LINER LTC486 Quad low power rs485 driver Datasheet

LTC486
Quad Low Power
RS485 Driver
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FEATURES
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DESCRIPTIO
Very Low Power: ICC = 110µA Typ
Designed for RS485 or RS422 Applications
Single 5V Supply
– 7V to 12V Bus Common-Mode Range Permits
± 7V GND Difference Between Devices on the Bus
Thermal Shutdown Protection
Power-Up/Down Glitch-Free Driver Outputs Permit
Live Insertion/Removal of Package
Driver Maintains High Impedance in Three-State or
with the Power Off
28ns Typical Driver Propagation Delays
with 5ns Skew
Pin Compatible with the SN75172, DS96172,
µA96172, and DS96F172
The LTC486 is a low power differential bus/line driver
designed for multipoint data transmission standard RS485
applications with extended common-mode range (12V to
–7V). It also meets RS422 requirements.
The CMOS design offers significant power savings over its
bipolar counterpart without sacrificing ruggedness against
overload or ESD damage.
The driver features three-state outputs, with the driver
outputs maintaining high impedance over the entire
common-mode range. Excessive power dissipation caused
by bus contention or faults is prevented by a thermal
shutdown circuit which forces the driver outputs into a
high impedance state.
Both AC and DC specifications are guaranteed from 0°C to
70°C (Commercial), – 40°C to 85°C (Industrial), over the
4.75V to 5.25V supply voltage range.
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APPLICATI
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S
Low Power RS485/RS422 Drivers
Level Translator
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TYPICAL APPLICATI
RS485 Cable Length Specification*
EN
EN
4
DI
1
DRIVER
12
2
120Ω
120Ω
4000 FT BELDEN 9841
4
RECEIVER
1
3
RO
CABLE LENGTH (FT)
10k
1k
100
1/4 LTC488
1/4 LTC486
EN
LTC486 • TA01
10
10k
100k
1M
2.5M
10M
DATA RATE (bps)
LTC486 • TA09
* APPLIES FOR 24 GAUGE, POLYETHYLENE
DIELECTRIC TWISTED PAIR
1
LTC486
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ABSOLUTE
PACKAGE/ORDER I FOR ATIO
RATI GS
(Note 1)
ORDER PART
NUMBER
TOP VIEW
Supply Voltage (VCC) ............................................... 12V
Control Input Voltages .................... – 0.5V to VCC + 0.5V
Driver Input Voltages ...................... – 0.5V to VCC + 0.5V
Driver Output Voltages ......................................... ± 14V
Control Input Currents ....................................... ± 25mA
Driver Input Currents ......................................... ± 25mA
Operating Temperature Range
LTC486C ................................................ 0°C to 70°C
LTC486I ............................................ – 40°C to 85°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
DI1
1
16 VCC
DO1A
2
15 DI4
DO1B
3
14 DO4A
EN
4
13 DO4B
DO2B
5
12 EN
DO2A
6
11 DO3B
DI2
7
10 DO3A
GND
8
9
LTC486CN
LTC486CSW
LTC486IN
LTC486ISW
DI3
N PACKAGE
S PACKAGE
16-LEAD PLASTIC DIP 16-LEAD PLASTIC SOL
TJMAX = 125°C, θJA = 70°C/W (N)
TJMAX = 150°C, θJA = 95°C/W (S)
Consult factory for Military grade parts
DC ELECTRICAL CHARACTERISTICS
VCC = 5V ± 5%, 0°C ≤ Temperature ≤ 70°C (Commercial), – 40°C ≤ Temperature ≤ 85°C (Industrial) (Note 2, 3)
SYMBOL
PARAMETER
CONDITIONS
VOD1
Differential Driver Output Voltage (Unloaded)
IO = 0
VOD2
Differential Driver Output Voltage (With Load)
R = 50Ω; (RS422)
VOD
Change in Magnitude of Driver Differential
Output Voltage for Complementary Output States
R = 27Ω or R = 50Ω
(Figure 1)
TYP
MAX
VOC
Driver Common-Mode Output Voltage
❘VOC❘
Change in Magnitude of Driver Common-Mode
Output Voltage for Complementary Output States
VIH
Input High Voltage
VIL
Input Low Voltage
0.8
V
IIN1
Input Current
±2
µA
ICC
Supply Current
5
R = 27Ω; (RS485) (Figure 1)
DI, EN, EN
No Load
MIN
2
UNITS
V
V
1.5
5
V
0.2
V
3
V
0.2
V
2.0
V
Output Enabled
110
200
µA
Output Disabled
110
200
µA
100
250
mA
IOSD1
Driver Short-Circuit Current, VOUT = High
VO = – 7V
IOSD2
Driver Short-Circuit Current, VOUT = Low
VO = 12V
100
250
mA
IOZ
High Impedance State Output Current
VO = – 7V to 12V
± 10
± 200
µA
UNITS
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SWITCHI G CHARACTERISTICS
VCC = 5V ± 5%, 0°C ≤ Temperature ≤ 70°C (Commercial), – 40°C ≤ Temperature ≤ 85°C (Industrial) (Note 2, 3)
SYMBOL
PARAMETER
CONDITIONS
tPLH
Driver Input to Output
tPHL
Driver Input to Output
RDIFF = 54Ω, CL1 = CL2 = 100pF
(Figures 2, 4)
tSKEW
Driver Output to Output
tr, tf
Driver Rise or Fall Time
tZH
Driver Enable to Output High
2
MIN
TYP
MAX
10
30
50
ns
10
30
50
ns
5
15
ns
15
25
ns
35
70
ns
5
CL = 100pF (Figures 3, 5) S2 Closed
LTC486
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SWITCHI G CHARACTERISTICS
VCC = 5V ± 5%, 0°C ≤ Temperature ≤ 70°C (Commercial), – 40°C ≤ Temperature ≤ 85°C (Industrial) (Note 2, 3)
SYMBOL
PARAMETER
CONDITIONS
TYP
MAX
tZL
Driver Enable to Output Low
CL = 100pF (Figures 3, 5) S1 Closed
MIN
35
70
UNITS
ns
tLZ
Driver Disable Time from Low
CL = 15pF (Figures 3, 5) S1 Closed
35
70
ns
tHZ
Driver Disable Time from High
CL = 15pF (Figures 3, 5) S2 Closed
35
70
ns
pins are negative. All voltages are referenced to device ground unless
otherwise specified.
Note 3: All typicals are given for VCC = 5V and temperature = 25°C.
Note 1: Absolute maximum ratings are those beyond which the safety of
the device cannot be guaranteed.
Note 2: All currents into device pins are positive; all currents out of device
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SWITCHI G TI E WAVEFOR S
3V
DI
f = 1MHz : t r < 10ns : t f < 10ns
1.5V
1.5V
0V
t PLH
t PHL
B
VO
A
t SKEW
1/2 VO
VO
80%
–VO
90%
VDIFF = V(A) – V(B)
10%
tr
20%
tf
Figure 4. Driver Propagation Delays
3V
EN
t SKEW
1/2 VO
LTC486 • TA05
f = 1MHz : t r ≤ 10ns : t f ≤ 10ns
1.5V
1.5V
0V
t ZL
5V
A, B
VOL
VOH
A, B
t LZ
2.3V
OUTPUT NORMALLY LOW
2.3V
OUTPUT NORMALLY HIGH
0.5V
0.5V
0V
tHZ
tZH
LTC486 • TA06
Figure 5. Driver Enable and Disable Times
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TYPICAL PERFOR A CE CHARACTERISTICS
Driver Output High Voltage
vs Output Current TA = 25°C
Driver Differential Output Voltage
vs Output Current TA = 25°C
64
–72
– 48
–24
80
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
–96
OUTPUT CURRENT (mA)
Driver Output Low Voltage
vs Output Current TA = 25°C
48
32
16
0
0
0
1
2
3
OUTPUT VOLTAGE (V)
4
LTC486 • TPC01
60
40
20
0
0
1
2
3
OUTPUT VOLTAGE (V)
4
LTC486• TPC02
0
1
2
3
OUTPUT VOLTAGE (V)
4
LTC486 • TPC03
3
LTC486
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TYPICAL PERFOR A CE CHARACTERISTICS
Driver Skew vs Temperature
1.63
5
1.61
4
TIME (ns)
INPUT THRESHOLD VOLTAGE (V)
TTL Input Threshold vs Temperature
1.59
1.57
3
2
1.55
–50
0
50
1
–50
100
TEMPERATURE (°C )
50
130
DIFFERENTIAL VOLTAGE (V)
120
110
100
0
50
2.1
1.9
1.7
1.5
–50
100
TEMPERATURE (°C )
0
50
TABLE
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ENABLES
OUTPUTS
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DI
EN
EN
OUTA
OUTB
H
L
H
L
X
H
H
X
X
L
X
X
L
L
H
H
L
H
L
Z
L
H
L
H
Z
4
100
LTC486 • TPC06
TEMPERATURE (°C )
INPUT
LTC486 • TPC05
2.3
90
–50
FU CTI
100
Driver Differential Output Voltage
vs Temperature RO = 54Ω
Supply Current vs Temperature
SUPPLY CURRENT (µA)
0
TEMPERATURE (°C )
LTC486 • TPC04
H: High Level
L: Low Level
X: Irrelevant
Z: High Impedance (Off)
LTC486 • TPC07
LTC486
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DI1 (Pin 1): Driver 1 Input. If Driver 1 is enabled, then a low
on DI1 forces the driver outputs DO1A low and DO1B high.
A high on DI1 with the driver outputs enabled will force
DO1A high and DO1B low.
DO1A (Pin 2): Driver 1 Output.
DO1B (Pin 3): Driver 1 Output.
EN (Pin 4): Driver Outputs Enabled. See Function Table for
details.
DO2B (Pin 5): Driver 2 Output.
DO2A (Pin 6): Driver 2 Output.
DI2 (Pin 7): Driver 2 Input. Refer to DI1.
TEST CIRCUITS
GND (Pin 8): Ground Connection.
DI3 (Pin 9): Driver 3 Input. Refer to DI1.
DO3A (Pin 10): Driver 3 Output.
DO3B (Pin 11): Driver 3 Output.
EN (Pin 12): Driver Outputs Disabled. See Function Table
for details.
DO4B (Pin 13): Driver 4 Output.
DO4A (Pin 14): Driver 4 Output.
DI4 (Pin 15): Driver 4 Input. Refer to DI1.
VCC (Pin 16): Positive Supply; 4.75V < VCC < 5.25V .
S1
EN
A
VCC
CI1
R
OUTPUT
UNDER TEST
A
VOD
DI
R
DRIVER
RDIFF
CL
VOC
B
500Ω
S2
B
CI2
LTC486 • TA04
LTC486 • TA03
EN
LTC486 • TA02
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APPLICATI
Figure 2. Driver Timing Test Circuit
Figure 3. Driver Timing Test Load #2
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Figure 1. Driver DC Test Load
S I FOR ATIO
Typical Application
A typical connection of the LTC486 is shown in Figure 6.
A twisted pair of wires connect up to 32 drivers and
receivers for half duplex data transmission. There are no
restrictions on where the chips are connected to the
wires, and it isn’t necessary to have the chips connected
at the ends. However, the wires must be terminated only
at the ends with a resistor equal to their characteristic
impedance, typically 120Ω. The optional shields around
the twisted pair help reduce unwanted noise, and are
connected to GND at one end.
Thermal Shutdown
The LTC486 has a thermal shutdown feature which protects the part from excessive power dissipation. If the
outputs of the driver are accidently shorted to a power
supply or low impedance source, up to 250mA can flow
through the part. The thermal shutdown circuit disables
the driver outputs when the internal temperature reaches
150°C and turns them back on when the temperature cools
to 130°C. If the outputs of two or more LTC486 drivers are
shorted directly, the driver outputs can not supply enough
current to activate the thermal shutdown. Thus, the thermal shutdown circuit will not prevent contention faults
when two drivers are active on the bus at the same time.
Cable and Data Rate
The transmission line of choice for RS485 applications is
a twisted pair. There are coaxial cables (twinaxial) made for
this purpose that contain straight pairs, but these are less
flexible, more bulky, and more costly than twisted pairs.
Many cable manufacturers offer a broad range of 120Ω
cables designed for RS485 applications.
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LTC486
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APPLICATI
EN
EN
4
DX
1
SHIELD
SHIELD
3
2
120Ω
DX
120Ω
2
4
3
RX
RX
1
12
12
EN
EN
1/4 LTC486
EN
4
DX
1
3
EN
1/4 LTC488
4
1
DX
3
RX
2
2
12
EN
1/4 LTC486
RX
12
EN
1/4 LTC488
LTC486 • TA07
Figure 6. Typical Connection
LOSS PER 100 FT (dB)
10
1k
100
10
10k
100k
1M
2.5M
10M
DATA RATE (bps)
1
LTC486 • TA09
Figure 8. Cable Length vs Data Rate
0.1
0.1
1
10
100
FREQUENCY (MHz)
LTC486 • TA08
Figure 7. Attenuation vs Frequency for Belden 9841
When using low loss cables, Figure 8 can be used as a
guideline for choosing the maximum line length for a given
data rate. With lower quality PVC cables, the dielectric loss
factor can be 1000 times worse. PVC twisted pairs have
terrible losses at high data rates (>100kbs) and greatly
reduce the maximum cable length. At low data rates
however, they are acceptable and much more economical.
Cable Termination
The proper termination of the cable is very important. If the
cable is not terminated with its characteristic impedance,
6
10k
CABLE LENGTH (FT)
Losses in a transmission line are a complex combination
of DC conductor loss, AC losses (skin effect), leakage, and
AC losses in the dielectric. In good polyethylene cables
such as the Belden 9841, the conductor losses and dielectric losses are of the same order of magnitude, with
relatively low overall loss (Figure 7).
distorted waveforms will result. In severe cases, distorted
(false) data and nulls will occur. A quick look at the output
of the driver will tell how well the cable is terminated. It is
best to look at a driver connected to the end of the cable,
since this eliminates the possibility of getting reflections
from two directions. Simply look at the driver output while
transmitting square wave data. If the cable is terminated
properly, the waveform will look like a square wave
(Figure 9).
If the cable is loaded excessively (e.g., 47Ω), the signal
initially sees the surge impedance of the cable and jumps
to an initial amplitude. The signal travels down the cable
and is reflected back out of phase because of the
mistermination. When the reflected signal returns to the
driver, the amplitude will be lowered. The width of the
pedestal is equal to twice the electrical length of the cable
(about 1.5ns/ft). If the cable is lightly loaded (e.g., 470Ω),
LTC486
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APPLICATI
PROBE HERE
Rt
DX
DRIVER
RECEIVER
RX
Rt = 120Ω
Rt = 47Ω
represents an electrical one-tenth wavelength. The value
of the coupling capacitor should therefore be set at 16.3pF
per foot of cable length for 120Ω cables. With the coupling
capacitors in place, power is consumed only on the signal
edges, not when the driver output is idling at a 1 or 0 state.
A 100nF capacitor is adequate for lines up to 4000 feet in
length. Be aware that the power savings start to decrease
once the data rate surpasses 1/(120Ω × C).
Receiver Open-Circuit Fail-Safe
Rt = 470Ω
LTC486 • TA10
Figure 9. Termination Effects
the signal reflects in phase and increases the amplitude at
the driver output. An input frequency of 30kHz is adequate
for tests out to 4000 ft. of cable.
AC Cable Termination
Cable termination resistors are necessary to prevent unwanted reflections, but they consume power. The typical
differential output voltage of the driver is 2V when the cable
is terminated with two 120Ω resistors. When no data is
being sent 33mA of DC current flows in the cable . This DC
current is about 220 times greater than the supply current
of the LTC486. One way to eliminate the unwanted current
is by AC coupling the termination resistors as shown in
Figure 10.
Some data encoding schemes require that the output of
the receiver maintains a known state (usually a logic 1)
when the data is finished transmitting and all drivers on
the line are forced into three-state. All LTC RS485
receivers have a fail-safe feature which guarantees the
output to be in a logic 1 state when the receiver inputs
are left floating (open-circuit). However, when the cable
is terminated with 120Ω, the differential inputs to the
receiver are shorted together, not left floating.
If the receiver output must be forced to a known state,
the circuits of Figure 11 can be used.
The termination resistors are used to generate a DC bias
which forces the receiver output to a known state, in this
case a logic 0. The first method consumes about
208mW and the second about 8mW. The lowest power
5V
110Ω
130Ω
130Ω
110Ω
RECEIVER
RX
RECEIVER
RX
RECEIVER
RX
5V
120Ω
1.5k
C
RECEIVER
RX
140Ω
C = LINE LENGTH (FT) × 16.3pF
1.5k
LTC486 • TA11
Figure 10. AC Coupled Termination
The coupling capacitor allows high frequency energy to
flow to the termination, but blocks DC and low frequencies. The dividing line between high and low frequency
depends on the length of the cable. The coupling capacitor
must pass frequencies above the point where the line
100k
C
5V
120Ω
LTC486 • TA12
Figure 11. Forcing “0” When All Dirvers Are Off
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 representation that the
interconnection of its circuits as described herein will not infringe on existing patent rights.
7
LTC486
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APPLICATI
S I FOR ATIO
solution is to use an AC termination with a pull-up resistor.
Simply swap the receiver inputs for data protocols ending
in logic 1.
Fault Protection
breakdown voltages and prices. Be sure to pick a breakdown voltage higher than the common-mode voltage
required for your application (typically 12V). Also, don’t
forget to check how much the added parasitic capacitance
will load down the bus.
All of LTC’s RS485 products are protected against ESD
transients up to ±2kV using the human body model
(100pF, 1.5kΩ). However, some applications need greater
protection. The best protection method is to connect a
bidirectional TransZorb® from each line side pin to ground
(Figure 12).
A TransZorb® is a silicon transient voltage suppressor that
has exceptional surge handling capabilities, fast response
time, and low series resistance. They are available from
General Semiconductor Industries and come in a variety of
Y
120Ω
DRIVER
Z
LTC486 • TA13
Figure 12. ESD Protection
TransZorb® is a registrated trademark of General Instruments, GSI
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TYPICAL APPLICATI
RS232 to RS485 Level Translator with Hysteresis
R = 220k
Y
10k
RS232 IN
120Ω
DRIVER
5.6k
Z
1/4 LTC486
⎜VY - VZ ⎜
19k
HYSTERESIS = 10k × ———— ≈ ——
R
R
LTC486 • TA14
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PACKAGE DESCRIPTIO
Dimensions in inches (millimeters) unless otherwise noted.
0.300 – 0.325
(7.620 – 8.255)
N Package
16-Lead Plastic DIP
0.009 – 0.015
(0.229 – 0.381)
(
0.130 ± 0.005
(3.302 ± 0.127)
0.015
(0.381)
MIN
+0.025
0.325 –0.015
8.255
0.005
(0.127)
RAD MIN
+0.635
–0.381
)
0.065
(1.651)
TYP
0.125
(3.175)
MIN
0.045 ± 0.015
(1.143 ± 0.381)
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
10
9
0.260 ± 0.010
(6.604 ± 0.254)
0.018 ± 0.003
(0.457 ± 0.076)
0.100 ± 0.010
(2.540 ± 0.254)
0.093 – 0.104
(2.362 – 2.642)
0.037 – 0.045
(0.940 – 1.143)
16
15
14
13
12
11
0° – 8° TYP
0.009 – 0.013
(0.229 – 0.330)
NOTE 1
0.050
(1.270)
TYP
0.014 – 0.019
0.016 – 0.050
(0.356 – 0.482)
(0.406 – 1.270)
TYP
NOTE:
1. PIN 1 IDENT, NOTCH ON TOP AND CAVITIES ON THE BOTTOM OF PACKAGES ARE THE MANUFACTURING OPTIONS.
THE PART MAY BE SUPPLIED WITH OR WITHOUT ANY OF THE OPTIONS.
2. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006 INCH (0.15mm).
8
16
0.398 – 0.413
(10.109 – 10.490)
(NOTE 2)
0.291 – 0.299
(7.391 – 7.595)
(NOTE 2)
0.010 – 0.029 × 45°
(0.254 – 0.737)
S Package
16-Lead Plastic SOL
0.770
(19.558)
MAX
0.045 – 0.065
(1.143 – 1.651)
Linear Technology Corporation
0.004 – 0.012
(0.102 – 0.305)
0.394 – 0.419
(10.007 – 10.643)
NOTE 1
1
2
3
4
5
6
7
8
sn486 486fas LT/GP 0294 5K REV A • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977
© LINEAR TECHNOLOGY CORPORATION 1994