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

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

“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.

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

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

Extended Common Mode

LVDS

TI Extended Common

Mode LVDS

RS-485

Telefunken Extended Common

Mode LVDS

2.4V

5V

12V

0 V

- 4V

- 7V7

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

TF048 Icc Leakage Tests

10

mA

Degrees C

No increase in leakage above 150C

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

LVDS Features

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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

Evaluation KitExtended Common Mode

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Eval Board includes 2

separate ground planes with

LVDS connections

configured via Cat 5e or ribbon

cable

M-LVDS

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

M-LVDS constant VOD

M-LVDS maintains constant output VOD as load varies

-100

-50

0

50

100

150

200

mV

M-LVDS Receiver Thresholds

HIGH

HIGH

LOWLOW

TYPE 1 TYPE 2

FAILSAFE

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

RS-485 and LVDS

Specifications and Electricals

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

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

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

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

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

TopologiesMulti-drop, Multi-point

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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

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

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

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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