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DS2786BG+

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21
DS2786B
Stand-Alone OCV-Based Fuel Gauge
10 ______________________________________________________________________________________
MSB—ADDRESS 0Ah LSB—ADDRESS 0Bh
S 2
9
2
8
2
7
2
6
2
5
2
4
2
3
2
2
2
1
2
0
X X X X X
MSB LSB MSB LSB
“S”: SIGN BIT, “X”: RESERVED UNITS: 0.125°C
Figure 7. Temperature Register Format
MSB—ADDRESS 0Eh LSB—ADDRESS 0Fh
S 2
10
2
9
2
8
2
7
2
6
2
5
2
4
2
3
2
2
2
1
2
0
X X X X
MSB LSB MSB LSB
“S”: SIGN BIT, “X”: RESERVED UNITS: 25μV/R
SNS
Figure 8. Current Register Formats
CURRENT RESOLUTION (1 LSB)
R
SNS
|V
SS
- V
SNS
|
20m
15m 10m 5m
25μV
1.25mA
1.667mA 2.5mA 5mA
CURRENT INPUT RANGE
R
SNS
V
SS
- V
SNS
20m 15m 10m 5m
±51.2mV ±2.56A ±3.41A ±5.12A ±10.24A
Table 1. Current Range and Resolution for Various R
SNS
Values
Temperature Measurement
The DS2786B uses an integrated temperature sensor to
measure battery temperature with a resolution of
0.125°C. Temperature measurements are updated
every 1760ms and placed in the Temperature Register
in two’s-complement form. The format of the
Temperature Register is shown in Figure 7. The ITEMP
bit in the Status/Configuration Register must be set to
enable the internal temperature measurement instead
of the AIN1 measurement.
Current Measurement
In the Active Mode of operation, the DS2786B continu-
ally measures the current flow into and out of the bat-
tery by measuring the voltage drop across a low-value
current-sense resistor, R
SNS
, connected between the
SNS and V
SS
pins. The voltage-sense range between
SNS and V
SS
is ±51.2mV. Note that positive current val-
ues occur when V
SNS
is less than V
SS
, and negative
current values occur when V
SNS
is greater than V
SS
.
Peak signal amplitudes up to 102mV are allowed at the
input as long as the continuous or average signal level
does not exceed ±51.2mV over the conversion-cycle
period. The ADC samples the input differentially and
updates the Current Register every 880ms at the com-
pletion of each conversion cycle. Figure 8 describes
the Current Measurement Register format and resolu-
tion for each option. Charge currents above the maxi-
mum register value are reported at the maximum value
(7FFFh = +51.2mV). Discharge currents below the mini-
mum register value are reported at the minimum value
(8000h = -51.2mV).
Every 1024th conversion, the ADC measures its input
offset to facilitate offset correction to improve current
accuracy. Offset correction occurs approximately every
15min. The resulting correction factor is applied to the
subsequent 1023 measurements. During the offset cor-
rection conversion, the ADC does not make a measure-
ment. The current measurement just prior to the offset
conversion is displayed in the Current Register. See
Table 1 for current range and resolution for various
R
SNS
values.
DS2786B
Stand-Alone OCV-Based Fuel Gauge
______________________________________________________________________________________ 11
Current Offset Bias
The Current Offset Bias Register (COBR) allows a pro-
grammable offset value to be added to raw current
measurements. The result of the raw current measure-
ment plus the COBR value is displayed as the current
measurement result in the Current Register, and is used
for current accumulation and detection of an OCV con-
dition. The COBR value can be used to correct for a sta-
tic offset error, or can be used to intentionally skew the
current results and therefore the current accumulation.
Read and write access is allowed to COBR. Whenever
the COBR is written, the new value is applied to all sub-
sequent current measurements. COBR can be pro-
grammed in 25µV steps to any value between
+3.175mV and -3.2mV. The COBR value is stored as a
two’s-complement value in nonvolatile (NV) memory.
The COBR factory default value is 00h. Figure 9 shows
the Current Offset Bias Register format.
Current Accumulation
An Internal Accumulated Current Register (IACR)
serves as an up/down counter holding a running count
of charge since the last OCV condition. Current mea-
surement results, plus a programmable bias value are
internally summed, or accumulated, at the completion
of each current measurement-conversion period. The
IACR has a range of ±204.8mVh. The IACR uses the
Initial or Learned Cell Capacity Registers to increment
or decrement the Relative Capacity Register as current
flows into or out of the battery. In this way, the fuel
gauge is accurate even when an OCV condition does
not occur for an extended time period. See Table 2 for
the accumulated current range for various R
SNS
values.
Cell-Capacity Estimation
The DS2786B uses a hybrid OCV measurement and
coulomb-counting algorithm to estimate remaining cell
capacity. During periods of charging or discharging the
cell, the DS2786B counts charge flow into and out of
the cell. When the application becomes inactive, the
DS2786B waits for the cell voltage to relax and then
adjusts the coulomb count based on an open-circuit
voltage cell model stored in device EEPROM. The
resulting calculation is reported to the system as a per-
centage value between 0 and 100%. As the cell ages, a
learn feature adjusts for changes in capacity.
The Relative Capacity Register reports remaining cell
charge as a percentage of full. Relative capacity is
reported with a resolution of 0.5% and is limited to a
value between 0% and 100%. The Relative Capacity
Register is updated each time the IC performs a current
measurement or open-circuit cell-voltage measurement.
See Figure 10.
ADDRESS 60h
S 2
6
2
5
2
4
2
3
2
2
2
1
2
0
MSB LSB
“S”: SIGN BIT UNITS: 25μV/R
SNS
Figure 9. Current Offset Bias Register Format
IACR RANGE
R
SNS
V
SS
- V
SNS
20m 15m 10m 5m
±204.8mVh ±10.24Ah ±13.65Ah ±20.48Ah ±40.96Ah
Table 2. Accumulated Current Range for
Various R
SNS
Values
ADDRESS 02h
2
7
2
6
2
5
2
4
2
3
2
2
2
1
2
0
MSB LSB
UNITS: 0.5%
Figure 10. Relative Capacity Register Format
DS2786B
Stand-Alone OCV-Based Fuel Gauge
12 ______________________________________________________________________________________
Prior to the first learn operation, the relative capacity
value is calculated by adding the IACR multiplied by the
initial capacity scaling factor (7Ah) to the last OCV rela-
tive capacity (16h). After the first learn operation, the rel-
ative capacity value is calculated by adding the IACR
multiplied by the learned capacity scaling factor (17h) to
the last OCV relative capacity (16h).
Each Capacity Scaling Factor Register has a resolution
of 78.125%/Vh and a maximum range of 0 to
19921.875%/Vh. During assembly, the Initial Capacity
Register should be programmed to the capacity of the
cell. For example, an application using a 1Ah cell and
0.015Ω sense resistor would set the Initial Capacity
Register to a value of (100%/(1Ah x 0.015Ω))/
78.125%/Vh = 55h. The Learned Capacity Scaling Factor
Register is controlled by the DS2786B. The power-up
value is 00h, and the register is updated with the calcu-
lated new cell capacity value after every learn operation.
See Figures 11 and 12.
OCV Detection
When the magnitude of the measured current (after
COBR is applied) is less than the value defined by the
OCV Threshold Register, the DS2786B begins dV/dt
measurement evaluation to detect an OCV voltage
condition. A threshold value that is below the minimum
operational current, but above the maximum idle cur-
rent of the application should be selected. The OCV
Threshold Register has a resolution of 25µV/R
SNS
, and
a range from 0mV/R
SNS
to 6.375mV/R
SNS
. The factory
default value is 28h. See Figure 13 for the OCV thresh-
old register format.
While the measured current is below the OCV threshold
level, the DS2786B actively searches for a relaxed cell
by calculating the change in cell voltage as reported in
the Voltage Register over 7.5min intervals (dV/dt). If the
7.5min dV/dt change of an average of four Voltage
Register readings is less than the value stored in the
OCV dV/dt Threshold Register, the DS2786B deter-
mines that the cell is now in a relaxed state and the
Relative Capacity Register is adjusted based on the
OCV cell model stored in parameter EEPROM. This
operation occurs repeatedly every 7.5min up to 1hr
after the cell enters a relaxed state.
The OCV dV/dt Threshold Register has a resolution of
0.61mV/7.5min and a range from 0mV/7.5min to
9.15mV/7.5min. The factory default value is
2.44mV/7.5min. Note that the upper 4 bits of the OCV
dV/dt Threshold Register are used to EEPROM back
bits from the Status/Configuration Register. Figure 14
shows the OCV dV/dt threshold register format.
ADDRESS 7Ah
2
7
2
6
2
5
2
4
2
3
2
2
2
1
2
0
MSB LSB
UNITS: 78.125%/Vh
Figure 11. Initial Capacity Scaling Factor Register Format
ADDRESS 17h
2
7
2
6
2
5
2
4
2
3
2
2
2
1
2
0
MSB LSB
UNITS: 78.125%/Vh
Figure 12. Learned Capacity Scaling Factor Register Format
ADDRESS 7Bh
2
7
2
6
2
5
2
4
2
3
2
2
2
1
2
0
MSB LSB
UNITS: 25μV/R
SNS
Figure 13. OCV Threshold Register Format
ADDRESS 7Ch
SMOD LDIS VODIS ITEMP 2
3
2
2
2
1
2
0
MSB LSB
UNITS: 0.61mV/7.5min
Figure 14. OCV dV/dt Threshold Register Format
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21

DS2786BG+

Mfr. #:
Buy DS2786BG+
Manufacturer:
Maxim Integrated
Description:
Battery Management Stand-Alone Fuel Gauge
Lifecycle:
New from this manufacturer.
Delivery:
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