SNVSCH2 September   2024 LM65645-Q1

ADVANCE INFORMATION  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Device Comparison Table
  6. 5 Pin Configuration and Functions
  7. 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 System Characteristics
  8. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Descriptions
      1. 7.3.1 Output Voltage Selection
      2. 7.3.2 EN Pin and Use as VIN UVLO
      3. 7.3.3 Mode Selection
        1. 7.3.3.1 MODE/SYNC Pin Uses for Synchronization
        2. 7.3.3.2 Clock Locking
      4. 7.3.4 Adjustable Switching Frequency
      5. 7.3.5 Dual Random Spread Spectrum (DRSS)
      6. 7.3.6 Internal LDO, VCC UVLO, and BIAS Input
      7. 7.3.7 Bootstrap Voltage (BST Pin)
      8. 7.3.8 Soft Start and Recovery From Dropout
      9. 7.3.9 Safety Features
        1. 7.3.9.1 Power-Good Monitor
        2. 7.3.9.2 Overcurrent and Short-Circuit Protection
        3. 7.3.9.3 Hiccup
        4. 7.3.9.4 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Mode
      2. 7.4.2 Active Mode
        1. 7.4.2.1 Peak Current Mode Operation
        2. 7.4.2.2 Auto Mode Operation
          1. 7.4.2.2.1 Diode Emulation
        3. 7.4.2.3 FPWM Mode Operation
  9. 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 Choosing the Switching Frequency
        3. 8.2.2.3 FB for Adjustable or Fixed Output Voltage Mode
        4. 8.2.2.4 Inductor Selection
        5. 8.2.2.5 Output Capacitor Selection
        6. 8.2.2.6 Input Capacitor Selection
        7. 8.2.2.7 CBOOT
        8. 8.2.2.8 External UVLO
        9. 8.2.2.9 Maximum Ambient Temperature
    3. 8.3 Best Design Practices
    4. 8.4 Power Supply Recommendations
    5. 8.5 Layout
      1. 8.5.1 Layout Guidelines
        1. 8.5.1.1 Ground and Thermal Considerations
      2. 8.5.2 Layout Example
  10. 9 Device and Documentation Support
    1. 9.1 Device Support
      1. 9.1.1 Third-Party Products Disclaimer
      2. 9.1.2 Development Support
        1. 9.1.2.1 Custom Design With WEBENCH® Tools
    2. 9.2 Documentation Support
      1. 9.2.1 Related Documentation
    3. 9.3 Receiving Notification of Documentation Updates
    4. 9.4 Support Resources
    5. 9.5 Trademarks
    6. 9.6 Electrostatic Discharge Caution
    7. 9.7 Glossary
  11. 10Revision History
  12. 11Mechanical, Packaging, and Orderable Information
    1. 11.1 Tape and Reel Information

Package Options

Mechanical Data (Package|Pins)
  • RZT|20
Thermal pad, mechanical data (Package|Pins)
Orderable Information

7.3.1 Output Voltage Selection

The LM656x5-Q1 features pin-selectable fixed output voltage or adjustable output voltage mode. In fixed output voltage mode, the output voltage is selected by the FB pin. Connect the FB pin to GND to select the fixed 3.3V output, or connect to VCC for a fixed 5V output. When the fixed output voltage mode is selected, the BIAS pin is connected directly to the output of the regulator. In this mode, the BIAS pin closes the feedback loop of the regulator and provides input power to the internal bias regulator. Because of the internal LDO is supplied through this pin, a reliable bode plot cannot be taken in fixed output voltage mode however this measurement can be take in adjustable mode. Connect BIAS to VOUT as shown in Figure 8-1.

Table 7-1 Output Voltage Selection
FBVOUT
Short to GND3.3V
Short to VCC5V
Connect to a feedback resistor divider (Figure 7-1)ADJ

In the adjustable output voltage mode, a voltage divider is connected between the regulator output voltage and the FB pin. The resistor values are calculated based on the desired output voltage and the 0.8V reference of the regulator. See Figure 7-1 for detailed connections.

LM65645-Q1 Setting Output Voltage of Adjustable VersionsFigure 7-1 Setting Output Voltage of Adjustable Versions

Use Equation 1 to select a value for RFBB, based on a desired value of RFBT. Limiting the value of RFBT to 100kΩ or less is best practice. Larger values of resistance are susceptible to leakage currents on the PCB, caused by environmental contamination, that can shift the desired output voltage. Values up to about 1MΩ can be used to reduce the no-load supply current, in those cases where excessive PCB leakage currents are not present.

Equation 1. RFBB=RFBT×0.8VOUT - 0.8

In some cases, when using the adjustable mode, a feed forward capacitor can be used to improve the loop phase margin or load transient response. The exact value of CFF is best selected empirically during the initial bench evaluation of the design. Leave a placeholder for this capacitor in the PCB layout if needed at some stage during development.