ROHM releases white paper on solutions for next-generation car cockpits

“In addition to car radios and car stereos, various sounds are emitted inside and outside the car. For example, turn on the turn indicator, the car will emit a “tick, tick” steering tone. In addition, when the braking system used to avoid collisions is activated, a warning sound will sound, which is one of the functions of the Advanced Driver Assistance System (ADAS). Recently, motor-driven vehicles such as xEV are equipped with an acoustic vehicle alert system (AVAS) when pedestrians approach the vehicle. In addition, the car will also emit various other voices, such as welcome speech when starting the engine, ETC prompt sound, etc.

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Preface

In addition to car radios and car stereos, various sounds are emitted inside and outside the car. For example, turn on the turn indicator, the car will emit a “tick, tick” steering tone. In addition, when the braking system used to avoid collisions is activated, a warning sound will sound, which is one of the functions of the Advanced Driver Assistance System (ADAS). Recently, motor-driven vehicles such as xEV are equipped with an acoustic vehicle alert system (AVAS) when pedestrians approach the vehicle. In addition, the car will also emit various other voices, such as welcome speech when starting the engine, ETC prompt sound, etc.

Figure 1. Various sounds required by the multifunctional car dashboard

The steering tone mentioned above, previously heard the switching sound of a mechanical relay, but after the realization of Electronic, even if the relay is not used, there will be a sound from the speaker. This is just an example of how we obtain important information from sound. In the future, with the development of AI or autonomous driving technology, there will be higher requirements for two-way communication between people and cars. Therefore, sound is used as a tool to promote two-way communication. One, its presence should be further increased.

1. The composition of the speaker system

Systems that output the above-mentioned ADAS and AVAS voices are roughly divided into systems that use buzzers and systems that use speakers. Although the cost of the former is very low, the frequency that can be played is limited. The latter, like audio equipment, can be played in a wider frequency band. For a long time, only the steering tone and buzzer sound around the cockpit were enough, and there was no need for diversified voices. But now, due to the diversification of the voice required around the cockpit, there is a situation where the system cannot be configured without a speaker.

Therefore, the speaker amplifier IC becomes essential. The speaker amplifier IC is used to amplify the voice signal output by SoC (System On a Chip), etc., and make the current flow to the speaker to realize the drive. There are two voice signal formats, one is the analog input type that inputs analog signals such as sine waves, and the other is the use of I²The digital input type of digital audio format such as S needs to be used separately according to the application system. In addition, the output mode of the speaker amplifier IC is roughly divided into a class AB amplifier and a class D amplifier. Although the power conversion efficiency of the class AB amplifier is low and the IC generates more heat, it has the advantage of not generating unnecessary radiation. While the Class D amplifier, although the power conversion efficiency is high, and the IC generates less heat, it will generate unnecessary radiation, so it is necessary to configure an LC filter at the output. Therefore, class AB amplifiers are usually used within the allowable heat generation range; class D amplifiers are used when larger output power is required and no heat is allowed.

2. Problems with speaker amplifiers

No matter what output method is adopted, the car speaker amplifier needs to have high reliability, and have both high power output (large volume) and safety. The importance of reliability is unquestionable, but there is a trade-off relationship between high-power output and safety, and it is very difficult to have both. To achieve high-power output, a large current needs to flow to the speaker, which is equivalent to increasing the size of the output transistor of the IC. However, if the size of the output transistor becomes larger, such as when a short Circuit occurs due to a speaker failure, a large amount of current may flow between the output pins. In some cases, the IC may even be damaged, thereby affecting the safety of the car. To prevent this from happening, it is necessary to mount an over-current protection circuit on the IC. The traditional speaker amplifier IC uses a method of limiting the load current. But when adopting this kind of method, must set the working threshold value of the overcurrent protection circuit to be smaller than the value that can output the maximum electric current, as shown in Fig. 2. Therefore, the maximum output power is limited, and waveform (sound) distortion occurs during high-power output.

Figure 2. Waveform diagram of overcurrent protection circuit

3. Rohm’s new product “BD783xxEFJ-M”

ROHM has developed the “BD783xxEFJ-M” speaker amplifier for automotive instrument panels that perfectly solves this problem with high power output and safety. This series of products operate with the most demanded 5V power supply in the dashboard, and the output power is 1~2W as the main target. The input method uses analog input, and the output method uses class AB with a small number of components. This series of products have three major advantages as shown below.

3-1. With over-current protection function, and realized 2.8W high-power output

The new product “BD783xxEFJ-M” adopts a newly developed over-current protection circuit, which takes into account both high-power output and safety. Under the condition of power supply voltage of 5V and load of 4Ω, the output power can reach 2.8W (THD+N

Figure 3. Output power comparison of speaker amplifiers equipped with overcurrent protection for automotive dashboards

Generally, the overcurrent protection circuit of class AB amplifiers mostly uses a “current limiter circuit”. As mentioned above, by limiting the output current, although the outflow of overcurrent can be prevented, the output power is also limited. In order not to limit the output power and protect the product from overcurrent, it is necessary to adopt a “peak current protection circuit”, which is a mechanism that detects the peak current and stops the output, as the literal meaning indicates. When the output current exceeds the maximum current, this method can be used to detect the current, but when the output bias voltage is low, such as at startup or under voltage, even if the output is short-circuited, the maximum output current will not exceed Threshold, so the protection circuit will not work. Therefore, the disadvantage of the peak current protection circuit is that the IC heating temperature is higher than the chip junction temperature, and the chip may even be damaged in the worst case. In other words, if a current limiting circuit is used, it will cause sound distortion; if a peak current protection circuit is used, there will be situations where it cannot be protected.

To solve this problem, ROHM has researched and designed a new type of overcurrent protection circuit (patent pending) that combines the advantages of these two Circuits. When the technology does not need to output high power during startup or under voltage, it will activate the current-limiting circuit to prevent IC heating; during normal operation, it will automatically switch to peak current protection. This is a technology that combines the advantages of two protection circuits and can achieve high-power output.

The new product “BD783xxEFJ-M” is equipped with this new type of overcurrent protection circuit, which can effectively protect the IC from load short-circuits and will not cause distortion when outputting high power.

3-2. High reliability, supporting harsh environments in vehicle applications

The new product meets the reliability standard AEC-Q100 for automotive Electronic products and supports an operating temperature of Ta=105°C, so it can be used with confidence in automotive applications where high reliability is pursued. This series of products adopts a power package (HTSOP-J8), which can achieve high power output even at 105°C operating temperature in a Class AB amplifier that usually generates a large amount of heat. Although the package is a lead frame type package with a small size of only 4.9mm×6.0mm×1.0mm, when using a 4-layer circuit board (according to JEDEC51-5,7 standards), the θ_JAOnly 45.2℃/W, the heat dissipation performance is very good. With this package, compared with ROHM’s previous products, the chip temperature rise is reduced by 80% (conditions: VCC=5V, RL=8Ω, THD

Figure 4. “BD783xxEFJ-M” that complies with the AEC-Q100 standard and has various protection functions

3-3. Reduce the number of components through built-in resistors

In class AB amplifiers with this output range, the input resistance and feedback resistance used to adjust the signal gain when setting the volume are usually external. The new product reduces the number of components and the mounting area of ​​the printed circuit board through the built-in resistor. In addition, there are 11 models in this series, with a gain range of 6dB～26dB (in 2dB increments), allowing fine gain adjustment. The 26dB product “BD78326EFJ-M” is only used for sample evaluation where the gain is frequently adjusted. By adding evaluation resistors to each input pin, evaluation can be easily performed without replacing the IC. Even with built-in resistors, it will not Increase design man-hours. At present, 6dB, 10dB, and 26dB models have begun mass production, and other products will be released one after another.

4. Future development trends

As mentioned at the beginning of this article, the multifunctional voice around the cockpit has become an inevitable demand. In the future, with the advent of the CASE (Connected, Autonomous, Shared, Electric) era, this demand is expected to grow further, and there will be more High power output demand. In addition, in the layout of the car, the boundary between the traditional cockpit and car audio will disappear, and the use of voice will become more diversified.

In order to meet these needs, ROHM needs to continuously expand its speaker amplifier product lineup. The BD783xxEFJ-M launched this time is a car class AB speaker amplifier, and ROHM is currently developing a car class D speaker amplifier. The target application product of BD783xxEFJ-M has a power supply voltage of less than 5V. The next automotive Class D speaker amplifier product will support connection with a 12V battery and can achieve a large output power of more than 4W. For automotive-grade speaker amplifier ICs that support 12V power supply, although the current mainstream product is a class AB amplifier, as the ECU (Electric Control Unit) used in the cockpit and the head unit has higher and higher requirements for space saving, heat dissipation The use of radiators is becoming a technical bottleneck. In order to solve this bottleneck problem, ROHM has made a great contribution to the realization of miniaturization by adopting a class D amplifier system in the speaker amplifier.

As the first wave of automotive Class D speaker amplifiers, like the BD783xxEFJ-M, in addition to the analog input type, 4 products in the product lineup, including the digital input type and the multi-channel type, are also planned to meet the needs of various application products. For example, the digital input type supports TDM (Time Division Multiplexing) format. Up to 8 channels of speakers can be connected in a system, which can increase the options for application design and improve the flexibility of application design.

The second wave of automotive class-D speaker amplifiers is scheduled to sell samples in 2021. In the future, ROHM will continue to devote itself to the development of automotive speaker amplifier ICs and contribute to improving the safety and comfort of automobiles.