SLAAEH8 October   2024 AFE781H1 , AFE782H1 , AFE881H1 , AFE882H1 , DAC8740H , DAC8741H , DAC8742H

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
    1. 1.1 The 4-20mA Loop
    2. 1.2 The HART Protocol
      1. 1.2.1 Adding HART to the 4-20mA Loop
      2. 1.2.2 HART FSK
  5. 2AFE881H1 HART Modem
    1. 2.1 AFE881H1 HART Transmitter
    2. 2.2 Detailed Schematic
      1. 2.2.1 Input Protection
      2. 2.2.2 Startup Circuit
      3. 2.2.3 Voltage-to-Current Stage
      4. 2.2.4 Voltage-to-Current Calculation
      5. 2.2.5 HART Signal Transmission
      6. 2.2.6 HART Input Protection
      7. 2.2.7 Current Consumption
      8. 2.2.8 HART Transmitter Board
      9. 2.2.9 HART Protocol Stack
  6. 3HART Testing and Registration
    1. 3.1  HART History and the FieldComm Group
    2. 3.2  HART Testing Overview
      1. 3.2.1 HART Protocol Specifications
      2. 3.2.2 HART Protocol Test Specifications
      3. 3.2.3 Remote Transmitter Device Testing
    3. 3.3  HART Test Equipment
    4. 3.4  HART Physical Layer Testing
      1. 3.4.1 FSK Sinusoid Test
      2. 3.4.2 Carrier Start and Stop Time Tests
      3. 3.4.3 Carrier Start and Stop Transient Tests
      4. 3.4.4 Output Noise During Silence
      5. 3.4.5 Analog Rate of Change Test
      6. 3.4.6 Receive Impedance Test
      7. 3.4.7 Noise Sensitivity Test
      8. 3.4.8 Carrier Detect Test
    5. 3.5  Data Link Layer Tests
      1. 3.5.1 Data Link Layer Test Specifications
      2. 3.5.2 Data Link Layer Test Logs
    6. 3.6  Universal Command Tests
    7. 3.7  Common-Practice Command Tests
    8. 3.8  Device Specific Command Tests
    9. 3.9  HART Protocol Test Submission
    10. 3.10 HART Registration
  7. 4Other TI HART Modem Designs
  8. 5Summary
  9. 6Acknowledgments
  10. 7References

2.2.2 Startup Circuit

The schematic in Figure 2-5 shows the startup circuit of the HART transmitter. When power is applied, voltage pulls up on LOOP+ to start up the TLV431B LDO to power the board.

Transmitter Board Startup Circuit Figure 2-5 Transmitter Board Startup Circuit

The 249kΩ resistor allows current to flow through the 3.6V Zener diode, the voltage pulls up on the base of Q1. The Zener voltage starts the current through Q1 to turn on the LDO. The TLV431B LDO sets up the 3.3V supply.

As the 3.3V supply comes up, the control circuitry pulls current from Q3. This turns on Q2, which then takes over supplying current to the LDO.

When the 3.3V LDO output reaches the final value, the voltage at the emitter of Q1 rises. With the 3.6V driving the base of Q1 and 3.3V at the emitter, this voltage difference reduces the base-emitter voltage of Q1 to 0.3V. With a low base emitter voltage, Q1 shuts off. Q2 the maintains the current needed to drive LDO output and shunt the extra loop current. Because Q2 is the primary path of current through the transmitter, the transistor must be a PNP capable of high-power dissipation.

Take care when selecting the Zener diode. The voltage across the Zener diode varies with the loop voltage and the temperature of the circuit. This variance can change the VBE across Q1 and change the total current going through the start-up circuit. If the voltage is too high, the Zener diode sets Q1 to continue to source current after the circuit starts up. If the voltage is too low, the Zener diode prevents the TLVH431B from turning on. Verify proper start up by checking that the 3.3V supply starts up, and that Q1 turns off when in operation