Automotive designs demand 65V synchronous buck converters with ultralow Iq

September 03, 2014 // By Jeff Gruetter, Linear Technology
The rapid growth of very complex electronic systems in cars and commercial vehicles has created ever-higher demands on power management ICs. One of the most notable is a single power IC which must accommodate both single- and dual-cell lead acid battery configurations and their required transient behaviour.

Globally, legislation continues to drive the development of next generation vehicle technology, offering further enhancements to emissions control and safety. Industry competition and consumer expectations are leading to higher levels of vehicle connectivity to the cloud and personal portable devices. As a result, demand for enabling semiconductor devices is expected to grow at a CAAGR (compound average annual growth rate) of 5% over the next seven years, with the total market worth over $41 billion by 2021 compared to $27.5 billion in 2013. The Strategy Analytics analysis also identifies that demand for microcontroller and power semiconductors will drive over 40% of revenues.” [Source: Strategy Analytics, May 2014]

Strategy Analytics [the analyst company] provides a very quantitative description of forecasting the growth of electronics content in cars and commercial vehicles, but more importantly the prevalent role that power ICs play in this growth. These new power IC designs must offer:

1) The highest efficiency possible to minimise thermal issues and optimise battery run-time.

2) Operation from a wide range of battery input voltages; both single-battery (automotive) and dual-battery (commercial vehicle) lead acid applications that can accommodate wide transient voltage swings.

3) Ultra-low quiescent current to enable always-on systems such as security, environmental control and infotainment systems to stay engaged without draining the vehicle's battery when its engine (alternator) is not running.

4) Switching frequencies of 2 MHz or greater to keep the switching noise out of the AM radio band and to keep solution footprints very small.

5) Lowest EMI/EMC emission possible to reduce noise interference concerns within electronic systems.

The goal of the increased performance levels of power ICs is to design increasingly complex and numerous electronic systems found in cars to maximise comfort, safety and performance while simultaneously minimising harmful emissions. Specific applications fuelling the growth for electronic content in cars are found in every aspect of the vehicle. For example, new safety systems, including lane monitoring, adaptive safety control and automatic turning, dimming headlights and infotainment systems (telematics) continue to evolve and pack more functionality into that space and must support an ever growing number of cloud applications. Advanced engine management systems implement stop/start systems and electronics-laden transmissions and engine control. Drive train and chassis management is aimed at simultaneously improving performance, safety and comfort. A few years ago these systems were only found in “high-end” luxury cars, but now they are commonly found in automobiles from every manufacturer, accelerating automotive power IC growth at even a faster rate.

One of the key drivers for the growth of electronics systems is the adoption of many complex electronic systems improving the performance, comfort and safety of vehicles. But many of these systems are also designed to be used in a myriad of commercial vehicles, including trucks, buses, forklifts and so on. These applications generally use double batteries. But designers of many automotive systems would like the same design to service both single-battery automotive applications and dual-battery commercial vehicles, leading to a requirement for a single power IC that can accommodate both configurations.

By using two lead acid batteries in series, the nominal battery voltage increases to 24V and requires transient protection to 60V during load dump compared to a nominal voltage of 12V for a car and its load dump requirement of 36V. Conversely, single-cell automobile applications require power ICs to operate with inputs as low as 3.5V to accommodate low starting voltages found in cold-crank and stop-start scenarios. In dual-battery applications, this low input requirement is greatly relaxed and a minimum of only 7V (battery voltage) is required. The wide temporary voltage swing during cold-crank/stop-start and load dump for single-cell lead-acid batteries can be seen in Figure 1. Dual cell applications look similar, but the maximum voltage during load dump is generally 60V and the minimum during cold-crank/stop-start is 7V.

Figure 1. LT8620 with 36V load dump transient and 4V cold crank scenario

High efficiency operation

High efficiency operation of power management ICs in automotive applications is of primary importance for two main reasons. First, the more efficient the power conversion, the less energy is wasted in the form of heat. As heat is the enemy of the long-term reliability of any electronic system, it must be managed effectively which generally requires heat sinks for cooling; this adds complexity, size and cost to the solution. Secondly, any wasted electrical energy in hybrids or EVs will directly reduce their range. Until recently, high voltage monolithic power management ICs and high-efficiency synchronous rectification designs were mutually exclusive as the required IC processes could not support both goals. Historically, the highest efficiency solutions were high voltage controllers, which used external MOSFETs for their synchronous rectification. However, these configurations are relatively complex and bulky for applications under 15W when compared to a monolithic alternative. Fortunately, new power management ICs that can offer both high voltage (to 65V) and high efficiency and internal synchronous rectification can be found in the marketplace.

next; always-on systems...