Power over LVDS

Joe Chisolm

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Apr 9, 2013, 12:55:55 PM4/9/13

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Anyone have any good reference material or app notes for Power over LVDS
lines. I ran across this the other day. Google does not give me much.
One usage seems to be powering rear view cameras in cars. I found a
power point presentation from National Semi that had a couple of
slides with a simple analysis of pushing 2A@12V on the diff pair.

--
Chisolm
Republic of Texas

Jim Thompson

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Apr 9, 2013, 1:03:44 PM4/9/13

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John Larkin

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Apr 9, 2013, 1:47:58 PM4/9/13

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On Tue, 09 Apr 2013 10:03:44 -0700, Jim Thompson
<To-Email-Use-Th...@On-My-Web-Site.com> wrote:

>On Tue, 09 Apr 2013 11:55:55 -0500, Joe Chisolm
><jchi...@earthlink.net> wrote:
>
>>Anyone have any good reference material or app notes for Power over LVDS
>>lines. I ran across this the other day. Google does not give me much.
>>One usage seems to be powering rear view cameras in cars. I found a
>>power point presentation from National Semi that had a couple of
>>slides with a simple analysis of pushing 2A@12V on the diff pair.
>
>Isn't "12V LVDS" an oxymoron?
>
>Link to the PowerPoint?
>
> ...Jim Thompson

AC couple the data, dummy. Like PoE.

--

John Larkin Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation

Joe Chisolm

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Apr 9, 2013, 4:12:20 PM4/9/13

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On Tue, 09 Apr 2013 10:03:44 -0700, Jim Thompson wrote:

> On Tue, 09 Apr 2013 11:55:55 -0500, Joe Chisolm
> <jchi...@earthlink.net> wrote:
>
>>Anyone have any good reference material or app notes for Power over LVDS
>>lines. I ran across this the other day. Google does not give me much.
>>One usage seems to be powering rear view cameras in cars. I found a
>>power point presentation from National Semi that had a couple of slides
>>with a simple analysis of pushing 2A@12V on the diff pair.
>
> Isn't "12V LVDS" an oxymoron?
>
> Link to the PowerPoint?
>
> ...Jim Thompson

http://tinyurl.com/cxjtg29

There was also a mention about it in a Intersil presser
for their ISL76321.

I would expect something a little more exotic than a Ethernet
style POE given the signaling rates.

josephkk

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Apr 13, 2013, 11:58:54 PM4/13/13

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On Tue, 09 Apr 2013 11:55:55 -0500, Joe Chisolm <jchi...@earthlink.net>
wrote:

Could be reasonable. Seems like it would be a variant of Power over
Ethernet (PoE).

?-0)

upsid...@downunder.com

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Apr 14, 2013, 6:08:18 AM4/14/13

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On Tue, 09 Apr 2013 11:55:55 -0500, Joe Chisolm
<jchi...@earthlink.net> wrote:

Sounds like phantom feed for condenser microphones or Power over
Ethernet. However, these use transformers to couple the signals.

To use transformers, the signal should not contain any data dependent
DC offsets, so you would have to use Manchester coding etc.

k...@attt.bizz

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Apr 14, 2013, 12:39:17 PM4/14/13

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On Sun, 14 Apr 2013 13:08:18 +0300, upsid...@downunder.com wrote:

>On Tue, 09 Apr 2013 11:55:55 -0500, Joe Chisolm
><jchi...@earthlink.net> wrote:
>
>>Anyone have any good reference material or app notes for Power over LVDS
>>lines. I ran across this the other day. Google does not give me much.
>>One usage seems to be powering rear view cameras in cars. I found a
>>power point presentation from National Semi that had a couple of
>>slides with a simple analysis of pushing 2A@12V on the diff pair.
>
>
>Sounds like phantom feed for condenser microphones or Power over
>Ethernet. However, these use transformers to couple the signals.

Sorta. The schemes I've seen use capacitors at each end to couple the
LVDS lines and inductors to isolate the DC. At the frequencies
usually encountered, the parts are small and cheap.

>To use transformers, the signal should not contain any data dependent
>DC offsets, so you would have to use Manchester coding etc.

I haven't seen transformers used for power over LVDS. Often there's
only one LVDS line, so the common center-tapped PoE thing doesn't
work.

Joe Chisolm

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Apr 14, 2013, 1:19:32 PM4/14/13

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The transmitters and receivers that support AC coupling are using
8b/10b Line Code to DC balance.

I'll look through this and see if there is anything interesting

AC-Coupling Between Differential LVPECL, LVDS, HSTL,and CML
www.ti.com/lit/an/scaa059c/scaa059c.pdf

The Intersil part ISL76321 shows ac coupling in the data sheet with
27nf caps. This is for their 1G link but no power injection.

When I have some time I'll sit down and work it out....

Jasen Betts

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Apr 15, 2013, 6:26:51 AM4/15/13

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isn't it +12 on the signal lines and ground return?

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k...@attt.bizz

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Apr 15, 2013, 12:39:03 PM4/15/13

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I'm not clear what the question is. The LVDS schemes I've seen are
power on one and ground on the other, similar to the phantom
microphone power scheme (above). I suppose you could put 12V on both
and rely on another common ground for the return. Sounds like an EMI
problem waiting to happen, though.

lang...@fonz.dk

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Apr 15, 2013, 1:18:22 PM4/15/13

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On Apr 15, 6:39 pm, k...@attt.bizz wrote:
> On 15 Apr 2013 10:26:51 GMT, Jasen Betts <ja...@xnet.co.nz> wrote:
>
>
>
>
>
>
>
>
>
> >On 2013-04-14, k...@attt.bizz <k...@attt.bizz> wrote:

xlr phantom power is gnd to gnd, ~48V via resistors to both signal
wires

think you could use similar scheme for lvds, use inductors and caps to
isolate power and signal so you can try to get proper differential
termination

should few volts of common mode voltage to play with

-Lasse