ZHCSKV1A February   2020  – July 2020 LM76005

PRODUCTION DATA  

  1. 1 特性
  2. 2 应用
  3. 3 说明
  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 Characteristics
    7. 6.7 Switching Characteristics
    8. 6.8 System Characteristics
    9. 6.9 Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  Fixed-Frequency, Peak-Current-Mode Control
      2. 7.3.2  Light Load Operation Modes — PFM and FPWM
      3. 7.3.3  Adjustable Output Voltage
      4. 7.3.4  Enable (EN Pin) and UVLO
      5. 7.3.5  Internal LDO, VCC UVLO, and Bias Input
      6. 7.3.6  Soft Start and Voltage Tracking (SS/TRK)
      7. 7.3.7  Adjustable Switching Frequency (RT) and Frequency Synchronization
      8. 7.3.8  Minimum On-Time, Minimum Off-Time, and Frequency Foldback at Dropout Conditions
      9. 7.3.9  Bootstrap Voltage and VBOOT UVLO (BOOT Pin)
      10. 7.3.10 Power Good and Overvoltage Protection (PGOOD)
      11. 7.3.11 Overcurrent and Short-Circuit Protection
      12. 7.3.12 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
      4. 7.4.4 CCM Mode
      5. 7.4.5 DCM Mode
      6. 7.4.6 Light Load Mode
      7. 7.4.7 Foldback Mode
      8. 7.4.8 Forced Pulse-Width-Modulation Mode
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1  Custom Design With WEBENCH® Tools
        2. 8.2.2.2  Output Voltage Setpoint
        3. 8.2.2.3  Switching Frequency
        4. 8.2.2.4  Input Capacitors
        5. 8.2.2.5  Inductor Selection
        6. 8.2.2.6  Output Capacitor Selection
        7. 8.2.2.7  Feedforward Capacitor
        8. 8.2.2.8  Bootstrap Capacitors
        9. 8.2.2.9  VCC Capacitors
        10. 8.2.2.10 BIAS Capacitors
        11. 8.2.2.11 Soft-Start Capacitors
        12. 8.2.2.12 Undervoltage Lockout Setpoint
        13. 8.2.2.13 PGOOD
        14. 8.2.2.14 Synchronization
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
      1. 10.1.1 Layout Highlights
      2. 10.1.2 Compact Layout for EMI Reduction
      3. 10.1.3 Ground Plane and Thermal Considerations
      4. 10.1.4 Feedback Resistors
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Development Support
        1. 11.1.1.1 Custom Design With WEBENCH® Tools
    2. 11.2 Receiving Notification of Documentation Updates
    3. 11.3 Support Resources
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Support Resources
  12. 12Mechanical, Packaging, and Orderable Information

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7.3.8 Minimum On-Time, Minimum Off-Time, and Frequency Foldback at Dropout Conditions

Minimum on-time, tON-MIN, is the smallest duration of time that the HS switch can be on. tON-MIN is typically 70 ns in the LM76005. Minimum off-time, tOFF-MIN, is the smallest duration that the HS switch can be off. tOFF-MIN is typically 100 ns in the LM76005. In CCM operation, tON-MIN and tOFF-MIN limits the voltage conversion range given a selected switching frequency. The minimum duty cycle allowed is:

Equation 7. DMIN = tON-MIN × fSW

And the maximum duty cycle allowed is:

Equation 8. DMAX = 1 – tOFF-MIN × fSW

Given fixed tON-MIN and tOFF-MIN, the higher the switching frequency the narrower the range of the allowed duty cycle. In the LM76005, frequency foldback scheme is employed to extend the maximum duty cycle when tOFF-MIN is reached. The switching frequency decreases once longer duty cycle is needed under low VIN conditions. Such a wide range of frequency foldback allows the LM76005 output voltage to stay in regulation with a much lower supply voltage VIN. This leads to a lower effective dropout voltage.

Given an output voltage, the choice of the switching frequency affects the allowed input voltage range, solution size, and efficiency. The maximum operational supply voltage can be found by:

Equation 9. VIN_MAX = VOUT / (fSW × tON-MIN)

At lower supply voltage, the switching frequency decreases once tOFF-MIN is tripped. The minimum VIN without frequency foldback can be approximated by:

Equation 10. VIN_MIN = VOUT / (fSW × tOFF-MIN)

Considering power losses in the system with heavy load operation, VIN-MIN is higher than the result calculated in Equation 10. With frequency foldback, VIN-MIN is lowered by decreased fSW. When the device is operating in auto mode at voltages near maximum rated input voltage and light load conditions, an increased output voltage ripple during load transient can be observed. For this reason, TI recommends that the device operating point be calculated with sufficient operational margin so that minimum on-time condition is not triggered.