65V Synchronous Buck Converters with Ultralow IQ

Technology News | July 9, 2014

By eeNews Europe

Introduction

“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 five percent 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 percent of revenues.” [Source: Strategy Analytics May 2014]

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

The highest efficiency possible to minimize thermal issues and optimize battery run-time.
Operation from a wide range of battery input voltages; both single-cell (automotive) and dual-cell (commercial vehicle) lead acid applications that can accommodate wide transient voltage swings.
Ultralow quiescent current to enable always-on systems such as security, environmental control and infotainment systems to stay engaged without draining the vehicles battery when its engine (alternator) is not running.
Switching frequencies of 2MHz or greater to keep the switching noise out of the AM Radio band and to keep solution footprints very small.
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 automobiles to maximize comfort, safety and performance while simultaneously minimizing harmful emissions. Specific applications fueling 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 forth. These applications generally use double batteries. But designers of many automotive systems would like the same design to service both single-cell automotive applications and dual-cell commercial vehicles, requiring 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 so operate with inputs as low as 3.5V to accommodate low starting voltages found in cold-crank and stop-start scenarios. In dual cell applications, this low input requirement is greatly relaxed and only a minimum of 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 & 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 which 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.

“Always-On” Systems Need Ultralow Supply Current

Many electronic subsystems are required to operate in “standby” or “keep alive” mode, drawing minimal quiescent current at a regulated voltage while in this state. These circuits can be found in most navigation, safety, security, and engine management electronic power systems. Each of these subsystems can use several microprocessors and microcontrollers. Most luxury cars have over 150 of these DSPs onboard and approximately 20% of these require always on operation.

In these systems, the power conversion ICs must operate in two different modes. First, when the car is running, the power conversion circuits that power these DSPs will generally operate at full current fed by the battery and charging system. However, when the car ignition is turned off, the microprocessors in these systems must be kept alive, requiring their power ICs to provide a constant voltage while drawing minimal current from the battery. Since there can be upwards of 30 of these always-on processors operating at once, there is a significant power demand on the battery even when the ignition is turned off.

Collectively, hundreds of milliamps (mA) of supply current can be required to power these always-on processors, which could completely drain a battery in a matter of days. For example, if a car’s high voltage step-down converters require 2mA of supply current each, combining 30 of these from security systems, GPS systems and remote keyless entry systems with other mandatory always-on systems such as ABS brakes and leakage current from electronically actuated windows could drain a battery after an extended 3 week business trip, rendering it unable to turn over the engine. Therefore, the quiescent current of these power supplies needs to be drastically reduced in order to preserve battery life without increasing the size or complexity of the electronic systems.

Until recently, the requirement of high input voltage capability and low quiescent currents were mutually exclusive parameters for a DC/DC converter. In order to better manage these requirements, a decade ago several automotive manufacturers created a low quiescent current target of <100uA for each always-on DC/DC converter, but today anything lower than 10uA is preferred. Luckily, a new generation of power ICs has been introduced, which offers quiescent currents of less than 3µA.

New Alternatives

Since the voltage range of single battery automobiles and dual battery commercial vehicle power bus can vary from under 3.5V to over 60V as they are exposed to different transient scenarios and configurations. The need for well regulated voltage rails in spite of this wide range of input voltages requires wide input voltage, high performance power conversion ICs. As the growth of electronic content in automobiles continues to accelerate with Electronic Control Modules (ECM) used for security, safety, navigation, chassis control and engine/transmission management so will the need for high voltage power management ICs which can offer high efficiency, low quiescent current, high frequency switching with very robust protection features and reliability. Fortunately, IC designs are already meeting these demanding requirements.

Linear Technology’s LT8620 is the first of a family of high voltage synchronous buck regulators. Its 3.4V to 65V input voltage range makes it ideal for automotive and commercial vehicle (single and dual battery) applications that are subjected to both low voltage transients such as cold-crank or stop-start scenarios as well as high voltage transients encountered during a load dump scenario. Its 2.0A continuous output current capability and ability to deliver outputs from 1V to slightly under VIN makes it ideal for many automotive rails that run directly from the single- or dual-cell battery bus. It’s very compact and simple solution footprint eliminates the need for any external diodes and can be seen below in figure 2.

Figure 2. LT8620 Typical Automotive/Commercial Vehicle Schematic for a 5V, 2A Output

Its synchronous rectification design includes internal top and bottom MOSFETs to deliver efficiencies as high as 94%. Figure 3 shows that it can deliver over 94% efficiency when powering a 5V load from a nominal 12V input, even with a relatively high 700kHz switching frequency. Similarly, it can deliver up to 92% efficiency when delivering 5V from a nominal 24V input. This high efficiency operation minimizes wasted power and eliminates the need for heat sinks even in the most space constrained applications. In electric vehicles and hybrids, this can directly translate into increased driving range between battery recharges.

Figure 3. LT8620 Efficiency Graph of Typical Automotive/Commercial Vehicle Schematic

Additionally, the LT8620’s Burst Mode operation reduces no load quiescent current to a mere 2.5uA making it ideal for always-on applications which must maintain constant voltage regulation even at no loads while maximizing battery life. This is of particular importance due to the growing number of always on systems including security, environmental control, data recording, safety and location. Additionally, a very low ripple Burst Mode operation topology minimizes output noise to below 10mVPK-PK, making it suitable for noise-sensitive applications. If the application requires external synchronization, the Burst Mode function can be replaced with a pulse-skipping frequency scheme.

The LT8620’s very low dropout performance is also very beneficial, particularly in applications which must regulate outputs through use stop/start or cold-crank conditions. Figure 4 shows that even when the input voltage drops below the programmed output voltage, 5V in this case, the output is always 500mV (at 2A) below the input voltage, once the input exceeds 2.9V. This is important because many ECMs drive require one or multiple microprocessors/microcontrollers. Although these are designed to operate from a nominal 5V, they continue to operate with supply voltages as low as 3V. So in a cold crank scenario, the input can drop as low as 3.4V and the microprocessor can continue to operate, enabling the ECU to operate seamlessly through cold crank.

Figure 4. LT8620 Dropout Performance

Furthermore, the LT8620’s fast minimum on-time of only 30ns enables 2MHz constant frequency operation from a 24V input to a 1.5V output so designers can optimize efficiency while avoiding critical noise-sensitive frequency bands such as AM radio. Even with an input voltage above 16V, the LT8620 will deliver a very well regulated output voltage to an output as low as 1V. As faster switching frequency operation reduces the size of external components, the LT8620’s 2.2MHz switching capability allows a very compact solution footprint. Additionally, special design techniques have been implemented to minimize potential EMI/EMC concerns

The LT8620 utilizes internal top and bottom high efficiency power switches with the necessary boost diode, oscillator, control and logic circuitry integrated into a single die. Special design techniques and a new high speed process enable high efficiency over a wide input voltage range and the LT8620s’s current-mode topology enables fast transient response and excellent loop stability. Other features include internal compensation, power good flag, robust short-circuit protection output soft start/tracking and thermal protection. The combination of its 24-lead 3mm x 5mm QFN or 16-lead thermally enhanced MSOP package and high switching frequency keeps external inductors and capacitors small, providing a very compact, thermally efficient footprint.

Conclusion

The rapid growth of very complex electronic systems in automobiles and commercial vehicles has created even 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 there required transient behavior with a single power IC including stop-start, cold-crank and load-dump conditions. Furthermore, it must be capable of accurately regulating an output voltage throughout these conditions. Additionally, some of these systems will operate in an always on standby mode, requiring minimal supply current. As more electronic systems are added in ever shrinking spaces, minimizing the solution footprint while maximizing efficiency is also critical. Fortunately the new generation of power ICs that meet these demands are already available paving the way for even higher electronic content in future vehicles.