Synchronous buck converters for advanced automotive designs

As automobiles continue to add an ever increasing number of electronic systems to enhance safety, comfort, efficiency and performance while minimizing harmful exhaust emissions, it comes as no surprise that they require physically smaller power solutions with dramatically higher power levels. Additionally, with the proliferation of more EMI-sensitive systems within the vehicle, reducing EMI emissions from switching power supplies is of paramount importance, thereby creating more challenges for switching regulator IC design.

According to Strategy Analytics, “the demand for enabling semiconductor devices is expected to grow at a CAAGR (compound average annual growth rate) of five percent per year over the next seven years, with the total market worth over $41 billion by 2021, compared to $28.9 billion in 2014. The Strategy Analytics analysis also identifies that demand for microcontroller and power semiconductors will drive over 40 percent of revenues.”

Strategy Analytics provides a very quantitative description of forecasting the growth of electronics content in automobiles, but more interesting is the prevalent role that power ICs play in this growth.

These new power IC designs must offer the following:

1) Robust performance across a wide range of voltages, including handling of transients in excess of 36V

2) Ultralow electromagnetic interference (EMI) emissions

3) The highest efficiency possible to minimize thermal issues and optimize battery run time.

4) The smallest solution footprints, demanding very high power densities with switching frequencies of 2MHz or greater needed to keep the switching noise out of the AM Radio band while keeping solution footprints very small

5) Ultralow quiescent current (<10µA) 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 switched off 

The goals for the increased performance levels of power ICs are to enable the design of increasingly complex and numerous electronic systems. Applications fueling this growth are found in every aspect of the vehicle. For example, new safety systems such as lane monitoring, adaptive safety control, and automatic turning and dimming headlights are demanding functions to implement. Infotainment systems (telematics), which continue to evolve and pack more functionality into an already tight space, must also support an ever-growing number of cloud applications.

Advanced engine management systems with the implementation of stop/start systems and electronics laden transmissions and engine control, plus drive train and chassis management aimed at simultaneously improving performance, safety and comfort. A decade ago, these systems were only found in high-end luxury cars but are now commonplace in automobiles from every manufacturer; further accelerating automotive power IC growth. Figure 1 below shows the multitude of electronic systems that are typically found in today’s cars.

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Figure 1. Electronic Systems Proliferation in Automobiles

Transients in automotive systems

Although the battery bus voltage in cars is nominally 12V (it varies from 9V to 16V depending on when the alternator is charging). Furthermore, the lead-acid battery voltage is subjected to wide variations during temporary conditions. Cold-crank and stop-start scenarios can pull the battery voltage down to 3.5V, whereas load dump can subject the battery bus to voltages as high as 36V. Therefore, power ICs must be able to accurately regulate an output through wide variations of input voltages. The wide temporary voltage swing during cold-crank/stop-start and load dump for single-cell lead-acid batteries is illustrated in Figure 2. Note that the proper power IC (the LT8640 in this case) accurately regulates the 3.3V output through both of these scenarios.

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Figure 2. LT8640 with 36V Load Dump Transient & 4V Cold Crank Scenario

Low EMI Operation

Because the automotive electrical environment is inherently noisy, with many applications being electromagnetic interference (EMI) sensitive, it is imperative that switching regulators don’t exacerbate EMI concerns.  Because a switching regulator is typically the first active component on the input power bus line, and regardless of downstream converters, it significantly impacts overall converter EMI performance.

Minimizing EMI is imperative. Historically, the solution was to use an EMI shielding box, but this adds significant cost and size to the solution footprint while complicating thermal management, testing and manufacturability. Another potential solution within the power management IC is to slow down the switching edges of the internal MOSFET. However, this has the undesired effect of reducing the efficiency and increasing the minimum on time which compromises the IC’s ability to deliver low duty cycles at switching frequencies at, or above 2MHz.

As both high efficiency and small solution footprints are desired, this isn’t a viable solution. Fortunately, some unique power IC designs have been introduced to enable fast switching frequencies, very high efficiency and low minimum on-times concurrently. These designs generally offer in excess on 20dB lower EMI emissions while offering 2MHz switching frequencies and 95% efficiency. Some also have spread spectrum capability, which can lower EMI emissions an additional 10dB. These effects are accomplished with no additional components or shielding, offering a significant breakthrough in switching regulator design.

High-efficiency operation

High-efficiency operation of power management ICs in automotive applications is important for two main reasons. First, the more efficient the power conversion, the less energy is wasted in the form of heat. Since heat is the enemy of 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 overall solution. Secondly, any wasted electrical energy in hybrids or EVs will directly reduce their driving 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 25W when compared to a monolithic alternative. Fortunately, new power management ICs that can offer both high voltage and high efficiency via internal synchronous rectification is now found in the marketplace.

Smaller power conversion circuits

There are a few ways to make power conversion circuits smaller. In general, the largest components in the circuit are not the power IC, but the external inductor and capacitors. By increasing the switching frequency of an IC from 400kHz to 2MHz, the size of these externals can be dramatically reduced (about 4x smaller solution footprint). But in order to do this effectively, the power IC must deliver high efficiency at higher frequencies, which historically has not been feasible.

Using new process and design techniques, synchronous power ICs have been developed that deliver efficiencies in excess of 95%, while switching at 2MHz. The high efficiency operation minimizes power loss, eliminating the need for heat sinks. It also has the added benefit of keeping switching noise out of the AM frequency band. 

“Always-On” systems demand ultra-low 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. Furthermore, 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 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.

Therefore, the quiescent current of these power ICs 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.  About a decade ago, several automotive manufacturers created a low quiescent current target of <100µA for each always-on DC/DC converter, but today lower than 10µA is preferred. Luckily, a new generation of power ICs is now available which offers quiescent currents of less than 2.5µA in standby mode. 

New alternatives

Until now, there was no sure way to guarantee that EMI could be suppressed and efficiency requirements attained via power IC selection. However, the LT8640 Silent Switcher regulator makes this possible. The LT8640 is the second device in a family of Silent Switcher high voltage synchronous buck regulators. It is a 5A (continuous current with 7A peaks), 42V input capable synchronous step-down switching regulator.

As can be seen in Figure 3, the EMI emissions are 10dB to 30dB below the automotive CISPER 25, Class 5 peak limits without spread spectrum enabled. Spread spectrum lowers these levels by another 5dB to 10dB across the most critical automotive frequency band.  This combination reduces EMI emissions by more than 25dB when compared to current state-of-the-art switching regulators. The LT8640 in the graph below was switching at 2MHz with a load current of 4A and no external EMI shielding was required.

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Figure 3. LT8640 Radiated EMI Performance With/Without Spread Spectrum (f_SW=2MHz, I_LOAD=4A)

The schematic for the LT8640 is shown in Figure 4.  Synchronous rectification eliminates the need for any external diodes, improving efficiency while simplifying the solution footprint. This particular schematic switches at a 1MHz switching frequency utilizing a 3.3µH inductor, delivering efficiency of 96%. However, as can be seen in Figure 5, running the LT8640 at 2MHz avoids any interference concerns with the AM radio band, and enables the use of a smaller 2.2µH inductor while still delivering 95% efficiency. The LT8640 uses a unique design, which minimizes switching losses enabling it to deliver this high efficiency at switching frequencies of 2MHz or higher.

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Figure 4. LT8640 Typical Automotive Schematic for a 5V Output

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Figure 5. LT8640 Efficiency Graph of Figure 4 Circuit at 1MHz, 2MHz & 3MHz

The LT8640’s 3.4V to 42V input voltage range makes it ideal for automotive and industrial applications. The internal high efficiency switches deliver up to 5A of continuous output current and peak loads of 7A to voltages as low as 0.97V. Its Burst Mode® operation offers only 2.5µA of quiescent current, making it well suited for applications such as automotive always-on systems, which need to extend operating battery life.

The LT8640’s unique design maintains a minimum dropout voltage of only 100mV (@1A) under all conditions, enabling it to excel in scenarios such as automotive cold-crank. Furthermore, a fast minimum on time of only 40ns enables 2MHz constant frequency switching from a 16V input to a 1.5V output, enabling designers to optimize efficiency while avoiding critical noise-sensitive frequency bands. The LT8640’s 20-lead 3mm x 4mm QFN package and high switching frequency keeps external inductors and capacitors small, providing a compact, thermally efficient footprint.

Looking forward

The rapid growth of extremely complex electronic systems in automobiles has created even higher demands on power management ICs. In the past, high load currents, fast switching frequencies and high efficiency designs collectively created big EMI challenges. However, the LT8640’s unique design offers high efficiency, fast switching, enabling a very compact solution footprint – all in ultralow EMI emissions, setting a new standard in power ICs. Fortunately, a new generation of synchronous power ICs is now available to pave the way for even higher electronic content in future vehicles.