TIDUCV2B April   2017  – January 2023

 

  1.   Description
  2.   Resources
  3.   Features
  4.   Applications
  5.   5
  6. 1System Overview
    1. 1.1 System Description
    2. 1.2 Key System Specifications
    3. 1.3 Highlighted Products
      1. 1.3.1 TPS82130
    4. 1.4 Design Considerations
      1. 1.4.1 Inverting Buck-Boost Topology Concept
      2. 1.4.2 VIN and VOUT Range
      3. 1.4.3 Maximum Output Current
        1. 1.4.3.1 Thermal Limits
        2. 1.4.3.2 Stability Limits and Output Capacitor Selection
      4. 1.4.4 Design Precautions
      5. 1.4.5 Enable Pin Configuration
      6. 1.4.6 Power Good Pin Configuration
      7. 1.4.7 Discharging Output Voltage
      8. 1.4.8 Input Capacitor Selection
  7. 2Getting Started Hardware
  8. 3Testing and Results
    1. 3.1 Test Results
  9. 4Design Files
    1. 4.1 Schematics
    2. 4.2 Bill of Materials
    3. 4.3 PCB Layout Recommendations
      1. 4.3.1 Layout Prints
    4. 4.4 Gerber Files
    5. 4.5 Assembly Drawings
  10. 5Related Documentation
  11. 6Trademarks
  12. 7Revision History

Stability Limits and Output Capacitor Selection

The 'Recommended' curve in Figure 3-1 shows the recommended maximum output current based on stability. The TIDA-01405 design should be operated at load currents below this line. In most applications, which have ambient temperatures above 25°C, the thermal limit lines move down below the 'Recommended' line (as explained in Section 1.4.3.1) and thus further limit the maximum output current.

The inverting buck-boost topology contains a right-half plane zero which significantly and negatively impacts the control loop response by adding an increase in gain along with a decrease in phase at a high frequency. This right-half plane zero can cause instability. Equation 6 estimates the frequency of the right-half plane zero.

Equation 6. GUID-55A9A489-9B61-42F5-90A0-BCD5DE3C03A7-low.gif

The TIDA-01405 design uses four 22-µF output capacitors, which have an effective capacitance of about 72 µF at the -1.8-V output voltage. This amount of capacitance pushes the crossover frequency of the control loop down to frequencies low enough so that the right-half plane zero is sufficiently higher in frequency for stability. While one of these output capacitors requires placement near the TPS82130 device, the others can be placed at the point of load and serve as their input decoupling capacitor. When three of the output capacitors are placed at the point of load, the solution size is below 50 mm2. If these three capacitors are included, the solution size of all components shown within the 'Active Circuitry' box on page 1 becomes around 75 mm2.

More output capacitance improves stability by increasing the separation between the right-half plane zero and crossover. The right-half plane zero frequency occurs at lower frequencies with lower input voltages, which have a higher duty cycle. Load transient testing is the best test for stability, as described in the Simplifying Stability Checks application report. Because the VOS pin of the TPS82130 is connected on the device, it is impossible to break the entire control loop and measure a bode plot.