SLVSIM9 September   2025 LM5125A-Q1

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Pin Configuration and 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 Electrical Characteristics
    6. 5.6 Timing Requirements
    7. 5.7 Typical Characteristics
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Feature Description
      1. 6.3.1  Device Configuration (CFG0-pin, CFG1-pin, CFG2-pin)
      2. 6.3.2  Device and Phase Enable/Disable (UVLO/EN, EN2)
      3. 6.3.3  Dual Device Operation
      4. 6.3.4  Switching Frequency and Synchronization (SYNCIN)
      5. 6.3.5  Dual Random Spread Spectrum (DRSS)
      6. 6.3.6  Operation Modes (BYPASS, DEM, FPWM)
      7. 6.3.7  VCC Regulator, BIAS (BIAS-pin, VCC-pin)
      8. 6.3.8  Soft Start (SS-pin)
      9. 6.3.9  VOUT Programming (VOUT, ATRK, DTRK)
      10. 6.3.10 Protections
        1. 6.3.10.1 VOUT Overvoltage Protection (OVP)
        2. 6.3.10.2 Thermal Shutdown (TSD)
      11. 6.3.11 Power-Good Indicator (PGOOD-pin)
      12. 6.3.12 Slope Compensation (CSP1, CSP2, CSN1, CSN2)
      13. 6.3.13 Current Sense Setting and Switch Peak Current Limit (CSP1, CSP2, CSN1, CSN2)
      14. 6.3.14 Input Current Limit and Monitoring (ILIM, IMON, DLY)
      15. 6.3.15 Maximum Duty Cycle and Minimum Controllable On-time Limits
      16. 6.3.16 Signal Deglitch Overview
      17. 6.3.17 MOSFET Drivers, Integrated Boot Diode, and Hiccup Mode Fault Protection (LOx, HOx, HBx-pin)
    4. 6.4 Device Functional Modes
      1. 6.4.1 Shutdown State
  8. 7 Application and Implementation
    1. 7.1 Application Information
      1. 7.1.1 Feedback Compensation
      2. 7.1.2 Non-synchronous Application
    2. 7.2 Typical Application
      1. 7.2.1 Design Requirements
      2. 7.2.2 Detailed Design Procedure
        1. 7.2.2.1  Determine the Total Phase Number
        2. 7.2.2.2  Determining the Duty Cycle
        3. 7.2.2.3  Timing Resistor RT
        4. 7.2.2.4  Inductor Selection Lm
        5. 7.2.2.5  Current Sense Resistor Rcs
        6. 7.2.2.6  Current Sense Filter RCSFP, RCSFN, CCS
        7. 7.2.2.7  Low-Side Power Switch QL
        8. 7.2.2.8  High-Side Power Switch QH
        9. 7.2.2.9  Snubber Components
        10. 7.2.2.10 Vout Programming
        11. 7.2.2.11 Input Current Limit (ILIM/IMON)
        12. 7.2.2.12 UVLO Divider
        13. 7.2.2.13 Soft Start
        14. 7.2.2.14 CFG Settings
        15. 7.2.2.15 Output Capacitor Cout
        16. 7.2.2.16 Input Capacitor Cin
        17. 7.2.2.17 Bootstrap Capacitor
        18. 7.2.2.18 VCC Capacitor CVCC
        19. 7.2.2.19 BIAS Capacitor
        20. 7.2.2.20 VOUT Capacitor
        21. 7.2.2.21 Loop Compensation
      3. 7.2.3 Application Curves
        1. 7.2.3.1 Efficiency
        2. 7.2.3.2 Steady State Waveforms
        3. 7.2.3.3 Step Load Response
        4. 7.2.3.4 Sync Operation
        5. 7.2.3.5 AC Loop Response Curve
        6. 7.2.3.6 Thermal Performance
    3. 7.3 Power Supply Recommendations
    4. 7.4 Layout
      1. 7.4.1 Layout Guidelines
      2. 7.4.2 Layout Example
  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

6.3.15 Maximum Duty Cycle and Minimum Controllable On-time Limits

To cover the non-ideal factors caused by resistive elements, a maximum duty cycle limit DMAX and a minimum forced off-time is implemented. In CCM operation the minimum supported input voltage VI_MIN for a programmed output voltage VOUT is defined by the maximum duty cycle DMAX (see Equation 22). In DEM operation the minimum input voltage VI_MIN is not limited by DMAX.

Equation 22. VI_MIN ≈ VOUT × (1 - DMAX ) + II_MAX × (RDCR + RSNS + RDS(ON))

where

  • II_MAX is the maximum input current at minimum input voltage VI_MIN
  • RDCR is the DC resistance of the inductor
  • RSNS is the resistance of the sense resistor
  • RDS(ON) is the on resistance of the MOSFET

LM5125A-Q1 Switching Frequency vs Maximum Duty Cycle Figure 6-24 Switching Frequency vs Maximum Duty Cycle

At very light load condition or when VI is close to VOUT the device skips the low-side driver pulses if the required on-time is less than tON-MIN to avoid VOUT runaway. This pulse skipping appears as a random behavior. If VI is further increased to the voltage higher than VOUT, the required on-time becomes zero and eventually the device enters bypass operation which turns on the high-side driver 100%.