42V, 6A, ultra-low EMI radiation, high efficiency... This voltage regulator is very suitable for automotive environment applications~

The LT8640S/LT8643S are 42V, 6A continuous current/7A peak current monolithic step-down regulators that feature the second generation Silent Switcher ^® 2 architecture. Silent Switcher regulators suppress EMI radiation by splitting the high frequency loop in two, with the magnetic fields generated by the separated loops canceling each other. The second generation Silent Switcher 2 integrates bypass ceramic capacitors within the package. These capacitors are located within the fast AC loops (V _IN , BST, and INTV _CC ), requiring precise and repeatable PCB layout to ensure EMI performance. Integrating these capacitors greatly simplifies PCB layout and manufacturing requirements. Even low-cost, two-layer boards can now have excellent EMI performance.

In automotive applications, designers prefer power supplies with 2 MHz or higher switching frequencies to avoid the AM band and minimize solution size. Unfortunately, high switching frequencies usually mean lower efficiency and higher power consumption, forcing designers to trade off small size and low EMI performance with efficiency. The LT8640S and LT8643S eliminate this trade-off: thanks to controlled and fast switching edges, they can operate with high efficiency and low power consumption even at high switching frequencies.

Figure 1 shows an ultralow EMI and high efficiency LT8640S 12 V to 5 V/6 A design. The internal regulator is powered from the 5 V output through the BIAS pin to reduce power dissipation. The switching frequency of this design is set to 2 MHz. Spread spectrum mode is enabled (SYNC/MODE = INTV ^CC ), allowing the switching frequency to vary from 2 MHz to 2.4 MHz with 3 kHz triangle modulation.

Figure 2 compares EMI emissions from a 2-layer and 4-layer board using the design shown in Figure 1. Both boards meet the stringent automotive CISPR 25 Class 5 radiated EMI requirements, requiring only a ferrite bead on the input side. Figure 3 shows the efficiency. At switching frequencies up to 2 MHz, the LT8640S achieves peak efficiencies of 95% for 12 V input and 92% for 24 V input.

Figure 4 shows the LT8640S design with a 9 V output. Figure 5 shows the thermal results at a 5 A load. At 45 W output power and 1 MHz switching frequency, the temperature rise of the LT8640S 4 mm × 4 mm LQFN package is less than 50°C due to enhanced heat dissipation techniques.

Multiple LT8643S devices can be paralleled to support output currents in excess of 7 A (peak). The LT8643S utilizes current mode control and external compensation to achieve balanced current sharing—very important in a parallel configuration. Current sharing is naturally achieved by connecting the output VC pins of all error amplifiers together. The CLKOUT and SYNC/MODE pins enable frequency synchronization without the need for an additional clock device.

Figure 6 shows the simplicity of a 12 V to 3.3 V/12 A LT8643S parallel design. The top LT8643S is set to forced continuous mode by leaving the SYNC/MODE pin floating, and its CLKOUT signal drives the bottom LT8643S SYNC/MODE pin for synchronization. Figure 7 shows the efficiency of this design, while Figure 8 shows the 8 A step transient response.

The LT8640S and LT8643S are 6 A (7 A peak), ultralow EMI monolithic synchronous switching regulators in a small 4 mm × 4 mm LQFN package. The patented Silent Switcher2 architecture ensures very low EMI emissions. Integrated loop capacitors eliminate PCB layout sensitivity, helping to save design effort and solution cost. Synchronous design and fast switching edges improve efficiency at heavy loads, while low quiescent current benefits light load efficiency. The wide 3.4 V to 42 V input range and low dropout voltage enable the LT8640S and LT8643S to meet the requirements of automotive cold crank or load shedding situations.