SNVSC77 December   2024 LM5125-Q1

ADVANCE INFORMATION  

  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. 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  Switching Frequency and Synchronization (SYNCIN)
      3. 6.3.3  Dual Random Spread Spectrum (DRSS)
      4. 6.3.4  Operation Modes (BYPASS, DEM, FPWM)
      5. 6.3.5  Dual- and Multi-phase Operation
      6. 6.3.6  BIAS (BIAS-pin)
      7. 6.3.7  Soft Start (SS-pin)
      8. 6.3.8  VOUT Programming (VOUT, ATRK, DTRK)
      9. 6.3.9  Protections
      10. 6.3.10 VOUT Overvoltage Protection (OVP)
      11. 6.3.11 Thermal Shutdown (TSD)
      12. 6.3.12 Power-Good Indicator (PGOOD-pin)
      13. 6.3.13 Current Sensing, Peak Current Limit, and Slope Compensation (CSP1, CSP2, CSN1, CSN2)
      14. 6.3.14 Current Sense Programming (CSP1, CSP2, CSN1, CSN2)
      15. 6.3.15 Input Current Limit and Monitoring (ILIM, IMON, DLY)
      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. 7.2 Typical Application
      1. 7.2.1 Application
      2. 7.2.2 Design Requirements
      3. 7.2.3 Detailed Design Procedure
        1. 7.2.3.1  Determine the Total Phase Number
        2. 7.2.3.2  Determining the Duty Cycle
        3. 7.2.3.3  Timing Resistor RT
        4. 7.2.3.4  Inductor Selection Lm
        5. 7.2.3.5  Current Sense Resistor Rcs
        6. 7.2.3.6  Current Sense Filter RCSFP, RCSFN, CCS
        7. 7.2.3.7  Low-Side Power Switch QL
        8. 7.2.3.8  High-Side Power Switch QH and Additional Parallel Schottky Diode
        9. 7.2.3.9  Snubber Components
        10. 7.2.3.10 Vout Programming
        11. 7.2.3.11 Input Current Limit (ILIM/IMON)
        12. 7.2.3.12 UVLO Divider
        13. 7.2.3.13 Soft Start
        14. 7.2.3.14 CFG Settings
        15. 7.2.3.15 Output Capacitor Cout
        16. 7.2.3.16 Input Capacitor Cin
        17. 7.2.3.17 Bootstrap Capacitor
        18. 7.2.3.18 VCC Capacitor CVCC
        19. 7.2.3.19 BIAS Capacitor
        20. 7.2.3.20 VOUT Capacitor
        21. 7.2.3.21 Loop Compensation
      4. 7.2.4 Application Curves
        1. 7.2.4.1 Efficiency
        2. 7.2.4.2 Steady State Waveforms
        3. 7.2.4.3 Step Load Response
        4. 7.2.4.4 Sync Operation
        5. 7.2.4.5 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
    1. 10.1 Tape and Reel Information
    2.     85

Package Options

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

7.2.3.15 Output Capacitor Cout

The output capacitors smooth the output voltage ripple and provide a source of charge during load transient conditions.

Ripple current rating of output capacitor must be carefully selected. In boost regulator, the output is supplied by discontinuous current and the ripple current requirement is usually high. In practice, the ripple current requirement can be dramatically reduced by placing high-quality ceramic capacitors earlier than the bulk aluminum capacitors close to the power switches.

The output voltage ripple is dominated by ESR of the output capacitors. Paralleling output capacitor is a good choice to minimize effective ESR and split the output ripple current into capacitors.

The single phase boost output RMS ripple current can be expressed as:

Equation 60. I 1 p _ r m s ≈ I o u t × D D '

The output RMS current is reduced with interleaving as shown in Figure 7-10. Dual phase interleaved boost output RMS ripple current can be expressed as:

Equation 61. I o u t _ 2 p _ r m s ≈ I o u t 2 × D × 1 - 2 D D ' ,     D < 0.5 I o u t 2 × 2 D - 1 D ' ,     D ≥ 0.5
LM5125-Q1 Normalized Output Capacitor RMS Ripple Current Figure 7-9 Normalized Output Capacitor RMS Ripple Current

Decoupling capacitors are critical for minimized voltage spike of the MOSFETs. This is also important from EMI view. Quite a few 0603/100nF ceramic capacitors are placed close to the MOSFETs following "vertical loop" concept. Refer to Improve High-Current DC/DC Regulator EMI Performance for Free With Optimized Power Stage Layout application brief for more details.

A few 10µF ceramic capacitors are also necessary to reduce the output voltage ripple and split the output ripple current.

Typically, aluminum capacitors are required for high capacitance. In this example, four 150µF aluminum capacitors are selected.

The output transient response is closely related to the bandwidth of the loop gain and the output capacitance. According to How to Determine Bandwidth from the Transient-response Measurement technical article, the overshoot or undershoot Vp can be estimated as:

Equation 62. Vp=ΔItran2π×fc×Cout

where ΔItran is the transient load current step.

Please be aware that Equation 62 is valid only if the converter is always operating in CCM or FPWM during load step. If the converter enters DCM or pulsing skip mode at light load, the overshoot is worse.

Due to the inherent path from input to output, unlimited inrush current can flow when the input voltage rises quickly and charges the output capacitor. The slew rate of input voltage rising must be controlled by a hot-swap or by starting the input power supply softly for the inrush current not to damage the inductor, sense resistor or high-side MOSFET.