With the growing popularity of modern vehicle sales, the automotive industry has significantly advanced in its process development. However, the demands on automotive power supply systems have become more stringent. It is well known that a specialized voltage regulator is an essential component within a car’s power system. As car manufacturing techniques continue to evolve, there is now a higher expectation for the performance of buck regulators used in these systems.
This article focuses on Linear Technology's LTC7138, a step-down regulator designed specifically for automotive environments. The LTC7138 is a 400mA buck converter capable of operating from an input voltage range of 4V to 140V, making it suitable for applications in industrial, telecommunications, and automotive sectors where voltage transients are common. Its hysteresis architecture eliminates the need for external compensation, simplifying design. The output voltage can be set to 1.8V, 3.3V, or 5.0V via internal pins, or adjusted down to 0.8V using an external resistor divider. With an internal P-channel FET, it supports 100% duty cycle operation, and it is housed in a thermally enhanced high-voltage MSOP package.
One application example is a simple 5V/400mA step-down converter with a wide VIN range. Figure 1 shows a circuit that uses only four external components, setting the output to 5V by connecting VPRG1 to SS and VPRG2 to ground. No external compensation is required, making it easy to implement.
The efficiency of this configuration remains high across a broad load range, thanks to the LTC7138’s Burst Mode operation and low 12μA quiescent current. At a 12V input, the efficiency peaks at 87% for a 10mA load and stays above 80% even at 0.4mA, which makes it ideal for always-on battery-powered systems.
Another configuration involves two LTC7138s connected in parallel to deliver a 24V/800mA output. By linking the master’s FBO pin to the slave’s VFB pin, both devices operate in sync, as shown in Figure 3. This setup achieves a full load efficiency of 93.6% when using a 48V input, as seen in Figure 5.
Additionally, the LTC7138 can be used as a surge suppressor for inputs up to 32V. When the input exceeds 32V, the regulator switches to maintain output regulation. An overvoltage lockout feature can be added by connecting the OVLO pin to a voltage divider, offering protection for downstream components in industrial, automotive, and avionics systems.
The LTC7138 also includes several useful features such as a RUN pin for external UVLO, an ILIM pin for current limit programming, and a soft start function (either internal or external). These make it a versatile choice for various applications.
In summary, the LTC7138 provides a robust, low-component-count solution for wide-input voltage applications. It features a 140V maximum input rating, a thermally enhanced MSOP package, burst mode operation, low IQ current, adjustable output voltage, no external compensation, and 100% duty cycle operation. It is an excellent choice for automotive and industrial power management systems.
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