LTC1685
52Mbps, Precision Delay,
RS485 Fail-Safe Transceiver
FEATURES
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DESCRIPTION
The LTC
®
1685 is a high speed, precision delay RS485
transceiver that can operate at data rates as high as 52Mbps.
The device also meets the requirements of RS422.
A unique architecture provides very stable propagation
delays and low skew over a wide common mode and
ambient temperature range.
The driver and receiver feature three-state outputs, with
disabled driver outputs maintaining high impedance over
the entire common mode range. A short circuit feature
detects shorted outputs and substantially reduces driver
output current. A similar feature also protects the receiver
output from short circuits. Thermal shutdown circuitry
protects from excessive power dissipation.
The receiver has a fail-safe feature that guarantees a high
output state when the inputs are shorted or are left floating.
The LTC1685 RS485 transceiver guarantees receiver fail-
safe operation over the
entire
common mode range (– 7V
to 12V). Input resistance will remain
≥
22k when the device
is unpowered or disabled.
The LTC1685 operates from a single 5V supply and draws
only 7mA of supply current.
, LTC and LT are registered trademarks of Linear Technology Corporation.
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Precision Propagation Delay Over Temperature:
Receiver/Driver: 18.5ns
±
3.5ns
High Data Rate:
52Mbps
Low t
PLH
/t
PHL
Skew:
Receiver/Driver: 500ps Typ
–7V to 12V RS485 Input Common Mode Range
Guaranteed Fail-Safe Receiver Operation Over the
Entire Common Mode Range
High Receiver Input Resistance:
≥
22k, Even When
Unpowered
Short-Circuit Protected
Thermal Shutdown Protected
Driver Maintains High Impedance in Three-State or
with Power Off
Single 5V Supply
Pin Compatible with LTC485
45dB CMRR at 26MHz
APPLICATIONS
s
s
s
s
s
High Speed RS485/RS422 Transceivers
Level Translator
Backplane Transceiver
STS-1/OC-1 Data Transceiver
Fast-20, Fast-40 SCSI Transceivers
TYPICAL APPLICATION
RO1
R
RE1
DE1
DI1
D
GND1
1V/DIV
10Mbps Data Pulse
400ft Category 5 UTP
V
CC1
Rt
2V/DIV
DRIVER INPUT
CABLE DELAY
RO2
R
RE2
DE2
DI2
D
Rt
V
CC2
5V/DIV
RECEIVER
OUTPUT
GND2
1685 TA01
U
U
U
RECEIVER
INPUT
100ns/DIV
1685 TA02
1
LTC1685
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
RO 1
R
RE 2
DE 3
DI 4
D
7
6
5
8
V
DD
B
A
GND
Supply Voltage (V
DD
) .............................................. 10V
Control Input Currents .................... – 100mA to 100mA
Control Input Voltages .................. – 0.5V to V
DD
+ 0.5V
Driver Input Voltages .................... – 0.5V to V
DD
+ 0.5V
Driver Output Voltages .................................. +12V/– 7V
Receiver Input Voltages ................................. +12V/– 7V
Receiver Output Voltages ............. – 0.5V to V
DD
+ 0.5V
Receiver Input Differential ...................................... 10V
Short-Circuit Duration (Driver V
OUT
: – 7V to 10V,
Receiver V
OUT
: 0V to V
DD
) ............................... Indefinite
Operating Temperature Range .................... 0°C to 70°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
ORDER PART
NUMBER
LTC1685CS8
S8 PART MARKING
1685
S8 PACKAGE
8-LEAD PLASTIC SO
T
JMAX
= 125°C,
θ
JA
= 150°C/ W
Consult factory for Industrial and Military grade parts.
DC ELECTRICAL CHARACTERISTICS
V
DD
= 5V
±
5%, unless otherwise noted. (Notes 2, 3)
SYMBOL
V
OD1
V
OD2
∆V
OD
PARAMETER
Differential Driver Output (Unloaded)
Differential Driver Output (With Load)
Change in Magnitude of Driver Differential
Output Voltage for Complementary
Output States
Driver Common Mode Output Voltage
Change in Magnitude of Driver Common
Mode Output Voltage for Complementary
Output States
Input High Voltage
Input Low Voltage
Input Current
Input Current (A, B)
Differential Input Threshold Voltage
for Receiver
Receiver Input Hysteresis
Receiver Output High Voltage
Receiver Output Low Voltage
Three-State (High Impedance) Output
Current at Receiver
Supply Current
Driver Short-Circuit Current, V
OUT
= HIGH
Driver Short-Circuit Current, V
OUT
= LOW
Receiver Short-Circuit Current
CONDITIONS
I
OUT
= 0
R = 50Ω (RS422)
R = 27Ω (RS485), Figure 1
R = 27Ω or 50Ω, Figure 1
q
q
q
MIN
2
1.5
TYP
MAX
V
DD
V
DD
0.2
UNITS
V
V
V
V
V
OC
∆V
OC
R = 27Ω or 50Ω, V
DD
= 5V, Figure 1
R = 27Ω or 50Ω, Figure 1
q
q
2
3
0.2
V
IH
V
IL
I
IN1
I
IN2
V
TH
∆V
TH
V
OH
V
OL
I
OZR
I
DD
I
OSD1
I
OSD2
I
OSR
DE, DI, RE
DE, DI, RE
DE, DI, RE
V
A
, V
B
= 12V, DE = 0, V
DD
= 0V or 5.25V
V
A
, V
B
= – 7V, DE = 0, V
DD
= 0V or 5.25V
– 7V
≤
V
CM
≤
12V
V
CM
= 0V
I
OUT
= – 4mA, V
ID
= 300mV
I
OUT
= 4mA, V
ID
= – 300mV
0.4V
≤
V
OUT
≤
2.4V
No Load, Pins 2, 3, 4 = 0V or V
DD
V
OUT
= – 7V or 10V (Note 5)
V
OUT
= – 7V or 10V (Note 5)
V
OUT
= 0V or V
DD
(Note 5)
q
q
q
q
q
q
2
0.8
–1
– 500
– 0.3
25
0.3
1
500
q
q
q
q
q
q
q
3.5
–1
4.8
0.4
1
7
12
20
20
20
2
U
V
V
V
V
µA
µA
µA
V
mV
V
V
µA
mA
mA
mA
mA
W
U
U
W W
W
LTC1685
DC ELECTRICAL CHARACTERISTICS
V
DD
= 5V
±5%,
unless otherwise noted. (Notes 2, 3)
SYMBOL
R
IN
C
IN
Fail-Safe
Time
CMRR
C
LOAD
PARAMETER
Input Resistance
Input Capacitance
Open-Circuit Input Voltage, Figure 5
Time to Detect Fail-Safe Condition
Receiver Input Common Mode
Rejection Ratio
Receiver and Driver Output
Load Capacitance
V
CM
= 2.6V, f = 26MHz
(Note 4)
q
CONDITIONS
– 7V
≤
V
CM
≤
12V
A, B Inputs, D, DE, RE
V
DD
= 5V (Note 4)
q
q
MIN
22
TYP
3
MAX
UNITS
kΩ
pF
3.2
3.3
2
45
3.4
V
µs
dB
500
pF
SWITCHING CHARACTERISTICS
V
DD
= 5V, unless otherwise noted. (Notes 2, 3)
SYMBOL
t
PLH
, t
PHL
t
SKEW
t
r
, t
f
t
ZH
t
ZL
t
LZ
t
HZ
t
PLH
, t
PHL
t
SQD
t
ZL
t
ZH
t
LZ
t
HZ
PARAMETER
Driver Input-to-Output Propagation Delay
Driver Output A-to-Output B Skew
Driver Rise/Fall Time
Driver Enable to Output High
Driver Enable to Output Low
Driver Disable from Low
Driver Disable from High
Receiver Skew
t
PLH
– t
PHL
Receiver Enable to Output Low
Receiver Enable to Output High
Receiver Disable from Low
Receiver Disable from High
Maximum Receiver Input
Rise/Fall Times
t
PKG-PKG
Package-to-Package Skew
Minimum Input Pulse Width
Maximum Data Rate
Maximum Input Frequency
CONDITIONS
R
DIFF
= 54Ω, C
L1
= C
L2
= 100pF,
Figures 3, 5
R
DIFF
= 54Ω, C
L1
= C
L2
= 100pF,
Figures 3, 5
R
DIFF
= 54Ω, C
L1
= C
L2
= 100pF,
Figures 3, 5
C
L
= 100pF, S2 Closed, Figures 4, 6
C
L
= 100pF, S1 Closed, Figures 4, 6
C
L
= 15pF, S1 Closed, Figures 4, 6
C
L
= 15pF, S2 Closed, Figures 4, 6
C
L
= 15pF, Figures 3, 7
C
L
= 15pF, S1 Closed, Figures 2, 8
C
L
= 15pF, S2 Closed, Figures 2, 8
C
L
= 15pF, S1 Closed, Figures 2, 8
C
L
= 15pF, S2 Closed, Figures 2, 8
(Note 4)
Same Temperature (Note 4)
V
DD
= 5V
±
5% (Note 4)
V
DD
= 5V
±
5% (Note 4)
V
DD
= 5V
±
5% (Note 4)
q
q
q
q
q
q
q
q
q
q
q
q
q
q
Receiver Input-to-Output Propagation Delay C
L
= 15pF, Figures 3, 7
The
q
denotes specifications which apply over the full operating
temperature range.
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
All currents into the device pins are positive; all currents out of the
device pins are negative.
U
MIN
15
TYP
18.5
500
3.5
25
25
25
25
MAX
22
UNITS
ns
ps
ns
50
50
50
50
22
50
50
50
50
2000
ns
ns
ns
ns
ns
ps
ns
ns
ns
ns
ns
ns
15
18.5
500
25
25
25
25
1.5
17
52
26
60
30
19.2
ns
Mbps
MHz
Note 3:
All typicals are given for V
DD
= 5V, T
A
= 25°C.
Note 4:
Guaranteed by design, but not tested.
Note 5:
Short-circuit current does not represent output drive capability.
When the output detects a short-circuit condition, output drive current is
significantly reduced (from hundreds of mA to 20mA max) until the short
is removed.
3
LTC1685
TYPICAL PERFORMANCE CHARACTERISTICS
Receiver Input CMRR
46.5
COMMON MODE REJECTION RATIO (dB)
46.0
SUPPLY CURRENT (mA)
45.0
44.5
44.0
43.5
43.0
42.5
T
A
= 25°C
42.0
10
1k
100k
FREQUENCY (Hz)
1M
1685 G01
50
40
30
20
10
0
1
10
30
DATA RATE (Mbps)
20
40
50
1685 G02
SUPPLY CURRENT (mA)
45.5
Receiver Propagation Delay
vs Load Capacitance
30
T
A
= 25°C
25
25
RECEIVER PROPAGATION DELAY (ns)
PROPAGATION DELAY (ns)
20
15
10
5
0
PROPAGATION DELAY (ns)
5
15
25
35
55
105
LOAD CAPACITANCE (pF)
Receiver Propagation Delay
vs Temperature
25
70
60
PROPAGATION DELAY (ns)
20
DATA RATE (Mbps)
15
10
5
10
0
–50 –25
4
U W
1685 G04
Supply Current vs Data Rate
70
BOTH DRIVER AND RECEIVER
ENABLED AND LOADED
60 T = 25°C
A
Supply Current vs Temperature
58
57
56
55
54
53
52
51
BOTH DRIVER AND RECEIVER
ENABLED AND LOADED
25Mbps DATA RATE
0
50
TEMPERATURE (°C)
25
75
100
1685 G03
50
– 25
Receiver Propagation Delay
vs Common Mode
25
Receiver Propagation Delay
vs Input Overdrive
T
A
= 25°C
20
T
A
= 25°C
20
15
15
10
10
5
5
205
0
8 10
4
–7 –4 –2 0
6
2
RECEIVER COMMON MODE (V)
12
0
0.3
0.5 0.7 1.0 1.25 1.5 2.0
RECEIVER INPUT OVERDRIVE (V)
2.5
1685 G05
1685 G06
Receiver Maximum Data Rate
vs Input Overdrive
T
A
= 25°C
50
40
30
20
50
75
0
25
TEMPERATURE (°C)
100
125
0
0.3
1.5
0.4
0.5 0.6 0.7 1.0
RECEIVER INPUT DIFFERENTIAL (V)
2.5
1680 G09
1685 G10
LTC1685
TYPICAL PERFORMANCE CHARACTERISTICS
Driver Propagation Delay
vs Temperature
25
PROPAGATION DELAY (ns)
PROPAGATION DELAY (ns)
t
LH
15
PROPAGATION DELAY (ns)
20
15
10
5
0
– 20
0
20
40
60
TEMPERATURE (°C)
PIN FUNCTIONS
RO (Pin 1):
Receiver Output. If A
≥
B by 300mV, then RO
will be high. If A
≤
B by 300mV, then RO will be low.
RE (Pin 2):
Receiver Enable. RE = Low enables the
receiver. RE = High forces receiver output into high
impedance state. Do not float.
DE (Pin 3):
Driver Enable. DE = High enables the driver.
DE = Low will force the driver output into a high impedance
state and the device will function as a line receiver if RE is
also low. Do not float.
DI (Pin 4):
Driver Input. Controls the states of the A and
B outputs only if DE = High. If DE = Low, DI will have no
effect on A and B pins. Do not float.
GND (Pin 5):
Ground.
A (Pin 6):
Noninverting Receiver Input/Driver Output.
B (Pin 7):
Inverting Receiver Input/Driver Output.
V
DD
(Pin 8):
Positive Supply, 5V to
±5%.
Bypass with
0.1µF ceramic capacitor.
U W
80
1685 G07
Driver Propagation Delay
vs Driver Input Voltage
25
V
DD
= 5V
INPUT THRESHOLD = 1.5V
T
A
= 25°C
t
HL
19.0
Driver Propagation Delay
vs Capacitive Load
T
A
= 25°C
18.5
18.0
17.5
17.0
16.5
16.0
20
10
5
100
0
2.5
3.0
4.0
4.5
3.5
DRIVER INPUT VOLTAGE (V)
5.0
1685 G08
5
15
25
50
75
100
LOAD CAPACITANCE (pF)
150
1685 G11
U
U
U
5
FPGA/CPLD
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