RE: two XTR116 with non isolated power supply

Hi Shrikant,

I split this post because it was slightly different question than Dhruv had.

My first question to you would be can you use 3-wire transmitters such as the XTR111 to accomplish your goals?  3-wire transmitters are located somewhere with a local power supply and GND which can be shared with many XTR111 products.

You can not combine the VREG currents to get 8mA to power the loop circuitry with a 2-wire transmitter.  You can use an external switch-mode power converter to increase the power on the loop side.  Let me know if you need additional information on this.

If you need to use 2-wire transmitters and need to share a common input, then the easiest way is to use a digital isolator between the MCU and each XTR.  The output of the isolator can be low-pass filtered and a DC voltage can be created.  The MCU side of the isolator will be powered between the MCU power and common.  The XTR side of each isolator will be powered between VREG and IRET. 

Hi Collin,

Thanks for the reply,

I can not use XTR111 as the unit receiving my current loops are functional and they accepts only one loop input for each of the two current loops and has common +24V loop supply.

My requirement is to give two current loops using XTR116 / XTR115 where loop supply will be same for both the loops.

There is a I2C based sensor which give two digital outputs which I need to calibrate into 4-20mA .

As you have suggested I can use two digital isolators for each XTR . Its XTRside will take power from Vreg pins of corresponding XTR ic's. But I need to run my MCU on the same +24v loop power which goes to xtr ic's.And in case of MCU side I dont have any common pin available for connecting MCU GND. Can you suggest me some solution for this case.

Regards

Shrikant Lokhande.

Hello Shrikant,

Is this what you want to do?

The microcontrollers are powered by the VReg of the XTRs individually. Their currents return into IRet. The XTR output currentss flow into load resistors returning to a common GND.

Regards,

JayantD

Hi Jayant!

Thanks for helping and good to see you on the E2E community again!  I was going to ask for a picture so we can get this circuit working correctly.

Shrikant,

Please confirm and/or correct Jayant's image so we know exactly what you're trying to build.  Then we can work to find a valid solution.

Thanks! 

Hello Collin and Jayant Sir,

Sorry for being late on this.

My application is different than what Jayant Sir has shown, I intended to use only one controller as my sensor give me two parameters Humidity and Temperature on I2C bus.

The controller will generate two PWM's (LPF following each PWM) to get analog voltages corresponding to the input parameters.

I need to convert these analog signals to 4-20mA loops using XTR115.

Earlier I was thinking to isolate both analog signals going to XTR IC's. But after thinking for some time I came to the conclusion that I could use Vreg pin of one of the xtr to power my controller and connect its GND to Iret pin of xtr.

The scheme is as follow:

Hello Collin and Jayant Sir,

Earlier I tried uploading the image but couldnt do that , Please go through this and let me know whether this scheme can work.

And let me know if anything needs to be done.

Regards

Shrikant L

Hello Collin,

Nice to be in touch through E2E!

I have a suggestion for Shrikant, who is a bright young engineer from my previous employment. Being in the same geographical area (India) gives me an advantage in replying before you. Please do correct me in case I'm off-track or something.. Thanks!

Shrikant,

I think the following solution will work for you.. subject to Collin's approval, of course!

8182.Document1.pdf

Regards,

Jayant 

Hello Jayant Sir,

Thanks for the comment.

In my case only three wires are available to the system, two wires that takes my 4-20mA loops to the remote GND and one with 24V supply(Which i have shown in the block dia.).

Here I dont have any GND wire running in so we can not give +5V and GND to the microcontroller.

So I must connect controller's VDD pin to VREF and VSS pin to IRET of one of the XTR ics.

As collin has already specified VREF pins of the two XTR ic's should not be connected to each other, that is why I have used only one isolator.

I think the block diagram you have suggested requires separate +5V supply for controller which is not available for me.

please confirm will the scheme which I have posted wili work or any issues will arise that I couldnt see at this point of time.

Collin,

I would like to here your opinion on this

And both of you please correct me if I'm missing anything.

Thanks and Regards,

Shrikant

Hello Shrikant,

I'm still not very clear about your application. What I understand from the discussions above:

1. You have a three wire current loop. 4-20mA, 24V and Ground. You may use these 24V & Ground wires to power a voltage regulator for the microcontroller.

2. The microcontroller PWM outputs are isolated, low pass filtered to get two isolated linear voltage outputs.

3. The voltages can be inputs to two XTR115 ICs. The VReg of the XTR can power the isolator, whose ground returns to IRet.

4. You may need to power the two XTRs using a separate 24V supply, or use the same one mentioned above (#1).

I'm not sure whether this will be reliable, let us wait for Collin to respond.

Regards,

JayantD

Hello Jayant Sir and Collin,

Sorry for my blurry explanations.

 I have only 3 wires available to my whole system.

1. 4-20mA output for Temperature.

2. 4-20mA output for Humidity.

3. +24V supply.

+24V supply comes from the same system which accepts two 4-20mA  loops.

So my system should get power from that  +24V supply and the Ground should be connected via one of the 4-20mA loops.

The sensor used here gets the power from my system only.

Thanks and Regards,

Shrikant L

Hey Shrikant,

This is a much better schematic, thanks for the clarification.

Let us lable the XTR shown on the left side of your diagram as XTR#1 and the other as XTR #2.
The VReg of XTR#1 has to power the MCU, the sensor and one side of the isolator power. The maximum current drawn by all of these together will flow back through IRet of XTR#1 and SHOULD NEVER exceed 4mA.(?) or so..( i don't have the data sheet in front of me right now, please double check .)  This could be an issue of concern.

The XTR#2 VReg powers only the other side of the isolator and should pose no problem. 

Apart from this there could be noise issues as this is somewhat different from a twisted pair loop - I'm not so sure about this.

I think Collin will have a ready solution for this. Let us see what he has to add.. it would be monday morning at his location soon. Good morning, Collin!

Regards,

JayantD

Hello Jayant,

Thanks as always for your assistance in the community!

I looked through the messages and I believe I understand what Shrikant is trying to accomplish.  I agree that the solution you proposed in the .PDF document would be the ideal solution because there won't be any possible issues with the microcontroller drawing too much current.  However, it seems that this solution will not work for Shrikant so we will work off of his last schematic which I've copied below:

Shrikant,

This concept is valid presuming we abide by a few restrictions, good design!  My main concern is the same as Jayant's that the VREG circuit on XTR#1 will have to provide power for the following:  MCU, Sensor, Isolator, and PWM.  For this circuit to operate correctly, the VREG output current must be kept lower than roughly 3.5 mA to ensure proper operation.  This may not be an issue if you use a very low-power MCU and sensor, but you will need to add the operational currents of the circuit items listed above and verify that they do not exceed 3.5mA.  If they do we will have to work towards another solution.  If they do not, then this should work.  

Good luck!  Please let us know how you plan to proceed and share results when they are obtained.  

Hello Collin,

Thanks for your support.. it would be interesting to know what approach Shrikant takes to tackle this.

Warm regards,

Jayant

Hello Collin,

Thanks for the confirmation, 

The ultra low power MSP430 series controller looks obvious choice for my application. The sensor m using  takes 850uA maximum current.

So only isolator remains which is the main concern for me now.

I'm searching for the isolator which consumes very low power as optocouplers may not suit this application due to their high current requirements.

Please suggest some low power digital isolators which suits this type of system.

While searching I found out one link in this forum where a step down convertor is used following the XTR115 ic to get more power.(some HART modem)

In one of the comments you said that you have succeeded doing so,It would be a favor for me If you can explain the concept  where I can get more current on the VREG side of the XTR IC.

As per my knowledge IRET pin can carry less than 3.5mA current so how u dealt with the extra current.

I would like to do some more work in the input side with extra current available using step down convertor.

Thanks and Regards

Shrikant.

Hi Shrikant,

I forgot to answer some of your questions about using the TPS54062 step down converter on the loop-powered side to get more power.  Since it looks like your system will work by using the VREG current I don't think we need to explore this route.  Basically though the SMPS is a power-converter instead of a standard LDO like the VREG circuit.  So if you use the +24V loop supply then you can turn 3mA at 24V into >10mA at +5V or +3.3V based on the efficiency of the converter.  The current from the converter returns into IRET and must still be less than ~3.5mA (3.75 for the XTR117).

Hello Collin,

Thanks for both the replies and suggestions.

I will design a prototype and let you know the results.

Once again thanks for the help and support.

Warm regards,

Shrikant L

Hello Collin,

I have some basic queries about the technique of powering the TPS54062 through the loop, and returning the current through IRet.  I leave it to you to continue this discussion or to move this as a new topic.

Well, my experience with switchers is that they are noisy, run hot, have inrush current, and require "zero" ground impedance. In this particular case, with the XTR11x IRet impedance (+ load)  "always" in series with the switcher's Ground pin, it seems that one has to live with some trade-offs here.

Will you please explain this application in more detail ? Will this clear EMI EMC tests for CE certification?

Thanks and Regards,

JayantD

Hi Collin,

Sorry for being late on the updates.

I have developed a PCB for the discussed system as per your suggestions.

My system requires 3.3V for microcontroller which I have taken from Vreg pin of XTR116 following two dioded in series. I have also kept provision for LDO to get 3.3V regulated supply for controller and external transister used is BCP56.

The ckt for LDO is kept disabled for now as I was trying to get the same 3.3V using two diodes in series with Vreg.

The output current of  XTR (one which feeds uC and sensor) shows 4.8mA when there is no input voltage given. I suspect it is because of the two diodes in series which causes the power dessipation.

Though I have an option of LDO TPS78233, but As per my calculations the two series diode would have serve the perpose but they are not.

One more thing I need to understand is, what is the voltage range required for XTR input for the 4-20mA operation.

Please share your views on this.

Regards

Shrikant L