SNVA856B May 2020 – October 2022 LM63615-Q1 , LM63625-Q1 , LM63635-Q1 , LMR33620 , LMR33620-Q1 , LMR33630 , LMR33630-Q1 , LMR33640 , LMR36006 , LMR36015 , TPS54360B , TPS54560B
Generating a negative output voltage rail from a positive input voltage rail can be done by reconfiguring an ordinary buck regulator. The result is an inverting buck-boost (IBB) topology implementation. This application report gives details regarding this conversion with examples.
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Many systems require a negative power supply rail, when all that is available is a positive supply with respect to ground. Examples of such systems include both medical ultrasound scanners and test and measurement equipment. A unique DC/DC converter called an inverting buck-boost (IBB) can be used to provide this negative rail from a positive supply, all with a common ground connection. Almost any ordinary buck regulator can be converted into an IBB with a few simple changes in line and load connections. This application report details the conversion from a buck to an IBB, the operation of the converter and things that you need to consider to make your power supply application a success.
The diagrams in Figure 2-1 show a comparison between an ordinary buck DC/DC converter and the IBB. The buck converter takes a positive input voltage and converts it to a positive output voltage of smaller magnitude. The IBB takes a positive input voltage and coverts it to a negative output voltage, with a common ground connection between input and output. It is easy to see the similarity between the two converters. The top schematic in Figure 2-2 shows the connections for taking the buck regulator and converting it into an IBB. Looking at the connections shown in red, we see that the buck output is now the system ground and the buck "ground" becomes the negative output. An additional input capacitor is added between the input supply and system ground. The lower schematic shows the connections when using a synchronous buck regulator IC. A non-synchronous converter can also be used. Notice that the "ground" reference for the IC is now the negative output voltage. This has consequences for the maximum input voltage and the control inputs when using this configuration. Also, notice that the feed-back connection to the regulator is not changed from that of an ordinary buck. Although the examples show a synchronous buck with an internal feedback divider, a non-synchronous IC, and/or external feedback dividers can also be used.
The connection changes are detailed in the following list:
The basic operation of the converter is shown in Figure 3-1. During the portion of the switching cycle in which the HS FET is on, the inductor voltage is equal to VIN. For the remainder of the switching cycle, the LS FET turns on and the inductor voltage is -VOUT. At this point the inductor energy is supplied to the load and the output capacitor. The controller regulates the output voltage by adjusting the duty cycle of the HS and LS FET switches. Performing a standard analysis on the circuit in Figure 3-1, we arrive at the conversion law found in Equation 1.
where
A plot of this equation is shown in Figure 3-2 for a typical duty cycle range of 0.1 to 0.9. The first thing to notice is that the conversion ratio can be less than or greater than one. This means that the IBB can either increase or decrease the input voltage, depending on the duty cycle D; hence the name "buck-boost". For example you can regulate to an output voltage of –5 V from an input voltage of from 5 V and 24 V or convert an input range of 12 V to 24 V to an output of –15 V. The controller smoothly moves from "buck" mode to "boost" mode as the input voltage changes, while regulating the output voltage. Rearranging Equation 1, we arrive at the duty cycle as a function of our input and output voltages; as found in Equation 2.
In the IBB topology, both the input and output currents are "chopped". In other words these currents are discontinuous and have very fast transition times. This means that the IBB may generate more voltage spikes in the output voltage than a buck. These issues can be addressed with properly sized output capacitors or post regulation filters.