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DS2781E+-DS2781E+T&R
1-Cell or 2-Cell Stand-Alone Fuel Gauge IC
GENERAL DESCRIPTION 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 on-
chip 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.
APPLICATIONS Digital Video Cameras
Commercial Two-Way Radios
Industrial PDAs and Handheld PC Data Terminals
Portable GPS Navigation Systems
SIMPLIFIED TYPICAL OPERATING
CIRCUIT DQ
SNS
VDD
VSS
Protection
Circuit
DS2781PIO
VB
VIN
OVD
PIN CONFIGURATIONS VSS
VSS
VIN
VDD
PIO
SNS
OVD
PAD
VSS
VIN
VDD
PIO
SNS
OVD
TSSOP-8
3mm x 4mm TDFN-10
TOP VIEW
FEATURES � Precision Voltage, Temperature, and Current
Measurement System � Operates in One-Cell or Two--Cell Applications � Accurate, Temperature Stable Internal Timebase � 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 Temperature Coefficient Calibration
Allows the Use of Low-Cost Sense Resistors � 24-Byte Parameter EEPROM � 16-Byte User EEPROM � Unique ID and Multidrop 1-Wire® Interface � Tiny 8-Pin TSSOP and 10-Pin TDFN (3mm x
4mm) Packages Embed Easily in Thin Prismatic
Cell Packs
ORDERING INFORMATION
PART PIN-PACKAGE TOP MARK DS2781E+ 8 TSSOP 2781
DS2781E+T&R 8 TSSOP 2781
DS2781G+ 10 TDFN-EP* 2781
DS2781G+T&R 10 TDFN-EP* 2781
+Denotes a lead-free/RoHS-compliant package.
T&R = Tape and reel.
DS2781
1-Cell or 2-Cell Stand-Alone
Fuel Gauge IC
DS2781
ABSOLUTE MAXIMUM RATINGS Voltage Range on VDD, VIN Relative to VSS -0.3V to +12V
Voltage Range on Any Pin Relative to VSS -0.3V to +6.0V
Continuous Sink Current, DQ, PIO 20mA
Operating Temperature Range -40°C to +85°C
Storage Temperature Range -55°C to +125°C
Soldering Temperature Refer to the IPC/JEDEC J-STD-020 Specification.
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 reliability.
RECOMMENDED DC OPERATING CHARACTERISTICS (VDD = 2.5V to 10V, TA = -20°C to +70°C, unless otherwise noted. Typical values are at TA = +25°C)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS Supply Voltage VDD (Note 1) +2.5 +10 V
VIN Voltage Range (Note 1) -0.3 VDD + 0.3 V
DQ, PIO Voltage Range (Note 1) -0.3 +5.5 V
VB Output Voltage VVB
VDD > 3.0V,
IVB = 500μA,
(Note 1)
2.5 2.8 3.1 V
OVD Voltage Range (Note 1) -0.3 VVB + 0.3 V
DC ELECTRICAL CHARACTERISTICS (VDD = 2.5V to 10V, TA = -20°C to +70°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS ACTIVE Current IACTIVE IVB = 0 70 95 μA
TA > +50°C, IVB = 0 10 SLEEP Mode Current ISLEEP IVB = 0, (Note 5) 3 5 μA
Input Logic-High:DQ, PIO VIH (Note 1) 1.5 V
Input Logic-Low: DQ, PIO VIL (Note 1) 0.6 V
Output Logic-Low: DQ, PIO VOL IOL = 4mA (Note 1) 0.4 V
Pulldown Current: DQ, PIO IPD VDQ, VPIO = 0.4V 0.2 μA
Input Logic-High: OVD VIH (Note 1) VVB - 0.2 V
Input Logic-Low: OVD VIL (Note 1) VSS + 0.2 V
VIN Input Resistance RIN 15 MΩ
DQ Capacitance CDQ (Note 4) 50 pF
DQ SLEEP Timeout tSLEEP DQ < VIL 1.5 2 2.5 s
UVTH = 1, (Note 1) 4.8 4.9 5.0 Undervoltage SLEEP
Threshold VSLEEP UVTH = 0, (Note 1) 2.40 2.45 2.50 V
ELECTRICAL CHARACTERISTICS: TEMPERATURE, VOLTAGE, CURRENT (VDD = 2.5V to 10V, TA = -20°C to +70°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS Temperature Resolution TLSB 0.125 °C
DS2781
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS Voltage Full-Scale VFS 0 9.9902 V
Voltage Error VERR ±100 mV
Current Resolution ILSB 1.56 µV
Current Full-Scale IFS ±51.2 mV
Current Gain Error IGERR (Note 2) ±1 % Full-
Scale
Current Offset Error IOERR 0°C ≤ TA ≤ +70°C,
(Note 4) - 7.82 + 12.5 μV
Accumulated Current Offset qOERR 0°C ≤ TA ≤ +70°C,
VSNS = VSS (Notes 3, 4) - 188 + 0 μVhr/
day
TA = +25°C, VDD = 7.6V ±1 Timebase Error tERR ±2
ELECTRICAL CHARACTERISTICS: 1-WIRE INTERFACE, STANDARD (VDD = 2.5V to 10V, TA = -20°C to +70°C.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS Time Slot tSLOT 60 120 μs
Recovery Time tREC 1 μs
Write-0 Low Time tLOW0 60 120 μs
Write-1 Low Time tLOW1 1 15 μs
Read Data Valid tRDV 15 μs
Reset Time High tRSTH 480 μs
Reset Time Low tRSTL 480 960 μs
Presence Detect High tPDH 15 60 μs
Presence Detect Low tPDL 60 240 μs
ELECTRICAL CHARACTERISTICS: 1-Wire INTERFACE, OVERDRIVE (VDD = 2.5V to 10V, TA = -20°C to +70°C.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS Time Slot tSLOT 6 16 μs
Recovery Time tREC 1 μs
Write-0 Low Time tLOW0 6 16 μs
Write-1 Low Time tLOW1 1 2 μs
Read Data Valid tRDV 2 μs
Reset-Time High tRSTH 48 μs
Reset-Time Low tRSTL 48 80 μs
Presence-Detect High tPDH 2 6 μs
DS2781
EEPROM RELIABILITY SPECIFICATION (VDD = 2.5V to 10V, TA = -20°C to +70°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS EEPROM Copy Time tEEC 15 ms
EEPROM Copy Endurance NEEC TA = +50°C 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
TSSOP TDFN-EP NAME FUNCTION 1 1 VB Internal Supply. Bypass to VSS with a 0.1µF capacitor.
2 2, 3 VSS
Device Ground. Connect directly to the negative terminal of the cell stack. Connect the sense resistor between VSS and SNS.
3 4 VIN
Voltage Sense Input. The voltage of the battery pack is monitored through this input pin with respect to the VSS pin.
4 5 VDD
Power-Supply Input. Connect to the positive terminal of the battery pack through a decoupling network.
5 6 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 7 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.
— 8 N.C.
No Connection 7 9 SNS
Sense Resistor Connection. Connect to the negative terminal of the battery pack. Connect the sense resistor between VSS and SNS.
8 10 PIO
Programmable I/O Pin. Can be configured as input or output to monitor or control user-defined external circuitry. Output driver is open drain. This pin
has a weak internal pulldown (IPD).
— EP EP Exposed Pad. Connect to VSS or leave floating.
DS2781
Figure 1. Block Diagram VPOR
1-Wire
Interface
BIAS/VREFTimebase
Temp
Voltage
ADC
Current ADC
15 bit + sign
Rate,
Temperature
Compensation
EEPROM
Status
Control
Accumulated
Current
SNSVSS
VDD
OVD
PIO
VIN
Voltage
Regulator
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 fuel-
gauging 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 Maxim 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 Maxim 1-Wire interface on the DS2781 supports multidrop capability so that
multiple slave devices may be addressed with a single pin.
DS2781
Figure 2. Typical Operating Circuit DQ
SNS
VDD
VSS
RSNS
PACK+
PACK-
DATA
DS2781PIO
0.1uF
VB
VIN
5001K
OVD
TSSOP-85.6V
0.1uF
1 or 2
Cell
Li-Ion
Stack
Protection
Circuit
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 stand-alone 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.
DS2781
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 complement 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 LSB—Address 0Dh
S 29 28 27 26 25 24 23 22 21 20 X X X X X
MSb LSb MSb LSb
“S”: sign bit(s), “X”: reserved Units: 9.76mV
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 LSB—Address 0Bh
S 29 28 27 26 25 24 23 22 21 20 X X X X X
MSb LSb MSb LSb
“S”: sign bit(s), “X”: reserved Units: 0.125°C
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).
DS2781
Figure 5. Current Register Format
CURRENT Read OnlyMSB—Address 0Eh LSB—Address 0Fh
S 214 213 212 211 210 29 28 27 26 25 24 23 22 21 20
MSb LSb MSb LSb
“S”: sign bit(s) Units: 1.5625μV/Rsns
CURRENT RESOLUTION (1 LSB)
RSNS VSS - VSNS 20mΩ 15mΩ 10mΩ 5mΩ
1.5625μV 78.13μA 104.2μA 156.3μA 312.5μA
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 1.56μV steps to any value between +198.1μV and -199.7μV.
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 21 20MSb LSb“S”: sign bit(s) Units: 1.56μV/Rsns
DS2781
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
the gain from 0 to 1.999 in steps of 0.001 (precisely 2-10). 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.
Temperature compensation adjustments are made when the Temperature register crosses 0.5oC 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 LSB—Address 09h
S 214 213 212 211 210 29 28 27 26 25 24 23 22 21 20
MSb LSb MSb LSb
“S”: sign bit(s) Units: 1.5625μV/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
6.25µVh. 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).
DS2781
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 & EEMSB—Address 10h LSB—Address 11h 15 214 213 212 211 210 29 28 27 26 25 24 23 22 21 20
MSb LSb MSb LSb Units: 6.25μVh/Rsns
Figure 9. Fractional/Low Accumulated Current Register Format, ACRL
ACRL Read OnlyMSB—Address 12h LSB—Address 13h 11 210 29 28 27 26 25 24 23 22 21 20 X X X X
MSb LSb MSb LSb
“X”: reserved Units:1.526nVHr/RSNS
ACR LSb
RSNS VSS - VSNS 20mΩ 15mΩ 10mΩ 5mΩ
6.25μVh 312.5μAh 416.7μAh 625μAh 1.250mAh
ACR RANGE
RSNS VSS - VSNS 20mΩ 15mΩ 10mΩ 5mΩ
409.6mVh 20.48Ah 27.30Ah 40.96Ah 81.92Ah
DS2781
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 complement 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 26 25 24 23 22 21 20 MSb LSb“S”: sign bit Units: 1.5625μV/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.
DS2781
Figure 11. Top-Level Algorithm Diagram
Capacity Look-up
Available Capacity Calculation
ACR Housekeeping
Age Estimator
Learn Function
Cell
Parameters
16 bytes
(EEPROM)
FULLFULL(T)(R)
Active EmptyAE(T)(R)
Standby EmptySE(T)(R)
Remaining Active Absolute
Capacity (RAAC) mAh (R)
Sense Resistor’
(RSNSP) (1byte EE)
Voltage(R)
Temperature(R)
Current(R)
Accumulated
Current (ACR) (R/W)
Aging Cap (AC)
(2 bytes EE)
Charge Voltage
(VCHG) (1 byte EE)
Remaining Stand-by Absolute
Capacity (RSAC) mAh (R)
Remaining Active Relative
Capacity (RARC) % (R)
Remaining Stand-by Relative
Capacity (RSRC) % (R)
Age Scalar (AS)
(1 bytes EE)
Min Chg Current
(IMIN) (1 byte EE)
Active Empty
Voltage (VAE)
(1 byte EE)
Active Empty
Current (IAE)
(1 byte EE)
Average Current (R)
DS2781
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 Application Note AN131: Lithium-Ion Cell Fuel Gauging with Dallas Semiconductor Battery Monitor
ICs. 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 +40°C, the segment 5 model
curves extend infinitely with zero slope, approximating the nearly flat change in capacity of Li-Ion cells at
temperatures above +40°C. Segment 4 of each model curves originates at +40°C 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 +1°C increments from -128°C
to +40°C. 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
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:
((1031mAh – 1009mAh) / (1051mAh / 1M)) / (18ºC - 0ºC) = 1163ppm/ºC
1 LSB of the slope registers equals 61ppm so the Full Segment 3 Slope register (location 0x6Dh) would be
TBP12TBP23TBP3440C
100%
Segment 1
Derivative
[ppm / C]
Active
Empty
Stand-by
Empty
FULL
Cell Characterization
data points
Seg. 2 Seg. 3 Seg. 4 Seg. 5
DS2781
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: 0.020Ω
TBP12 TBP23 TBP34
Segment
Break Points -12ºC 0ºC 18ºC +40°C
Nominal
[mAh]
Seg. 1
ppm/°C
Seg. 2
ppm/°C
Seg. 3
ppm/°C
Seg. 4
ppm/°C
Full 1051 3601 3113 1163 854
Active Empty 2380 1099 671 305
Standby Empty 1404 427 244 183
Figure 13. Lookup Function Diagram
Cell Model
Parameters
(EEPROM)
Temperature
Look-up
Function
FULL(T)
AE(T)
SE(T)
DS2781
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 1Ω to 3.922mΩ. 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 50µV and can range from 0 to 12.75mV. Assuming RSNS =
20mΩ, 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 Modeling Cell Stack 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 200μV
and can range from 0 to 51.2mV. Assuming RSNS = 20mΩ, IAE can be programmed from 0mA to 2550mA in
10mA steps. IAE is located in the Parameter EEPROM block. See the Modeling Cell Stack 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 (6.25μVh). 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
value that represents values between 49.2% and 100%. The lsb is weighted at 0.78% (precisely 2-7). 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
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.
DS2781
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.
