Rev 0; 10/07
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
The DS2788 measures voltage, temperature, and current, and estimates available capacity for rechargeable
lithium-ion (Li+) and Li+ 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 is reported
in mAh remaining and percentage of full.
LED display drivers and a debounced input make display of the capacity information easy. The LED pins can
directly sink current, requiring only a resistor for setting
the current in the LED display, thus reducing space and
cost.
Applications
Power Tools
Features
♦ Five 30mA Open-Drain Drivers for Driving LED
Fuel-Gauge Display
♦ Debounced Fuel-Gauge Display Enable
♦ Internal Voltage Measurement Gain Register for
Trimming External Voltage-Divider
♦ Pin for Driving FETs to Enable Voltage-Divider
Only During Voltage Measurement, Conserving
Power
♦ Precision Voltage, Temperature, and Current
Measurement System
♦ 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
Electric Bicycles
♦ Automatic Backup of Coulomb Count and Age
Estimation to Nonvolatile (NV) EEPROM
Electric Vehicles
Uninterruptible Power Supply
♦ Gain and Tempco Calibration Allows the Use of
Low-Cost Sense Resistors
Digital Cameras
♦ 24-Byte Battery/Application Parameter EEPROM
Pin Configuration
♦ 16-Byte User EEPROM
♦ Unique ID and Multidrop 1-Wire® Interface
TOP VIEW
♦ 14-Pin TSSOP Package
+
LED2
1
14
LED3
LED1
2
13
LED4
DVSS
3
12
LED5
PART
TEMP RANGE
PIN-PACKAGE
11
PIO
DS2788E+
-25°C to +70°C
14 TSSOP
10
VIN
DS2788E+T&R
-25°C to +70°C
14 TSSOP
VDD
4
OVD
5
DS2788
VSS
6
9
SNS
DQ
7
8
VMA
Ordering Information
+Denotes a lead-free package.
T&R = Tape and reel.
TSSOP
Typical Operating Circuit appears at end of data sheet.
1-Wire is a registered trademark of Dallas Semiconductor Corp.,
a wholly owned subsidiary of Maxim Integrated Products, Inc.
________________________________________________________________ Maxim Integrated Products
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
1
DS2788
General Description
DS2788
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
ABSOLUTE MAXIMUM RATINGS
Operating Temperature Range ...........................-40°C to +85°C
Storage Temperature Range .............................-55°C to +125°C
Soldering Temperature (10s) ................Refer to IPC/JEDEC-020
Specification.
Voltage Range on Any Pin Relative to VSS..............-0.3V to +6.0V
Voltage Range on VIN, VMA Relative to VSS ...-0.3V to VDD + 0.3V
DVSS to VSS .....................................................................-0.3V to +0.3V
LED1–5.................................................................60mA each pin
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
absolute maximum rating conditions for extended periods may affect device reliability.
RECOMMENDED DC OPERATING CHARACTERISTICS
(VDD = 2.5V to 5.5V, TA = -25°C to +70°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
MAX
UNITS
(Note 1)
+2.5
+5.5
V
VIN, VMA Voltage Range
(Note 1)
0
VDD
V
DQ, PIO, OVD, LED1–LED5
Voltage Range
(Note 1)
0
+5.5
V
MAX
UNITS
Supply Voltage
SYMBOL
VDD
CONDITIONS
MIN
TYP
DC ELECTRICAL CHARACTERISTICS
(VDD = 2.5V to 5.5V, TA = -25°C to +70°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
ACTIVE Current
IACTIVE
SLEEP Mode Current
I SLEEP
CONDITIONS
MIN
2.5V VDD 4.2V
TYP
70
95
105
1
μA
3
μA
Input Logic-High: DQ, PIO
VIH
(Note 1)
Input Logic-Low: DQ, PIO
VIL
(Note 1)
0.6
V
Output Logic-Low: DQ, PIO, VMA
VOL
I OL = 4mA (Note 1)
0.4
V
Output Logic-High: VMA
VOH
I OH = 1mA (Note 1)
VMA Precharge Time
tPRE
Pulldown Current: DQ, PIO
I PD
VDQ, VPIO = 0.4V
Output Logic-Low: LED1–LED5
VOL
I OL = -30mA (Note 1)
Input Logic-High: OVD
VIH
(Note 1)
Input Logic-Low: OVD
VIL
(Note 1)
VIN Input Resistance
1.5
V
VDD 0.5
V
13.3
RIN
0.2
14.2
ms
5
μA
1
V
VDD 0.2
V
VSS +
0.2
15
V
M
DQ SLEEP Timeout
t SLEEP
DQ < VIL
1.8
2.0
2.2
Undervoltage SLEEP Threshold
VSLEEP
(Note 1)
2.40
2.45
2.50
V
130
ms
PIO Switch Debounce
LED1 Display Blink Rate
LED Display-On Time
2
100
50% duty cycle
s
0.9
1.0
1.1
Hz
3.6
4.0
4.4
s
_______________________________________________________________________________________
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
DS2788
ELECTRICAL CHARACTERISTICS: TEMPERATURE, VOLTAGE, CURRENT
(VCC = 2.5V to 5.5V, TA = -25°C to +70°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
Temperature Resolution
TLSB
Temperature Error
T ERR
Voltage Resolution
VLSB
Voltage Full-Scale
VFS
CONDITIONS
MIN
TYP
MAX
0.125
°C
±3
4.88
0
UNITS
°C
mV
4.992
V
±50
mV
±51.2
mV
±1
% Full
Scale
Voltage Error
VERR
Current Resolution
ILSB
Current Full-Scale
IFS
Current Gain Error
I GERR
(Note 2)
Current Offset Error
I OERR
0°C TA +70°C, 2.5V VDD 4.2V
(Notes 3, 4)
-7.82
+12.5
μV
Accumulated Current Offset
qOERR
0°C TA +70°C, 2.5V VDD 4.2V,
VSNS = VSS (Notes 3, 4, 5)
-188
0
μVhr/
day
Timebase Error
t ERR
1.56
μV
VDD = 3.8V, TA = +25°C
±1
0°C TA +70°C, 2.5V VDD 4.2V
±2
%
±3
ELECTRICAL CHARACTERISTICS: 1-Wire INTERFACE, STANDARD
(VCC = 2.5V to 5.5V, TA = -25°C to +70°C.)
PARAMETER
Time Slot
SYMBOL
t SLOT
Recovery Time
CONDITIONS
MIN
60
TYP
MAX
UNITS
120
μs
tREC
1
Write-0 Low Time
tLOW0
60
120
μs
μs
Write-1 Low Time
tLOW1
1
15
μs
Read Data Valid
tRDV
Reset-Time High
tRSTH
480
15
μs
Reset-Time Low
tRSTL
480
960
Presence-Detect High
t PDH
15
60
μs
Presence-Detect Low
t PDL
60
240
μs
μs
μs
_______________________________________________________________________________________
3
DS2788
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
ELECTRICAL CHARACTERISTICS: 1-Wire INTERFACE, OVERDRIVE
(VCC = 2.5V to 5.5V, TA = -25°C to +70°C.)
PARAMETER
Time Slot
Recovery Time
SYMBOL
CONDITIONS
MIN
tSLOT
6
TYP
MAX
UNITS
16
μs
tREC
1
Write-0 Low Time
tLOW0
6
16
μs
μs
Write-1 Low Time
tLOW1
1
2
μs
Read Data Valid
tRDV
Reset-Time High
tRSTH
48
2
μs
Reset-Time Low
tRSTL
48
Presence-Detect High
tPDH
2
6
μs
Presence-Detect Low
tPDL
8
24
μs
MAX
UNITS
μs
80
μs
EEPROM RELIABILITY SPECIFICATION
(VCC = 2.5V to 5.5V, TA = -25°C to +70°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
EEPROM Copy Time
t EEC
EEPROM Copy Endurance
NEEC
Note 1:
Note 2:
Note 3:
Note 4:
Note 5:
Note 6:
4
CONDITIONS
MIN
TYP
10
TA = +50°C (Note 6)
50,000
All voltages are referenced to VSS.
Factory-calibrated accuracy. Higher accuracy can be achieved by in-system calibration by the user.
Parameters guaranteed by design.
At a constant regulated VDD voltage, the Current Offset Bias register can be used to obtain higher accuracy.
Accumulation Bias register set to 00h.
EEPROM data retention is 10 years at +50°C.
_______________________________________________________________________________________
ms
Cycles
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
PIN
NAME
1
LED2
Display Driver. Connect to an LED connected to VDD for display of relative pack capacity.
FUNCTION
2
LED1
Display Driver. Connect to an LED connected to VDD for display of relative pack capacity.
3
DVSS
4
VDD
Power-Supply Input. Connect to the positive terminal of the battery cell through a decoupling network.
5
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 (STD) timing. On a multidrop bus, all devices must operate at the same speed.
6
VSS
Device Ground. Connect directly to the negative terminal of the battery cell. Connect the sense resistor
between VSS and SNS.
7
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.
8
VMA
Voltage Measurement Active. Output is driven high before the start of a voltage conversion and driven low at
the end of the conversion cycle.
9
SNS
Sense Resistor Connection. Connect to the negative terminal of the battery pack. Connect the sense resistor
between VSS and SNS.
10
VIN
Voltage Sense Input. The voltage of the battery cell is monitored through this input pin.
11
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). When configured as an input,
upon recognition of a rising edge, the fuel-gauge display is enabled.
12
LED5
Display Driver. Connect to an LED connected to VDD for display of relative pack capacity. Leave floating in
LED4 configuration.
13
LED4
Display Driver. Connect to an LED connected to VDD for display of relative pack capacity.
14
LED3
Display Driver. Connect to an LED connected to VDD for display of relative pack capacity.
Display Ground. Ground connection for the LED display drivers. Connect to VSS.
VDD
EN
BIAS/VREF
TIME BASE
VPOR
LED
DRIVERS
PIO
DQ
OVD
1-Wire
INTERFACE
STATUS
AND
CONTROL
TEMP
AND
VOLTAGE
ADC
LED5
LED4
LED3
LED2
LED1
DVSS
VMA
VIN
EEPROM
DS2788
ACCUMULATED
CURRENT
CURRENT ADC
15-BIT + SIGN
SNS
RATE AND
TEMPERATURE
COMPENSATION
VSS
Figure 1. Block Diagram
_______________________________________________________________________________________
5
DS2788
Pin Description
DS2788
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
Detailed Description
The DS2788 operates directly from 2.5V to 5.5V and
supports single-cell Li+ battery packs. As shown in
Figure 2, the DS2788 accommodates multicell applications by adding a trim register for calibration of an
external voltage-divider for VIN. NV 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.
A 1-Wire interface provides serial communication at the
standard 16kbps or overdrive 140kbps speeds, allowing access to data registers, control registers, and user
memory. A unique, factory-programmed, 64-bit regis-
tration number (8-bit family code + 48-bit serial number
+ 8-bit CRC) assures that no two parts are alike and
enables absolute traceability. The 1-Wire interface on
the DS2788 supports multidrop capability so that multiple slave devices can be addressed with a single pin.
Power Modes
The DS2788 has two power modes: ACTIVE and
SLEEP. On initial power-up, the DS2788 defaults to
ACTIVE mode. While in ACTIVE mode, the DS2788 is
fully functional with measurements and capacity estimation continuously updated. In SLEEP mode, the
DS2788 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
undervoltage enable (UVEN) bit.
The PMOD type SLEEP is entered if the PMOD bit is set
and DQ is low for tSLEEP (2s nominal). The condition of
DQ low for tSLEEP can be used to detect a pack discon-
PK+
IN
ENABLE
0.1μF
MAX6765TTLD2+
RST
OUT
330Ω
330Ω
330Ω
330Ω
GND
TIMEOUT
10kΩ
330Ω
10-CELL
Li+ BATTERY
BSS84
B3F-1000
LEDs
LED1
LED2
LED3
LED4
DS2788
LED5
150Ω
DATA
10kΩ
PIO
VDD
DQ
SNS
900kΩ
VMA
VIN
OVD
DVSS
VSS
2N7002
10μF
0.1μF
100kΩ
5.6V
RSNS
20mΩ
PK-
Figure 2. Multicell Application Example
6
_______________________________________________________________________________________
PROTECTION
CIRCUIT
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
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 DS2788
remains in SLEEP and therefore does not measure or
accumulate current when a battery is charged on a
charger that fails to properly drive DQ.
Initiating Communication
in Sleep
When beginning communication with a DS2788 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
DS2788 is prepared to receive a 1-Wire reset from the
master. In the first two cases with DQ low during
SLEEP, the DS2788 does not respond to the first rising
edge of DQ with a presence pulse.
Voltage Measurement
Battery voltage is measured at the V IN input with
respect to VSS over a range of 0 to 4.992V, with a resolution of 4.88mV. The result is updated every 440ms
and placed in the Voltage (VOLT) 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. Figure 3 shows the format of the
Voltage register.
VIN is usually connected to the positive terminal of a
single-cell Li+ battery by a 1kΩ resistor. The input
impedance is sufficiently large (15MΩ) 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 2,
the value of R can be up to 1MΩ without incurring significant error due to input loading. The VMA pin is driven high tPRE before the voltage conversion begins.
This allows an external switching element to enable the
voltage-divider, and allows settling to occur before the
start of the conversion.
READ ONLY
VOLT
MSB—ADDRESS 0Ch
S
29
28
27
26
MSb
“S”: SIGN BIT(S), “X”: RESERVED
LSB—ADDRESS 0Dh
25
24
23
22
LSb
MSb
21
20
X
X
X
X
X
LSb
UNITS: 4.88mV
Figure 3. Voltage Register Format
_______________________________________________________________________________________
7
DS2788
nection or system shutdown, in which no charge or discharge current flows. A PMOD SLEEP condition transitions back to ACTIVE mode when DQ is pulled high.
The second option for entering SLEEP is an undervoltage condition. When the UVEN bit is set, the DS2788
transitions to SLEEP if the voltage on VIN is less than
VSLEEP (2.45V nominal) and DQ is stable at a low or
high logic level for tSLEEP. An undervoltage condition
occurs when a pack is fully discharged, where loading
on the battery should be minimized. UVEN SLEEP
relieves the battery of the IACTIVE load until communication on DQ resumes.
DS2788
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
Temperature Measurement
The DS2788 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 (TEMP)
register in two’s complement form. Figure 4 shows the
format of the Temperature register.
Current Measurement
In the ACTIVE mode of operation, the DS2788 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 (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).
READ ONLY
TEMP
MSB—ADDRESS 0Ah
S
29
28
27
26
LSB—ADDRESS 0Bh
25
24
MSb
23
22
LSb
MSb
21
20
X
X
X
X
X
LSb
“S”: SIGN BIT(S), “X”: RESERVED
UNITS: 0.125°C
Figure 4. Temperature Register Format
READ ONLY
CURRENT
MSB—ADDRESS 0Eh
S
214
213
212
211
LSB—ADDRESS 0Fh
210
29
MSb
28
27
LSb
MSb
26
25
24
23
22
20
LSb
“S”: SIGN BIT(S)
UNITS: 1.5625μV/RSNS
CURRENT RESOLUTION (1 LSB)
VSS - VSNS
1.5625μV
RSNS
20m
15m
10m
5m
78.13μA
104.2μA
156.3μA
312.5μA
Figure 5. Current Register Format
8
21
_______________________________________________________________________________________
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
Current Offset Bias
The Average Current (IAVG) 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 eight preceding
Current register updates. Figure 6 shows the format of
the Average Current register. 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).
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.
COB allows read and write access. 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 nonvolatile memory.
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
(ACR) register 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 Measurement
Calibration
The DS2788’s current measurement gain can be
adjusted through the RSGAIN register, which is factorycalibrated 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, nonprecision
READ ONLY
IAVG
MSB—ADDRESS 08h
S
214
213
212
211
LSB—ADDRESS 09h
210
29
MSb
28
27
LSb
MSb
26
25
24
23
22
21
20
LSb
“S”: SIGN BIT(S)
UNITS: 1.5625μV/RSNS
Figure 6. Average Current Register Format
R/W AND EE
COB
ADDRESS 7Bh
S
26
MSb
“S”: SIGN BIT(S)
25
24
23
22
21
20
LSb
UNITS: 1.56μV/RSNS
Figure 7. Current Offset Bias Register Format
_______________________________________________________________________________________
9
DS2788
Average Current Measurement
DS2788
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
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 (address B0h and B1h) is provided to preserve the factory value only and is not used
in the current measurement.
Sense Resistor Temperature
Compensation
The DS2788 is capable of temperature compensating
the current-sense resistor to correct for variation in a
sense resistor’s value over temperature. The DS2788 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/°C in steps of
30.5ppm/°C. The user must program RSTC cautiously to
ensure accurate current measurement.
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 DS2788 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 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 (least significant
bit) 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 MSB (most significant byte) first, then
LSB (least significant byte). 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 the ACR clears the fractional values
in ACRL. ACR’s format is shown in Figure 8, and
ACRL’s format is shown in Figure 9.
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 3 for specific address location and
backup frequency.
R/W AND EE
ACR
MSB—ADDRESS 10h
215
214
213
212
211
210
LSB—ADDRESS 11h
29
MSb
28
27
LSb
MSb
26
25
24
23
22
21
20
LSb
UNITS: 6.25μVh/RSNS
Figure 8. Accumulated Current Register (ACR) Format
10
______________________________________________________________________________________
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
MSB—ADDRESS 12h
211
DS2788
READ ONLY
ACRL
210
29
28
27
LSB—ADDRESS 13h
26
25
MSb
24
23
22
LSb
MSb
21
20
X
X
X
X
LSb
“X”: reserved
Units: 1.526nVHr/RSNS
ACR LSb
RSNS
VSS - VSNS
6.25μVh
20m
15m
10m
5m
312.5μAh
416.7μAh
625μAh
1.250mAh
ACR RANGE
RSNS
VSS - VSNS
±409.6mVh
20m
15m
10m
5m
±20.48Ah
±27.30Ah
±40.96Ah
±81.92Ah
Figure 9. Fractional/Low Accumulated Current Register (ACRL) Format
EE
AB
ADDRESS 61h
S
26
25
24
MSb
“S”: SIGN BIT(S)
23
22
21
20
LSb
UNITS: 1.5625μV/RSNS
Figure 10. Accumulation Bias Register Formats
Current Blanking
Accumulation Bias
The current blanking feature modifies the 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. Chargecurrent 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.
The Accumulation Bias (AB) register 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
DS2788 device. The AB register allows a user-programmed positive or negative constant 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.
Figure 10 shows the format of the AB register.
______________________________________________________________________________________
11
DS2788
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
Capacity Estimation Algorithm
Remaining capacity estimation uses real-time measured
values and stored parameters describing the cell characteristics and application operating limits. Figure 11
describes the algorithm inputs and outputs.
Modeling Cell Stack
Characteristics
To achieve reasonable accuracy in estimating remaining capacity, the cell stack performance characteristics
over temperature, load current, and charge termination
VOLTAGE
(R)
TEMPERATURE
(R)
point must be considered. Since the behavior of Li+
cells is nonlinear, 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 DS2788 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
DS2788. Full and empty points are retrieved in a lookup
process that retraces piece-wise linear model consist-
FULL
FULL(T)
(R)
ACTIVE EMPTY
AE(T)
(R)
STANDBY EMPTY SE(T)
(R)
CAPACITY LOOKUP
CURRENT
(R)
ACCUMULATED
CURRENT (ACR) (RW)
AVAILABLE CAPACITY CALCULATION
ACR HOUSEKEEPING
AGE ESTIMATOR
AVERAGE CURRENT
(R)
CELL PARAMETERS
16 BYTES
(EEPROM)
LEARN FUNCTION
REMAINING ACTIVE ABSOLUTE
CAPACITY (RAAC) mAh
(R)
REMAINING STANDBY ABSOLUTE
CAPACITY (RSAC) mAh
(R)
REMAINING ACTIVE RELATIVE
CAPACITY (RARC) %
(R)
REMAINING STANDBY RELATIVE
CAPACITY (RSRC) %
(R)
AGING CAPACITY (AC)
(2 BYTES EE)
AGE SCALAR (AS)
(1-BYTE EE)
SENSE RESISTOR PRIME (RSNSP)
(1 -BYTE EE)
CHARGE VOLTAGE (VCHG)
(1-BYTE EE)
MINIMUM CHARGE CURRENT (IMIN)
(1-BYTE EE)
ACTIVE EMPTY VOLTAGE (VAE)
(1-BYTE EE)
ACTIVE EMPTY CURRENT (IAE)
(1-BYTE EE)
Figure 11. Top Level Algorithm Diagram
FuelPack is a trademark of Dallas Semiconductor Corp., a
wholly owned subsidiary of Maxim Integrated Products, Inc.
12
______________________________________________________________________________________
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
SEGMENT 1
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
DS2788 reconstructs the full line from cell characteristic
table values to determine the full capacity of the battery
at each temperature. Reconstruction occurs in onedegree temperature increments. Full values are stored
as ppm change per °C. For example, if a cell had a
nominal capacity of 1214mAh at +50°C, a full value of
1199mAh at +25°C, and 1182mAh at 0°C (TBP23), the
slope for segment 3 would be:
((1199mAh - 1182mAh) / (1214mAh / 1M)) /
(25°C - 0°C) = 560ppm/°C
1 LSB of the slope registers equals 61ppm so the full
segment 3 slope register (location 0x6Dh) would be
programmed with a value of 0x09h. Each Slope register
has a dynamic range 0ppm to 15555ppm.
SEGMENT 3
SEGMENT 2
SEGMENT 4
SEGMENT 5
100%
FULL
DERIVATIVE
(ppm/°C)
CELL
CHARACTERIZATION
ACTIVE
EMPTY
TBP12
STANDBY
EMPTY
TBP23
+25°C
+50°C
Figure 12. Cell Model Example Diagram
______________________________________________________________________________________
13
DS2788
ing of three curves named full, active empty, and
standby empty. Each model curve is constructed with
five line segments, numbered 1 through 5. Above
+50°C, the segment 5 model curves extend infinitely
with zero slope, approximating the nearly flat change in
capacity of Li+ cells at temperatures above +50°C.
Segment 4 of each model curves originates at +50°C
on its upper end and extends downward in temperature
to +25°C. 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 °C. The two junctions or breakpoints that join the segments (labeled TBP12 and
TBP23 in Figure 12) are programmable in 1°C increments from -128°C to +25°C. They are stored in two’s
complement format, TBP23 at 7Ch, and TBP12 at 7Dh.
The slope or derivative for segments 1, 2, 3, and 4 are
also programmable.
DS2788
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
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
DS2788 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.
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.
The standby load current and voltage are used for
determining the cell characteristics but are not programmed into the DS2788. The DS2788 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 +50°C. The cell parameter
EEPROM block stores the initial values, the +50°C full
value in mVh units, and the +50°C active empty value
as a fraction of the +50°C value. Standby empty at
+50°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. Table 1 shows an example of data
stored in this manner.
CELL MODEL
PARAMETERS
15 BYTES
(EEPROM)
FULL(T)
LOOKUP
FUNCTION
AE(T)
SE(T)
TEMPERATURE
Figure 13. Lookup Function Diagram
Table 1. Example Cell Characterization Table (Normalized to +50°C)
Manufacturer’s Rated Cell Capacity: 1220mAh
Charge Voltage: 4.2V
Charge Current: 500mA
Active Empty (V, I): 3.0V, 500mA
Termination Current: 50mA
Standby Empty (V, I): 3.0V, 4mA
Sense Resistor: 0.020
SEGMENT BREAKPOINTS
TBP12 = -12°C
TBP23 = 0°C
+50°C NOMINAL
(mAh)
SEGMENT 1
(ppm/°C)
SEGMENT 2
(ppm/°C)
SEGMENT 3
(ppm/°C)
SEGMENT 4
(ppm/°C)
1214
488
549
1587
2686
Active Empty
854
1526
2686
3113
Standby Empty
244
183
916
244
CALCULATED VALUE
Full
14
______________________________________________________________________________________
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
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 19.52mV and can range
from 0 to 4.978V. VCHG should be set marginally less
than the cell 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 0 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 19.52mV and can range from 0 to 4.978V. VAE
is located in the parameter EEPROM block.
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.
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 unaged 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 cyclecount-based age estimation introduced above and by
the capacity learn function. The host system has read
and write access to AS, however caution should be
exercised when writing AS to ensure that the cumulative aging estimate is not overwritten with an incorrect
value. Typically, it is not necessary for the host to write
AS because the DS2788 automatically saves AS to
EEPROM on a periodic basis. (See the Memory section
for details.) The EEPROM-stored value of AS is recalled
on power-up.
Capacity Estimation Utility
Functions
Aging Estimation
As previously discussed, 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%. See the AC register description 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. Second, once charging starts, the charge must
______________________________________________________________________________________
15
DS2788
Application Parameters
DS2788
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
continue uninterrupted until the battery is charged to
full. Upon detecting full, LEARNF is cleared, the chargeto-full (CHGTF) flag is set, and the age scalar (AS) is
adjusted according to the learned capacity of the cell.
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, 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, 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
nonzero. 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 (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 bit (LEARNF) in STATUS.
16
Result Registers
The DS2788 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 +50°C full
value. This 15-bit value reflects the cell model full value
at the given temperature. FULL(T) reports values
between 100% and 50% with a resolution of 61ppm
(precisely 2-14). 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 +50°C full value. This 13-bit value reflects
the cell model active empty at the given temperature.
AE(T) reports values between 0% and 49.8% with a
resolution of 61ppm (precisely 2-14).
Standby Empty, SE(T): The standby empty capacity of
the battery at the present temperature is reported normalized to the +50°C full value. This 13-bit value
reflects the cell model standby empty value at the current temperature. SE(T) reports values between 0% and
49.8% with a resolution of 61ppm (precisely 2-14).
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 (mAh). 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 mAh. 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.
______________________________________________________________________________________
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
RAAC [mAh] = (ACR[mVh] - AE(T) × FULL50[mVh]) × RSNSP [mhos]
RSAC [mAh] = (ACR[mVh] - SE(T) × FULL50[mVh]) × RSNSP [mhos]
RARC [%] = 100% × (ACR[mVh] - AE(T) × FULL50[mVh]) / {(AS × FULL(T) - AE(T)) × FULL50[mVh]}
RSRC [%] = 100% × (ACR[mVh] - SE(T) × FULL50[mVh]) / {(AS × FULL(T) - SE(T)) × FULL50[mVh]}
Status Register
The Status register contains bits that report the device
status. The bits can be set internally by the DS2788.
The CHGTF, AEF, SEF, LEARNF, and VER bits are
ADDRESS
Field
read-only bits that can be cleared by hardware. The
UVF and PORF bits can only be cleared through the 1Wire interface.
01h
Bit
BIT DEFINITION
Format
Allowable Values
CHGTF
7
Read Only
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%
AEF
6
Read Only
Active Empty Flag
Set to 1 when: VOLT < VAE
Cleared to 0 when: RARC > 5%
SEF
5
Read Only
Standby Empty Flag
Set to 1 when: RSRC < 10%
Cleared to 0 when: RSRC > 15%
LEARNF
4
Read Only
Learn Flag—When 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).
Reserved
3
Read Only
Undefined
UVF
2
Read/Write*
Undervoltage Flag
Set to 1 when: VOLT < VSLEEP
Cleared to 0 by: User
PORF
1
Read/Write*
Power-On Reset Flag—Useful for reset detection, see text below.
Set to 1 when: upon power-up by hardware.
Cleared to 0 by: User
Reserved
0
Read Only
Undefined
*This bit can be set by the DS2788, and can only be cleared through the 1-Wire interface.
**LEARNF is only cleared if ACR reaches 0 after VOLT < VAE.
Figure 14. Status Register Format
______________________________________________________________________________________
17
DS2788
Calculation of Results
DS2788
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
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
ADDRESS
60h
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.
BIT DEFINITION
Field
Bit
Format
Allowable Values
NBEN
7
Read/Write
Negative Blanking Enable
0: Allows negative current readings to always be accumulated.
1: Enables blanking of negative current readings up to -25μV.
UVEN
6
Read/Write
Undervoltage SLEEP Enable
0: Disables transition to SLEEP mode based on VIN voltage.
1: Enables transition to SLEEP mode if VIN < VSLEEP and DQ are stable at either
logic level for t SLEEP.
PMOD
5
Read/Write
Power Mode Enable
0: Disables transition to SLEEP mode based on DQ logic state.
1: Enables transition to SLEEP mode if DQ is at a logic-low for t SLEEP.
RNAOP
4
Read/Write
Read Net Address Op Code
0: Read net address command = 33h.
1: Read net address command = 39h.
DC
3
Read/Write
Display Control
0: Enables LED5 fuel-gauge display.
1: Enables LED4 fuel-gauge display.
Reserved
0:2
Undefined
Figure 15. Control Register Format
18
______________________________________________________________________________________
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
All Special Feature register bits are read and write accessible, with default values specified in each bit definition.
ADDRESS
15h
Field
Bit
Reserved
1:7
PIOSC
0
BIT DEFINITION
Format
Allowable Values
Undefined
Read/Write
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 high-Z).
Note: PIO pin has weak pulldown.
Figure 16. Special Feature Register Format
Fuel-Gauge Display
The DS2788 provides five open-drain drivers capable
of sinking 30mA. These can be used to directly drive
either 4 or 5 LEDs to display Remaining Active Relative
Pack Capacity (RARC). The LEDs are enabled when
the PIO is configured as an input and the PIO pin recognizes a rising edge. The display lights for 4s and
then is disabled regardless of the state of the PIO pin.
Further presses or releases of the PIO pin after the
100ms debounce delay causes the display to be
enabled (the display does not light continuously if the
button is held down).
Table 2 summarizes how the LEDs are enabled. B signifies that the LED is blinking at a 50% duty cycle, 0.5s
on, 0.5s off, to be repeated for the display time of 4s. L
signifies the pin is pulled low, and the LED is lit. X signifies the pin is high impedance, and the LED is unlit.
Table 2. Fuel-Gauge Display Summary
CAPACITY
5 LEDs, DC: 0
LED5–LED1
4 LEDs, DC: 1
LED4–LED1
RARC 10
XXXXB
XXXB
10 < RARC 20
XXXXL
XXXL
20 < RARC 25
XXXLL
XXXL
25 < RARC 40
XXXLL
XXLL
40 < RARC 50
XXLLL
XXLL
50 < RARC 60
XXLLL
XLLL
60 < RARC 75
XLLLL
XLLL
75 < RARC 80
XLLLL
LLLL
80 < RARC 100
LLLLL
LLLL
______________________________________________________________________________________
19
DS2788
Special Feature Register
DS2788
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
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
ADDRESS
Field
EEC
1Fh
Bit
7
LOCK
6
Reserved
2:6
BL1
BL0
1
0
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.
BIT DEFINITION
Format
Allowable Values
Read Only
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
Read/Write
to 1
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 is not the
command issued immediately following the Write command used to set the lock
enable bit.
Power-up default: 0
Undefined
Read Only
EEPROM Block 1 Lock Flag (Parameter EEPROM 60h–7Fh)
0: EEPROM is not locked.
1: EEPROM block is locked.
Factory default: 0
Read Only
EEPROM Block 0 Lock Flag (User EEPROM 20h–2Fh)
0: EEPROM is not locked.
1: EEPROM block is locked.
Factory default: 0
Figure 17. EEPROM Register Format
20
______________________________________________________________________________________
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
The DS2788 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 two-byte registers in order to store 16-bit values. The MSB of the 16-bit value is located at a even
address and the 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 NV 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. To modify the
data stored in the EEPROM cells, data must be written to
the shadow RAM and then copied to the EEPROM. 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.
User EEPROM
A 16-byte user EEPROM memory (block 0, addresses
20h–2Fh) provides NV memory that is uncommitted to
other DS2788 functions. Accessing the user EEPROM
block does not affect the operation of the DS2788. 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 and 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 DS2788 to be located outside the protection FETs. In this manner, if a protection device is
triggered, the DS2788 cannot lose more that 4% of
charge or discharge data.
Table 3. Memory Map
ADDRESS (HEX)
DESCRIPTION
READ/WRITE
00
Reserved
R
01
STATUS: Status Register
02
RAAC: Remaining Active Absolute Capacity MSB
R
03
RAAC: Remaining Active Absolute Capacity LSB
R
04
RSAC: Remaining Standby Absolute Capacity MSB
R
05
RSAC: Remaining Standby Absolute Capacity LSB
R
06
RARC: Remaining Active Relative Capacity
R
07
RSRC: Remaining Standby Relative Capacity
R
08
IAVG: Average Current Register MSB
R
R/W
09
IAVG: Average Current Register LSB
R
0A
TEMP: Temperature Register MSB
R
0B
TEMP: Temperature Register LSB
R
0C
VOLT: Voltage Register MSB
R
0D
VOLT: Voltage Register LSB
R
0E
CURRENT: Current Register MSB
R
0F
CURRENT: Current Register LSB
10
ACR: Accumulated Current Register MSB
R/W*
11
ACR: Accumulated Current Register LSB
R/W*
R
______________________________________________________________________________________
21
DS2788
Memory
DS2788
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
Table 3. Memory Map (continued)
ADDRESS (HEX)
DESCRIPTION
READ/WRITE
12
ACRL: Low Accumulated Current Register MSB
13
ACRL: Low Accumulated Current Register LSB
14
AS: Age Scalar
R/W*
15
SFR: Special Feature Register
R/W
16
FULL: Full Capacity MSB
R
17
FULL: Full Capacity LSB
R
18
AE: Active Empty MSB
R
19
AE: Active Empty LSB
R
1A
SE: Standby Empty MSB
R
1B
SE: Standby Empty LSB
R
Reserved
—
1C to 1E
1F
R
R
EEPROM: EEPROM Register
R/W
20 to 2F
User EEPROM, Lockable, Block 0
R/W
30 to 5F
Reserved
—
60 to 7F
Parameter EEPROM, Lockable, Block 1
80 to AD
Reserved
—
AE
FVGAIN: Factory Voltage Gain MSB
R
AF
FVGAIN: Factory Voltage Gain LSB
R
B0
FRSGAIN: Factory Sense Resistor Gain MSB
R
B1
FRSGAIN: Factory Sense Resistor Gain LSB
R
Reserved
—
B2 to FF
R/W
*Register value is automatically saved to EEPROM during ACTIVE mode operation and recalled from EEPROM on power-up.
Table 4. Parameter EEPROM Memory Block 1
ADDRESS (HEX)
22
DESCRIPTION
ADDRESS (HEX)
DESCRIPTION
60
CONTROL: Control Register
70
AE Segment 4 Slope
61
AB: Accumulation Bias
71
AE Segment 3 Slope
62
AC: Aging Capacity MSB
72
AE Segment 2 Slope
63
AC: Aging Capacity LSB
73
AE Segment 1 Slope
64
VCHG: Charge Voltage
74
SE Segment 4 Slope
65
IMIN: Minimum Charge Current
75
SE Segment 3 Slope
66
VAE: Active Empty Voltage
76
SE Segment 2 Slope
67
IAE: Active Empty Current
77
SE Segment 1 Slope
68
Active Empty 50
78
RSGAIN: Sense Resistor Gain MSB
69
RSNSP: Sense Resistor Prime
79
RSGAIN: Sense Resistor Gain LSB
6A
Full 50 MSB
7A
RSTC: Sense Resistor Temp Coefficient
6B
Full 50 LSB
7B
COB: Current Offset Bias
6C
Full Segment 4 Slope
7C
TBP23
6D
Full Segment 3 Slope
7D
TBP12
6E
Full Segment 2 Slope
7E
VGAIN: Voltage Gain MSB
6F
Full Segment 1 Slope
7F
VGAIN: Voltage Gain LSB
______________________________________________________________________________________
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
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 DS2788 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 DS2788 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 (32h for DS2788).
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
DS2788 to communicate through the 1-Wire protocol
detailed in the 1-Wire Bus System section.
CRC Generation
The DS2788 has an 8-bit CRC stored in the MSB 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 DS2788. The host system is
responsible for verifying the CRC value and taking
action as a result. The DS2788 does not compare CRC
8-BIT 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 19, or it can be generated in software.
Additional information about the Dallas 1-Wire CRC is
available in Application Note 27: Understanding and
Using Cyclic Redundancy Checks with Dallas
Semiconductor iButton Products (www.maximic.com/appnoteindex).
In the circuit in Figure 19, the shift register bits are initialized to 0. Then, starting with the LSb 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.
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 three-state output drivers. The
DS2788 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,
48-BIT SERIAL NUMBER
MSb
8-BIT FAMILY
CODE (32h)
LSb
Figure 18. 1-Wire Net Address Format
INPUT
MSb
XOR
XOR
LSb
XOR
Figure 19. 1-Wire CRC Generation Block Diagram
iButton is a registered trademark of Dallas Semiconductor
Corp., a wholly owned subsidiary of Maxim Integrated
Products, Inc.
______________________________________________________________________________________
23
DS2788
1-Wire Bus System
DS2788
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
separate output and input pins can be connected
together.
The 1-Wire bus must have a pullup resistor at the busmaster end of the bus. For short line lengths, the value
of this resistor should be approximately 5kΩ. 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 120μs (16μs for overdrive speed), slave devices on the bus begin to interpret the low period as a reset pulse, effectively
terminating the transaction.
The DS2788 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.
Transaction Sequence
The protocol for accessing the DS2788 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 DS2788 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 op code
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 DS2788’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 op code for this command, with RNAOP = 0
indicating 33h and RNAOP = 1 indicating 39h.
VPULLUP
(2.0V TO 5.5V)
DS2788 1-Wire PORT
BUS MASTER
4.7kΩ
Rx
Rx
0.2μA
(TYP)
Tx
Rx = RECEIVE
Tx = TRANSMIT
Tx
100Ω MOSFET
Figure 20. 1-Wire Bus Interface Circuitry
24
______________________________________________________________________________________
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
Skip Net Address [CCh]. This command saves time
when there is only one DS2788 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 1-Wire net addresses of all slave devices on the
bus. The search process involves the repetition of a
simple three-step 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 iButton
Standards for a comprehensive discussion of a net
address search, including an actual example
(www.maxim-ic.com/ibuttonbook).
Resume [A5h]. This command increases data throughput in multidrop environments where the DS2788 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
DS2788 is accessed. After successfully executing a
Match Net Address command or Search Net Address
command, an internal flag is set in the DS2788. When
the flag is set, the DS2788 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.
Function Commands
After successfully completing one of the net address
commands, the bus master can access the features of
the DS2788 with any of the function commands
described in the following paragraphs. The name of
each function is followed by the 8-bit op code for that
command in square brackets. Table 5 summarizes the
function commands.
Read Data [69h, XX]. This command reads data from
the DS2788 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
DS2788 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.
______________________________________________________________________________________
25
DS2788
Match Net Address [55h]. This command allows the
bus master to specifically address one DS2788 on the
1-Wire bus. Only the addressed DS2788 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.
DS2788
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
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.
Table 5. Function Commands
COMMAND
DESCRIPTION
COMMAND
PROTOCOL
BUS STATE AFTER
COMMAND
PROTOCOL
BUS DATA
Read Data
Reads data from memory starting at address XX.
69h, XX
Master Rx
Up to 256
bytes of data
Write Data
Writes data to memory starting at address XX.
6Ch, XX
Master Tx
Up to 256
bytes of data
Copy Data
Copies shadow RAM data to EEPROM block containing
address XX.
48h, XX
Master Reset
None
Recall Data
Recalls EEPROM block containing address XX to RAM.
B8h, XX
Master Reset
None
Permanently locks the block of EEPROM
containing address XX.
6Ah, XX
Master Reset
None
Lock
26
______________________________________________________________________________________
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
DS2788
MASTER Tx
RESET PULSE
DS2788 Tx
PRESENCE PULSE
MASTER Tx NET
ADDRESS COMMAND
33h/39h
READ
NO
NO
55h
MATCH
F0h
SEARCH
YES
YES
YES
CCh
SKIP
YES
NO
A5h
RESUME
NO
YES
DS2788 Tx BIT 0
MASTER Tx
BIT 0
DS2788 Tx
FAMILY CODE
1 BYTE
NO
DS2788 Tx BIT 0
MASTER Tx BIT 0
CLEAR RESUME
RESUME
FLAG SET?
NO
YES
DS2788 Tx
SERIAL NUMBER
6 BYTES
BIT O
MATCH?
NO
NO
YES
DS2788 Tx
CRC
1 BYTE
BIT O
MATCH?
MASTER Tx
FUNCTION COMMAND
MASTER Tx
FUNCTION COMMAND
YES
DS2788 Tx BIT 1
MASTER Tx
BIT 1
DS2788 Tx BIT 1
MASTER Tx BIT 1
BIT 1
MATCH?
CLEAR RESUME
YES
MASTER Tx
FUNCTION COMMAND
MASTER Tx
BIT 63
NO
NO
BIT 1
MATCH?
YES
DS2788 Tx BIT 63
DS2788 Tx BIT 63
MASTER Tx BIT 63
SET RESUME
FLAG
YES
BIT 1
MATCH?
NO
CLEAR RESUME
Figure 21. Net Address Command Flowchart
______________________________________________________________________________________
27
DS2788
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
1-Wire Signaling
The 1-Wire bus requires strict signaling protocols to
ensure data integrity. The four protocols used by the
DS2788 are as follows: the initialization sequence (reset
pulse followed by presence pulse), write-0, write-1, and
read data. All these types of signaling except the presence pulse are initiated by the bus master.
Figure 22 shows the initialization sequence required to
begin any communication with the DS2788. A presence
pulse following a reset pulse indicates that the DS2788
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 DS2788 waits for tPDH and then transmits the presence pulse for tPDL.
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 1µs minimum recovery time, t REC ,
between cycles. The DS2788 samples the 1-Wire bus
line between 15µs and 60µs (between 2µs and 6µs 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 15µs (2µs 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 1µs and then release it to allow the DS2788 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 DS2788 releases the bus
line and allows it to be pulled high by the external
pullup resistor. All read-time slots must be t SLOT in
duration with a 1µs minimum recovery time, t REC ,
between cycles. See Figure 23 for more information.
tRSTL
tRSTH
tPDH
tPDL
PK+
DQ
PK-
LINE TYPE LEGEND:
BUS MASTER ACTIVE LOW
DS2788 ACTIVE LOW
RESISTOR PULLUP
Figure 22. 1-Wire Initialization Sequence
28
______________________________________________________________________________________
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
DS2788
WRITE-0 SLOT
WRITE-1 SLOT
tSLOT
tSLOT
tLOW0
tLOW
tREC
VPULLUP
GND
MIN
> 1μs
DS2788 SAMPLE WINDOW
TYP
MAX
MIN
DS2788 SAMPLE WINDOW
TYP
MAX
MODE:
STANDARD
15μs
15μs
30μs
15μs
15μs
30μs
OVERDRIVE
2μs
1μs
3μs
2μs
1μs
3μs
READ-0 SLOT
READ-1 SLOT
tSLOT
tSLOT
tREC
VPULLUP
GND
> 1μs
MASTER SAMPLE WINDOW
tRD
MASTER SAMPLE WINDOW
tRD
LINE TYPE LEGEND:
BUS MASTER ACTIVE LOW
DS2788 ACTIVE LOW
BOTH BUS MASTER AND
DS2788 ACTIVE LOW
RESISTOR PULLUP
Figure 23. 1-Wire Write- and Read-Time Slots
______________________________________________________________________________________
29
DS2788
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
Typical Operating Circuit
PK+
5V
REGULATOR
DISPLAY
LED1
LED2
LED3
LED4
DS2788
LED5
150Ω
DATA
(N – 1) RΩ
PIO
VDD
DQ
SNS
N-CELL
Li+ BATTERY
VMA
VIN
OVD
DVSS
VSS
0.1μF
RΩ
PROTECTION
CIRCUIT
5.6V
RSNS
PK-
Package Information
(For the latest package outline information, go to
www.maxim-ic.com/DallasPackInfo.)
PACKAGE TYPE
DOCUMENT NO.
14 TSSOP
56-G2015-000
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
30 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2007 Maxim Integrated Products
is a registered trademark of Maxim Integrated Products, Inc.