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
–
+
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
DESIGN FEATURES L
0
REJECTION (dB)
0
REJECTION (db)
10M
CELL12
CELL1
100Ω
LTC6802-1
TP0610K
0 = REF_EN
GPIO2
0 = CELL1
GPIO1
WDTB
1M
V
REG
−
V
C1
TP0610K
Figure 7. Improving accuracy with calibration
1µF
1M
2N7002
150Ω
100nF
TP0610K
2.2M
1M 10M
2N7002
TP0610K
at least 14% more weight, cost and
electronics to travel an equivalent
distance as a vehicle with the more
accurate BMS. Batteries are expensive. It takes about $4000 worth of
batteries to drive 50 miles, so the
increased measurement error means
$560 in additional cells. This is why
BMS designers scrutinize every 0.01%
of measurement error. Figure 6 shows
a simple battery cost model as a function of BMS accuracy.
Adding a low drift reference, an initial factory calibration, and a periodic
self-calibration routine can improve
the measurement accuracy of the
LTC6802 to 0.03%. For example, in
Figure 7 the LT1461A-4 is periodically
applied to channel C1. The temperature stable LT1461 measurement is
used to correct temperature drift in
the LTC6802. The initial error of the
LTC6802 and LT1461A is corrected by
measuring and storing a calibration
reference after board assembly.
Inverter noise can seriously interfere with cell voltage measurements.
When a 100-cell stack is loaded by an
electric motor it can have a 370V open
circuit voltage and up to 100V switching transients (Figure 8). Spreading
the transient equally over the 100 cells
means the top cell has 370V of common mode voltage, 100V of common
mode transients, 1V of differential
transients and an average DC value
1M
V
STACK12
LT1461A-4
DNC
DNC
V
DNC
90.9k
IN
V
SD
OUT
GND
DNC
+
LT1636
SD
–
1M
2.2µF
4.096V
DD
SELCH1CH0V
TC4W53FU
VSSVEEINHCOM
of 3.7V, which we need to measure
to 4mV. Breaking the battery stack
into 12-cell modules further reduces
The LTC6802’s 0.1% total
measurement error from
–20°C to 60°C translates to
4mV precision for a 3.7V cell.
Batteries are expensive. It
takes about $4000 worth
of batteries to drive 50
miles, so just increasing
measurement error to 10mV
means $560 in additional
cells. This is why BMS
designers scrutinize every
0.01% of measurement error.
V
= 5V
CM(IN)
72dB REJECTION
–10
CORRESPONDS TO
LESS THAN 1 BIT
AT ADC OUTPUT
–20
–30
–40
–50
–60
–70
10 10k
P-P
1M100k1k100
FREQUENCY (Hz)
Figure 9. Cell measurement
common mode rejection
370V
Figure 8. Inverter noise example
6µs
270V
10kHz
the common mode voltage. In a pack
like Figure 2, each LTC6802 (one
per module) sees up to 12V common
mode transients and 1V differential
transients per cell. The transients
are at the PWM frequency of 10kHz
to 20kHz. The LTC6802 has excellent
common mode rejection (Figure 9) to
eliminate this error term. The SINC2
filter inherent in the delta-sigma ADC
attenuates the differential noise by
40dB (Figure 10). External filtering or
measurement averaging can be used to
further reduce the differential noise.
Diagnostic Features of the
LTC6802 Improve Robustness
Automotive systems require that “no
bad cell reading be misinterpreted
as a good cell reading.” Two of the
more common faults that can cause
false readings are open circuits and
IC failures. If there is an open circuit
in the wiring harness and if there is
a filter capacitor on the ADC input
(Figure 11), the capacitor will tend
to hold the input voltage at a point
midway between the adjacent cells.
Some type of open wire detection or
cell resistance measuring function
is necessary. The LTC6802 includes
100µA current sources to load the cell
inputs. The current source will cause
large changes in cell readings if there
is an open circuit in the harness.
–10
–20
–30
–40
–50
–60
–70
10 10k 100k1k100
Figure 10. Cell measurement filtering
FREQUENCY (Hz)
Linear Technology Magazine • March 2009
5
L DESIGN FEATURES
The host controller must be able
to run diagnostics on all the modules
during normal operation to detect IC
failures. If these periodic self-tests fail,
then the control algorithm is suspect
and the battery pack must be taken off
line. The LTC6802 includes a built-in
self-test in combination with external
support circuits to allow the BMS to
completely verify the data acquisition
system. See the LTC6802 data sheets
for more details.
The LTC6802 Isolates
Communications from
Swings in Ground Potential
Breaking a ~100 cell pack into modules makes it easier to integrate the
analog circuits. Unfortunately, we are
left with the task of getting the data
from measurement IC to the host controller when the difference in ground
potential exceeds 300V. The LTC6802
can solve this problem in a number of
ways, depending on the specific needs
of the application.
The LTC6802 comes in two flavors,
depending on the desired data communication scheme. The LTC6802-1
offers a built-in stackable serial
peripheral interface (SPI) solution
designed for easy daisy chaining of the
interface. The addressable LTC6802-2
is designed for bus-oriented (parallel)
SPI communication, but it can also be
used in a parallel-addressable, daisy
chained interface for a robust and rela-
C4
B4
B3
C
F4
C
F3
Figure 11. Current sources help detect open circuits.
C3
C2
C1
–
V
tively inexpensive solution. All three
schemes are described below.
SPI Bus Communication with
the Addressable LTC6802-2
and Digital Isolators
The most straightforward approach is
to use a bus communications scheme,
with a digital isolator between each
module and the host controller. Figure 12 shows a 96-cell pack using
eight multicell modules monitored
by the LTC6802. The physical layer
is a 4-wire SPI bus. An addressing
scheme allows the control module to
talk to the battery modules separately
or in unison. The data buses on the
modules are isolated from one another.
This is a robust scheme, but it has
one major drawback: digital isolators
are expensive and require an isolated
LTC6802-1
MUX
100µA
power supply so that the battery cells
don’t have to provide the power to the
cell side of the isolator.
Daisy Chaining the SPI Interface
with the LTC6802-1
The LTC6802-1 provides fixed 1mA
signaling between stacked devices to
enable easy implementation a daisy
chained SPI interface with inexpensive
support circuitry. The digital isolators
are eliminated as shown in Figure 13.
The interface exploits the fact that the
positive supply of module “N” is the
same voltage as the ground of module
“N+1.” A 1mA current is used to transmit data between adjacent modules.
Like the analog circuits, the modular
approach means the data bus has to
deal with a fraction of the total pack
voltage.
BATTERY MODULE 8
LTC6802
12 Li-Ion
SERIES
BATTERIES
12 Li-Ion
SERIES
BATTERIES
6
BATTERY
MONITOR
LTC6802
BATTERY
MONITOR
Figure 12. Using digital isolators to communicate to the LTC6802
DIGITAL
ISOLATOR
BATTERY MODULE 1
DIGITAL
ISOLATOR
CONTROL MODULE
µCONTROLLER
SPI
CAN
CAN
TRANSCEIVER
GALVANIC
ISOLATOR
TO VEHICLE
CAN BUS
Linear Technology Magazine • March 2009
LTC6802
V
500kbps MAX DATA RATE
ALL SCHOTTKY: CMD5H2-3
DESIGN FEATURES L
BATTERY MODULE 8
12 Li-Ion
SERIES
BATTERIES
12 Li-Ion
SERIES
BATTERIES
BATTERY
MONITOR
LTC6802
BATTERY
MONITOR
Figure 13. Using the daisy chained SPI to eliminate digital isolators
The disadvantage of any pure daisy
chain is that a fault in one module
results in a loss of communications
with all the modules above it in the
stack. Also, since there is no galvanic
isolation between modules, the interface needs to handle large voltages
that occur during fault conditions.
For example if the “service switch” in
Figure 1 is open and there is a load
on the pack then the data bus connection between modules 4 and 5 will
see a reverse voltage equal to the total
pack voltage (–300V to –400V). The
LTC6802 interface relies on external
discrete diodes to block the reverse
voltage during fault conditions.
BATTERY MODULE 1
CONTROL MODULE
µCONTROLLER
SPI
CAN
TRANSCEIVER
4mA to energize low power auxiliary
circuitry.
Figure 14 shows a complete stacked
LTC6802-2 SPI interface for a 36cell application. The stack can be
increased in size by replicating the
BATT
LTC6802-2
IC #3
V
WDT
SCKI
CSBI
SDO
REG
SDI
NDC7002N
−
V
GALVANIC
ISOLATOR
CAN
TO VEHICLE
CAN BUS
circuit of the middle IC. In Figure 14,
the V
REG
and V
–
pins of each stacked
IC are used to bias common-base
connected transistors to form a signal
translation current for each SPI data
line. Each LTC6802 can monitor up
2.2k2.2k1.8k1M
2.2k
ALL NPN: CMPT8099
ALL PNP: CMPT8599
ALL PN: RS07J
The Best of Both Worlds:
Daisy Chained, Addressable
Interface with the LTC6802-2
With inexpensive external circuitry,
the LTC6802-2 can also be used in
a stacked SPI configuration like the
LTC6802-1, but with more flexibility
in the operating parameters.
The SPI port of the LTC6802-2 is
a 4-wire connection: chip select in
(CSBI), clock in (SCKI), data in (SDI),
and data out (SDO). The inputs are
conventional CMOS levels and the
output is an open-drain NMOS. The
SDO pin must have an external pull-up
current or added resistance suitable
for the intended data rate. The IC also
provides a versatile always-on 5V output (V
Linear Technology Magazine • March 2009
), which can produce up to
REG
LTC6802-2
IC #2
LTC6802-2
IC #1
Figure 14. Inexpensive SPI daisy chain for parallel-addressed LTC6802-2
V
SCKI
CSBI
SDO
V
SCKI
CSBI
SDO
REG
SDI
REG
SDI
2.2k2.2k100Ω
2.2k
−
V
100Ω
R12
−
V
2.2k
2.2k2.2k
2.2k
CS
CK
HOST µP
DI
DO
7
L DESIGN FEATURES
to 12 cell-potentials, which could sum
to 60V in certain instances, so the
transistors selected for the SPI translation need to have a V
over 60V, but
CBO
they should be the highest available fT
to prevent undue slowing of the logic
signals. A suitable NPN candidate is
the CMPT8099, while the CMPT8599
is its PNP complement, both from
Central Semiconductor. These are fast
80V devices (fT > 150MHz).
Sending Signals Upwards
At the bottom-of-stack IC, the logic
signal is furnished by the host connection, be it a microprocessor or an
SPI isolation device. By simply pulling
down the emitter leg of an NPN having
a V
base potential through a known
REG
resistance, a specific current is formed
for a logic low input signal. In the case
of the component values shown, the
current is about 2mA for a logic low,
and conversely, the transistor is essentially turned off with a logic high
(~0mA for 5V logic).
Since the collector current is nearly
identical to the emitter current, the
same current pulls on the next higher
cascode circuit. Since that next circuit
is the same as the first, the voltage on
the upper emitter resistor reproduces
that of the bottom circuit logic level for
the upper IC. This continues up the
daisy chain, eventually terminating at
the top potential of the battery stack.
Since each IC is provided the same input waveforms, this structure forms a
parallel bus from a logical perspective,
even though each IC is operating at a
different potential in the stack.
The NPN transistors at the top IC
source the logic current directly from
the battery stack. Only small base
currents flow from any V
output.
REG
The 600V collector diodes provide reverse-voltage protection in the event a
battery group interconnection is lost,
perhaps during service (these are not
required for functionality and could
be omitted in some situations).
Bringing Data Down the Stack
The SDO cascode chain is similar in
concept, except the current starts at
the top of the stack and flows downward. At the top IC, a PNP transistor
with its base connected to the local
–
V
pin has current injected into its
emitter by a pullup resistor. Here
again, the collector current is essentially identical to the emitter current,
and so current flows downward
through each successive PNP and terminates into a resistor at the bottom
of stack. In this case, the presence of
the current in the termination resistor,
about 2mA for the component values
shown, forms a logic high potential
for the host interface.
A Schottky diode is connected from
each SDO pin to the emitter of a local
PNP thereby allowing any LTC6802
on the stack to divert the pullup current to the local V
–
when outputting
a logic low. This effectively turns off
the emitter current to the local PNP
transistor and all points lower in the
stack, so the voltage on the bottom
termination resistor then drops to a
logic low level. Since each SDO pin
can force a low level, this forms a
wire-OR function that is equivalent
to paralleled connections as far as
the host interface is concerned. Note
the bottom of stack SDO diode is connected slightly differently; it forms a
direct wire-OR at the host interface.
Since the LTC6802-2 is designed to
use addressed readback commands,
this line is properly multiplexed and
no inter-IC contention occurs.
To eliminate the pull-up current
during standby, a general purpose
N-channel MOSFET is used to interrupt the top PNP emitter current when
the watchdog timer bit goes low. The
watchdog timeout will release when
clock activity is present, so the SDO
line will reactivate as needed. Here
again, an NPN is used at the top of
stack to ensure the pull-up current
comes directly from the battery, rather
than loading V
REG
.
Collector diodes are added here as
well to provide a high reverse voltage
protection capability, plus some added
series resistance is included to protect
the lower transistor emitters from
transient energy (once again, these
protection parts don’t add any other
functionality to the data transmission and could be omitted in some
circumstances).
External SPI Advantages
Since the LTC6802-2 uses a parallel
addressable SPI protocol, the conventional method of connecting multiple
devices in a stack is to provide isolation
for each SPI connection, then parallel
the signals on the host side. Isolators
are relatively expensive and often need
extra power circuitry, thus adding significantly to the total solution cost. The
transistor circuitry shown here is quite
inexpensive and offers the option to
make certain design tradeoffs as well.
With the propagation delays involved
and desire to keep power fairly low, this
circuit as shown still communicates
at over 500kbps. Lower SPI currents
could be chosen in applications that
don’t demand the high data rate by
simply raising the resistance values
accordingly.
The main feature of the transistorized SPI bus is the wide compliance
range that is afforded by the unconstrained collector -base operating
range of the transistors. In normal
operation the VCB ranges from just
less than the cells connected to the
LTC6802, to some five volts below that,
depending on the logic level transmitted. This becomes important since
voltage fluctuations on the battery,
due to load dynamics or switching
transients, affect the VCB of the transistors even though the V+ and ADC cell
inputs may be filtered. Some vehicle
manufacturers are requiring that a
BMS tolerate 1V steps with 200ns
rise/fall time per cell in the stack, so
this is a 12V waveform edge as seen by
the transistors in a typical application.
With the low collector capacitance and
2mA logic level of the transistor chain,
SPI transmissions remain error free
with even this high level of noise.
Conclusion
EVs and HEVs are here to stay. Inherently safe lithium batteries, which
combine energy density, power density, and cycle life, will continue to
evolve to improve the performance of
these vehicles. Battery management
systems using the LTC6802 extract
the most driving distance and lifetime
from the battery pack while lowering
system cost.
L
8
Linear Technology Magazine • March 2009
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