2-wire 4-20mA sensor transmitters: Increasing Sensor Power with Switching Power Supplies (Part 6)

RE: 2-wire 4-20mA sensor transmitters: Increasing Sensor Power with Switching Power Supplies (Part 6)

Collin Wells — Tue, 03 Jan 2017 19:35:23 GMT

Hi Benjamin,

Your comments don't appear to be related to this blog. I saw you also posted similar questions on the forum. I've answered your questions there.

RE: 2-wire 4-20mA sensor transmitters: Increasing Sensor Power with Switching Power Supplies (Part 6)

Benjamin Ko — Wed, 28 Dec 2016 03:37:03 GMT

Also, does Rload and Rshunt the same thing?

In my understand, Rshunt is used as sensing the current, Rload can be either sensing or rising up the potential so it will not the same as Vloop(-), so, can I put Rshunt any series place of the loop, as long as there is a Rload connect to the Vloop(-)?

RE: 2-wire 4-20mA sensor transmitters: Increasing Sensor Power with Switching Power Supplies (Part 6)

Benjamin Ko — Mon, 26 Dec 2016 15:45:33 GMT

Hello Colin.

I found all the 4-20mA receiver's Ref circuits is taking the method of 'Low-Side Measurements', which connects R_shunt to Vloop(-).

(Ref to page 3, www.ti.com/.../slyb194b.pdf )

While in my design, I use the 'High-Side' method to ensure the load is shorted to GND, so R_shunt is connected to Vloop(+). I am using a 24.9ohm resistor and an AD629 to sense the loop current.

Now I'm uncertain about my design, is it fit or unfit to most of the industry's 4-20mA transmitter? Is there any fatal defects or disadvantages? To design a transmitter, what factor(s) shall be considered?

Thanks.

Benjamin Gao

RE: 2-wire 4-20mA sensor transmitters: Increasing Sensor Power with Switching Power Supplies (Part 6)

Collin Wells — Tue, 28 Apr 2015 19:59:04 GMT

Hi Rob,

Thanks for reading! The TIDA-00349 reference design includes more information about the incremental input impedance differences between a typical inductor based buck converter versus a simple charge-pump buck. This the main reason that inductor based buck converters typically have stability issues in these systems if there is a current limiter or other protection circuitry in the transmitter loop.

We know that applications like the one described in this post need a higher voltage solution than the solution presented in the TIDA-00349 or TIDA-00167 reference designs. I checked with our charge pump team, and our current portfolio of charge pumps are focused on standard single battery sizes and not the higher voltages required for this application. However, there a few products available on the market which are specifically meant for this application. While many sensor transmitters are input-isolated (as described in Part 5 of this series) this application does not require isolation and can be achieved with a non-isolated buck charge-pump. For isolated designs, the two reference designs I linked to include a recommended transformer in the BOM.

RE: 2-wire 4-20mA sensor transmitters: Increasing Sensor Power with Switching Power Supplies (Part 6)

rob taylor1 — Mon, 27 Apr 2015 17:08:28 GMT

Hi Colin

This is definitely of interest to me. I want to make a 2 wire transmitter with a sensor that doesn't need isolation, and needs 5.5mA @ 5VDC regulated, using XTR11X. I would also have a low power micro, A/D, D/A, total budget 500uA.

I have failed twice using regulated inductor buck devices, stability issues just as you say. So an unregulated buck charge pump, post linear sounds great. The problem is all the charge pumps I see are low input volts, no help at 28VDC, many so-called bucks are rail splitters with limited drive for all source, no sink.

Your note is largely isolated apps, if that's how it must be, do you have a suggested SMT transformer.

Thanks

Rob Taylor