Discrete custom design of RS485 driver

Hi

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

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.

We only need 115k baud, so we could use a tiny logic level FET as the output stage. Shortcircuit protection would be done with a current limit circuit along with a low value supply capacitance (to reduce peak power in the FETs)

Backfeed would need to be solved with a beefy diode to a defined clamp voltage.

So, anyone been down this road, designing your own RS485 driver?

Cheers

Klaus

Reply to
Klaus Kragelund
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rther large drop voltage when loaded with the defined bus load for Modbus o f 54ohms, and this causes problems for our design since we have limited pow er available for driving the bus

, so we can reduce the supply down to 2V and still comply with minimum 1.5V differential voltage into 54ohms.

put stage. Shortcircuit protection would be done with a current limit circu it along with a low value supply capacitance (to reduce peak power in the F ETs)

ltage.

A rough first draft:

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Reply to
Klaus Kragelund

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

we can reduce the supply down to 2V and still comply with minimum 1.5V differential voltage into 54ohms.

stage. Shortcircuit protection would be done with a current limit circuit along with a low value supply capacitance (to reduce peak power in the FETs)

voltage.

With only a 2V supply, how do you get enough drive for the P-channel device? Do you have a more negative supply available?

An amusing thought... I know that open drain output logic exists, at least with an N-channel output. Does such a thing exist with a P-channel open-drain? ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     | 
| Analog Innovations, Inc.                         |     et      | 
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Reply to
Jim Thompson

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).

Reply to
upsidedown

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

we can reduce the supply down to 2V and still comply with minimum 1.5V differential voltage into 54ohms.

stage. Shortcircuit protection would be done with a current limit circuit along with a low value supply capacitance (to reduce peak power in the FETs)

Just use a cmos quad xor gate; two paralleled sections for one phase, two for the other, with maybe 3.3 volt supply and 30 ohm source terminations. There's no need to use discrete fets.

We recently did this:

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The basic line driver is a couple of tiny-logic gates driven from complementary FPGA outputs. The downstream junk is selectable line driver equalization, to partially correct for CAT5 cable losses. This runs up to 125 MHz.

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John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    
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John Larkin

Just a comment: Diodes are already in the FETs, in the form of body diodes.

One thought would be whether a hysteretic sync-buck IC could be pressed into service here. I haven't needed one this low in voltage yet but they should come for very low supply voltages (processor core supplies and such).

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

rarther large drop voltage when loaded with the defined bus load for Modbu s of 54ohms, and this causes problems for our design since we have limited power available for driving the bus

nts, so we can reduce the supply down to 2V and still comply with minimum 1 .5V differential voltage into 54ohms.

output stage. Shortcircuit protection would be done with a current limit ci rcuit along with a low value supply capacitance (to reduce peak power in th e FETs)

voltage.

The voltage rail for the FET are driven by 2V and I will generate an additi onal supply voltage to drive the gates, about 3V.

The majority of the power goes for the bus, driving the 54ohms load (120//1

20//1500 ohms in parallel, that is two termination resistors and the 32 uni t load impedance).

Right now the implementation is using a standard RS485 driver running at 3V supply, but with 54 ohms resistance along with the driver impedance, draws 90mW during transmission.

A low RDSon driver at 2V would reduce that to about 60mW

Regards

Klaus

Reply to
Klaus Kragelund

Yes, I added parallel more sturdy diodes, to direct the current away from the low current body diodes.

Regards

Klaus

Reply to
Klaus Kragelund

On Friday, December 21, 2012 5:57:52 PM UTC+1, snipped-for-privacy@downunder.com wrote :

arther 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 po wer available for driving the bus

Yes, but to conform to the Modbus standard, the termination resistors are a dded without diodes

Cheers

Klaus

Reply to
Klaus Kragelund

arther 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 po wer available for driving the bus

s, so we can reduce the supply down to 2V and still comply with minimum 1.5 V differential voltage into 54ohms.

tput stage. Shortcircuit protection would be done with a current limit circ uit along with a low value supply capacitance (to reduce peak power in the FETs)

oltage.

for

re's no

entary

to

Maybe a good point, if I can find a logic device that has low RDSon at 2V.

The ones I have found have 10ohms RDSon (NC7SZ74), but could parallel some of those to bring down the RDSon to the 2-3 ohms range

Regards

Klaus

Reply to
Klaus Kragelund

Are you expecting huge common mode transients? MOSFET diodes have been rated at, or above, the channel current for ages. FDV301N says 0.29A diode, 0.22A channel (both I'm sure depend on thermal resistance, it's only an SOT-23). I've never used external diodes in an inductively loaded inverter and never found any reason to: the body diodes do a fine job. They just aren't good at hard switching (slow recovery).

Have you considered BJTs for this? They tend to be easier to drive at lower voltages. With Vceo as low, you can easily find fast transistors with high hFE, so even with saturated operation, you don't have to worry about switching speed or error in the current source. You may still need a bootstrap (using all NPNs, or a negative bootstrap for the PNP pair), but only one at least.

The TL431 as shown clamps about 5V, which is way more than your supply -- are you sure about this? If it's for ESD, it's only clamping 100mA, and takes a moment to respond. A zener TVS would be a bit sloppier (a 3.3V rated device might break down at 5V and carry a heavy load at, say,

8V...), but much faster and more robust. You could also use a diode back to the +2V supply, which is probably as transient-resistant.

Tim

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Website: http://seventransistorlabs.com
Reply to
Tim Williams

Yes, the RS485 line is subjected to hot swapping, termination resistors inserted "live" and must be tested against surges/bursts. I am also worried about injected DC voltages from user wrongful installation.

The big diodes is used to divert current to the clamp using the 1ohms resistor to allow for the external diodes to draw the biggest portion of the current.

Yes, could be a good idea, just need to add circuitry to draw the carriers out of the base to switch them off fast.

Regards

Klaus

Reply to
Klaus Kragelund

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

so we can reduce the supply down to 2V and still comply with minimum 1.5V differential voltage into 54ohms.

stage. Shortcircuit protection would be done with a current limit circuit along with a low value supply capacitance (to reduce peak power in the FETs)

voltage.

no

complementary

those to bring down the RDSon to the 2-3 ohms range

Right. Parallel the sections of some 10-cent TinyLogic gate.

--
John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
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Reply to
John Larkin

low current body diodes.

Usually they are about as sturdy as the channel in the FET, can take a similar current.

Diverting current away from those only works (to some extent) if you hang a Schottky of sigifnicant size in parallel. The best method would be to steer the FET conductive while some massive surge current shows up for some reason.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

And generally not safe to use for repetitive pulses. Some discrete FET's I was using at Zarlink came with a Schottky in the same package.

...Jim Thompson

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| James E.Thompson, CTO                            |    mens     | 
| Analog Innovations, Inc.                         |     et      | 
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Reply to
Jim Thompson

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

so we can reduce the supply down to 2V and still comply with minimum 1.5V differential voltage into 54ohms.

stage. Shortcircuit protection would be done with a current limit circuit along with a low value supply capacitance (to reduce peak power in the FETs)

voltage.

no

complementary

those to bring down the RDSon to the 2-3 ohms range

Do you need to tri-state the driver? If so, Larkin's suggestion doesn't work. Even with tri-state you have to watch out for "kick" above/below rails. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     | 
| Analog Innovations, Inc.                         |     et      | 
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Reply to
Jim Thompson

What diodes ? I was suggesting using capacitors.

What Modbus "standard" ? The closest that I can think as electric Modbus standard is the

formatting link
"MODBUS over serial line specification and implementation guide V1.02"

Look at page 28

Polarization = "Fail safe termination" in RS-485 speak.

Reply to
upsidedown

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

so we can reduce the supply down to 2V and still comply with minimum 1.5V differential voltage into 54ohms.

output stage. Shortcircuit protection would be done with a current limit circuit along with a low value supply capacitance (to reduce peak power in the FETs)

voltage.

for

There's no

complementary

those to bring down the RDSon to the 2-3 ohms range

Why not? Use tri-state tiny-logic drivers.

Even with tri-state you have to watch out for "kick"

Kick? Logic chips can't drive transmission lines? Add some protection if you expect lightning bolts.

Logic chips have ESD diodes in both directions. Discretes usually don't. You're being a jerk, as usual.

--
John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
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Reply to
John Larkin

Why that? They are often used as regular power current paths. The current rating is roughly the same as the FET itself, usually.

With RF stuff all bets are off, RF transistors can be like the princess on the pea. 3V reverse Vbe ... poof ... gone. Or gaoan, as they'd say at one client.

[...]
--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

I meant to write capacitor, sorry :-)

Yes, the Modbus standard defines that, but the widespread industry standard is

120 ohms and no capacitor. (adopted from the RS485 standard)

Cheers

Klaus

Reply to
Klaus Kragelund

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