SNVA994A February   2022  – March 2023 LM5157 , LM5157-Q1 , LM51571-Q1 , LM5158 , LM5158-Q1 , LM51581 , LM51581-Q1

 

  1.   1
  2.   Trademarks
  3. 1Introduction
  4. 2Example Application
  5. 3Calculations and Component Selection
    1. 3.1 Switching Frequency
    2. 3.2 Transformer Selection
      1. 3.2.1 Maximum Duty Cycle and Turns Ratio Selection
      2. 3.2.2 Primary Winding Inductance Selection
    3. 3.3 Slope Compensation Check
    4. 3.4 Diode Selection
    5. 3.5 Output Capacitor Selection
    6. 3.6 Input Capacitor Selection
    7. 3.7 UVLO Resistor Selection
    8. 3.8 Control Loop Compensation
      1. 3.8.1 Crossover Frequency (fcross) Selection
      2. 3.8.2 RCOMP Selection
      3. 3.8.3 CCOMP Selection
      4. 3.8.4 CHF Selection
  6. 4Component Selection Summary
    1. 4.1 Application Circuit
    2. 4.2 Bill of Materials
  7. 5Small Signal Frequency Analysis
    1. 5.1 Flyback Regulator Modulator Modeling
    2. 5.2 Compensation Modeling
  8. 6Revision History

3.8.2 RCOMP Selection

The RCOMP value directly affects the crossover frequency of the control loop. The higher the crossover frequency, the faster the control loop reacts to transient conditions. Knowing the desired loop crossover frequency, 5 kHz, RCOMP is calculated using Equation 23.

Equation 23. RCOMP= 2×π× ACs × COUT_total × NS1Np × VLOAD1 × fCROSSGCOMP × gm ×(1- DVIN_min) RCOMP= 2×π× 0.095 × 300µF × 1.21 × 10V × 5kHzGCOMP × gm ×(1-0.51)=10.96kΩ

where

  • gm is the transconductance of the error amplifier, 2 mA/V (see data sheet)
  • ACS is gain of the internal sensed current, 0.095 (see data sheet)
  • GCOMP is COMP to PWM gain, 1 V/V

RCOMP is selected to be 10 kΩ. Decreasing the RCOMP resistance value lowers the crossover frequency but helps ensure the control loop remains stable over the specified supply voltage range.