In the buck configuration, the average inductor current equals the average output current because the inductor always supplies current to the load during both the on and off times of the control MOSFET. However, in the inverting buck boost configuration, the load is supplied with current only from the output capacitor and is completely disconnected from the inductor during the on time of the control MOSFET. During the off time, the inductor connects to both the output capacitor and the load (see Figure 1-3).

The peak current of the MOSFET and inductor can easily be calculated, as follows in Equation 1, Equation 2, Equation 3, and Equation 4.

Where:

When V_IN is increased and V_OUT is kept constant, the duty cycle, D, and I_Lavg decrease, while ΔI_L increases. You can see that the sum of I_Lavg and ΔI_L decreases. So, when V_IN is at the minimum, you can get the maximum I_peak. You must choose an applicable inductor, L, to keep the maximum I_peak lower than the minimum current limit, I_cl(min) of the device. Therefore, you get Equation 5, as follows:

You can get the I_OUTmax versus L_min graph of the TPS6293x device, shown in Figure 4. For the TPS62933 device, I_{lim_HS} = 4.2 A and choose f_s = 500 kHz. From Figure 4, you can see that by increasing the inductor and V_INmin, or decreasing the output voltage level, this device can hold more output current in the buck-boost application.