telefunken lvds/m-lvds as an alternative to rs-485/422
TRANSCRIPT
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Telefunken
LVDS/M-LVDS as an alternative to RS-485/422
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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
3
100 350mV
3.5 mA
LOGIC “1”
3.5 mA
LOGIC “0”
Close spacing of differential pair and opposite currents
minimize EMI
Noise coupled onto both lines cancels thus maintaining the
differential voltage and boosting noise immunity
LVDS is used extensively for reliable, low power, mid-range performance
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“Common Mode”
400 mV
GND
2.4V
1.4V
1.0V
Common Mode
Voltage Range where the Rx is guaranteed
to operate
Tx Rx
4
GND
In noisy or distributed applications, there can be significant variation in local “GND” potential due to return resistance or ground bounce.
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Common Mode Example
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LVDS Tx LVDS Rx
Chasis Gnd “A”
Chasis Gnd “B”
Eg: Automotive
Value of R changes over vehicle life
R
Several Volts of potential may develop between A & B
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Extended Common Mode
GND @ Tx GND @ Rx
2.4V
Industry LVDS Spec Guarantees Operation Between Ground and
2.4V
>1V of noise or Ground
Potential Difference at Rx Causes Fault !!!
1V
Telefunken LVDS extends the common
mode to -7 to +12V , the same as RS-485
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Extended Common Mode
LVDS
TI Extended Common
Mode LVDS
RS-485
Telefunken Extended Common
Mode LVDS
2.4V
5V
12V
0 V
- 4V
- 7V7
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Robust Telefunken LVDS
• Telefunken LVDS is manufactured using our in-house proprietary Silicon-on-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
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P-Substrat
P-Well
N+
NBL
P-Well
N+
NBL
N+
NBLPBL
P-Well
n+poly
Fox N+N+ P+ Fox
DrainSource/
BulkBox BoxBox
SiO2Insulator
SOI
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)
Conventional bulk substrate with parasitic PNs
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TF048 Icc Leakage Tests
10
mA
Degrees C
No increase in leakage above 150C
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TF048 VT threshold
11
mV
Degrees C
SPEC
NOTE: VTH (max) is, Vcc = 3.6V (max)Common mode = 12V (max)
NOTE: VTH (min) is, Vcc = 3.0V (min)Common mode = -7V (min)
Worst case thresholds stay close to 0V above 150C
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LVDS Features
12
LVDS EIA/TIA-644A
Internal Termination
Ext Common Mode -4 to +5
Ext Common Mode -7 to +12V
Robust Latch-up Free SOI
Fairchild X
National X
Maxim X X
Texas Instruments X X X
Telefunken X X X X X
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Evaluation KitExtended Common Mode
13
Eval Board includes 2
separate ground planes with
LVDS connections
configured via Cat 5e or ribbon
cable
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M-LVDS
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M-LVDS Features
50
Smooth and balanced edges essential for driving backplanes (tr/tf ~2 ns)
Must drive ~ 500mV Vod into distributed loads between 30 and 50ohms (glitchfree)
Rx Common Mode spec is -1V to 3.4V 15
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M-LVDS constant VOD
M-LVDS maintains constant output VOD as load varies
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-100
-50
0
50
100
150
200
mV
M-LVDS Receiver Thresholds
HIGH
HIGH
LOWLOW
TYPE 1 TYPE 2
FAILSAFE
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M-LVDS Type 2 Receiver – “Wired Or”
Type 2 Receivers with offset can provide “Wired-Or” 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
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RS-485 and LVDS
Specifications and Electricals
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RS-485V
OD
MIN
= 1
.5V
-1V
3V
2.1V
0.3V
1.375V
1.125V
VO
S R
ange
VO
D M
AX
= 5
V
VO
D M
IN =
0.4
8V
VO
S R
ange
M-LVDS
VO
D M
AX
= 0
.65V
VO
S R
ange
VO
D M
IN =
0.2
5V
VO
D M
AX
= 0
.45V
LVDS
Driver Comparison
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High
Low Low
0 V
2.4 V
-2.4 V
50 mV
-50 mV
150 mV
Transition Region
M-LVDS Type 1
M-LVDS Type 2
High
High
Low
RS-485
0 V
5 V
-5 V
200 mV
-200 mV
VID
Low
0 V
2.4 V
-2.4 V
-100 mV
100 mV
LVDS
High
Receiver Comparison
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TopologiesPoint to Point
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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
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VOD (amplitude)
VOS (offset)
IOD (drive)
Rise/Fall(typical)
Datarate(typical)
VIDCommon
Mode
EX CM LVDS
250-450 mV1.125 to 1.375 V
2.5 to 4.5 mA .5 ns
DC to 1000 Mbps
.1 to 1 Volts
-7 to +12 Volts
RS-485 1.5 to 5 Volts
-1 to 3 V
28 to 93 mA 5 to 50 ns
DC to 10 Mbps
.4 to 5 Volts
- 7 to +12 Volts
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Pt to pt TranslationRS-485/422 to Extended CM LVDS
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RS-485 TF048
22Ω
22Ω
11Ω
Resistor-divider network guarantees LVDS Vin amplitude of between 300mV
and 1Volt
Common mode of -7 to +12V meets RS-485 spec
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TopologiesMulti-drop, Multi-point
25
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
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VOD (amplitude)
VOS (offset)
IOD (drive)
Rise/Fall(typical)
Datarate(typical)
VIDCommon
Mode
M-LVDS 480-650 mV.3 to 2.1
V9 to 13
mA1.5 ns
DC to 250 Mbps
.1 to 2.4 Volts
-1 to 3.4 Volts
RS-485 1.5 to 5 Volts
-1 to 3 V
28 to 93 mA 5 to 50 ns
DC to 10 Mbps
.4 to 5 Volts
- 7 to +12 Volts
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Multidrop TranslationRS-485 to M-LVDS
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RS-485
Need to assess common mode, M-LVDS -1 to 3.4V
TF176
43Ω
43Ω
18ΩTF176
43Ω
43Ω
18Ω
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SummaryComparing 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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