SLLSFW9 April   2024 ISO7741TA-Q1

ADVANCE INFORMATION  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Pin Configuration and Functions
    1.     Pin Functions
  6. 5 Specifications
    1. 5.1  Absolute Maximum Ratings
    2. 5.2  ESD Ratings
    3. 5.3  Recommended Operating Conditions
    4. 5.4  Thermal Information
    5. 5.5  Power Ratings
    6. 5.6  Insulation Specifications
    7. 5.7  Safety-Related Certifications
    8. 5.8  Safety Limiting Values
    9. 5.9  Electrical Characteristics Transformer
    10. 5.10 Electrical Characteristics—5-V Supply
    11. 5.11 Supply Current Characteristics—5-V Supply
    12. 5.12 Electrical Characteristics—3.3-V Supply
    13. 5.13 Supply Current Characteristics—3.3-V Supply
    14. 5.14 Electrical Characteristics—2.5-V Supply 
    15. 5.15 Supply Current Characteristics—2.5-V Supply
    16. 5.16 Switching Characteristics—5-V Supply
    17. 5.17 Switching Characteristics—3.3-V Supply
    18. 5.18 Switching Characteristics—2.5-V Supply
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Feature Description
      1. 6.3.1 Electromagnetic Compatibility (EMC) Considerations
      2. 6.3.2 Push-Pull Converter
      3. 6.3.3 Core Magnetization
    4. 6.4 Device Functional Modes
      1. 6.4.1 Device I/O Schematics
      2. 6.4.2 Start-Up Mode
      3. 6.4.3 Operating Mode
      4. 6.4.4 Spread Spectrum Clocking
  8. 7 Application and Implementation
    1. 7.1 Application Information
    2. 7.2 Typical Application
      1. 7.2.1 Design Requirements
      2. 7.2.2 Detailed Design Procedure
        1. 7.2.2.1 Drive Capability
        2. 7.2.2.2 LDO Selection
        3. 7.2.2.3 Diode Selection
        4. 7.2.2.4 Capacitor Selection
        5. 7.2.2.5 Transformer Selection
          1. 7.2.2.5.1 V-t Product Calculation
          2. 7.2.2.5.2 Turns Ratio Estimate
          3. 7.2.2.5.3 Recommended Transformers
      3. 7.2.3 Application Curve
        1. 7.2.3.1 Insulation Lifetime
      4. 7.2.4 System Examples
        1. 7.2.4.1 Higher Output Voltage Designs
  9. 8 Device and Documentation Support
    1. 8.1 Documentation Support
      1. 8.1.1 Related Documentation
    2. 8.2 Receiving Notification of Documentation Updates
    3. 8.3 Support Resources
    4. 8.4 Trademarks
    5. 8.5 Electrostatic Discharge Caution
    6. 8.6 Glossary
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information

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発注情報

7.2.2.2 LDO Selection

The minimum requirements for a suitable low dropout regulator are:

  • The current drive capability must slightly exceed the specified load current of the application to prevent the LDO from dropping out of regulation. Therefore, for a load current of 550mA, choose a 600mA to 700mA LDO. While regulators with higher drive capabilities are acceptable, these regulators also typically possess higher dropout voltages that reduce the overall converter efficiency.
  • The internal dropout voltage, VDO, at the specified load current must be as low as possible to maintain efficiency. For a low-cost 700mA LDO, a VDO of 600mV at 700mA is common. Be aware; however, that this lower value is typically specified at room temperature and can increase by a factor of 2 over temperature, which in turn raises the required minimum input voltage.
  • The required minimum input voltage preventing the regulator from dropping out of line regulation is given with:
    Equation 1. VI-min = VDO-max + VO-max

    To determine VI for worst-case condition, the user must take the maximum values for VDO and VO specified in the LDO data sheet for rated output current (that is, 600mA) and add these values together. The user must also specify that the output voltage of the push-pull rectifier at the specified load current is equal or higher than VI-min. If the output voltage is not, the LDO loses line-regulation and any variations at the input passes straight through to the output. Hence, below VI-min the output voltage follows the input and the regulator behaves like a simple conductor.

  • The maximum regulator input voltage must be higher than the rectifier output under no-load. Under this condition there is no secondary current reflected back to the primary, thus making the voltage drop across RDS-on negligible and allowing the entire converter input voltage to drop across the primary. At this point, the secondary reaches the maximum voltage of
    Equation 2. VS-max = VIN-max × n

with VIN-max as the maximum converter input voltage and n as the transformer turns ratio. Thus to prevent the LDO from damage the maximum regulator input voltage must be higher than VS-max. Table 7-3 lists the maximum secondary voltages for various turns ratios commonly applied in push-pull converters.

Table 7-3 Required Maximum LDO Input Voltages for Various Push-Pull Configurations
PUSH-PULL CONVERTERLDO
CONFIGURATIONVIN-max [V]TURNS-RATIOVS-max [V]VI-max [V]
3.3VIN to 3.3VOUT3.61.5 ± 3%5.66 to 10
3.3VIN to 5VOUT3.62.2 ± 3%8.210
5VIN to 5VOUT5.51.5 ± 3%8.510