SN888C
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Bus-Polarity Correcting RS-485 Transceiver for E-Meters
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FEATURES
APPLICATIONS
•
•
•
•
•
1
•
•
•
Exceeds Requirements of EIA-485 Standard
Bus-Polarity Correction within 76 ms
Data Rate: 300 bps to 250 kbps
Works with Two Configurations:
– Failsafe Resistors Only
– Failsafe and Termination Resistors
Up to 256 Nodes on a Bus
SOIC-8 Package for Backward Compatibility
Bus-Pin Protection:
– ±16 kV HBM protection
– ±12 kV IEC61000-4-2 Contact Discharge
– +4 kV IEC61000-4-4 Fast Transient Burst
R
The SN888C is a low-power RS-485 transceiver with
bus-polarity correction and transient protection. Upon
hot plug-in the device detects and corrects the bus
polarity within the first 76 ms of bus idling. On-chip
transient protection protects the device against
IEC61000 ESD and EFT transients.
The SN888C is available in an SOIC-8 package. The
device is characterized from –40°C to 85°C.
DE
A
120
B
1k
D
POLCOR
A
B
R
R
1k
RE
D
DESCRIPTION
Cross-wire
fault 0
Vcc
R
E-meters
A
B
A
D
B
Master
SN65HVD82
RE
DE
D
Slave
SN888C
R
R
D
D
POLCOR
R
RE DE
Slave
SN888C
POLCOR
D
R
RE DE
D
Slave
SN888C
Figure 1. Typical Network Application With Polarity Correction (POLCOR)
1
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 2013, Texas Instruments Incorporated
SN888C
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These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
SOIC-8
(TOP VIEW)
1
2
3
4
8
7
6
5
Vcc
B
A
GND
R
RE
DE
D
Vcc
POLCOR
R
RE
DE
D
Block Diagram
B
A
GND
DRIVER PIN FUNCTIONS
INPUT
ENABLE
D
DE
OUTPUTS
A
DESCRIPTION
B
NORMAL MODE
H
H
H
L
Actively drives bus high
L
H
L
H
Actively drives bus low
X
L
Z
Z
Driver disabled
X
OPEN
Z
Z
Driver disabled by default
OPEN
H
H
L
Actively drives bus high
POLARITY-CORRECTING MODE (1)
(1)
H
H
L
H
Actively drives bus low
L
H
H
L
Actively drives bus high
X
L
Z
Z
Driver disabled
X
OPEN
Z
Z
Driver disabled by default
OPEN
H
L
H
Actively drives bus low
The polarity-correcting mode is entered when VID < VIT– and t > tFS and DE = low. This state is latched when /RE turns from low to high.
RECEIVER PIN FUNCTIONS
DIFFERENTIAL
INPUT
ENABLE
OUTPUT
VID = VA – VB
/RE
R
VIT+ < VID
L
H
Receive valid bus high
VIT– < VID < VIT+
L
?
Indeterminate bus state
VID < VIT–
L
L
Receive valid bus low
X
H
Z
Receiver disabled
X
OPEN
Z
Receiver disabled
Open, short, idle bus
L
?
Indeterminate bus state
DESCRIPTION
NORMAL MODE
POLARITY-CORRECTING MODE (1)
VIT+ < VID
L
L
Receive valid bus low
VIT– < VID < VIT+
L
?
Indeterminate bus state
VID < VIT–
L
H
Receive polarity corrected bus high
X
H
Z
Receiver disabled
X
OPEN
Z
Receiver disabled
Open, short, idle bus
L
?
Indeterminate bus state
(1)
2
The polarity-correcting mode is entered when VID < VIT– and t > tFS and DE = low. This state is latched when /RE turns from low to high.
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ABSOLUTE MAXIMUM RATINGS (1)
VALUE
UNIT
MIN
VCC
MAX
Supply voltage
–0.5
7
Input voltage range at any logic pin
–0.3
5.7
Voltage input range, transient pulse, A and B, through 100 Ω
–100
100
Voltage range at A or B inputs
–18
18
Receiver output current
–24
24
Continuous total-power dissipation
±12
IEC 61000-4-4 EFT (Fast transient or burst) bus terminals and GND
±4
IEC 60749-26 ESD (HBM), bus terminals and GND
±16
Test Method A114 (HBM), all pins
TJ
Junction temperature
TSTG
Storage temperature
(1)
mA
See THERMAL INFORMATION table
IEC 61000-4-2 ESD (Contact Discharge), bus terminals and GND
JEDEC Standard 22
V
kV
±8
Test Method C101 (Charged Device Model), all pins
±1.5
Test Method A115 (Machine Model), all pins
±200
V
170
–65
°C
150
Stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. These are stress ratings
only and functional operation of the device at these or any other conditions beyond those indicated under recommended operating
conditions is not implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
THERMAL INFORMATION
SN888C
THERMAL METRIC (1)
PACKAGE SOIC
(D)
θJA
Junction-to-ambient thermal resistance
116.1
θJCtop
Junction-to-case (top) thermal resistance
60.8
θJB
Junction-to-board thermal resistance (2)
57.1
ψJT
Junction-to-top characterization parameter
13.9
ψJB
Junction-to-board characterization parameter
56.5
θJCbot
Junction-to-case (bottom) thermal resistance
NA
(1)
(2)
UNITS
°C/W
For more information about traditional and new thermal metrics, see the IC Package Thermal Metrics application report, SPRA953.
The junction-to-board thermal resistance is obtained by simulating in an environment with a ring cold plate fixture to control the PCB
temperature, as described in JESD51-8.
POWER DISSIPATION
PARAMETER
PD
TEST CONDITIONS
Unterminated
Power dissipation
Driver and receiver enabled,
VCC = 5.5 V, TJ = 150°C
RS-422 load
50% duty cycle square-wave signal at
250-kbps signaling rate:
RS-485 load
VALUE
RL = 300 Ω,
CL = 50 pF (driver)
164
RL = 100 Ω,
CL = 50 pF (driver)
247
RL = 54 Ω,
CL = 50 pF (driver)
316
UNITS
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RECOMMENDED OPERATING CONDITIONS
MIN
NOM
MAX
VCC
Supply voltage
4.5
5
VID
Differential input voltage
–12
12
VI
Input voltage at any bus terminal (separate or common mode) (1)
–7
12
VIH
High-level input voltage (driver, driver-enable, and receiver-enable inputs)
2
VCC
VIL
Low-level input voltage (driver, driver-enable, and receiver-enable inputs)
0
0.8
Driver
–60
60
Receiver
–8
8
UNIT
5.5
V
IO
Output current
CL
Differential load capacitance
50
pF
RL
Differential load resistance
60
Ω
1/tUI
Signaling rate
0.3
250
TJ
Junction temperature
–40
150
TA (2)
Operating free-air temperature (see THERMAL INFORMATION for additional
information)
–40
85
(1)
(2)
mA
kbps
°C
The algebraic convention in which the least positive (most negative) limit is designated as minimum is used in this data sheet.
Operation is specified for internal (junction) temperatures up to 150°C. Self-heating due to internal power dissipation should be
considered for each application. Maximum junction temperature is internally limited by the thermal shut-down (TSD) circuit which
disables the driver outputs when the junction temperature reaches 170°C.
ELECTRICAL CHARACTERISTICS
over operating free-air temperature range (unless otherwise noted)
PARAMETER
│VOD│
Driver differential-output
voltage magnitude
TEST CONDITIONS
RL = 60 Ω, 375 Ω on each
output from –7 to +12 V
RL = 54 Ω (RS-485)
RL = 100 Ω (RS-422)
See Figure 2
See Figure 3
RL = 54 Ω, CL = 50 pF
MIN
TYP
1.5
2.5
1.5
2.5
2
3
–0.2
0
Δ│VOD
│
Change in magnitude of
driver differential-output
voltage
VOC(SS)
Steady-state common-mode
output voltage
ΔVOC
Change in differential driver
common-mode output
voltage
VOC(PP)
Peak-to-peak driver commonmode output voltage
COD
Differential output
capacitance
8
Positive-going receiver
differential-input voltage
threshold
35
VIT+
VIT–
Negative-going receiver
differential-input voltage
threshold
VHYS (1)
Receiver differential-input
voltage threshold hysteresis
(VIT+ – VIT– )
VOH
Receiver high-level output
voltage
IOH = –8 mA
VOL
Receiver low-level output
voltage
IOL = 8 mA
II
Driver input, driver enable,
and receiver enable input
current
(1)
4
MAX
V
0.2
See Figure 3
Center of two 27-Ω load
resistors
UNIT
V
1
VCC / 2
3
–0.2
0
0.2
V
See Figure 3
mV
850
pF
100
mV
–100
–35
mV
40
60
mV
2.4
VCC – 0.3
0.2
–2
V
0.4
V
2
µA
Under any specific conditions, VIT+ is ensured to be at least VHYS higher than VIT–.
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ELECTRICAL CHARACTERISTICS (continued)
over operating free-air temperature range (unless otherwise noted)
PARAMETER
TEST CONDITIONS
MIN
TYP
IOZ
Receiver high-impedance
output current
VO = 0 V or VCC, /RE at VCC
│IOS│
Driver short-circuit output
current
│IOS│ with VA or VB from –7 to +12 V
II
Bus input current (driver
disabled)
VCC = 4.5 to 5.5 V or
VI = 12 V
VCC = 0 V, DE at 0 V
VI = –7 V
Driver and receiver enabled
DE = VCC, /RE =
GND, No load
Driver enabled, receiver
disabled
DE = VCC, /RE = VCC,
No load
650
Driver disabled, receiver
enabled
DE = GND, /RE =
GND, No load
750
Driver and receiver disabled
DE = GND, D = GND
/RE = VCC, No load
ICC
Supply current (quiescent)
Supply current (dynamic)
–10
MAX
10
150
75
–100
125
–40
750
UNIT
µA
mA
µA
900
0.4
5
MIN
TYP
MAX
400
700
1200
90
700
1000
25
200
µA
See
SWITCHING CHARACTERISTICS
3.3 ms > bit time > 4 μs (unless otherwise noted)
PARAMETER
TEST CONDITIONS
UNIT
DRIVER
tr, tf
Driver differential-output rise and
fall times
tPHL, tPLH
Driver propagation delay
tSK(P)
Driver pulse skew, |tPHL – tPLH|
tPHZ, tPLZ
Driver disable time
tPHZ, tPLZ
Driver enable time
RL = 54 Ω, CL = 50
pF
Receiver enabled
See Figure 4
See Figure 5 and
Figure 6
Receiver disabled
50
500
500
1000
3
9
ns
ns
µs
RECEIVER
tr, tf
Receiver output rise and fall times
tPHL, tPLH
Receiver propagation delay time
tSK(P)
Receiver pulse skew, |tPHL – tPLH|
1
15
tPHZ, tPLZ
Receiver disable time
50
500
tPZL(1),
tPZH(1)
tPZL(2),
tPZH(2)
tFS
CL = 15 pF
See Figure 7
18
30
85
195
ns
Driver enabled
See Figure 8
20
130
ns
Receiver enable time
Driver disabled
See Figure 9
2
8
µs
Bus failsafe time
Driver disabled
See Figure 10
58
76
ms
44
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PARAMETER MEASUREMENT INFORMATION
DRIVER
375
Vcc
DE
A
D
0V or 5 V
VOD
-7V < Vtest < 12 V
60
B
375
Figure 2. Measurement of Driver Differential-Output Voltage With Common-Mode Load
A
0V or 5 V
RL/2
A
D
VA
B
VB
VOD
RL/2
B
VOC(PP)
CL VOC
ûVOC(SS)
VOC
Figure 3. Measurement of Driver Differential and Common-Mode Output With RS-485 Load
5V
Vcc
DE
A
D
Input
Generator
VI
VOD
50
50%
VI
54
B
CL=
50 pF
0V
tPLH
tPHL
90%
50%
10%
VOD
tr
§2V
§-2V
tf
Figure 4. Measurement of Driver Differential-Output Rise and Fall Times and Propagation Delays
A
D
VI
50%
VI
B
DE
Input
Generator
5V
VO
S1
RL=
110
CL=
50 pF
50
0V
tPZH
90%
VO
VOH
50%
§0V
tPHZ
Figure 5. Measurement of Driver Enable and Disable Times With Active-High Output and Pull-Down Load
5V
5V
D
DE
Input
Generator
VI
A
RL= 110
S1
B
50
CL=
50 pF
VO
50%
VI
0V
tPZL
VO
tPLZ
§5V
50%
10%
VOL
Figure 6. Measurement of Driver Enable and Disable Times With Active-Low Output and Pull-Up Load
6
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PARAMETER MEASUREMENT INFORMATION (continued)
RECEIVER
5V
A
Input
Generator
VI
50 1.5V
R VO
0V
tPLH
B
tPHL
90%
50%
10%
CL= 15 pF
RE
0V
50%
VI
VOD
tr
VOH
VOL
tf
Figure 7. Measurement of Receiver Output Rise and Fall Times and Propagation Delays
Vcc
Vcc
DE
0V or 5 V
B
Input
Generator
50%
0V
A
D
5V
VI
R VO
S1
VO
CL= 15 pF
RE
tPHZ
tPZH(1)
1 k
§0V
tPZL(1)
50
VI
D at 5V
S1 to GND
VOH
90%
50%
tPLZ
VO
50%
D at 0V
S1 to VCC
VCC
10%
VOL
Figure 8. Measurement of Receiver Enable and Disable Times With Driver Enabled
5V
Vcc
1.5 V or 0 V
Input
Generator
B
50%
0V
A
0V or 1.5 V
VI
R VO
RE
tPZH(2)
1 k
VOH
S1
VO
CL= 15 pF
A at 1.5V
B at 0V
S1 to GND
50%
§0V
tPZL(2)
VI
VCC
50
VO
A at 0V
B at 1.5V
S1 to VCC
50%
VOL
Figure 9. Measurement of Receiver Enable Times With Driver Disabled
VI
A
Input
Generator
VI
50 1.5V
B
RE
0V
(DE = Low)
R
50%
0V
VO
tPHL
tFS
10 k
VCC
VO
50%
Figure 10. Measurement of Receiver Polarity-Correction Time With Driver Disabled
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EQUIVALENT INPUT AND OUTPUT SCHEMATIC DIAGRAMS
D and RE Inputs
DE Input
Vcc
Vcc
Vcc
R Output
100k
1k
1k
D,RE
1k
DE
9V
R
9V
100k
9V
Vcc
Receiver Inputs
Driver Outputs
Vcc
16V
16V
R2
R2
R1
A
A
R
R1
B
B
16V
8
R3
R3
16V
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DEVICE INFORMATION
Low-Power Standby Mode
When the driver and the receiver are both disabled (DE = low and RE = high) the device enters standby mode. If
the enable inputs are in the disabled state for only a brief time (for example: less than 100 ns), the device does
not enter standby mode, preventing the SN888C device from entering standby mode during driver or receiver
enabling. Only when the enable inputs are held in the disabled state for a duration of 300 ns or more does the
device enter low-power standby mode. In this mode most internal circuitry is powered down, and the steady-state
supply current is typically less than 400 nA. When either the driver or the receiver is re-enabled, the internal
circuitry becomes active. During VCC power-up, when the device is set for both driver and receiver disabled
mode, the device may consume more than 5-µA of ICC disabled current because of capacitance charging
effects. This condition occurs only during VCC power-up.
Bus Polarity Correction
The SN888C device automatically corrects a wrong bus-signal polarity caused by a cross-wire fault. In order to
detect the bus polarity, all three of the following conditions must be met:
• A failsafe-biasing network (commonly at the master node) must define the signal polarity of the bus.
• A slave node must enable the receiver and disable the driver (/RE = DE = low).
• The bus must idle for the failsafe time, tFS-max.
After the failsafe time has passed, the polarity correction is complete and applied to both the receive and transmit
channels. The status of the bus polarity latches within the transceiver and maintains for subsequent data
transmissions.
NOTE
Avoid data string durations of consecutive 0s or 1s exceeding tFS-min, which can accidently
trigger a wrong polarity correction.
Figure 11 shows a simple point-to-point data link between a master node and a slave node. Because the master
node with the failsafe biasing network determines the signal polarity on the bus, an RS-485 transceiver without
polarity correction, such as SN65HVD82, suffices. All other bus nodes, typically performing as slaves, require the
SN888C transceiver with polarity correction.
VS-Master
Master
node
Vdd
VS-Master
VS-Slave
VSM
Vcc
RxD
DIR
TxD
DGND
Vdd
R
RFS
A
RE
DE
B
D
VS-Slave
Vcc
A
RT
RT
(opt.)
(opt.)
POLCOR
MCU
R
Slave
node
B
GND
GND
MCU
RE
DE
D
RFS
RxD
DIR
TxD
DGND
Figure 11. Point-To-Point Data Link With Cross-Wire Fault
Prior to initiating data transmission the master transceiver must idle for a time span that exceeds the maximum
failsafe time, tFS-max, of a slave transceiver. To accomplish this idle time, drive the direction control line, DIR, low.
After a time, t > tFS-max, the master begins transmitting data.
Because of the indicated cross-wire fault between master and slave, the slave node receives bus signals with
reversed polarity. Assuming the slave node has just been connected to the bus, the direction-control pin is
pulled-down during power-up, and then is actively driven low by the slave MCU. The polarity correction begins as
soon as the slave supply is established and ends after approximately 44 to 76 ms.
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DIRm
Master
signals
Low due to pull-down
or actively driven
high Z
Dm
VAm
VFS
0V
-Vod
+Vod
+Vid
-Vid
VBm
VBs
VFS
0V
VAs
VSs
Slave
signals
DIRs
Low due to pull-down and then actively driven
Rs
tFS
Uncorrected R output:
R is in phase with
wrong VID polarity
Corrected R output:
R is reversed to
wrong VID polarity
Figure 12. Polarity Correction Timing Prior to a Data Transmission
Initially the slave receiver assumes that the correct bus polarity is applied to the inputs and performs no polarity
reversal. Because of the reversed polarity of the bus-failsafe voltage, the output of the slave receiver, RS, turns
low. After tFS has passed and the receiver has detected the wrong bus polarity, the internal POLCOR logic
reverses the input signal and RS turns high.
At this point, all incoming bus data with reversed polarity are polarity-corrected within the transceiver. Because
polarity correction is also applied to the transmit path, the data sent by the slave MCU are reversed by the
POLCOR logic, then fed into the driver.
The reversed data from the slave MCU are reversed again by the cross-wire fault in the bus, and the correct bus
polarity is reestablished at the master end.
This process repeats each time the device powers up and detects an incorrect bus polarity.
10
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APPLICATION INFORMATION
Device Configuration
The SN888C device is a half-duplex RS-485 transceiver operating from a single 5-V ±10% supply. The driver
and receiver enable pins that allow for the configuration of different operating modes.
DE
D
D
B
RE
A
DE
D
GND
R
D
R
Vcc
B
RE
A
DE
D
GND
Vcc
B
A
D
GND
c) Receiver always on
b) Combined enable signals for
use as directional control pin
a) Independent driver and
receiver enable signals
R
POLCOR
RE
R
Vcc
POLCOR
R
POLCOR
R
Figure 13. Transceiver Configurations
Using independent enable lines provides the most flexible control as the lines allow for the driver and the
receiver to be turned on and off individually. While this configuration requires two control lines, it allows for
selective listening to the bus traffic, whether the driver is transmitting data or not. Only this configuration allows
the SN888C device to enter low-power standby mode because it allows both the driver and receiver to be
disabled simultaneously.
Combining the enable signals simplifies the interface to the controller by forming a single direction-control signal.
Thus, when the direction-control line is high, the transceiver is configured as a driver, while when low, the device
operates as a receiver.
Tying the receiver enable to ground and controlling only the driver-enable input also uses only one control line. In
this configuration, a node not only receives the data on the bus sent by other nodes, but also receives the data
sent on the bus, enabling the node to verify the correct data has been transmitted.
Bus Design
An RS-485 bus consists of multiple transceivers connected in parallel to a bus cable. To eliminate line
reflections, each cable end is terminated with a termination resistor, RT, whose value matches the characteristic
impedance, Z0, of the cable. This method, known as parallel termination, allows for relatively high data rates over
long cable length.
Common cables used are unshielded twisted pair (UTP), such as low-cost CAT-5 cable with Z0 = 100 Ω, and
RS-485 cable with Z0 = 120 Ω. Typical cable sizes are AWG 22 and AWG 24.
The maximum bus length is typically given as 4000 ft or 1200 m, and represents the length of an AWG 24 cable
whose cable resistance approaches the value of the termination resistance, thus reducing the bus signal by half
or 6 dB. Actual maximum usable cable length depends on the signaling rate, cable characteristics, and
environmental conditions.
Table 1. VID With a Failsafe Network and Bus Termination
VCC
5V
RL Differential
Termination
54 Ω
RFS Pull-Up
RFS Pull-Down
VID
560 Ω
560 Ω
230 mV
1 KΩ
1 KΩ
131 mV
4.7 KΩ
4.7 KΩ
29 mV
10 KΩ
10 KΩ
13 mV
An external failsafe-resistor network must be used to ensure failsafe operation during an idle bus state. When the
bus is not actively driven, the differential receiver inputs could float allowing the receiver output to assume a
random output. A proper failsafe network forces the receiver inputs to exceed the VIT threshold, thus forcing the
SN888C receiver output into the failsafe (high) state. Table 1 shows the differential input voltage (VID) for various
failsafe networks with a 54-Ω differential bus termination.
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Cable Length Versus Data Rate
There is an inverse relationship between data rate and cable length, which means the higher the data rate, the
shorter the cable length; and conversely, the lower the data rate, the longer the cable length. While most RS-485
systems use data rates between 10 kbps and 100 kbps, applications such as e-metering often operate at rates of
up to 250 kbps even at distances of 4000 ft and longer. Longer distances are possible by allowing for small
signal jitter of up to 5 or 10%.
CABLE LENGTH (FT)
vs
DATA RATE (BPS)
10000
CABLE LENGTH - ft
5,10,20 % Jitter
1000
Conservative
Characteristics
100
10
100
1k
10k
100k
1M
10M
100M
DATA RATE - bps
Stub Length
When connecting a node to the bus, the distance between the transceiver inputs and the cable trunk, known as
the stub, should be as short as possible. The reason for the short distance is because a stub presents a nonterminated piece of bus line, which can introduce reflections if the distance is too long. As a general guideline,
the electrical length or round-trip delay of a stub should be less than one-tenth of the rise time of the driver, thus
leading to a maximum physical stub length as shown in Equation 1.
LStub ≤ 0.1 × tr × v × c
where
•
•
•
tr is the 10 / 90 rise time of the driver
c is the speed of light (3 × 108 m/s or 9.8 × 108 ft/s)
v is the signal velocity of the cable (v = 78%) or trace (v = 45%) as a factor of c
(1)
Based on Equation 1, with a minimum rise time of 400 ns, Equation 2 shows the maximum cable-stub length of
the SN888C device.
LStub ≤ 0.1 × 400 × 10-9 × 3 × 108 × 0.78 = 9,4 m (or 30.6 ft)
(2)
LS
A
B
R
D
R RE DE D
Figure 14. Stub Length
12
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3-V to 5-V Interface
Interfacing the SN888C device to a 3-V controller is easy. Because the 5-V logic inputs of the transceiver accept
3-V input signals, they can be directly connected to the controller I/O. The 5-V receiver output, R, however, must
be level-shifted by a Schottky diode and a 10-k resistor to connect to the controller input (see Figure 15). When
R is high, the diode is reverse biased and the controller supply potential lies at the controller RxD input. When R
is low, the diode is forward biased and conducts. Only in this case, the diode forward voltage of 0.2 V lies at the
controller RxD input.
3.3V
10k
5V
BAS70
MCU
RxD
1
RCV
2
RE
DRV
3
DE
TxD
4
D
Vcc
8
A
7
B
6
GND
5
R
XCVR
0.1µF
Figure 15. 3-V to 5-V Interface
Noise Immunity
The input sensitivity of a standard RS-485 transceiver is ±200 mV. When the differential input voltage, VID, is
greater than +200 mV, the receiver output turns high, for VID < –200 mV the receiver outputs low.
The SN888C transceiver implements high receiver noise-immunity by providing a typical positive-going input
threshold of 35 mV and a minimum hysteresis of 40 mV. In the case of a noisy input condition, a differential
noise voltage of up to 40 mVPP can be present without causing the receiver output to change states from high to
low.
Transient Protection
The bus terminals of the SN888C transceiver family possess on-chip ESD protection against ±16 kV HBM and
±12 kV IEC61000-4-2 contact discharge. The International Electrotechnical Commision (IEC) ESD test is far
more severe than the HBM ESD test. The 50% higher charge capacitance, CS, and 78% lower discharge
resistance, RD of the IEC model produce significantly higher discharge currents than the HBM model.
High-Voltage
Pulse
Generator
RC
RD
50M
(1M)
330Ω
(1.5k)
CS
150pF
(100pF)
Device
Under
Test
Current - A
As stated in the IEC 61000-4-2 standard, contact discharge is the preferred transient protection test method.
Although IEC air-gap testing is less repeatable than contact testing, air discharge protection levels are inferred
from the contact discharge test results.
40
35
30
25
20
15
10
5
0
10kV IEC
10kV HBM
0
50
100
150
200
250
300
Time - ns
Figure 16. HBM and IEC-ESD Models and Currents in Comparison (HBM Values in Parenthesis)
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The on-chip implementation of IEC ESD protection significantly increases the robustness of equipment. Common
discharge events occur because of human contact with connectors and cables. Designers may choose to
implement protection against longer duration transients, typically referred to as surge transients. Figure 10
suggests two circuit designs providing protection against short and long-duration surge transients, in addition to
ESD and Electrical Fast Transients (EFT) transients. Table 2 lists the bill of materials for the external protection
devices.
EFTs are generally caused by relay-contact bounce, or the interruption of inductive loads. Surge transients often
result from lightning strikes (direct strike or an indirect strike which induces voltages and currents), or the
switching of power systems, including load changes and short circuits switching. These transients are often
encountered in industrial environments, such as in factory automation and power-grid systems.
Figure 17 compares the pulse-power of the EFT and surge transients with the power caused by an IEC ESD
transient. In the diagram on the left of Figure 17, the tiny blue blip in the bottom left corner represents the power
of a 10-kV ESD transient, which is low compared to the significantly higher EFT power spike, and certainly lower
than the 500-V surge transient. This type of transient power is well representative of factory environments in
industrial and process automation. The diagram on the right of Figure 17 compares the enormous power of a 6kV surge transient, most likely occurring in e-metering applications of power generating and power grid systems,
with the aforementioned 500-V surge transient.
22
20
18
16
14
12
10
8
6
4
2
0
Pulse Power - MW
Pulse Power - kW
NOTE
The unit of the pulse-power changes from kW to MW, thus making the power of the 500-V
surge transient almost disappear from the scale.
0.5kV Surge
4kV EFT
10kV ESD
0
5
10
15
20
25
30
35
40
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
6kV Surge
0.5kV Surge
0
5
10
15
20
25
30
35
40
Time - μs
Time - μs
Figure 17. Power Comparison of ESD, EFT, and Surge Transients
In the case of surge transients, hgih-energy content is signified by long pulse duration and slow-decaying pulse
power
The electrical energy of a transient that is dumped into the internal protection cells of the transceiver is converted
into thermal energy. This thermal energy heats the protection cells and literally destroys them, thus destroying
the transceiver. Figure 18 shows the large differences in transient energies for single ESD, EFT, and surge
transients as well as for an EFT pulse train, commonly applied during compliance testing.
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1000
100
Surge
Pulse Energy - Joule
10
1
EFT Pulse Train
0.1
0.01
EFT
10-3
10-4
ESD
10-5
10-6
0.5
1
2
4
6
8 10
15
Peak Pulse Voltage - kV
Figure 18. Comparison of Transient Energies
Table 2. Bill of Materials
Device
Function
Order Number
Manufacturer
XCVR
5-V, 250-kbps RS-485 Transceiver
SN888C
R1, R2
10-Ω, Pulse-Proof Thick-Film Resistor
CRCW0603010RJNEAHP
Vishay
TVS
Bidirectional 400-W Transient Suppressor
CDSOT23-SM712
Bourns
TBU1, TBU2
Bidirectional.
TBU-CA-065-200-WH
Bourns
MOV1, MOV2
200mA Transient Blocking Unit 200-V, MetalOxide Varistor
MOV-10D201K
Bourns
Vcc
Vcc
Vcc
10k
1
R
2
RE
DIR
3
DE
TxD
4
D
RxD
MCU
TI
Vcc
8
A
7
B
6
GND
5
XCVR
0.1μF
R1
1
R
2
RE
DIR
3
DE
TxD
4
D
RxD
TVS
MCU
Vcc
8
A
7
B
6
GND
5
XCVR
R2
10k
Vcc
10k
0.1μF
R1
TBU1
MOV1
TVS
MOV2
R2
10k
TBU2
Figure 19. Transient Protections Against ESD, EFT, and Surge Transients
The left circuit shown in Figure 19 provides surge protection of ≥ 500-V transients, while the right protection
circuits can withstand surge transients of 5 kV.
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Design and Layout Considerations for Transient Protection
Because ESD and EFT transients have a wide frequency bandwidth from approximately 3 MHz to 3 GHz, highfrequency layout techniques must be applied during PCB design.
In order for PCB design to be successful, begin with the design of the protection circuit in mind.
1. Place the protection circuitry close to the bus connector to prevent noise transients from penetrating your
board.
2. Use Vcc and ground planes to provide low-inductance. Note that high-frequency currents follow the path of
least inductance, not the path of least impedance.
3. Design the protection components into the direction of the signal path. Do not force the transients currents to
divert from the signal path to reach the protection device.
4. Apply 100-NF to 220-nF bypass capacitors as close as possible to the VCC-pins of transceiver, UART,
controller ICs on the board.
5. Use at least two vias for VCC and ground connections of bypass capacitors and protection devices to
minimize effective via-inductance.
6. Use 1-k to 10-k pull-up or pull-down resistors for enable lines to limit noise currents in these lines during
transient events.
7. Insert pulse-proof resistors into the A and B bus lines, if the TVS clamping voltage is higher than the
specified maximum voltage of the transceiver bus terminals. These resistors limit the residual clamping
current into the transceiver and prevent it from latching up.
– While pure TVS protection is sufficient for surge transients up to 1 kV, higher transients require metaloxide varistors (MOVs), which reduce the transients to a few-hundred volts of clamping voltage, and
transient blocking units (TBUs) that limit transient current to 200 mA.
16
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Isolated Bus Node Design
Many RS-485 networks use isolated bus nodes to prevent the creation of unintended ground loops and their
disruptive impact on signal integrity. An isolated bus node typically includes a micro controller that connects to
the bus transceiver through a multi-channel, digital isolator (Figure 20).
0.1μF
2
Vcc D2 3
1:1.33 MBR0520L
SN6501
GND D1
N
3
1
10μF
IN
OUT
1
3.3VISO
TLV70733
10μF 0.1μF
4,5
L1
4
EN
GND
2
10μF
7
R1
6
R2
MBR0520L
ISO-BARRIER
3.3V
0.1μF
PSU
0.1μF
PE
0.1μF
4.7k
PE
2
DVcc
5
6
XOUT
XIN
UCA0RXD
P3.0
MSP430
F2132
DVss
P3.1
UCA0TXD
4
16
11
12
15
1
16
Vcc1
Vcc2
7
10
EN1 ISO7241 EN2
6
11
OUTD
IND
3
14
INA
OUTA
4
13
INB
OUTB
5
12
INC
OUTC
GND1
GND2
2,8
0.1μF
4.7k
1
2
3
4
8
Vcc
R
RE
DE
B
SN888C
A
D GND2
5
TVS
9,15
R HV
Short thick earth wire or chassis
Protective Earth Ground,
Equipment Safety Ground
Floating RS-485 Common
A.
C HV
PE
island
R1,R2, TVS: see A. below
RHV = 1MΩ, 2kV high-voltage resistor, TT electronics, HVC 2010 1M0 G T3
CHV = 4.7nF, 2kV high-voltage capacitor, NOVACAP, 1812 B 472 K 202 N T
See Table 2.
Figure 20. Isolated Bus Node With Transient Protection
Power isolation is accomplished using the push-pull transformer driver SN6501 and a low-cost LDO, TLV70733.
Signal isolation uses the quadruple digital isolator ISO7241. Notice that both enable inputs, EN1 and EN2, are
pulled-up via 4.7-k resistors to limit input currents during transient events.
While the transient protection is similar to the one in Figure 19 (left circuit), an additional high-voltage capacitor
diverts transient energy from the floating RS-485 common further towards protective earth (PE) ground. This
diversion is necessary as noise transients on the bus are usually referred to earth potential.
RVH refers to a high-voltage resistor, and in some applications, even a varistor. This resistance is applied to
prevent charging of the floating ground to dangerous potentials during normal operation.
Occasionally varistors are used instead of resistors in order to rapidly discharge CHV, if expecting that fast
transients might charge CHV to high-potentials.
Note that the PE island represents a copper island on the PCB for the provision of a short, thick earth wire
connecting this island to PE ground at the entrance of the power supply unit (PSU).
In equipment designs using a chassis, the PE connection is usually provided through the chassis itself. Typically
the PE conductor is tied to the chassis at one end, while the high-voltage components, CHV and RHV, connect to
the chassis at the other end.
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PACKAGE OPTION ADDENDUM
www.ti.com
10-Dec-2020
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
SN888CD
ACTIVE
SOIC
D
8
75
RoHS & Green
NIPDAU
Level-1-260C-UNLIM
-40 to 85
RS485N
EESA
SN888CDR
ACTIVE
SOIC
D
8
2500
RoHS & Green
NIPDAU
Level-1-260C-UNLIM
-40 to 85
RS485N
EESA
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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