How to select class D Stereo Audio Amplifier IC
Time:2022-09-10
Views:1726
Choosing the right device for electronic products is a very complex thing, which often requires repeated compromises in many aspects, such as class D audio amplifier.
The following describes the process of selecting such amplifiers through a hypothetical application, such as a mobile phone or a personal media player (PMP) base powered by a wall power adapter or an alkaline battery.
Marketing needs
Marketing personnel may put forward many specific suggestions for products, ranging from a very small detail (for example, stereo speakers should be used) to a bunch of guidelines that require products to be comparable to competitive products. Market personnel may also propose to make the product meet the performance standards of the industry (such as Microsoft Windows Vista or Dolby Digital Certification).
In our project (PMP base or external speaker), we assume that the marketing personnel require that when the product is powered by batteries, the sound should be as loud as possible and the working time should be as long as possible. Moreover, when the battery is running out, the performance should be moderately reduced. Two 8 Ω speakers with a diameter of 50mm and a power of 3W are used. We further assume that the market personnel have not put forward more opinions on the transmission bandwidth, linearity or signal-to-noise ratio (SNR) of the base.
Circuit constraints
If the product has two power supplies, one is a regulated 12V wall power adapter, and the other is an optional alkaline battery pack (the nominal voltage when using a new battery is 9V, and the voltage when the power is almost exhausted is 6V).
output
Power determines the topology of the output stage required by the amplifier. In order to provide 3W power per channel to an 8 Ω load, a supply voltage of about 4.90vrms or a peak voltage of 6.93v is required. Since the single end (half bridge) layout cannot generate the required power even under the best power supply conditions, the BTL (full bridge) topology must be adopted.
Further, even the BTL amplifier cannot generate the 6.93v peak voltage required to maintain the 3W output at the worst (6V) supply voltage. However, this may not be a problem, because it is generally impossible for a battery that is nearly fully discharged to provide the current required for two 3W speakers. The final product will be able to realize the characteristics of "mild degradation" described by the marketing personnel.
Physical / mechanical constraints
The efficiency of class D audio amplifier is much higher than that of linear (class AB) amplifier. In other words, if a given output power is to be provided to the load, the power wasted by the class D amplifier in heat dissipation is far less than that of the class AB amplifier with the same output power.
It is necessary to check whether a special thermal design is required. Suppose that there is now a suitable Stereo Class D amplifier, which can operate at 4.5V, powered by a normal power supply, and can output 2 × 3 W of sufficient power.
At 2 × Under the condition of 3W output power (driving two channels), the relationship between efficiency and output power is shown in the figure. It can be seen from the figure that the typical efficiency of the device is 82%. After having this index, the heat dissipation power under these conditions can be obtained from the following formula.
η= Pout/Pin=Pout/(Pout+Pdiss)
Pdiss=Pout × (1- η)/η
here, η Is a percentage.
Relationship curve between efficiency and output power
Fig. relationship curve between efficiency and output power
Stereo output is 6W (2 × 3W), the efficiency is 82%, and pdiss = 1.32W can be calculated, which is not a small number. Using the package recommended by the device, it is not easy to dissipate heat, so the only way to dissipate heat is through copper plating on the PCB.
Therefore, in the PCB layout stage, attention should be paid to ensure that the pins exposed under the devices are connected with the PCB board and connected to the copper grounding on the solder side through multiple paths. If cost and packaging constraints allow, the copper surface exposed on the PCB should be as large as possible to dissipate the heat generated by the amplifier.
At this time, the evaluation board of class D amplifier is very helpful. The PCB layout and placement position of components provided by the manufacturer can be used as a good reference, and the temperature rise of the device under load can be known through laboratory tests, and the device will not be turned off or misoperated at high temperatures.
EMI constraints
The output stage of the class D amplifier will be quickly switched on and off between the power rails, so as to achieve the unique high efficiency of such devices. For other equipment and circuits placed near the amplifier, the EMI generated by this rapid rise and fall time is very harmful. Although the detailed analysis of how to make a compromise to meet the RF radiation standard when using class D amplifiers is beyond the scope of this article, some guidance for reference is listed below.
First of all, it is necessary to understand the radiation standards that must be met by the product and the margin that we want to leave on the premise of meeting the standards.
Do not assume that the switching frequency of the class D amplifier used is 300kHz, and it will not encounter 30MHz or above. Radiation beyond this frequency band is often detected. EMI radiation that is difficult to eliminate will occur in the audio frequency band 100 times the basic switching frequency.
The speaker lead is actually an effective RF radiation antenna. The longer the lead, the lower the frequency of EMI problems.
If an LC filter is used in front of the speaker with a cutoff frequency of 20kHz, it is necessary to check whether the inductance can work normally within the entire EMI radiation frequency range.
If there is enough space and sufficient PCB area in the base shell, there can be multiple filter design schemes when designing the pcb prototype, so that different schemes can be tried when EMC testing the product to achieve the required performance.
Some amplifier suppliers have released the RF radiation performance test results of the Evaluation Kit under the given cable length and filter element conditions, which greatly facilitates engineers to start designing. In order to reduce EMI from the source, most new class D amplifiers use patented modulation technology to ensure audio quality and efficiency.
Supply chain constraints
Unfortunately, after carefully selecting a device for a product, many non engineering factors have to decide whether to adopt such a device. Here are some possible problems.
● has the supplier passed the procurement certification?
● are the technical support personnel of the device supplier competent?
● does the follow-up cost meet the target requirements?
● are you confident that the supplier can provide stable and reliable products in the required quantity?
Detail design
In any audio system design, fine appearance can increase (or decrease) the overall experience of the user to the product. When selecting the gain of each audio component, the overall gain structure shall be able to achieve high volume under the condition of avoiding speaker clipping distortion. But the gain should not be too large to avoid noise exceeding an acceptable volume level.
The transient noise during startup and shutdown should be minimized, but the product, price and user experience will change to suppress the noise. The performance of suppressing clicking sound is ultimately determined by the amplifier. Some manufacturers have given typical click suppression parameters in the data sheet, so that different devices can be compared before hearing tests.
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