April 2010 - Tiny, Accurate Battery Gas Gauges with Easy-to-Use I2C Interface and Optional Integrated Precision Sense Resistors

Tiny, Accurate Battery Gas Gauges with Easy-to-Use I2C
Interface and Optional Integrated Precision Sense Resistors
Christoph Schwoerer, Axel Klein and Bernhard Engl
Imagine your daughter is catching her first wave on a
surfboard and your video camera shuts down because the
battery—reading half full when you started filming a few
minutes ago—is suddenly empty. The problem is inaccurate
battery gas gauging. Inaccurate fuel gauging is a common
nuisance as many portable devices derive remaining
battery capacity directly from battery voltage. This method
is cheap but inaccurate since the relationship between
battery voltage and capacity has a complex dependency
on temperature, load conditions and usage history.
More accurate battery gauging can be
achieved by monitoring not only the
battery voltage but also by tracking the
charge that goes in and out of the battery. For applications requiring accurate
battery gas gauging, the LTC2941 and
LTC2942 coulomb counters are tiny and
easy-to-use solutions. These feature-rich
devices are small and integrated enough to
fit easily into the latest handheld gadgets.
The LTC2941 is a battery gas gauge device
designed for use with single Li-Ion cells
and other battery types with terminal
voltages between 2.7V and 5.5V. A precision coulomb counter integrates current
through a sense resistor between the
battery’s positive terminal and the load
or charger. The high side sense resistor
avoids splitting the ground path in the
application. The state of charge is continuously updated in an accumulated
charge register (ACR) that can be read out
via an SMBus/I2C interface. The LTC2941
also features programmable high and
low thresholds for accumulated charge.
If a threshold is exceeded, the device
12 | April 2010 : LT Journal of Analog Innovation
ANALOG INTEGRATOR ALLOWS
PRECISE COULOMB COUNTING
Charge is the time integral of current. The LTC2941 and LTC2942 use
a continuous time analog integrator
to determine charge from the voltage drop developed across the sense
resistor RSENSE as shown in Figure 2.
The differential voltage between SENSE+
and SENSE– is applied to an autozeroed
differential integrator to convert the
measured current to charge, as shown
in Figure 2. When the integrator output
ramps to REFHI or REFLO levels, switches
S1, S2, S3 and S4 toggle to reverse the ramp
direction. By observing the condition
of the switches and the ramp direction,
polarity is determined. A programmable
prescaler adjusts integration time to
match the capacity of the battery. At each
underflow or overflow of the prescaler,
the ACR value is incremented or decremented one count. The value of accumulated charge is read via the I2C interface.
communicates an alert using either
the SMBus alert protocol or by setting
a flag in the internal status register.
The LTC2942 adds an ADC to the coulomb
counter functionality of the LTC2941. The
ADC measures battery voltage and chip
temperature and provides programmable
thresholds for these quantities as well.
The LTC2941 and LTC2942 are pin compatible and come in tiny 6-pin 2mm × 3mm
DFN packages. Each consumes only
75µA in normal operation. Figure 1
shows the LTC2942 monitoring the charge
status of a single cell Li-ion battery.
500mA
VIN
5V
BAT
VCC
1µF
Figure 1. Monitoring
the charge status of a
single-cell Li-Ion battery with the LTC2942
The use of an analog integrator distinguishes the LTC coulomb counters from
most other gas gauges available on the
2k
LTC4057-4.2
(CHARGER)
PROG SHDN
GND
2
LOAD
0.1µF
3.3V
2k
VDD
µP
2k
2k
SENSE+
LTC2942
AL/CC
SDA
SENSE–
SCL
GND
RSENSE
100mΩ
+
1-CELL
Li-Ion
design features
Figure 2. Coulomb counter
section of LTC2942
LTC2942
LOAD
1
SENSE+
S1
2
market. It is common to use an ADC to
periodically sample the voltage drop over
the sense resistor and digitally integrate
the sampled values over time. This implementation has two major drawbacks.
First, any current spikes occurring inbetween sampling instants is lost, which
leads to rather poor accuracy—especially in applications with pulsed loads.
Second, the digital integration limits the
precision to the accuracy of the available
time base—typically low if not provided
by additional external components.
In contrast, the coulomb counter of
the LTC2941 and LTC2942 achieves an
accuracy of better than 1% over a wide
range of input signals, battery voltages
and temperatures without such external
components, as shown in Figures 3 and 4.
SENSE–
ACR
+
S4
REFLO
GND
POLARITY
DETECTION
–
TEMPERATURE AND VOLTAGE
MEASUREMENT
gral nonlinearity of the ADC is typically
below 0.5LSB as shown in Figure 6.
The LTC2942 includes a 14-bit No Latency
DS™ analog-to-digital converter with internal clock and voltage reference circuits to
measure battery voltage. The integrated
reference circuit has a temperature coefficient typically less than 20ppm/°C, giving
an ADC gain error of less than 0.3% from
–45°C to 85°C (see Figure 5). The inte-
The ADC is also used to read the output
of the on-chip temperature sensor. The
sensor generates a voltage proportional
to temperature with a slope of 2.5mV/°C,
resulting in a voltage of 750mV at 27°C.
The total temperature error is typically below ±2°C as shown in Figure 7.
3
1.00
0.75
2
1
0
–1
–2
INTEGRATED SENSE RESISTOR
VERSIONS LTC2941-1 AND LTC2942‑1
1
10
0.50
0.25
0
–0.25
–0.50
–0.75
VSENSE+ = 2.7V
VSENSE+ = 4.2V
–3
0.1
100
–1.00
–50
VSENSE = –50mV
VSENSE = –10mV
VSENSE (mV)
25
0
50
TEMPERATURE (°C)
Figure 3. Charge error vs VSENSE
Figure 4. Charge error vs temperature
10
–25
75
100
1.0
8
TOTAL UNADJUSTED ERROR (mV)
The accuracy of the charge monitoring
depends not only on the accuracy of the
chosen battery gas gauge but also on
the precision of the sense resistor. The
LTC2941-1 and LTC2942-1 remove the need
for a high precision external resistor by
including an internal, factory trimmed
50mΩ sense resistor. Proprietary internal
circuitry compensates the temperature
coefficient of the integrated metal resistor
to a residual error of only 50ppm/°C which
makes the LTC2941-1 and LTC2942-1 by
far the most precise internal sense resistor battery gas gauges available today.
M
PRESCALER
CHARGE ERROR (%)
6
+
CONTROL
LOGIC
–
+
S3
CHARGE ERROR (%)
BATTERY
–
+
S2
RSENSE
IBAT
REFHI
VCC
TA = 85°C
6
0.5
TA = 85°C
4
2
0
INL (VLSB)
CHARGER
TA = –45°C
–2
–4
–6
0
TA = –40°C
TA = 25°C
–0.5
TA = 25°C
–8
–10
2.5
3.0
3.5
4.0 4.5
5.0
VSENSE– (V)
5.5
6.0
Figure 5. ADC total unadjusted error
–1.0
2.5
3.0
3.5
4.0 4.5 5.0
VSENSE– (V)
5.5
6.0
Figure 6. ADC integrated nonlinearity
April 2010 : LT Journal of Analog Innovation | 13
USB CHARGING
Figure 9 shows a portable application
designed to charge a Li-Ion battery
from a USB connection. The LTC2942-1
monitors the charge status of a singlecell Li-Ion battery in combination
with the LTC4088-1 high efficiency battery charger/USB power manager.
Once a charge cycle is completed, the
LTC4088-1 releases the CHRG pin. The
microcontroller detects this and sets the
accumulated charge register to full either
by writing it via the I2C interface or by
applying a pulse to the charge complete (AL/CC) pin of the LTC2942-1 (if it
is configured as input). Once initialized,
the LTC2942-1 accurately monitors the
3
MONITORING BATTERY STACKS
The LTC2941 and LTC2942 are not
restricted to single cell Li-Ion applications.
They can also monitor the charge state of
a battery stack as shown in Figure 11.
2
TEMPERATURE ERROR (°C)
Conversion of either temperature or
voltage is triggered by setting the control
register via the I2C interface. The LTC2942
also features an optional automatic
mode where a voltage and a temperature
conversion are executed once a second.
At the end of each conversion the corresponding registers are updated before
the converter goes to sleep to minimize
quiescent current. Figure 8 shows a block
diagram of the LTC2942, with the coulomb counter and its ACR, the temperature
sensor, the ADC with the corresponding
data registers and the I2C interface.
1
0
In this configuration, the power consumption of the gas gauge might lead
to an unacceptable imbalance between
the lower and the upper Li-Ion cells
in the stack. This imbalance can be
eliminated by supervising every cell
individually, as shown in Figure 12.
–1
–2
–3
–50
–25
0
25
50
TEMPERATURE (°C)
75
100
Figure 7. Temperature sensor error
charge flowing in and out of the battery,
and the microcontroller can monitor the
state of charge by reading the accumulated charge register via the I2C interface.
By monitoring each cell’s state of
charge, the LTC2942-1 provides enough
information to balance the cells
while charging and discharging.
ENERGY MONITORING
TRACKING BATTERY CAPACITY WITH
TEMPERATURE AND AGING
Real time energy monitoring is increasingly used in non-portable, wall powered
systems such as servers or networking
equipment. The LTC2941 and LTC2942
are just as well suited to monitor energy
flow in any 3.3V or 5V rail application
as they are in battery powered applications. Figure 10 shows an example.
Battery capacity varies with temperature
and aging. There is a wide variety of
approaches and algorithms to track the
battery capacity tailored to specific applications and the chemistry of the battery.
The LTC2942 measures all physical quantities—charge, voltage, temperature and (by
differentiating charge) current—necessary
to model the effects of temperature and
aging on battery capacity. The measured
quantities are easily accessible by reading
the corresponding registers with standard
With a constant supply voltage, the
charge flowing through the sense resistor is proportional to the energy consumed by the load. Thus several LTC2941
devices can help determine exactly
where system energy is consumed.
Figure 8. Block diagram
of the LTC2942
1
SENSE+
VSUPPLY
COULOMB COUNTER
REF
TEMPERATURE
SENSOR
6
2
SENSE–
MUX
CC
CLK
REFERENCE
GENERATOR
OSCILLATOR
REF+
CLK
IN
ACCUMULATED
CHARGE
REGISTER
AL
I2C/
SMBus
AL/CC
SCL
SDA
ADC
DATA AND
CONTROL
REGISTERS
REF–
GND
LTC2942
14 | April 2010 : LT Journal of Analog Innovation
5
3
4
design features
I2C commands. No special instruction
language or programming is required.
3.3uH
The LTC2941 and LTC2942 do not impose
a particular approach or algorithm to
determine battery capacity from the
measured quantities, but instead allow
the system designer to implement algorithms tailored to the special needs of
the system via the host controller. The
microcontroller can then simply adapt
the charge thresholds of LTC2941 and
LTC2942 based on these calculations.
SW
VBUS
USB
10µF
VOUT
LOAD
10µF
VOUTS
GATE
BAT
LT4088-1
0.1µF
3.3V
D0
VDD
D1
SENSE+
2k
2k
8.2k
0.1µF
PROG C/X
+
1 CELL
LI-ION
AL/CC
SDA
µP
CHRG
_
LTC2942-1
D2
CLPROG
SENSE
GND NTC
SCL
GND
2.94k
2k
CONCLUSION
Battery gas gauging is one feature that lags
behind other technological improvements
in many portable electronics. The LTC2941
and LTC2942 integrated coulomb counters
solve this problem with accurate battery
gas gauging that is easy to implement and
fit into the latest portable applications.
For high accuracy in the tightest spots,
the LTC2941-1 and LTC2942-1 versions
integrate factory trimmed and temperature compensated sense resistors for the
ultimate small coulomb counters. This new
family of accurate coulomb counters can
help prevent you from ever again missing
those priceless vacation moments due to
inaccurate battery charge monitoring. n
Figure 11. Using the
LTC2942 in a battery
stack
CHARGER
Figure 9. Battery gas gauge with USB charger
TO WALL
ADAPTER
3.3V OUT
GND
SCL
_
SENSE
GND
SENSE+ SENSE
_
2k
2k
2k
2k
2k
2k
LTC2941
VDD
AL/CC
SDA
0.1µF
SENSE+ SENSE
_
GND SCL
µP
AL/CC
SDA
GND SCL
Figure 10. Monitoring system energy flow using LTC2941s at the loads
+
SENSE+
I2C/SMBus
TO HOST
3.3V
5A LOAD
LTC2941
+
AL/CC
SDA
0.1µF
DC/DC
5V
10A LOAD
5mΩ
5V OUT
10mΩ
+
LTC2942
VIN
LOAD
1 CELL
LI-ION
1 CELL
LI-ION
1 CELL
LI-ION
Figure 12. Individual
cell monitoring of a
2-cell stack
CHARGER
LTC2942-1
SENSE+
I2C/SMBus
TO HOST
RSENSE
+
LOAD
LEVEL–
SHIFT
AL/CC
SDA
SCL
1 CELL
LI-ION
SENSE
GND
_
+
1 CELL
LI-ION
LTC2942-1
SENSE+
I2C/SMBus
TO HOST
AL/CC
SDA
SCL
SENSE
GND
_
+
1 CELL
LI-ION
April 2010 : LT Journal of Analog Innovation | 15
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