Optimize High-Current Buck-Boost DC/DC Converter Efficiency

Many electronic designs, such as battery-powered systems, require robust DC/DC converters to maintain a stable output voltage as input voltage fluctuates. While a four-switch buck-boost topology is a popular choice for its flexibility and power density, scaling these systems for high-current applications introduces significant design challenges. Designers must carefully weigh the architectural tradeoffs regarding integration within the buck-boost regulator. Specifically, the integration of inductors and current-sensing mechanisms can significantly affect the circuit's overall size, complexity, and efficiency.

This article provides a brief overview of the challenges and trade-offs facing power system designers. It then introduces solutions from Analog Devices’ lineup of buck-boost regulators and shows how they can address those challenges and optimize designs. The article also highlights evaluation kits and software that designers can use to accelerate prototyping and development.

Integration tradeoffs in high-current buck-boost design

In a four-switch buck-boost converter, the power stage requires four MOSFETs, a power inductor, and a current-sensing mechanism. How these components are partitioned between the module package and the printed circuit board (pc board) is the central architectural decision for designers.

Placing the inductor and sense resistor externally on the pc board provides designers full control over component selection. Inductor size, core material, and saturation current can be precisely matched to the application. However, this flexibility comes at a cost: external components consume board space, complicate layout, and require careful routing to minimize noise in the current sense path.

Integrating the inductor and sense resistor into the module package simplifies design and layout, reducing the component count and pc board footprint. The tradeoff here is that the inductor is constrained by the package dimensions, which can limit the maximum output current and thermal performance.

It is also possible to eliminate the sense resistor entirely by replacing it with a lossless current sensing scheme. This improves power efficiency, resulting in a more complex integrated circuit (IC) design for the buck-boost module.

How three module families address buck-boost integration challenges

As part of its broad µModule product lineup, Analog Devices offers a variety of DC/DC modules that enable designers to choose between these integration strategies. This article focuses on four-switch buck-boost modules (Figure 1): the LTM4607, LTM4605, and LTM4609; the LTM8055, LTM8056, and LTM8054; and the LTM4712. Each of these targets a different region of the input voltage and the output current space.

Figure 1: Shown are four-switch buck-boost µModules that take different architectural approaches to target various input voltages and output currents. (Image source: Analog Devices, modified by Kenton Williston)

DC/DC converter with external inductor and sense resistor

The LTM4607, LTM4605, and LTM4609 integrate the controller and MOSFETs within the µModule package, with the power inductor and current sense resistor placed externally on the pc board (Figure 2). This architecture gives designers flexibility in selecting inductor and sense resistor values to match specific application requirements.

Figure 2: Shown is the package (left) for the LTM4607, LTM4605, and LTM4609, along with the corresponding power-stage schematic (right) that highlights the external inductor and sense resistor. (Image source: Analog Devices)

The LTM4607, LTM4605, and LTM4609 come in pin-compatible 15 mm × 15 mm × 2.82 mm LGA packages. The LTM4605 is designed for lower-voltage applications, with an input voltage range of 4.5 V to 20 V and an output current of 12 A (buck mode). The LTM4607 and LTM4609 extend the input range to 36 V at 10 A (buck mode), with the LTM4609 providing the widest output voltage range among the three, from 0.8 V to 34 V.

DC/DC converter with an integrated inductor and sense resistor

The LTM8055, LTM8056, and LTM8054 (Figure 3) integrate the power inductor and the current sense resistor into the µModule package, simplifying design and layout by reducing the number of external components on the pc board.

Figure 3: Shown is the module (left) for the LTM8055, LTM8054, and LTM8056 devices, alongside the schematic layout (right) highlighting the integrated inductor and sense resistor. (Image source: Analog Devices)

Of the three distinct families discussed here, this family has the lowest output current: 5.4 A for the LTM8054, 5.5 A for the LTM8056, and 8.5 A for the LTM8055 (in buck mode). The LTM8056 has an input range of 5 V to 60 V, the widest among the devices discussed here, and has the highest output voltage of 48 V. The LTM8054 is the most compact, with a footprint of 15 mm × 11.25 mm and a height of 3.42 mm for space-constrained designs. The LTM8055 and LTM8056 come in a 15 mm × 15 mm × 4.92 mm package.

DC/DC converter with an integrated inductor and lossless current sensing

The LTM4712 (Figure 4) takes a different approach to current sensing. Instead of a discrete sense resistor, it uses a proprietary lossless current sensing scheme integrated into the module. This eliminates the power loss associated with a dedicated sense resistor.

Figure 4: Shown is the LTM4712 module (left) alongside its schematic diagram (right), highlighting the integrated inductor and lossless current sensing. (Image source: Analog Devices)

The power inductor is integrated using component-on-package technology in a 16 × 16 × 8.34 mm BGA package. The LTM4712 accepts 5 V to 36 V input and delivers 1 V to 36 V output at 12 A in buck mode.

DC/DC converter spec and efficiency comparison

Table 1 summarizes the key specifications of the seven µModule devices discussed here.

Table 1: Shown are the key specifications for the µModule devices discussed. (Image source: Analog Devices)

A comparison of the efficiency of the LTM8055, LTM4607, and LTM4712 (Figure 5) illustrates the practical impact of their architectural differences. The comparison is across three operating conditions: 6 V input (boost mode), 12 V input (buck-boost mode), and 24 V input (buck mode), all delivering a 12 V output.

Figure 5: A comparison of efficiency across three input voltages shows how the LTM8055, LTM4607, and LTM4712 perform in boost, buck-boost, and buck modes. (Image source: Analog Devices)

Parallel operation, constant current regulation, and redundant inputs

The preceding sections covered the baseline operation of the three µModule buck-boost families. These devices can also be configured for more advanced applications, such as parallel operation for higher current, constant current regulation, and redundant input power. The LTM4712 illustrates all three capabilities.

Designers considering parallel designs can take advantage of the LTM4712’s peak current-mode control. This fast, cycle-by-cycle control provides reliable protection and facilitates excellent current sharing when parallel configurations are used for higher-current applications.

In a scenario with four modules in parallel, they can be configured for a 90° phase shift, offering optimal interleaving. Additionally, a clock output from one module can be tied to the SYNC input of a second module to enable frequency synchronization.

The EVAL-LTM4712-A2Z evaluation kit (Figure 6) demonstrates this capability with four LTM4712 modules. This board is a useful platform for experimenting with current sharing, validating thermal performance, and driving prototype circuits.

The board operates the four LTM4712 modules in an interleaved parallel configuration, generating 12 V at 48 A from a 5 V to 36 V input, with the full 48 A available in buck and buck-boost modes, and 24 A in boost mode. It also includes an optional constant current feature that delivers a precise, regulated current to a variable load.

Figure 6: The EVAL-LTM4712-A2Z evaluation board features four LTM4712 modules configured in parallel for a 48 A output in buck and buck-boost modes. (Image source: Analog Devices)

The constant current mode is also available on individual LTM4712 modules. In this configuration, a voltage proportional to the load current develops across an external sense resistor. When this voltage reaches a threshold set by a control pin, the module automatically reduces its output voltage to hold the current at the target level. This feature is useful for applications like LED driving or battery charging, where maintaining a precise current is more critical than maintaining a fixed voltage.

The LTM4712 also supports redundant input configurations, in which two modules powered by independent sources share a common output. This is useful for systems requiring backup power supplies or those drawing from distinct input sources to support a common load. In this scenario, two modules are connected in parallel, with the modules’ compensation pins tied together. If either input drops off, the remaining module maintains output regulation.

DC/DC conversion evaluation boards and design tools

To help designers get started, Analog Devices offers evaluation kits for its µModules. For example, the DC3189A (Figure 7) is a single-module platform for evaluating the LTM4712 across its full 5 V to 36 V input and 1 V to 36 V output.

Figure 7: The DC3189A evaluation board provides a single-module platform for evaluating the LTM4712. (Image source: Analog Devices)

Software tools are also available to accelerate the design process. The LTpowerCAD design tool aids in component selection, efficiency estimation, loop compensation, and load transient analysis. Designs can be exported to LTspice for time-domain simulation and dynamic analysis.

Conclusion

Modern buck-boost regulators offer developers numerous options when scaling DC/DC conversion across a wide range of high-current applications. Analog Devices’ four-switch µModule converters feature wide input and output ranges and flexible integration options. Supported by evaluation boards and software, these modules allow designers to quickly select and implement the architecture that best fits their design needs.