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In this section, we will look at the buck-boost DC-DC topology for automotive front end power conversion. A wide-vin buck-boost, such as LM50175 offers single stage conversion with higher efficiency than the two stage approaches. A buck-boost offers inherent short circuit protection and inrush current limiting, which is not present in a boost converter.

Another advantage of using buck-boost compared to a boost converter is that the output voltage is well regulated. So the downstream components never see the overvoltages present on the input rail. This allows the downstream components to be rated for lower voltage. In terms of the automotive transients on the battery rail, a wide-vin buck-boost converter can enable the downstream circuits to operate through warm and cold cranks, jump start, as well as load dump conditions.

The LM5176-Q1 is wide-vin buck-boost controller from Texas Instruments with a wide-vin range of 3.5 volts to 42 volts and integrates many useful features, such as optional frequency dithering for reduced EMI, programmable UVLO, clock synchronization capability, and hicccup mode current limit for reducing thermal stress in case of overload.

The LM5175 four-switch buck-boost converter provides higher efficiency compared to two stage approaches, as well as compared to other topologies used for buck-boost DC to DC conversion. It achieves higher efficiency, because of single stage conversion and single inductor; synchronous rectification, which reduces the losses in the rectifier diodes; single switching event per cycle in buck-boost mode; lower circulating currents; and by allowing the designer to select lower voltage switches compared to a flyback or a SEPIC converter. The graph on the right-hand side shows that the force which buck-boost converter achieves is greater than 98% efficiency.

This slide shows the response of a buck-boost converter for a load dump test. The input voltage to the converter represents the battery voltage and is shown in yellow. The output voltage of the DC-DC converter is regulated at 10 volts. And a three amp load is present at the output of the converter. As the test pulse simulating load dump condition is applied, the input voltage ramps quickly from 13.5 volts to 27 volts with a rise time of two milliseconds, shown in the expanded time scale on the right side. The output voltage is well regulated throughout the elevated battery voltage. Similarly, the output voltage remains well regulated when the battery rail voltage turns to 13.5 volts at the end of load dump transient.

This slide shows the lab results for a cold-crank test profile, as per ISO 16750-2. The input voltage, shown in yellow, falls from its nominal 12 volts to close to three volts in two milliseconds, simulating a start up condition. The output voltage of the buck-boost converter remains well regulated, even as the input goes through the start up transient profile.

This slide summarizes the wide-vin automotive related buck-boost controllers and converters available from Texas Instruments. The LM5175 is a four-switch buck-boost controller rated for 42 volt input. That can provide over 100 watts to the load. LM5118 and LM(2)5118 are non-synchronous two-switch buck-boost controllers for medium power applications. The TPIC74100 family of converters have integrated one amp FETs for applications that require five volt output and lower currents.

For more information on these parts, please visit their respective product web pages on TI.com. [? 175 ?] buck-boost converter. The product web page at www.TI.com/LM5175 includes the datasheet, user guide, and app notes. In the Tools and Software section, you will find the EVM, the reference designs, as well as the piecewise model and the Quickstart Calculator tool.

There is a web bench applet on the right-hand side that allows you to start the design within a few minutes. The Technical Documentation tab relates many useful app notes, user guides, white papers, and external articles that contain good design practices for the part, as well as many potential applications.

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