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.

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.