How to deal with the harsh automotive environment? These solutions can easily solve it

• Automotive Input Transients

• Input voltage range

• Output voltage/current

• Low quiescent current (I _Q )

• Electromagnetic Interference (EMI)

Figure 1 shows a complete power solution that meets the demanding requirements of automotive applications. On the front end, the LT8672 acts as an ideal diode, protecting the circuit from the harsh environment under the hood and preventing destructive faults such as reverse polarity. Following the ideal diode is a series of low quiescent current (I _Q ) step-down regulators with a wide input range—operating from as low as 3V to as high as 42V—to provide regulated voltages for the core, I/O, DDR, and other power rails required by peripherals.

Figure 1. Overview of ADI Power solutions for automotive electronics transient immunity requirements

Figure 2 shows a traditional automotive electrical system where the engine drives an alternator. The alternator is essentially a three-phase generator whose AC output is rectified by a full diode bridge. The output of this rectifier is used to charge the lead-acid battery and power 12V circuits and devices. Typical loads include ECUs, fuel pumps, brakes, fans, air conditioning, audio systems, and lighting. More and more ADAS are being added to the 12V bus, including peripherals, I/O, DDR, processors and their power supplies.

Figure 2. Typical electrical system in a car.

Electric vehicles change the above picture to some extent. The engine is replaced by an electric motor, and a DC-DC converter converts the 400V high-voltage lithium-ion (Li-Ion) battery pack voltage to 12V instead of an alternator. However, the traditional 12V alternator device and its transient pulses (including fast pulses) still exist.

The engine operates at peak efficiency over a narrow RPM range, so in most situations (more on this below), the alternator steady-state output and battery voltage are relatively constant, such as about 13.8 V. Every circuit powered directly from the car battery must operate reliably within the 9 V to 16 V range, but a robust automotive electronics design must also operate under abnormal conditions, which inevitably occur during the most troublesome times.

While the output of the alternator is nominally stable, it is not stable enough to power other vehicle systems without conditioning. Unexpected voltage spikes or transients are harmful to downstream electronic systems and can cause these systems to malfunction or be permanently damaged if not handled properly. Over the past few decades, many automotive standards have emerged to define the spikes and voltage transients that automotive power supplies face and to set expectations for design, such as ISO7637-2, ISO16750-2, LV124, and TL82066.

One of the most important and challenging high voltage transients is load dump. In automotive electronics, load dump occurs when the vehicle battery is disconnected from the alternator while the battery is charging. During a load dump transient, the alternator excitation field remains high due to the large time constant – the alternator is still outputting high power even though there is no load. The battery is a large capacitor that normally absorbs the extra energy, but when loose terminals or other problems cause the battery to be disconnected, it can no longer provide this service. As a result, all other electronic equipment sees a voltage surge and must be able to survive a load dump event. An unsuppressed load dump event can generate voltages in excess of 100V. Fortunately, modern automotive alternators use avalanche rated rectifier diodes, and the load dump voltage is limited to 35V – still a significant departure from the norm. Load dump events can last up to 400ms.

Another high-voltage event is jump starting. Some tow trucks use two batteries in series to ensure effective jump starting to activate the car's dead battery, so the car's circuits must be able to withstand double the nominal battery voltage (28V) for several minutes. Many high-voltage step-down regulators, such as the Silent Switcher® ^and Silent Switcher2 families, including the LT8650S and LT8640S, operate up to 42V, exceeding these requirements. In contrast, devices with lower voltage ratings require clamping circuits, which increase cost and reduce efficiency. Some regulators, such as the LT8645S and LT8646S, are rated at 65V to support truck and aircraft applications (where 24V systems are standard).

Table 1. Silent Switcher and Silent Switcher 2 Monolithic Step-Down Regulators for Automotive Applications

When the driver starts the car, the starter draws hundreds of amps from the battery, causing another voltage transient. This pulls the battery voltage down for a short period of time. With a traditional car, this only happens when the car is started by the driver; for example, starting the car to drive to the supermarket and then starting it again to drive home. Modern cars have start-stop features to save fuel, and during a trip to and from the supermarket, there may be multiple start-stop events—for example, at every stop sign and every red light. The additional start-stop events put much more stress on the battery and starter than in a traditional car.

Figure 3. LT8672 response to reverse battery polarity.

Additionally, if you start the car on a cold morning, the starter draws more current than it would if the ambient temperature was higher, and the battery voltage drops to 3.2V or less for about 20ms—known as a cold crank. Some functions must remain active even under cold crank conditions. The good news is that such critical functions are typically not designed to require much power. Integrated solutions, such as the LT8603 multichannel converter, can maintain regulation even when its input voltage drops below 3V.

ISO7637-2 and TL82066 define many other pulses. Some have not only high positive or negative voltages, but also high source impedances. These pulses have relatively low energy compared to the above events and can be filtered or clamped by properly selecting the input TVS.

The LT8672 active rectifier controller features high input voltage rating (+42V, -40V), low quiescent current, ultra-fast transient response, and ultra-low external FET voltage drop control, providing protection in 12V automotive systems with extremely low power consumption.

Whenever the battery terminals are disconnected, there is a chance that the polarity of the car battery could be reversed by mistake, and the electronic system could be damaged by the negative battery voltage. Blocking diodes are often placed in series with the power input to prevent reverse power, but blocking diodes have a voltage drop that causes system inefficiency and reduces input voltage, especially during cold cranking.

The LT8672 is an ideal replacement for passive diodes to protect downstream systems from negative voltages, as shown in Figure 3.

Under normal conditions, the LT8672 controls an external N-channel MOSFET to form an ideal diode. The GATE amplifier senses DRAIN and SOURCE, driving the MOSFET gate to regulate the forward voltage to 20mV. D1 protects SOURCE in the positive direction during load steps and overvoltage conditions. When a negative voltage appears at the input and SOURCE goes negative, GATE is pulled to SOURCE, turning off the MOSFET and isolating DRAIN from the negative input. With the fast pull-down (FPD) function, the LT8672 can quickly turn off the external MOSFET.

Figure 4. LT8672 response waveform to reverse polarity.

A common disturbance on the battery rail is a superimposed ac voltage. This ac component can be an artifact of the rectified alternator output or the result of frequent switching of high current loads such as motors, light bulbs, or PWM controlled loads. According to automotive specifications ISO16750 and LV124, ECUs can be affected by ac ripple (up to 30 kHz in frequency and 6Vp-p in amplitude) superimposed on their power supply. In Figure 5, a high frequency ac ripple is superimposed on the battery line voltage. A typical ideal diode controller would be too slow to respond, but the LT8672 can generate high frequency gate pulses up to 100kHz, which can control external FETs as needed to suppress these ac ripples.

Figure 5. LT8672 response waveform to superimposed AC voltage

The LT8672’s unique ability to reject common AC components on the power rail is a result of its fast pull-up (FPU) and fast pull-down (FPD) control strategies and its strong gate drive capability, which is powered by an integrated boost regulator. Compared to charge pump gate supply solutions, the boost regulator enables the LT8672 to maintain a regulated 11V voltage and keep the external FET on, while providing a large gate source current to reduce switching losses for high-frequency AC ripple rectification. Its 50mA source current capability supports ultra-fast turn-on of the FET for minimal power dissipation, and its 300mA sink current capability supports fast turn-off for minimal reverse current conduction. In addition, this significantly reduces the ripple current in the output capacitor. A typical rectified waveform with superimposed AC voltage is shown in Figure 6.

Figure 6. LT8672 response waveform to superimposed AC voltage

In addition, the LT8672 effectively reduces conduction losses compared to traditional Schottky diode solutions under the same load conditions. As shown in the thermal image of Figure 7, the solution using the LT8672 is nearly 20°C cooler than the traditional diode-based solution. It not only improves efficiency but also eliminates the need for a large heat sink.

High-peak narrow pulses appearing at the input of automotive electronic systems typically come from two sources:

• Disconnect the input power supply when an inductive load is connected in series or parallel

• The switching process of the load affects the distributed capacitance and inductance of the wiring harness.

  
  

Figure 7. Thermal performance comparison

Some of these pulses may have high voltage peaks. For example, pulse 3a defined in ISO7632-2 is a negative spike with a peak voltage exceeding -220V, while pulse 3b has a maximum peak voltage of 150V plus the initial voltage of the battery. Although they have large internal impedance and very short duration, downstream electronic equipment can be easily damaged if they encounter these pulses.

In order to suppress this spike, two TVS of appropriate size are installed at the front end. In fact, some low energy pulses can be directly absorbed by the filter effect of input capacitance and parasitic line inductance.

Figure 8. Severe cold start events for 12 V systems as defined in LV 124.

Figure 9. Cold start event

The LT8602 is a compact solution that provides up to four regulated rails (e.g., 5V, 3.3V, 1.8V, 1.2V) with an input voltage range of 5V to 42V for functions that do not necessarily need to be on during cold crank. For functions that must also operate during cold crank, such as spark plug controllers or alarms, solutions such as the LT8603 should be used, which can operate with input voltages as low as 3V (or lower).

LV124 defines the worst-case cold crank scenario, as shown in Figure 8. It states that the minimum battery voltage can be as low as 3.2V for 19ms during vehicle startup. The specification requires applications to operate down to 2.5V when faced with the additional diode drop caused by reverse battery protection in traditional (non-ideal diode) solutions. Passive diode protection schemes may require a buck-boost regulator instead of a less complex and more efficient buck regulator to provide the regulated 3V supply often required by many microcontrollers.

The LT8672 controller has a minimum input operating voltage of 3V V _BATT , allowing the active rectifier to ride through the cold crank pulse with minimal voltage drop (20mV) between input and output. The downstream supply input voltage during cold crank does not drop below 3V, so a step-down regulator with a minimum operating voltage of 3V and low dropout characteristics (such as the LT8650S) can be used to generate the 3V supply.

Like the LT8650S, many of ADI's automotive ICs are rated for a minimum input voltage of 3V.

Figure 9 compares a 1.8V supply using the LT8672 and using a traditional diode. The buck regulator operates down to 3V. As shown, with a traditional diode, when the battery voltage V _BATT drops to 3.2V, the buck regulator’s V _IN drops to around 2.7V because the high voltage drop of the diode triggers the downstream switching regulator UVLO shutdown and its 1.8V output collapses. In contrast, the LT8672 output voltage remains nearly constant during cold crank, and the downstream buck regulator is able to maintain a 1.8V output.

Many critical functions require regulated 5V and 3.3V rails, as well as sub-2V rails, to power components, processor I/Os, and cores in analog and digital ICs. When powered directly from V _{BATT , a pure buck regulator will lose regulation if V BATT} drops below its output or V _IN (MIN). However, such critical functions typically do not require much power, so highly integrated, compact solutions can be used, such as the 6mm × 6mm, quad-output, triple-channel monolithic buck converter and boost controller LT8603.

The LT8603’s integrated boost controller can operate below 2V, making it an ideal pre-regulator for the other three buck regulators. Figure 10 shows ADI Power’s state-of-the-art solution for these applications, which rides through cold crank events. The two high-voltage buck regulators are powered by the pre-boost converter. When V _BATT drops below 8.5V, the boost controller starts switching and the output (OUT4) is regulated to 8V. Once started, the output voltage remains regulated with input voltages as low as 3V. Thus, the two high-voltage buck regulators ride through cold crank conditions while providing constant 5V and 3.3V outputs, as shown in Figure 11. Once V _BATT recovers above 8.5V from the cold crank condition, the boost controller acts like a pass-through diode. The high-voltage buck regulator can handle V _BATT up to 42V . The low-voltage buck regulator is powered by OUT2 and provides 1.2V during a cold crank event.

Figure 10. LT8672 and LT8603 solutions can withstand and ride through cold crank events.

Figure 11. The LT8672 and LT8603 combination produces 5V and 3.3V outputs to ride through cold crank events.

Burst Mode efficiency at a given light load is primarily affected by switching losses, which are a function of switching frequency and gate voltage. Since a certain amount of energy is required to turn the MOSFET on and off and keep the internal logic active, reducing the switching frequency reduces gate charge losses and improves efficiency. The switching frequency is primarily determined by the Burst Mode current limit, the inductor value, and the output capacitor. For a given load current, increasing the burst current limit increases the energy delivered in each switching cycle, and the corresponding switching frequency will be lower. For a given burst current limit, a larger value inductor can store more energy than a smaller value inductor, and the switching frequency will be lower. For the same reason, a larger output capacitor will store more energy and take longer to discharge.

Figure 12. The low I _Q LT8650S can maintain very high light load efficiency to support always-on applications without significantly draining the battery.

Figure 12 shows the ultralow I _Q synchronous step-down regulator LT8650S used in a high efficiency, wide input voltage and load current range application. With integrated MOSFETs, this device can provide up to 8A of total output current at a fixed output voltage of 3.3V or 5V. Although the overall design and layout are simple, the converter also includes other options that can be used to optimize the performance of specific applications in battery-powered systems.

Table 1 lists low I _Q monolithic regulators suitable for the automotive market, with input voltages up to 42 V or 65 V. Thanks to the low I _Q technology developed by ADI, these devices have a typical quiescent current of only 2.5 μA. With a minimum on-time of 35 ns, these regulators can provide a 3.3 V output voltage from a 42 V input at a switching frequency of 2 MHz, which is common in the automotive industry.

Automotive applications require systems that do not generate electromagnetic noise that could interfere with the normal operation of other automotive systems. For example, switching power supplies are high-efficiency power converters, but they generate unwelcome high-frequency signals that can affect other systems. Switching regulator noise occurs at the switching frequency and its harmonics.

Ripple is the noise component that appears on the output and input capacitors. Low ESR and ESL capacitors and low-pass LC filters can reduce ripple. Higher frequency noise components caused by fast switching of power MOSFETs are much more difficult to handle. As designs focus on small solution size and high efficiency, switching operating frequencies are now pushed up to 2MHz to reduce passive component size and avoid the audible frequency band. In addition, switching transition times have been shortened to nanoseconds to improve efficiency by reducing switching losses and duty cycle losses.

Parasitic capacitance and inductance in packages and PCB layouts play a significant role in noise distribution, and if noise is present, it is difficult to eliminate it. Switching noise covers a range from tens of MHz to over GHz, making EMI prevention very complicated. Sensors and other instruments affected by such noise may not operate properly, causing audible noise or serious system failures. Therefore, strict standards have been established to regulate EMI. The most commonly used standard is CISPR25Class5, which details acceptable limits at frequencies from 150kHz to 1GHz.

Meeting EMI requirements at high currents typically involves a complex design and test process that includes trade-offs in solution size, overall efficiency, reliability, and complexity. Traditional approaches to controlling EMI slow down switching edges or reduce switching frequency, which results in reduced efficiency, increased minimum switching times, and larger solution size. Alternative mitigation solutions include bulky and complex EMI filters, snubbers, or metal shielding, which significantly increase board space, component and assembly costs, and complicate thermal management and testing.

Our Silent Switcher technology solves the EMI problem in an innovative way, enabling excellent EMI performance in high-frequency, high-power power supplies. The second-generation Silent Switcher2 devices simplify board design and manufacturing by integrating the hot loop capacitors into the package. For a step-down regulator such as the 42V/4A LT8650S, the hot loop includes an input capacitor and the top and bottom switches. Other noise loops include the gate drive circuit and the boost capacitor charging circuit. In Silent Switcher2 devices, the hot loop and warm loop capacitors are integrated into the package and laid out in a way that minimizes EMI. This reduces the impact of the final board layout on EMI, simplifying design and manufacturing. Using the optional spread spectrum feature integrated in these devices, peak EMI can be further reduced, making it easier to meet stringent EMI standards.

Figure 13. LT8672 and LT8650S configured for high output current

Figure 13 shows a low I _Q , low noise solution that supports high current applications for automotive I/O and peripherals. The LT8672 on the front end protects the circuit from reverse battery faults and high frequency AC ripples with a forward voltage drop of only tens of mV. The LT8650S switches at 400kHz with an input range of 3V to 40V and an output capability of 8A when two channels are operated in parallel. Two decoupling capacitors are placed close to the input pins of the LT8650S. Due to the Silent Switcher2 technology, high frequency EMI performance is excellent even without an EMI filter installed. The system meets the CISPR25Class5 peak and average limits with a large margin. Figure 14 shows the radiated EMI average test results for vertical polarization in the range of 30MHz to 1GHz. The complete solution features a simple schematic, very low total component count, compact size, and EMI performance is not affected by changes in board layout (Figure 15).

Figure 14. LT8672 and LT8650S EMI Performance: 30 MHz to 1 GHz

Figure 15. Complete power solution with 3.3 V and 5 V outputs from the car battery.

Automotive applications require low-cost, high-performance, reliable power solutions. The harsh under-hood environment requires power designers to come up with robust solutions that consider a variety of potentially destructive electrical and thermal events. Electronic boards connected to the 12 V battery must be carefully designed to achieve high reliability, small solution size, and high performance. The ADI Power device catalog contains innovative solutions specifically targeted at automotive requirements: ultra-low quiescent current, ultra-low noise, low EMI, high efficiency, wide operating range, small size, and wide temperature range. By removing complexity and improving performance, ADI Power solutions shorten power supply design time, reduce solution cost, and accelerate time to market.