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DS2782E+
Stand-Alone Fuel Gauge IC
GENERAL DESCRIPTION The DS2782 measures voltage, temperature and
current, and estimates available capacity for rechargeable lithium ion and lithium-ion polymer
batteries. Cell 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 reported in mAh remaining and percentage of full.
APPLICATIONS Digital Still Cameras
Sub-Notebook Computers
Handheld PC Data Terminals
3G Multimedia Wireless Handsets
TYPICAL OPERATING CIRCUIT
FEATURES Precision Voltage, Temperature, and Current
Measurement System Accurate 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 User EEPROM or 16-Byte User
EEPROM and Unique 64-bit ID Industry 2-Wire Interface with Programmable
Slave Address Tiny 8-Pin TSSOP Package Embeds Easily in
Battery Packs Using Thin Prismatic Cells
PIN CONFIGURATION ORDERING INFORMATION
Note: To order devices with the unique 64-bit ID option, contact Maxim/Dallas Semiconductor sales.
DS2782
Stand-Alone Fuel Gauge IC
DS2782: Stand-Alone Fuel Gauge IC
ABSOLUTE MAXIMUM RATINGS Voltage Range on Any Pin Relative to VSS -0.3V to +6.0V
Voltage on VIN Relative to VSS -0.3V to VDD+0.3Operating Temperature Range -40°C to +85°C
Storage Temperature Range -55°C to +125°CSoldering Temperature See IPC/JEDEC J-STD-020A 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.
RECOMMENDED DC OPERATING CHARACTERISTICS (VDD = 2.5V to 5.5V; TA = -20°C to +70°C, unless otherwise noted. Typical values are at TA = +25°C.)
DC ELECTRICAL CHARACTERISTICS (VDD = 2.5V to 5.5V; TA = -20°C to +70°C, unless otherwise noted. Typical values are at TA = +25°C.)
ELECTRICAL CHARACTERISTICS: TEMPERATURE, VOLTAGE, CURRENT (VCC = 2.5V to 5.5V; TA = -20°C to +70°C, unless otherwise noted. Typical values are at TA = +25°C.)
DS2782: Stand-Alone Fuel Gauge IC
ELECTRICAL CHARACTERISTICS: 2-WIRE INTERFACE (2.5V � VDD � 5.5V, TA = -20�C to +70�C.)
EEPROM RELIABILITY SPECIFICATION (VCC = 2.5V to 5.5V; TA = -20°C to +70°C, unless otherwise noted. Typical values are at TA = +25°C.)
DS2782: Stand-Alone Fuel Gauge IC
Note 1: All voltages are referenced to VSS.
Note 2: To properly enter sleep mode the application should hold the bus low for longer than the maximum tSLEEP. Note 3: Factory calibrated accuracy. Higher accuracy can be achieved by in-system calibration by the user.
Note 4: Accumulation bias register set to 00h. Note 5: Parameters guaranteed by design.
Note 6: Timing must be fast enough to prevent the DS2782 from entering sleep mode due to bus low for period > tSLEEP.
Note 7: fSCL must meet the minimum clock low time plus the rise/fall times. Note 8: The maximum tHD:DAT has only to be met if the device does not stretch the LOW period (tLOW) of the SCL signal.
Note 9: This device internally provides a hold time of at least 300 ns for the SDA signal (referred to the VIHmin of the SCL signal) to bridge the undefined region of the falling edge of SCL.
Note 10: Filters on SDA and SCL suppress noise spikes at the input buffers and delay the sampling instant.
Note 11: Cb – total capacitance of one bus line in pF.
Figure 1. I2C Bus Timing Diagram DS2782: Stand-Alone Fuel Gauge IC
PIN DESCRIPTION DS2782: Stand-Alone Fuel Gauge IC
Figure 2. Block Diagram DETAILED DESCRIPTION The DS2782 operates directly from 2.5V to 5.5V and supports single cell Lithium-ion battery packs. As shown in
Figure 3, the DS2782 accommodates multicell applications by adding a voltage regulator for VDD and voltage divider for VIN. 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 NV storage of system or battery usage statistics. Through its 2-Wire interface, the DS2782 gives the host system read/write access to status and control registers,
instrumentation registers, and general-purpose data storage. The 7-bit slave address is field programmable, thus allowing up to 128 devices to be distinctly addressed by the host system. A unique, factory programmed 64-bit
registration number (8-bit family code + 48-bit serial number + 8-bit CRC) option assures that no two parts are alike and enables absolute traceability.
DS2782: Stand-Alone Fuel Gauge IC
Figure 3. Multicell Application Example
POWER MODES
The DS2782 has two power modes: ACTIVE and SLEEP. On initial power up, the DS2782 defaults to ACTIVE
mode. While in ACTIVE mode, the DS2782 is fully functional with measurements and capacity estimation continuously updated. In SLEEP mode, the DS2782 conserves power by disabling measurement and capacity
estimation functions, but preserves register contents. SLEEP mode is entered under two different conditions and an enable bit for each condition makes entry into SLEEP optional. SLEEP mode can be enabled using the Power
Mode (PMOD) bit or the Under Voltage Enable (UVEN) bit.
The PMOD type SLEEP is entered if the PMOD bit is set AND a bus low condition occurs. A bus low condition, where both SDA AND SCL 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 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 SDA and/or SCL. The DS2782 transitions from PMOD SLEEP to ACTIVE mode when either SDA or SCL is pulled high. The second option for entering SLEEP is an under voltage condition measured on VIN. When the UVEN bit is set,
the DS2782 will transition to SLEEP if the voltage on VIN is less than VSLEEP (2.45V nominal) AND the 2-Wire bus is in a bus high or a bus low condition for tSLEEP. UVEN SLEEP relieves the battery of the DS2782 load until
communication resumes to prevent over discharging the battery. The DS2782 transitions from UVEN SLEEP to ACTIVE mode when either SDA or SCL change logic state. The bus master should initiate a transaction after
charging of a depleted battery begins.
Note: PMOD and UVEN SLEEP features must be disabled when a battery is charged on an external charger that does not connect to SDA and/or SCL. PMOD SLEEP can be used if the charger pulls the bus high. The DS2782
remains in SLEEP and therefore does not measure or accumulate current when a battery is charged on a charger that fails to properly drive the communication bus.
DS2782: Stand-Alone Fuel Gauge IC
INITIATING COMMUNICATION IN SLEEP
When beginning communication with a DS2782 in PMOD SLEEP, the bus must be pulled up before a START bit
can be issued by the master. In UVEN SLEEP, the procedure depends on the bus state when UVEN SLEEP was entered. If the bus was low, it must be pulled up before a START bit can be issued by the master as required with
PMOD SLEEP. If the bus was high when UVEN SLEEP was entered, then the DS2782 is prepared to receive a START bit from the master. A standard procedure of issuing a START – STOP – START when the host system is powered up on the charger input properly initiates communication from both PMOD and UVEN SLEEP modes.
VOLTAGE MEASUREMENT
Battery voltage is measured at the VIN input with respect to VSS over a range of 0V to 4.992V, with a resolution of 4.88mV. 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 4.
Figure 4. Voltage Register Format
VIN is usually connected to the positive terminal of a single cell Lithium-Ion battery via a 1k� resistor. The input
impedance is sufficiently large (15M�) to be connected to a high impedance voltage divider in order to support multiple cell applications. The pack voltage should be divided by the number of series cells to present a single cell
average voltage to the VIN input. In Figure 3, the value of R can be up to 1M� without incurring significant error
due to input loading.
TEMPERATURE MEASUREMENT
The DS2782 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 5.
Figure 5. Temperature Register Format
DS2782: Stand-Alone Fuel Gauge IC
CURRENT MEASUREMENT
In the ACTIVE mode of operation, the DS2782 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). Figure 6. Current Register Format
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
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
DS2782: Stand-Alone Fuel Gauge IC
CURRENT MEASUREMENT CALIBRATION
The DS2782’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 from the factory, the gain calibration value is stored in two separate
locations in the Parameter EEPROM Block, RSGAIN which is reprogrammable and FRSGAIN which is read only. RSGAIN determines the gain used in the current measurement. The read-only FRSGAIN is provided to preserve
the factory value only and is not used in the current measurement.
SENSE RESISTOR TEMPERATURE COMPENSATION
The DS2782 is capable of temperature compensating the current sense resistor to correct for variation in a sense resistor’s value over temperature. The DS2782 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 DS2782 package if possible. 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 timebase. 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).
Charge currents (positive Current register values) less than 100�V are not accumulated in order to mask the effect of accumulating small positive offset errors over long periods. This limits the minimum charge current, for coulomb-
counting purposes, to 5mA for RSNS = 0.020� and 20mA for RSNS = 0.005�.
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
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.
DS2782: Stand-Alone Fuel Gauge IC
Figure 8. Accumulated Current Register Format, ACR
Figure 9. Fractional/Low Accumulated Current Register Format, ACRL 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 DS2782 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.
DS2782: Stand-Alone Fuel Gauge IC
Figure 10. Accumulation Bias Register Formats
CAPACITY ESTIMATION ALGORITHM
Remaining capacity estimation uses real-time measured values and stored parameters describing the cell
characteristics and application operating limits. The following diagram describes the algorithm inputs and outputs.
Figure 11. Top Level Algorithm Diagram
DS2782: Stand-Alone Fuel Gauge IC
MODELING CELL CHARACTERISTICS
In order to achieve reasonable accuracy in estimating remaining capacity, the cell performance characteristics over
temperature, load current, and charge termination point must be considered. Since the behavior of Li-ion cells is non-linear, even over a limited temperature range of 10°C to 35°C, these characteristics must be included in the
capacity estimation to achieve a reasonable accuracy. See Applications Note AN131 “Li+ Fuel Gauging with Dallas Semiconductor Devices” for general information on the FuelPack™ method used in the DS2782. To facilitate efficient implementation in hardware, a modified version of the method outlined in AN131 is used to store cell
characteristics in the DS2782. Full and empty points are retrieved in a lookup process which re-traces a piece-wise
linear model. Three model curves are stored: Full, Active Empty and Standby Empty. Each model curve is constructed with 4 line segments and spans from 0°C to 40°C. Operation outside the 0°C to 40°C model span is
supported by the model with minimal loss of accuracy. Above 40°C, the 40°C fixed points are extended with zero slope. This achieves a conservative capacity estimate for temperatures above 40°C. Below 0°C, the model curves are extended using the slope of each 0°C to 10°C segment. If low temperature operation is expected, the 0°C to
10°C slopes can be selected to optimize the model accuracy. A diagram of example battery cell model curves is
shown if Figure 12.
Figure 12. Cell Model Example Diagram
Full: The Full curve defines how the full point of a given cell depends on temperature for a given charge termination. The charge termination method used in the application is used to determine the table values. The
DS2782 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. 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 220ms average to correspond to values read from the Voltage
register. The DS2782 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.
