It is common knowledge in the DC/DC converter domain that a buck converter or regulator integrated circuit (IC), such as the LM5017 family, can create a negative VOUT from a positive VIN. At first glance, the schematic of an inverting buck-boost converter using a buck regulator IC looks deceivingly similar to a buck converter (Figure 1a and 1c). But there are important differences in the two circuits, both in terms of voltage and current levels, switching-current flow, and layout.
I discussed the differences between VIN range, VOUT range and available output current IOUT max in an earlier blog post. The difference in layout arising from the difference in switching-current flow paths of an inverting buck-boost converter and a buck converter – although critical – is not as well understood.
Figure 1 illustrates the difference in switching-current flow in a buck converter and an inverting buck-boost converter. In the buck converter (Figure 1a and 1b), the input loop – comprising the input capacitor CIN, high-side switch QH and synchronous rectifier QL, carries high di/dt switching current. The output loop, comprising the synchronous rectifier QL, inductor L1 and output capacitor COUT, has a relatively continuous current. Thus, while optimizing the input-current loop area is critical, it’s not as important to optimize the output-current loop area.
Figure 1: Switching-current flow in a buck converter (a, b); and an inverting buck-boost converter (c, d)
The input- and output-current loops in an inverting buck-boost converter comprise the same elements as those in a buck converter (Figure 1c and 1d). The input loop has the input capacitor CIN, control FET QH and synchronous rectifier QL. The output-current loop consists of the synchronous rectifier QL, the filter inductor L1 and the output capacitor COUT. In an inverting buck-boost converter, however, both the input- and output-current loops carry high di/dt switching current because the filter inductor switches from CIN to COUT between the switching subintervals.
Because of the similarity in the buck and inverting schematics, the difference in switching-current paths often gets overlooked, and many inverting buck-boost designs and layouts are done just as they are for a buck converter, with only the input-current loop optimized for a small loop area. The buck to inverting buck-boost transition is often treated as a mere reconnection of VOUT and ground pins. But this approach fails to consider that the different current flows in a simple buck and an inverting buck-boost converter (using the same regulator IC) result in these issues:
Figure 2: The ripple current in the output filter capacitor of a buck converter (a, b) is small, as the inductor is always connected to the output node. The ripple current in the output filter capacitor of an inverting buck-boost converter (c, d) is much higher due to the discontinuous nature of current flowing through the output capacitor.
Figure 3 shows how to optimize an inverting buck-boost power stage to achieve lower di/dt input and output loops. Figure 4 shows an example inverting buck-boost power-stage layout using the LM5017, a 100V synchronous buck regulator.
Figure 3: Optimization of power-stage components to minimize switching-current loop area (a) identifying current loops (b) minimizing current loops
Figure 4: An example layout of an inverting buck-boost converter based on the LM5017 synchronous buck regulator
Designers often use a buck regulator to create an inverting buck-boost regulator. But there are critical differences in the switching-current flow between buck and inverting buck-boost circuits. In particular, designers should pay attention to output filter capacitor selection and the switching-current loop layout to achieve the best reliability and noise performance.
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