MAXIM DS2781 User Manual

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 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.

PIN CONFIGURATION

APPLICATIONS

Digital Video Cameras

Commercial Two-Way Radios

Industrial PDAs and Handheld PC Data Terminals

Portable GPS Navigation Systems

TYPICAL OPERATING CIRCUIT

P+

DQ

P-

Protection

Circuit

DQ

PIO

OVD

SNS

VDD

VIN

VB

VSS

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
 Tiny TSSOP-8 and TDFN-10 Packages Embed

Easily in Thin Prismatic Cell packs

ORDERING INFORMATION

PART MARKING PACKAGE

DS2781E+ 2781 TSSOP-8
DS2781E+T&R 2781 DS2781E+ in Tape-and-Reel
DS2781G+ 2781 3mm x 4mm TDFN-10
DS2781G+T&R 2781 DS2781G+ in Tape-and-Reel

+ Denotes lead-free package.
1-Wire is a registered trademark of Dallas Semiconductor.

DS2781

1-Cell or 2-Cell

Standalone Fuel Gauge IC

®

Interface

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

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050907

DS2781: Two-Cell Standalone Fuel Gauge IC

ABSOLUTE MAXIMUM RATINGS

Voltage on VDD, VIN Relative to VSS
Voltage Range on Any Pin Relative to V

-0.3V to +6.0V

SS

Continuous Sink Current, DQ, PIO 20mA
Operating Temperature Range
Storage Temperature Range
Soldering Temperature

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.

See JEDEC J-STD-020

-40°C to +85°C

-55°C to +125°C

-0.3V to +12V

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 V

+ 0.3 V

DD

DQ, PIO Voltage Range (Note 1) -0.3 +5.5 V

V

> 3.0V,

DD

VB Output Voltage VVB

= 500µA,

I

VB

2.5 2.8 3.1 V

(Note 1)

OVD Voltage Range (Note 1) -0.3 V

+ 0.3 V

VB

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 I

IVB = 0 70 95 µA

ACTIVE

TA > +50°C, IVB = 0 10

SLEEP Mode Current I

SLEEP

I

= 0, (Note 5) 3 5

VB

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, V

Input Logic High: OVD VIH (Note 1) V

= 0.4V 0.2 µA

PIO

- 0.2 V

VB

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 t
Undervoltage SLEEP
Threshold

DQ < VIL 1.5 2 2.5 s

SLEEP

V

SLEEP

UVTH = 1, (Note 1) 4.8 4.9 5.0
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 T

Temperature Error T

Voltage Resolution V

0.125 °C

LSB

±3 °C

ERR

9.76 mV

LSB

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DS2781: Two-Cell Standalone Fuel Gauge IC

PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS

Voltage Full-Scale VFS 0 9.9902 V

Voltage Error V

Current Resolution I

±100 mV

ERR

1.56 µV

LSB

Current Full-Scale IFS ±51.2 mV

Current Gain Error I

Current Offset Error I

Accumulated Current Offset q

Timebase Error t

(Note 2) ±1

GERR

OERR

OERR

ERR

0°C ≤ T
(Note 4)
0°C ≤ T
V

SNS

TA = +25°C, V

≤ +70°C,

A

≤ +70°C,

A

= VSS (Notes 3, 4)

= 7.6V ±1

DD

- 7.82 + 12.5 µV

- 188 + 0

% Full-

Scale

µVhr/

day

%

±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 t

Recovery Time t

Write-0 Low Time t

Write-1 Low Time t

Read Data Valid t

Reset Time High t

Reset Time Low t

Presence Detect High t

Presence Detect Low t

60 120

SLOT

1

REC

60 120

LOW0

1 15

LOW1

15

RDV

480

RSTH

480 960

RSTL

15 60

PDH

60 240

PDL

μs

μs

μs

μs

μs

μs

μs

μs

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 t

Recovery Time t

Write-0 Low Time t

Write-1 Low Time t

Read Data Valid t

Reset-Time High t

Reset-Time Low t

Presence-Detect High t

Presence-Detect Low t

6 16

SLOT

1

REC

6 16

LOW0

1 2

LOW1

2

RDV

48

RSTH

48 80

RSTL

2 6

PDH

8 24

PDL

μs

μs

μs

μs

μs

μs

μs

μs

μs

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DS2781: Two-Cell Standalone Fuel Gauge IC

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 t

EEPROM Copy Endurance N

15 ms

EEC

TA = +50°C 50,000 cycles

EEC

Note 1: All voltages are referenced to V

SS

.

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

NAME TSSOP PIN TDFN PIN FUNCTION

VB 1 1 Internal Supply. Bypass to V

VSS 2 2, 3

VIN 3 4

VDD 4 5

Device Ground. Connect directly to the negative terminal of the cell stack.

Connect the sense resistor between V

Voltage Sense Input. The voltage of the battery pack is monitored through

this input pin with respect to the V

Power-Supply Input. Connect to the positive terminal of the battery pack

through a decoupling network.

Data Input/Output. 1-Wire data line. Open-drain output driver. Connect this

DQ 5 6

pin to the DATA terminal of the battery pack. This pin has a weak internal
pulldown (I

) for sensing pack disconnection from host or charger.

PD

1-Wire Bus Speed Control.

OVD 6 7

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.

with a 0.1µF capacitor.

SS

and SNS.

SS

pin.

SS

Input logic level selects the speed of the 1-Wire

N.C. — 8 No Connection. (Only present on TDFN package).

SNS 7 9

Sense Resistor Connection. Connect to the negative terminal of the battery

pack. Connect the sense resistor between V

and SNS.

SS

Programmable I/O Pin. Can be configured as input or output to monitor or

PIO 8 10

PAD — PAD

control user-defined external circuitry. Output driver is open drain. This pin has
a weak internal pulldown (I

Exposed Pad. Connect to V

).

PD

or leave floating. (Only present on TDFN

SS

package).

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Figure 1. Block Diagram

DS2781: Two-Cell Standalone Fuel Gauge IC

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.

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DS2781: Two-Cell Standalone Fuel Gauge IC

Figure 2: Typical Operating 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 t
shutdown in which the bus pull-up voltage, V

(2s nominal), is used to detect a pack disconnection or system

SLEEP

, is not present. PMOD type SLEEP assumes that no charge or

PULLUP

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 V
Enable (UVEN) bit is set, the DS2781 will transition to SLEEP if the VIN voltage is less than V
or 4.9V). The bus must be in a static state, that is with DQ either high or low for t

). When the Under Voltage

IN

(Selectable 2.45

SLEEP

. UVEN SLEEP reduces

SLEEP

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.

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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 LSB—Address 0Dh

S 2

9

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

MSB—Address 0Ah LSB—Address 0Bh

Read Only

S 2

9

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, R
SNS and V

is ±51.2mV. The input linearly converts peak signal amplitudes up to 102.4mV as long as the

SS

. The voltage-sense range between

SNS

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).

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Figure 5. Current Register Format

DS2781: Two-Cell Standalone Fuel Gauge IC

CURRENT

S 2

MSb LSb MSb LSb

“S”: sign bit(s)

MSB—Address 0Eh LSB—Address 0Fh

14

213 212 211 210 29 28 27 26 25 24 23 22 21 20

Units: 1.5625μV/Rsns

Read Only

CURRENT RESOLUTION (1 LSB)

R

V

- V

SNS

20mΩ 15mΩ 10mΩ 5mΩ

SS

1.5625μV 78.13μA 104.2μA 156.3μA 312.5μA

SNS

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 2

0

MSb LSb

“S”: sign bit(s) Units: 1.56μV/Rsns

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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
the gain from 0 to 1.999 in steps of 0.001 (precisely 2
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.

-10

). The user must program RSGAIN cautiously to ensure

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 0.5
temperature compensation is most effective with the resistor placed as close as possible to the V
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.

o

C boundaries. The

terminal to

SS

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

S 2

MSb LSb MSb LSb

“S”: sign bit(s)

MSB—Address 08h LSB—Address 09h

14

213 212 211 210 29 28 27 26 25 24 23 22 21 20

Units: 1.5625μV/Rsns

R/W

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).

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

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