SNVSAP9A March   2017  – February 2018 LM25141-Q1

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Simplified Schematic
  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 Switching Characteristics
    7. 6.7 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  High Voltage Start-Up Regulator
      2. 7.3.2  VCC Regulator
      3. 7.3.3  Oscillator
      4. 7.3.4  Synchronization
      5. 7.3.5  Frequency Dithering (Spread Spectrum)
      6. 7.3.6  Enable
      7. 7.3.7  Power Good
      8. 7.3.8  Output Voltage
        1. 7.3.8.1 Minimum Output Voltage Adjustment
      9. 7.3.9  Current Sense
      10. 7.3.10 DCR Current Sensing
      11. 7.3.11 Error Amplifier and PWM Comparator
      12. 7.3.12 Slope Compensation
      13. 7.3.13 Hiccup Mode Current Limiting
      14. 7.3.14 Standby Mode
      15. 7.3.15 Soft Start
      16. 7.3.16 Diode Emulation
      17. 7.3.17 High- and Low-Side Drivers
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      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 Inductor Calculation
        3. 8.2.2.3 Current Sense Resistor
        4. 8.2.2.4 Output Capacitor
        5. 8.2.2.5 Input Filter
          1. 8.2.2.5.1 EMI Filter Design
          2. 8.2.2.5.2 MOSFET Selection
          3. 8.2.2.5.3 Driver Slew Rate Control
          4. 8.2.2.5.4 Frequency Dithering
        6. 8.2.2.6 Control Loop
          1. 8.2.2.6.1 Feedback Compensator
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
      1. 10.1.1 Layout Procedure
    2. 10.2 Layout Examples
  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 Documentation Support
      1. 11.2.1 Related Documentation
        1. 11.2.1.1 PCB Layout Resources
        2. 11.2.1.2 Thermal Design Resources
    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

7.3.17 High- and Low-Side Drivers

The LM25141-Q1 contains N-channel MOSFET gate drivers and an associated high-side level shifter to drive the external N-channel MOSFETs. The high-side gate driver works in conjunction with an external bootstrap diode DBST, and bootstrap capacitor CBST (refer to Figure 27). During the on-time of the low-side MOSFET, the SW pin voltage is approximately 0 V and CBST is charged from VCC through the DBST. A 0.1-μF or larger ceramic capacitor, connected with short traces between the HB and SW pin is recommended.

The LO and HO outputs are controlled with an adaptive dead-time methodology which ensures that both outputs (HO and LO) are never enabled at the same time, preventing cross conduction. When the controller commands LO to be enabled, the adaptive dead-time logic first disables HO and waits for the HO-SW voltage to drop below 2.5 V typical. LO is then enabled after a small delay (HO falling to LO rising delay). Similarly, the HO turnon is delayed until the LO voltage has dropped below 2.5 V. HO is then enabled after a small delay (LO falling to HO rising delay). This technique ensures adequate dead-time for any size N-channel MOSFET device or parallel MOSFET configurations. Caution is advised when adding series gate resistors, as this may decrease the effective dead-time. Each of the high and low-side drivers have independent driver source and sink output pins. This allows the user to adjust drive strength to optimize the switching losses for maximum efficiency and to control the slew rate for reduced EMI. The selected N-channel high-side MOSFET determines the appropriate boost capacitance values CBST in the Figure 27 according to Equation 14.

Equation 14. LM25141-Q1 equation_13_snvsaj6.gif

where

  • QG is the total gate charge of the high-side MOSFET
  • ΔVBST is the voltage variation allowed on the high-side MOSFET driver after turnon

Choose ΔVBST such that the available gate-drive voltage is not significantly degraded when determining CBST. A typical range of ΔVBST is 100 mV to 300 mV. The bootstrap capacitor should be a low-ESR ceramic capacitor. A minimum value of 0.1 μF to 0.47 μF is best in most cases. The gate threshold of the high-side and low-side MOSFETs should be a logic level variety appropriate for 5-V gate drive.

LM25141-Q1 drivers_snvsaj6.gifFigure 27. Drivers