JAJSFY2B August   2018  – June 2021 LM5146-Q1

PRODUCTION DATA  

  1. 特長
  2. アプリケーション
  3. 概要
  4. Revision History
  5. 概要 (続き)
  6. Pin Configuration and Functions
    1. 6.1 Wettable Flanks
  7. Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings 
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 Switching Characteristics
    7. 7.7 Typical Characteristics
  8. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1  Input Range (VIN)
      2. 8.3.2  Output Voltage Setpoint and Accuracy (FB)
      3. 8.3.3  High-Voltage Bias Supply Regulator (VCC)
      4. 8.3.4  Precision Enable (EN/UVLO)
      5. 8.3.5  Power Good Monitor (PGOOD)
      6. 8.3.6  Switching Frequency (RT, SYNCIN)
        1. 8.3.6.1 Frequency Adjust
        2. 8.3.6.2 Clock Synchronization
      7. 8.3.7  Configurable Soft Start (SS/TRK)
        1. 8.3.7.1 Tracking
      8. 8.3.8  Voltage-Mode Control (COMP)
      9. 8.3.9  Gate Drivers (LO, HO)
      10. 8.3.10 Current Sensing and Overcurrent Protection (ILIM)
      11. 8.3.11 OCP Duty Cycle Limiter
    4. 8.4 Device Functional Modes
      1. 8.4.1 Shutdown Mode
      2. 8.4.2 Standby Mode
      3. 8.4.3 Active Mode
      4. 8.4.4 Diode Emulation Mode
      5. 8.4.5 Thermal Shutdown
  9. Application and Implementation
    1. 9.1 Application Information
      1. 9.1.1 Design and Implementation
      2. 9.1.2 Power Train Components
        1. 9.1.2.1 Inductor
        2. 9.1.2.2 Output Capacitors
        3. 9.1.2.3 Input Capacitors
        4. 9.1.2.4 Power MOSFETs
      3. 9.1.3 Control Loop Compensation
      4. 9.1.4 EMI Filter Design
    2. 9.2 Typical Applications
      1. 9.2.1 Design 1 – 12-A High-Efficiency Synchronous Buck DC/DC Regulator for Automotive Applications
        1. 9.2.1.1 Design Requirements
        2. 9.2.1.2 Detailed Design Procedure
        3. 9.2.1.3 Custom Design With WEBENCH® Tools
        4. 9.2.1.4 Custom Design With Excel Quickstart Tool
        5. 9.2.1.5 Application Curves
      2. 9.2.2 Design 2 – High Density, 12-V, 8-A Rail From 48-V Automotive Battery Power
        1. 9.2.2.1 Design Requirements
        2. 9.2.2.2 Detailed Design Procedure
        3. 9.2.2.3 Application Curves
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
      1. 11.1.1 Power Stage Layout
      2. 11.1.2 Gate Drive Layout
      3. 11.1.3 PWM Controller Layout
      4. 11.1.4 Thermal Design and Layout
      5. 11.1.5 Ground Plane Design
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Device Support
      1. 12.1.1 Third-Party Products Disclaimer
      2. 12.1.2 Development Support
        1. 12.1.2.1 Custom Design With WEBENCH® Tools
    2. 12.2 Documentation Support
      1. 12.2.1 Related Documentation
        1. 12.2.1.1 PCB Layout Resources
        2. 12.2.1.2 Thermal Design Resources
    3. 12.3 Receiving Notification of Documentation Updates
    4. 12.4 サポート・リソース
    5. 12.5 Trademarks
    6. 12.6 Electrostatic Discharge Caution
    7. 12.7 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Current Sensing and Overcurrent Protection (ILIM)

The LM5146-Q1 implements a lossless current sense scheme designed to limit the inductor current during an overload or short-circuit condition. Figure 8-8 portrays the popular current sense method using the on-state resistance of the low-side MOSFET. Meanwhile, Figure 8-9 shows an alternative implementation with current shunt resistor, RS. The LM5146-Q1 senses the inductor current during the PWM off-time (when LO is high).

Figure 8-8 MOSFET RDS(on) Current Sensing
Figure 8-9 Shunt Resistor Current Sensing

The ILIM pin of the LM5146-Q1 sources a reference current that flows in an external resistor, designated RILIM, to program of the current limit threshold. A current limit comparator on the ILIM pin prevents further SW pulses if the ILIM pin voltage goes below GND. Figure 8-10 shows the implementation.

Resistor RILIM is tied to SW to use the RDS(on) of the low-side MOSFET as a sensing element (termed RDS(on) mode). Alternatively, RILIM is tied to a shunt resistor connected at the source of the low-side MOSFET (termed RSENSE mode). The LM5146-Q1 detects the appropriate mode at start-up and sets the source current amplitude and temperature coefficient (TC) accordingly.

The ILIM current with RDS-ON sensing is 200 µA at 27°C junction temperature and incorporates a TC of +4500 ppm/°C to generally track the RDS(on) temperature variation of the low-side MOSFET. Conversely, the ILIM current is a constant 100 µA in RSENSE mode. This controls the valley of the inductor current during a steady-state overload at the output. Depending on the chosen mode, select the resistance of RILIM using Equation 6.

Equation 6. GUID-0C015054-847A-4361-8241-443B6D1C25D3-low.gif

where

  • ΔIL is the peak-to-peak inductor ripple current
  • RDS(on)Q2 is the on-state resistance of the low-side MOSFET
  • IRDSON is the ILIM pin current in RDS-ON mode
  • RS is the resistance of the current-sensing shunt element
  • IRS is the ILIM pin current in RSENSE mode

Given the large voltage swings of ILIM in RDS(on) sensing mode, a capacitor designated CILIM connected from ILIM to PGND is essential to the operation of the valley current limit circuit. Choose this capacitance such that the time constant RILIM· CILIM is approximately 6 ns.

GUID-E02816FF-E203-4CEB-8463-EF50B903B151-low.gifFigure 8-10 OCP Setpoint Defined by Current Source IRDSON and Resistor RILIM in RDS-ON Mode

Note that current sensing with a shunt component is typically implemented at lower output current levels to provide accurate overcurrent protection. Burdened by the unavoidable efficiency penalty, PCB layout, and additional cost implications, this configuration is not usually implemented in high-current applications (except where OCP setpoint accuracy and stability over the operating temperature range are critical specifications).