Telefunken
LVDS/M-LVDS as an alternative
to RS-485/422
Why is LVDS attractive ?
• For short haul (<50m) LVDS offers a
huge improvement in bandwidth
• LVDS provides significant power savings
• LVDS generates much less EMI
• LVDS is a standard I/O in FPGA and
ASIC libraries simplifying translation
• Telefunken LVDS has extended common
mode equaling the -7 to +12V of RS-485
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Pervasive LVDS
LOGIC “1”
Close spacing of differential
pair and opposite currents
minimize EMI
3.5 mA
350mV
100
3.5 mA
LOGIC “0”
Noise coupled onto both lines
cancels thus maintaining the
differential voltage and boosting
noise immunity
3
LVDS is used extensively for reliable, low power, mid-range performance
“Common Mode”
2.4V
1.4V
Common
Mode
400
mV
Tx
Rx
1.0V
GND
Voltage Range
where the Rx
is guaranteed
to operate
GND
In noisy or distributed applications, there can be significant variation
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in local “GND” potential due to return resistance or ground bounce.
Common Mode Example
Eg: Automotive
LVDS Tx
LVDS Rx
R
Value of R changes over vehicle life
Chasis Gnd “B”
Several Volts of potential may develop between A & B
Chasis Gnd “A”
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Extended Common Mode
2.4V
Industry LVDS Spec
Guarantees Operation
Between Ground and
2.4V
>1V of noise or
Ground
Potential
Difference at Rx
Causes Fault !!!
Telefunk
extends th
mode to -7 t
same as
1V
GND @ Tx
GND @ Rx
Extended Common Mode
12V
5V
2.4V
LVDS
TI
Extended
Common
Mode
LVDS
Telefunken
Extended
Common
Mode LVDS
RS-485
0V
- 4V
- 7V
7
Robust Telefunken LVDS
• Telefunken LVDS is manufactured
using our in-house proprietary Siliconon-Insulator process. This provides:
– Extended common mode -7 to +12V
– Complete immunity to Latch-up
– Minimal leakage and consistent operation up to
~150C
– 8KV ESD
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Silicon on Insulator (SOI)
Process Technology
Source/
Bulk
Drain
SOI eliminates
parasitics and leakage
paths for a very robust
and quiet signal path.
Latch-up immune and
excellent high-temp
performance (used for
extended common
mode LVDS)
Box
Box
Box
N+
N+
N+
n+poly
N+
Fox
PBL
N+ P+ Fox
P-Well
P-Well
P-Well
NBL
NBL
NBL
P-Substrat
SiO2Insulator
SOI
Conventional bulk substrate with parasitic PNs
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TF048 Icc Leakage Tests
mA
Degrees C
No increase in leakage above 150C
10
TF048 VT threshold
NOTE: VTH (max) is,
Vcc = 3.6V (max)
Common mode = 12V (max)
SPEC
mV
NOTE: VTH (min) is,
Vcc = 3.0V (min)
Common mode = -7V (min)
Degrees C
Worst case thresholds stay close to 0V above 150C
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LVDS Features
LVDS
EIA/TIA-644A
Internal
Termination
Ext Common
Mode -4 to +5
Fairchild
X
National
X
Maxim
X
X
Texas
Instruments
X
X
X
Telefunken
X
X
X
Ext Common
Mode -7 to +12V
Robust
Latch-up
Free SOI
X
X
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Evaluation Kit
Extended Common Mode
Eval Board
includes 2
separate ground
planes with
LVDS
connections
configured via
Cat 5e or ribbon
cable
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M-LVDS
M-LVDS Features
Smooth and balanced edges
essential for driving backplanes
(tr/tf ~2 ns)
Must drive ~ 500mV Vod into distributed
loads between 30 and 50ohms (glitchfree)
50
Rx Common Mode spec is -1V to 3.4V
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M-LVDS constant VOD
M-LVDS maintains constant output VOD as load varies
M-LVDS Receiver Thresholds
200
HIGH
150
100
mV
HIGH
50
FAILSAFE
0
-50
-100
LOW
TYPE 1
LOW
TYPE 2
M-LVDS Type 2 Receiver – “Wired Or”
Type 2 Receivers with offset can provide “WiredOr” function for control signals.
Floating bus has 0 V differential and M-LVDS type 2 Rx
produces LOW output. Any driver can pull HI to interrupt
RS-485 and LVDS
Specifications and Electricals
-1V
VOS Range
VOD MAX = 0.65V
2.1V
LVDS
1.375V
1.125V
VOD MAX = 0.45V
VOD MIN = 0.25V
0.3V
VOD MIN = 0.48V
VOS Range
VOD MIN = 1.5V
VOS Range
VOD MAX = 5V
Driver Comparison
RS-485
3V
M-LVDS
Receiver Comparison
RS-485
5V
M-LVDS
Type 1
High
High
M-LVDS
Type 2
High
200 mV
LVDS
2.4 V
150 mV
High
100 mV
50 mV
0V
VID
0V
0V
-50 mV
-200 mV
Low
Low
Transition Region
-100 mV
Low
-2.4 V
Low
-5 V
2.4 V
-2.4 V
Topologies
Point to Point
Note : One Tx & Rx, terminated as close to the Rx
as possible. Provides cleanest environment capable
of the highest performance, datarate & jitter
Suitable technologies – RS-485, RS-422, LVDS
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RS-485 – Extended Common Mode
LVDS Comparison
EX CM
LVDS
RS-485
VOD
VOS
IOD
(amplitude)
(offset)
(drive)
250-450 mV
1.125 to
1.375 V
2.5 to 4.5
mA
1.5 to 5
Volts
-1 to 3
V
28 to 93
mA
VID
Common
Mode
DC to 1000
Mbps
.1 to 1
Volts
-7 to +12
Volts
DC to 10
Mbps
.4 to 5
Volts
- 7 to +12
Volts
Rise/Fall
(typical)
Datarate
.5 ns
5 to 50 ns
(typical)
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Pt to pt Translation
RS-485/422 to Extended CM LVDS
22Ω
RS-485
Resistor-divider
network guarantees
LVDS Vin amplitude
of between 300mV
and 1Volt
11Ω
TF048
22Ω
Common mode of -7 to +12V meets RS-485 spec
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Topologies
Multi-drop, Multi-point
Multiple Rx (multi-drop) and/or multiple Tx (multipoint)
Note : termination typically at each end of
transmission line, eg 100Ω for 50Ω effective load.
Suitable technologies – (capable of driving
multiple distributed loads) – RS-485, M-LVDS
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RS-485 – Multi-drop LVDS
Comparison
VOD
VOS
IOD
(amplitude)
(offset)
(drive)
M-LVDS
480-650 mV
.3 to 2.1
V
9 to 13
mA
RS-485
1.5 to 5
Volts
-1 to 3
V
28 to 93
mA
VID
Common
Mode
DC to 250
Mbps
.1 to 2.4
Volts
-1 to 3.4
Volts
DC to 10
Mbps
.4 to 5
Volts
- 7 to +12
Volts
Rise/Fall
(typical)
Datarate
1.5 ns
5 to 50 ns
(typical)
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Multidrop Translation
RS-485 to M-LVDS
RS-485
TF176
43Ω
43Ω
18Ω
18Ω
43Ω
43Ω
TF176
Need to assess common mode, M-LVDS -1 to 3.4V
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Summary
Comparing RS-485 and LVDS/M-LVDS
• LVDS offers the following advantages:
–
–
–
–
Higher datarate (at distances up to 50m)
Significant Power savings
Significantly less EMI generation
Simplified interface with FPGAs & ASICs
• Telefunken LVDS matches the RS-485 common
mode and is on latch-up free SOI
• Telefunken LVDS is an excellent alternative for short
haul point-to-point links
– Voltage divider termination needed if RS-485 driver used
• Telefunken M-LVDS is an alternative for multi-drop
applications
– Common mode and voltage divider termination/M-LVDS drive strength
needs to be evaluated for mixed RS-485/M-LVDS network.
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