DEMO MANUAL DC1496C
LTC2941/LTC2942:
Battery Gas Gauge with I
2
C
Interface and 14-Bit ADC
DESCRIPTION
Demonstration circuit 1496C-A (Figure 1) features the
LTC
®
2941.
Demonstration circuit 1496C-B features the
LTC2942.
Demonstration circuit 1496C-C features the
LTC2941-1.
Demonstration circuit 1496C-D features the
LTC2942-1.
The C, D options have parts that use an internal
sense resistor while the A, B options rely on an external
precision sense resistor. All four devices measure battery
charge state in handheld PC and portable product applica-
tions. The operating range is perfectly suited for single
cell Li-Ion batteries. A precision analog coulomb counter
integrates current through a sense resistor between the
battery’s positive terminal and the load or charger. The
LTC2942 adds battery voltage and on-chip temperature
measurement with an internal 14-bit No Latency
∆Σ™
ADC. The three measured quantities (charge, voltage and
temperature) are stored in internal registers accessible via
the onboard SMBus/I
2
C interface.
The LTC2941/LTC2941-1 has programmable high and
low thresholds for accumulated charge. The LTC2942/
LTC2942-1 has programmable high and low thresholds for
all three measured quantities. If a programmed threshold
is exceeded, the device reports an alert using either the
SMBus alert protocol or by setting a flag in the internal
status register.
The LTC2941 and LTC2942 require only a single low
value sense resistor to set the measured current range.
The LTC2941-1 and the LTC2942-1 have their own 50mΩ
internal resistor. The default value assembled on the
DC1496 is 100mΩ for a maximum current measurement
of 500mA. Both parts have a software-configurable charge
complete/alert pin. When the pin is set for charge com-
plete, a jumper connects the pushbutton which simulates
a logic high input to indicate a full battery. When the pin is
configured for alert, the same jumper is used to connect
a red LED that indicates an alert is present.
The DC1496C is a part of the QuikEval™ system for quick
evaluation with a host controller through a PC.
Design files for this circuit board are available at
http://www.linear.com/demo/DC1496C.
L,
LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and
No Latency
∆Σ
and QuikEval are trademarks of Linear Technology Corporation. All other
trademarks are the property of their respective owners.
Figure 1. DC1496C
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DEMO MANUAL DC1496C
QUICK START PROCEDURE
Demonstration circuit 1496C is easy to set up to evaluate
the performance of the LTC2941/LTC2942/LTC2941-1/
LTC2942-1. Refer to Figure 2 for proper measurement
equipment setup and follow the procedure below.
1. Connect a 1-cell Li-Ion battery across V_BAT and GND.
2. Connect a load across V_CHRG/LD and GND for battery
discharge measurement. Up to 500mA supplied from
the battery can be measured with the board default
100mΩ sense resistor or 50mΩ internal resistor. Use
SENSE
+
and SENSE
–
test points to read voltage across
the sense resistor.
3. Connect a 2.7V to 5.5V battery charger supply across
V_CHRG/LD and GND. Up to 500mA supplied to the
battery can be measured with the board default 100mΩ
sense resistor. Use SENSE
+
and SENSE
–
test points to
read voltage across the sense resistor.
4. Connect a DC590 to 14-pin connector J1 for evaluation
with QuikEval, or connect a host controller I
2
C bus to
the SDA, SCL and GND test turrets.
5. Set JP1 to QuikEval if a DC590 is present. Otherwise
set JP1 to Bat/Chrg for bus pull-up to the battery, or
float JP1 and supply a bus pull-up voltage to VP.
6. Read and write to the LTC2941/LTC2942 through I
2
C.
7. Through I
2
C, configure the
AL/CC
pin. Set JP2
accordingly.
8 If
AL/CC
is set for charge complete, use pushbutton
switch S1 to simulate a logic high from a controller to
indicate a fully charged battery.
V
SENSE
CHARGER
+
–
+
LOAD
–
BATTERY
1-CELL
Li-Ion
BUS PULL-UP
VOLTAGE
TO QuikEval
TO MICROCONTROLLER
Figure 2. DC1496C Basic Setup
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DEMO MANUAL DC1496C
QuikEval INTERFACE
The DC1496C can be connected to a DC590 and used
with the QuikEval software. The DC590 connects to a PC
through USB. QuikEval automatically detects the demo
board and brings up the LTC2941/LTC2942 evaluation
software interface (Figure 3).
Compact and Detailed Form
When the interface is brought up, a compact form is
first shown with a display for the accumulated charge
register (ACR), voltage ADC and temperature ADC.
To expand the form for a more detailed display of the
LTC2941/LTC2942/LTC2941-1/LTC2942-1 registers and
board configurations, click on Detail. To go back to the
compact form, click on Hide.
Start/Refresh
Click on Start to begin a polling routine that refreshes the
interface every 1 second. Click on Stop to halt the poll-
ing. For a single update, click on Refresh. Each refresh
scans through the internal I
2
C registers and updates the
respective displays.
LTC2941 and LTC2942 Display
On a refresh or poll, the software reads Status bit A[7] to
determine communication with an LTC2941, LTC2942,
LTC2941-1 or LTC2942-1. When an LTC2941 is detected,
the voltage and temperature ADC and threshold displays
are not shown. Control bits B[7:6] configure VBAT Alert
for the LTC2941 and ADC Mode for the LTC2942.
ACR Display
The data in the ACR (registers C and D) is displayed in one
of three selected formats: Counter in coulombs, Counter
in mA • hour, battery gas gauge in mA • hour, and battery
gas gauge in charge percentage of battery. The two gas
gauge displays correspond to the battery gas gauge full
battery configuration set in the detailed form.
Voltage and Temperature ADC (LTC2942, LTC2942-1)
Data from the Voltage ADC (registers I and J) and the
Temperature ADC (registers M and N) is displayed here
in Volts and Celsius.
Figure 3. LTC2941/LTC2942 QuikEval Interface
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DEMO MANUAL DC1496C
QuikEval INTERFACE
Address/I
2
C Status
The write address for the LTC2941/LTC2942/LTC2941-1/
LTC2942-1 is C8h and the read address is C9h. The
alert response address (ARA) is 19h. If an error occurs
while reading from the LTC2941/LTC2942/LTC2941-1/
LTC2942-1, the I
2
C status will read as an error. Otherwise,
the status is good. If the LTC2941/LTC2942/LTC2941-1/
LTC2942-1
AL/CC
pin is set for alert mode and an alert
has been latched, the device will pull down this pin. Click
on ARA to send out an ARA on to the bus lines and the
device will respond with its address. The Alert pin will then
be cleared if the alert is no longer present.
Status
The individual status bits A[0:7] and their states are shown
here. A red indicator next to bits A[0:5] indicates the re-
spective alert is currently present and will latch the Alert
pin if configured for alert. Bit A[7] shows if an LTC2941,
LTC2942, LTC2941-1 or LTC2942-1 is detected.
Sense Resistor
Enter here the sense resistor value used in the application.
The default for the DC1496C is a 100mΩ sense resistor.
Check LTC2941-1/LTC2942-1 if one of these devices is
used in place of the default IC. This sets the sense resistor
value to 50mΩ, the value of the internal sense resistor
in these devices. The sense resistor can only be changed
when not polling. The software only accepts sense resis-
tors between 0.1mΩ to 5Ω.
Battery Gas Gauge
The battery capacity in the application is entered here. The
ACR full scale (FFFFh) is set to this value and affects the
two Gas Gauge ACR display options. Instead of counting
up from 0 as in the Coulomb Counters, the Gas Gauge
is used to count down from a full battery. The battery
capacity can only be entered when not polling. The data
in the ACR when a battery should be empty is calculated
based off of R
SENSE
, and pre-scaler M, and displayed in
hexadecimal below the ACR full scale.
Control
Configurations done in the Control section write to the
Control register (register B). For the LTC2941, the Control
bits B[7:6] enables a battery monitor to one of three set
voltage thresholds (2.8V, 2.9V, or 3V) or disables this
battery voltage alert. The ADC mode with the LTC2942 is
default to Sleep where both Voltage and Temperature ADCs
(LTC2942) are disabled. Setting ADC Mode to Automatic
Mode enables full-time the Voltage and Temperature ADC.
Selecting Manual Voltage or Temperature mode enables the
respective ADC once and returns the ADC to Sleep mode.
Select a pre-scaler M value to scale the ACR according
to battery capacity and maximum current. Changing the
pre-scaler will halt the poll. A calculator tool is provided
in the tool bar under Tools to assist in calculating a pre-
scaler value and sense resistor (Figure 4).
The AL#/CC pin can be configured for Alert mode, Charge
Complete mode, or disabled. Select the corresponding
settings on the DC1496C jumper J2.
The Shutdown Analog Section is checked to disable the
Analog portion of the LTC2941/42/41-1/42-1 and set the
device in a low current state.
Register Read/Write
Data in the internal registers of the LTC2941/LTC2942/
LTC2941-1/LTC2942-1 is displayed here in hexadecimal
or appropriate units. Data can also be entered and written
to the write registers. Enter data to be written in hexa-
decimal, or select Unit and enter data in decimal form.
Data in decimal scale is auto corrected if the maximum
or minimum full scale is exceeded. Select the ACR display
in Counter (Coulombs) to be able to write to the ACR and
charge thresholds in Coulombs, or select Counter (mAh)
to be able to write to the ACR and charge thresholds in
mA • hour. Voltage and Temperature High thresholds are
rounded down in the calculations to the nearest lower
count, while the low thresholds are rounded up to nearest
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dc1496cfb
DEMO MANUAL DC1496C
QuikEval INTERFACE
higher count. Switching back and forth between Hex and
Unit can be used as a conversion tool.
The LSB value for the 16-bit ACR and charge thresholds
is displayed on the bottom. This value is adjusted with the
sense resistor and pre-scaler M. The units are in mAh or
mC depending on the selected ACR display. Shown for
the LTC2942/LTC2942-1 is the LSB for the 14-bit voltage
ADC, 10-bit temperature ADC, and 8-bit high and low
thresholds for voltage and temperature.
Calculator Tool
A calculator tool is available in the tool bar options under
Tools. In this calculator (Figure 4), enter the maximum cur-
rent passed through the sense and the maximum battery
capacity. Click on Calculate to calculate a recommended
sense resistor and pre-scaler (M) value. The display shows
the battery capacity in comparison to ACR full scale and
provides an LSB value in mAh. Also shown is the recom-
mended equation to use to determine an appropriate sense
resistor as a function of the maximum battery charge and
maximum current.
Figure 4. LTC2941/LTC2942 Pre-Scaler and Sense Resistor Calculator
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