SNVSCL1 November   2023 LM25185

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Description (continued)
  6. 5 Pin Configuration and Functions
  7. 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 Typical Characteristics
  8. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  Power MOSFET Gate Driver
      2. 7.3.2  PSR Flyback Modes of Operation
      3. 7.3.3  High Voltage VCC Regulator
      4. 7.3.4  Setting the Output Voltage
        1. 7.3.4.1 Diode Thermal Compensation
      5. 7.3.5  Control Loop Error Amplifier
      6. 7.3.6  Precision Enable
      7. 7.3.7  Configurable Soft Start
      8. 7.3.8  Minimum On-Time and Off-Time
      9. 7.3.9  Current Sensing and Overcurrent Protection
      10. 7.3.10 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Mode
      2. 7.4.2 Standby Mode
      3. 7.4.3 Active Mode
  9. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
      1. 8.2.1 Design 1: Wide VIN, Low IQ PSR Flyback Converter Rated at 16.4 V, 1 A
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
          1. 8.2.1.2.1  Custom Design With WEBENCH® Tools
          2. 8.2.1.2.2  Custom Design With Excel Quickstart Tool
          3. 8.2.1.2.3  Flyback Transformer T1 and Current-Sense Resistor (RCS)
          4. 8.2.1.2.4  Flyback Diode – DFLY
          5. 8.2.1.2.5  Leakage Inductance Clamp Circuit – DF, DCLAMP
          6. 8.2.1.2.6  Feedback Resistor – RFB
          7. 8.2.1.2.7  Thermal Compensation Resistor – RTC
          8. 8.2.1.2.8  UVLO Resistors – RUV1, RUV2
          9. 8.2.1.2.9  Soft-Start Capacitor – CSS
          10. 8.2.1.2.10 Compensation Components
        3. 8.2.1.3 Application Curves
    3. 8.3 Power Supply Recommendations
    4. 8.4 Layout
      1. 8.4.1 Layout Guidelines
      2. 8.4.2 Layout Examples
  10. 9 Device and Documentation Support
    1. 9.1 Device Support
      1. 9.1.1 Development Support
        1. 9.1.1.1 Custom Design With WEBENCH® Tools
    2. 9.2 Documentation Support
      1. 9.2.1 Related Documentation
    3. 9.3 Receiving Notification of Documentation Updates
    4. 9.4 Support Resources
    5. 9.5 Trademarks
    6. 9.6 Electrostatic Discharge Caution
    7. 9.7 Glossary
  11. 10Revision History
  12. 11Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

8.3 Power Supply Recommendations

The LM25185 flyback controller operates over a wide input voltage range from 4.5 V to 42 V. The characteristics of the input supply must be compatible with the design specification ranges. In addition, the input supply must be capable of delivering the required input current to the fully-loaded regulator. Estimate the average input current with Equation 36.

Equation 36. IIN=VOUT×IOUTVIN×η

where

  • η is the efficiency

If the converter is connected to an input supply through long wires or PCB traces with a large impedance, special care is required to achieve stable performance. The parasitic inductance and resistance of the input cables can have an adverse effect on converter operation. The parasitic inductance in combination with the low-ESR ceramic input capacitors form an underdamped resonant circuit. This circuit can cause overvoltage transients at VIN each time the input supply is cycled ON and OFF. The parasitic resistance causes the input voltage to dip during a load transient. If the regulator is operating close to the minimum input voltage, this dip can cause false UVLO fault triggering and a system reset. The best way to solve such issues is to reduce the distance from the input supply to the regulator and use an aluminum electrolytic input capacitor in parallel with the ceramics. The moderate ESR of the electrolytic capacitors helps damp the input resonant circuit and reduce any voltage overshoots. A capacitance in the range of 22 µF to 100 µF is usually sufficient to provide input damping and helps to hold the input voltage steady during large load transients. A typical ESR of 200 mΩ provides enough damping for most input circuit configurations.

An EMI input filter is often used in front of the regulator that, unless carefully designed, can lead to instability as well as some of the effects mentioned above. The application report Simple Success with Conducted EMI for DC-DC Converters provides helpful suggestions when designing an input filter for any switching regulator.