LINEAR TECHNOLOGY LTM4606 Technical data
LINEAR TECHNOLOGY
LINEAR TECHNOLOGY
LINEAR TECHNOLOGY
MARCH 2009 VOLUME XIX NUMBER 1
SPI
CURRENT
SENSOR
SERVICE SWITCH
CAN
12-CELL BATTERY
MODULE
–
–
DATA BUS
+
+
BATTERY
MONITORING
& BALANCING
HOST
CONTROLLER
–
+
–
+
–
+
–
+
–
+
–
+
–
+
12-CELL BATTERY
MODULE
DATA BUS
BATTERY
MONITORING
& BALANCING
12-CELL BATTERY
MODULE
DATA BUS
12-CELL BATTERY
MODULE
DATA BUS
12-CELL BATTERY
MODULE
DATA BUS
12-CELL BATTERY
MODULE
DATA BUS
12-CELL BATTERY
MODULE
DATA BUS
12-CELL BATTERY
MODULE
DATA BUS
BATTERY
MONITORING
& BALANCING
BATTERY
MONITORING
& BALANCING
BATTERY
MONITORING
& BALANCING
BATTERY
MONITORING
& BALANCING
BATTERY
MONITORING
& BALANCING
BATTERY
MONITORING
& BALANCING
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
...................27
APD
2
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.
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
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
..........39
OUT
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.
Figure 1. 96-cell battery pack
, Operational
SENSE
L LINEAR IN THE NEWS
Linear in the News…
EDN Highlights Linear for Innovation Awards
EDN magazine in February chose several Linear Technology
products as finalists for their annual Innovation Awards, to
be announced later this month. And the nominees are:
Best Contributed Article—“High Voltage,
Low-Noise DC/DC Converters” by Jim Williams
You can find the article in its entirety on the EDN website
at www.edn.com/jimwilliams.
Battery ICs Category—LTC6802 Battery Stack Monitor
The LTC6802 is a highly integrated multicell battery
monitoring IC capable of precisely measuring the voltages
of up to 12 series-connected battery cells. Using a novel
stacking technique, multiple LTC6802s can be placed in
series without optocouplers or isolators. See the cover
article of this issue for an overview of this part.
Power ICs Category—LTC3642 50mA
Synchronous Step-Down Converter
The LTC3642 uses a unique high voltage synchronous
rectification design, capable of continuous input voltages of 45V and offers transient protection up to 60V. Its
internal synchronous rectification and its programmable
peak current mode control feature enable it to deliver up
to 93% efficiency, maximizing battery run time.
Power ICs: Modules—LTM4606 Ultralow EMI,
6A DC/DC µModule Regulator
The LTM4606 DC/DC µModule™ regulator significantly
reduces switching regulator noise by attenuating conducted
and radiated energy at the source. The µModule device
is a complete DC/DC system-in-a-package, including the
inductor, controller IC, MOSFETs, input and output capacitors and the compensation circuitry, housed in an enclosed
surface-mount plastic package resembling an IC.
The LTM4606 reduces switching regulator noise at the source.
Linear CEO Comments on Growth Markets
Last month in EE Times, Linear Technology CEO Lothar
Maier discussed the challenging market conditions and
the bright spots on the horizon: “In these times our customers will continue to invest in new products and new
product development. Innovation will return growth to the
semiconductor market—specifically to analog. Now is the
time to get new products out, to be first to market and to
have products that target emerging growth markets.” He
discussed several key markets:
q
Automotive. “Automotive manufacturers are
forecasting automotive electronic content to grow 2–3
times over the next few years, so we will continue
to provide new products to the automotive area. In
addition, every major automotive manufacturer in the
world is now working on hybrid vehicles, which will
add even more electronic content in cars. We have
just introduced an innovative device, the LTC6802,
a highly integrated battery stack monitor that
significantly eases the design of battery monitoring
systems for hybrid/electric vehicles.”
q
Green Growth Markets. “Products targeted toward
energy conservation or energy harvesting will
see growth opportunities and are insulated from
the current market conditions. Energy costs and
environmental concerns, as well as the need to
extend battery life for mobile devices, have led to a
focus on power optimization. Our energy-efficient
products enable customers to convert power more
efficiently, consume less power and extend battery
life. Our LED drivers enable a new generation of
low power lighting for a range of applications, from
cars and medical instruments to laptops and office
lighting. Our efficient analog solutions will help drive
innovative cleantech markets such as solar and wind
power systems.”
q
Communications Infrastructure. “Wireless systems
continue to produce significant market opportunities
for products in wireless and network infrastructure.
Our high speed data converters and high frequency
products are designed into the next generation of
cellular basestations. And our Hot Swap™ and Power
over Ethernet products are proliferating in networks.”
q
Industrial. “The broad industrial market continues
to provide a solid core of business and is somewhat
more insulated from market swings. Linear’s analog
products are used in a broad range of industrial
systems, including factory automation, industrial
process control, medical, instrumentation and
security.”
Lothar Maier concluded, “Finally, I believe that Linear’s
strategy of customer, market and geographic diversity will
be a hedge against the current market conditions and will
provide the conduit to future growth.”
L
2
2
Linear Technology Magazine • March 2009
DESIGN FEATURES L
MG1 INVERTER MG2 INVERTERBATTERY
AXLES
DIFFERENTIAL
GASOLINE
ENGINE
ELECTRIC MOTOR/
GENERATOR 1 (MG1)
ELECTRIC MOTOR/
GENERATOR 2 (MG2)
POWER SPLIT
DEVICE
FRONT
WHEELS
REDUCTION
GEARS
SILENT
CHAIN
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
Figure 2. Toyota Prius “split power” hybrid drive train
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.
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
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
Linear Technology Magazine • March 2009
3
L DESIGN FEATURES
DISCHARGE (%)
0
CELL VOLTAGE (V)
3.5
4.0
70
3.0
2.5
20 40
10 90
30 508060 100
2.0
1.5
4.5
1C
2C
5C
10C
20C
50C
DISCHARGE (%)
0
CELL VOLTAGE (V)
3.5
4.0
70
3.0
2.5
20 40
10 90
30 508060 100
2.0
1.5
4.5
–20°C
0°C
30°C
60°C
MEASUREMENT ERROR (%)
TEMPERATURE (°C)
125–50
0.30
–0.30
–25 0 25 50 75 100
–0.20
–0.25
–0.10
0.10
0
0.20
–0.15
–0.05
0.15
0.05
0.25
7 REPRESENTATIVE
UNITS
COST OF TYPICAL BATTERY PACK ($)
MEASUREMENT ERROR (mV)
300
9k
3k
5
10 15
20 25
4k
5k
7k
6k
8k
MUX
DIE TEMP
12-CELL
BATTERY
STRING
NEXT 12-CELL
PACK ABOVE
NEXT 12-CELL
PACK BELOW
V
+
V
–
100k NTC
100k
EXTERNAL
TEMP
SERIAL DATA
TO LTC6802-1
ABOVE
SERIAL DATA
TO LTC6802-1
BELOW
LTC6802-1
VOLTAGE
REFERENCE
REGISTERS
AND
CONTROL
12-BIT
∆∑ ADC
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).
Figure 3. State of charge vs current and temperature for a typical Li-ion cell
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
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
Figure 5. Typical measurement accuracy
vs temperature of seven samples
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
Figure 6. High BMS accuracy is important to
keeping battery costs in check, as shown in
this cost vs measurement error model.
Linear Technology Magazine • March 2009
LTC6802-1
GPIO2
V
−
C1
V
REG
GPIO1
0 = REF_EN
0 = CELL1
V
STACK12
CELL1
CELL12
1M
1M
2N7002
2N7002
TP0610K
TP0610K
TP0610K
TP0610K
1µF
WDTB
LT1461A-4
DNC
DNC
V
OUT
DNC
DNC
V
IN
SD
GND
1M 10M
1M
90.9k
150Ω
100Ω
2.2M
1M
100nF
2.2µF
4.096V
+
–
TC4W53FU
LT1636
SD
SELCH1CH0V
DD
VSSVEEINHCOM
0
–10
–30
–50
–20
–40
–60
–70
FREQUENCY (Hz)
REJECTION (dB)
10 10k 100k1k100
0
–10
–30
–50
–20
–40
–60
–70
FREQUENCY (Hz)
REJECTION (db)
10 10k 10M1M100k1k100
V
CM(IN)
= 5V
P-P
72dB REJECTION
CORRESPONDS TO
LESS THAN 1 BIT
AT ADC OUTPUT
370V
270V
10kHz
6µs
Figure 7. Improving accuracy with calibration
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
Linear Technology Magazine • March 2009
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.
Figure 9. Cell measurement
common mode rejection
DESIGN FEATURES L
Figure 8. Inverter noise example
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.
Figure 10. Cell measurement filtering
5
L DESIGN FEATURES
MUX
C4
C3
C
F4
C
F3
C2
C1
V
–
100µA
B4
B3
LTC6802-1
LTC6802
BATTERY
MONITOR
12 Li-Ion
SERIES
BATTERIES
BATTERY MODULE 8
CAN
TRANSCEIVER
SPI
µCONTROLLER
GALVANIC
ISOLATOR
TO VEHICLE
CAN BUS
CONTROL MODULE
CAN
LTC6802
BATTERY
MONITOR
DIGITAL
ISOLATOR
DIGITAL
ISOLATOR
12 Li-Ion
SERIES
BATTERIES
BATTERY MODULE 1
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-
Figure 11. Current sources help detect open circuits.
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
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.
6
Figure 12. Using digital isolators to communicate to the LTC6802
Linear Technology Magazine • March 2009
DESIGN FEATURES L
LTC6802
BATTERY
MONITOR
12 Li-Ion
SERIES
BATTERIES
BATTERY MODULE 8
CAN
TRANSCEIVER
SPI
µCONTROLLER
GALVANIC
ISOLATOR
TO VEHICLE
CAN BUS
CONTROL MODULE
CAN
LTC6802
BATTERY
MONITOR
12 Li-Ion
SERIES
BATTERIES
BATTERY MODULE 1
2.2k2.2k1.8k1M
NDC7002N
2.2k
LTC6802-2
IC #3
V
REG
V
BATT
WDT
SDI
SCKI
CSBI
SDO
V
−
2.2k2.2k100Ω
100Ω
2.2k
LTC6802-2
IC #2
V
REG
SDI
SCKI
CSBI
SDO
V
−
2.2k2.2k
R12
2.2k
2.2k
LTC6802-2
IC #1
V
REG
SDI
SCKI
CSBI
SDO
CS
CK
DI
DO
HOST µP
500kbps MAX DATA RATE
ALL NPN: CMPT8099
ALL PNP: CMPT8599
ALL PN: RS07J
ALL SCHOTTKY: CMD5H2-3
V
−
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.
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
REG
Linear Technology Magazine • March 2009
), which can produce up to
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
Figure 14. Inexpensive SPI daisy chain for parallel-addressed LTC6802-2
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
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
DESIGN FEATURES L
GND
0
V
OUT
100V/DIV
I
IN(AVG)
2A/DIV
20ms/DIV
VIN = 24V
C
OUT
= 100µF
2V/DIV
250ns/DIV
GND
CHARGE
CLAMP
V
CC
DONE
FAULT
UVLO1
OVLO1
UVLO2
OVLO2
RDCM
RV
OUT
HVGATE
LVGATE
CSP
CSN
FB
RV
TRANS
T1*
1:10
D1
V
OUT
50V TO 450V
V
TRANS
10V TO 24V
V
CC
TO µP
V
CC
LT3751
GND RBG
R6
40.2k
OFF ON
C3
680µF
C2
2.2µF
s5
C1
10µF
•
•
R7
18.2k
R8
40.2k
M1
R5
6mΩ
1W
D2
+
+
C4
100µF
R9
V
TRANS
R1,
154k
R2, 475k
DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY
C5
0.47µF
ALL RESISTORS ARE 0805,
1% RESISTORS UNLESS
OTHERWISE NOTED
D1,D2: VISHAY MURS260
M1: IRF3710Z
T1: WURTH 750310349
LIMIT OUTPUT POWER TO
40W FOR 65°C T1 MAX
AMBIENT OPERATION
*
4.7nF
Y RATED
DC/DC Converter, Capacitor Charger
Takes Inputs from 4.75V to 400V
Introduction
High voltage power supplies and capacitor chargers are readily found in
a number of applications, including
professional photoflashes, security
control systems, pulsed radar systems,
satellite communication systems, and
explosive detonators. The LT3751
makes it possible for a designer to
meet the demanding requirements
of these applications, including high
reliability, relatively low cost, safe
operation, minimal board space and
high performance.
The LT3751 is a general purpose
flyback controller that can be used as
either a voltage regulator or as a capacitor charger. The LT3751 operates in
boundary-mode, between continuous
conduction mode and discontinuous
conduction mode. Boundary-mode
operation allows for a relatively small
transformer and an overall reduced
PCB footprint. Boundary-mode also
reduces large signal stability issues
that could arise from using voltagemode or PWM techniques. Regulation
is achieved with a new dual, overlapping modulation technique using both
by Robert Milliken and Peter Liu
Figure 1. Gate driver waveform
in a typical application
peak primary current modulation and
duty-cycle modulation, drastically reducing audible transformer noise.
The LT3751 features many safety
and reliability functions, including
two sets of undervoltage lockouts
(UVLO), two sets of overvoltage
lockouts (OVLO), no-load operation,
over-temperature lockout (OTLO), internal Zener clamps on all high voltage
pins, and a selectable 5.6V or 10.5V
internal gate driver voltage clamp (no
external components needed). The
LT3751 also adds a start-up/shortcircuit protection circuit to protect
against transformer or external FET
damage. When used as a regulator, the
LT3751’s feedback loop is internally
compensated to ensure stability. The
LT3751 is available in two packages,
either a 20-pin exposed pad QFN or a
20-lead exposed pad TSSOP.
New Gate Driver with Internal
Clamp Requires No External
Components
There are four main concerns when
using a gate driver: output current
drive capability, peak output voltage,
power consumption and propagation
delay. The LT3751 is equipped with a
1.5A push-pull main driver, enough to
drive +80nC gates. An auxiliary 0.5A
PMOS pull-up only driver is also integrated into the LT3751 and is used in
parallel with the main driver for VCC
voltages of 8V and below. This PMOS
driver allows for rail-to-rail operation.
Above 8V, the PMOS driver must be
deactivated by tying its drain to VCC.
Most discrete FETs have a VGS limit
of 20V. Driving the FET higher than
20V could cause a short in the internal gate oxide, causing permanent
Figure 2. Isolated high voltage capacitor charger from 10V to 24V input
Linear Technology Magazine • March 2009
Figure 3. Isolated high voltage capacitor
charger charging waveform
9
L DESIGN FEATURES
R
N
V V
R
OUT TRIP DIODE
9 8
0 98
=
•
+
•
.
( )
0
GND
V
DRAIN
20V/DIV
I
PRIMARY
5A/DIV
10µs/DIV
V
OUT
(V)
EFFICIENCY (%)
LOAD CURRENT (mA)
1000
90
60
20 40 60 80
65
70
80
75
85
402
399
400
401
LOAD REGULATION
EFFICIENCY
0
GND
V
DRAIN
20V/DIV
I
PRIMARY
5A/DIV
10µs/DIV
CHARGE
CLAMP
V
CC
DONE
FAULT
UVLO1
OVLO1
UVLO2
OVLO2
RDCM
RV
OUT
HVGATE
LVGATE
CSP
CSN
FB
RV
TRANS
T1**
1:10
D1
V
OUT
400V
V
TRANS
10V TO 24V
V
CC
TO µP
V
CC
LT3751
GND RBG
R6
40.2k
OFF ON
C3
680µF
R10*
499k
R11
1.54k
C2
2.2µF
s5
C1
10µF
•
•
R7
18.2k
R8
40.2k
M1
R5
6mΩ
1W
D2
+
+
C4
100µF
R9
787Ω
V
TRANS
R1,154k
R2, 475k
DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY
C5
0.47µF
C6
10nF
ALL RESISTORS ARE 0805,
1% RESISTORS UNLESS
OTHERWISE NOTED
C4: CDE 380LX101M500J042
C5: TDK CKG57NX7R2J474M
D1,D2: VISHAY MURS260
M1: IRF3710Z
T1: WURTH 750310349
USE TWO SERIES 1206,
1% RESISTORS FOR R10
R10: 249k s2
LIMIT OUTPUT POWER TO
40W FOR 65°C T1 MAX
AMBIENT OPERATION
*
**
Figure 4. A 10V to 24V input, 400V regulated power supply
damage. To alleviate this issue, the
LT3751 has an internal, selectable
5.6V or 10.5V gate driver clamp. No
external components are needed, not
even a capacitor. Simply tie the CLAMP
pin to ground for 10.5V operation or
tie to VCC for 5.6V operation. Figure
1 shows the gate driver clamping at
10.5V with a VCC voltage of 24V.
Not only does the internal clamp
protect the FET from damage, it also
reduces the amount of energy injected
into the gate. This increases overall
efficiency and reduces power consumption in the gate driver circuit. The
gate driver overshoot is very minimal,
as seen in Figure 1. Placing the external
FET closer to the LT3751 HVGATE pin
reduces overshoot.
a. Switching waveform for I
10
High Voltage, Isolated
Capacitor Charger from
10V to 24V Input
The LT3751 can be configured as
a fully isolated stand-alone capacitor charger using a new differential
discontinuo us-conductio n-mode
(DCM) comparator—used to sense
the boundary-mode condition—and
a new differential output voltage
(V
) comparator. The differential
OUT
operation of the DCM comparator and
V
comparator allow the LT3751 to
OUT
accurately operate from high voltage
input supplies of greater than 400V.
Likewise, the LT3751’s DCM comparator and V
input supplies down to 4.75V. This
accommodates an unmatched range
of power sources.
= 100mA b. Switching waveform for I
OUT
Figure 5. High voltage regulator performance
OUT
comparator can work with
= 10mA c. Efficiency and load regulation
OUT
Figure 2 shows a high voltage capacitor charger driven from an input
supply ranging from 10V to 24V. Only
five resistors are needed to operate
the LT3751 as a capacitor charger.
The output voltage trip point can be
continuously adjusted from 50V to
450V by adjusting R9 given by:
The LT3751 stops charging the
output capacitor once the programmed
output voltage trip point (V
OUT(TRIP)
) is
reached. The charge cycle is repeated
by toggling the CHARGE pin. The
maximum charge/discharge rate in
Linear Technology Magazine • March 2009
DESIGN FEATURES L
P
C FREQUENCY
V V V
AVG
OUT
OUT TRIP RIPPLE
=
••
•
1
2
2
( )
–
RRIPPLE
W240
(
)
≤
V
CC
R3, 154k
R4, 475k
CHARGE
CLAMP
V
CC
DONE
FAULT
UVLO1
OVLO1
UVLO2
OVLO2
RV
OUT
HVGATE
LVGATE
CSP
CSN
FB
RV
TRANS
T1***
1:3
D1
V
OUT
500V
V
TRANS
100V TO 400V DC
V
CC
10V TO 24V
TO µP
V
CC
LT3751
GND RBG
R6*
625k
OFF ON
C3
100µF
450V
C2
2.2µF
630V
s5
C1
10µF
•
•
R8
137k ×3
R7
88.7k + 7.5k
R10*
208k
R13,20Ω
M1
FQB4N80
R12
68mΩ
1/4W
D2
+
+
C4
220µF
550V
R5
1.11k
V
TRANS
R1**
1.5M
R2**, 9M
DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY
C5
0.47µF
630V
RDCM
F1, 1A
R9
66.5k
R11
14.7k +
17.4k
ALL RESISTORS ARE 0805,
1% RESISTORS UNLESS
OTHERWISE NOTED
C4: HITACHI PS22L221MSBPF
C5: TDK CKG57NX7R2J474M
T1: COILCRAFT HA4060-AL
D1,D2: VISHAY US1M
F1: BUSSMANN PCB-1-R
* USE THREE SERIES 1206, 0.1%
RESISTORS FOR R6 & R10
R6: 249k ×2 + 127k
R10: 66.5k ×2 + 75k
** USE TWO SERIES 1206, 1%
RESISTORS FOR R1 & R2
R1: 750k ×2
R2: 4.53M ×2
*** OUTPUT POWER LIMITED TO
20W FOR 65°C T1 AMBIENT
OPERATION
4.7nF
Y RATED
0
V
OUT
AC RIPPLE
10V/DIV
I
IN(AVG)
20mA/DIV
2s/DIV
CHARGE TIME (ms)
V
OUT,TRIP
(V)
INPUT VOLTAGE (V)
400100
530
490
200 300
500
520
510
1000
400
850
550
700
V
OUT,TRIP
CHARGE TIME
the output capacitor is limited by the
temperature rise in the transformer.
Limiting the transformer surface temperature in Figure 2 to 65°C with no
air flow requires the average output
power to be ≤40W given by:
where V
voltage, V
OUT(TRIP)
is the output trip
is the ripple voltage
RIPPLE
on the output node, and frequency is
the charge/discharge frequency. Two
techniques are used to increase the
available output power: increase the
airflow across the transformer, or increase the size of the transformer itself.
Figure 3 shows the charging waveform
and average input current for a 100µF
output capacitor charged to 400V in
less than 100ms (R
= 976Ω).
9
For output voltages higher than
450V, the transformer in Figure 2 must
be replaced with one having higher
primary inductance and a higher
turns ratio. Consult the LT3751 data
Figure 6. The LT3751 protecting the
output during a no-load condition
sheet for proper transformer design
procedures.
High Voltage Regulated Power
Supply from 10V to 24V Input
The LT3751 can also be used to convert
a low voltage supply to a much higher
voltage. Placing a resistor divider from
the output node to the FB pin and
ground causes the LT3751 to operate as a voltage regulator. Figure 4
shows a 400V regulated power supply
operating from an input supply range
of 10V to 24V.
The LT3751 uses a regulation control scheme that drastically reduces
audible noise in the transformer and
the input and output ceramic bulk
capacitors. This is achieved by using
an internal 26kHz clock to synchronize
the primary winding switch cycles.
Within the clock period, the LT3751
modulates both the peak primary
current and the number of switching cycles. Figures 5a and 5b show
heavy-load and light-load waveforms,
respectively, while Figure 5c shows
efficiency over most of the operating
range for the application in Figure 4.
The clock forces at least one switch
cycle every period which would overcharge the output capacitor during a
no-load condition. The LT3751 handles no-load conditions and protects
against over-charging the output node.
Figure 6 shows the LT3751 protecting
during a no-load condition.
Resistors can be added to RV
OUT
and
RBG to add a second layer of protection, or they can be omitted to reduce
component count by tying RV
OUT
and
RBG to ground. The trip level for the
V
comparator is typically set 20%
OUT
higher than the nominal regulation
voltage. If the resistor divider were to
fail, the V
comparator would disable
OUT
switching when the output climbed to
20% above nominal.
Linear Technology Magazine • March 2009
Figure 7. A 100V to 400V input, 500V output, isolated capacitor charger
Figure 8. Isolated capacitor charger V
and charge time with respect to input voltage
OUT(TRIP)
11
L DESIGN FEATURES
OUTPUT VOLTAGE (V)
INPUT VOLTAGE (V)
400100
398
395
200 300
396
397
I
OUT
= 10mA
I
OUT
= 25mA
I
OUT
= 50mA
EFFICIENCY (%)
OUTPUT CURRENT (mA)
750
90
40
50
25 50
60
70
80
VIN = 100V
VIN = 250V
VIN = 400V
V
CC
R3, 154k
R4, 475k
CHARGE
CLAMP
V
CC
DONE
FAULT
UVLO1
OVLO1
UVLO2
OVLO2
RV
OUT
HVGATE
LVGATE
CSP
CSN
FB
RV
TRANS
T1***
1:3
D1
V
OUT
400V
V
TRANS
100V TO 400V DC
V
CC
10V TO 24V
TO µP
V
CC
LT3751
GND RBG
R6*
615k
OFF ON
C3
100µF
C2
2.2µF
s5
C1
10µF
C6
10nF
•
•
R8*
411k
R13,20Ω
M1
FQB4N80
R10
68mΩ
¼W
D2
+
+
C4
100µF
R12
1.54k
R11**
499k
V
TRANS
R1**, 1.5M
R2**, 9M
DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY
C5
0.47µF
RDCM
F1, 1A
R9
66.5k
ALL RESISTORS ARE 0805,
1% RESISTORS UNLESS
OTHERWISE NOTED
C4: CDE 380LX101M500J042
C5: TDK CKG57NX7R2J474M
T1: COILCRAFT HA4060-AL
D1,D2: VISHAY US1M
F1: BUSSMANN PCB-1-R
* USE THREE SERIES 1206, 1%
RESISTORS FOR R6 & R8
R6: 205k ×3
R8: 137k ×3
** USE TWO SERIES 1206, 1%
RESISTORS FOR R1, R2 & R11
R1: 750k ×2
R2: 4.53M ×2
R11: 249k ×2
*** OUTPUT POWER LIMITED TO
20W FOR 65°C T1 AMBIENT
OPERATION
R7
95.3k
can also be used for a capacitor
charger. The LT3751 operates as a
capacitor charger until the FB pin
reaches 1.225V, after which the
LT3751 operates as a voltage regulator.
This keeps the capacitor topped-off
until the application needs to use its
energy. The output resistor divider
forms a leakage path from the output
capacitor to ground. When the output
voltage droops, the LT3751 feedback
circuit will keep the capacitor topped-
12
Figure 9. A 100V to 400V input, 400V output, capacitor charger and voltage regulator
Note that the FB pin of the LT3751
a. Overall efficiency b. Line regulation
Figure 10. High voltage input and output regulator performance
off with small, low current bursts of
charge as shown in Figure 6.
High Input Supply Voltage,
Isolated Capacitor Charger
As mentioned above, the LT3751 differential DCM and V
allow the part to accurately work from
very high input supply voltages. An
offline capacitor charger, shown in
Figure 7, can operate with DC input
voltages from 100V to 400V. The transformer provides galvanic isolation from
comparators
OUT
the input supply to output node—no
additional magnetics required.
Input voltages greater than 80V
require the use of resistor dividers
on the DCM and V
comparators
OUT
(charger mode only). The accuracy of
the V
trip threshold is heightened
OUT
by increasing current IQ through R10
and R11; however, the ratio of R6/R7
should closely match R10/R11 with
tolerances approaching 0.1%. A trick
is to use resistor arrays to yield the
desired ratio. Achieving 0.1% ratio accuracy is not difficult and can reduce
the overall cost compared to using
individual 0.1% surface mount resistors. Note that the absolute value of
the individual resistors is not critical,
only the ratio of R6/R7 and R10/R11.
The DCM comparator is less critical
and can tolerate resistance variations
greater than 1%.
The 100V to 400VDC input capacitor charger has an overall V
accuracy of better than 6% over the
entire operating range using 0.1% resistor dividers. Figure 8 shows a typical
performance for V
OUT(TRIP)
and charge
time for the circuit in Figure 7.
Linear Technology Magazine • March 2009
OUT(TRIP)
DESIGN FEATURES L
V
TRANS
100V TO 200V DC
V
CC
V
CC
R11, 84.5k
R12, 442k
UVLO1
OVLO1
UVLO2
OVLO2
DONE
FAULT
CHARGE
CLAMP
V
CC
HVGATE
LVGATE
CSP
CSN
FB
RV
TRANS
TO µP
V
CC
LT3751
LT4430
GND RBG
R3
210k
OFF ON
C3
22µF
350V
s2
C2
1µF
C1
100pF
C4
1µF
250V
s2
C7
400µF
330V
C8
22nF
R17
3.16k
R14
249k
V
OUT
282V
225mA
C6
0.1µF
630V
ISOLATION
BOUNDARY
C5
0.01µF
630V
C9
3.3µF
C10
0.47µF
•
M2
M1
D1
R8
2.49k
R7
475Ω
R18
274Ω
R6
40mΩ
1/4W
V
TRANS
R9, 2.7M
R5, 210k
R13
5.11Ω
R16, 1k
R10, 4.3M
DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY
RDCM
RV
OUT
U1
ALL RESISTORS ARE 0805,1% RESISTORS
UNLESS OTHERWISE NOTED
C7: 330FK400M22X38
D1: 12V ZENER
D2: MURS140
D3: P6kE200A
D4, D5: STTH112A
D6: BAT54
D7: BAS516
M1: IRF830
M2: STB11NM60FD
T1: TDK SRW24LQ-UxxH015
(Np:Ns:Npb:Nsb=1:2:0.08:0.08)
U1: PS2801-1
U2: LT4430
R4, 105k
R2, 10ΩD2
D3
D5
D6
D4
Np
Ns
Nsb
U2
VINCOMP
GND
OC FB
OPTO
Npb
T1
D7
R15
221k
R1
49.9k
1/2W
•
•
•
+
+
1
1 1 1
1
11
1
1
1
2
2
2
2
2
2
2
F1, 2A
1 2
4.7nF
Y RATED
0
GND
V
DRAIN
100V/DIV
I
PRIMARY
2A/DIV
20µs/DIV
0
GND
V
DRAIN
100V/DIV
I
PRIMARY
2A/DIV
20µs/DIV
R
V
A
OUT TRIP
11
1 225
50
=
−
( )
.
µ
Figure 11. Fully isolated, high output voltage regulator
High Input Supply Voltage,
Non-Isolated Capacitor
Charger/Regulator
The FB pin of the LT3751 can also
be configured for charging a capacitor from a high input supply voltage.
Simply tie a resistor divider from the
output node to the FB pin. The resistor dividers on the R
pins can tolerate 5% resistors, and all
the R
removed. This lowers the number and
and RBG pin resistors are
V(OUT)
the tolerance of required components,
reducing board real estate and overall
design costs. With the output voltage
resistor divider, the circuit in Figure
9 is also a fully functional, high-efficiency voltage regulator with load
Linear Technology Magazine • March 2009
VTRANS
a. I
and R
= 225mA b. I
OUT
DCM
and line regulation better than 1%.
Efficiency and line regulation for the
circuit in Figure 9 are shown in Figure
10a and Figure 10b, respectively.
Alternatively, a resistor can be tied
from V
pin. This mimics the V
to the OVLO1 pin or OVLO2
OUT
compara-
OUT
tor, stopping charging once the target
voltage is reached. The FB pin is tied
to ground. The CHARGE pin must be
toggled to initiate another charge sequence, thus the LT3751 operates as
a capacitor charger only. Resistor R12
is omitted from Figure 9 and resistor
R11 is tied from V
or OVLO2. R11 is calculated using the
following equation:
Figure 12. Switching waveforms
OUT
directly to OVLO1
Note that OVLO1 or OVLO2 will
cause the FAULT pin to indicate a
fault when the target outpaut voltage,
V
OUT(TRIP) ,
is reached.
High Voltage Input/Output
Regulator with Isolation
Using a resistor divider from the output
node to the FB pin allows regulation
but does not provide galvanic isolation.
Two auxiliary windings are added to
the transformer in circuit shown in
Figure 11 to drive the FB pin, the
OUT
= 7.1mA
continued on page 42
13
L DESIGN FEATURES
V
REF
+IN
–IN
GND
8
7
6
5
1
2
3
4
V
CC
CLK
D
OUT
CS/SHDN
LTC1286
V
IN
5V
0.1µF
5V
18k
LT1634-4.096
µC/µP
SERIAL
INTERFACE
0.1µF
How to Choose a Voltage Reference
by Brendan Whelan
Why Voltage References?
It is an analog world. All electronic
devices must in some way interact with
the “real” world, whether they are in
an automobile, microwave oven or cell
phone. To do that, electronics must be
able to map real world measurements
(speed, pressure, length, temperature)
to a measurable quantity in the electronics world (voltage). Of course, to
measure voltage, you need a standard
to measure against. That standard is
a voltage reference. The question for
any system designer is not whether he
needs a voltage reference, but rather,
which one?
A voltage reference is simply that—a
circuit or circuit element that provides
a known potential for as long as the circuit requires it. This may be minutes,
hours or years. If a product requires
information about the world, such
as battery voltage or current, power
consumption, signal size or characteristics, or fault identification, then the
signal in question must be compared
to a standard. Each comparator, ADC,
DAC, or detection circuit must have a
voltage reference in order to do its job
(Figure 1). By comparing the signal of
interest to a known value, any signal
may be quantified accurately.
Figure 1. Typical use of a voltage reference for an ADC
Reference Specifications
Voltage references come in many forms
and offer different features, but in
the end, accuracy and stability are
a voltage reference’s most important
features, as the main purpose of the
reference is to provide a known output
voltage. Variation from this known
value is an error. Voltage reference
specifications usually predict the
uncertainty of the reference under
Table 1. Specifications for high performance voltage references
Temperature
Coefficient
Initial
Accuracy
I
S
Architecture V
OUT
Voltage
Noise*
Long-Term
Drift
Package
LT1031 5ppm/°C 0.05% 1.2mA Buried Zener 10V 0.6ppm 15ppm/kHr H
LT1019 5ppm/°C 0.05% 650µA Bandgap
LT1027 5ppm/°C 0.05% 2.2mA Buried Zener 5V 0.6ppm
2.5V, 4.5V,
5V, 10V
2.5ppm SO-8, PDIP
20ppm/
month
SO-8, PDIP
LT1021 5ppm/°C 0.05% 800µA Buried Zener 5V, 7V, 10V 0.6ppm 15ppm/kHr SO-8, PDIP, H
1.25V, 2.048V,
LTC6652 5ppm/°C 0.05% 350µA Bandgap
2.5V, 3V, 3.3V,
2.1ppm 60ppm/√kHr
MSOP
4.096V, 5V
LT1236 5ppm/°C 0.05% 800µA Buried Zener 5V, 10V 0.6ppm 20ppm/kHr SO-8, PDIP
LT1461 3ppm/°C 0.04% 35µA Bandgap
LT1009 15ppm/°C 0.2% 1.2mA Bandgap 2.5V 20ppm/kHr
LT1389 20ppm/°C 0.05% 700nA Bandgap
LT1634 10ppm/°C 0.05% 7µA Bandgap
2.5V, 3V, 3.3V,
4.096V, 5V
1.25V, 2.5V,
4.096V, 5V
1.25V, 2.5V,
4.096V, 5V
8ppm 60ppm/√kHr
MSOP-8,
SO-8, Z
20ppm SO-8
6ppm
SO-8,
MSOP-8, Z
LT1029 20ppm/°C 0.20% 700µA Bandgap 5V 20ppm/kHr Z
LM399 1ppm/°C 2% 15mA Buried Zener 7V 1ppm 8ppm/√kHr
LTZ1000 0.05ppm/°C 4% Buried Zener 7.2V 0.17ppm 2µV/√kHr H
*0.1Hz–10Hz, Peak-to-Peak
14
Linear Technology Magazine • March 2009
SO-8
H
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