DAC8750: Using in an Intrisically Safe design

Hi Mike,

I am less versed in intrinsic safety than many other topics in the factory automation and control space, but have been learning more over the past several months (or at least trying to). That said, you're probably the lead expert in this conversation so please be gentle as I may ask or suggest something silly :)

In my understanding, very few components are really "trusted" in the context of intrinsic safety, essentially only diodes and Zener barriers.

In your consideration of DAC8750 you menitoned 3.3V so I would assume that you're planning on using an external DVDD supply and your MCU or control will be on the 3.3V domain. Based on the logic threshold specifications of your controller could using the internal DVDD LDO of the DAC8750 be an option to provide some relief? I understand there would be other connections from the DAC to the 3.3V powered device(s), but maybe there is less capacitance on those lines versus the power plane? From what I've been lead to understand only "single failures" are typically considered, so it seemed like a failure on the DAC which "fed into" the digital communication lines would be sort of a second-order failure mechanism that may not be considered or would be treated more mildly?

Otherwise, a lot of the reaction I have observed to this point leads me to believe that the current reaction from most designers is to consider a discrete approach instead, or perhaps an approach based on the XTRxxx devices, which give you some more opportunities to place resistors / Zener barriers at critical points vs a fully integrated solution.

Is the 21V supply requirement derived from loading conditions or is that just the standard supply available in your systems? Generally speaking I see IS come up more in the context of sensors vs PLCs, and usually the sensor is specified with a smaller maximum load for the current output. In such a case, maybe another angle for us to look at would be to consider regulating 21V down to something lower.

Hello MIke,

I will be looking into this further however I want some clarification first. Are you saying IS rules allow for shorts between adjacent pins or between any two device pins? If it is the former, then it is not possible to short AVDD to any pins in the DVDD domain because of the pinout.

Regards,

Reza

Kevin,

You are correct that certain limited components have 'privileges' - they typically also have to be 2/3 derated for voltage, current, and power!  Using the DVDD LDO of the DAC8750 would not help.  Typically they mass fault all capacitance into one lump sum and unless you protect it from getting to the higher voltage area - say through enough series resistance, then it's available as an energy source to create a spark if something is shorted or opened.

The number of faults (failures) that can occur are a bit more complicated.  Depending on the area the equipment is installed in or connected to, you have to handle either 1 or 2 "Countable" faults and an unlimited number of 'non-countable' faults.  So let's say you are trying to protect internal capacitance from feeding back to a supply that is an IS supply, you would use 2 or 3 series diodes - 2 for Zone 1 and 3 for Zone 0.  They would have to be 2/3 derated and have appropriate spacing as dictated by the standard - otherwise they could be faulted with a non-countable fault.  In some cases they relax the double redundancy and call a pair of diodes a protective assembly that is not faultable - assuming all conditions are met for the assembly....whew - easy peasy right!?

The 21V requirement is because we are designing a 4-20mA HART AO and AI card so we need the voltage to supply the field device.

The only way I can see we can use the DAC8750 is if it can be configured to NOT have the 21V connected to the chip.

If you have any reference designs for using other parts, please share with me.

Mike.

Reza,

For semiconductors, the IS rules allow any number of shorts or opens internally on the part.  This is because spacing is very small in ICs.  There are a few cases when ICs have been IS certified.  In this case, the certification agency has analyzed the internal structure of the part to verify it meets the IS rules - I'm thinking of a digital signal isolation chip.  Then the manufacturer cannot change anything about the IC unless approved by the certification agency in advance.

The only way I can see we can use the DAC8750 is if it can be configured to NOT have the 21V connected to the chip.

Mike.

Hello Mike,

Thanks for the detailed explanation. The DAC8750 can be used with a minimum of 10V for the analog supply and unfortunately that wouldn't help solve the problem. Please check out TIPD155. It uses a partially discrete approach to generate two current outputs from a low voltage DAC (DAC8563) and the XTR300. The pins within each IC can be shorted since the low voltage & high voltage domains are separate.

I am still not sure if this will work for the IS application however it may help with some other ideas. Thanks.

Regards,

Reza

Mike,

Just to continue to follow up on this topic, can you walk us through any example of how this problem has been solved in the past with a block diagram or something? Maybe that would help us better see a path towards any solution or maybe even still using the DAC8760.

If you'd prefer to do that via email we can contact you at the email address you used to create your myTI.com account.

Yes, please email me to continue the conversation.

Mike.