"Klaus Kragelund" <klauskvik@hotmail.com> wrote in message news:3a305d28-7df1-431d-b20e-99feb5e22fb5@googlegroups.com...> Yes, I added parallel more sturdy diodes, to direct the current away > from the low current body diodes.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 -- Deep Friar: a very philosophical monk. Website: http://seventransistorlabs.com
Discrete custom design of RS485 driver
Started by ●December 21, 2012
Reply by ●December 21, 20122012-12-21
Reply by ●December 21, 20122012-12-21
On Friday, December 21, 2012 9:10:45 PM UTC+1, Tim Williams wrote:> "Klaus Kragelund" <klauskvik@hotmail.com> wrote in message > > news:3a305d28-7df1-431d-b20e-99feb5e22fb5@googlegroups.com... > > > Yes, I added parallel more sturdy diodes, to direct the current away > > > from the low current body diodes. > > > > 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). >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.> > > 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.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 by ●December 21, 20122012-12-21
On Fri, 21 Dec 2012 11:11:58 -0800 (PST), Klaus Kragelund <klauskvik@hotmail.com> wrote:>On Friday, December 21, 2012 6:16:02 PM UTC+1, John Larkin wrote: >> On Fri, 21 Dec 2012 03:02:40 -0800 (PST), Klaus Kragelund >> >> <klauskvik@hotmail.com> wrote: >> >> >> >> >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 >> >> >> >> >> >> 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: >> >> >> >> https://dl.dropbox.com/u/53724080/Circuits/ESM/Line_Drivers.pdf >> >> >> >> 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. > >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 >Right. Parallel the sections of some 10-cent TinyLogic gate. -- 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
Reply by ●December 21, 20122012-12-21
Klaus Kragelund wrote:> On Friday, December 21, 2012 7:12:56 PM UTC+1, Joerg wrote: >> Klaus Kragelund wrote: >> >>> On Friday, December 21, 2012 12:02:40 PM UTC+1, Klaus Kragelund >>> wrote: >>>> 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? >>> A rough first draft: >>> www.electronicsdesign.dk/tmp/RS485_Custom.pdf >> >> >> >> >> 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. 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 by ●December 21, 20122012-12-21
On Fri, 21 Dec 2012 10:12:56 -0800, Joerg <invalid@invalid.invalid> wrote:>Klaus Kragelund wrote: >> On Friday, December 21, 2012 12:02:40 PM UTC+1, Klaus Kragelund >> wrote: >>> 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? >>> >> >> A rough first draft: >> >> www.electronicsdesign.dk/tmp/RS485_Custom.pdf > > >Just a comment: Diodes are already in the FETs, in the form of body diodes.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.> >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)....Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●December 21, 20122012-12-21
On Fri, 21 Dec 2012 11:11:58 -0800 (PST), Klaus Kragelund <klauskvik@hotmail.com> wrote:>On Friday, December 21, 2012 6:16:02 PM UTC+1, John Larkin wrote: >> On Fri, 21 Dec 2012 03:02:40 -0800 (PST), Klaus Kragelund >> >> <klauskvik@hotmail.com> wrote: >> >> >> >> >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 >> >> >> >> >> >> 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: >> >> >> >> https://dl.dropbox.com/u/53724080/Circuits/ESM/Line_Drivers.pdf >> >> >> >> 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. > >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 > >KlausDo 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 | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●December 21, 20122012-12-21
On Fri, 21 Dec 2012 11:03:25 -0800 (PST), Klaus Kragelund <klauskvik@hotmail.com> wrote:>> 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). > >Yes, but to conform to the Modbus standard, the termination resistors are >added without diodesWhat diodes ? I was suggesting using capacitors. What Modbus "standard" ? The closest that I can think as electric Modbus standard is the http://www.modbus.org/docs/Modbus_over_serial_line_V1_02.pdf "MODBUS over serial line specification and implementation guide V1.02" Look at page 28>Line termination may be a 150 ohms value ( 0.5 W ) resistor.>A serial capacitor ( 1 nF, 10 V minimum ) with a 120 Ohms ( 0.25 W ) >resistor is a better choice when a polarization of the pair must >be implemented (see here after).Polarization = "Fail safe termination" in RS-485 speak.
Reply by ●December 21, 20122012-12-21
On Fri, 21 Dec 2012 14:17:44 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:>On Fri, 21 Dec 2012 11:11:58 -0800 (PST), Klaus Kragelund ><klauskvik@hotmail.com> wrote: > >>On Friday, December 21, 2012 6:16:02 PM UTC+1, John Larkin wrote: >>> On Fri, 21 Dec 2012 03:02:40 -0800 (PST), Klaus Kragelund >>> >>> <klauskvik@hotmail.com> wrote: >>> >>> >>> >>> >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 >>> >>> >>> >>> >>> >>> 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: >>> >>> >>> >>> https://dl.dropbox.com/u/53724080/Circuits/ESM/Line_Drivers.pdf >>> >>> >>> >>> 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. >> >>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 > >Do you need to tri-state the driver? If so, Larkin's suggestion >doesn't work.Why not? Use tri-state tiny-logic drivers. Even with tri-state you have to watch out for "kick">above/below rails.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 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
Reply by ●December 21, 20122012-12-21
Jim Thompson wrote:> On Fri, 21 Dec 2012 10:12:56 -0800, Joerg <invalid@invalid.invalid> > wrote: > >> Klaus Kragelund wrote: >>> On Friday, December 21, 2012 12:02:40 PM UTC+1, Klaus Kragelund >>> wrote: >>>> 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? >>>> >>> A rough first draft: >>> >>> www.electronicsdesign.dk/tmp/RS485_Custom.pdf >> >> Just a comment: Diodes are already in the FETs, in the form of body diodes. > > And generally not safe to use for repetitive pulses. ...Why that? They are often used as regular power current paths. The current rating is roughly the same as the FET itself, usually.> ... Some discrete > FET's I was using at Zarlink came with a Schottky in the same package. >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 by ●December 21, 20122012-12-21
On Friday, December 21, 2012 11:04:06 PM UTC+1, upsid...@downunder.com wrote:> On Fri, 21 Dec 2012 11:03:25 -0800 (PST), Klaus Kragelund > > <klauskvik@hotmail.com> wrote: > > > > >> 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). > > > > > >Yes, but to conform to the Modbus standard, the termination resistors are > > >added without diodes > > > > What diodes ? I was suggesting using capacitors. > >I meant to write capacitor, sorry :-)> > What Modbus "standard" ? > > The closest that I can think as electric Modbus standard is the > > http://www.modbus.org/docs/Modbus_over_serial_line_V1_02.pdf > > "MODBUS over serial line specification and implementation guide V1.02" > > > > Look at page 28 > > > > >Line termination may be a 150 ohms value ( 0.5 W ) resistor. > >Yes, the Modbus standard defines that, but the widespread industry standard is 120 ohms and no capacitor. (adopted from the RS485 standard) Cheers Klaus
