LINEAR TECHNOLOGY LTC6802 Technical data
LINEAR TECHNOLOGY
SERVICE SWITCH
LINEAR TECHNOLOGY
LINEAR TECHNOLOGY
MARCH 2009 VOLUME XIX NUMBER 1
IN THIS ISSUE…
COVER ARTICLE
Battery Stack Monitor
Extends Life of Li-Ion Batteries
in Hybrid Electric Vehicles ..................1
Michael Kultgen and Jon Munson
Linear in the News… ...........................2
DESIGN FEATURES
DC/DC Converter, Capacitor Charger
Takes Inputs from 4.75V to 400V ........9
Robert Milliken and Peter Liu
How to Choose a Voltage Reference ...14
Brendan Whelan
1.2A Monolithic Buck Regulator
Shrinks Supply Size and Cost with
Programmable Output Current Limit
.........................................................20
Tom Sheehan
Boost Converters for Keep-Alive Circuits
Draw Only 8.5μA of Quiescent Current
.........................................................22
Xiaohua Su
Industrial/Automotive Step-Down
Regulator Accepts 3.6V to 36V and
Includes Power-On Reset and Watchdog
Timer in 3mm × 3mm QFN ................24
Ramanjot Singh
Complete APD Bias Solution in 60mm
with On-the-Fly Adjustable Current
Limit and Adjustable V
Xin (Shin) Qi
DESIGN IDEAS
Don’t Want to Hear It? Avoid the Audio
Band with PWM LED Dimming at
Frequencies Above 20kHz ..................30
Eric Young
Eliminate EMI Worries with 2A,
15mm × 9mm × 2.82mm μModule™
Step-Down Regulator ........................33
David Ng
Diode Turn-On Time Induced Failures
in Switching Regulators ....................34
Jim Williams and David Beebe
μModule Regulator Fits a (Nearly)
Complete Buck-Boost Solution in
15mm × 15mm × 2.8mm for
4.5V–36V VIN to 0.8V–34V V
Judy Sun, Sam Young and Henry Zhang
New Device Cameos ...........................41
Design Tools ......................................43
Sales Offices .....................................44
...................27
APD
OUT
2
..........39
Battery Stack Monitor
Extends Life of Li-Ion
Batteries in Hybrid
Electric Vehicles
by Michael Kultgen and Jon Munson
Introduction
The cost of running a car on electricity
is equivalent to paying $0.75/gallon
for gasoline, and if that electricity
comes from carbon neutral sources,
car owners are saving both money
and the environment (gasoline combustion produces 9kg of CO2 per US
gallon). Advancements in battery
technology (see sidebar), especially
with Lithium-based chemistries, hold
the greatest promise for converting
the worldwide fleet of cars to hybrid
or fully electric.
12-CELL BATTERY
MODULE
+
CURRENT
SENSOR
CAN
HOST
CONTROLLER
–
L
, LT, LTC, LTM, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered trademarks of Linear Technology
Corporation. Adaptive Power, Bat-Track, BodeCAD, C-Load, DirectSense, Easy Drive, FilterCAD, Hot Swap, LinearView,
µModule, Micropower SwitcherCAD, Multimode Dimming, No Latency ΔΣ, No Latency Delta-Sigma, No R
Filter, PanelProtect, PowerPath, PowerSOT, SmartStart, SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT, TimerBlox, True
Color PWM, UltraFast and VLDO are trademarks of Linear Technology Corporation. Other product names may be trademarks
of the companies that manufacture the products.
+
MONITORING
& BALANCING
SPI
MONITORING
& BALANCING
–
12-CELL BATTERY
BATTERY
DATA BUS
DATA BUS
BATTERY
MODULE
12-CELL BATTERY
MODULE
+
–
BATTERY
MONITORING
& BALANCING
DATA BUS
DATA BUS
BATTERY
MONITORING
& BALANCING
–
+
12-CELL BATTERY
MODULE
Figure 1. 96-cell battery pack
Lithium battery packs offer the
highest energy density of any cur rent battery technology, but high
performance is not guaranteed simply by design. In real world use, a
battery management system (BMS)
makes a significant difference in the
performance and lifetime of Li-Ion
batteries—arguably more so than
the design of the battery itself. The
LTC6802 multicell battery stack
monitor is central to any BMS for the
continued on page 3
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+
12-CELL BATTERY
MODULE
+
BATTERY
MONITORING
& BALANCING
DATA BUS
DATA BUS
BATTERY
MONITORING
& BALANCING
–
12-CELL BATTERY
MODULE
–
+
12-CELL BATTERY
MODULE
+
BATTERY
MONITORING
& BALANCING
DATA BUS
DATA BUS
BATTERY
MONITORING
& BALANCING
–
12-CELL BATTERY
MODULE
SENSE
–
+
, Operational
DESIGN FEATURES L
LTC6802, continued from page 1
large battery stacks common in electric vehicles (EVs) and hybrid electric
vehicles (HEVs). Its robust design and
high accuracy helps guarantee the
performance and lifetime of expensive
battery packs.
lifetime is traded against the need
to use as few kg of batteries as possible—the most expensive component
in any EV. Only a well-designed BMS
can maximize battery performance and
lifetime in the face 200A peak charge
and discharge currents.
For instance, to meet a 15-year,
5000 charge cycle goal, only a portion
(say 40%) of the battery pack’s cellcapacity can be used. Of course, using
only 40% of the capacity essentially
lowers the energy density of the pack.
This is the problem: increasing battery
Battery Management System
Optimizes Li-Ion Run Time
and Lifetime
In any battery stack, the more accurately you know state of charge (SOC)
of each cell, the more cell capacity you
Li-ion Batteries in Electric Vehicles and Hybrids
So why aren’t all cars electric? One
reason is energy density. Gasoline
holds 80 times the energy per kg as
Li-ion batteries (Table 1) and refuels
in three minutes, essentially allowing
indefinite driving. Even a big lithium
pack only gives a passenger car
about a 100-miles after an 8-hour
charging cycle. To drive a passenger
car further than 100 miles you still
need a gasoline engine, but even
so, batteries improve gas mileage in
hybrid electric vehicles (HEVs). The peak efficiency of
the Otto cycle engine is only 30% at high RPMs and the
average efficiency is about 12%. Using batteries to supply torque during acceleration and recover joules during
MG1 INVERTER MG2 INVERTERBATTERY
GASOLINE
FRONT
WHEELS
ENGINE
ELECTRIC MOTOR/
GENERATOR 1 (MG1)
AXLES
REDUCTION
GEARS
POWER SPLIT
DEVICE
DIFFERENTIAL
Table 1. Energy density comparison
Medium Wh/kg
Diesel Fuel 12,700
Gasoline 12,200
Li-Ion Battery 150
NiMh Battery 100
Lead Acid Battery 25
Li-ion batteries take energy density another step forward,
by offering another 50% improvement. The safety of Liion was a concern, but new battery technologies like the
A123 nanophosphate cell, the EnerDel Spinel-Titanate
chemistry, the GS Yuasa EH6 design and others are as
safe as NiMh, offer extremely high power (200A peak discharge rates), and last 10 to 15 years with proper charge
management. By model year 2012, the majority of hybrid
cars and trucks will use lithium battery technology.
SILENT
CHAIN
Figure 1 shows a shows a block diagram of the battery pack with a BMS, and Figure 2 shows a typical HEV
power train. The battery pack building block is a 2.5V
to 3.9V, 4Ahr to 40Ahr Li-ion cell. 100 to 200 cells are
connected in series to bring the battery pack voltage into
the hundreds of volts. This DC power source drives a
ELECTRIC MOTOR/
GENERATOR 2 (MG2)
30kW to 70kW electric motor. The pack voltage is high
so that the average current is low for a given power level.
Lower current reduces I2R power losses, so cables can
be smaller, thus reducing weight and cost. The pack
should be able to deliver 200A under peak conditions
and be quickly rechargeable. In other words, the battery
needs to offer high energy density and high power density, specifications that can be met by Li-ion batteries.
Systems for busses and tractor-trailers use up to four
parallel packs of 640V each.
can use while still maximizing cell life.
In a laptop computer, gas gauging
comes from monitoring cell voltage
and counting coulombs in and out of
the stack of four to eight cells. Voltage, current, time and temperature
are combined in a robust algorithm
to give an indication of the SOC. Unfortunately, it’s nearly impossible to
count coulombs in a car. The battery
drives an electric motor, not a motherboard, so it must handle current
spikes of 200A, followed by low level
idling. Furthermore, you have from 96
regenerative braking means the gas
engine runs less often and runs at a
higher efficiency, effectively doubling
the mpg.
In the 1970s the only available high
power battery chemistry was lead
acid, too heavy to reasonably power
anything larger than a golf cart. Then
came NiMh batteries, which improved
energy density enough to enable the
first commercially successful HEVs,
like the Toyota Prius and Ford Escape.
L
Figure 2. Toyota Prius “split power” hybrid drive train
Linear Technology Magazine • March 2009
3
L DESIGN FEATURES
CELL VOLTAGE (V)
CELL VOLTAGE (V)
MEASUREMENT ERROR (%)
0.30
COST OF TYPICAL BATTERY PACK ($)
9k
NEXT 12-CELL
TO LTC6802-1
TO LTC6802-1
to 200 cells in series, in groups of 10
or 12. The cells age at different rates,
were manufactured from multiple lots,
and vary in temperature. Their capacities diverge constantly. Different cells
with the same coulomb count can have
wildly different charge levels.
That’s why the BMS focuses on
cell voltage. If you can accurately
measure the voltage of every cell, you
can know the cell’s SOC with reasonable accuracy (Figure 3). The trick is
to improve the accuracy of the voltage
measurement by taking into account
temperature effects on battery ESR
and capacity. By constantly measuring
each cell’s voltage, you keep a running
estimation of each cell’s charge level.
If some cells are overcharged and
some under, they can be balanced by
bleeding off charge (passive balancing) or redistributing charge (active
balancing).
Accurate Monitoring is Key to
Raising Battery Performance
while Lowering Costs
The LTC6802 (Figure 4) is a precision data acquisition IC optimized for
measuring the voltage of every cell in
a large string series-connected batteries. In the BMS, the LTC6802 does the
heavy lifting analog function, passing
digital voltage and temperature measurements to the host processor for
SOC computation. The LTC6802’s high
accuracy, excellent noise rejection,
high voltage tolerance, and extensive
self-diagnostics make it robust and
easy-to-use. The high level of integration means a substantial cost savings
for customers when compared to
discrete component data acquisition
designs.
Increasing measurement accuracy
reduces battery cost, as illustrated
by the following example. Figure 5
shows the typical performance of the
LTC6802, where 0.1% total error from
–20°C to 60°C translates to 4mV precision for a 3.7V cell. Suppose that to
achieve a 15-year battery lifetime, you
are limited to 40% of a cell’s capacity
per charge cycle, and assume the cell
voltage vs charge level of the battery
is very flat, e.g., 1.25mV/%SOC. A
measurement error of 4mV means the
4
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1C
2C
5C
10C
20C
50C
20 40
0
30 508060 100
10 90
DISCHARGE (%)
Figure 3. State of charge vs current and temperature for a typical Li-ion cell
PACK ABOVE
12-CELL
BATTERY
STRING
NEXT 12-CELL
PACK BELOW
70
+
V
MUX
–
V
100k NTC
Figure 4. Simplified block diagram of the LTC6802
estimation of SOC is accurate to 3%.
The BMS must charge cells to no more
than 37% (40% – 3%) of their capacity
to guarantee the 15-year lifetime.
Now consider a monitor IC with
10mV error over similar conditions.
In this case, the BMS can only use
0.25
0.20
0.15
0.10
0.05
0
–0.05
–0.10
–0.15
–0.20
–0.25
–0.30
Figure 5. Typical measurement accuracy
vs temperature of seven samples
7 REPRESENTATIVE
UNITS
–25 0 25 50 75 100
TEMPERATURE (°C)
125–50
4.5
4.0
3.5
3.0
2.5
–20°C
2.0
1.5
EXTERNAL
TEMP
0°C
30°C
60°C
20 40
0
100k
30 508060 100
10 90
DISCHARGE (%)
LTC6802-1
DIE TEMP
REGISTERS
AND
CONTROL
12-BIT
∆∑ ADC
VOLTAGE
REFERENCE
SERIAL DATA
SERIAL DATA
70
ABOVE
BELOW
32% (40% – 10mV • 1%/1.25mV) of
the cells’ capacity and still guarantee a
15-year life. This seemingly negligible
increase in measurement error results
in a significant 14% reduction in the
usable capacity. That is, a vehicle
requires least 14% more batteries, or
8k
7k
6k
5k
4k
3k
Figure 6. High BMS accuracy is important to
keeping battery costs in check, as shown in
this cost vs measurement error model.
10 15
5
MEASUREMENT ERROR (mV)
Linear Technology Magazine • March 2009
20 25
300
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