The inverting buck – boost topology is very similar to the buck topology. In the buck configuration shown in Figure 1-1, the positive connection (V_OUT) is connected to the inductor and the return is connected to the integrated circuit (IC) ground. However, in the inverting buck – boost configuration shown in Figure 1-2, the IC ground is used as the negative output voltage (labeled as -V_OUT). What used to be the positive output in buck configuration is used as the ground (GND). This inverting buck – boost topology allows the output voltage to be inverted and is always lower than the ground.

The circuit operation is different in the inverting buck - boost topology than in the buck topology. Figure 1-3 (a) illustrates that the output voltage terminals are reversed, though the components are wired the same as a buck converter. During the on time of the control MOSFET, shown in Figure 1-3 (b), the inductor is charged with current while the output capacitor supplies the load current. The inductor does not provide current to the load during that time. During the off time of the control MOSFET and the on time of the synchronous MOSFET, shown in Figure 1-3 (c), the inductor provides current to the load and the output capacitor. These changes affect many parameters described in the upcoming sections.

The average inductor current is affected in this topology. 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). Knowing that the off time is 1-D of the switching period, then the average inductor current is:

The duty cycle for the typical buck converter is simply V_OUT / V_IN but the duty cycle for an inverting buck - boost converter becomes:

The efficiency term in Equation 2 adjusts the equations in this section for power conversion losses and yields a more accurate maximum output current result. Where V_OUT is a negative value. The peak to peak inductor ripple current is calculated as:

Where,

∆I_L (A): Peak to peak inductor ripple current

D: Duty cycle

η: Efficiency

f_S (MHz): Switching frequency

L (µH): Inductance

V_IN (V): Input voltage with respect to ground, instead of IC ground or -V_OUT.

Finally, the maximum inductor current becomes:

For an output voltage of –3.3 V, 2.5 MHz switching frequency, 2.2 µH inductor, and an input voltage of 12 V, the following calculations produce the maximum allowable output current that can be delivered based on the 1.3 A minimum current limit (I_LIM) of TPS629210-Q1. The efficiency term is estimated at 85%.

Rearranging Equation 4 and setting I_L(max) equal to the minimum value of I_LIM, as specified in the data sheet, gives:

This result is then used in Equation 1 to calculate the maximum achievable output current:

Table 1-1 provides several examples of the calculated maximum output current for different output voltages (–5 V, –3.3 V and –1.2 V) based on an inductor value of 2.2 μH and 3.3 µH with 2.5 MHz switching frequency, respectively. Increasing the inductance and/or input voltage allows higher output current in the inverting buck - boost topology. The maximum output current for the TPS629210-Q1 in the inverting buck - boost topology are frequently lower than 1 A due to the fact that the average inductor current is higher than that of a typical buck. The output current for the same three output voltages and different input voltages are displayed in Figure 1-4.