The need to generate negative voltage rails for electronic designs is a common challenge. The wide 3-V to 18-V input voltage range of the TPS62903-Q1 makes it ideal for an inverting buck-boost (IBB) circuit, where the output voltage is inverted or negative with respect to ground.

The circuit operation in the IBB topology differs from that in the traditional buck topology. Though the components are connected the same as with a traditional buck converter, the output voltage terminals are reversed. See Figure 9-68.

The maximum input voltage that can be applied to an IBB converter is less than the maximum voltage that can be applied to the TPS62903-Q1 in a typical buck configuration. This is because the ground pin of the IC is connected to the (negative) output voltage. Therefore, the input voltage across the device is V_IN to V_OUT, and not V_IN to ground. Thus, the input voltage range of the TPS62903-Q1 in an IBB configuration becomes 3 V to 18 V + V_OUT, where V_OUT is a negative value.

The output voltage range is the same as when configured as a buck converter, but only negative. Thus, the output voltage for a TPS62903-Q1 in an IBB configuration can be set between –0.4 V and –5.5 V.

The maximum output current for the TPS62903-Q1 in an IBB topology is normally lower than a traditional buck configuration due to the average inductor current being higher in an IBB configuration. Traditionally, lower input or (more negative) output voltages results in a lower maximum output current. However, using a larger inductor value or the higher 2.5-MHz frequency setting can be used to recover some or all of this lost maximum current capability.

When implementing an IBB design, it is important to understand that the IC ground is tied to the negative voltage rail, and in turn, the electrical characteristics of the TPS62903-Q1 device are referenced to this rail. During power up, as there is no charge in the output capacitor and the IC GND pin (and V_OUT) are effectively 0 V, thus parameters such as the V_IN UVLO and EN thresholds are the same as in a typical buck configuration. However, after the output voltage is in regulation, due to the negative voltage on the IC GND pin, the device traditionally continues to operate below what can appear to be the normal UVLO or EN falling thresholds relative to the system ground. Take care if the user is using the dynamic mode change feature on the MODE pin of the TPS62903-Q1 or driving the EN pin from an upstream microcontroller as the high and low thresholds are relative to the negative rail and not the system ground.

More information on using a DCS regulator in an IBB configuration can be found in the Description Compensating the Current Mode Boost Control Loop, Using the TPS6215x in an Inverting Buck-Boost Topology, and Using the TPS629210 in an Inverting Buck-Boost Topology application notes.

Figure 9-68 IBB Example with Adjustable Feedback