ZHCSP80 December   2022 AFE78101 , AFE88101

PRODUCTION DATA  

  1. 1 特性
  2. 2 应用
  3. 3 说明
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
  6. 6 Specifications
    1. 6.1  Absolute Maximum Ratings
    2. 6.2  ESD Ratings
    3. 6.3  Recommended Operating Conditions
    4. 6.4  Thermal Information
    5. 6.5  Electrical Characteristics
    6. 6.6  Timing Requirements
    7. 6.7  Timing Diagrams
    8. 6.8  Typical Characteristics: VOUT DAC
    9. 6.9  Typical Characteristics: ADC
    10. 6.10 Typical Characteristics: Reference
    11. 6.11 Typical Characteristics: Power Supply
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Digital-to-Analog Converter (DAC) Overview
        1. 7.3.1.1 DAC Resistor String
        2. 7.3.1.2 DAC Buffer Amplifier
        3. 7.3.1.3 DAC Transfer Function
        4. 7.3.1.4 DAC Gain and Offset Calibration
        5. 7.3.1.5 Programmable Slew Rate
        6. 7.3.1.6 DAC Register Structure and CLEAR State
      2. 7.3.2 Analog-to-Digital Converter (ADC) Overview
        1. 7.3.2.1 ADC Operation
        2. 7.3.2.2 ADC Custom Channel Sequencer
        3. 7.3.2.3 ADC Synchronization
        4. 7.3.2.4 ADC Offset Calibration
        5. 7.3.2.5 External Monitoring Inputs
        6. 7.3.2.6 Temperature Sensor
        7. 7.3.2.7 Self-Diagnostic Multiplexer
        8. 7.3.2.8 ADC Bypass
      3. 7.3.3 Programmable Out-of-Range Alarms
        1. 7.3.3.1 Alarm Action Configuration Register
        2. 7.3.3.2 Alarm Voltage Generator
        3. 7.3.3.3 Temperature Sensor Alarm Function
        4. 7.3.3.4 Internal Reference Alarm Function
        5. 7.3.3.5 ADC Alarm Function
        6. 7.3.3.6 Fault Detection
      4. 7.3.4 IRQ
      5. 7.3.5 Internal Reference
      6. 7.3.6 Integrated Precision Oscillator
      7. 7.3.7 One-Time Programmable (OTP) Memory
    4. 7.4 Device Functional Modes
      1. 7.4.1 DAC Power-Down Mode
      2. 7.4.2 Reset
    5. 7.5 Programming
      1. 7.5.1 Communication Setup
        1. 7.5.1.1 SPI Mode
        2. 7.5.1.2 UART Mode
      2. 7.5.2 Serial Peripheral Interface (SPI)
        1. 7.5.2.1 SPI Frame Definition
        2. 7.5.2.2 SPI Read and Write
        3. 7.5.2.3 Frame Error Checking
        4. 7.5.2.4 Synchronization
      3. 7.5.3 UART
        1. 7.5.3.1 UART Break Mode (UBM)
      4. 7.5.4 Status Bits
      5. 7.5.5 Watchdog Timer
    6. 7.6 Register Maps
      1. 7.6.1 AFEx8101 Registers
  8. 8 Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Multichannel Configuration
    2. 8.2 Typical Application
      1. 8.2.1 4-mA to 20-mA Current Transmitter
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
          1. 8.2.1.2.1 Start-Up Circuit
          2. 8.2.1.2.2 Current Loop Control
          3. 8.2.1.2.3 Input Protection and Rectification
          4. 8.2.1.2.4 System Current Budget
        3. 8.2.1.3 Application Curves
    3. 8.3 Initialization Set Up
    4. 8.4 Power Supply Recommendations
    5. 8.5 Layout
      1. 8.5.1 Layout Guidelines
      2. 8.5.2 Layout Example
  9. 9 Device and Documentation Support
    1. 9.1 Documentation Support
      1. 9.1.1 Related Documentation
    2. 9.2 接收文档更新通知
    3. 9.3 支持资源
    4. 9.4 Trademarks
    5. 9.5 Electrostatic Discharge Caution
    6. 9.6 术语表
  10. 10Mechanical, Packaging, and Orderable Information

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8.2.1.2.1 Start-Up Circuit

When the loop is applied to the terminals, the loop power starts up the board. Transistor Q4 from Figure 8-2 pulls current from the start-up and current-shunt regulator sections of the transmitter. The start-up circuit is shown in Figure 8-4.

Figure 8-4 Start-Up Circuit

In the start-up circuit, the 3.6-V Zener diode sets the voltage at the base of Q1. If the TLVH431B shunt regulator has not started, apply voltage to LOOP+ and LOOP– to turn on Q1 and source current to the shunt regulator. As the shunt regulator turns on and approaches the set voltage of 3.3 V, the base-emitter voltage (VBE) of Q1 becomes smaller. The collector current of Q2 drives the current of the shunt regulator to set the LOOP current going through the 40.2-Ω resistor in the current loop control circuit shown in the following section. After the start-up circuit has started, Q1 stops supplying current because the VBE is restricted. Q1 shuts off, leaving several microamps of current flowing through the 3.6-V Zener diode.

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 3.3-V supply starts up, and that Q1 turns off when in operation.

When the circuit starts up and the 3V3 line comes up to the desired 3.3-V supply level, the current through the TLVH431B is primarily sourced through Q2. The Q2 transistor must be able to dissipate enough power to handle the high current (> 20 mA) and the high voltage (> 30 V) in the loop. Because the biased transistor, Q2, is responsible for sourcing most of the output current, choose the components in the path of this current flow with appropriate power ratings. In this case, the 8.2-Ω resistor is rated to 0.25 W.

The current mirror is set up so that the current gain from Q3 to Q2 is approximately a factor of 60 ×. The exact current gain is not important as long as the current through Q3 is low.