Possible Latchup problem with RS485 transciever

I have a project where the input of an RS-485 receiver can have voltage on it before the main power supply comes on.

My project has worked well in the past when my cable's bus voltage was

12v and I used some 12v to 5vDC Astrodyne DC-DC converters.

I am now using a cable bus voltage of 48vDC and some MeanWell DC-DC converters.

I have experienced strange part failures since switching over to the

48vDC supplies. I think the source of the problem may either be latchup or the regulated output of the DC-DC converter rising too slowly. It takes about 200us to go from 0v to 6v and within 50us has settled to 5v. None of the parts should be damaged from 6v, even continuously. The parts rarely survive 5-10 power cycles.

I have two signals, one that is about 50% duty cycle at 1MHz, and the other that is "on" for about 2us and "off" for about 5ms. So far I have not experienced a problem with the RS-485 receiver connected to the 2us/5ms signal, just the 1MHz signal. The duty of the signal shouldn't matter with RS-485, right? Considering A will be what B is not...

The part failure seems to be where one of the RS-485 inputs either sinks or sources current into the transmission line. With one part I observed a 10 ohm resistance from an input pin to ground. This part does seem to still have some functionality since if both inputs of the receiver have the same signal the "R" output of the receiver will mirror its inputs, however there is still current being either sinked or sourced depending on the specific chip and how it decided to die, I guess...

The RS-485 devices that I have observed this with are the 75ALS180 and the MAX3467. Neither are specifically "Hot Swappable".

From reading the Maxim IC data sheet

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I don't understand what makes the MAX3467 not hot swappable and the

3468 hot swappable. As far as I can tell, the feature only makes sure the transmit stage doesn't activate during processor initialization. This isn't my concern since I only receive a signal.

I've read about solutions to protect the ICs against latchup. One of them involved series resistors. I may try to place 10k resistors in series with the A+B signals after the termination resistor. I can't imagine that these devices should behave like this, latchup shouldn't occur on a device like this from the differential input pair.

Can anyone suggest what may be causing my problems?.Which one of these ideas should I use? Pull up/down resistors? The RS-485 standard already allows for voltages above VCC and below GND, so if I'm not crazy I shouldn't be having issues with these parts! The only part I have left with enough quantity to work with is the LTC485.

Sorry for the long unorganized message. Its been a very long day! I'll be working on this project all weekend...

Thanks, Grant

Reply to
logjam
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6V is abs max for the chip you mention below. I would definitely get rid of that overshoot.

Make sure you don't exceed the input voltage abs max. They do not mention any subtrate diode maximum current which is kind of strange. I'd ask them about it.

For series resistors to work you would need to know the substrate diode ratings.

Can you post a schematic?

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Joerg

Perhaps you have problems with some inductance somewhere causing the voltage spike. Connect a 10u to 100uf capacitor directly across the RS485 chip. I've had a similar problem with a circuit and placing the extra capacitor solved the problem nicely.

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Reply to
Nico Coesel

Right now I have a 1u tantalum near the ICs and (2) .1u ceramics as close to each driver/receiver as possible. I've just swapped in the LTC485 and I haven't been able to kill it. I almost gave myself carpel tunnel twisting the cable on and off... : )

The cable that connects to this board contains 48vDC and the two RS-485 pairs. I need the circuit to withstand live connections/ disconnections with any order of power/ground/485 connections.

I have been working on the issue for a few hours now, and have a lot more information to add. (schematic, scope captures) I will update this thread soon.

Thanks!

Grant

Reply to
Grant Stockly

Thanks for the response! I have investigated the problem further this afternoon and have some new information. I will add it below the responses to your questions.

How long is it "OK" to have a ringing signal or transient exceed the VCC or GND limits? Not at all? I'm primarily a digital person. Will a zener diode on the power supply clamp fast enough to remove the transients? I suppose a zener along with a very low ESR electrolytic would help. Right now all I have is a 1u tantalum, and whatever is in the output stage of those switching DC-DC power supplies.

Yes, here it is:

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-I included 5 "load" resistors on the PCB to satisfy the minimum load required to meet the regulation specification of the power supplies.

New information:

-The 75ALS180 ICs in their "dead" eventually reach a temperature of

130F.

-Quite frequently, but not all the time, I experience transients and/ or fairly long negative voltage periods when first making the connection with the circuit.

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The negative part of the first to examples worries me more than the positive part. The last capture was VCC during connecting the circuit. At 100ns/div, would a zener be fast enough to protect against these positive spikes?

-When NO power supply is connected to the PCB, only Clock A/B and/or Line A/B, the 75ALS180 in this "dead" state (I don't have any good ones left) will cause the VCC pin to reach 2vDC. It does this approximately 10ms after the PCB has been connected. ALL of the capture examples in this section are from a chip that I consider to be dead. I will specifically state when a capture is from a good environment.

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The above screen capture shows the 48vDC and 5v VCC voltages when the A/B data pairs are NOT connected. The DC-DC converter turns on approximately 350ms after 48v is present

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Sometimes, but not always, the above example occurs.

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When plugged in, before the DC-DC converter turns on, the bus ramps up to 2v.

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The above screen capture shows the 48vDC applied to the power supply, VCC ramping up to 2v, and then about 80ms later VCC ramping up to 5v. The DC-DC converter hadn't been disconnected for very long, as you can see it had about 8v in its capacitors when I powered it up. This may explain the faster turn on time compared to other examples.

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If I leave the bus connected, but disconnect the power supply, VCC will remain at 2v. If 48v is applied after the bus was connected, the above capture shows the DC/DC converters response. Doesn't look high impedance to me! : )

GOOD NEWS:

-I can't manage to kill the LTC485. Its data inputs remain high impedance no matter how poorly I try to connect and disconnect the cable. What killed 75ALS180/MAX3467 IC after IC last night doesn't seem to hurt this part.

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Here is VCC coming up and the "R" output of the LTC485 activating

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The "R" output. This is good. The other ICs when dead do not give me an output, they only load the lines.

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This is the A and B lines. The B line did not have a ground clamp, only A.

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This is the A and B lines (A on scope probe, B on probe ground) when terminated. The faint gray trace is out of circuit, but terminated, and the black trace is connected to the LTC485 and terminated. I'm not sure if the signal was being offset by the scope's ground. I had to disconnect the scope's RS-232 port from my computer (which shares a ground with the RS-485 circuit). Before disconnecting the RS-232 cable it looked a lot worse. The scope's own ground could still be impacting the signal presentation for all I know...

So, I know the MAXIM and TI ICs are damaged. I don't know HOW they were damaged. It could be a latchup type failure from seeing voltage on the RS-485 inputs before VCC came up. I hope that isn't the case since in the "real world" a device could be connected to a network and powered down, right?

The second possibility is that the transients and negative voltage periods damage the 75ALS180 and MAX3467, but not the LTC485. The LTC485 may be designed with better internal protection to such events? The LTC485 specifies a maximum input voltage of 12v, which would help with any possible positive voltage periods which would otherwise damage a 5v part.

All that aside, the LTC485 data sheet specifically says "Power-Up/Down Glitch-Free Driver Outputs Permit Live Insertion or Removal of Transceiver" and "Driver Maintains High Impedance in Three-State or with the Power Off". Since this is a half duplex device, driver and receiver share the same inputs. Therefore the data sheet clearly says that the "driver" pins can see voltage when the power is off.

The 75ALS180 data sheet says "Glitch-Free Power-Up and Power-Down Protection", which is not quite as spelled out, but from a RS-485 point of view indicates to me that it won't disturb network traffic during power-up and down. The MAXIM data sheet doesn't specifically speak about power-up and power-down except when referring to their "Hot-Swap Versions", but in that case it is only referring to a set- reset setup inside the IC which prevents a high impedance driver enable from accidentally disturbing the bus during power up. The device I'm using is not a hot swap version, but I'm also not having and will never have a problem with two devices trying to drive the same line.

In closing, I would hope that all 3 of the RS-485 devices would be perfectly capable of handling voltage on their inputs when the power supply is off. If this is not the case, please tell me! It only seems logical! : ) The common mode voltage can already exceed VCC and go below ground, so it shouldn't care when the power is off, right? Ground+12v and Ground-7v is always the same regardless of where VCC is...

Is it possible, or logical, to assume that a transient or negative voltage could cause an input to conduct power to VCC? Maybe an input is conducting through a damaged protection diode to VCC???

I hope that I have described the problem clear enough that someone can tell me without too much effort what my problem is! I would like to fix the problem so that I can no longer kill 75ALS180s on demand. I will then switch to using the LTC parts which have proven to be at least a little better in this environment. I figure that if the circuit will work with the suicidal parts, it will work even better with the LTC parts. I'm not comfortable with using the LTC parts until I'm sure the problem has been fixed. : )

Thanks for your time, Grant

Reply to
Grant Stockly

I am not sure how it all sits together but let me give some pointers anyway.

The "real" abs max is usually a few percent higher but that depends on how conservative a manufacturer is. Once you reach "real" abs max levels you must not exceed that at all. Never, ever.

1uF is not a lot, try more. And personally I do not use tantalums in supply rails, ever.

There are just parts and ports, not sure where it all goes. Consider protection measures especially if the data line is long or runs in a noisy environment. You might need transzorbs, ferrites and the like.

That is quite typical after an overvoltage meltdown. If the power supply had more punch you'd probably see some smoke.

Ouch!

Yes but that won't work for production because zeners have too much tolerance. You could concoct something around a TL431 but why not use more bypass capacitance? It's cheap.

The negative part can be capped by a nice Schottly diode.

That is normal. It is being fed via the substrate diodes. That can kill chips if VCC ratchets up past abs max.

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LTC makes good quality chips. Still, don't push your luck there either ;-)

That's a lot of ringing. I'd consider a little AC termination here.

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Depends on your setup but my impression is that your chips were killed by overvoltage.

I think the MAX was spec'd for 13V or so abs max but I don't know what happens when that's exceeded.

Keep in mind that there always comes a point where it either breaks down or where a substrate diode begins to conduct and then quietly feeds your supply rail. And it doesn't know when to stop ;-)

Yes, very possible. I'd ask the vendors at what point that will happen. Hard to say with datasheets as skimpy as for the MAX. I'd ask. Email often works well (but don't necessarily hold your breath with TI).

Take a look at the 75ALS180 datasheet, schematic of inputs and outputs:

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You can see 18K and 3K and yes, those in series will feed the supply rail but it seems two diodes in series with 1.1K should keep that down to a few volts, depending on how much arrives on the bus lines.

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Regards, Joerg

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Joerg

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