508653F [ETC]
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| 型号： | 508653F |
| 厂家： | 
ETC |
| 描述： | IC
|
| 文件： | 总8页 (文件大小：202K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC1487
Ultra-Low Power RS485
with Low EMI, Shutdown
and High Input Impedance
U
DESCRIPTIO
EATURE
High Input Impedance: Up to 256 Transceivers
on the Bus
S
F
The LTC®1487 is an ultra-low power differential line trans-
ceiverdesignedwithhighimpedanceinputsallowingupto
256 transceivers to share a single bus. It meets the
requirementsofRS485andRS422.TheLTC1487features
output drivers with controlled slew rate, decreasing the
EMI radiated from the RS485 lines, and improving signal
fidelity with misterminated lines. The CMOS design offers
significant power savings without sacrificing ruggedness
against overload or ESD damage. Typical quiescent cur-
rent is only 80µA while operating and 1µA in shutdown.
■
■
■
■
■
■
■
Low Power: ICC = 120µA Max with Driver Disabled
ICC = 200µA Max with Driver Enabled, No Load
1µA Quiescent Current in Shutdown Mode
Controlled Slew Rate Driver for Reduced EMI
Single 5V Supply
ESD Protection to ±10kV On Receiver Inputs and
Driver outputs
–7V to 12V Common-Mode Range Permits ±7V
Ground Difference Between Devices on the Data Line
Thermal Shutdown Protection
Power Up/Down Glitch-Free Driver Outputs Permit
Live Insertion or Removal of Transceiver
Driver Maintains High Impedance in Three-State
or with the Power Off
■
The driver and receiver feature 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. The receiver has a fail-safe
feature which guarantees a high output state when the
inputsareleftopen. I/Opinsareprotectedagainstmultiple
ESD strikes of over ±10kV using the Human Body Model.
■
■
■
■
Pin Compatible with the LTC485
O U
PPLICATI
A
S
The LTC1487 is fully specified over the commercial tem-
perature range and is available in 8-pin DIP and SO
packages.
■
■
■
Battery-Powered RS485/RS422 Applications
Low Power RS485/RS422 Transceiver
Level Translator
, LTC and LT are registered trademarks of Linear Technology Corporation.
U
O
TYPICAL APPLICATI
LTC1487
LTC1487
R
1
2
3
1
2
3
RO
RE
DE
R
RO
RE
DE
2000 FEET OF TWISTED-PAIR WIRE
DI
7
6
7
4
4
120Ω
120Ω
DI
D
D
DI
A
B
6
330Ω
4.7nF
RO
EQUIVALENT LOAD OF 256
LTC1487 TRANSCEIVERS
LTC1487 • TA01
LTC1487 • TA02
1
LTC1487
W W W
U
W
U
ABSOLUTE AXI U RATI GS
/O
PACKAGE RDER I FOR ATIO
(Note 1)
Supply Voltage (VCC) .............................................. 12V
Control Input Voltage..................... –0.5V to VCC + 0.5V
Driver Input Voltage....................... –0.5V to VCC + 0.5V
Driver Output Voltage ........................................... ±14V
Receiver Input Voltage.......................................... ±14V
Receiver Output Voltage ................ –0.5V to VCC + 0.5V
Operating Temperature Range ............. 0°C ≤ TA ≤ 70°C
Lead Temperature (Soldering, 10 sec)................. 300°C
TOP VIEW
ORDER PART
NUMBER
RO
RE
DE
DI
1
2
3
4
V
B
A
8
7
6
5
CC
R
LTC1487CN8
LTC1487CS8
D
GND
N8 PACKAGE
8-LEAD PDIP
S8 PACKAGE
S8 PART MARKING
1487
8-LEAD PLASTIC SO
TJMAX = 125°C, θJA = 130°C/ W (N8)
JMAX = 125°C, θJA = 150°C/ W (S8)
T
Consult factory for Industrial and Military grade parts.
0°C ≤ TA ≤ 70°C, VCC = 5V (Notes 2, 3) unless otherwise noted.
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
OD1
V
OD2
Differential Driver Output Voltage (Unloaded)
Differential Driver Output Voltage (with Load)
I = 0
●
5
V
O
R = 50Ω (RS422)
R = 27Ω (RS485), Figure 1
●
●
2.0
1.5
V
V
5
∆V
Change in Magnitude of Driver Differential Output
Voltage for Complementary Output States
R = 27Ω or R = 50Ω, Figure 1
●
0.2
V
OD
V
Driver Common-Mode Output Voltage
R = 27Ω or R = 50Ω, Figure 1
R = 27Ω or R = 50Ω, Figure 1
●
●
3
V
V
OC
∆ V
Change in Magnitude of Driver Common-Mode
Output Voltage for Complementary Output States
0.2
OC
V
V
Input High Voltage
Input Low Voltage
Input Current
DE, DI, RE
DE, DI, RE
DE, DI, RE
●
●
●
2
V
V
IH
0.8
IL
I
I
±2
µA
IN1
IN2
Input Current (A, B)
DE = 0, V = 0V or 5.25V, V = 12V
●
●
0.30
–0.15
mA
mA
CC
IN
DE = 0, V = 0V or 5.25V, V = –7V
CC
IN
V
Differential Input Threshold Voltage for Receiver
Receiver Input Hysteresis
–7V ≤ V ≤ 12V
●
●
●
●
●
–0.2
3.5
0.2
V
mV
V
TH
CM
∆V
V
= 0V
CM
45
96
TH
V
V
Receiver Output High Voltage
I = –4mA, V = 200mV
O ID
OH
Receiver Output Low Voltage
I = 4mA, V = –200mV
0.4
V
OL
O
ID
I
Three-State (High Impedance) Output
Current at Receiver
V
= Max, 0.4V ≤ V ≤ 2.4V
±1
µA
OZR
CC
O
R
Receiver Input Resistance
Supply Current
–7V ≤ V ≤ 12V
●
70
kΩ
IN
CM
I
CC
No Load, Output Enabled
No Load, Output Disabled
●
●
120
80
200
120
µA
µA
I
I
I
I
Supply Current in Shutdown Mode
DE = 0V, RE = V
1
10
250
250
85
µA
mA
mA
mA
SHDN
OSD1
OSD2
OSR
CC
Driver Short-Circuit Current, V
Driver Short-Circuit Current, V
Receiver Short-Circuit Current
= HIGH
= LOW
–7V ≤ V ≤ 12V
●
●
●
35
35
7
OUT
OUT
O
–7V ≤ V ≤ 12V
O
0V ≤ V ≤ V
O
CC
2
LTC1487
–40°C ≤ TA ≤ 85°C, VCC = 5V (Note 4) unless otherwise noted.
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
V
Differential Driver Output Voltage (Unloaded)
Differential Driver Output Voltage (with Load)
I = 0
●
5
V
OD1
OD2
O
R = 50Ω (RS422)
R = 27Ω (RS485), Figure 1
●
●
2.0
1.5
V
V
5
3
V
V
Driver Common-Mode Output Voltage
Differential Input Threshold Voltage for Receiver
Receiver Input Hysteresis
R = 27Ω or R = 50Ω, Figure 1
●
●
●
V
V
OC
TH
–7V ≤ V ≤ 12V
–0.2
0.2
CM
∆V
V
= 0V
CM
45
mV
TH
I
Supply Current
No Load, Output Enabled
No Load, Output Disabled
●
●
120
80
200
120
µA
µA
CC
I
t
t
t
Supply Current in Shutdown Mode
Driver Input to Output
DE = 0V, RE = V
1
10
µA
ns
SHDN
PLH
CC
R
= 54Ω, C = C = 100pF,
●
●
●
●
●
●
●
●
150
150
1200
1200
600
DIFF
L1
L2
(Figures 3, 5)
Driver Input to Output
ns
PHL
Driver Output to Output
Driver Rise or Fall Time
Receiver Input to Output
Receiver Input to Output
100
ns
SKEW
t , t
150
30
2000
250
ns
r
f
PLH
PHL
SKD
MAX
t
t
t
f
R
= 54Ω, C = C = 100pF,
140
140
13
ns
DIFF
L1
L2
(Figures 3, 7)
30
250
ns
t
– t
Differential Receiver Skew
PHL
ns
PLH
Maximum Data Rate
250
kbps
U
0°C ≤ TA ≤ 70°C, VCC = 5V (Notes 2, 3) unless otherwise noted.
SWITCHI G CHARACTERISTICS
SYMBOL PARAMETER
CONDITIONS
MIN
150
150
TYP
MAX
1200
1200
600
UNITS
ns
t
t
t
Driver Input to Output
R
DIFF
= 54Ω, C = C = 100pF,
●
●
●
●
●
●
●
●
●
●
●
●
●
●
●
●
●
PLH
L1
L2
(Figures 3, 5)
Driver Input to Output
ns
PHL
Driver Output to Output
Driver Rise or Fall Time
Driver Enable to Output High
Driver Enable to Output Low
Driver Disable Time from Low
Driver Disable Time from High
Receiver Input to Output
Receiver Input to Output
250
ns
SKEW
t , t
150
100
100
150
150
30
1200
1500
1500
1500
1500
250
ns
r
f
t
t
t
t
t
t
t
t
t
t
t
f
t
C = 100pF (Figures 4, 6), S2 Closed
L
ns
ZH
ZL
LZ
HZ
C = 100pF (Figures 4, 6), S1 Closed
L
ns
C = 15pF (Figures 4, 6), S1 Closed
L
ns
C = 15pF (Figures 4, 6), S2 Closed
L
ns
R
DIFF
= 54Ω, C = C = 100pF,
140
140
13
ns
PLH
PHL
SKD
ZL
L1
L2
(Figures 3, 7)
30
250
ns
t
– t
Differential Receiver Skew
PHL
ns
PLH
Receiver Enable to Output Low
Receiver Enable to Output High
Receiver Disable from Low
Receiver Disable from High
Maximum Data Rate
C
RL
C
RL
C
RL
C
RL
= 15pF (Figures 2, 8), S1 Closed
= 15pF (Figures 2, 8), S2 Closed
= 15pF (Figures 2, 8), S1 Closed
= 15pF (Figures 2, 8), S2 Closed
20
50
50
50
50
ns
20
ns
ZH
20
ns
LZ
20
ns
HZ
250
50
kbps
ns
MAX
SHDN
Time to Shutdown
DE = 0, RE =
200
600
3
LTC1487
U
0°C ≤ TA ≤ 70°C, VCC = 5V (Notes 2, 3) unless otherwise noted.
SWITCHI G CHARACTERISTICS
SYMBOL PARAMETER
CONDITIONS
MIN
TYP
MAX
2000
2000
2000
2000
UNITS
ns
t
t
t
t
Driver Enable from Shutdown to Output High
Driver Enable from Shutdown to Output Low
Receiver Enable from Shutdown to Output High
Receiver Enable from Shutdown to Output Low
C = 100pF (Figures 4, 6), S2 Closed
●
●
●
●
ZH(SHDN)
ZL(SHDN)
ZH(SHDN)
ZL(SHDN)
L
C = 100pF (Figures 4, 6), S1 Closed
ns
L
C = 15pF (Figures 2, 8), S2 Closed
ns
L
C = 15pF (Figures 2, 8), S1 Closed
ns
L
Note 3: All typicals are given for V = 5V and T = 25°C.
The
●
denotes specifications which apply over the full operating
CC
A
temperature range.
Note 1: Absolute maximum ratings are those beyond which the safety of
the device cannot be guaranteed.
Note 4: The LTC1487 is not tested and is not quality-assurance sampled at
–40°C and at 85°C. These specifications are guaranteed by design,
correlation, and/or inference from 0°C, 25°C and/or 70°C tests.
Note 2: All currents into device pins are positive; all currents out ot device
pins are negative. All voltages are referenced to device ground unless
otherwise specified.
W
U
TYPICAL PERFORMANCE CHARACTERISTICS
Driver Differential Output Voltage
vs Output Current
Driver Differential Output Voltage
vs Temperature
Supply Current vs Temperature
2.24
2.22
2.20
2.18
2.16
2.14
2.12
2.10
2.08
2.06
2.04
2.02
2.00
450
400
350
300
250
200
150
100
50
80
70
60
50
40
30
20
10
0
R
= 54Ω
T
= 25°C
L
A
THERMAL SHUTDOWN
WITH DRIVER ENABLED
AND NOMINAL LOAD
DRIVER ENABLED
WITH NO LOAD
DRIVER DISABLED WITH NO LOAD
0
2.0 2.5
0
0.5 1.0 1.5
3.0 3.5 4.0 4.5
50
TEMPERATURE (°C)
125
–50
0
25
75 100
–25
–50 –25
0
25
TEMPERATURE (°C)
125 150 175
50 75 100
OUTPUT VOLTAGE (V)
LTC1487 • TPC02
LTC1487 • TPC03
LTC1487 • TPC01
Driver Output Low Voltage
vs Output Current
Driver Output High Voltage
vs Output Current
Driver Skew vs Temperature
120
100
500
450
0
–10
T
A
= 25°C
T
A
= 25°C
–20
–30
–40
–50
–60
–70
–80
–90
–100
400
350
300
250
200
80
60
40
20
0
150
0
1
2
3
4
–50 –25
0
25
50
75
100 125
0
1
2
3
4
5
OUTPUT VOLTAGE (V)
TEMPERATURE (°C)
OUTPUT VOLTAGE (V)
LTC1487 • TPC04
LTC1487 • G06
LTC1487 • TPC05
4
LTC1487
U
U
U
PIN FUNCTIONS
RO (Pin 1): Receiver Output. If the receiver output is
enabled (RE LOW), and A > B by 200mV, RO will be HIGH.
If A < B by 200mV, then RO will be LOW.
DI (Pin 4): Driver Input. If the driver outputs are enabled
(DEHIGH)thenaLOWonDIforcestheoutputsALOWand
B HIGH. A HIGH on DI with the driver outputs enabled will
force A HIGH and B LOW.
RE (Pin 2): Receiver Output Enable. A LOW enables the
receiver output, RO. A HIGH input forces the receiver
output into a high impedance state.
GND (Pin 5): Ground.
A (Pin 6): Driver Output/Receiver Input.
B (Pin 7): Driver Output/Receiver Input.
DE (Pin 3): Driver Outputs Enable. A HIGH on DE enables
thedriveroutput.AandBandthechipwillfunctionasaline
driver. ALOWinputwillforcethedriveroutputsintoahigh
impedance state and the chip will function as a line
receiver. If RE is HIGH and DE is LOW, the part will enter
a low power (1µA) shutdown state.
V
CC (Pin 8): Positive Supply. 4.75V < VCC < 5.25V.
U
U
FU CTIO TABLES
LTC1487 Transmitting
INPUTS
LTC1487 Receiving
INPUTS
OUTPUTS
OUTPUTS
RE
X
DE
1
DI
1
B
0
A
RE
0
DE
0
A – B
≥0.2V
RO
1
1
X
1
0
1
0
0
0
≤–0.2V
Inputs Open
X
0
0
0
X
X
Z
Z
0
0
1
1
0
Z*
Z*
1
0
Z*
*Shutdown mode
*Shutdown mode
TEST CIRCUITS
A
S1
TEST POINT
1k
R
R
RECEIVER
OUTPUT
V
CC
V
OD
1k
C
RL
V
OC
S2
B
LTC1487 • F01
LTC1487 • F02
Figure 1. Driver DC Test Load
Figure 2. Receiver Timing Test Load
3V
DE
A
A
B
C
C
L1
L2
S1
DI
RO
R
V
CC
DIFF
B
500Ω
OUTPUT
UNDER TEST
RE
15pF
S2
C
L
LTC1487 • F03
LTC1487 • F04
Figure 3. Driver/Receiver Timing Test Circuit
Figure 4. Driver Timing Test Load
5
LTC1487
U
W
W
SWITCHI G TI E WAVEFOR S
3V
f = 1MHz, t ≤ 10ns, t ≤ 10ns
DI
1.5V
1.5V
r
f
0V
1/2 V
O
t
t
PHL
PLH
B
A
V
O
t
t
SKEW
1/2 V
SKEW
O
V
O
O
90%
10%
90%
V
DIFF
= V(A) – V(B)
0V
10%
–V
LTC1487 • F05
t
t
f
r
Figure 5. Driver Propagation Delays
3V
0V
5V
f = 1MHz, t ≤ 10ns, t ≤ 10ns
1.5V
1.5V
DE
A, B
A, B
r
f
t
t
, t
ZL(SHDN) ZL
LZ
2.3V
OUTPUT NORMALLY LOW
0.5V
0.5V
V
OL
OH
0V
V
OUTPUT NORMALLY HIGH
, t
2.3V
t
t
LTC1487 • F06
HZ
ZH(SHDN) ZH
Figure 6. Driver Enable and Disable Times
V
OH
1.5V
1.5V
RO
OUTPUT
V
OL
t
f = 1MHz, t ≤ 10ns, t ≤ 10ns
t
PHL
r
f
PLH
V
A – B
–V
OD2
OD2
0V
0V
INPUT
LTC1487 • F07
Figure 7. Receiver Propagation Delays
3V
0V
5V
1.5V
1.5V
RE
RO
RO
f = 1MHz, t ≤ 10ns, t ≤ 10ns
r
f
t
, t
t
ZL(SHDN) ZL
LZ
1.5V
OUTPUT NORMALLY LOW
0.5V
0.5V
OUTPUT NORMALLY HIGH
, t
1.5V
t
0V
t
LTC1487 • F08
HZ
ZH(SHDN) ZH
Figure 8. Receiver Enable and Disable Times
6
LTC1487
U U
W
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APPLICATIO S I FOR ATIO
V
CC
High Input Impedance
SD3
SD4
The LTC1487 is designed with a 96kΩ (typ) input imped-
ance to allow up to 256 transceivers to share a single
RS485 differential data bus. The RS485 specification
requires that a transceiver be able to drive as many as 32
“unit loads.” One unit load (UL) is defined as an imped-
ancethatdrawsamaximumof1mAwithupto12Vacross
it. Typical RS485 transceivers present between 0.5 and 1
unit load at their inputs. The 96kΩ input impedance of the
LTC1487 will draw only 125µA under the same 12V
condition,presentingonly0.125ULtothebus.Asaresult,
256 LTC1487 transceivers (32UL/0.125UL = 256) can be
connected to a single RS485 data bus without exceeding
the RS485 driver load specification. The LTC1487 meets
all other RS485 specifications, allowing it to operate
equally well with standard RS485 transceiver devices or
high impedance transceivers.
P1
D1
OUTPUT
LOGIC
D2
N1
LTC1487 • F10
Figure 10. LTC1487 Output Stage
When two or more drivers are connected to the same
transmission line, a potential condition exists whereby
more than two drivers are simultaneously active. If one or
more drivers is sourcing current while another driver is
sinking current, excessive power dissipation may occur
within either the sourcing or sinking element. This condi-
tion is defined as driver contention, since multiple drivers
are competing for one transmission line. The LTC1487
provides a current limiting scheme to prevent driver
contention failure. When driver contention occurs, the
current drawn is limitedtoabout70mA, preventingexces-
sive power dissipation within the drivers.
CMOS Output Driver
The RS485 specification requires that a transceiver with-
stand common-mode voltages of up to 12V or –7V at the
RS485lineconnections.Additionally,thetransceivermust
be immune to both ESD and latch-up. This rules out
traditional CMOS drivers, which include parasitic diodes
fromtheirdriveroutputstoeachsupplyrail(Figure9). The
LTC1487 uses a proprietary process enhancement which
adds a pair of Schottky diodes to the output stage (Figure
10), preventing current from flowing when the common-
mode voltage exceeds the supply rails. Latch-up at the
output drivers is virtually eliminated and the driver is
prevented from loading the line under RS485 specified
fault conditions. A proprietary output protection structure
protects the transceiver line terminals against ESD strikes
(Human Body Model) of up to ±10kV.
The LTC1487 has a thermal shutdown feature which
protects the part from excessive power dissipation. Under
extreme fault conditions, up to 250mA can flow through
the part, causing rapid internal temperature rise. The
thermal shutdown circuit will disable the driver outputs
when the internal temperature reaches 150°C and turns
them back on when the temperature cools to 130°C. This
cycle will repeat as necessary until the fault condition is
removed.
V
Receiver Inputs
CC
The LTC1487 receiver features an input common-mode
range covering the entire RS485 specified range of –7V to
12V. Internal96kinputresistorsfromeachlineterminalto
ground provide the 0.125UL load to the RS485 bus.
Differential signals of greater than ±200mV within the
specified input common-mode range will be converted to
a TTL-compatible signal at the receiver output. A small
amount of input hysteresis is included to minimize the
P1
D1
OUTPUT
D2
LOGIC
N1
LTC1487 • F09
Figure 9. Conventional CMOS Output Stage
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 represen-
tation that the interconnection of circuits as described herein will not infringe on existing patent rights.
7
LTC1487
APPLICATIO S I FOR ATIO
U U
W
U
effects of noise on the line signals. If the line is terminated
or the receiver inputs are shorted together, the receiver
output will retain the last valid line signal due to the 45mV
of hysteresis incorporated in the receiver circuit. If the
LTC1487transceiverinputsareleftfloating(unterminated),
an internal pull-up of 10µA at the A input will force the
receiver output to a known high state.
In shutdown the LTC1487 typically draws only 1µA of
supply current. In order to guarantee that the part goes
into shutdown, RE must be HIGH and DE must be LOW for
at least 600ns simultaneously. If this time duration is less
than50nsthepartwillnotentershutdownmode. Toggling
either RE or DE will wake the LTC1487 back up within
3.5µs.
If the driver is active immediately prior to shutdown, the
supply current will not drop to 1µA until the driver
outputs have reached a steady state; this can take as long
as 2.6µs under worst case conditions. If the driver is
disabled prior to shutdown the supply current will drop
to 1µA immediately.
Low Power Operation
The LTC1487 draws very little supply current whenever
the driver outputs are disabled. In shutdown mode, the
quiescent current is typically less than 1µA. With the
receiveractiveandthedriveroutputsdisabled,theLTC1487
will typically draw 80µA quiescent current. With the driver
outputs enabled but unterminated, quiescent current will
rise slightly as one of the two outputs sources current into
the internal receiver input resistance. With the minimum
receiver input resistance of 70k and the maximum output
swing of 5V, the quiescent current will rise by a maximum
of 72µA. Typical quiescent current rise with the driver
enabled is about 40µA.
Slew Rate and Propagation Delay
Many digital encoding schemes are dependent upon the
difference in the propagation delay times of the driver and
receiver. Figure 11 shows the test circuit for the LTC1487
propagation delay.
100pF
BR
RECEIVER
R
TTL IN
f
D
OUT
R
100Ω
t , t < 6ns
The quiescent current rises significantly if the driver is
enabled when it is externally terminated. With 1/2
termination load (120Ω between the driver outputs), the
quiescent current will jump to at least 13mA as the drivers
forceaminimumof1.5Vacrosstheterminationresistance.
With a fully terminated 60Ω line attached, the current will
rise to greater than 25mA with the driver enabled,
completely overshadowing the extra 40µA drawn by the
internal receiver inputs.
r
LTC1487 • F11
100pF
Figure 11. Receiver Propagation Delay Test Circuit
The receiver delay times are:
tPLH – tPHL = 13ns Typ, VCC = 5V
The LTC1487 drivers feature controlled slew rate to reduce
system EMI and improve signal fidelity by reducing reflec-
tions due to misterminated cables.
Shutdown Mode
Both the receiver output (RO) and the driver outputs (A, B)
can be placed in three-state mode by bringing RE HIGH
and DE LOW respectively. In addition, the LTC1487 will
enter shutdown mode when RE is HIGH and DE is LOW.
The driver’s skew times are:
Skew = 250ns Typ, VCC = 5V
600ns Max, VCC = 5V, TA = –40°C to 85°C
U
PACKAGE DESCRIPTION
For package descriptions consult the 1994 Linear Databook Volume III.
LT/GP 0395 10K • PRINTED IN THE USA
LINEAR TECHNOLOGY CORPORATION 1995
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
8
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(408) 432-1900 FAX: (408) 434-0507 TELEX: 499-3977
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