May 2004 An Accurate Battery Gas Gauge

DESIGN FEATURES
An Accurate Battery Gas Gauge
by James Herr
Introduction
A battery fuel gauge can be implemented in a variety of ways. The
most popular is to derive the remaining battery capacity from the battery
voltage. This method has advantages
in that it is easy to implement and
relatively low in cost, but it does have
one major drawback: It is relatively
inaccurate. Battery voltage has, at
best, an inconsistent relationship
to battery capacity—the relationship
varies greatly depending on battery
discharge rate and temperature.
The latest portable devices, though,
require more accurate battery gas
gauging. For instance, a portable
computer or PDA may need to save
data, or state information, and shut
down when the battery reaches a
critical discharge point. Accurate
prediction of this point allows the
device to safely run longer on battery
power. For applications that require
accurate gauging, the LTC4150
coulomb counter is a compact and
easy-to-implement solution.
The LTC4150 measures the charge
flowing into and out of the battery
through a sense resistor. A voltageto-frequency converter transforms the
current sense voltage into a series of
output pulses. Each pulse corresponds
to a fixed quantity of charge flowing
CHARGER
into or out of the battery. The device
indicates the charge polarity as the
battery is depleted or charged. The
status of the battery can be accurately
predicted by a microcontroller, connected via a simple 1-wire or 2-wire
interface.
Precision Integrator
Enables Charge Measurement
Charge is the time integral of current. The LTC4150 measures battery
current by monitoring the voltage developed across a sense resistor and
then integrates this information to
determine charge. The block diagram
shown in Figure 1 shows how.
The current measurement is filtered
by capacitor CF connected across CF+
and CF– pins. This averages all fast
changes in current arising from ripple,
noise and spikes in the load, charge
current, or Burst Mode® operation of a
switching regulator. The filter’s output
is applied to an integrator with the
amplifier and 100pF capacitor at its
core. Switches S1 and S2 reverse the
ramp direction once the integrator’s
output reaches the REFHI or REFLO
levels. By observing the condition of
S1, S2, and the ramp direction, the
polarity is determined.
VDD
CF
RSENSE
IBAT
200k
CF –
SENSE –
2k
COUNTER
CONTROL
LOGIC
AMPLIFIER
+
200k
S2
(1)
Where:
OFLOW/
UFLOW
–
–
+
CF
f = GVF • | VSENSE |
(2)
INT
+
100pF
S1
200k
+
The LTC4150’s transfer function is
quantified as a voltage to frequency
gain GVF, where the output frequency
is the number of interrupts per second
and the input voltage is the voltage
V SENSE across the SENSE+ and
SENSE– pins. The number of interrupts per second is:
REFHI
1.7V
S3
2k
Coulomb Counting
VSENSE = IBATTERY • RSENSE
LOAD
SENSE +
A counter is used to effectively
increase the integration time by
a factor of 1024, greatly reducing
microcontroller overhead required
to service the interrupts from the
LTC4150. At each counter underflow
or overflow, the I N
 T
 output latches low,
while simultaneously, the POL output
is latched to indicate the polarity of the
charge count. Once the interrupt is recognized, the microcontroller resets I N
 T

output with a low going pulse at CLR
pin. To simplify the connections, INT
and CLR pins can also be connected
together. In this case, the interrupt
signal lasts at least 1µs, enough time
for the microcontroller to register the
data, before the INT pin resets automatically.
POLARITY
DETECTION
S
Q
R
CLR
UP/DN
CHARGE
POL
DISCHARGE
–
REFLO
0.95V
SHDN
GND
Figure 1. LTC4150 block diagram shows how measured current, at the sense resistor, is integrated and converted to an integer count of charge.
Linear Technology Magazine • May 2004
11
DESIGN FEATURES
POWER-DOWN
SWITCH
VSENSE = 50mV for the LTC4150,
therefore:
f = GVF • | IBATTERY • RSENSE |
2.5V
(3)
Since I • t = Q, the coulombs of
battery charge per INT pulse (interrupt interval) can be derived from
Equation 4:
1
One INT =
Coulombs
G VF • R SENSE
1
RSENSE
0.1Ω
2-CELL
Li-Ion
6V ~ 8.4V
(4)
One INT =
1
3600 • G VF • R SENSE
Ah
or
1Ah = 3600 • GVF • RSENSE Interrupts
(6)
(7)
The charge measurement can then
be scaled with a microcontroller.
High Side Sensing up to 8.5V
Figure 2 shows a typical application design for a 2-cell lithium-ion
battery system with 500mA of maximum load current. Using Equation 2
to calculate RSENSE = 50mV/0.5A =
0.1Ω. With RSENSE = 0.1Ω, Equation
6 shows that each interrupt corresponds to 0.085mAh of charge with
GVF = 32.55 Hz/V. A battery with
850mAh of capacity takes a total of
10,000 INT assertions to fully charge
or discharge.
LTC4055, continued from page 10
current if the die temperature attempts
to rise above a preset value of approximately 105°C. Another benefit of the
LTC4055 thermal regulation is that
charge current can be set according
to typical, not worst-case, ambient
temperatures for a given application
with the assurance that the charger
will automatically reduce the current
in worst-case conditions. Thermal
regulation simplifies design, maximizes charge current and prevents
overheating.
12
3
CF
4.7µF
4
SENSE –
VDD
CF +
GND
CF –
SHDN
POL
RL
3k
LOAD
RL
3k
9
8
C2
4.7µF
7
µP
6
SHUTDOWN
(5)
Combining Equations 4 and 5:
2
5
Figure 2. A 2-cell lithium-ion battery gas gauge
The LTC4150 can be shut down,
when not needed, to a low current
mode (1.5µA max) reducing the drain
on the battery.
Accurate Prediction
of Battery Capacity
The factors that affect the accuracy of
the capacity prediction are the input
0.5
0.4
offset voltage, the integral nonlinearity
error (INL), the tolerance of the sense
resistor, and the self-discharge of the
battery. The self-discharge rate of a
Li-Ion type of battery is around 2%–4%
per month at room temperature. The
LTC4150 has 0.3% of INL error across
the input and common mode range,
see Figure 3, and 150µV of input offset
voltage.
Conclusion
0.3
ERROR (% FULL SCALE)
1Ah = 3600 Coulombs
INT
LTC4150 CLR
+
Battery capacity is most often expressed in ampere-hours:
SENSE +
10
CL
47µF
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
–50
–25
25
0
CURRENT SENSE VOLTAGE (mV)
50
The LTC4150 offers a simple and compact solution for high side coulomb
counting/battery gas gauging for battery voltages up to 8.5V(2-cell Li-Ion
or 6-cell NiCd or NiMH batteries). The
only required external components are
the sense resistor and a filter capacitor
to average out transient events and
ripple current.
Figure 3. Integral nonlinearity of
the LTC4150 is within 0.3% over
the entire sense voltage range.
Conclusion
The LTC4055 is a complete PowerPath
controller and Li-Ion battery charger
for portable USB applications. The
LTC4055 is designed to provide device power and Li-Ion battery charging
from the USB while maintaining the
current limits imposed by the USB
specification. This is accomplished
by reducing battery charge current
as output/load current is increased.
The available bus current is maximized
to minimize battery charge times.
The LTC4055’s versatility, simplicity, high level of integration and
small size makes it an ideal choice
for many portable USB applications.
The LTC4055 is available in a small
16-lead low profile 4mm × 4mm QFN
package.
for
the latest information
on LTC products,
visit
www.linear.com
Linear Technology Magazine • May 2004