The need to generate negative voltage rails for electronic designs is a common challenge. The wide 3-V to 17-V input voltage range of the TPS629206 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 9-83 and Figure 9-84.

The maximum input voltage that can be applied to an IBB converter is less than the maximum voltage that can be applied to the TPS629206 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 TPS629206 in an IBB configuration becomes 3 V to 17 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 TPS629206 in an IBB configuration may be set between –0.4 V and –5.5 V.

The maximum output current for the TPS629206 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 TPS629206 device are referenced to this rail. During power up, as there is no charge in the output capacitor, 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 could appear to be the normal UVLO/EN falling thresholds relative to the system ground. Thus, special care needs to be taken if the user is utilizing the dynamic mode change feature on the MODE pin of the TPS629206 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 Application Note and Using the TPS6215x in an Inverting Buck-Boost Topology Application Note.