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
7.2.2.5.2 Turns Ratio Estimate

Assuming that the rectifier diodes and linear regulator are selected and that the transformer selected must have a V-t product of at least 11Vμs. However, before searching the manufacturer web sites for a suitable transformer, the user still needs to know the minimum turns ratio that allows the push-pull converter to operate flawlessly over the specified current and temperature range. This minimum transformation ratio is expressed through the ratio of minimum secondary to minimum primary voltage multiplied by a correction factor that takes the transformer typical efficiency of 97% into account:

Equation 4. VP-min = VIN-min - VDS-max

VS-min must be large enough to allow for a maximum voltage drop, VF-max, across the rectifier diode and still provide sufficient input voltage for the regulator to remain in regulation. From the LDO Selection section, this minimum input voltage is known and by adding VF-max gives the minimum secondary voltage with:

Equation 5. VS-min = VF-max + VDO-max + VO-max
ISO7741TA-Q1 ISO7741TB-Q1 Establishing the Required Minimum Turns Ratio Through Nmin = 1.031 × VS-min / VP-minFigure 7-4 Establishing the Required Minimum Turns Ratio Through Nmin = 1.031 × VS-min / VP-min

Then calculating the available minimum primary voltage, VP-min, involves subtracting the maximum possible drain-source voltage of the device, VDS-max, from the minimum converter input voltage VIN-min:

Equation 6. VP-min = VIN-min – VDS-max

VDS-max however, is the product of the maximum RDS(on) and ID values for a given supply specified in the data sheet:

Equation 7. VDS-max = RDS-max × IDmax

Then inserting Equation 7 into Equation 6 yields:

Equation 8. VP-min = VIN-min - RDS-max x IDmax

and inserting Equation 8 and Equation 5 into Equation 4 provides the minimum turns ration with:

Equation 9. ISO7741TA-Q1 ISO7741TB-Q1

Example:

For a 3.3VIN to 5VOUT converter using the rectifier diode MBR0520L and the 5V LDO, the data sheet values taken for a load current of 600mA and a maximum temperature of 85°C are VF-max = 0.2V,
VDO-max = 0.5V, and VO-max = 5.1V.

Then assuming that the converter input voltage is taken from a 3.3V controller supply with a maximum ±2% accuracy makes VIN-min = 3.234V. Finally the maximum values for drain-source resistance and drain current at 3.3V are taken from the data sheet with RDS-max = 0.45Ω and ID-max = 700mA.

Inserting the values above into Equation 10 yields a minimum turns ratio of:

Equation 10. n m i n = 1.031 × 0.2 V + 0.5 V + 5.1 V 3.234 V - 0.45 Ω × 700 m A

Most commercially available transformers for 3V to 5V push-pull converters offer turns ratios between 2.0 and 2.3 with a common tolerance of ±3%.