Discrete custom design of RS485 driver

On Saturday, December 22, 2012 5:48:56 PM UTC+1, Joerg wrote:
> dagmargoodboat@yahoo.com wrote:
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> > On Dec 21, 3:48 pm, Joerg <inva...@invalid.invalid> wrote:
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> >> Klaus Kragelund wrote:
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> >>> On Friday, December 21, 2012 7:12:56 PM UTC+1, Joerg wrote:
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> >=20
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> >>>> Just a comment: Diodes are already in the FETs, in the form of body =
diodes.
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> >>> Yes, I added parallel more sturdy diodes, to direct the current away =
from the low current body diodes.
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> >> Usually they are about as sturdy as the channel in the FET, can take a
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> >> similar current.
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> >=20
>=20
> > Note: I don't see a spec for the FDV304P's trr.  Body diodes' trr can
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> > be horrendously slow--at least once upon a time on power FETs they
>=20
> > often were...
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> >=20
>=20
>=20
>=20
> Datasheets aren't what they used to be, lots of specs lacking. The
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> question that needs to be pondered though is, does trr really matter in
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> Klaus' case? RS485 isn't exactly like a Maserati so even if ringing
>=20
> pings the body diodes a little it probably won't matter.
>=20

Very likely not, like Tim pointed out.

Our max speed is 115k, which corresponds to 9us. The drivers are allowed to=
 drift by 2%, so for 11 bits, that would be a single character drift at the=
 end of the bit stream of close to 25% from the ideal mid bit sampling. So =
it could probably tolerate plenty of ringing after the bit transition.

Some UARTS today implement averaging, oversampling the RX by 8 to 16 times,=
 so ringing can affect the data quality closer to the transition, but nowhe=
re near the 100ns switching times as discussed

Regards

Klaus

On Saturday, December 22, 2012 9:49:00 PM UTC+1, upsid...@downunder.com wrote:
> On Sat, 22 Dec 2012 09:08:51 -0800, Joerg <invalid@invalid.invalid>
> 
> wrote:
> 
> 
> 
> >Klaus Kragelund wrote:
> 
> >> On Saturday, December 22, 2012 7:43:22 AM UTC+1, miso wrote:
> 
> >>> If supply voltage is the only issue, have you investigated using a DC/DC 
> 
> >>>
> 
> >>> plus an off the shelf 485?
> 
> >> 
> 
> >> Yes, currently we are using off the shelf RS485, but the bus voltage is high due to 3V supply, so the power budget is exceeded
> 
> >> 
> 
> >> We have seen no devices below 3V that can drive 54 ohms
> 
> >> 
> 
> >
> 
> >There could be hope:
> 
> >
> 
> >http://www.electronicsweekly.com/Articles/13/07/2012/54115/intersil-has-rs-485-transceiver-running-off-1.8v.htm
> 
> >
> 
> >Maybe give them a ring but then I'd also check for availability.
> 
> 
> 
> I must say, I am really impressed by those specs.
> 
> 
> 
> However, you have to play the Devil's advocate to select a usable
> 
> product. 
> 
> 
> 
> A quick look suggests that you have to use at least Vcc > 3.0 V to be
> 
> within RS-485 common mode range. I have not looked at other
> 
> parameters.

Actually the device we are using now is an Intersil part and we have had the FAE visit and in direct contact with the design engineers. They suggested that part, but it only works at 1.8V with no load (no termination resistors)

Cheers

Klaus

On Saturday, December 22, 2012 8:58:39 PM UTC+1, whit3rd wrote:
> On Friday, December 21, 2012 3:02:40 AM UTC-8, Klaus Kragelund wrote:
> 
> 
> 
> > The standard RS485 drivers available has a minimum voltage of 3V and a rarther large drop voltage when loaded with the defined bus load ...
> 
> > So, we are thinking about designing our own driver in discrete components, so we can reduce the supply down to 2V and still comply with minimum 1.5V differential voltage into 54ohms.
> 
> 
> 
> RS-485 is a multidrop bus, with termination resistors; do you control
> 
> the termination resistors, or are they external to your design?
> 

No, that is up to the system configurator/responsible

> 
> 
> Your 'supply down to 2V' circuitry will have to be capable of safely handling the RS-485 bus
> 
> signal range, -7V to +12V, or you can discard the standard entirely, and use something
> 
> more appropriate to low-power implementation.

As long as I supply minimum 1.5V (A-B or B-A) within the compliance range, I should be ok. 

Since I have galvanic isolation the output (TX) should be well behaved. As for the RX I will use a commercial part for the detection that has the required compliance.

Cheers

Klaus

On Dec 22, 12:15=A0am, k...@att.bizzz wrote:
> On Fri, 21 Dec 2012 19:06:43 -0800 (PST), dagmargoodb...@yahoo.com
> wrote:
>
> >On Dec 21, 3:48=A0pm, Joerg <inva...@invalid.invalid> wrote:
> >> Klaus Kragelund wrote:
> >> > On Friday, December 21, 2012 7:12:56 PM UTC+1, Joerg wrote:
>
> >> >> Just a comment: Diodes are already in the FETs, in the form of body=
 diodes.
>
> >> > Yes, I added parallel more sturdy diodes, to direct the current away=
 from the low current body diodes.
>
> >> Usually they are about as sturdy as the channel in the FET, can take a
> >> similar current.
>
> >Note: I don't see a spec for the FDV304P's trr. =A0Body diodes' trr can
> >be horrendously slow--at least once upon a time on power FETs they
> >often were...
>
> Define "slow". =A0This is the one I'm currently using (Trr=3D40ns) for a
> power supply. =A0I'm going to have to go to a 60V device, though.
>
> http://www.ti.com/lit/ds/symlink/csd18501q5a.pdf

40nS is fast.  Slow?  Well, if they don't specify, you've got to
measure.  I don't remember the figure, but I got burned on a proto
trying to use a body diode as  a flyback rectifier once upon a time.
It worked, but it got HOT and wasted a lot of power.  A schottky fixed
it.

--
Cheers,
James Arthur

On Dec 22, 1:37=A0am, miso <m...@sushi.com> wrote:
> Is it possible the VC is going to stupid internet social media garbage?
> Ya think?
>
> Why build a market for a hardware device, which hey, is work, when you
> can sell nonsense. The current mania is photographs that disappear, when
> it used in sexting. Hey, I know of a few congressmen who could have used
> such a service.

Sounds like a weiner^H^H^H^Hinner!

--
Cheers,
James Arthur

On Dec 22, 6:23=A0am, "Tim Williams" <tmoran...@charter.net> wrote:
> <k...@att.bizzz> wrote in message
>
> news:m4gad85lueriqmjs5qj7im8caa8uvk6ih3@4ax.com...
>
> >>Note: I don't see a spec for the FDV304P's trr. =A0Body diodes' trr can
> >>be horrendously slow--at least once upon a time on power FETs they
> >>often were...
>
> > Define "slow". =A0This is the one I'm currently using (Trr=3D40ns) for =
a
> > power supply. =A0I'm going to have to go to a 60V device, though.
>
> >http://www.ti.com/lit/ds/symlink/csd18501q5a.pdf
>
> FYI, note their switching speed measurements specify "RG=3D0", which real=
ly
> just means the figures they give are utterly meaningless.
>
> My experience is, t_rr is roughly proportional to Vds(max), and usually
> about 2-3 times the rating of the fastest junction diode of comparable
> ratings.
>
> Example:
> - The above does 60V and 100A, and the body diode is specified at 25A.
> There aren't any junction diodes available to compare with that, they're
> all schottky. =A0(A plain old UF4001 is rated 50V, 1A, and 50ns, but it's=
 in
> the same bracket as UF4004, which ought to be 50ns. =A0A true, optimized
> UF4001 should be quite fast indeed, basically 3-4 x 1N914 in parallel.)
> - A better comparison would be made between, say, 200V, 600V and 1200V
> devices. =A0Offhand, I think the ratings for something like a 200V, 10A
> MOSFET will be maybe 100ns, and a 200V, 10A diode, 50ns; at 600V 10A, mor=
e
> like 250ns and 100ns; and at 1200V 10A, around 350ns and 150ns.

My basic assumption is that if a MOSFET's trr isn't spec'd, it's
probably not good.  If they do spec it, then there's nothing to worry
about.

I've seen they're making lots of the big FETs' body diodes fast these
days, I'm just not sure about the little guys, like Klaus' FDV30P.
Anyway, he's got clamp diodes; my point was that he might well need
them.

--
Cheers,
James Arthur

On Dec 22, 3:53=A0pm, Klaus Kragelund <klausk...@hotmail.com> wrote:
> On Saturday, December 22, 2012 5:48:56 PM UTC+1, Joerg wrote:
> > dagmargoodb...@yahoo.com wrote:
>
> > > On Dec 21, 3:48 pm, Joerg <inva...@invalid.invalid> wrote:
>
> > >> Klaus Kragelund wrote:
>
> > >>> On Friday, December 21, 2012 7:12:56 PM UTC+1, Joerg wrote:
>
> > >>>> Just a comment: Diodes are already in the FETs, in the form of bod=
y diodes.
>
> > >>> Yes, I added parallel more sturdy diodes, to direct the current awa=
y from the low current body diodes.
>
> > >> Usually they are about as sturdy as the channel in the FET, can take=
 a
>
> > >> similar current.
>
> > > Note: I don't see a spec for the FDV304P's trr. =A0Body diodes' trr c=
an
>
> > > be horrendously slow--at least once upon a time on power FETs they
>
> > > often were...
>
> > Datasheets aren't what they used to be, lots of specs lacking. The
>
> > question that needs to be pondered though is, does trr really matter in
>
> > Klaus' case? RS485 isn't exactly like a Maserati so even if ringing
>
> > pings the body diodes a little it probably won't matter.
>
> Very likely not, like Tim pointed out.
>
> Our max speed is 115k, which corresponds to 9us. The drivers are allowed =
to drift by 2%, so for 11 bits, that would be a single character drift at t=
he end of the bit stream of close to 25% from the ideal mid bit sampling. S=
o it could probably tolerate plenty of ringing after the bit transition.
>
> Some UARTS today implement averaging, oversampling the RX by 8 to 16 time=
s, so ringing can affect the data quality closer to the transition, but now=
here near the 100ns switching times as discussed

Good point.  Agreed.  So, if you forge ahead, you might be able to
save a few clamp diodes.

--
Cheers,
James Arthur

On Sun, 23 Dec 2012 06:33:31 -0800 (PST), dagmargoodboat@yahoo.com
wrote:

>On Dec 22, 12:15�am, k...@att.bizzz wrote:
>> On Fri, 21 Dec 2012 19:06:43 -0800 (PST), dagmargoodb...@yahoo.com
>> wrote:
>>
>> >On Dec 21, 3:48&#4294967295;pm, Joerg <inva...@invalid.invalid> wrote:
>> >> Klaus Kragelund wrote:
>> >> > On Friday, December 21, 2012 7:12:56 PM UTC+1, Joerg wrote:
>>
>> >> >> Just a comment: Diodes are already in the FETs, in the form of body diodes.
>>
>> >> > Yes, I added parallel more sturdy diodes, to direct the current away from the low current body diodes.
>>
>> >> Usually they are about as sturdy as the channel in the FET, can take a
>> >> similar current.
>>
>> >Note: I don't see a spec for the FDV304P's trr. &#4294967295;Body diodes' trr can
>> >be horrendously slow--at least once upon a time on power FETs they
>> >often were...
>>
>> Define "slow". &#4294967295;This is the one I'm currently using (Trr=40ns) for a
>> power supply. &#4294967295;I'm going to have to go to a 60V device, though.
>>
>> http://www.ti.com/lit/ds/symlink/csd18501q5a.pdf
>
>40nS is fast.  Slow?  Well, if they don't specify, you've got to
>measure.  I don't remember the figure, but I got burned on a proto
>trying to use a body diode as  a flyback rectifier once upon a time.
>It worked, but it got HOT and wasted a lot of power.  A schottky fixed
>it.

Well, the body diode is a Si P-N junction so it's going to be .7-1V,
vs the .3-.5ish for the Schottky.  The Schottky saves energy?  Well,
yeah.  So does turning on the FET (Vf*Id < Rds(on) * Id^2).  ;-)

On Fri, 21 Dec 2012 18:57:52 +0200, upsidedown@downunder.com wrote:

>On Fri, 21 Dec 2012 03:02:40 -0800 (PST), Klaus Kragelund
><klauskvik@hotmail.com> wrote:
>
>>The standard RS485 drivers available has a minimum voltage of 3V and a rarther large drop voltage when loaded with the defined bus load for Modbus of 54ohms, and this causes problems for our design since we have limited power available for driving the bus
>
>This is not a Modbus specific issue, but rather RS-485 specific issue
>with a twisted pair bus with characteristic impedance of 100-120 ohms.
>In order to avoid reflections at the open ends of the bus cable,
>termination resistors are typically used at both ends with the same
>value as the cable characteristic impedance. 
>
>For DC, those two resistors are effectively in parallel and hence the
>45 ohm total load.
>
>However, those termination resistors are needed only to avoid the
>reflections from voltage _transitions_. Thus, putting a capacitor in
>series with the termination resistor(s) should reduce the idle power
>consumption, when no data is being sent. Of course, without DC
>continuity, the end to end signal ground conductor is essential.
>
>There are application notes describing even more elaborate termination
>methods, describing their advantages and disadvantages. You should
>also look for various termination techniques used on CAN bus (which is
>essentially RS-485).
> 

I will get back to you after i reread some of my TIA-485 standard.  I
don't think you understand it properly.

?-)

On 12/24/2012 11:18 AM, eatshit@dropdead.com wrote:
> On Fri, 21 Dec 2012 18:57:52 +0200, upsidedown@downunder.com wrote:
>
>> On Fri, 21 Dec 2012 03:02:40 -0800 (PST), Klaus Kragelund
>> <klauskvik@hotmail.com> wrote:
>>
>>> The standard RS485 drivers available has a minimum voltage of 3V and a rarther large drop voltage when loaded with the defined bus load for Modbus of 54ohms, and this causes problems for our design since we have limited power available for driving the bus
>>
>> This is not a Modbus specific issue, but rather RS-485 specific issue
>> with a twisted pair bus with characteristic impedance of 100-120 ohms.
>> In order to avoid reflections at the open ends of the bus cable,
>> termination resistors are typically used at both ends with the same
>> value as the cable characteristic impedance.
>>
>> For DC, those two resistors are effectively in parallel and hence the
>> 45 ohm total load.
>>
>> However, those termination resistors are needed only to avoid the
>> reflections from voltage _transitions_. Thus, putting a capacitor in
>> series with the termination resistor(s) should reduce the idle power
>> consumption, when no data is being sent. Of course, without DC
>> continuity, the end to end signal ground conductor is essential.
>>
>> There are application notes describing even more elaborate termination
>> methods, describing their advantages and disadvantages. You should
>> also look for various termination techniques used on CAN bus (which is
>> essentially RS-485).
>>
>
> I will get back to you after i reread some of my TIA-485 standard.  I
> don't think you understand it properly.
>
> ?-)
>

The reason that you do not understand it is that you eat too much shit.