SNVSCQ8B November   2025  – July 2026 LM654A5-Q1 , LM654B0-Q1

PRODMIX  

  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 Descriptions
      1. 7.3.1  Output Voltage Selection
      2. 7.3.2  EN Pin and Use as VIN UVLO
      3. 7.3.3  Device Configuration
      4. 7.3.4  Mode Selection
        1. 7.3.4.1 MODE/SYNC Pin Uses for Synchronization
        2. 7.3.4.2 Clock Locking
      5. 7.3.5  Adjustable Switching Frequency and Phase Shift
      6. 7.3.6  Dual Random Spread Spectrum (DRSS)
      7. 7.3.7  Internal LDO, VCC UVLO, and BIAS Input
      8. 7.3.8  Bootstrap Voltage (BST Pin)
      9. 7.3.9  Soft Start and Recovery From Dropout
      10. 7.3.10 Safety Features
        1. 7.3.10.1 Power-Good Monitor
        2. 7.3.10.2 Overcurrent and Short-Circuit Protection
        3. 7.3.10.3 Hiccup
        4. 7.3.10.4 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 Example Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Choosing the Switching Frequency
        2. 8.2.2.2 Inductor Selection
        3. 8.2.2.3 Output Capacitors
        4. 8.2.2.4 Input Capacitor Selection
        5. 8.2.2.5 Setting the Output Voltage
        6. 8.2.2.6 Compensation Components
        7. 8.2.2.7 Feed-forward Capacitor (CFF)
        8. 8.2.2.8 Maximum Ambient Temperature
      3. 8.2.3 Application Curves
    3. 8.3 Power Supply Recommendations
    4. 8.4 Layout
      1. 8.4.1 Layout Guidelines
      2. 8.4.2 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

8.2.2.7 Feed-forward Capacitor (CFF)

The value of the CFF capacitor is calculated so that the resulting phase margin of the overall system is improved. The addition of the CFF capacitor does not change the response of the system at the DC level or at lower frequencies. At higher frequencies, the capacitor helps reduce the impedance from VOUT to FB. This action helps propagate any high frequency change due to a fast load transient at the output to the feedback, and allows the error amplifier to correct for this.

In the frequency domain, the addition of the CFF capacitor creates one zero and one pole. The zero helps increase the gain by 20dB/decade and gives a phase boost at the loop crossover frequency, that increases the phase margin of the loop. This extra phase boost is most effective when there is a wide ratio between the output voltage and the reference voltage values. Adding a CFF to a low VOUT application does not provide much help to the phase margin of the loop. The pole from CFF helps to roll off the frequency response and improve the loop gain margin.

Phase boost values at the loop crossover frequency in the range of 5º to 20º are reasonable. The loop crossover frequency is generally increased slightly by the use of a CFF, so only moderate values of phase boost must be used. Use Equation 21 to help select a starting point value for CFF for a desired phase boost, θB, in the above range, in degrees.

Equation 21. C F F ≅ θ B 360 × R F B T × f C × 1 - 0.8 V O U T

Where:

  • CFF = Feed forward capacitor value (F)
  • RFBT = Upper feedback resistor value (Ω)
  • θB = Desired phase boost (º)
  • fC = Loop crossover frequency (Hz)
  • VOUT = Output voltage (V)

For the conditions of this example and assuming a desired phase boost of 10º, we get a value of 2.5pF. In this case a 5pFcapacitor can be used and the results evaluated on the bench.

This method of selecting a CFF gives only an estimate. In addition, a CFF can not be needed in every case. The best way to select CFF, if used, is to follow the recommendations in the quick start calculator tool for this family of devices. In any case, a place for a CFF must be provided on the PCB.