In addition to the car radio and car stereo, there are various sounds inside and outside the car. For example, turn on the turn indicator light, and the car will make a "tick-tock, tick-tock" turn prompt sound. In addition, when the braking system to avoid collision is started, it will give a warning sound, which is also one of the functions of ADAS. Recently, when pedestrians approach vehicles, vehicles such as xEV are equipped with acoustic vehicle warning system (AVAS). In addition, the car will make other sounds, such as the welcome speech when starting the engine, the prompt sound of ETC and so on.
Figure 1. Various sounds required by the instrument panel of multifunctional vehicles
The steering prompt tone mentioned above used to be the switch sound of mechanical relay. After electronization, even if the relay is not used, there will be sound coming from the speaker. This is just an example of how we can get 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 vehicles. Therefore, sound, as one of the tools to promote two-way communication, should further increase the sense of existence.
1. Composition of speaker system
Systems that output the above ADAS and AVAS sounds can be roughly divided into systems that use buzzers and systems that use loudspeakers. Although the cost of the former is very low, the frequency of playing is limited. The latter, like audio equipment, can be played in a wider frequency band. For a long time, all that is needed around the cockpit is turn signals and buzzers, and there is no need for diversified sounds. But now, due to the diversity of sounds needed around the cockpit, it is impossible to configure the system without speakers.
Therefore, the speaker amplifier IC becomes essential. Speaker amplifier IC is used to amplify SoC (system? Open? Answer? Chip), etc., so that the current flows to the speaker to realize the driving IC. There are two voice signal formats, one is the analog input type that inputs analog signals such as sine wave, and the other is the digital input type that uses digital audio formats such as I2S, which need to be different according to different application systems. In addition, the output mode of speaker amplifier IC can be roughly divided into class AB amplifier and class D amplifier. Although the power conversion efficiency of class AB amplifier is low, and the IC is seriously heated, its advantage is that it will not produce unnecessary radiation. Class-D amplifier has high power conversion efficiency and less IC heating, but it will produce unnecessary radiation, so it is necessary to configure LC filter at the output. Therefore, Class AB amplifier is usually used within the allowable calorific value range; When large output power is needed and heating is not allowed, class D amplifier is used.
2. The topic of loudspeaker amplifier
No matter what output mode is adopted, the automobile loudspeaker amplifier must have high reliability, high power output (large volume) and safety. There is no doubt about the importance of reliability, but there is a trade-off between high power output and safety, and it is difficult to have both. To achieve high power output, it is necessary to let a large current flow to the speaker, which is equivalent to increasing the size of the IC output transistor. However, if the size of the output transistor becomes larger, for example, when the speaker fails and is short-circuited, a large amount of current may flow between the output pins. In some cases, it will even damage the IC and affect the safety of the car. In order to prevent this from happening, it is necessary to carry an overcurrent protection circuit on the IC. The traditional loudspeaker amplifier IC adopts the way of limiting the load current. However, when this method is adopted, the working threshold of the overcurrent protection circuit must be set to a value less than the maximum output current, as shown in Figure 2. Therefore, there are problems that the maximum output power is limited and waveform (sound) distortion occurs at high power output.
Figure 2. Waveform diagram of overcurrent protection circuit.
3. Rohm's new product "BD 783 XEFJ-M"
Rohm Company developed a loudspeaker amplifier "BD 783 XEFJ-M" for vehicle dashboard with high power output and high safety, which perfectly solved this problem. This series of products use 5V power supply, which is the most needed in the instrument panel, and the output power is 1 ~ 2w. The input mode is analog input, and the output mode is AB with fewer components. This series of products has the following three advantages.
3- 1.？ It has over-current protection function and realizes 2.8W high power output.
The new product "BD 783 XEFJ-M" adopts a newly developed overcurrent protection circuit, which gives consideration to high power output and safety. The output power can reach 2.8W (THD+N
Figure 3. Comparison of output power of speaker amplifier equipped with over-current protection function in vehicle dashboard.
Usually, the overcurrent protection circuit of class AB amplifier mostly adopts "current limiting circuit". As mentioned above, by limiting the output current, although overcurrent can be prevented from flowing out, 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 "peak current protection circuit", which literally means the mechanism of detecting peak current and stopping output. When the output current exceeds the maximum current, the current can be detected in this way. However, when the output bias voltage is low, such as when the output is short-circuited, the maximum output current will not exceed the threshold, so the protection circuit will not work. Therefore, the disadvantage of peak current protection circuit is that the heating temperature of IC is higher than the junction temperature of chip, and even the chip will be damaged in the worst case. That is to say, if the current limiting circuit is used, it will lead to sound distortion; If the peak current protection circuit is used, there is no protection.
In order to solve this problem, ROHM Company has researched and designed a new over-current protection circuit which combines the advantages of these two circuits (patent application is pending). This technology will start the current limiting circuit to prevent the IC from heating when it does not need to output high power when it is started or undervoltage. During normal operation, it will automatically switch to peak current protection. This is a technology that combines the advantages of two kinds of protection circuits and can realize high power output.
The new product "BD 783 XEFJ-M" is equipped with this new over-current protection circuit, which can effectively protect IC from load short circuit and will not be distorted when outputting high power.
3-2.？ High reliability, supporting the harsh environment in vehicle application.
The new product conforms to the reliability standard AEC-Q 100 for automotive electronic products, and can support the working temperature up to? Ta= 105℃, so it can also be used safely in the field of vehicle applications that pursue high reliability. This series of products adopts power package (HTSOP-J8), which can achieve high power output even at the working temperature of 105℃ in class AB amplifier with high heat value. Although the package is a lead frame package, the size is very small, only 4.9mm×6.0mm× 1.0mm, but when using a four-layer circuit board (according to JedC51-5,7 standard), θJA is only 45.2℃/W, and the heat dissipation performance is excellent. Compared with the products before ROHM, the chip temperature is reduced by 80% (condition: VCC=5V,? RL = 8Ω，？ THD< 10%), even under the harsh conditions of Ta= 105℃, can output voice without destroying the function, which is impossible in previous packaging. In addition, in terms of function, in order to improve its reliability, in addition to the overcurrent protection circuit, other protection circuits are also equipped. When abnormal heating occurs, the temperature protection function can prevent IC from being damaged; When the battery is disconnected instantly, the undervoltage protection function can prevent accidental sonic boom. Therefore, this series of products is helpful to build a robust system that can be applied to various environments.
Figure 4. "BD 783xEFJ-M" conforms to AEC-Q 100 standard and has many protection functions.
3-3.？ Reduce the number of components through built-in resistors.
In the class AB amplifier 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 installation area of printed circuit board by constructing resistors. In addition, this series of products * * * has 1 1 model, and the gain range is 6dB~26dB (increment is 2dB), which can be fine-tuned. The product "BD78326EFJ-M" with 26dB will be used only in the sample evaluation with frequent gain adjustment. By adding an evaluation resistor to each input pin, the evaluation can be easily performed without replacing the IC, and even if the resistor is built in, the design man-hour will not be increased. At present, 6dB, 10dB and 26dB models have started mass production, and other products will be released one after another.
4. Future development trend
As mentioned at the beginning of this paper, multifunctional voice around the cockpit has become an inevitable demand, and in the future, with the chassis (connection,? Autonomy? Share? With the advent of the electric age, it is expected that this demand will further increase and there will be more demand for power output. In addition, in terms of interior layout, the boundary between traditional cockpit and car audio will disappear, and the use of voice will become more diversified.
In order to meet these requirements, Rohm needs to continuously expand the product lineup of speaker amplifiers. The BD 783xEFJ-M launched this time is a class AB loudspeaker amplifier for vehicles. At present, ROHM is developing a class D loudspeaker amplifier for vehicles. The power supply voltage of the target application product of BD 783xEFJ-M is below 5V, and the next vehicle-mounted Class D speaker amplifier product will support the connection with 12V battery, which can achieve a large output power of over 4W. For car speaker power amplifier IC supporting 12V power supply, although the mainstream product is class AB power amplifier at present, with the development of ECU (electric? Control? The requirement of saving space is getting higher and higher, and the radiator for heat dissipation is becoming a technical bottleneck. In order to solve this bottleneck problem, ROHM adopted class D amplifier system in the loudspeaker amplifier, which made great contribution to miniaturization.
As the first wave of vehicle-mounted class D loudspeaker amplifier, like BD 783xEFJ-M, in the product lineup, it is planned to introduce four products, including digital input type and multi-channel type, to meet the needs of various application products. For example, the digital input type supports TDM (time? Organization? Multiplex) format. 1 The system can connect up to 8 speakers, which increases the options of application design and improves the flexibility of application design.
The second wave of car-mounted class D speaker power amplifier is planned to be 202 1 for sale. In the future, Luomen will continue to devote itself to the development of car speaker amplifier IC and contribute to the improvement of car safety and comfort.
This article comes from car home, the author of the car manufacturer, and does not represent car home's position.