CN-0197: Lithium Ion Battery Stack Monitor with Both Signal and Power Isolation PDF

Circuit Note
CN-0197
Devices Connected/Referenced
Circuits from the Lab™ reference circuits are engineered and
tested for quick and easy system integration to help solve today’s
analog, mixed-signal, and RF design challenges. For more
information and/or support, visit www.analog.com/CN0197.
AD7280A
Lithium Ion Battery Monitoring System
ADuM5401
Quad-Channel Isolator with Integrated
DC-to-DC Converter
ADuM1201
Dual-Channel Digital Isolator
ADG849
SPDT Switch
Lithium Ion Battery Stack Monitor with Both Signal and Power Isolation
EVALUATION AND DESIGN SUPPORT
CIRCUIT DESCRIPTION
Circuit Evaluation Boards
AD7280A Evaluation Board (EVAL-AD7280AEDZ)
Converter Evaluation and Development Board
(EVAL-CED1Z)
Design and Integration Files
Schematics, Layout Files, Bill of Materials
The AD7280A daisy chain obtains its power from the battery
cells it monitors. The ADuM5401 includes an integrated dc-todc converter, which is used to power the high voltage side of the
ADuM1201, provide the VDRIVE supply to the AD7280A SPI
interface, and provide the power-down signal to the AD7280A
daisy chain circuit. If the +5 V supply is pulled low on the low
voltage side of the BMS, the isolators and the chain of
AD7280A’s will power down. Likewise, if the PD signal from the
BMC is taken low, the low voltage supply to the ADuM5401
routed through the ADG849 switch is driven low and also
provides a hardware power down of the isolators and the chain
of AD7280A devices.
CIRCUIT FUNCTION AND BENEFITS
Lithium ion (Li-Ion) battery stacks contain a large number of
individual cells that must be monitored correctly in order to
enhance the battery efficiency and prolong the battery life. The
6-channel AD7280A devices in the circuit shown in Figure 1 act
as the primary monitor providing accurate measurement data
to the Battery Management Controller (BMC).
The AD7280A contains an internal ±3 ppm reference that
allows a cell voltage measurement accuracy of ±1.6 mV. The
ADC resolution is 12 bits and allows conversion of up to 48
cells within 7 μs.
The AD7280A, which resides on the high voltage side of the
Battery Management System (BMS) has a daisy-chain interface,
which allows up to eight AD7280A’s to be stacked together and
allows for 48 Li-Ion cell voltages to be monitored. Adjacent
AD7280A's in the stack can communicate directly, passing data
up and down the stack without the need for isolation. The
AD7280A master device on the bottom of the stack uses the SPI
interface to communicate with the BMC, and it is only at this
point that high voltage galvanic isolation is required in order to
protect the low-voltage side of the BMS. The ADuM1201 digital
isolator and the ADuM5401 isolator with integrated dc-to-dc
converter combine to provide the required six channels of
isolation in a compact and cost effective solution.
To optimize the performance of the daisy-chain communication
under noisy conditions, for example, when experiencing
electromagnetic interference, the daisy-chain signals are
shielded on an inner layer of the printed circuit board (PCB).
Shielding is provided above and below by a VSS supply plane,
which is connected to the VSS pin of the upper device in the chain.
Figure 2 shows the top layer of the EVAL-AD7280AEDZ PCB
which contains the upper shielding. Figure 3 shows the inner
layer (Layer 2), which contains the shielded daisy-chain signals,
and the shielding below is carried out on Layer 3 as shown in
Figure 4. Individual 22 pF capacitors are placed on each daisychain connection and are terminated to either the VSS pin of the
upper device or the VDD pin of the lower device, depending on
the direction in which data is flowing on the daisy chain. The
PD, CS, SCLK, SDI, and CNVST daisy-chain connections pass
data up the chain, and the 22 pF capacitors on these pins are
terminated to the VSS of the upper device in the chain.
The SDOlo and ALERTlo daisy-chain connections pass data
down the chain, and the 22 pF capacitors on these pins are
terminated to the VDD of the lower device in the chain. A direct
low impedance trace is used to connect the VDD of the lower
device with the VSS of the upper device to hold the two
potentials as close as possible together in a noisy environment.
Rev. 0
Circuits from the Lab™ circuits from Analog Devices have been designed and built by Analog Devices
engineers. Standard engineering practices have been employed in the design and construction of
each circuit, and their function and performance have been tested and verified in a lab environment at
room temperature. However, you are solely responsible for testing the circuit and determining its
suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices
be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause
whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page)
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
www.analog.com
Fax: 781.461.3113
©2011 Analog Devices, Inc. All rights reserved.
CN-0197
Circuit Note
1kΩ
VDD1
VDD1
100nF
10kΩ
100nF
SDOhi
SDOhi
VIN5
VREG
DVCC
AVCC
VDRIVE
VIN4
ALERT
AD7280A
VIN3
SDO
VIN2
10kΩ
100nF
10kΩ
100nF
MASTER
VIN1
10kΩ
100nF
10kΩ
100nF
10kΩ
100nF
ALERTlo
SDOlo
CNVST
SDI
SCLK
SDOhi
SDOhi
SCLKhi
VIN5
VREG
DVCC
AVCC
1µF
VOB
GND2
VIB
GND1
0.1µF
ADuM5401
ALERT
AD7280A
VIN3
VISO
CNVST
PD
VIN2
GNDISO
VID
0.1µF
SDOlo
VREF
VSS
CREF
VIN1
1µF
ALERTlo
SDO
VSS0
+5V
VDRIVE
VIN4
VIN0
VDD1
VOA
VIA
ALERThi
100nF
ADuM1201
VDD2
CNVSThi
10kΩ
0.1µF
VDD0
CShi
MASTER
VIN6
10kΩ
CS
PD
VSS
VDD
100nF
100nF
1kΩ
10kΩ
10kΩ
100nF
100nF
VREF
CREF
22pF
22pF
22pF
22pF
22pF
22pF
22pF
VDD0
10kΩ
1kΩ
1µF
VIN0
10kΩ
1µF
0.1µF
SCLK
VOA
VOB
SDI
CS
1kΩ
4-WIRE
SPI INTERFACE
VOC
VSEL
GNDISO
ALERT
VDD1
CNVST
GND1
VOD
SDO
SCLK
SDI
VIA
VIB
VIC
CS
RCOUT
GND1
ADG849
PD
IN
+5V
S2
09683-001
D
S1
09683-012
Figure 1. AD7280A Daisy-Chain Configuration Circuit with Isolation (Simplified Schematic: All Connections and Decoupling Not Shown)
Figure 2. Top Layer of the EVAL-AD7280AEDZ PCB Contains the Upper Shielding for the Daisy-Chain Signals
Rev. 0 | Page 2 of 5
BATTERY MANAGEMENT
CONTROLLER INTERFACE
100nF
10kΩ
ALERThi
10kΩ
CNVSThi
VIN6
100nF
10µF
CShi
PDhi
10kΩ
10kΩ
SCLKhi
VDD0
VDD
100nF
100nF
PDhi
10µF
CN-0197
09683-013
Circuit Note
09683-014
Figure 3. Layer 2 of the EVAL-AD7280AEDZ PCB Contains the Shielded Daisy-Chain Signals
Figure 4. Layer 3 of the EVAL-AD7280AEDZ PCB Contains the Shielding Below the Daisy-Chain Signals
Rev. 0 | Page 3 of 5
CN-0197
Circuit Note
A ground fence at the isolation barrier is used to enclose the
low voltage side, which consists of the left hand side of the PCB.
This fence consists of a guard ring laced together by vias and
connects to the digital ground on all layers throughout the
board. Noise on power and ground planes that reach the edge of
the circuit board can radiate causing emissions, but with this
shielded structure the noise is reflected back.
Input-to-output dipole radiation can also be generated when
driving a current source across a gap between ground planes. To
help minimize this, a continuous shield is used at the isolation
gap whereby the ground planes are extended on all layers
throughout the PCB to create a cross-barrier coupling using
overlapping shields; and the isolation gap on each layer is kept
to a minimum, with a gap of 0.4 mm used on the tested board.
For further recommendations to control radiated emissions
with isoPower® devices, such as the ADuM5401 used in this
circuit, please refer to Application Note AN-0971.
Test Results
An important measure of the performance of the circuit is the
amount of noise in the final output voltage measurement.
Figure 5 shows a histogram of 10,000 measurement samples
taken for the VIN3−VIN2 channel. This data was taken with
the AD7280 Evaluation Board connected to the EVAL-CED1Z
Converter Evaluation and Development Board. Details of the
setup are described in the Circuit Evaluation and Test section of
this circuit note
A resistor divider string driven by the supply voltage was used
to simulate the cell voltages. The captured code of 2675
represents 3.612 V, which is representative of a typical lithium
ion cell voltage. Note that there are only a small percentage of
codes that fall outside the primary bin due to noise.
9000
7818
NUMBER OF OCCURRENCES
8000
The circuit is proven to work with good stability and accuracy.
Other combinations of isolated channels can be used with the
iCoupler® isolation products. For example, an extra channel of
isolation could be added to accommodate the PD signal to the
AD7280A instead of saving on a channel and using the VISO
output supply to drive the PD signal on the AD7280A, as in this
circuit. The signals chosen to pass through which isolator can
also vary. In this circuit, the 4 SPI signals (SCLK, CS, SDI, and
SDO) are passed through the ADuM5401 isolator, and the
CNVST and ALERT signals are passed through the ADuM1201
isolator.
CIRCUIT EVALUATION AND TEST
The circuit shown in Figure 1 is used on the AD7280A
Evaluation Board. Details of the AD7280A Evaluation board
and test methods can be found in the Evaluation Board User
Guide UG-252.
Equipment Needed
When using the AD7280A Evaluation Board with the
EVAL-CED1Z board, all supplies, with the exception of the
battery connections, are provided from the EVAL-CED1Z
through a 96-way connector. When the board is shipped, the
assumption is that the user will operate with the EVAL-CED1Z
board. The appropriate links are set so that all power supplies
and control signals are supplied by the EVAL-CED1Z.
Software to communicate with the EVAL-CED1Z and
AD7280A is provided with the AD7280A evaluation board
package.
The EVAL-CED1Z board provides all the supplies for the
evaluation board. It is powered from a +7 V, 15 W "wall wart"
power supply that accepts input voltages from 100 V to 240 V
ac and contains the relevant adaptors for worldwide use. The
power supply is provided with the EVAL-CED1Z.
Connection between the EVAL-CED1Z and the USB port of a
PC is via a standard USB 2.0 connection cable that is provided
as part of the EVAL-CED1Z package.
7000
6000
5000
Getting Started
4000
Complete details of setting up the hardware and installing the
software are contained in UG-252.
3000
2076
2000
Functional Block Diagram
1000
69
A block diagram of the AD7280A evaluation board is shown in
Figure 1 of this circuit note and in UG-252.
37
2671 2672 2673 2674 2675 2676 2677 2678 2679 2680 MORE
CODE
09683-005
0
COMMON VARIATIONS
Setup and Test
Figure 5. Histrogram of Codes for 10,000 Samples, VIN3 – VIN2 Channel
The basic test setup consists of the AD7280A Evaluation Board
connected to the EVAL-CED1Z Converter Evaluation Board.
The only other connections required are to the lithium-ion
battery stack. The battery stack can be simulated with a resistor
divider, which is driven by a precision dc supply voltage.
Rev. 0 | Page 4 of 5
Circuit Note
CN-0197
LEARN MORE
Data Sheets and Evaluation Boards
CN0197 Design Support Package:
http://www.analog.com/CN0197-DesignSupport
AD7280A Evaluation Board (EVAL-AD7280AEDZ)
Cantrell, Mark. Application Note AN-0971, Recommendations
for Control of Radiated Emissions with isoPower Devices,
Analog Devices.
AD7280A Data Sheet
Converter Evaluation and Development Board (EVAL-CED1Z)
Chen, Baoxing. 2006. iCoupler® Products with isoPower™
Technology: Signal and Power Transfer Across Isolation
Barrier Using Microtransformers. Analog Devices, Inc.
ADuM1201 Data Sheet
ADG849 Data Sheet
MT-004 Tutorial, The Good, the Bad, and the Ugly Aspects of ADC
Input Noise—Is No Noise Good Noise? Analog Devices, Inc.
Wayne, Scott. “iCoupler Digital Isolators Protect RS-232, RS485, and CAN Buses in Industrial, Instrumentation, and
Computer Applications.” Analog Dialogue (October 2005).
®
ADuM5401 Data Sheet
REVISION HISTORY
4/11—Revision 0: Initial Version
(Continued from first page) Circuits from the Lab circuits are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. While you
may use the Circuits from the Lab circuits in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual property by
application or use of the Circuits from the Lab circuits. Information furnished by Analog Devices is believed to be accurate and reliable. However, "Circuits from the Lab" are supplied "as is"
and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability, noninfringement or fitness for a particular
purpose and no responsibility is assumed by Analog Devices for their use, nor for any infringements of patents or other rights of third parties that may result from their use. Analog Devices
reserves the right to change any Circuits from the Lab circuits at any time without notice but is under no obligation to do so.
©2011 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
CN09683-0-4/11(0)
Rev. 0 | Page 5 of 5
Similar pages