Step-Down Regulators for Automotive, Industrial Applications

Author:
Dong Wang, Analog Devices, Inc.

Date
03/30/2020

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The LT8609, LT8609A, LT8609B, and LT8609S are synchronous monolithic step-down regulators that feature a wide 3 V to 42 V input range.

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Figure 1. Ultralow EMI emission LT8609S 12 V to 5 V synchronous step-down converter

This device family is optimized for applications requiring low EMI, high efficiency, and a small solution size—suitable for demanding automotive, industrial, computing, and communications applications. All regulators in this series have the same 2 A continuous, 3 A transient (<1 second) load current capability. Their features are summarized in Table 1.

The LT8609, LT8609A, and LT8609S feature 2.5 µA ultralow quiescent current, which is important for battery-powered systems. With integrated top and bottom N-channel MOSFETs, the regulators exhibit impressive light load efficiency. The LT8609B operates in pulse-skipping mode only, with higher quiescent current than the other devices, and it offers lower ripple during light load operation.

All of these devices can pass CISPR 25 Class 5 radiated EMI regulation, the most rigorous EMI standard for automotive equipment. Furthermore, the LT8609, LT8609A, and LT8609S feature spread spectrum frequency operation to reduce EMI peaks. The LT8609S displays the mostimpressive EMI performance in this family, based on its proprietary Silent Switcher 2 technology.

5.5 V to 42 V Input, Low EMI, High Efficiency 5 V, 2 A Supply

A 5.5 V to 42 V input to 5 V/2 A output power supply is shown in Figure 1. This solution features a 16-lead LT8609S regulator with a 2 MHz switching frequency. Only a few components are required for the complete solution, including inductor L1 and a few passive components. Figure 2 shows that this solution can achieve 92.9% peak efficiency.

Burst Mode Operation to Improve Light Load Efficiency

During light load operation and no-load standby mode, high efficiency and low idle current are important for battery-powered applications. The LT8609, LT8609A, and LT8609S feature a low 2.5 µA quiescent current in Burst Mode® operation. During light load and no-load conditions, the switching frequency is gradually reduced, greatly reducing power loss while keeping the output voltage ripple relatively low. Figure 2 showsthat light load efficiency remains above 85% while the power loss approaches zero at minimal loads.

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Figure 2. Efficiency vs. load current for LT8609/LT8609A/LT8609S-based 12 VIN to 5 VOUT step-down converter

High Switching Frequency with Ultralow EMI Emission and Improved Thermal Performance

EMI compliance is a concern in a number of environments, including automotive systems. With integrated MOSFETs, advanced process technology, and up to 2.2 MHz operation, all of the devices in this family can achieve a small solution size while satisfying the most stringent EMI standards. Spread spectrum frequency operation, which reduces EMI peaks, is available in all but the LT8609B. Furthermore, the LT8609S incorporates Silent Switcher 2 technology. Silent Switcher 2 devices feature integrated hot loop and warm loop caps to make EMI performance insensitive to board layout and the number of board layers. A board with fewer layers can be used to reduce manufacturing costs without sacrificing EMI and thermal performance.

Figure 2 shows that the LT8609S features the best peak and full load efficiency of the device family. Figure 3 and Figure 4 show a CISPR 25 EMI and thermal performance comparison of the Figure 1 solution on 2- and 4-layer boards.

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Figure 3. CISPR 25 radiated EMI performance comparison between 2- and 4-layer boards for the circuit in Figure 1

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Figure 4. Thermal performance comparison between 2- and 4-layer boards for the circuit in
Figure 1                                                                                                                                                                            

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Conclusion

The devices in the LT8609 family are easy to use monolithic step-down regulators with integrated power MOSFETs and built-in compensation circuits.Theyareoptimizedforapplicationswithwideinputvoltagerange and low EMI noise requirements. Low, 2.5 µA quiescent current and Burst Mode operation make them excellent battery-powered step-down converter solutions. A 200 kHz to 2.2 MHz switching frequency range make them suitable for most low power to micropower applications. Integrated MOSFETs and up to 2.2 MHz switching frequency capability minimize solution size. CISPR 25 scanning results show excellent radiated EMI performance, compliant with the most stringent EMI standards. Silent Switcher 2 technology in the LT8609S makes its performance immune to layout and layer changes, which greatly reduces development and manufacturingcosts.

Analog Devices

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