LTC6820
1
6820fb
For more information www.linear.com/LTC6820
TYPICAL APPLICATION
FEATURES DESCRIPTION
isoSPI Isolated
Communications Interface
The LT C
®
6820 provides bidirectional SPI communications
between two isolated devices through a single twisted-
pair connection. Each LTC6820 encodes logic states into
signals that are transmitted across an isolation barrier to
another LTC6820. The receiving LTC6820 decodes the
transmission and drives the slave bus to the appropriate
logic states. The isolation barrier can be bridged by a simple
pulse transformer to achieve hundreds of volts of isolation.
The LTC6820 drives differential signals using matched
source and sink currents, eliminating the requirement
for a transformer center tap and reducing EMI. Precision
window comparators in the receiver detect the differential
signals. The drive currents and the comparator thresholds
are set by a simple external resistor divider, allowing the
system to be optimized for required cable lengths and
desired signal-to-noise performance.
Microcontroller to SPI Slave Isolated Interface Data Rate vs Cable Length
APPLICATIONS
n 1Mbps Isolated SPI Data Communications
n Simple Galvanic Isolation Using Standard
Transformers
n Bidirectional Interface Over a Single Twisted Pair
n Supports Cable Lengths Up to 100 Meters
n Very Low EMI Susceptibility and Emissions
n Configurable for High Noise Immunity or Low Power
n Engineered for ISO26262 Compliant Systems
n Requires No Software Changes in Most SPI Systems
n Ultralow, 2µA Idle Current
n Automatic Wake-Up Detection
n Operating Temperature Range: –40°C to 125°C
n 2.7V to 5.5V Power Supply
n Interfaces to All Logic from 1.7V to 5.5V
n Available in 16-Lead QFN and MSOP Packages
n Industrial Networking
n Battery Monitoring Systems
n Remote Sensors
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and isoSPI
is a trademark of Linear Technology Corporation. All other trademarks are the property of their
respective owners. Protected by U.S. patents, including 8908779.
IP
120Ω
100 METERS
TWISTED PAIR
LTC6820
MSTR
MOSI
MISO
SCK
CS IM
MASTER
µC
SDO
SDI
SCK
CS
IP
120Ω
6820 TA01a
LTC6820
MSTR
MOSI
MISO
SCK
CS
IM
REMOTE
SLAVE IC
SDI
SDO
SCK
CS
CABLE LENGTH (METERS)
1
0
DATA RATE (Mbps)
0.2
0.4
0.6
0.8
1.2
10 100
6820 TA01b
1.0
CAT-5 ASSUMED
LTC6820
2
6820fb
For more information www.linear.com/LTC6820
PIN CONFIGURATION
ABSOLUTE MAXIMUM RATINGS
Input Supply Voltages (VDD and VDDS) to GND ...........6V
Pin Voltages
SCK, CS, EN ...............–0.3V to VDDS + 0.3V (6V Max)
IBIAS, SLOW, IP, IM .....–0.3V to VDD + 0.3V (6V Max)
All Other Pin Voltages .............................. –0.3V to 6V
Maximum Source/Sink Current
IP, IM .................................................................30mA
MOSI, MISO, SCK, CS ........................................20mA
Operating Temperature Range
LTC6820I .............................................–40°C to 85°C
LTC6820H .......................................... –40°C to 125°C
(Notes 1, 2, 3)
16 15 14 13
5678
TOP VIEW
17
UD PACKAGE
16-LEAD (3mm × 3mm) PLASTIC QFN
9
10
11
12
4
3
2
1MOSI
MISO
SCK
CS
SLOW
MSTR
IP
IM
EN
IBIAS
ICMP
GND
VDDS
POL
PHA
VDD
TJMAX = 150°C, θJA = 58.7°C/W
EXPOSED PAD (PIN 17) PCB CONNECTION TO GND IS OPTIONAL
1
2
3
4
5
6
7
8
EN
MOSI
MISO
SCK
CS
VDDS
POL
PHA
16
15
14
13
12
11
10
9
IBIAS
ICMP
GND
SLOW
MSTR
IP
IM
VDD
TOP VIEW
MS PACKAGE
16-LEAD PLASTIC MSOP
TJMAX = 150°C, θJA = 120°C/W
ORDER INFORMATION
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION SPECIFIED TEMPERATURE RANGE
LTC6820IUD#PBF LTC6820IUD#TRPBF LGFM 16-Lead (3mm × 3mm) Plastic QFN –40°C to 85°C
LTC6820HUD#PBF LTC6820HUD#TRPBF LGFM 16-Lead (3mm × 3mm) Plastic QFN –40°C to 125°C
LTC6820IMS#PBF LTC6820IMS#TRPBF 6820 16-Lead Plastic MSOP –40°C to 85°C
LTC6820HMS#PBF LTC6820HMS#TRPBF 6820 16-Lead Plastic MSOP –40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/. Some packages are available in 500 unit reels through
designated sales channels with #TRMPBF suffix.
Specified Temperature Range
LTC6820I .............................................–40°C to 85°C
LTC6820H .......................................... –40°C to 125°C
Storage Temperature Range .................. –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
MSOP ............................................................... 300°C
http://www.linear.com/product/LTC6820#orderinfo
LTC6820
3
6820fb
For more information www.linear.com/LTC6820
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full specified
temperature range, otherwise specifications are at TA = 25°C. VDD = 2.7V to 5.5V, VDDS = 1.7V to 5.5V, RBIAS = 2k to 20k unless
otherwise specified. All voltages are with respect to GND.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Power Supply
VDD Operating Supply Voltage Range l2.7 5.5 V
VDDS IO Supply Voltage Range (Level Shifting) Affects CS, SCK, MOSI, MISO and EN Pins l1.7 5.5 V
IDD Supply Current, READY/ACTIVE States
(Note 4)
RBIAS = 2kΩ (IB = 1mA) 1/tCLK = 0MHz
1/tCLK = 1MHz
l4 4.8
7
5.8 mA
mA
RBIAS = 20kΩ (IB = 0.1mA) 1/tCLK = 0MHz
1/tCLK = 1MHz
l1.3 2
2.4
2.9 mA
mA
Supply Current, IDLE State MSTR = 0V
MSTR = VDD
l
l
2
1
6
3
µA
µA
IDDS IO Supply Current (Note 5) SPI Inputs and EN Pin at 0V or VDDS,
SPI Outputs Unloaded
l1 µA
Biasing
VBIAS Voltage on IBIAS Pin READY/ACTIVE State
IDLE State
l1.9 2.0
0
2.1 V
V
IBIsolated Interface Bias Current (Note 6) RBIAS = 2k to 20k lVBIAS/RBIAS mA
AIB Isolated Interface Current Gain VA ≤ 1.6V IB = 1mA
IB = 0.1mA
l
l
18
18
20
20
22
24
mA/mA
mA/mA
VATransmitter Pulse Amplitude VA = |VIP – VIM| VDD < 3.3V
VDD ≥ 3.3V
l
l
VDD – 1.7V
1.6
V
V
VICMP Threshold-Setting Voltage on ICMP Pin VTCMP = ATCMP • VICMP l0.2 1.5 V
ILEAK(ICMP) Leakage Current on ICMP Pin VICMP = 0V to VDD l±1 µA
ILEAK(IP/IM) Leakage Current on IP and IM Pins IDLE State, VIP = VIM = 0V to VDD l±2 µA
ATCMP Receiver Comparator Threshold Voltage
Gain
VCM = VDD/2 to VDD – 0.2V,
VICMP = 0.2V to 1.5V
l0.4 0.5 0.6 V/V
VCM Receiver Common Mode Bias IP/IM Not Driving (VDD – VICMP/3 – 167mV) V
RIN Receiver Input Resistance Single-Ended to IP or IM l26 35 42 kΩ
Idle/Wake-Up (See Figures 13, 14, 15)
VWAKE Differential Wake-Up Voltage
(See Figure 13)
tDWELL = 240ns l240 mV
tDWELL Dwell Time at VWAKE VWAKE = 240mV l240 ns
tREADY Start-Up Time After Wake Detection l8 µs
tIDLE Idle Time-Out Duration l4 5.7 7.5 ms
Digital I/O
VIH(CFG) Digital Voltage Input High, Configuration
Pins (PHA, POL, MSTR, SLOW)
VDD = 2.7V to 5.5V (POL, PHA, MSTR, SLOW) l0.7 • VDD V
VIL(CFG) Digital Voltage Input Low, Configuration
Pins (PHA, POL, MSTR, SLOW)
VDD = 2.7V to 5.5V (POL, PHA, MSTR, SLOW) l0.3 • VDD V
VIH(SPI) Digital Voltage Input High, SPI Pins
(CS, SCK, MOSI, MISO)
VDDS = 2.7V to 5.5V
VDDS = 1.7V to 2.7V
l
l
0.7 • VDDS
0.8 • VDDS
V
V
VIL(SPI) Digital Voltage Input Low, SPI Pins
(CS, SCK, MOSI, MISO)
VDDS = 2.7V to 5.5V
VDDS = 1.7V to 2.7V
l
l
0.3 • VDDS
0.2 • VDDS
V
V
VIH(EN) Digital Voltage Input High, EN Pin VDDS = 2.7V to 5.5V
VDDS = 1.7V to 2.7V
l
l
2
0.85 • VDDS
V
V
VIL(EN) Digital Voltage Input Low, EN Pin VDDS = 2.7V to 5.5V
VDDS = 1.7V to 2.7V
l
l
0.8
0.25 • VDDS
V
V
VOH Digital Voltage Output High (CS and SCK) VDDS = 3.3V, Sourcing 2mA
VDDS = 1.7V, Sourcing 1mA
l
l
VDDS – 0.2
VDDS – 0.25
V
V
VOL Digital Voltage Output Low
(MOSI, MISO, CS, SCK)
VDDS = 3.3V, Sinking 3.3mA
VDDS = 1.7V, Sinking 1mA
l
l
0.2
0.2
V
V
LTC6820
4
6820fb
For more information www.linear.com/LTC6820
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full specified
junction temperature range, otherwise specifications are at TA = 25°C. VDD = 2.7V to 5.5V, VDDS = 1.7V to 5.5V, RBIAS = 2k to 20k
unless otherwise specified. All voltages are with respect to GND.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
ILEAK(DIG) Digital Pin Input Leakage Current PHA, POL, MSTR, SLOW = 0V to VDD
CS, SCK, MOSI, MISO, EN = 0V to VDDS
l±1 µA
CI/O Input/Output Pin Capacitance (Note 9) 10 pF
Isolated Pulse Timing (See Figure 2)
t1/2PW(CS) Chip-Select Half-Pulse Width l120 150 180 ns
tINV(CS) Chip-Select Pulse Inversion Delay l200 ns
tDEL(CS) Chip-Select Response Delay l140 190 ns
t½PW(D) Data Half-Pulse Width l40 50 60 ns
tINV(D) Data Pulse Inversion Delay l70 ns
tDEL(D) Data Response Delay (Note 8) l75 120 ns
isoSPI™ Timing—Master (See Figures 3, 4)
tCLK SCK Latching Edge to SCK Latching Edge (Note 7) SLOW = 0
SLOW = 1
l
l
1
5
µs
µs
t1MOSI Setup Time Before SCK Latching Edge (Note 8) l25 ns
t2MOSI Hold Time After SCK Latching Edge l25 ns
t3SCK Low tCLK = t3 + t4 ≥ 1µs l50 ns
t4SCK High tCLK = t3 + t4 ≥ 1µs l50 ns
t5CS Rising Edge to CS Falling Edge l0.6 µs
t6SCK Latching Edge to CS Rising Edge (Note 7) l1 µs
t7CS Falling Edge to SCK Latch Edge (Note 7) l1 µs
t8SCK Non-Latch Edge to MISO Valid (Note 8) l55 ns
t9SCK Latching Edge to Short ±1 Transmit l50 ns
t10 CS Transition to Long ±1 Transmit l55 ns
t11 CS Rising Edge to MISO Rising (Note 8) l55 ns
isoSPI Timing—Slave (See Figures 3, 4)
t12 isoSPI Data Recognized to SCK
Latching Edge
(Note 8) SLOW = 0
SLOW = 1
l
l
110
0.9
145
1.1
185
1.4
ns
µs
t13 SCK Pulse Width SLOW = 0
SLOW = 1
l
l
90
0.9
115
1.1
150
1.4
ns
µs
t14 SCK Non-Latch Edge to isoSPI Data Transmit (Note 8) SLOW = 0
SLOW = 1
l
l
115
0.9
145
1.1
190
1.4
ns
µs
t15 CS Falling Edge to SCK Non-Latch Edge PHA = 1 SLOW = 0
SLOW = 1
l
l
90
0.9
120
1.1
160
1.4
ns
µs
t16 CS Falling Edge to isoSPI Data Transmit SLOW = 0
SLOW = 1
l
l
200
1.8
265
2.2
345
2.8
ns
µs
t17 CS Rising Edge to SCK Latching Edge PHA = 1 SLOW = 0
SLOW = 1
l
l
90
0.9
120
1.1
160
1.4
ns
µs
t18 CS Rising Edge to MOSI Rising Edge (Note 8) l35 ns
tRTN Data Return Delay SLOW = 0
SLOW = 1
l
l
485
3.3
625
4
ns
µs
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: All currents into pins are positive, and all voltages are referenced
to GND unless otherwise specified.
Note 3: The LTC6820I is guaranteed to meet specified performance
from –40°C to 85°C. The LTC6820H is guaranteed to meet specified
performance from –40°C to 125°C.
Note 4: Active supply current (IDD) is dependent on the amount of time
that the output drivers are active on IP and IM. During those times IDD will
increase by the 20 • IB drive current. For the maximum data rate 1MHz,
the drivers are active approximately 10% of the time if MSTR = 1, and 5%
LTC6820
5
6820fb
For more information www.linear.com/LTC6820
TYPICAL PERFORMANCE CHARACTERISTICS
Supply Current (IDLE)
vs Supply Voltage
Supply Current (IDLE)
vs Temperature
Supply Current (READY/ACTIVE)
vs Clock Frequency
Supply Current (READY)
vs Temperature
Input Voltage Threshold
(Except EN Pin)
vs Supply Voltage (VDD or VDDS)
ELECTRICAL CHARACTERISTICS
of the time if MSTR = 0. See Applications Information section for more
detailed information.
Note 5: The IO supply pin, VDDS, provides power for the SPI inputs and
outputs, including the EN pin. If the inputs are near 0V or VDDS (to avoid
static current in input buffers) and the outputs are not sourcing current,
then IDDS includes only leakage current.
Note 6: The LTC6820 is guaranteed to meet specifications with RBIAS
resistor values ranging from 2k to 20k, with 1% or better tolerance. Those
resistor values correspond to a typical IB that can range from 0.1mA
(for 20k) to 1mA (for 2k).
Note 7: These timing specifications are dependent on the delay through
the cable, and include allowances for 50ns of delay each direction. 50ns
corresponds to 10m of CAT-5 cable (which has a velocity of propagation of
66% the speed of light). Use of longer cables would require derating these
specs by the amount of additional delay.
Note 8: These specifications do not include rise or fall time. While fall
time (typically 5ns due to the internal pull-down transistor) is not a
concern, rising-edge transition time tRISE is dependent on the pull-up
resistance and load capacitance. In particular, t12 and t14 require tRISE
< 110ns (if SLOW = 0) for the slave’s setup and hold times. Therefore,
the recommended time constant is 50ns or less. For example, if the
total capacitance on the data pin is 25pF (including self capacitance
CI/O of 10pF), the required pull-up resistor value is RPU ≤ 2kΩ. If these
requirements can’t be met, use SLOW = 1.
Note 9: Guaranteed by design. Not tested in production.
VDD = VDDS, unless otherwise noted.
FREQUENCY (kHz)
0
1
SUPPLY CURRENT (mA)
2
3
4
5
6
7
200 400 600 800
6820 G01
1000
VDD = 5V, IB = 1mA
VDD = 5V, IB = 0.1mA
VDD = 3V, IB = 1mA
VDD = 3V, IB = 0.1mA
MSTR = 1
TEMPERATURE (°C)
–50 –25
4.8
SUPPLY CURRENT (mA)
5.0
5.3
050 75
6820 G02
4.9
5.2
5.1
25 100 125
VDD = 5V
VDD = 3V
IB = 1mA
SUPPLY VOLTAGE (V)
1.5 2.5
0
INPUT VOLTAGE THRESHOLD (V)
0.5
1.0
1.5
2.0
4.0
VIH
VIL
3.02.0 3.5 4.0 4.5
6820 G03
5.0 5.5
3.5
3.0
2.5
ONLY
SPI PINS HIGH
LOW
SUPPLY VOLTAGE (V)
2.5
0
SUPPLY CURRENT (µA)
0.5
1.0
1.5
2.0
3.0
3.0 3.5 4.0 4.5
6820 G04
5.0 5.5
2.5
SLAVE (MSTR = 0)
MASTER (MSTR = 1)
TEMPERATURE (°C)
–50
SUPPLY CURRENT (µA)
3.0
25 75
6820 G05
2.0
–25 0 50 100 125
1.0
0
2.5
1.5
0.5
VDD = 5V
SLAVE (MSTR = 0)
MASTER (MSTR = 1)
Output Resistance vs Supply
Voltage (VOH/VOL)
SUPPLY VOLTAGE (V)
1.5
OUTPUT RESISTANCE (Ω)
40
60
5.5
6820 G19
20
02.5 3.5 4.5
100
80
OUTPUT
SOURCING 2mA CURRENT
OUTPUT SINKING 3.3mA CURRENT
LTC6820
6
6820fb
For more information www.linear.com/LTC6820
TYPICAL PERFORMANCE CHARACTERISTICS
Driver Current Gain vs Amplitude
Driver Current Gain
vs IBIAS Current (IB)
Driver Current Gain
vs Supply Voltage
Driver Current Gain
vs Temperature
Driver Common Mode Voltage
vs Temperature
Driver Common Mode Voltage
vs Pulse Amplitude
IBIAS Voltage vs Temperature IBIAS Voltage Load Regulation IBIAS Voltage vs Supply Voltage
VDD = VDDS, unless otherwise noted.
IBIAS CURRENT (mA)
0
IBIAS PIN VOLTAGE (V)
2.000
2.005
0.8
6820 G08
1.995
1.990 0.2 0.4 0.6 1.0
2.010 VDD = 3V
SUPPLY VOLTAGE (V)
2.5
1.990
IBIAS PIN VOLTAGE (V)
1.995
2.000
2.005
2.010
3 3.5 4 4.5
6820 G09
5 5.5
IB = 0.1mA
IB = 1mA
PULSE AMPLITUDE VA (V)
0
17
CURRENT GAIN AIB (mA/mA)
18
19
20
21
22
23
0.5 1.0 1.5
6820 G10
2.0
VDD = 3V
IB = 1mA
VDD = 5V
IB = 1mA VDD = 5V
IB = 0.1mA
VDD = 3V
IB = 0.1mA
VA(MAX) = 1.6V
FOR VDD > 3.3V
VA(MAX) = 1.3V
FOR VDD = 3V
IBIAS CURRENT (mA)
0
CURRENT GAIN (mA/mA)
19.5
20.0
20.5
0.6 1.0
VA = 1V
6820 G11
19.0
18.5
18.0 0.2 0.4 0.8
21.0
21.5
22.0
VDD = 5V
VDD = 3V
SUPPLY VOLTAGE (V)
2.5
CURRENT GAIN (mA/mA)
20.0
6820 G12
19.0
18.0 3.5 4.5
34 5
21.0
22.0
19.5
18.5
20.5
21.5
VA = 1V
IB = 0.1mA
IB = 1mA
5.5
TEMPERATURE (°C)
–50
CURRENT GAIN (mA/mA)
21.5
25
6820 G13
20.0
19.0
–25 0 50
18.5
18.0
22.0
21.0
20.5
19.5
75 100 125
IB = 0.1mA, VDD = 5V
VA = 1V
IB = 0.1mA, VDD = 3V
IB = 1mA, VDD = 5V
IB = 1mA, VDD = 3V
TEMPERATURE (°C)
–50 –25
0
DRIVER COMMON MODE (V)
2
5
050 75
6820 G14
1
4
3
25 100 125
IB = 0.1mA, VDD = 5V
IB = 0.1mA, VDD = 3V
IB = 1mA, VDD = 5V
IB = 1mA, VDD = 3V
VA = 1V
PULSE AMPLITUDE (V)
0
DRIVER COMMON MODE (V)
3.0
4.0
5.0
6820 G15
2.0
2.5
3.5
4.5
1.5
1.0 0.5 1.0 1.5 2.0
IB = 0.1mA, VDD = 5V
IB = 0.1mA, VDD = 3V
IB = 1mA, VDD = 5V
IB = 1mA, VDD = 3V
TEMPERATURE (°C)
–50 –25
1.96
IBIAS PIN VOLTAGE (V)
1.98
2.04
050 75
6820 G07
2.02
2.00
25 100 125
VDD = 3V
3 PARTS
IB = 1mA
IB = 0.1mA
LTC6820
7
6820fb
For more information www.linear.com/LTC6820
TYPICAL PERFORMANCE CHARACTERISTICS
VDD = VDDS, unless otherwise noted.
Wake-Up Pulse Amplitude
vs Dwell Time
Comparator Threshold Gain
vs ICMP Voltage
Comparator Threshold Gain
vs Common Mode
Comparator Threshold Gain
vs Temperature
SPI Signal and isoSPI Pulses, MSTR = 1 SPI Signal and isoSPI Pulses, MSTR = 0
Start-Up Time
CS
5V/DIV
SCK
5V/DIV
MOSI
5V/DIV
MIS0
5V/DIV
IP-IM
2V/DIV
1.2µs/DIV 6820 G21
VDD = 5V
VDDS = 3.3V
PHA = 1
POL = 1
CS
5V/DIV
SCK
5V/DIV
MOSI
5V/DIV
MIS0
5V/DIV
IP-IM
2V/DIV
1.2µs/DIV 6820 G22
VDD = 5V
VDDS = 5V
PHA = 0
POL = 0
CS
5V/DIV
IBIAS
2V/DIV
IP-IM
1V/DIV
1µs/DIV 6820 G06
VDDS = 5V
MSTR = 1
RBIAS = 2k
3.6µs
ICMP VOLTAGE (V)
0
COMPARATOR THRESHOLD GAIN (V/V)
0.52
0.50
0.54
0.56
0.6 1.0 1.6
6820 G16
0.48
0.46
0.44
0.2 0.4 0.8 1.2 1.4
3 PARTS
VDD = 3V
VDD = 5V
COMMON MODE VOLTAGE (V)
1.5 2.0
COMPARATOR THRESHLD GAIN (V/V)
2.5 3.5 4.0
6820 G17
3.0 4.5 5.55.0
VICMP = 1V
VDD = 3V
VICMP = 0.2V
VDD = 5V
VICMP = 0.2V
VDD = 3V
VICMP = 1V
VDD = 5V
0.52
0.50
0.54
0.56
0.48
0.46
0.44
TEMPERATURE (°C)
–50 –25
COMPARATOR THRESHLD GAIN (V/V)
050 75
6820 G18
25 100 125
VDD = 3V
3 PARTS
VICMP = 1V
VICMP = 0.2V
0.52
0.50
0.54
0.56
0.48
0.46
0.44
WAKE-UP DWELL TIME, tDWELL (ns)
0
WAKE-UP PULSE AMPLITUDE, VWAKE (mV)
150
200
600
6820 G20
100
50 150 300 450
300
250 GUARANTEED
WAKE-UP REGION
VDD = 3V
LTC6820
8
6820fb
For more information www.linear.com/LTC6820
PIN FUNCTIONS
(QFN/MSOP)
MOSI (Pin 1/Pin 2): SPI Master Out/Slave In Data. If
connected on the master side of a SPI interface (MSTR
pin high), this pin receives the data signal output from
the master SPI controller. If connected on the slave side
of the interface (MSTR pin low), this pin drives the data
signal input to the slave SPI device. The output is open
drain, so an external pull-up resistor to VDDS is required.
MISO (Pin 2/Pin 3): SPI Master In/Slave Out Data. If con-
nected on the master side of a SPI interface (MSTR pin
high), this pin drives the data signal input to the master SPI
controller
. If connected on the slave side of the interface
(MSTR pin low), this pin receives the data signal output
from the slave SPI device. The output is open drain, so
an external pull-up resistor to VDDS is required.
SCK (Pin 3/Pin 4): SPI Clock Input/Output. If connected on
the master side of the interface (MSTR pin high), this pin
receives the clock signal from the master SPI controller.
This input should not be pulled above VDDS. If connected
on the slave side of the interface (MSTR pin low), this pin
outputs the clock signal to the slave device. The output
driver is push-pull; no external pull-up resistor is needed.
CS (Pin 4/Pin 5): SPI Chip Select Input/Output. If connected
on the master side of the interface (MSTR pin high), this
pin receives the chip select signal from the master SPI
controller. This input should not be pulled above VDDS. If
connected on the slave side of the interface (MSTR pin
low), this pin outputs the chip select signal to the slave
device. The output driver is push-pull; no external pull-up
resistor is needed.
VDDS (Pin 5/Pin 6): SPI Input/Output Power Supply Input.
The output drivers for the SCK and CS pins use the VDDS
input as their positive power supply. The input threshold
voltages of SCK, CS, MOSI, MISO and EN are determined
by VDDS. May be tied to VDD or to a supply above or below
VDD to level shift the SPI I/O. If separate from VDD, con-
nect a bypass capacitor of at least 0.01μF directly between
VDDS and GND.
POL (Pin 6/Pin 7): SPI Clock Polarity Input. Tie to VDD or
GND. See Operation section for details.
PHA (Pin 7/Pin 8): SPI Clock Phase Input. Tie to VDD or
GND. See Operation section for details.
VDD (Pin 8/Pin 9): Device Power Supply Input. Connect
a bypass capacitor of at least 0.01μF directly between
VDD and GND.
IM (Pin 9/Pin 10): Isolated Interface Minus Input/Output.
IP (Pin 10/Pin 11): Isolated Interface Plus Input/Output.
MSTR (Pin 11/Pin 12): Serial Interface Master/Slave
Selector Input. Tie this pin to VDD if the device is on the
master side of the isolated interface. Tie this pin to GND
if the device is on the slave side of the isolated interface.
SLOW (Pin 12/Pin 13): Slow Interface Selection Input. For
clock frequencies at or below 200kHz, or if slave devices
cannot meet timing requirements, this pin should be tied
to VDD. For clock frequencies above 200kHz, this pin
should be tied to GND.
GND (Pin 13/Pin 14): Device Ground.
ICMP (Pin 14/Pin 15): Isolated Interface Comparator
Voltage Threshold Set. Tie this pin to the resistor divider
between IBIAS and GND to set the voltage threshold of the
interface receiver comparators. The comparator thresholds
are set to 1/2 the voltage on the ICMP pin.
IBIAS (Pin 15/Pin 16): Isolated Interface Current Bias.
Tie IBIAS to GND through a resistor divider to set the
interface output current level. When the device is enabled,
this pin is approximately 2V. When transmitting pulses,
the sink current on each of the IP and IM pins is set to
20 times the current sourced from pin IBIAS to GND.
Limit the capacitance on the IBIAS pin to less than 50pF
to maintain the stability of the feedback circuit regulating
the IBIAS voltage.
EN (Pin 16/Pin 1): Device Enable Input. If high, this pin
forces the LTC6820 to stay enabled, overriding the internal
IDLE mode function. If low, the LTC6820 will go into IDLE
mode after the CS pin has been high for 5.7ms (when
MSTR pin is high) or after no signal on the IP/IM pins for
5.7ms (when MSTR pin is low). The LTC6820 will wake-up
less than 8µs after CS falls (MSTR high) or after a signal
is detected on IP/IM (MSTR low).
Exposed Pad (Pin 17, QFN Package Only): Exposed pad
may be left open or connected to device GND.
LTC6820
9
6820fb
For more information www.linear.com/LTC6820
BLOCK DIAGRAM
OPERATION
+
–
IDRV
IP
IBIAS IB
ICMP
IM
RM
6820 BD
RPU
RB1
RB2
Tx = –1
CS
Tx • 20 • IB
Tx = +1
Rx = +1
Rx = –1
–
+
THRESHOLD 0.5x
35k
35k
VDD
READY
RBIAS = RB1 + RB2
OPEN
WHEN
IDLE
+ 167mV
VICMP
3
IDLE TIMEOUT
WAKE DETECT
CS
EN
2V
SCK
GND
MISO
MOSI
EN
VDDS
VDDS
VDD
PHA
POL
SLOW
MSTR
VDD-POWERED CONFIGURATION INPUTSVDDS-POWERED SPI PIN TRANSLATION
TIMING PULSE QUALIFICATION LOGIC
VDD
0.1µF
(TO MISO IF MSTR = 1)
(TO MOSI IF MSTR = 0)
The LTC6820 creates a bidirectional isolated serial port
interface (isoSPI) over a single twisted pair of wires, with
increased safety and noise immunity over a nonisolated
interface. Using transformers, the LTC6820 translates
standard SPI signals (CS, SCK, MOSI and MISO) into pulses
that can be sent back and forth on twisted-pair cables.
A typical system uses two LTC6820 devices. The first is
paired with a microcontroller or other SPI master. Its IP
and IM transmitter/receiver pins are connected across an
isolation barrier to a second LTC6820 that reproduces the
SPI signals for use by one or more slave devices.
The transmitter is a current-regulated differential driver.
The voltage amplitude is determined by the drive current
and the equivalent resistive load (cable characteristic
impedance and termination resistor, RM).
The receiver consists of a window comparator with a
differential voltage threshold, VTCMP. When VIP – VIM
is greater than +VTCMP, the comparator detects a logic
+1. When VIP – VIM is less than –VTCMP, the comparator
detects a logic –1. A logic 0 (null) indicates VIP – VIM is
between the positive and negative thresholds.
The comparator outputs are sent to pulse timers (filters)
that discriminate between short and long pulses.
Selecting Bias Resistors
The adjustable signal amplitude allows the system to trade
power consumption for communication robustness, and
the adjustable comparator threshold allows the system to
account for signal losses.
LTC6820
10
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For more information www.linear.com/LTC6820
OPERATION
Figure 1. Typical System Using Two LTC6820 Devices
RM
IP MSTR
IM
IBIAS
ICMP
6820 F01
MOSI
MISO
SCK
CS
SDI
SDO
SCK
CS
LTC6820 SLAVE
RM
RB1
RB2
RB1
RB2
IPMSTR
IM
IBIAS
ICMP
MOSI
MISO
SCK
CS
SDO
SDI
SCK
CS
LTC6820
TWISTED-PAIR CABLE
WITH CHARACTERISTIC IMPEDANCE RM
ISOLATION BARRIER
MASTER
The transmitter drive current and comparator voltage
threshold are set by a resistor divider (RBIAS = RB1 + RB2)
between the IBIAS pin and GND, with the divided voltage
tied to the ICMP pin. When the LTC6820 is enabled (not
IDLE), IBIAS is held at 2V, causing a current, IB, to flow
out of the IBIAS pin. The IP and IM pin drive currents are
20 • IB. The comparator threshold is half the voltage on
the ICMP pin (VICMP).
As an example, if divider resistor RB1 is 1.21k and resistor
RB2 is 787Ω (so that RBIAS = 2k), then:
IB=
2V
RB1 +RB2
=1mA
IDRV = IIP = IIM = 20 • IB = 20mA
V
ICMP =2V •
R
B2
R
B1
+R
B2
=IB• RB2 =788mV
VTCMP = 0.5 • VICMP = 394mV
In this example, the pulse drive current IDRV will be 20mA,
and the receiver comparators will detect pulses with IP-IM
amplitudes greater than ±394mV.
If the isolation barrier uses 1:1 transformers connected
by a twisted pair and terminated with 100Ω resistors on
each end, then the transmitted differential signal amplitude
(±) will be:
VA=IDRV •
R
M
2
=1V
(This result ignores transformer and cable losses, which
will reduce the amplitude).
isoSPI Pulse Detail
The isoSPI transmitter can generate three voltage levels:
+VA, 0V, and –VA. To eliminate the DC signal component
and enhance reliability, isoSPI pulses are defined as
symmetric pulse pairs. A +1 pulse pair is defined as a
+VA pulse followed by a –VA pulse. A –1 pulse pair is –VA
followed by +VA.
The duration of each pulse is defined as t1/2PW. (The total
isoSPI pulse duration is 2 • t1/2PW). The LTC6820 allows
for two different t1/2PW values so that four types of pulses
can be transmitted, as listed in Table 1.
Table 1. isoSPI Pulse Types
PULSE TYPE FIRST LEVEL SECOND LEVEL ENDING LEVEL
Long +1 +VA (150ns) –VA (150ns) 0V
Long –1 –VA (150ns) +VA (150ns) 0V
Short +1 +VA (50ns) –VA (50ns) 0V
Short –1 –VA (50ns) +VA (50ns) 0V
Long pulses are used to transmit CS changes. Short pulses
transmit data (MOSI or MISO). An LTC6820 detects four
types of communication events from the SPI master: CS
falling, CS rising, SCK latching MOSI = 0, and SCK latch-
ing MOSI = 1. It converts each event into one of the four
pulse types, as shown in Table 2.
Table 2. Master Communication Events
SPI MASTER EVENT TRANSMITTED PULSE
CS Rising Long +1
CS Falling Long –1
SCK Latching Edge, MOSI = 1 Short +1
SCK Latching Edge, MOSI = 0 Short –1
LTC6820
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OPERATION
On the other side of the isolation barrier (i.e., the other end
of the cable) another LTC6820 is configured to interface
with a SPI slave. It receives the transmitted pulses and
reconstructs the SPI signals on its output port, as shown
in Table 3. In addition, the slave device may transmit a
return data pulse to the master to set the state of MISO.
See isoSPI Interaction and Timing for additional details.
Table 3. Slave SPI Port Output
RECEIVED PULSE SPI PORT ACTION RETURN PULSE
Long +1 Drive CS High None
Long –1 Drive CS Low Short –1 Pulse
if MISO = 0
(No Return Pulse
if MISO = 1)
Short +1 1. Set MOSI = 1
2. Pulse SCK
Short –1 1. Set MOSI = 0
2. Pulse SCK
A slave LTC6820 never transmits long (CS) pulses. Fur-
thermore, a slave will only transmit a short –1 pulse (when
MISO = 0), never a +1 pulse. This allows for multiple slave
devices on a single cable without risk of collisions (see
Multidrop section).
isoSPI Pulse Specifications
Figure 2 details the timing specifications for the +1 and
–1 isoSPI pulses. The same timing specifications apply to
either version of these symmetric pulses. In the Electrical
VA
+1 PULSE
–1 PULSE
–VA
MOSI, MISO OR CS
VTCMP t1/2PW
t1/2PW
–VTCMP
VIP – VIM
VA
–VA
MOSI, MISO OR CS
VTCMP
–VTCMP
VIP – VIM t1/2PW
t1/2PW
tINV
tINV
tDEL
tDEL
6820 F02
Characteristics table, these specifications are further
separated into CS (long) and Data (short) parameters.
A valid pulse must meet the minimum spec for t1/2PW and
the maximum spec for tINV. In other words, the half-pulse
width must be long enough to pass through the appropriate
pulse timer, but short enough for the inversion to begin
within the valid window of time.
The response observed at MOSI, MISO or CS will occur
after delay tDEL from the pulse inversion.
Setting Clock Phase and Polarity (PHA and POL)
SPI devices often use one clock edge to latch data and
the other edge to shift data. This avoids timing problems
associated with clock skew. There is no standard to specify
whether the shift or latch occurs first. There is also no
requirement for data to be latched on a rising or falling
clock edge, although latching on the rising edge is most
common. The LTC6820 supports all four SPI operating
modes, as configured by the PHA and POL Pins.
Table 4. SPI Modes
MODE POL PHA DESCRIPTION
0 0 0 SCK Idles Low, Latches on Rising (1st) Edge
1 0 1 SCK Idles Low, Latches on Falling (2nd) Edge
2 1 0 SCK Idles High, Latches on Falling (1st) Edge
311SCK Idles High, Latches on Rising (2nd) Edge
Figure 2. isoSPI Differential Pulse Detail
LTC6820
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OPERATION
If POL = 0, SCK idles low. Data is latched on the rising
(first) clock edge if PHA = 0 and on the falling (second)
clock edge if PHA = 1.
If POL =1, SCK idles high. Data is latched on the falling
(first) clock edge if PHA = 0 and on the rising (second)
clock edge if PHA = 1.
The two most common configurations are mode 0 (PHA= 0
and POL = 0) and mode 3 (PHA = 1 and POL = 1) because
these modes latch data on a rising clock edge.
isoSPI Interaction and Timing
The timing diagrams in Figures 3 and 4 show how an iso-
SPI in master mode (connected to a SPI master) interacts
with an isoSPI in slave mode (connected to a SPI slave).
Figure3 details operation with PHA = 0 (and shows SCK
signals for POL = 0 or 1). Figure 4 provides the timing
diagram for PHA = 1. Although not shown, it is acceptable
to use different SPI modes (PHA and POL settings) on the
master and slave devices.
A master SPI device initiates communication by lowering
CS. The LTC6820 converts this transition into a Long –1
pulse on its IP/IM pins. The pulse traverses the isolation
barrier (with an associated cable delay) and arrives at the
IP/IM pins of the slave LTC6820. Once validated, the Long
–1 pulse is converted back into a falling CS transition, this
time supplied to the slave SPI device. If slave PHA = 1,
SCK will also leave the idle state at this time.
Before the master SPI device supplies the first latching
clock edge (usually a rising edge, but see Table 4 for
exceptions), the slave LTC6820 must transmit the initial
slave data bit SN, which it determines by sampling the
state of MISO after a suitable delay.
If MISO = 0, the slave will transmit a Short –1 pulse to the
master. The master LTC6820 will receive and decode the
pulse and set the master MISO = 0 (matching the slave).
However, if the slave MISO=1, the slave does not transmit
a pulse. The master will interpret this null response as a1
and set the master MISO = 1. This makes it possible to
connect multiple slave LTC6820’s to a single cable with
no conflicting signals (see Multidrop section).
After the falling CS sequence, every latching clock edge
on the master converts the state of the MOSI pin into an
isoSPI data pulse (MN, MN–1, … M0) while simultaneously
latching the slave’s data bit. As the slave LTC6820 receives
each data bit it will set the slave MOSI pin to the proper
state and then generate an SCK pulse before returning the
slave’s MISO data (either as a Short –1 pulse, or as a null).
At the end of communication, the final data bit sent by
the slave (either as a pulse or null) will be ignored by
the master controller. (The slave LTC6820 must return a
data bit since it cannot predict when communications will
cease.) The master SPI device can then raise CS, which
is transmitted to the slave in the form of a Long +1 pulse.
The process ends with the slave LTC6820 transitioning
CS high, and returning SCK to the idle state if PHA = 1.
Rise Time
MOSI and MISO outputs have open-drain drivers. The rise
time tRISE for the data output is determined by the pull-up
resistance and load capacitance. RPU must be small enough
to provide adequate setup and hold times.
Slow Mode
When configured for slave operation, the LTC6820 provides
two operating modes to ensure compatibility with a wide
range of SPI timing scenarios. These modes are referred
to as fast and slow mode, and are set using the SLOW
pin. When configured for master operation, the SLOW
pin setting has no effect on the LTC6820 operation. In
this case, it is recommended to tie the SLOW pin to GND.
In fast mode (SLOW pin tied to GND), the LTC6820 can
operate at clock rates up to 1MHz (tCLK = 1µs). However,
some SPI slave devices can’t respond quickly enough to
support this data rate. Fast mode requires a slave to operate
with setup and response times of 100ns, as well as 100ns
clock widths. In addition, allowances must be made for the
RC rise time of MOSI and MISO’s open-drain outputs. In
slow mode (SLOW pin tied to V+), the timing requirement
are relaxed at the expense of maximum data rate. As indi-
cated in the Electrical Characteristics, the clock pulses and
required setup and response times are increased to 0.9µs
minimum. Accordingly, the minimum tCLK (controlled by
the master) must be limited to 5µs. The SLOW pin setting
has no effect on the master LTC6820 (with MSTR = 1).
LTC6820
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For more information www.linear.com/LTC6820
OPERATION
SCK (POL = 0)
ISO
ISO
MISO
MOSI
SCK (POL = 1)
SCK (POL = 0)
t7
t4
t1
CS
CS
SCK (POL = 1)
MOSI
MISO
5000 1000 1500 2000 2500
TIME (ns)
3000
SAMPLESAMPLE
3500 4000 4500 5000
6820 F03
tCLK t6
t3t5
t2
t8
t11
t10
t9tDEL(D)
tRISE
tRISE
t10
t16
t14
t13
t18
tDEL(CS)
SAMPLE
tRTN
tDEL(CS)
CSB = 0 MNMN-1 M0CSB = 1
SNSN-1 SN-2 IGNORED
SLAVE DOES NOT
TRANSMIT +1
t12
tDEL(D)
Figure 3. Transceiver Timing Diagram (PHA = 0)
LTC6820
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For more information www.linear.com/LTC6820
OPERATION
Figure 4. Transceiver Timing Diagram (PHA = 1)
t7
CS
SCK (POL = 1)
tCLK
t4t3
t6
t5
t1
t8
t9tDEL(D)
tDEL(CS)
t10
t11
t10
t2
t16
t15
tDEL(CS)
t13
t14
t17
t18
SAMPLESAMPLE
500 1000 1500 2000 2500
TIME (ns)
3000 3500 4000 4500 50000
SCK (POL = 0)
MOSI
MISO
ISO
ISO
CS
SCK (POL = 1)
SCK (POL = 0)
MOSI
MISO
CSB = 0 MNMN-1 MOCSB = 1
SNSN-1 SN-2 IGNORED
6820 F04
tRISE
t12
tDEL(D) SLAVE DOES NOT
TRANSMIT +1
SAMPLE
tRTN
tRISE
LTC6820
15
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For more information www.linear.com/LTC6820
OPERATION
Figure 6 demonstrates slow mode, as compared to fast
mode in Figure 5.
Figure 5. Fast Mode (SLOW = 0)
Figure 6. Slow Mode (SLOW = 1)
IP-IM
2V/DIV
SCK
5V/DIV
MOSI
5V/DIV
MISO
5V/DIV
200ns/DIV 6820 F05
VDD = 5V
VDDS = 5V
IP-IM
2V/DIV
SCK
5V/DIV
MOSI
5V/DIV
MISO
5V/DIV
1µs/DIV 6820 F06
VDD = 5V
VDDS = 5V
IP and IM Pulse Driver
The IP and IM pins transmit and receive the isoSPI pulses.
The transmitter uses a current-regulated driver (see
Figure7) to establish the pulse amplitude, as determined
by the IBIAS pin current, IB, and the load resistance. The
sinking current source is regulated to 20x the bias current IB.
The sourcing current source operates in a current-starved
(resistive) manner to maintain the sourcing pin’s voltage
near VDD, as shown in Figures 8 and 9. The common mode
voltage (while driving) is dependent on bias current and
output amplitude.
The output driver will regulate the common mode and
peak swing of IP and IM to the proper levels, allowing for
a broad range of output amplitude with fairly flat gain, as
shown in Figure 10.
+
–
RM
35k
OPEN WHEN IDLE
+ 167mV
VDD VDD
35k
6820 F07
POS
NEG
20 • IB
POS
NEG
VIP – VIM
IP
VICMP
3
IM
Figure 7. Pulse Driver
VIP OR VIM (V)
0
0
SOURCE/SINK CURRENT (mA)
5
10
15
20
25
0.5 1 1.5 2
6820 F08
2.5 3
SOURCING OUTPUT
1V AMPLITUDE
SINKING OUTPUT
VDD = 3V
IB = 1mA
Figure 8. Drive Source/Sink vs Output Voltage
PULSE AMPLITUDE (V)
0
0
OUTPUT VOLTAGE (V)
0.5
1.0
1.5
2.0
3.0
VCM
0.5 1 1.5 2
6820 F09
2.5 3
2.5 SOURCING
OUTPUT
SINKING
OUTPUT
VDD = 3V
IB = 1mA
Figure 9. Output Voltages and Common Mode vs Amplitude
LTC6820
16
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For more information www.linear.com/LTC6820
OPERATION
Figure 10. AIB Current Gain vs Amplitude
PULSE AMPLITUDE (V)
0
0
CURRENT GAIN (mA/mA)
5
10
15
20
25
0.5 1 1.5 2
6820 F10
2.5 3
VDD = 3V
IS = 1mA
This type of driver does not require a center-tapped
transformer, but such a transformer may improve noise
immunity, especially if it has a common mode choke. See
the Applications Information section for additional details.
Receiver Common Mode Bias
When not transmitting, the output driver maintains IP
and IM near VDD with a pair of 35k (RIN) resistors to a
voltage of VDD – VICMP/3 – 167mV. This weak bias net-
work holds the outputs near their desired operating point
without significantly loading the cable, which allows a large
number of LTC6820’s to be paralleled without affecting
signal amplitude.
Figure 11 shows the differential and single-ended IP and
IM signals while transmitting and receiving data. The
driver forces the common mode voltage it needs while
transmitting, then it returns to the bias level with a time
constant of RIN • CLOAD/2, where CLOAD is the sum of the
capacitance at the IP and IM pins.
When the LTC6820 is in low power IDLE mode, the bias
voltage is disconnected from the 35k resistors, resulting
in a 70k differential load.
State Diagram
During periods of no communication, a low current IDLE
(or shutdown) state is available to reduce power. In the
IDLE state the LTC6820 shuts down most of the circuitry.
A slave device uses a low current comparator to monitor
for activity, so it has larger IDLE current.
Figure 11. Transmitting and Receiving Data
Figure 12. State Diagram
TIME (ns)
0
–1.5
VOLTAGE (V)
–1.0
0
0.5
1.0
400 800 1000
3.0
IP IM
IP-IM
6820 F11
–0.5
200 600
1.5
2.0
2.5
TRANSMIT SHORT +1
RECEIVE SHORT –1
VDD = 3V
IB = 1mA
IDLE
READY
WAKE-UP SIGNAL
(tREADY)
IDLE
TIMEOUT
(tIDLE)
NO ACTIVITY
ON isoSPI
PORT
TRANSMIT/RECEIVE
6820 F12
ACTIVE
In the READY state all circuitry is enabled and ready to
transmit or receive, but is not actively transmitting on IP
and IM.
Supply current increases when actively communicating,
so this condition is referred to as the ACTIVE state.
Supply Current
Table 5 provides equations for estimating IDD in each state.
The results are for average supply current (as opposed
to peak currents), and make the assumption that a slave
is returning an equal number of 0s and 1s (significant
because the slave doesn’t generate +1 data pulses, so the
average driver current is smaller).
LTC6820
17
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For more information www.linear.com/LTC6820
OPERATION
Table 5. IDD Equations
STATE MSTR ESTIMATED IDD
IDLE 0 (slave) 2µA
1 (master) 1µA
READY 0 or 1 1.7mA + 3 • IB
ACTIVE 0 (slave)
2mA +3+20 • 100ns • 0.5
tCLK
•IB
1 (master)
2mA +3+20 • 100ns
tCLK
•IB
IDLE Mode and Wake-Up Detection
To conserve power, an LTC6820 in slave mode (MSTR=0)
will enter an IDLE state after 5.7ms (tIDLE) of inactivity
on the IP/IM pins. In this condition IDD is reduced to less
than 6µA and the SPI pins are idled (CS = 1, MOSI = 1
and SCK = POL).
The LTC6820 will continue monitoring the IP and IM
pins using a low power AC-coupled detector. It will wake
up when it sees a differential signal of 240mV or greater
that persists for 240ns or longer. In practice, a long (CS)
isoSPI pulse is sufficient to wake the device up. Once the
comparator generates the wake-up signal it can take up
to 8µs (tREADY) for bias circuits to stabilize.
Figure 14 details the sequence of waking up a slave LTC6820
(placing it in the READY state), using it to communicate,
then allowing it to return to the low power IDLE state.
A LTC6820 in master mode (MSTR = 1) doesn’t use the
wake-up detection comparator. A falling edge on CS will
enable the isoSPI port within tREADY, and the LTC6820
will transmit a long (CS) pulse as it leaves the IDLE state.
(The polarity of the pulse matches the CS state at the end
of tREADY).
The master LTC6820 will remain in the READY/ACTIVE
state as long as CS = 0. If CS transitions high and EN=0
it will enter the IDLE state, but not until tIDLE expires.
This prevents the device from shutting down between
data packets.
In either master or slave mode the IDLE feature may be
disabled by driving EN high. This forces the device to
remain “ready” at all times.
Figure 15 demonstrates a simple procedure for waking
a master (MSTR = 1) LTC6820 and its connected slave
(MSTR = 0). A negative edge on CS causes the master
to drive IBIAS to 2V and, after a short delay, transmit a
long +1 pulse. (If CS remains low throughout tREADY, the
LTC6820 would first generate a –1 pulse, then the +1
pulse when CS returns high). The long pulse serves as a
wake-up signal for the slave device, which responds by
driving its IBIAS pin to 2V and entering the READY state.
|IPAC–IMAC| > 240mV
IPAC
IMAC
WAKE-UP
IP
240mV
IM
EN
CS
240ns
240ns DELAY
(FILTER)
SLAVE
MASTER
tREADY
tIDLE
IDLE TIMER
READY
(IBIAS = 2V)
6820 F13
Figure 13. Wake-Up Detection and IDLE Timer
Figure 14. Slave LTC6820 Wake-Up/Idle Timing
Figure 15. Master and Slave Wake-Up/Idle Sequence
REJECTS
COMMON MODE
NOISE
IP
IM
IP-IM
READY
tDWELL tREADY
tIDLE
6820 F14
OK TO COMMUNICATE
SLAVE CS
SLAVE
IBIAS
IP-IM
MASTER
IBIAS
MASTER
CS
tDWELL
ALLOW >2 • t
READY
TO WAKE
MASTER AND SLAVE
tREADY
6820 F15
tIDLE
tIDLE
tREADY
LTC6820
18
6820fb
For more information www.linear.com/LTC6820
IP
RM
LTC6820
MSTR
MOSI
MISO
SCK
CS IM
MASTER
SDO
SDI
SCK
CS
1
1 1
2 2
3 3
2
3
IP
MSTR
MOSI
MISO
SCK
CS
IM
SLAVE 1LTC6820
SDI
SDO
SCK
CS
IP
MSTR
MOSI
MISO
SCK
CS
IM
SLAVE 2LTC6820
SDI
SDO
SCK
CS
RM
IP
MSTR
MOSI
MISO
SCK
CS
IM
SLAVE 3
6820 F16
LTC6820
SDI
SDO
SCK
CS
Figure 16. Multidropping Multiple Slaves on a Single Cable
OPERATION
Multidrop
Multiple slaves can be connected to a single master by con-
necting them in parallel (multidrop configuration)along one
cable. As shown in Figure16, the cable should be terminated
only at the beginning (master) and the end. In between, the
additional LTC6820’s and their associated slave devices will
be connected to “stubs” on the cable. These stubs should
be kept short, with as little capacitance as possible, to avoid
degrading the termination along the cable.
The multidrop scheme is only possible if the SPI slaves
have certain characteristics:
n The SPI slaves must be addressable, because they will
all see the same CS signal (as decoded by each slave
LTC6820).
n When not addressed, the slave SDO must remain high.
When a slave is not addressed, its LTC6820 will not trans-
mit data pulses as long as MISO (the SPI device’s SDO)
remains high. This eliminates the possibility for collisions,
as only the addressed slave device will ever be returning
data to the master
.
LTC6820
19
6820fb
For more information www.linear.com/LTC6820
APPLICATIONS INFORMATION
isoSPI Setup
The LTC6820 allows each application to be optimized for
power consumption or for noise immunity. The power
and noise immunity of an isoSPI system is determined
by the programmed IB current. The IB current can range
from 0.1mA to 1mA. A low IB reduces the isoSPI power
consumption in the READY and ACTIVE states, while a
high IB increases the amplitude of the differential signal
voltage VA across the matching termination resistor, RM.
IB is programmed by the sum of the RB1 and RB2 resis-
tors connected between the IBIAS pin and GND. For most
applications setting IB to 0.5mA is a good compromise
between power consumption and noise immunity. Using
this IB setting with a 1:1 transformer and RM = 120Ω, RB1
should be set to 2.8k and RB2 set to 1.2k. In a typical CAT5
twisted pair these settings will allow for communication
up to 50m.
For applications that require cables longer than 50m it is
recommended to increase the amplitude VA by increasing
IB to 1mA. This compensates for the increased insertion
loss in the cable and maintains high noise immunity. So
when using cables over 50m and, again, using a trans-
former with a 1:1 turns ratio and RM = 120Ω, RB1 would
be 1.4k and RB2 would be 600Ω.
Other IB settings can be used to reduce power consumption
or increase the noise immunity as required by the applica-
tion. In these cases when setting VICMP and choosing RB1
and RB2 resistor values the following rules should be used:
For cables 50 meters or less:
IB = 0.5mA
VA = (20 • IB) • (RM/2)
VTCMP = 1/2 • VA
VICMP = 2 • VTCMP
RB2 = VICMP/IB
RB1 =2V
IB
– RB2
For cables over 50 meters:
IB = 1mA
VA = (20 • IB) • (RM/2)
VTCMP = 1/4 • VA
VICMP = 2 • VTCMP
RB2 = VICMP/IB
RB1 =2V
IB
– RB2
The maximum data rate of an isoSPI link is determined by
the length of the cable used. For cables 10 meters or less
the maximum 1MHz SPI clock frequency is possible. As
the length of the cable increases the maximum possible
SPI clock rate decreases. This is a result of the increased
propagation delays through the cable creating possible
timing violations.
Cable delay affects three timing specifications, tCLK, t6, and
t7. In the Electrical Characteristics table, each is derated by
100ns to allow for 50ns of cable delay. For longer cables,
the minimum timing parameters may be calculated as
shown below:
tCLK, t6, and t7 > 0.9µs + 2 • tCABLE
Pull-Up Resistance Considerations
The data output (MOSI if MSTR = 0, MISO if MSTR =
1) requires a pull-up resistor, RPU. The rise time tRISE is
determined by RPU and the capacitance on the pin. RPU
must be small enough to provide adequate setup and hold
times. For a slave device, the time constant must be less
than t12 and t14. In fast mode, 50ns is recommended.
RPU < 50ns/CLOAD
Larger pull-up resistances, up to 5k, can be used in slow
mode.
LTC6820
20
6820fb
For more information www.linear.com/LTC6820
APPLICATIONS INFORMATION
Transformer Selection Guide
As shown in Figure 1, a transformer or a pair of transform-
ers are used to isolate the IP and IM signals between the
two LTC6820’s. The isoSPI signals have programmable
pulse amplitudes up to 1.6V, and pulse widths of 50ns
and 150ns. To meet these requirements, choose a trans-
former having a magnetizing inductance ranging from
50µH to 350µH, and a 1:1 or 2:1 turns ratio. Minimizing
transformer insertion loss will reduce required transmit
power; generally an insertion loss of less than –1.5dB is
recommended.
For optimal common mode noise rejection, choose a center-
tapped transformer or a transformer with an integrated
common mode choke. The center tap can be tied to a 27pF
or smaller capacitor (larger will restrict the driver’s ability
to set the common mode voltage). If the transformer has
both a center tap and common mode choke on the primary
side, a larger capacitor may be used.
Table 7 shows a recommended list of transformers for use
with the LTC6820. 10/100BaseTX Ethernet transformers
are inexpensive and work very well in this application.
Ethernet transformers often include a common mode
choke, which will improve common mode rejection as
compared to other transformers.
Table 6. Typical RB1 and RB2 Values
MAX CABLE
LENGTH
TURNS
RATIO
TERMINATION
RESISTANCE IBVAVTCMP VICMP RB2 RB1 IDRV
READY
CURRENT
100m 1 :1 120Ω 1mA 1.2V 0.3V 0.6V 604Ω 1.4k 20mA 4.7mA
50m 1 :1 120Ω 0.5mA 0.6V 0.3V 0.6V 1.21k 2.8k 10mA 3.2mA
100m 1 :1 75Ω 1mA 0.75V 0.19V 0.38V 374Ω 1.62k 20mA 4.7mA
50m 1 :1 75Ω 0.5mA 0.375V 0.19V 0.38V 750Ω 3.24k 10mA 3.2mA
Table 7. Recommended Transformers
MANUFACTURER PART NUMBER ISOLATION VOLTAGE TURNS RATIO CENTER TAP CM CHOKE
PCA EPF8119SE 1500VRMS 1:1 Yes Yes
Halo TG110-AE050N5LF 1500VRMS 1:1 Yes Yes
Pulse PE-68386NL 1500V DC 1:1 No No
Murata 78613/3C 1000VRMS 1:1 Yes No
Murata 78604/3C 1000VRMS 2:1 No No
Pulse HX1188NL 1500VRMS 1:1 Yes Yes
EPCOS B82804A0354A110 1500V DC 1:1 No No
IP
480Ω
LTC6820
2:1 1:2
MSTR
MOSI
MISO
SCK
CS IM
µC
SDO
SDI
SCK
CS
480Ω
IP
MSTR
MOSI
MISO
SCK
CS
IM
LTC2452LTC6820
2:1 Transformers
SDO
SCK
CS
IP
120Ω
LTC6820
MSTR
MOSI
MISO
SCK
CS IM
µC
SDO
SDI
SCK
CS
120Ω
6820 F17
IP
MSTR
MOSI
MISO
SCK
CS
IM
LTC6802LTC6820
Single-Transformer Isolation
SDI
SDO
SCK
CS
Figure 17. Alternative Isolation Barriers
LTC6820
21
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For more information www.linear.com/LTC6820
APPLICATIONS INFORMATION
Capacitive Isolation Barrier
In some applications, where the environment is relatively
noise free and only galvanic isolation is required, capaci-
tors can be used in place of transformers as the isolation
barrier. With capacitive coupling, the twisted pair cable
is driven by a voltage and is subject to signal loss with
cable length. This low cost isolated solution can be suit-
able for short distance interconnections (1 meter or less),
such as between adjacent circuit boards or across a large
PCB. The capacitors will provide galvanic isolation, but no
common mode rejection. This option uses the drivers in
a different way, by using pull up resistors to maintain the
common mode near VDD, only the sinking drive current
has any effect. Figure 18 shows an example application
circuit using a capacitive isolation barrier capable of driv-
ing 1 meter of cable.
MANUFACTURER PART NUMBER CAPACITANCE
VOLTAGE
RATING
Murata GCM188R72A104KA64 100nF 100V
EMC
When using the LTC6820, for the best electromagnetic
compatibility (EMC) performance it is recommended to
use a transformer with a center tap and a common mode
choke as shown in Figure 19. The center tap of the trans-
former should be bypassed with a 27pF capacitor. The
center tap capacitor will help attenuate common mode
signals. Large center tap capacitors should be avoided as
they will prevent the isoSPI transmitters common mode
voltage from settling.
To improve common mode current rejection a common
mode choke should also be placed in series with the IP
and IM lines of the LTC6820. The common mode choke
will both increase EMI immunity and reduce EMI emission.
When choosing a common mode choke, the differential
mode impedance should be 20Ω or less for signals 50MHz
and below. Generally common mode chokes similar to
those used in Ethernet applications are recommended.
Table 8. Recommended Common Mode Chokes
MANUFACTURER PART NUMBER
DIFFERENTIAL
IMPEDANCE AT
50MHz
COMMON MODE
IMPEDANCE AT
50MHz
TDK ACT45B-220-2P 20Ω 5000Ω
LTC6820µC
MSTR
MOSI
MISO
SCK
CS
LTC2640LTC6820
SDI
IP
100nF
Capacitive Isolation
IM
MSTR
MOSI
MISO
SCK
CS
IP
IM SCK
CS
100nF
6820 F18
SDO
SDI
SCK
CS
Figure 18. Capacitive Isolation Barrier
120Ω
IP
IM
LTC6820
6820 F19
27pF
Figure 19. Connection of Transformer and Common Mode Choke
LTC6820
22
6820fb
For more information www.linear.com/LTC6820
APPLICATIONS INFORMATION
Figure 20. Example Layout
Layout of the isoSPI signal lines also plays a significant
role in maximizing the immunity of a circuit. The following
layout guidelines should be followed:
1. The transformer should be placed as close to the isoSPI
cable connector as possible. The distance should be
kept less than 2cm. The LTC6820 should be placed at
least 1cm to 2cm away from the transformer to help
isolate the IC from the magnetic coupling fields.
2. On the top layer, no ground plane should be placed
under the magnetic, the isoSPI connector, or in between
the transformer and the connector.
3. The IP and IM traces should be isolated from surround-
ing circuits. No traces should cross the IP and IM lines,
unless separated by a ground plane within the printed
circuit board.
The isoSPI drive currents are programmable and allow
for a tradeoff between power consumption and noise
immunity. The noise immunity of the LTC6820 has been
evaluated using a bulk current injection (BCI) test. The
BCI test injects current into the twisted-pair lines at set
levels over a frequency range of 1MHz to 400MHz. With
the minimum IB current, 0.1mA, the isoSPI serial link has
been shown to pass a 40mA BCI test with no bit errors.
A 40mA BCI test level is sufficient for most industrial ap-
plications. Automotive applications tend to have a higher
BCI requirement so the recommended IB is set to 1mA, the
maximum power level. The isoSPI system has been shown
to pass a 200mA BCI test with no transmitted bit errors.
The 200mA test level is typical for automotive testing.
Software Layer
The isoSPI physical layer has high immunity to EMI and
is not particularly susceptible to bit errors induced by
noise, but for best results in a high noise environment it
is recommended to implement a software layer that uses
an error detection code like a cyclic redundancy check
or check sum. Error detection codes will allow software
detection of any bit error and will notify the system to retry
the last erroneous serial communication.
1.5cm
IP
IM
1cm
CONNECTOR
6820 F20
LTC6820
23
6820fb
For more information www.linear.com/LTC6820
TYPICAL APPLICATIONS
15
HX1188NL HX1188NL
120Ω
1.21k 2.8k
100nF
2
1
3
2
1
3
14
16
15
14
16
100nF
IBIAS
ICMP
GND
SLOW
MSTR
IP
IM
EN
CS
SCK
MISO
MOSI
PHA
VDDS
POL
VDD
1µF
3V
100nF
+
–
TO SENSOR SCK
MISO
IN+
IN–CS
6820 TA02
3.6V
LTC6820
+
LT6656-3
VCC
VREF
LTC2452
2.8k
1.21k
5V
LTC6820
I
Q
SHUTDOWN = 3.7µA
120Ω
2k
IBIAS
ICMP
GND
SLOW
VDD
IP
IM
POL
MSTR
PHA
EN
MISO
MOSI
SCK
CS
VDDS
5V
Remote Sensor Monitor with Micropower Shutdown
LTC6820
24
6820fb
For more information www.linear.com/LTC6820
100 Meter Remote DAC Control
15
HX1188NL HX1188NL
120Ω
604Ω 1.4k
100nF
2
1
3
2
1
3
14
16
15
14
16
100nF
IBIAS
ICMP
GND
SLOW
MSTR
IP
IM
EN
CS
SCK
MISO
PHA
VDDS
MOSI
POL
VDD
1µF
3V
100nF
VOUT SCK
SDI
CS
6820 TA03
3.6V
LTC6820
+
LT6656-3
VCC
GND
VREF
LTC2640
1.4k
604Ω
5V
LTC6820
120Ω
2k
IBIAS
ICMP
GND
SLOW
VDD
IP
IM
POL
MSTR
PHA
EN
MISO
MOSI
SCK
CS
VDDS
3V
OUT
2k
TYPICAL APPLICATIONS
LTC6820
25
6820fb
For more information www.linear.com/LTC6820
806Ω
1.21k
IPA
IBIAS
IMA
ICMP
VM
6820 TA05
LTC6804-2
120Ω
VREG
ISOMD
A3
A2
A1
A0
VREG
ISOMD
A3
A2
A1
A0
806Ω
1.21k
IPA
IBIAS
IMA
ICMP
VM
LTC6804-2
1
2
0
806Ω
1.21k
IPA
IBIAS
IMA
ICMP
VM
LTC6804-2
VREG
ISOMD
A3
A2
A1
A0
1
2
0
100nF
100nF
806Ω
1.21k
5V
LTC6820
120Ω
2k
IBIAS
ICMP
GND
SLOW
VDD
IP
IM
POL
MSTR
PHA
EN
MISO
MOSI
SCK
CS
VDDS
5V
Interfacing to Addressable Stack of LTC6804-2 Multicell Battery Monitors
TYPICAL APPLICATIONS
LTC6820
26
6820fb
For more information www.linear.com/LTC6820
TYPICAL APPLICATIONS
120Ω 2k
100Ω
806Ω1.21k
IBIAS
ICMP
GND
SLOW
MSTR
LTC6820 GND3
GND3
LTC6803-2
V
STACK3
VREG
CZT3055
MOSI
MISO
SCK
CS
A3
A2
A1
A0
2k
100Ω
806Ω1.21k
806Ω1.21k
VDD2
VDD2
VDD1
VDD1
IBIAS
ICMP
GND
SLOW
MSTR
EN
MOSI
MISO
SCK
CS
LTC6820
LTC6803-2
GND2
GND2
GND1
GND1
GND1
GND2
GND3
VSTACK2
VREG
CZT3055
MOSI
MISO
SCK
CS
A3
A2
A1
A0
2k
100Ω
IBIAS
ICMP
GND
SLOW
MSTR
EN
MOSI
MISO
SCK
CS
LTC6820
LTC6803-2 VSTACK1
VREG
CZT3055
6820 TA04
MOSI
MISO
SCK
CS
A3
A2
A1
A0
VDD3
VDD3
100nF
100nF
806Ω
1.21k
5V
LTC6820
120Ω
2k
IBIAS
ICMP
GND
SLOW
VDD
IP
IM
POL
MSTR
PHA
EN
MISO
MOSI
SCK
CS
VDDS
5V
EN
MOSI
MISO
SCK
CS
VDDS
POL
PHA
VDDS
POL
PHA
VDDS
POL
PHA
IP
IM
VDD
IP
IM
VDD
IP
IM
VDD
2k
2k
2k
Battery Monitoring System Using a Multidrop isoSPI Link
LTC6820
27
6820fb
For more information www.linear.com/LTC6820
PACKAGE DESCRIPTION
3.00 ±0.10
(4 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
1.65 ±0.05
(4 SIDES)
NOTE:
1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WEED-4)
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
PIN 1
TOP MARK
(NOTE 6)
0.40 ±0.10
BOTTOM VIEW—EXPOSED PAD
1.65 ±0.10
(4-SIDES)
0.75 ±0.05 R = 0.115
TYP
0.25 ±0.05
1
PIN 1 NOTCH R = 0.20 TYP
OR 0.25 × 45° CHAMFER
15 16
2
0.50 BSC
0.200 REF
2.10 ±0.05
3.50 ±0.05
0.70 ±0.05
0.00 – 0.05
(UD16 VAR A) QFN 1207 REV A
0.25 ±0.05
0.50 BSC
PACKAGE OUTLINE
UD Package
16-Lead Plastic QFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1700 Rev A)
Exposed Pad Variation AA
Please refer to http://www.linear.com/product/LTC6820#packaging for the most recent package drawings.
LTC6820
28
6820fb
For more information www.linear.com/LTC6820
PACKAGE DESCRIPTION
MSOP (MS16) 0213 REV A
0.53 ±0.152
(.021 ±.006)
SEATING
PLANE
0.18
(.007)
1.10
(.043)
MAX
0.17 –0.27
(.007 – .011)
TYP
0.86
(.034)
REF
0.50
(.0197)
BSC
16151413121110
12345678
9
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
0.254
(.010) 0° – 6° TYP
DETAIL “A”
DETAIL “A”
GAUGE PLANE
5.10
(.201)
MIN
3.20 – 3.45
(.126 – .136)
0.889 ±0.127
(.035 ±.005)
RECOMMENDED SOLDER PAD LAYOUT
0.305 ±0.038
(.0120 ±.0015)
TYP
0.50
(.0197)
BSC
4.039 ±0.102
(.159 ±.004)
(NOTE 3)
0.1016 ±0.0508
(.004 ±.002)
3.00 ±0.102
(.118 ±.004)
(NOTE 4)
0.280 ±0.076
(.011 ±.003)
REF
4.90 ±0.152
(.193 ±.006)
MS Package
16-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1669 Rev A)
Please refer to http://www.linear.com/product/LTC6820#packaging for the most recent package drawings.
LTC6820
29
6820fb
For more information www.linear.com/LTC6820
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
REVISION HISTORY
REV DATE DESCRIPTION PAGE NUMBER
A 06/13 Web hyperlinks added.
Note 8 added to Electrical Characteristics section.
1 to 30
5
B 01/17 Patent Information added.
Web Links updated.
1
All
LTC6820
30
6820fb
For more information www.linear.com/LTC6820
LINEAR TECHNOLOGY CORPORATION 2012
LT 0117 REV B• PRINTED IN USA
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LTC6820
TYPICAL APPLICATION
RELATED PARTS
PART NUMBER DESCRIPTION COMMENTS
LTC6803-2/
LTC6803-4
Multicell Battery Stack Monitor with an Individually
Addressable SPI Interface
Functionality Equivalent to LTC6803-1/LTC6803-3, Allows for Parallel
Communication Battery Stack Topologies
LTC6803-1/
LTC6803-3
Multicell Battery Stack Monitor with Daisy-Chained
SPI Interface
Functionality Equivalent to LTC6803-2/LTC6803-4, Allows for Multiple
Devices to Be Daisy Chained
LTC6903 1kHz to 68MHz Programmable Silicon Oscillator with
SPI Interface
Frequency Resolution of 0.01%. No External Components Required.
Operates on 2.7V to 5.5V.
LTC6804-1/
LTC6804-2
Multicell Battery Stack Monitor with Built-In isoSPI
Interface
Includes isoSPI Interfaces for Communication with Master LTC6820 and to
other LTC6804 Devices
806Ω
1.21k
IPA
IBIAS
IMA
ICMP
VM
6820 TA06
LTC6804-1
120Ω
120Ω
120Ω
VREG
ISOMD
806Ω
1.21k
IPA
IBIAS
IMA
ICMP
VM
GND3
GND2
IPB
IMB
IPB
IMB
LTC6804-1
120Ω
120Ω
VREG
ISOMD
806Ω
1.21k
IPA
IBIAS
IMA
ICMP
VM
GND4
IPB
IMB
LTC6804-1 VREG
ISOMD
100nF
100nF
806Ω
1.21k
5V
LTC6820
120Ω
2k
IBIAS
ICMP
GND
SLOW
VDD
IP
IM
POL
MSTR
PHA
EN
MISO
MOSI
SCK
CS
VDDS
5V
Interfacing to Daisy-Chained Stack of LTC6804-1 Multicell Battery Monitors