DS2781
1-Cell or 2-Cell
Standalone Fuel Gauge IC
GENERAL DESCRIPTION
FEATURES
The DS2781 measures voltage, temperature, and
current, and estimates available capacity for
rechargeable Lithium-Ion and Lithium-Ion Polymer
batteries. Cell stack characteristics and application
parameters used in the calculations are stored in onchip EEPROM. The available capacity registers
report a conservative estimate of the amount of
charge that can be removed given the current
temperature, discharge rate, stored charge and
application parameters. Capacity estimation is
reported in milliamp hours remaining and percentage
of full.
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APPLICATIONS
Digital Video Cameras
Precision Voltage, Temperature, and Current
Measurement System
Operates in One-Cell or Two--Cell Applications
Accurate, Temperature Stable Internal Time
Base
Absolute and Relative Capacity Estimated from
Coulomb Count, Discharge Rate, Temperature,
and Battery Cell Characteristics
Accurate Warning of Low Battery Conditions
Automatic Backup of Coulomb Count and Age
Estimation to Nonvolatile (NV) EEPROM
Gain and Tempco Calibration Allows the Use of
Low-Cost Sense Resistors
24-Byte Battery/Application Parameter EEPROM
16-Byte User EEPROM
Unique ID and Multidrop 1-WireÒ Interface
Tiny 8-Pin TSSOP Package Embeds Easily in
Battery Packs Using Thin Prismatic Cells
Portable GPS Navigation Systems
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TYPICAL OPERATING CIRCUIT
PIN CONFIGURATION
Commercial Two-Way Radios
Industrial PDAs and Handheld PC Data Terminals
TOP VIEW
P+
TSSOP-8
DS2781
DQ
DQ
VDD
PIO
VIN
OVD
VB
SNS
VSS
VB
1
8
PIO
VSS
2
7
SNS
VIN
3
6
OVD
VPLS
4
5
DQ
Protection
Circuit
P-
ORDERING INFORMATION
PART
DS2781E+
DS2781E+T&R
MARKING
2781
2781
PACKAGE
TSSOP
Tape-and-Reel
+ Denotes lead-free package.
1-Wire is a registered trademark of Dallas Semiconductor.
Note: Some revisions of this device may incorporate deviations from published specifications known as errata. Multiple revisions of any device
may be simultaneously available through various sales channels. For information about device errata, click here: www.maxim-ic.com/errata.
1 of 30
REV: 010906
DS2781: Two-Cell Standalone Fuel Gauge IC
ABSOLUTE MAXIMUM RATINGS
Voltage on VPLS, VIN Relative to VSS
Voltage Range on Any Pin Relative to VSS
Continuous Sink Current, DQ, PIO
Operating Temperature Range
Storage Temperature Range
Soldering Temperature (10s)
-0.3V to +12V
-0.3V to +6.0V
20mA
-40°C to +85°C
-55°C to +125°C
+260°C
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only,
and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is
not implied. Exposure to the absolute maximum rating conditions for extended periods may affect device.
RECOMMENDED DC OPERATING CHARACTERISTICS
(VPLS = 2.5V to 10V, TA = -20°C to +70°C, unless otherwise noted. Typical values are at TA = +25°C)
PARAMETER
Supply Voltage
VIN Voltage Range
SYMBOL
VPLS
DQ, PIO Voltage Range
VB Output Voltage
VVB
OVD Voltage Range
CONDITIONS
MIN
TYP
MAX
UNITS
(Note 1)
(Note 1)
+2.5
-0.3
+10
VPLS + 0.3
V
V
(Note 1)
VPLS > 3.0V,
IVB = 500µA,
(Note 1)
(Note 1)
-0.3
+5.5
V
3.1
V
VVB + 0.3
V
2.5
2.8
-0.3
DC ELECTRICAL CHARACTERISTICS
(VPLS = 2.5V to 10V, TA = -20°C to +70°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
ACTIVE Current
IACTIVE
SLEEP Mode Current
ISLEEP
Input Logic High: DQ, PIO
Input Logic Low: DQ, PIO
Output Logic Low: DQ, PIO
Pulldown Current: DQ, PIO
Input Logic High: OVD
Input Logic Low: OVD
VIN Input Resistance
DQ Capacitance
DQ SLEEP Timeout
Undervoltage SLEEP
Threshold
VIH
VIL
VOL
IPD
VIH
VIL
RIN
CDQ
tSLEEP
VSLEEP
CONDITIONS
MIN
IVB = 0
TYP
MAX
UNITS
70
95
µA
TA > +50°C, IVB = 0
10
IVB = 0, (Note 5)
(Note 1)
(Note 1)
IOL = 4mA (Note 1)
VDQ, VPIO = 0.4V
(Note 1)
(Note 1)
3
5
1.5
0.6
0.4
0.2
0.7 x VVB
0.3 x VVB
15
(Note 4)
DQ < VIL
UVTH = 1, (Note 1)
UVTH = 0, (Note 1)
1.5
4.8
2.40
50
2
4.9
2.45
2.5
5.0
2.50
µA
V
V
V
µA
V
V
MW
pF
s
V
ELECTRICAL CHARACTERISTICS: TEMPERATURE, VOLTAGE, CURRENT
(VPLS = 2.5V to 10V, TA = -20°C to +70°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
Temperature Resolution
TLSB
Temperature Error
TERR
Voltage Resolution
VLSB
CONDITIONS
MIN
TYP
MAX
0.125
°C
±3
9.76
2 of 30
UNITS
°C
mV
DS2781: Two-Cell Standalone Fuel Gauge IC
PARAMETER
SYMBOL
Voltage Full-Scale
VFS
Voltage Error
VERR
Current Resolution
ILSB
Current Full-Scale
IFS
Current Gain Error
IGERR
Current Offset Error
IOERR
Accumulated Current Offset
qOERR
Timebase Error
tERR
CONDITIONS
MIN
TYP
0
MAX
UNITS
9.9902
V
±100
mV
1.56
µV
±51.2
(Note 2)
0°C ≤ TA ≤ +70°C,
(Note 4)
0°C ≤ TA ≤ +70°C,
VSNS = VSS (Notes 3, 4)
±1
mV
% FullScale
- 7.82
+ 12.5
µV
- 188
+0
µVhr/
day
TA = +25°C, VPLS = 7.6V
±1
±2
%
ELECTRICAL CHARACTERISTICS: 1-WIRE INTERFACE, STANDARD
(VPLS = 2.5V to 10V, TA = -20°C to +70°C.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
120
ms
Time Slot
tSLOT
60
Recovery Time
tREC
1
Write-0 Low Time
tLOW0
60
120
ms
Write-1 Low Time
tLOW1
1
15
ms
Read Data Valid
tRDV
15
ms
Reset Time High
tRSTH
480
Reset Time Low
tRSTL
480
960
ms
Presence Detect High
tPDH
15
60
ms
Presence Detect Low
tPDL
60
240
ms
MAX
UNITS
16
ms
ms
ms
ELECTRICAL CHARACTERISTICS: 1-WIRE INTERFACE, OVERDRIVE
(VPLS = 2.5V to 10V, TA = -20°C to +70°C.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
Time Slot
tSLOT
6
Recovery Time
tREC
1
Write-0 Low Time
tLOW0
6
16
ms
Write-1 Low Time
tLOW1
1
2
ms
Read Data Valid
tRDV
2
ms
Reset-Time High
tRSTH
48
Reset-Time Low
tRSTL
48
80
ms
Presence-Detect High
tPDH
2
6
ms
Presence-Detect Low
tPDL
8
24
ms
3 of 30
ms
ms
DS2781: Two-Cell Standalone Fuel Gauge IC
EEPROM RELIABILITY SPECIFICATION
(VPLS = 2.5V to 10V, TA = -20°C to +70°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
EEPROM Copy Time
tEEC
EEPROM Copy Endurance
NEEC
CONDITIONS
TA = +50°C
MIN
TYP
MAX
UNITS
15
ms
50,000
cycles
Note 1: All voltages are referenced to VSS.
Note 2: Factory calibrated accuracy. Higher accuracy can be achieved by in-system calibration by the user.
Note 3: Accumulation Bias register set to 00h. Current Offset Bias register set to 00h. NBEN bit = 0.
Note 4: Parameters guaranteed by design.
Note 5: Internal voltage regulator active.
PIN DESCRIPTION
PIN
NAME
FUNCTION
Internal Supply. Bypass to VSS with a 0.1µF capacitor.
1
VB
2
VSS
3
VIN
4
VPLS
5
DQ
Data Input/Output. 1-Wire data line. Open-drain output driver. Connect this pin to the
DATA terminal of the battery pack. This pin has a weak internal pulldown (IPD) for sensing
pack disconnection from host or charger.
6
OVD
1-Wire Bus Speed Control. Input logic level selects the speed of the 1-Wire bus. Logic 1
selects overdrive (OVD) and Logic 0 selects standard timing (STD). On a multidrop bus,
all devices must operate at the same speed.
7
SNS
Sense Resistor Connection. Connect to the negative terminal of the battery pack.
Connect the sense resistor between VSS and SNS.
8
PIO
Programmable I/O Pin. Can be configured as input or output to monitor or control userdefined external circuitry. Output driver is open drain. This pin has an weak internal
pulldown (IPD).
Device Ground. Connect directly to the negative terminal of the cell stack. Connect the
sense resistor between VSS and SNS.
Voltage Sense Input. The voltage of the battery pack is monitored through this input pin
with respect to the VSS pin.
Power-Supply Input. Connect to the positive terminal of the battery pack through a
decoupling network.
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DS2781: Two-Cell Standalone Fuel Gauge IC
Figure 1. Block Diagram
VB
VPLS
Voltage
Regulator
V POR
BIAS/VREF
Timebase
EN
PIO
VIN
DQ
OVD
1-Wire
Interface
Status
&
Control
Temp
&
Voltage
ADC
EEPROM
SNS
Accumulated
Current
Current ADC
15 bit + sign
Rate,
Temperature
Compensation
VSS
DETAILED DESCRIPTION
The DS2781 operates directly from 2.5V to 10V and supports single or dual cell Lithium-ion battery packs.
Nonvolatile storage is provided for cell compensation and application parameters. Host side development of fuelgauging algorithms is eliminated. On-chip algorithms and convenient status reporting of operating conditions
reduce the serial polling required of the host processor.
Additionally, 16 bytes of EEPROM memory are made available for the exclusive use of the host system and/or
pack manufacturer. The additional EEPROM memory can be used to facilitate battery lot and date tracking and
non-volatile storage of system or battery usage statistics.
A Dallas 1-Wire interface provides serial communication at the standard 16kbps or overdrive 140kbps speeds
allows access to data registers, control registers and user memory. A unique, factory programmed 64-bit
registration number (8-bit family code + 48-bit serial number + 8-bit CRC) assures that no two parts are alike and
enables absolute traceability. The Dallas 1-Wire interface on the DS2781 supports multidrop capability so that
multiple slave devices may be addressed with a single pin.
5 of 30
DS2781: Two-Cell Standalone Fuel Gauge IC
Figure 2: Typical Operating Circuit
PACK+
DS2781
DATA
5.6V
PACK-
500
1K
150
DQ
VDD
PIO
VIN
OVD
VB
SNS
VSS
TSSOP-8
1 or 2
Cell
Li-Ion
Stack
0.1uF
0.1uF
Protection
Circuit
RSNS
POWER MODES
The DS2781 has two power modes: ACTIVE and SLEEP. On initial power up, the DS2781 defaults to ACTIVE
mode. While in ACTIVE mode, the DS2781 is fully functional with measurements and capacity estimation
continuously updated.
In SLEEP mode, the DS2781 conserves power by disabling measurement and capacity estimation functions, but
preserves register contents. SLEEP mode is entered under two different conditions. An enable bit makes entry into
SLEEP optional for each condition. The first condition in which SLEEP is entered is a bus low condition. The Power
Mode (PMOD) bit must be set to enter SLEEP when a bus low condition occurs. (PMOD = 1 AND BUS_LOW). A
bus low condition, where the DQ pin is low for tSLEEP (2s nominal), is used to detect a pack disconnection or
system shutdown in which the bus pull-up voltage, VPULLUP, is not present. PMOD type SLEEP assumes that no
charge or discharge current will flow and therefore coulomb counting is not necessary. A system with PMOD
SLEEP enabled must ensure that a standalone or cradle charger includes a pull-up on DQ. The DS2781 transitions
from PMOD SLEEP to ACTIVE mode when DQ is pulled high, as would happen when a battery is inserted into a
system.
The second condition to enter SLEEP is an under voltage condition (measured on VIN). When the Under Voltage
Enable (UVEN) bit is set, the DS2781 will transition to SLEEP if the VIN voltage is less than VSLEEP (Selectable
2.45 or 4.9V). The bus must be in a static state, that is with DQ either high or low for tSLEEP. UVEN SLEEP reduces
battery drain due to the DS2781 to prevent over discharge. The DS2781 transitions from UVEN SLEEP to ACTIVE
mode when DQ changes logic state. The bus master should initiate communication when charging of a depleted
battery begins to ensure that the DS2781 enters ACTIVE mode from UVEN SLEEP.
NOTE: PMOD and UVEN SLEEP features must be disabled when a battery is charged on an external charger that
does not connect to the DQ pin. PMOD SLEEP can be used if the charger pulls DQ high. UVEN SLEEP can be
used if the charger toggles DQ. The DS2781 remains in SLEEP and therefore does not measure or accumulate
current when a battery is charged on a charger that failures properly drive DQ.
INITIATING COMMUNICATION IN SLEEP
When beginning communication with a DS2781 in PMOD SLEEP, DQ must be pulled up first and then a 1-Wire
Reset pulse must be issued by the master. In UVEN SLEEP, the procedure depends on the state of DQ when
UVEN SLEEP was entered. If DQ was low, DQ must be pulled up and then a 1-Wire Reset pulse must be issued
by the master as with PMOD SLEEP. If DQ was high when UVEN SLEEP was entered, then the DS2781 is
prepared to receive a 1-Wire reset from the master. In the first two cases with DQ low during SLEEP, the DS2781
does not respond to the first rising edge of DQ with a presence pulse.
6 of 30
DS2781: Two-Cell Standalone Fuel Gauge IC
VOLTAGE MEASUREMENT
Battery voltage is measured at the VIN input with respect to VSS over a range of 0V to 9.9902V, with a resolution
of 9.76mV. The result is updated every 440ms and placed in the VOLTAGE register in two’s compliment form.
Voltages above the maximum register value are reported at the maximum value; voltages below the minimum
register value are reported at the minimum value. The format of the voltage register is shown in Figure 3.
Figure 3. Voltage Register Format
VOLT
Read Only
MSB—Address 0Ch
S
9
2
8
2
7
2
6
2
LSB—Address 0Dh
5
2
4
3
2
MSb
2
2
2
LSb
MSb
1
2
0
2
X
X
X
X
X
LSb
Units: 9.76mV
“S”: sign bit(s), “X”: reserved
TEMPERATURE MEASUREMENT
The DS2781 uses an integrated temperature sensor to measure battery temperature with a resolution of 0.125°C.
Temperature measurements are updated every 440ms and placed in the temperature register in two’s complement
form. The format of the temperature register is shown in Figure 4.
Figure 4. Temperature Register Format
TEMP
Read Only
MSB—Address 0Ah
S
9
2
8
2
7
2
6
2
MSb
LSB—Address 0Bh
5
2
4
2
3
2
2
2
LSb
MSb
1
2
0
2
X
X
X
X
X
LSb
Units: 0.125°C
“S”: sign bit(s), “X”: reserved
CURRENT MEASUREMENT
In the ACTIVE mode of operation, the DS2781 continually measures the current flow into and out of the battery by
measuring the voltage drop across a low-value current-sense resistor, RSNS. The voltage-sense range between
SNS and VSS is ±51.2mV. The input linearly converts peak signal amplitudes up to 102.4mV as long as the
continuous signal level (average over the conversion cycle period) does not exceed ±51.2mV. The ADC samples
the input differentially at 18.6kHz and updates the Current register at the completion of each conversion cycle.
The Current register is updated every 3.515s with the current conversion result in two’s complement form. Charge
currents above the maximum register value are reported at the maximum value (7FFFh = +51.2mV). Discharge
currents below the minimum register value are reported at the minimum value (8000h = -51.2mV).
7 of 30
DS2781: Two-Cell Standalone Fuel Gauge IC
Figure 5. Current Register Format
CURRENT
Read Only
MSB—Address 0Eh
S
14
2
13
2
12
2
11
2
10
2
LSB—Address 0Fh
9
8
2
MSb
7
2
2
LSb
MSb
6
2
5
2
4
2
3
2
2
2
1
2
0
2
LSb
“S”: sign bit(s)
Units:
1.5625mV/Rsns
CURRENT RESOLUTION (1 LSB)
VSS VSNS
1.5625mV
RSNS
20mW
78.13mA
15mW
104.2mA
10mW
156.3mA
5mW
312.5mA
CURRENT OFFSET CORRECTION
Every 1024th conversion, the ADC measures its input offset to facilitate offset correction. Offset correction occurs
approximately once per hour. The resulting correction factor is applied to the subsequent 1023 measurements.
During the offset correction conversion, the ADC does not measure the sense resistor signal. A maximum error of
1/1024 in the accumulated current register (ACR) is possible; however, to reduce the error, the current
measurement made just prior to the offset conversion is displayed in the current register and is substituted for the
dropped current measurement in the current accumulation process. This results in an accumulated current error
due to offset correction of less than 1/1024.
CURRENT OFFSET BIAS
The Current Offset Bias (COB) register allows a programmable offset value to be added to raw current
measurements. The result of the raw current measurement plus COB is displayed as the current measurement
result in the CURRENT register, and is used for current accumulation. COB can be used to correct for a static
offset error, or can be used to intentionally skew the current results and therefore the current accumulation.
Read and write access is allowed to COB. Whenever the COB is written, the new value is applied to all subsequent
current measurements. COB can be programmed in 1.56mV steps to any value between +198.1mV and -199.7mV.
The COB value is stored as a two’s complement value in volatile memory, and must be initialized through the
interface on power-up.
Figure 6. Current Offset Bias Register Format
Address 7B
S
26
25
24
23
22
MSb
21
20
LSb
“S”: sign bit(s)
Units:
8 of 30
1.56mV/Rsns
DS2781: Two-Cell Standalone Fuel Gauge IC
CURRENT MEASUREMENT CALIBRATION
The DS2781’s current measurement gain can be adjusted through the RSGAIN register, which is factory-calibrated
to meet the data sheet specified accuracy. RSGAIN is user accessible and can be reprogrammed after module or
pack manufacture to improve the current measurement accuracy. Adjusting RSGAIN can correct for variation in an
external sense resistor’s nominal value, and allows the use of low-cost, non-precision current sense resistors.
RSGAIN is an 11 bit value stored in 2 bytes of the Parameter EEPROM Memory Block. The RSGAIN value adjusts
-10
the gain from 0 to 1.999 in steps of 0.001 (precisely 2 ). The user must program RSGAIN cautiously to ensure
accurate current measurement. When shipped, the same unique factory gain calibration value is stored in RSGAIN
and in a read only location, FSGAIN (B0h and B1h).The original factory gain value can be restored to the device at
any time by writing the value of FSGAIN back into RSGAIN.
SENSE RESISTOR TEMPERATURE COMPENSATION
The DS2781 is capable of temperature compensating the current sense resistor to correct for variation in a sense
resistor’s value over temperature. The DS2781 is factory programmed with the sense resistor temperature
coefficient, RSTC, set to zero, which turns off the temperature compensation function. RSTC is user accessible
and can be reprogrammed after module or pack manufacture to improve the current accuracy when using a high
temperature coefficient current-sense resistor. RSTC is an 8-bit value stored in the Parameter EEPROM Memory
Block. The RSTC value sets the temperature coefficient from 0 to +7782ppm/ºC in steps of 30.5ppm/ºC. The user
must program RSTC cautiously to ensure accurate current measurement.
o
Temperature compensation adjustments are made when the Temperature register crosses 0.5 C boundaries. The
temperature compensation is most effective with the resistor placed as close as possible to the VSS terminal to
optimize thermal coupling of the resistor to the on-chip temperature sensor. If the current shunt is constructed with
a copper PCB trace, run the trace under the DS2781 package if possible.
AVERAGE CURRENT MEASUREMENT
The Average Current register reports an average current level over the preceding 28 seconds. The register value is
updated every 28s in two’s complement form, and is the average of the 8 preceding Current register updates. The
format of the Average Current register is shown in Figure 7. Charge currents above the maximum register value
are reported at the maximum value (7FFFh = +51.2mV). Discharge currents below the minimum register value are
reported at the minimum value (8000h = -51.2mV).
Figure 7. Average Current Register Format
IAVG
R/W
MSB—Address 08h
S
14
2
13
2
12
2
11
2
MSb
LSB—Address 09h
10
2
9
2
8
7
2
2
LSb
MSb
“S”: sign bit(s)
6
2
5
2
4
2
3
2
2
2
1
2
0
2
LSb
Units:
1.5625mV/Rsns
CURRENT ACCUMULATION
Current measurements are internally summed, or accumulated, at the completion of each conversion period with
the results displayed in the Accumulated Current Register (ACR). The accuracy of the ACR is dependent on both
the current measurement and the conversion time base. The ACR has a range of 0 to 409.6mVh with an LSb of
6.25 Vh. Additional read-only registers (ACRL) hold fractional results of each accumulation to avoid truncation
errors. Accumulation of charge current above the maximum register value is reported at the maximum register
value (7FFFh); conversely, accumulation of discharge current below the minimum register value is reported at the
minimum value (8000h).
Read and write access is allowed to the ACR. The ACR must be written MSByte first then LSByte. Whenever the
ACR is written, the fractional accumulation result bits are cleared. The write must be completed within 3.515s (one
9 of 30
DS2781: Two-Cell Standalone Fuel Gauge IC
ACR register update period). A write to the ACR forces the ADC to perform an offset correction conversion and
update the internal offset correction factor. Current measurement and accumulation begins with the second
conversion following a write to the ACR. Writing ACR clears the fractional values in ACRL. The Format of the ACR
register is shown in Figure 8, and the format of ACRL is shown in Figure 9.
In order to preserve the ACR value in case of power loss, the ACR value is backed up to EEPROM. The ACR
value is recovered from EEPROM on power-up. See the Memory Map in Table 2 for specific address location and
backup frequency.
Figure 8. Accumulated Current Register Format, ACR
ACR
R/W & EE
MSB—Address 10h
15
2
14
2
13
2
12
2
11
2
10
2
LSB—Address 11h
9
2
MSb
8
7
2
2
LSb
MSb
6
2
5
2
4
2
3
2
2
2
1
2
0
2
LSb
Units:
6.25mVh/Rsns
Figure 9. Fractional/Low Accumulated Current Register Format, ACRL
ACRL
Read Only
MSB—Address 12h
11
2
10
2
9
2
8
2
7
2
6
2
LSB—Address 13h
5
2
MSb
4
3
2
2
LSb
MSb
2
2
1
2
0
2
X
X
X
X
LSb
“X”: reserved
Units:1.526nVHr/RSNS
ACR LSb
VSS VSNS
6.25 Vh
RSNS
20m
312.5 Ah
15m
10m
416.7 Ah
625 Ah
5m
1.250mAh
ACR RANGE
VSS VSNS
±409.6mVh
RSNS
20m
±20.48Ah
15m
±27.30Ah
10 of 30
10m
±40.96Ah
5m
±81.92Ah
DS2781: Two-Cell Standalone Fuel Gauge IC
ACCUMULATION BIAS
The Accumulation Bias register (AB) allows an arbitrary bias to be introduced into the current-accumulation
process. The AB can be used to account for currents that do not flow through the sense resistor, estimate currents
too small to measure, estimate battery self-discharge or correct for static offset of the individual DS2781 device.
The AB register allows a user programmed constant positive or negative polarity bias to be included in the current
accumulation process. The user-programmed two’s compliment value, with bit weighting the same as the current
register, is added to the ACR once per current conversion cycle. The AB value is loaded on power-up from
EEPROM memory. The format of the AB register is shown in Figure 10.
Figure 10. Accumulation Bias Register Formats
AB
EE
Address 61h
S
6
2
5
2
4
3
2
2
MSb
“S”: sign bit
2
2
1
2
0
2
LSb
Units:
1.5625mV/Rsns
CURRENT BLANKING
The Current Blanking feature modifies current measurement result prior to being accumulated in the ACR. Current
Blanking occurs conditionally when a current measurement (raw current + COB) falls in one of two defined ranges.
The first range prevents charge currents less than 100µV from being accumulated. The second range prevents
discharge currents less than 25µV in magnitude from being accumulated. Charge current blanking is always
performed, however, discharge current blanking must be enabled by setting the NBEN bit in the Control register.
See the register description for additional information.
CAPACITY ESTIMATION ALGORITHM
Remaining capacity estimation uses real-time measured values and stored parameters describing the cell stack
characteristics and application operating limits. The following diagram describes the algorithm inputs and outputs.
11 of 30
DS2781: Two-Cell Standalone Fuel Gauge IC
Figure 11. Top Level Algorithm Diagram
Voltage
(R)
Temperature
(R)
Current
(R)
Accumulated
Current (ACR) (R/W)
Average Current (R)
Capacity Look-up
FULL
FULL(T)
(R)
Active Empty
AE(T)
(R)
Standby Empty SE(T)
(R)
Available Capacity Calculation
ACR Housekeeping
Age Estimator
Remaining Active Absolute
Capacity (RAAC) mAh
(R)
Learn Function
Remaining Stand-by Absolute
Capacity (RSAC) mAh
(R)
Cell
Parameters
16 bytes
(EEPROM)
Remaining Active Relative
Capacity (RARC) %
(R)
Remaining Stand-by Relative
Capacity (RSRC) %
(R)
Aging Cap (AC)
(2 bytes EE)
Age Scalar (AS)
(1 bytes EE)
Sense Resistor’
(RSNSP) (1byte EE)
Charge Voltage
(VCHG) (1 byte EE)
Min Chg Current
(IMIN) (1 byte EE)
Active Empty
Voltage (VAE)
(1 byte EE)
Active Empty
Current (IAE)
(1 byte EE)
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DS2781: Two-Cell Standalone Fuel Gauge IC
MODELING CELL STACK CHARACTERISTICS
In order to achieve reasonable accuracy in estimating remaining capacity, the cell stack performance
characteristics over temperature, load current and charge termination point must be considered. Since the behavior
of Li-ion cells is non-linear, these characteristics must be included in the capacity estimation to achieve an
acceptable level of accuracy in the capacity estimation. The FuelPack™ method used in the DS2781 is described
in general in Applications Note AN131 “Li+ Fuel Gauging with Dallas Semiconductor Devices”. To facilitate efficient
implementation in hardware, a modified version of the method outlined in AN131 is used to store cell
characteristics in the DS2781. Full and empty points are retrieved in a look-up process which re-traces piece-wise
linear model consisting of three model curves named Full, Active Empty and Stand-by Empty. Each model curve is
constructed with 5 line segments, numbered 1 through 5. Above 40C, the segment 5 model curves extend infinitely
with zero slope, approximating the nearly flat change in capacity of Li-Ion cells at temperatures above 40C.
Segment 4 of each model curves originates at +40C on its upper end and extends downward in temperature to the
junction with segment 3. Segment 3 joins with segment 2, which in turn joins with segment 1. Segment 1 of each
model curve extends from the junction with segment 2 to infinitely colder temperatures. Segment slopes are stored
as µVh PPM change per ºC. The three junctions or breakpoints that join the segments (labeled TBP12, TBP23 and
TBP34 in figure 12) are programmable in 1C increments from -128C to +40C. They are stored in two’s complement
format in locations 0x7C, 0x7D, and 0x7E respectively The slope or derivative for segments 1, 2, 3 and 4 are also
programmable. One the lower (cold) end of each model curve, segment 1 extends from breakpoint TBP12 to
infinitely to colder temperatures.
Figure 12. Cell Model Example Diagram
Segment 1
Seg. 2
100%
Seg. 3
Seg. 5
Seg. 4
FULL
Derivative
[ppm / C]
Active
Empty
Cell Characterization
data points
Stand-by
Empty
TBP12
TBP23
TBP34
40C
FuelPack is a trademark of Dallas Semiconductor.
Full: The Full curve defines how the full point of a given cell stack depends on temperature for a given charge
termination. The charge termination method used in the application is used to determine the table values. The
DS2781 reconstructs the Full line from cell characteristic table values to determine the Full capacity of the battery
at each temperature. Reconstruction occurs in one-degree temperature increments. Full values are stored as ppm
change per ºC. For example if a cell had a nominal capacity of 1051mAh at 40ºC, a full value of 1031mAh at 18ºC
(TBP34) and 1009mAh at 0ºC (TBP23), the slope for segment 3 would be:
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DS2781: Two-Cell Standalone Fuel Gauge IC
((1031mAh – 1009mAh) / (1051mAh / 1M)) / (18ºC - 0ºC) = 1163 ppm/ºC
1 LSB of the slope registers equals 61 ppm so the Full Segment 3 Slope register (location 0x6Dh) would be
programmed with a value of 0x13h. Each slope register has a dynamic range 0 ppm to 15555 ppm.
Active Empty: The Active Empty curve defines the temperature variation in the empty point of the discharge profile
based on a high level load current (one that is sustained during a high power operating mode) and the minimum
voltage required for system operation. This load current is programmed as the Active Empty current (IAE), and
should be a 3.5s average value to correspond to values read from the Current register, and the specified minimum
voltage, or Active Empty voltage (VAE) should be a 250ms average to correspond to values read from the Voltage
register. The DS2781 reconstructs the Active Empty line from cell characteristic table values to determine the
Active Empty capacity of the battery at each temperature. Reconstruction occurs in one-degree temperature
increments. Active Empty segment slopes are stored the same as described for the Full segments above.
Standby Empty: The Standby Empty curve defines the temperature variation in the empty point in the discharge
defined by the application standby current and the minimum voltage required for standby operation. Standby Empty
represents the point that the battery can no longer support a subset of the full application operation, such as
memory data retention or organizer functions on a wireless handset. Standby Empty segment slopes are stored the
same as described for the Full segments above.
The standby load current and voltage are used for determining the cell characteristics but are not programmed into
the DS2781. The DS2781 reconstructs the Standby Empty line from cell characteristic table values to determine
the Standby Empty capacity of the battery at each temperature. Reconstruction occurs in one-degree temperature
increments.
CELL STACK MODEL CONSTRUCTION
The model is constructed with all points normalized to the fully charged state at +40°C. Initial values, the +40°C
Full value in mVh units and the +40°C Active Empty value as a fraction of the +40°C Full are stored in the cell
parameter EEPROM block. Standby Empty at +40°C is by definition zero and therefore no storage is required. The
slopes (derivatives) of the 4 segments for each model curve are also stored in the cell parameter EEPROM block
along with the break point temperatures of each segment. An example of data stored in this manner is shown in
Table 1.
Table 1. Example Cell Characterization Table (Normalized to +40°C)
Manufacturers Rated Cell Capacity: 1000mAh
Charge Voltage: 8.4V
Charge Current: 500mA
Termination Current: 50mA
Active Empty (V, I): 6.0V, 300mA
Standby Empty (V, I): 6.0V, 4mA
Sense Resistor: 0.020W
Segment
Break Points
Full
Active Empty
Standby Empty
TBP12
TBP23
TBP34
-12ºC
0ºC
18ºC
Seg. 1
ppm/°C
Seg. 2
ppm/°C
Seg. 3
ppm/°C
Seg. 4
ppm/°C
3601
2380
1404
3113
1099
427
1163
671
244
854
305
183
+40°C
Nominal
[mAh]
1051
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DS2781: Two-Cell Standalone Fuel Gauge IC
Figure 13. Lookup Function Diagram
FULL(T)
Cell Model
Parameters
Look-up
(EEPROM)
Function
AE(T)
SE(T)
Temperature
APPLICATION PARAMETERS
In addition to cell model characteristics, several application parameters are needed to detect the full and empty
points, as well as calculate results in mAh units.
Sense Resistor Prime (RSNSP): RSNSP stores the value of the sense resistor for use in computing the absolute
capacity results. The value is stored as a 1-byte conductance value with units of mhos. RSNSP supports resistor
values of 1W to 3.922mW. RSNSP is located in the Parameter EEPROM block.
Charge Voltage (VCHG): VCHG stores the charge voltage threshold used to detect a fully charged state. The
value is stored as a 1-byte voltage with units of 39.04mV and can range from 0V to 9.956V. VCHG should be set
marginally less than the cell stack voltage at the end of the charge cycle to ensure reliable charge termination
detection. VCHG is located in the Parameter EEPROM block.
Minimum Charge Current (IMIN): IMIN stores the charge current threshold used to detect a fully charged state.
The value is stored as a 1-byte value with units of 50V and can range from 0 to 12.75mV. Assuming RSNS =
20mW, IMIN can be programmed from 0mA to 637.5mA in 2.5mA steps. IMIN should be set marginally greater than
the charge current at the end of the charge cycle to ensure reliable charge termination detection. IMIN is located in
the Parameter EEPROM block.
Active Empty Voltage (VAE): VAE stores the voltage threshold used to detect the Active Empty point. The value
is stored in 1-byte with units of 39.04mV and can range from 0V to 9.956V. VAE is located in the Parameter
EEPROM block. See the Cell Characteristics section for more information.
Active Empty Current (IAE): IAE stores the discharge current threshold used to detect the Active Empty point.
The unsigned value represents the magnitude of the discharge current and is stored in 1-byte with units of 200mV
and can range from 0 to 51.2mV. Assuming RSNS = 20mW, IAE can be programmed from 0mA to 2550mA in
10mA steps. IAE is located in the Parameter EEPROM block. See the Cell Characteristics section for more
information.
Aging Capacity (AC): AC stores the rated battery capacity used in estimating the decrease in battery capacity that
occurs in normal use. The value is stored in 2-bytes in the same units as the ACR (6.25mVh). Setting AC to the
manufacturer’s rated capacity sets the aging rate to approximately 2.4% per 100 cycles of equivalent full capacity
discharges. Partial discharge cycles are added to form equivalent full capacity discharges. The default estimation
results in 88% capacity after 500 equivalent cycles. The estimated aging rate can be adjusted by setting AC to a
different value than the cell manufacturer’s rating. Setting AC to a lower value, accelerates the estimated aging.
Setting AC to a higher value, retards the estimated aging. AC is located in the Parameter EEPROM block.
Age Scalar (AS): AS adjusts the capacity estimation results downward to compensate for cell aging. AS is a 1-byte
-7
value that represents values between 49.2% and 100%. The lsb is weighted at 0.78% (precisely 2 ). A value of
100% (128 decimal or 80h) represents an un-aged battery. A value of 95% is recommended as the starting AS
value at the time of pack manufacture to allow learning a larger capacity on batteries that have an initial capacity
greater than the nominal capacity programmed in the cell characteristic table. AS is modified by the cycle count
based age estimation introduced above and by the capacity Learn function. The host system has read and write
access to AS, however caution should exercised when writing AS to ensure that the cumulative aging estimate is
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DS2781: Two-Cell Standalone Fuel Gauge IC
not overwritten with an incorrect value. Usually, writing AS by the host is not necessary because AS is
automatically saved to EEPROM on a periodic basis by the DS2781. (See the Memory section for details.) The
EEPROM stored value of AS is recalled on power-up.
CAPACITY ESTIMATION UTILITY FUNCTIONS
Aging Estimation
As discussed above, the AS register value is adjusted occasionally based on cumulative discharge. As the ACR
register decrements during each discharge cycle, an internal counter is incremented until equal to 32 times AC. AS
is then decremented by one, resulting in a decrease in the scaled full battery capacity of 0.78%. Refer to the AC
register description above for recommendations on customizing the age estimation rate.
Learn Function
Since Li+ cells exhibit charge efficiencies near unity, the charge delivered to a Li+ cell from a known empty point to
a known full point is a dependable measure of the cell capacity. A continuous charge from empty to full results in a
“learn cycle”. First, the Active Empty point must be detected. The Learn Flag (LEARNF) is set at this point. Then,
once charging starts, the charge must continue uninterrupted until the battery is charged to full. Upon detecting full,
LEARNF is cleared, the Charge to Full (CHGTF) flag is set and the Age Scalar (AS) is adjusted according to the
learned capacity of the cell stack.
ACR Housekeeping
The ACR register value is adjusted occasionally to maintain the coulomb count within the model curve boundaries.
When the battery is charged to full (CHGTF set), the ACR is set equal to the age scaled full lookup value at the
present temperature. If a learn cycle is in progress, correction of the ACR value occurs after the age scalar (AS) is
updated.
When an empty condition is detected (AEF or LEARNF set), the ACR adjustment is conditional. If AEF is set and
LEARNF is not, then the Active Empty Point was not detected and the battery is likely below the Active Empty
capacity of the model. The ACR is set to the Active Empty model value only if it is greater than the Active Empty
model value. If LEARNF is set, then the battery is at the Active Empty Point and the ACR is set to the Active Empty
model value.
Full Detect
Full detection occurs when the Voltage (VOLT) readings remain continuously above the VCHG threshold for the
period between two Average Current (IAVG) readings, where both IAVG readings are below IMIN. The two
consecutive IAVG readings must also be positive and non-zero. This ensures that removing the battery from the
charger does not result in a false detection of full. Full Detect sets the Charge to Full (CHGTF) bit in the Status
register.
Active Empty Point Detect
Active Empty Point detection occurs when the Voltage register drops below the VAE threshold and the two
previous Current readings are above IAE. This captures the event of the battery reaching the Active Empty point.
Note that the two previous Current readings must be negative and greater in magnitude than IAE, that is, a larger
discharge current than specified by the IAE threshold. Qualifying the Voltage level with the discharge rate ensures
that the Active Empty point is not detected at loads much lighter than those used to construct the model. Also,
Active Empty must not be detected when a deep discharge at a very light load is followed by a load greater than
IAE. Either case would cause a learn cycle on the following charge to full to include part of the Standby capacity in
the measurement of the Active capacity. Active Empty detection sets the Learn Flag (LEARNF) bit in the Status
register.
RESULT REGISTERS
The DS2781 processes measurement and cell characteristics on a 3.5s interval and yields seven result registers.
The result registers are sufficient for direct display to the user in most applications. The host system can produce
customized values for system use, or user display by combining measurement, result and User EEPROM values.
FULL(T) [ ]: The Full capacity of the battery at the present temperature is reported normalized to the 40°C Full
value. This 15-bit value reflects the cell stack model Full value at the given temperature. FULL(T) reports values
16 of 30
DS2781: Two-Cell Standalone Fuel Gauge IC
-14
between 100% and 50% with a resolution of 61ppm (precisely 2 ). Though the register format permits values
greater than 100%, the register value is clamped to a maximum value of 100%.
Active Empty, AE(T) [ ]: The Active Empty capacity of the battery at the present temperature is reported
normalized to the 40°C Full value. This 13-bit value reflects the cell stack model Active Empty at the given
-14
temperature. AE(T) reports values between 0% and 49.8% with a resolution of 61ppm (precisely 2 ).
Standby Empty, SE(T) [ ]: The Standby Empty capacity of the battery at the present temperature is reported
normalized to the 40°C Full value. This 13-bit value reflects the cell stack model Standby Empty value at the
-14
current temperature. SE(T) reports values between 0% and 49.8% with a resolution of 61ppm (precisely 2 ).
Remaining Active Absolute Capacity (RAAC) [mAh]: RAAC reports the capacity available under the current
temperature conditions at the Active Empty discharge rate (IAE) to the Active Empty point in absolute units of milliamp-hours. RAAC is 16 bits.
Remaining Standby Absolute Capacity (RSAC) [mAh]: RSAC reports the capacity available under the current
temperature conditions at the Standby Empty discharge rate (ISE) to the Standby Empty point capacity in absolute
units of milli-amp-hours. RSAC is 16 bits.
Remaining Active Relative Capacity (RARC) [%]: RARC reports the capacity available under the current
temperature conditions at the Active Empty discharge rate (IAE) to the Active Empty point in relative units of
percent. RARC is 8 bits.
Remaining Standby Relative Capacity (RSRC) [%]: RSRC reports the capacity available under the current
temperature conditions at the Standby Empty discharge rate (ISE) to the Standby Empty point capacity in relative
units of percent. RSRC is 8 bits.
Calculation of Results
RAAC [mAh] = (ACR[mVh] - AE(T) * FULL40[mVh]) * RSNSP [mhos]
RSAC [mAh] = (ACR[mVh] - SE(T) * FULL40[mVh]) * RSNSP [mhos]
RARC [%] = 100% * (ACR[mVh] - AE(T) * FULL40[mVh]) /
{(AS * FULL(T) - AE(T)) * FULL40[mVh]}
RSRC [%] = 100%* (ACR[mVh] - SE(T) * FULL40[mVh]) /
{(AS * FULL(T) - SE(T)) * FULL40[mVh]}
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DS2781: Two-Cell Standalone Fuel Gauge IC
STATUS REGISTER
The STATUS register contains bits that report the device status. The bits can be set internally by the DS2781. The
CHGTF, AEF, SEF, LEARNF and VER bits are read only bits that can be cleared by hardware. The UVF and
PORF bits can only be cleared via the 1-Wire interface.
Figure 14. Status Register Format
Address
Field
01h
Bit
Format
CHGTF
7
Read Only
AEF
6
Read Only
SEF
5
Read Only
LEARNF
4
Read Only
Reserved
3
Read Only
UVF
2
Read / Write *
PORF
1
Read / Write *
Reserved
0
Read Only
Bit Definition
Allowable Values
Charge Termination Flag
Set to 1 when: ( VOLT > VCHG ) AND ( 0 < IAVG < IMIN ) continuously
for a period between two IAVG register updates ( 28s to 56s ).
Cleared to 0 when: RARC < 90%
Active Empty Flag
Set to 1 when: VOLT < VAE
Cleared to 0 when: RARC > 5%
Standby Empty Flag
Set to 1 when: RSRC < 10%
Cleared to 0 when: RSRC > 15%
Learn Flag¾When set to 1, a charge cycle can be used to learn battery
capacity.
Set to 1 when: ( VOLT falls from above VAE to below VAE ) AND
( CURRENT > IAE )
Cleared to 0 when: ( CHGTF = 1 ) OR ( CURRENT < 0 ) OR
( ACR = 0 **) OR ( ACR written or recalled from EEPROM) OR ( SLEEP
Entered )
Undefined
Undervoltage Flag
Set to 1 when: VOLT < VSLEEP
Cleared to 0 by: User
Power-On Reset Flag¾Useful for reset detection, see text below.
Set to 1 upon Power-Up by hardware.
Cleared to 0 by: User
Undefined
* - This bit can be set by the DS2781, and may only be cleared through the 1-Wire interface.
** - LEARNF is only cleared if ACR reaches 0 after VOLT < VAE.
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DS2781: Two-Cell Standalone Fuel Gauge IC
CONTROL REGISTER
All CONTROL register bits are read and write accessible. The CONTROL register is recalled from Parameter
EEPROM memory at power-up. Register bit values can be modified in shadow RAM after power-up. Shadow RAM
values can be saved as the power up default values by using the Copy Data command.
Figure 15. Control Register Format
Address
Field
60h
Bit
Format
NBEN
7
Read/Write
UVEN
6
Read/Write
PMOD
5
Read/Write
RNAOP
4
Read/Write
UVTH
3
Read/Write
Reserved
0:2
Bit Definition
Allowable Values
Negative Blanking Enable
0: Allows negative current readings to always be accumulated
1: Enables blanking of negative current readings up to -25µV
Under Voltage SLEEP Enable
0: Disables transition to SLEEP mode based on VIN voltage
1: Enables transition to SLEEP mode if,
VIN < VSLEEP AND DQ stable at either logic level for tSLEEP
Power Mode Enable
0: Disables transition to SLEEP mode based on DQ logic state
1: Enables transition to SLEEP mode if DQ at a logic low for tSLEEP
Read Net Address Opcode
0: Read Net Address Command = 33h
1: Read Net Address Command = 39h
Under Voltage Threshold Select
0: Selects an Under Voltage Sleep threshold of 2.45V
1: Selects an Under Voltage Sleep threshold of 4.9V
Undefined
SPECIAL FEATURE REGISTER
All Special Feature Register bits are read and write accessible, with default values specified in each bit definition.
Figure 16. Special Feature Register Format
Address
Field
Reserved
PIOSC
15h
Bit
1:7
Format
0
Read/Write
Bit Definition
Allowable Values
Undefined
PIO Sense and Control
Read values
0: PIO pin ≤ Vil
1: PIO pin ≥ Vih
Write values
0: Activates PIO pin open-drain output driver, forcing the PIO pin low
1: Disables the output driver, allowing the PIO pin to be pulled high or
used as an input
Power-up and SLEEP mode default: 1 (PIO pin is hi-Z)
Note: PIO pin has weak pulldown
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DS2781: Two-Cell Standalone Fuel Gauge IC
EEPROM REGISTER
The EEPROM register provides access control of the EEPROM blocks. EEPROM blocks can be locked to prevent
alteration of data within the block. Locking a block disables write access to the block. Once a block is locked, it
cannot be unlocked. Read access to EEPROM blocks is unaffected by the lock/unlock status.
Figure 17. EEPROM REGISTER FORMAT
Address
1Fh
Field
Bit
Format
EEC
7
Read Only
LOCK
6
Read /
Write to 1
Reserved
2:6
BL1
1
Read Only
BL0
0
Read Only
Bit Definition
Allowable Values
EEPROM Copy Flag
Set to 1 when: Copy Data command executed
Cleared to 0 when: Copy Data command completes
Note: While EEC = 1, writes to EEPROM addresses are ignored
Power-up default: 0
EEPROM Lock Enable
Host write to 1: Enables the Lock command. Host must issue Lock
command as next command after writing Lock Enable bit to 1.
Cleared to 0 when: Lock command completes or when Lock command
not the command issued immediately following the Write command
used to set the Lock Enable bit.
Power-up default: 0
Undefined
EEPROM Block 1 Lock Flag (Parameter EEPROM 60h–7Fh)
0: EEPROM is not locked
1: EEPROM block is locked
Factory default: 0
EEPROM Block 0 Lock Flag (User EEPROM 20h–2Fh)
0: EEPROM is not locked
1: EEPROM block is locked
Factory default: 0
MEMORY
The DS2781 has a 256 byte linear memory space with registers for instrumentation, status, and control, as well as
EEPROM memory blocks to store parameters and user information. Byte addresses designated as “Reserved”
return undefined data when read. Reserved bytes should not be written. Several byte registers are paired into twobyte registers in order to store 16-bit values. The most significant byte (MSB) of the 16 bit value is located at a
even address and the least significant byte (LSB) is located at the next address (odd) byte. When the MSB of a
two-byte register is read, the MSB and LSB are latched simultaneously and held for the duration of the read data
command to prevent updates to the LSB during the read. This ensures synchronization between the two register
bytes. For consistent results, always read the MSB and the LSB of a two-byte register during the same read data
command sequence.
EEPROM memory consists of the non-volatile EEPROM cells overlaid with volatile shadow RAM. The Read Data
and Write Data commands allow the 1-Wire interface to directly accesses only the shadow RAM. The Copy Data
and Recall Data function commands transfer data between the shadow RAM and the EEPROM cells. In order to
modify the data stored in the EEPROM cells, data must be written to the shadow RAM and then copied to the
EEPROM. In order to verify the data stored in the EEPROM cells, the EEPROM data must be recalled to the
shadow RAM and then read from the shadow RAM.
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DS2781: Two-Cell Standalone Fuel Gauge IC
USER EEPROM
A 16 byte User EEPROM memory (block 0, addresses 20h - 2Fh) provides non-volatile memory that is
uncommitted to other DS2781 functions. Accessing the User EEPROM block does not affect the operation of the
DS2781. User EEPROM is lockable, and once locked, write access is not allowed. The battery pack or host system
manufacturer can program lot codes, date codes and other manufacturing, warranty, or diagnostic information and
then lock it to safeguard the data. User EEPROM can also store parameters for charging to support different size
batteries in a host device as well as auxiliary model data such as time to full charge estimation parameters.
PARAMETER EEPROM
Model data for the cells, as well as application operating parameters are stored in the Parameter EEPROM
memory (block 1, addresses 60h - 7Fh). The ACR (MSB and LSB) and AS registers are automatically saved to
EEPROM when the RARC result crosses 4% boundaries. This allows the DS2781 to be located outside the
protection FETs. In this manner, if a protection device is triggered, the DS2781 cannot lose more that 4% of charge
or discharge data.
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DS2781: Two-Cell Standalone Fuel Gauge IC
Table 2. MEMORY MAP
ADDRESS (HEX)
00
01
02
03
DESCRIPTION
Reserved
STATUS - Status Register
RAAC - Remaining Active Absolute Capacity MSB
RAAC - Remaining Active Absolute Capacity LSB
READ/WRITE
R
R/W
R
R
04
05
06
07
08
09
RSAC - Remaining Standby Absolute Capacity MSB
RSAC - Remaining Standby Absolute Capacity LSB
RARC - Remaining Active Relative Capacity
RSRC - Remaining Standby Relative Capacity
IAVG - Average Current Register MSB
IAVG - Average Current Register LSB
R
R
R
R
R
R
0A
0B
0C
0D
0E
0F
TEMP - Temperature Register MSB
TEMP - Temperature Register LSB
VOLT - Voltage Register MSB
VOLT - Voltage Register LSB
CURRENT - Current Register MSB
CURRENT - Current Register LSB
R
R
R
R
R
R
10
11
12
13
14
15
ACR - Accumulated Current Register MSB
ACR - Accumulated Current Register LSB
ACRL – Low Accumulated Current Register MSB
ACRL – Low Accumulated Current Register LSB
AS - Age Scalar
SFR - Special Feature Register
16
17
18
19
1A
1B
FULL - Full Capacity MSB
FULL - Full Capacity LSB
AE - Active Empty MSB
AE - Active Empty LSB
SE - Standby Empty MSB
SE - Standby Empty LSB
R/W*
R/W *
R
R
R/W *
R/W
R
R
R
R
R
R
1C to 1E
1F
20 to 2F
30 to 5F
60 to 7F
80 to AF
Reserved
EEPROM - EEPROM Register
User EEPROM, Lockable, Block 0
Reserved
Parameter EEPROM, Lockable, Block 1
Reserved
B0 to B1
B2 to FF
FSGAIN – Factory Gain Calibration Value
Reserved
—
R/W
R/W
—
R/W
—
R
—
* Register value is automatically saved to EEPROM during ACTIVE mode operation and recalled from EEPROM
on power up.
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DS2781: Two-Cell Standalone Fuel Gauge IC
Table 3. PARAMETER EEPROM MEMORY BLOCK 1
ADDRESS
(HEX)
60
61
62
63
64
65
66
67
68
69
6A
6B
6C
6D
6E
6F
DESCRIPTION
CONTROL - Control Register
AB - Accumulation Bias
AC - Aging Capacity MSB
AC - Aging Capacity LSB
VCHG - Charge Voltage
IMIN - Minimum Charge Current
VAE - Active Empty Voltage
IAE - Active Empty Current
ADDRESS
(HEX)
70
71
72
73
DESCRIPTION
AE Segment 4 Slope
AE Segment 3 Slope
AE Segment 2 Slope
AE Segment 1 Slope
Active Empty 40
RSNSP - Sense Resistor Prime
74
75
76
77
78
79
SE Segment 4 Slope
SE Segment 3 Slope
SE Segment 2 Slope
SE Segment 1 Slope
RSGAIN - Sense Resistor Gain MSB
RSGAIN - Sense Resistor Gain LSB
Full 40 MSB
Full 40 LSB
Full Segment 4 Slope
Full Segment 3 Slope
Full Segment 2 Slope
Full Segment 1 Slope
7A
7B
7C
7D
7E
7F
RSTC - Sense Resistor Temp. Coeff.
COB – Current Offset Bias
TBP34
TBP23
TBP12
Reserved
1-WIRE BUS SYSTEM
The 1-Wire bus is a system that has a single bus master and one or more slaves. A multidrop bus is a 1-Wire bus
with multiple slaves. A single-drop bus has only one slave device. In all instances, the DS2781 is a slave device.
The bus master is typically a microprocessor in the host system. The discussion of this bus system consists of four
topics: 64-bit net address, hardware configuration, transaction sequence, and 1-Wire signaling.
64-BIT NET ADDRESS
Each DS2781 has a unique, factory-programmed 1-Wire net address that is 64 bits in length. The first eight bits are
the 1-Wire family code (3Dh for DS2781). The next 48 bits are a unique serial number. The last eight bits are a
cyclic redundancy check (CRC) of the first 56 bits (see Figure 18). The 64-bit net address and the 1-Wire I/O
circuitry built into the device enable the DS2781 to communicate through the 1-Wire protocol detailed in the 1-Wire
Bus System section of this data sheet.
Figure 18. 1-Wire Net Address Format
8-BIT CRC
MSb
48-BIT SERIAL NUMBER
8-BIT FAMILY
CODE (3Dh)
LSb
CRC GENERATION
The DS2781 has an 8-bit CRC stored in the most significant byte of its 1-Wire net address. To ensure error-free
transmission of the address, the host system can compute a CRC value from the first 56 bits of the address and
compare it to the CRC from the DS2781. The host system is responsible for verifying the CRC value and taking
action as a result. The DS2781 does not compare CRC values and does not prevent a command sequence from
proceeding as a result of a CRC mismatch. Proper use of the CRC can result in a communication channel with a
very high level of integrity.
The CRC can be generated by the host using a circuit consisting of a shift register and XOR gates as shown in
Figure 18, or it can be generated in software. Additional information about the Dallas 1-Wire CRC is available in
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DS2781: Two-Cell Standalone Fuel Gauge IC
Application Note 27, Understanding and Using Cyclic Redundancy Checks with Dallas Semiconductor Touch
Memory Products. (This application note can be found on the Maxim/Dallas Semiconductor website at
www.maxim-ic.com.)
In the circuit in Figure 19, the shift register bits are initialized to 0. Then, starting with the least significant bit of the
family code, one bit at a time is shifted in. After the 8th bit of the family code has been entered, then the serial
number is entered. After the 48th bit of the serial number has been entered, the shift register contains the CRC
value.
Figure 19. 1-Wire CRC Generation Block Diagram
INPUT
MSb
XOR
LSb
XOR
XOR
HARDWARE CONFIGURATION
Because the 1-Wire bus has only a single line, it is important that each device on the bus be able to drive it at the
appropriate time. To facilitate this, each device attached to the 1-Wire bus must connect to the bus with open-drain
or tri-state output drivers. The DS2781 uses an open-drain output driver as part of the bidirectional interface
circuitry shown in Figure 20. If a bidirectional pin is not available on the bus master, separate output and input pins
can be connected together.
The 1-Wire bus must have a pullup resistor at the bus-master end of the bus. For short line lengths, the value of
this resistor should be approximately 5kW. The idle state for the 1-Wire bus is high. If, for any reason, a bus
transaction must be suspended, the bus must be left in the idle state to properly resume the transaction later. If the
bus is left low for more than 120ms (16ms for overdrive speed), slave devices on the bus begin to interpret the low
period as a reset pulse, effectively terminating the transaction.
The DS2781 can operate in two communication speed modes, standard and overdrive. The speed mode is
determined by the input logic level of the OVD pin with a logic 0 selecting standard speed and a logic 1 selecting
overdrive speed. The OVD pin must be at a stable logic level of 0 or 1 before initializing a transaction with a reset
pulse. All 1-Wire devices on a multinode bus must operate at the same communication speed for proper operation.
1-Wire timing for both standard and overdrive speeds are listed in the Electrical Characteristics: 1-Wire Interface
tables.
Figure 20. 1-Wire Bus Interface Circuitry
BUS MASTER
Vpullup
(2.0V to 5.5V)
DS2781 1-WIRE PORT
4.7kW
RX
Rx
0.2mA
(typ)
Tx
RX = RECEIVE
TX = TRANSMIT
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TX
100W
MOSFET
DS2781: Two-Cell Standalone Fuel Gauge IC
TRANSACTION SEQUENCE
The protocol for accessing the DS2781 through the 1-Wire port is as follows:
§
§
§
§
Initialization
Net Address Command
Function Command
Transaction/Data
The sections that follow describe each of these steps in detail.
All transactions of the 1-Wire bus begin with an initialization sequence consisting of a reset pulse transmitted by the
bus master followed by a presence pulse simultaneously transmitted by the DS2781 and any other slaves on the
bus. The presence pulse tells the bus master that one or more devices are on the bus and ready to operate. For
more details, see the 1-Wire Signaling section.
NET ADDRESS COMMANDS
Once the bus master has detected the presence of one or more slaves, it can issue one of the net address
commands described in the following paragraphs. The name of each ROM command is followed by the 8-bit
opcode for that command in square brackets. Figure 21 presents a transaction flowchart of the net address
commands.
Read Net Address [33h or 39h]. This command allows the bus master to read the DS2781’s 1-Wire net address.
This command can only be used if there is a single slave on the bus. If more than one slave is present, a data
collision occurs when all slaves try to transmit at the same time (open drain produces a wired-AND result). The
RNAOP bit in the status register selects the opcode for this command, with RNAOP = 0 indicating 33h, and
RNAOP = 1 indicating 39h.
Match Net Address [55h]. This command allows the bus master to specifically address one DS2781 on the 1-Wire
bus. Only the addressed DS2781 responds to any subsequent function command. All other slave devices ignore
the function command and wait for a reset pulse. This command can be used with one or more slave devices on
the bus.
Skip Net Address [CCh]. This command saves time when there is only one DS2781 on the bus by allowing the
bus master to issue a function command without specifying the address of the slave. If more than one slave device
is present on the bus, a subsequent function command can cause a data collision when all slaves transmit data at
the same time.
Search Net Address [F0h]. This command allows the bus master to use a process of elimination to identify the 1Wire net addresses of all slave devices on the bus. The search process involves the repetition of a simple threestep routine: read a bit, read the complement of the bit, then write the desired value of that bit. The bus master
performs this simple three-step routine on each bit location of the net address. After one complete pass through all
64 bits, the bus master knows the address of one device. The remaining devices can then be identified on
®
additional iterations of the process. See Chapter 5 of the Book of DS19xx iButton Standards for a comprehensive
discussion of a net address search, including an actual example. (This publication can be found on the
Maxim/Dallas Semiconductor website at www.maxim-ic.com.)
Resume [A5h]. This command increases data throughput in multidrop environments where the DS2781 needs to
be accessed several times. Resume is similar to the Skip Net Address command in that the 64-bit net address
does not have to be transmitted each time the DS2781 is accessed. After successfully executing a Match Net
Address command or Search Net Address command, an internal flag is set in the DS2781. When the flag is set,
the DS2781 can be repeatedly accessed through the Resume command function. Accessing another device on the
bus clears the flag, thus preventing two or more devices from simultaneously responding to the Resume command
function.
iButton is a registered trademark of Dallas Semiconductor.
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DS2781: Two-Cell Standalone Fuel Gauge IC
FUNCTION COMMANDS
After successfully completing one of the net address commands, the bus master can access the features of the
DS2781 with any of the function commands described in the following paragraphs. The name of each function is
followed by the 8-bit opcode for that command in square brackets. The function commands are summarized in
Table 4.
Read Data [69h, XX]. This command reads data from the DS2781 starting at memory address XX. The LSb of the
data in address XX is available to be read immediately after the MSb of the address has been entered. Because
the address is automatically incremented after the MSb of each byte is received, the LSb of the data at address XX
+ 1 is available to be read immediately after the MSb of the data at address XX. If the bus master continues to read
beyond address FFh, data is read starting at memory address 00 and the address is automatically incremented
until a reset pulse occurs. Addresses labeled “Reserved” in the memory map contain undefined data values. The
read data command can be terminated by the bus master with a reset pulse at any bit boundary. Reads from
EEPROM block addresses return the data in the shadow RAM. A Recall Data command is required to transfer data
from the EEPROM to the shadow. See the Memory section for more details.
Write Data [6Ch, XX]. This command writes data to the DS2781 starting at memory address XX. The LSb of the
data to be stored at address XX can be written immediately after the MSb of address has been entered. Because
the address is automatically incremented after the MSb of each byte is written, the LSb to be stored at address XX
+ 1 can be written immediately after the MSb to be stored at address XX. If the bus master continues to write
beyond address FFh, the data starting at address 00 is overwritten. Writes to read-only addresses, reserved
addresses and locked EEPROM blocks are ignored. Incomplete bytes are not written. Writes to unlocked EEPROM
block addresses modify the shadow RAM. A Copy Data command is required to transfer data from the shadow to
the EEPROM. See the Memory section for more details.
Copy Data [48h, XX]. This command copies the contents of the EEPROM shadow RAM to EEPROM cells for the
EEPROM block containing address XX. Copy data commands that address locked blocks are ignored. While the
copy data command is executing, the EEC bit in the EEPROM register is set to 1 and writes to EEPROM
addresses are ignored. Reads and writes to non-EEPROM addresses can still occur while the copy is in progress.
The copy data command takes tEEC time to execute, starting on the next falling edge after the address is
transmitted.
Recall Data [B8h, XX]. This command recalls the contents of the EEPROM cells to the EEPROM shadow memory
for the EEPROM block containing address XX.
Lock [6Ah, XX]. This command locks (write-protects) the block of EEPROM memory containing memory address
XX. The LOCK bit in the EEPROM register must be set to 1 before the lock command is executed. To help prevent
unintentional locks, one must issue the lock command immediately after setting the LOCK bit (EEPROM register,
address 1Fh, bit 06) to a 1. If the LOCK bit is 0 or if setting the lock bit to 1 does not immediately precede the lock
command, the lock command has no effect. The lock command is permanent; a locked block can never be written
again.
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DS2781: Two-Cell Standalone Fuel Gauge IC
Table 4. Function Commands
COMMAND
Read Data
Write Data
Copy Data
Recall Data
Lock
DESCRIPTION
Reads data from
memory starting at
address XX
Writes data to
memory starting at
address XX
Copies shadow RAM
data to EEPROM
block containing
address XX
Recalls EEPROM
block containing
address XX to RAM
Permanently locks
the block of EEPROM
containing address
XX
COMMAND
PROTOCOL
BUS STATE
AFTER
COMMAND
PROTOCOL
69h, XX
Master Rx
Up to 256 bytes of
data
6Ch, XX
Master Tx
Up to 256 bytes of
data
48h, XX
Master Reset
None
B8h, XX
Master Reset
None
6Ah, XX
Master Reset
None
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BUS DATA
DS2781: Two-Cell Standalone Fuel Gauge IC
Figure 21. Net Address Command Flow Chart
MASTER TX
RESET PULSE
DS2781 Tx
PRESENCE PULSE
MASTER Tx NET
ADDRESS
COMMAND
33h / 39h
READ
NO
55h
MATCH
YES
F0h
SEARCH
NO
YES
YES
MASTER TX
BIT 0
DS2781 Tx
FAMILY CODE
1 BYTE
NO
CCh
SKIP
YES
NO
A5h
RESUME
YES
DS2781 Tx BIT 0
DS2781 Tx BIT 0
CLEAR RESUME
RESUME
FLAG SET ?
MASTER Tx BIT 0
DS2781 Tx
SERIAL NUMBER
6 BYTES
YES
BIT 0
MATCH ?
DS2781 Tx
CRC
1 BYTE
NO
NO
YES
BIT 0
MATCH ?
YES
MASTER TX
BIT 1
DS2781 Tx BIT 1
DS2781 Tx BIT 1
CLEAR RESUME
MASTER Tx BIT 1
BIT 1
MATCH ?
YES
MASTER TX
BIT 63
MASTER TX
FUNCTION
COMMAND
NO
NO
BIT 1
MATCH ?
YES
DS2781 Tx BIT 63
DS2781 Tx BIT 63
MASTER Tx BIT 63
SET
RESUME
FLAG
YES
NO
BIT 63
MATCH ?
NO
CLEAR RESUME
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MASTER TX
FUNCTION
COMMAND
MASTER TX
FUNCTION
COMMAND
NO
DS2781: Two-Cell Standalone Fuel Gauge IC
1-WIRE SIGNALING
The 1-Wire bus requires strict signaling protocols to ensure data integrity. The four protocols used by the DS2781
are as follows: the initialization sequence (reset pulse followed by presence pulse), write 0, write 1, and read data.
All of these types of signaling except the presence pulse are initiated by the bus master.
The initialization sequence required to begin any communication with the DS2781 is shown in Figure 22. A
presence pulse following a reset pulse indicates that the DS2781 is ready to accept a net address command. The
bus master transmits (Tx) a reset pulse for tRSTL. The bus master then releases the line and goes into receive mode
(Rx). The 1-Wire bus line is then pulled high by the pullup resistor. After detecting the rising edge on the DQ pin,
the DS2781 waits for tPDH and then transmits the presence pulse for tPDL.
Figure 22. 1-Wire Initialization Sequence
tRSTL
tRSTH
tPDH
tPDL
PACK+
DQ
PACKLINE TYPE LEGEND:
BUS MASTER ACTIVE LOW
DS2781 ACTIVE LOW
BOTH BUS MASTER AND
DS2781 ACTIVE LOW
RESISTOR PULLUP
WRITE-TIME SLOTS
A write-time slot is initiated when the bus master pulls the 1-Wire bus from a logic-high (inactive) level to a logic-low
level. There are two types of write-time slots: write 1 and write 0. All write-time slots must be tSLOT in duration with a
1ms minimum recovery time, tREC, between cycles. The DS2781 samples the 1-Wire bus line between 15ms and
60ms (between 2ms and 6ms for overdrive speed) after the line falls. If the line is high when sampled, a write 1
occurs. If the line is low when sampled, a write 0 occurs (see Figure 23). For the bus master to generate a write 1
time slot, the bus line must be pulled low and then released, allowing the line to be pulled high within 15ms (2ms for
overdrive speed) after the start of the write-time slot. For the host to generate a write 0 time slot, the bus line must
be pulled low and held low for the duration of the write-time slot.
READ-TIME SLOTS
A read-time slot is initiated when the bus master pulls the 1-Wire bus line from a logic-high level to a logic-low level.
The bus master must keep the bus line low for at least 1ms and then release it to allow the DS2781 to present valid
data. The bus master can then sample the data tRDV from the start of the read-time slot. By the end of the read-time
slot, the DS2781 releases the bus line and allows it to be pulled high by the external pullup resistor. All read-time
slots must be tSLOT in duration with a 1ms minimum recovery time, tREC, between cycles. See Figure 23 for more
information.
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DS2781: Two-Cell Standalone Fuel Gauge IC
Figure 23. 1-Wire Write- And Read-Time Slots
WRITE 0 SLOT
WRITE 1 SLOT
tSLOT
tSLOT
tLOW0
tLOW1
tREC
VPULLUP
GND
Mode
MIN
DS2781 Sample Window
TYP
MAX
MIN
>1ms
DS2781 Sample Window
TYP
MAX
Standard
15ms
15ms
30ms
15ms
15ms
30ms
Overdrive
2ms
1ms
3ms
2ms
1ms
3ms
READ 0 SLOT
READ 1 SLOT
tSLOT
tSLOT
tREC
VPULLUP
GND
Master Sample Window
>1ms
tRDV
Master Sample Window
tRDV
LINE TYPE LEGEND:
Bus master active low
DS2781 active low
Both bus master and
DS2781 active low
Resistor pullup
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