MSP430 datasheet(74/88 Pages) TI

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8.2.1.1 Design Requirements

The design requirements are:

• Transmitter with a current output range of 4 mA to 20 mA for a process variable signal

• Out-of-range current output capability from 3 mA to 25 mA for error or fault signal levels

• Operation with standard industrial automation supply voltages from 12 V to 30 V

• Current and voltage outputs with TUE less than 0.5% at 25°C

• Total on-board current must be less than or equal to 3 mA

8.2.1.2 Detailed Design Procedure

Figure 8-3 shows a block diagram of a loop-powered, 4-mA to 20-mA current transmitter.

Power

and

Digital

Isolation

Loop

Protection

AFE88101

MCU

MSP430FR5969

Start-Up and

V-to-I

Converter

+VTERMINAL

–VTERMINAL

LDO

Sensor

and

AFE

4-mA to

20-mA

Loop

Terminals

Boost

Constructed Section of the Circuit

Figure 8-3. Block Diagram of a Loop-Powered, 4-mA to 20-mA Current Transmitter

The terminals connected to the loop are shown on the right side of the block diagram. This connection to the

loop powers the entire transmitter. A bridge rectifier at the input protects against reverse connection to the

loop. The rectified loop voltage powers a start-up circuit that provides power to an LDO, that in turn powers

the AFE88101. The LDO powers a flyback converter acting as a boost and supplies power across an isolation

barrier. On the other side of the isolation barrier, another LDO powers the MCU and any sensor connected to the

transmitter. The LDOs also power the digital signal isolation on each side of the barrier.

The AFE88101 controls the loop current through the voltage-to-current (V-to-I) converter block. The DAC voltage

sets the output from 0.3 V to 2.5 V. The output is sent through a V-to-I converter block using an OPA333 and an

NPN bipolar junction transistor (BJT).

AFE78101, AFE88101

SLASF21 – DECEMBER 2022

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Product Folder Links: AFE78101 AFE88101

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