Author:
Ling Jiang, Wesley Ballar, Anjan Panigrahy, and Henry Zhang, Analog Devices
Date
10/21/2024
PDF
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Figure 1: The LTM4607 power stage schematic
There are many applications with wide input and/or output voltage ranges, such as a battery-powered system. The power supply needs to regulate its output voltage while the input voltage can be lower or higher than the output voltage. A four-switch buck-boost topology can offer the highest efficiency and power density for such an application, as long as ground isolation is not required. Furthermore, a buck-boost power regulator is very flexible and can be used as a buck-only (step-down) or boost-only (step-up with short circuit protection) power supply.
Analog Devices’ µModule group has developed several buckboost regulators. The LTM8045, LTM8049, LTM8083, and LTM4693 target lower current applications. High current applications (up to 12A) are supported by the LTM4607 family, LTM8055 family, and the newly released LTM4712 buck-boost module. The LTM4607/LTM4605/LTM4609 family incorporates the controller and MOSFETs internally, requiring an external power inductor and sensing resistor (RSENSE) on the PCB to form a complete power solution, as depicted in Figure 1. Alternatively, the LTM8054, LTM8055, and LTM8056 integrate the power inductor and RSENSE into the µModule package, simplifying the design and layout efforts for customers, as highlighted in Figure 2. The LTM8055 family offers a smaller solution size compared to the LTM4607 family, but its output current is constrained by a small-size integrated inductor, therefore limiting thermal and efficient performances.
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Figure 2: The LTM8055 typical application circuit
In 2023, a new high current, four-switch buck-boost regulator, LTM4712, was released. It is a 36 VIN(max), 12 A power module in a high density 16 mm × 16 mm × 8.34 mm BGA package. It integrates a high performance power inductor into the package using advanced ADI proprietary component-on-package technology. Figure 3 shows a proprietary current sensing scheme integrated in the module. This not onlysavesspace but also minimizes additional power loss. Leveraging a cutting-edge buck-boost controller and the advanced ADI package, this device achieves the highest power level, power density, efficiency, and excellent thermal performance across a wide input and output voltage range.
The LTM4712’s fast, cycle-by-cycle current-mode control enables reliable protection and smooth mode transitions. It facilitates excellent current sharing when parallel configurations are employed for higher current applications. Additionally, this new device supports an optional constant output current mode for battery charging applications and allows redundant inputs, enhancing its versatility as a redundant power supply.
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Figure 3. The LTM4712 typical application circuit
Efficiency Gain and Thermal Improvements
Figure 4 illustrates the efficiency comparison among the LTM4607, LTM8055, and the LTM4712 under 6VIN, 12VIN, and 24VIN for 12VOUT conditions. The assessment is conducted using standard evaluation boards available online. Based on the test results, the LTM4712 not only exhibits significantly higher efficiency compared to the other two but also enhances current capability.
The LTM4712 maintains a temperature of 30°C lower while doubling the power, in contrast to the LTM8055 under 12VIN and 12VOUT. For applications requiring a 12A current, employing two or three LTM8055 devices in parallel is necessary, depending on the cooling system. On the other hand, a single LTM4712 suffices, significantly reducing PCB footprint and simplifying circuit design.
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Figure 4: Efficiency comparison among the LTM4712, LTM4607, and LTM8055
Good Current Sharing in Parallel Configuration
The LTM4712 can be configured in parallel to achieve higher output power with a straightforward setup. Thanks to its current-mode control, it delivers excellent current sharing performance. The EVAL-LTM4712-A2Z evaluation kit showcases the operation of four modules in parallel, providing a total output current of 48 A.
When implementing a parallel configuration with the device, it is essential to refer to the design and configuration of the evaluation board. For effective current sharing, connect the COMP pins and FB pins when paralleling modules. The PHMODE pin can be utilized to configure the phase shift. In a scenario with four modules in parallel, tying the PHMODE pin to INTVCC results in a 90° phase shift, offering optimal interleaving. Additionally, for frequency synchronization, connect the CLOCKOUT signal of the first module to the SYNC pin of the second module.
Optional Constant Current Regulation
The LTM4712 can be used as a constant output current source, making it suitable for applications such as battery chargers or LED drivers. To set the load current, the ISET pin and an external output current sensing resistor near the output are employed. The equation VSENSE = IOUT × RSENSE_IOUT defines the voltage that represents the average output current. The maximum VSENSE is determined by the voltage on the ISET pin ranging from 0.2 V to 1.2 V, corresponding linearly to 0 mV to 50 mV. The ISET pin voltage VISET is established by a 15 μA internal current source and a resistor RISET connected between the ISET pin and ground, represented by VISET=15μA×RISET. The output current is then calculated as IOUT =(VISET-0.2V)/(20×RSENSE_IOUT).
It’s essential to note that the maximum VSENSE is internally limited to 50 mV whenthe ISET pin is floating or when the ISET pin voltage exceeds 1.2 V. Due to the ripple current in the output, an RC filter must be applied to the ISP and ISN pins for precise average current sensing. Moreover, when selecting the feedback resistor between the FB pin and ground, ensure it results in a higher output voltage than the targeted VOUT.
Redundant Power Supply
The LTM4712 accommodates applications necessitating redundant inputs. It can be utilized for systems requiring backup power supplies or those drawing from distinct input sources to support a common load. Figure 5 illustrates an example circuit where two modules, powered by different inputs (VIN1 and VIN2), collectively deliver a 12V output with a 24A load. Notably, any drop-off in either input has no impact on output regulation, and peak inductor current sharing remains effective through the connection of COMP pins.
In Figure 6, bench-tested waveforms under two different conditions are presented. Figure 6a showcases a scenario where both Phase 1 and Phase 2 operate in buck-boost mode. Initially, Phase 1 handles a 4 A load current, gradually sharing half of the load current with Phase 2 upon activation. The IMON waveform confirms equal load current sharing between both phases when both are operational.
Figure 6b replicates the same load condition but with different inputs. Here, Phase 1 operates in boost mode while Phase 2 operates in buck mode. Due to the tied COMP pins, both phases exhibit the same peak inductor current. Consequently, the output current of Phase 1 (boost) is lower than that of Phase 2 (buck). Specific load currents from each phase can be calculated based on parameters such as inductance, switching frequency, VIN, VOUT, and total load current. In this example, Phase 1 provides a 1.4 A load current, while Phase 2 provides 2.6 A.
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Figure 5: Input redundancy application circuit
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Figure 6: Input redundancy application circuit
Conclusion
The LTM4712 facilitates easy parallelization for higher power applications, and it also shows good current sharing capabilities in parallel configurations. It can be adeptly configured to deliver a constant current output, making it feasible for battery charging systems or LED applications. Additionally, its compatibility with redundant input setups further enhances its adaptability.
