The magnitude of the negative voltage
generated by inverting buck-boost
circuits can be higher or lower than
that of the available positive voltage.
"
"
Figure 2 shows a buck-boost circuit with an ADP2386 buck
regulator. If a buck regulator IC is used for the inverting circuit, the
ground connection of the IC is at the generated negative voltage.
The original output voltage from the buck regulator is connected to
the system ground. The buck regulator in the inverting topology
references its own ground to the set negative voltage because the
output voltage is connected to the system ground. The reference
ground for the IC (GND in Figure 2) is not connected to the system
ground. As a result, these two grounds are not at the same potential.
The switching regulator IC ground becomes the generated negative
voltage. All pins on the switching regulator IC are now referenced to
the generated negative voltage rather than to the system ground. As
a result, the communication lines and connections from the system
to the IC and vice versa need level shifting to guarantee safe
communication and prevent damage. Typically, the relevant signals
are SYNC, PGOOD, TRACKING, MODE, EN, UVLO, and RESET.
Additionally, Figure 2 shows a possible level shift circuit with two
bipolar transistors and seven resistors (in blue) for one signal. This
circuit requires a certain amount of space and adds complexity to
the circuitry as well as costs. Such a level shifter would have to be
implemented separately for all signals previously mentioned. It is
especially complicated when a switching regulator IC uses a digital
bus such as the Power Management Bus (PMBus
®
). Then, the entire bus
connection has to be operated with level shifting or galvanic isolation.
One way to avoid this external circuitry is to use a switching
regulator IC that has been designed specifically for inverting
voltages. Analog Devices offers a family of switching regulator
ICs that are variations of buck regulator ICs. They are designed to
facilitate communication between the system—that is, the entire
electronic circuitry—and the inverting switching regulator IC.
External level shifting, as shown in Figure 2, is not needed.
Figure 2: An external level shifter for supplying the switching regulator IC with an external clock for
synchronization. (Source: Analog Devices)
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