How to meet the increasingly complex automotive electronic systems? Here are reliable and compact power supply solutions~

In automotive applications, regulated 5 V, 3.3 V, and sub-2 V rails are needed to power a wide variety of analog and digital ICs, which may require different rails for content, processor I/O, and core. These rails are generated from the 12 V nominal automotive battery voltage, V _BAT , which is typically in the 8 V to 16 V range. High-efficiency step-down regulators are adequate for most situations, but during cold crank, V _BAT drops to 2 V for tens of milliseconds, and a step-down regulator alone will lose regulation if powered directly from V _BAT .

The LT8603 boost controller can operate down to 2 V, making it ideal as a preregulator to power the buck regulator. When _VBAT drops below 8.5 V, the boost controller output (OUT4) is regulated to 8 V. The two high voltage buck regulators ride out the cold-crank process and provide constant 5 V and 3.3 V outputs, as shown in Figure 1. Once VBAT _recovers above 8 V from a cold-crank, the boost controller simply acts as a punch-through diode. The high voltage buck regulator can handle VBAT up to 42 V. _In Figure 1, the low voltage buck regulator is powered from OUT2 and provides 1.2 V during a cold-crank event.

VBAT can be high for long periods of time, such as during a dual-battery boost start or in a 24V system. This has no effect on the boost regulator in Figure 1, which _{passes VBAT when it} is above 8V , but the current-delivery capability of the two high-voltage buck regulators is typically thermally limited at higher VBAT _conditions due to increased switching losses, especially at the 2MHz switching frequency commonly used in automotive applications.

The temperature rise can be controlled by reducing the switching frequency or lowering the operating voltage of the buck regulator. In Figure 2, the fourth channel is set up as a SEPIC to power the high voltage buck regulator. The output of this channel is regulated at 12 V, which is the best for the efficiency of the buck regulator. By running the buck regulator at optimal efficiency, the temperature rise can be kept under good control. Figure 2 shows a simple method to generate four accurately regulated outputs. Under light load conditions, the circuit can maintain regulation with inputs as low as 2 V.

Some automotive applications require a regulated high voltage (such as 54 V). One way to generate this regulated high voltage rail is to use the output of one of the high voltage buck regulators to drive the boost regulator, as shown in Figure 3. As long as V _BAT is above the minimum input voltage of the high voltage buck regulator, all four outputs are regulated. The buck regulator limits the maximum current of the boost converter, thereby protecting the boost converter from short circuit damage and providing cycle-by-cycle current limiting.

A charge pump circuit can be added to the SEPIC circuit to provide another regulated output as shown in Figure 4. The regulation curves for different input voltages are shown in Figure 4. Similarly, a negative output charge pump can be added to generate a negative supply rail.

The LT8603 uses a two-phase clock. Channel 1 and Channel 2 run 180° out of phase, reducing the peak input current of the buck regulator and helping to reduce EMI. The high density of electronic components requires careful balance between thermal and EMI performance. The LT8603 demonstration circuit DC2114A illustrates a layout optimized for low EMI and passes the CISPR 25 Class 5 peak limit requirements. Figure 5 shows the radiated EMI results (over the frequency range of 30 MHz to 1000 MHz) using vertical polarization. The input is 14 V with a 1 A load in each output.

The LT8603 provides a versatile and compact power solution by integrating three buck regulators and a boost controller in a tiny 6 mm x 6 mm QFN package. Each buck regulator features an internal power switch, cycle-by-cycle current limit, and tracking/soft-start control. Its synchronous rectification topology provides up to 94% efficiency. Burst Mode ^® operation maintains quiescent current below 30 µA (all channels on), making it ideal for always-on systems. The wide 2 V to 42 V input range and comprehensive features make the LT8603 ideal for automotive and other demanding applications.