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Telefunken

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

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Telefunken

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  1. Telefunken LVDS/M-LVDS as an alternative to RS-485/422

  2. 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

  3. Pervasive LVDS Close spacing of differential pair and opposite currents minimize EMI LOGIC “1” 3.5 mA 100 350mV 3.5 mA Noise coupled onto both lines cancels thus maintaining the differential voltage and boosting noise immunity LOGIC “0” LVDS is used extensively for reliable, low power, mid-range performance

  4. 400 mV “Common Mode” 2.4V 1.4V Common Mode Voltage Range where the Rx is guaranteed to operate Tx Rx 1.0V GND GND In noisy or distributed applications, there can be significant variation in local “GND” potential due to return resistance or ground bounce.

  5. 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”

  6. Extended Common Mode 2.4V >1V of noise or Ground Potential Difference at Rx Causes Fault !!! Telefunken LVDS extends the common mode to -7 to +12V , the same as RS-485 Industry LVDS Spec Guarantees Operation Between Ground and 2.4V 1V GND @ Tx GND @ Rx

  7. Extended Common Mode 12V 5V 2.4V TI Extended Common Mode LVDS Telefunken Extended Common Mode LVDS LVDS RS-485 0 V - 4V - 7V

  8. 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

  9. Silicon on Insulator (SOI) Process Technology 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) SiO2Insulator SOI Conventional bulk substrate with parasitic PNs

  10. TF048 Icc Leakage Tests mA Degrees C No increase in leakage above 150C

  11. 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

  12. LVDS Features

  13. Evaluation KitExtended Common Mode Eval Board includes 2 separate ground planes with LVDS connections configured via Cat 5e or ribbon cable

  14. M-LVDS

  15. 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

  16. M-LVDS constant VOD M-LVDS maintains constant output VOD as load varies

  17. M-LVDS Receiver Thresholds HIGH HIGH FAILSAFE LOW LOW TYPE 1 TYPE 2

  18. 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

  19. RS-485 and LVDS Specifications and Electricals

  20. Driver Comparison

  21. Receiver Comparison

  22. TopologiesPoint 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

  23. RS-485 – Extended Common Mode LVDS Comparison

  24. Pt to pt TranslationRS-485/422 to Extended CM LVDS 22Ω Resistor-divider network guarantees LVDS Vin amplitude of between 300mV and 1Volt 11Ω TF048 RS-485 22Ω Common mode of -7 to +12V meets RS-485 spec

  25. TopologiesMulti-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

  26. RS-485 – Multi-drop LVDS Comparison

  27. Multidrop TranslationRS-485 to M-LVDS 43Ω RS-485 43Ω 18Ω TF176 TF176 18Ω 43Ω 43Ω Need to assess common mode, M-LVDS -1 to 3.4V

  28. 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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