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Options Multiply in Auto Power Supply Design

The trend to higher levels of power supply integration seen in many industries is present in automotive systems. However, the growth in applications and highly variable requirements for power supplies to drive automotive electronics systems means that all possible solutions, from discretes to multi-output supplies and high-integration system basis chips (SBCs) are part of the design engineers’ tool kit. In this post, we’ll look at selection criteria for linear and switching solutions, system supplies and SBCs.

High rates of innovation in automotive systems are one reason for continued growth in applications for both discrete linear and DCDC switching regulators. In many applications discretes offer greater flexibility than higher integration level ICs. Engineering teams can more rapidly achieve specific requirements for new and relatively unique automotive systems. For example, the growing use of start/stop cranking to meet more demanding emissions and fuel consumption goals created demand for linear regulators that can operate with inputs of 3V or even 2.75V, compared to the traditional 4V minimum threshold that is still the spec for higher integration ICs. In body electronics, low quiescent current operation in stand-by mode is increasingly desirable to reduce demand on the battery. Process improvements in dedicated power transistors are at the leading edge of the envelope in this area.

Improvements in the latest generation of linear regulators also reflect continued emphasis on cost reduction by lowering the cost and quantity of external components. Transient robustness of the power transistors achieves this goal. Infineon offers two families of regulators that require external capacitors rated at just 1µF. Furthermore, the TLS820xx/850xx family exhibits a very rapid reaction to load changes, as illustrated in Figure 1.

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As is typical for linear regulators, current handling capacities range from loads of 50 – 100 mA with the TLS 805x/810x devices to 200 mA – 500 mA in the TLS820xx/850xx family. Stand-by consumption is very low, at just 5-10µA for the 50-100 mA family and typically 40µA for the higher current load devices. Above 500 mA current, switching regulators that offer higher efficiency and thus better manage heat loads are used.

Switching regulators designed for step down conversion directly from the auto battery support a range of current loads and output voltages. For a buck converter in the 0.5 A range, a typical device would support adjustable switching frequency in a relatively high range for design flexibility. High switching frequencies, adjustable in a range from 1 MHz to 2.5 MHz are an acceptable trade-off in order to reduce the need for external components. At this current level, applications are in body electronics, instrument clusters and decentralized lighting modules. In these “always on” areas, low quiescent current is a key optimization to conserve battery life.

In the 2A operating range, switching frequencies are reduced to limit heat dissipation. It is still useful to have the flexibility of adjustable frequency, in some cases by synchronizing the converter to an external source. This is a feature of the TLE83366, a 3.3V output voltage device which nominally operates at 370 kHz and can be synched with sources from 200 to 530 kHz.

Higher current loads, up to 10A, play a role in distributed power topologies that combine a switching supply as a pre-regulator and a series of linear post-regulator devices. In this scheme, a high current output device is used as a stable source by regulators driving loads of multiple voltage and current loads (Figure 2). This is a route to high efficiency in complex modules, such as a radar system. As shown in Figure 2, the post-regulator block might include a series of linear regulators and/or trackers. The tracker is a variant of regulators used to deliver high accuracy power while protecting the target component from circuit disruptions. These were discussed briefly in an earlier post in this series, Look Closer at Automotive Power Supplies , where they were illustrated in use with a system supply IC.

High current buck converters are used when output currents in excess of 5A are required. In general the discrete solution will offer higher efficiency than more integrated solutions. However designers can reduce complexity with both system supplies and with even higher integration system basis chips (SBCs). In particular a simplified system bill of materials often translates to lower cost of ownership for the design.

Supply ICs targeted at current loads up to 1A typically drive multiple loads with a combination of boost and/or buck converter capability, linear regulator and voltage tracker conditioning circuits, and monitoring and protection circuits. The on-chip linear drivers support various voltage and current outputs, usually including at least one output to additional external drivers. In the instance referenced in our earlier article, a system supply optimized for use in safety critical systems features a safe state controller and a dedicated communication supply for CAN or FlexRay transceivers.

SBCs are targeted at requirements for system power in modules that include a microcontroller and networked communications. They add communications transceivers to the combination of 3.3V and 5V voltage regulators used to drive coms and an external MCU, protection circuitry, and fail-safe functions in a 7 mm x 7 mm package. As with other power devices discussed here, SBCs are broadly available from automotive IC manufacturers. However, the details of each supplier’s portfolio vary in important ways, particularly in regards to support for evolving communications standards, such as CAN FD and CAN partial networking mode.

The mix of business models in automotive engineering and the rapid pace of innovation across the industry mean that any conceivable system design is likely to be considered. Component suppliers thus will continue to offer multiple paths to reach a design solution. That’s why nearly 30 years after the advent of single output linear regulators, today’s designer has literally hundreds of options available when developing power architectures.

Authors:

Vikram Patel, Product Manager, Infineon Technologies

Andres Zavala Sanchez, Segment Marketing Manager, Infineon Technologies

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