................... 20
6.1
6.2
6.3
6.4
6.5
6.6
6.7
STC3115 task procedure ................................................................ 28
6.5.1
How often the STC3115 has to be called in run mode..................... 28
STOP procedure ............................................................................. 28
Power saving mode software management .................................... 29
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Contents
7
Revision history ............................................................................ 30
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Architectural
overview
AN4324
1
Architectural overview
The system architecture has been separated into two main parts in order to facilitate the
most flexible integration. The STC3115 takes physical measurements and reports the
battery state of charge (SOC) through I²C interface while the software layer is concerned
with battery parameter initialization and compensation using all the dedicated features of
the chipset.
Depending on the targeted accuracy of the application environment, the STC3115 can be
used alone without software compensation. In this configuration, the application software
uses only the STC3115 register information to display the battery SOC to the user. If the
temperature and aging effects need to be compensated, the software layer uses the
STC3115 dedicated features.
Figure 1: System architecture overview
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AN4324
STC3115
selection guide
2
STC3115 selection guide
Two versions, in two different packages, of the STC3115 device currently exist. They are:
1.
2.
STC3115AIJT / STC3115AIQT
STC3115IJT / STC3115IQT
Either version can be selected to plug onto the application.
2.1
STC3115 version selection
The STC3115 can lend its functionalities to different battery types with different voltage
ranges. Depending on the battery voltage, either the STC3115AIJT / STC3115AIQT or the
STC3115IJT / STC3115IQT can be selected.
Table 1: STC3115 version selection
STC3115 version
STC3115AIJT / STC3115AIQT
STC3115IJT / STC3115IQT
Battery voltage (typ)
3.7 V
3.8 V
Battery voltage (max)
4.2 V
4.35 V
The STC3115 behavior is the same with the different versions but the device uses a
different initial internal open circuit voltage (OCV) curve to track the battery state of charge.
The table below describes the internal OCV curve of each STC3115 version.
Table 2: STC3115 internal OCV curve references
State of charge (%)
100
90
80
70
65
60
50
40
30
25
20
15
10
6
3
0
STC3115AIJT / STC3115AIQT
4.176
4.076
3.991
3.929
3.899
3.863
3.802
3.778
3.761
3.747
3.721
3.695
3.658
3.618
3.541
3.300
STC3115IJT / STC3115IQT
4.313
4.197
4.088
3.990
3.953
3.914
3.825
3.790
3.770
3.761
3.743
3.710
3.675
3.651
3.571
3.300
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