SNVS615K January   2010  – February 2018 LM27402

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Typical Application Circuit
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
    1.     Pin 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 Switching Characteristics
    8. 6.8 Typical Performance Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  Wide Input Voltage Range
      2. 7.3.2  UVLO
      3. 7.3.3  Precision Enable
      4. 7.3.4  Soft-Start and Voltage Tracking
      5. 7.3.5  Output Voltage Setpoint and Accuracy
      6. 7.3.6  Voltage-Mode Control
      7. 7.3.7  Power Good
      8. 7.3.8  Inductor-DCR-Based Overcurrent Protection
      9. 7.3.9  Current Sensing
      10. 7.3.10 Power MOSFET Gate Drivers
      11. 7.3.11 Pre-Bias Start-up
    4. 7.4 Device Functional Modes
      1. 7.4.1 Fault Conditions
        1. 7.4.1.1 Thermal Protection
        2. 7.4.1.2 Current Limit
        3. 7.4.1.3 Negative Current Limit
        4. 7.4.1.4 Undervoltage Threshold (UVT)
        5. 7.4.1.5 Overvoltage Threshold (OVT)
  8. 8 Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1  Converter Design
      2. 8.1.2  Inductor Selection (L)
      3. 8.1.3  Output Capacitor Selection (COUT)
      4. 8.1.4  Input Capacitor Selection (CIN)
      5. 8.1.5  Using Precision Enable
      6. 8.1.6  Setting the Soft-Start Time
      7. 8.1.7  Tracking
      8. 8.1.8  Setting the Switching Frequency
      9. 8.1.9  Setting the Current Limit Threshold
      10. 8.1.10 Control Loop Compensation
      11. 8.1.11 MOSFET Gate Drivers
      12. 8.1.12 Power Loss and Efficiency Calculations
        1. 8.1.12.1 Power MOSFETs
        2. 8.1.12.2 High-Side Power MOSFET
        3. 8.1.12.3 Low-Side Power MOSFET
        4. 8.1.12.4 Gate-Charge Loss
        5. 8.1.12.5 Input and Output Capacitor ESR Losses
        6. 8.1.12.6 Inductor Losses
        7. 8.1.12.7 Controller Losses
        8. 8.1.12.8 Overall Efficiency
    2. 8.2 Typical Applications
      1. 8.2.1 Example Circuit 1
        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
        3. 8.2.1.3 Application Curves
      2. 8.2.2 Example Circuit 2
      3. 8.2.3 Example Circuit 3
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
      1. 10.1.1 Power Stage Layout
      2. 10.1.2 Gate Drive Layout
      3. 10.1.3 Controller Layout
      4. 10.1.4 Thermal Design and Layout
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Third-Party Products Disclaimer
      2. 11.1.2 Development Support
        1. 11.1.2.1 Custom Design With WEBENCH® Tools
    2. 11.2 Documentation Support
      1. 11.2.1 Related Documentation
    3. 11.3 Receiving Notification of Documentation Updates
    4. 11.4 Community Resources
    5. 11.5 Trademarks
    6. 11.6 Electrostatic Discharge Caution
    7. 11.7 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

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

8.1.6 Setting the Soft-Start Time

Adding a soft-start capacitor reduces inrush currents and provides a monotonic start-up. The soft-start capacitance is calculated by Equation 10:

Equation 10. LM27402 30092614.gif

As shown, the CSS capacitance is set by the desired soft-start time tss, the soft-start current Iss (3 µA) and the nominal feedback (FB) voltage level of 0.6 V. If VVIN and VVDD are above their UVLO voltage levels (2.9 V) and EN is above the precision enable threshold (1.17 V), the soft-start sequence begins. The LM27402 defaults to a minimum start-up time of 1.28 ms when a soft-start capacitor is not connected. In other words, the LM27402 will not start-up faster than 1.28 ms. The soft-start capacitor is discharged when enable is cycled, during UVLO, OTP, or when the LM27402 enters hiccup mode from an overcurrent event.

There is a delay between EN transitioning above 1.17 V and the beginning of the soft-start sequence. The delay allows the LM27402 to initialize its internal circuitry. Once the output has charged to 94% of the nominal output voltage and SS/TRACK has exceeded 564 mV, the PGOOD indicator transitions high as illustrated in Figure 29.

LM27402 30092643.gifFigure 29. Soft-Start Timing