Referencing Control Pins in an Inverting Buck Boost Circuit

A buck regulator reused in an inverting buck boost circuit can have its local ground connected to the negative output. Control pins that were easy to connect in a positive output design now sit in a different voltage domain. Review each interface as a voltage between its actual pins, not only as a label relative to system ground.

Draw both reference nodes

Label system return and regulator local ground separately on the schematic. For every pin connected outside the power stage, write its voltage relative to local ground during normal operation, startup, shutdown, and an unpowered condition.

The TI application note on using TPSM636xx devices in an inverting configuration explains that features referenced to the regulator ground may need level shifting to the system return. Enable and power good connections therefore require explicit review when the topology changes.

Calculate a concrete interface case

If regulator local ground is at minus 5 V, a system controller output at 0 V is 5 V above that local reference. A system output at 3.3 V is 8.3 V above it. These simple differences can be very different from the logic levels suggested by the controller’s own supply.

Compare those voltages with the selected regulator’s operating and absolute maximum limits. Do not infer acceptability from a logic threshold alone. Also review current paths through protection structures when either side lacks power.

Design the translation for all states

For an enable translator, define the default state before the negative rail develops. Check whether the circuit can start, remain disabled intentionally, and recover after the rail collapses. A translator that works only after both domains are established can create a startup dependency.

For an open drain status output, identify the allowed pullup voltage reference and the receiver thresholds. Choose a translation arrangement appropriate to the documented pin behavior. The words open drain do not imply unlimited voltage tolerance or unrestricted operation while unpowered.

Measure without changing the references

Plan probe connections before energizing the circuit. A grounded instrument lead attached to the regulator’s negative local ground may short it to system or protective earth through another connection. Use measurement equipment and methods appropriate to the complete setup.

Capture system return, negative output, and translated control behavior during the relevant sequences. Save a state table with expected pin voltages and observed transitions. The review is complete when every external control and status connection has a defined reference and acceptable behavior throughout the sequence, rather than only at the final regulated voltage.