SLVSIY7 August   2025 LM5168E

PRODUCTION DATA  

  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 Typical Characteristics
  8. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  Control Architecture
      2. 7.3.2  Internal VCC Regulator and Bootstrap Capacitor
      3. 7.3.3  Internal Soft Start
      4. 7.3.4  On-Time Generator
      5. 7.3.5  Current Limit
      6. 7.3.6  N-Channel Buck Switch and Driver
      7. 7.3.7  Synchronous Rectifier
      8. 7.3.8  Enable/Undervoltage Lockout (EN/UVLO)
      9. 7.3.9  Power Good (PGOOD)
      10. 7.3.10 Thermal Protection
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Mode
      2. 7.4.2 Active Mode
      3. 7.4.3 Sleep Mode
  9. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Buck Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Switching Frequency (RT)
        2. 8.2.2.2 Buck Inductor Selection
        3. 8.2.2.3 Setting the Output Voltage
        4. 8.2.2.4 Type 3 Ripple Network
        5. 8.2.2.5 Output Capacitor Selection
        6. 8.2.2.6 Input Capacitor Considerations
        7. 8.2.2.7 CBST Selection
        8. 8.2.2.8 Example Design Summary
      3. 8.2.3 Application Curves
    3. 8.3 Power Supply Recommendations
    4. 8.4 Layout
      1. 8.4.1 Layout Guidelines
        1. 8.4.1.1 Compact PCB Layout for EMI Reduction
        2. 8.4.1.2 Feedback Resistors
      2. 8.4.2 Thermal Considerations
      3. 8.4.3 Layout Example
  10. 9 Device and Documentation Support
    1. 9.1 Device Support
      1. 9.1.1 Third-Party Products Disclaimer
    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

8.2.2.4 Type 3 Ripple Network

This example uses a Type 3 ripple injection network. This network uses an RC filter consisting of RA and CA across SW and VOUT to generate a triangular ramp that is in phase with the inductor current. This triangular ramp is then AC-coupled into the feedback node through capacitor CB. Type 3 ripple injection is an excellent choice for applications where low output voltage ripple is crucial, and allows the use of low-ESR ceramic output capacitors.

Use Equation 20 to calculate CA. With the values used in this example, CA > 184pF. A value of 3300pF is selected to keep RA within practical limits. In general, the user needs 20mV of ripple at the feedback pin for reliable operation, calculated at nominal input voltage. The minimum value of ripple must not be less than 12mV at minimum input voltage. Using Equation 21 with nominal input voltage, a value of RA > 120kΩ was found and a value of 121kΩ is selected.

Equation 20. C A ≥ 10 F S W × ( R F B B | | R F B T )
Equation 21. RA≤(VIN-VOUT)×VOUT0.02×VIN×FSW×CA

While the magnitude of the generated ripple does not affect the output voltage ripple, the magnitude produces a DC error of approximately half the amplitude of the generated ripple, scaled by the feedback divider ratio. Therefore, the amount of DC offset, tolerable in the output voltage, imposes an upper bound on the feedback ripple.

Finally, use Equation 22 to calculate the coupling capacitance CB. In the equation, tR is the approximate settling time of the control loop to a load transient disturbance. This was taken as 50μs.

Equation 22. C B ≥ t R 3 × R F B T

where

  • tR = 50μs (typical)

A value greater than 37pF was calculated for CB and a value of 56pF is selected for this example. This value avoids excessive coupling capacitor discharge by the feedback resistors during sleep intervals when operating at light loads. Note that the minimum value of CB is 47pF.