The switch node, inductor current, and the output voltage ripple during steady state are signals that need to be checked first for the stability of the system. Oscillations on the output voltage or the inductor current as well as jitter on the switch node are good indicators of the instability of the system. Figure 5-7 shows both the switch node and output voltage ripple of this topology. Load transient response is another good test for stability, as described in the Simplifying Stability Checks application note.

The recommended nominal inductor and output capacitor values to use for this topology are in the range of 15 µH to 22 µH and from 22 µF to 100 µF, respectively. In this application note, a 22-µH inductor and 2 x 22-µF capacitors are used.

The inverting buck-boost topology contains a Right Half Plane (RHP) 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 can cause instability. Equation 7 estimates the frequency of the RHP zero.

Equation 7.

It is recommended to keep the loop crossover frequency to, at most, 1/4th of the RHP zero frequency. Doing this requires either decreasing the inductance to increase the RHP zero frequency or increasing the output capacitance to decrease the crossover frequency. Note that the RHP zero frequency occurs at lower frequencies with lower input voltages, which have a higher duty cycle. How to Measure the Control Loop of DCS-Control™Devices application note explains how to measure the control loop of a DCS-Control™ device while Figure 4-2 shows the bode plot of Figure 5-1.

Figure 4-2 Bode Plot with V_IN = 5 V and 120-mA Load