SLVUAP3A April   2016  – October 2021 TPS54202

 

  1.   Trademarks
  2. 1Introduction
    1. 1.1 Background
    2. 1.2 Performance Specification Summary
    3. 1.3 Modifications
      1. 1.3.1 Output Voltage Set Point
      2. 1.3.2 Output Capacitor and Feed-Forward Capacitor
      3. 1.3.3 Adjustable UVLO
  3. 2Test Setup and Results
    1. 2.1 Input/Output Connections
    2. 2.2 Efficiency
    3. 2.3 Output Voltage Load Regulation
    4. 2.4 Output Voltage Line Regulation
    5. 2.5 Load Transients
    6. 2.6 Output Voltage Ripple
    7. 2.7 Input Voltage Ripple
    8. 2.8 Powering Up
    9. 2.9 Powering Down
  4. 3Board Layout
    1. 3.1 Layout
  5. 4Schematic and Bill of Materials
    1. 4.1 Schematic
    2. 4.2 Bill of Materials
  6. 5Revision History

1.3.2 Output Capacitor and Feed-Forward Capacitor

Considering the loop stability and the effect of the internal parasitic parameters, choose a crossover frequency less than 40 kHz, without considering the feed-forward capacitor. A simple estimation for the crossover frequency without feed-forward capacitor C8 is shown in Equation 2, assuming COUT has small ESR.

Equation 2. GUID-5B56A147-4D0A-44C5-BBC2-EF3FF34471DD-low.gif

Depending on VOUT, if the output capacitor, COUT, is dominated by low-ESR (ceramic types) capacitors, a low phase margin could result. To improve the phase boost, an external feed-forward capacitor, C8, can be added in parallel with R2. C8 is chosen such that phase margin is boosted at the crossover frequency.

C8 is calculated in Equation 3:

Equation 3. GUID-EDCC5F71-EFA1-4AF4-A543-C02991E30156-low.gif

For this design, C8 = 75 pF. C8 is not needed when COUT has high ESR, and C8 calculated from Equation 3 should be reduced with medium ESR. Use Table 1-3 as a starting point.