ST AN3064 Application note

AN3064

Application note

Using the STC3100 battery monitor for gas gauge applications

Introduction

Handheld devices such as mobile phones, music and video players, as well as portable
navigation devices, tend to integrate ever more multimedia functions, all of which have
different power needs. As a consequence, it has become difficult to manage the battery’s
state-of-charge (SoC) and predict the remaining operating time with no dedicated gas
gauge support. The STC3100 is a new device that includes battery monitoring features
(voltage, current and temperature) and a Coulomb counter to implement a gas gauge
system in existing or future applications.

This application note is intended to assist product designers by taking advantage of using
the STC3100 in one-cell Li-Ion battery handheld applications to implement the gas gauge
function. The document provides:

■ a brief description of the STC3100 device and typical applications,

■ information on how to use the battery monitoring functions of the STC3100,

■ a general flow chart of the tasks needed to implement a simple gas gauge system.

October 2009 Doc ID 16263 Rev 1 1/15

www.st.com

Using the STC3100 AN3064

1 Using the STC3100

1.1 Brief description of the STC3100

The STC3100 battery monitoring function includes the measurement of the battery voltage
and current, measurement of the IC’s die temperature and computation of the battery
charge variation (Coulomb counter). An external sense resistor is used in series with the
battery to adapt the current measurement to the application requirements.

Figure 1 shows a typical application using the STC3100.

Figure 1. Typical application diagram for the STC3100

Battery State-of-charge

Remaining percentage

Remaining time

2:06

uC with

Gas Gauge

firmware

80 %

32768 Hz

clock

I2C bus

Battery data

Current

Voltage

Temperature

ROSC

SCL

SDA

0.32 A

3.71 V

STC3100

GND

27 ° C

VCC

VIN

CG

IO0

Battery

Rcg

AM04578

The STC3100 mode and operation are controlled through an I²C interface. SCL and SDA
are the clock and data signals of the I²C interface.

1.2 Internal or external 32,768 Hz timebase

The Coulomb counter system needs an accurate 32,768 Hz timebase to compute the level
of charge flowing from and to the battery. The STC3100 can operate from an internal
oscillator or use an external RTC signal for highest accuracy. To use the internal oscillator, a
200 kΩ 0.1% resistor must be connected to the ROSC pin. The accuracy of the internal
oscillator is 2.5% maximum over the supply voltage and temperature ranges. An external
RTC signal can be applied to the ROSC pin (no resistor required in this case). The signal
must be a square wave with a low level below 0.5 V and a high level above 1.5 V.

2/15 Doc ID 16263 Rev 1

AN3064 Using the STC3100

Figure 2. ROSC connection for internal or external timebase

200 kΩ, 0.1%

ROSC

STC3100

The STC3100 automatically recognizes the presence of the external clock signal and uses it
if present. The internal oscillator, however, can be inhibited to force the use of an external
signal by setting the SEL_EXT_CLK bit in the REG_MODE register.

1.3 STC3100 external components

The Rcg resistor is used to sense the current flowing into or from the battery. The value of
the Rcg resistor must be selected based on the maximum peak current in the application;
the voltage drop across the Rcg resistor must not exceed +/-80 mV.

Rcg resistance (Ohm) <= 0.08/peak current (A).

For instance, a current of up to +/- 2 A can be monitored using a 33 mΩ resistor.

The STC3100 has separate pins for the power supply (VCC) and voltage measurement
(VIN) inputs, providing an easy way of implementing an input filter or additional ESD
protection without affecting the accuracy of the voltage measurement.

32,768 Hz RTC

ROSC

STC3100

AM04579

Figure 3. STC3100 connections with VIN input filter and additional ESD protection

System supply

R1

R2

Battery

Rcg

Gnd

AM04580

STC3100

GND

VCC

VIN

CG

C1C2D1

C1 is the normal recommended decoupling capacitor (1 µF).

R1 and D1 provide additional ESD protection to the STC3100’s VCC pin. A typical value for
R1 is 150 Ω and D1 is a 5.6 V Zener diode.

R2 provides additional ESD protection to the STC3100’s VIN pin. A typical value for R2 is
1 kΩ.. The input impedance of the VIN pin is approximately 500 kΩ; an R2 resistor value of
1 kΩ will not affect the accuracy of the battery voltage measurement. The VIN input must be
connected directly (through the R2 resistor) to the battery’s positive terminal.

Doc ID 16263 Rev 1 3/15

Using the STC3100 AN3064

The C2 capacitor is optional and can be used if additional filtering is required, with typical
values of 47 to 220 nF.

The STC3100’s ground connection (GND) is used as the reference input for the current
measurement and must be connected to the ground side of the sense resistor by a
dedicated track. No current, except the supply current of the STC3100 itself, must flow in
this track to avoid creating an offset error in the current measurement.

4/15 Doc ID 16263 Rev 1

AN3064 Battery monitoring functions

2 Battery monitoring functions

The STC3100 includes the following battery monitoring functions.

● A Coulomb counter for automatic computation of the battery’s charge variation: this is

done by integrating the battery current versus the time and accumulating the resulting
charge variation into the accumulator register. The last current measurement is always
available.

● Automatic periodic measurement of the battery voltage.

● Automatic periodic measurement of the temperature using the STC3100’s internal

sensor.

The battery monitoring functions are controlled by the GG_RUN bit in the REG_MODE
register. Since GG_RUN is reset at power-up, the battery monitoring functions are disabled,
ensuring a low standby current when the battery is inserted until the application is started.

The Coulomb counter function uses a 12/14-bit ADC and runs continuously after it has been
enabled by the GG_RUN bit.

Values held in consecutive registers (such as the charge value in the REG_CHARGE_LOW
and REG_CHARGE_HIGH registers) must be read with a single I
integrity. Multiple values can be read in one I²C access; all values will then be consistent,
that is, will be relative to the same time.

2.1 Coulomb counter operation

The voltage drop between the CG and GND pins is integrated during a conversion period
and input to a 12- to 14-bit AD converter. The output conversion is accumulated into a 28-bit
accumulator. The system controller can control the Coulomb counter and read the data
(upper 16 bits of the accumulator) through the I

2

C control registers.

2

C access to ensure data

Doc ID 16263 Rev 1 5/15