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In portable and miniaturized consumer products, the use of Class D audio power amplifiers has become very common. This paper introduces the design principle of the output low-pass filter of the class D audio amplifier, and gives the calculation method and the considerations of the selection of the inductance and capacitance values ​​in the filter. This article also uses National Semiconductor's Class D audio amplifiers LM4668 and LM4680 as an example to describe the specific output filter design method, and introduces the circuit block diagram and characteristics of the upcoming LM4681.
Audio signals in electronic systems have historically been represented by analog electrical signals. Although digital processing and digital amplification techniques have been used in today's systems, audio/sound signals must be converted back to analog signals to meet the needs of the human auditory system to listen to music.
Figure 1: The composition of a single-chip Class D audio amplifier. |
Currently, switching mode (Class D) audio power amplifiers are common in most portable and miniaturized consumer products such as MP3, portable DVD and flat panel displays. Due to the lower power consumption of the Class D amplifier, higher efficiency can be achieved. It extends the battery life of portable devices and reduces the size of the heat sink and the area of ​​the PCB, saving system cost. Therefore, many large flat panel displays and consumer audio products are more willing to use such amplifiers.
However, Class D amplifiers are based on digital modulation techniques that use high switching frequency signals to achieve efficient amplification of the signal. The modulation frequency is typically up to hundreds of kHz, which is well beyond the audio range.
Since we need to recover the desired real audio signal (music) from the digitized or modulated signal, an output low pass filter is required to filter out the high frequency components to reproduce the real analog signal that matches the human auditory system.
Here, we will elaborate some considerations and recommendations for the output low-pass filtering design.
Figure 2: BTL half-circuit model. |
Class D Amplifiers: Monolithic Class D audio amplifiers include analog audio inputs, modulators, power transistors, etc. (see Figure 1).
Output Filter Design: Since we need to recover the required audio signal, it is important to design an excellent output low-pass filter to filter out high frequency components (unwanted signals) and obtain high quality analog sound. We must design an output filter with a specific reactive output impedance to match the load impedance. The BTL half-edge circuit model is shown in Figure 2.
The output filter of a Class D amplifier is typically a second-order, LC-type Butterworth filter. This is because the Butterworth filter provides a relatively flat passband frequency response and requires a small number of components. Here is a reference graph showing the LPF response of the Butterworth, Bessel and Chebyshev type filters (Figure 3).
Calculation of inductance and capacitance values: The general transfer function of the second-order Butterworth filter is:
use? ? Instead of inductors and capacitors, substitute for the S domain. The transfer function becomes:
Used to simplify these equations and conclude that:
Figure 3: Comparison of low-pass filter responses of Butterworth, Bessel, and Chebyshev filters. |
For an actual BTL circuit, the output filter is shown in Figure 4.
The derived BTL filter equation is:
The 3dB cutoff frequency of the LC filter is:
According to the above equation, Table 1 lists the inductance (L) value and capacitance (C) value corresponding to the specific f c and R L .
Inductor selection: In the output filter, the inductor is the key component. It is related to the DC resistance and rated peak current specifications of Class D audio power amplifier systems. The DC resistance reflects the efficiency of the total output power. The efficiency of the system can be estimated by:
Where: R L is the DC resistance of the speaker, R DSON is the transistor on-resistance of the output driver inside the Class D amplifier; R IND is the DC resistance of the inductor.
Figure 4: Actual BTL circuit output filter. |
In addition to choosing the appropriate inductor value to achieve a particular cutoff frequency, the maximum DC resistance of the output inductor is another key parameter that affects overall efficiency. Therefore, it is strongly recommended to use an inductor with a lower DC resistance.
Another important parameter that must be considered for an inductor is its maximum rated current. If the inductor's rated current is not sufficient to maintain the output current of the device, the inductor will short-circuit. This will cause damage to the device or speaker due to large currents.
Figure 5: Application block diagram of the LM4680 (determination of the value of the LC output filter). |
Finally, it is worth mentioning that in order to reduce distortion, EMI and crosstalk, shielded inductors (eg pot core inductors) are recommended.
The pot core is known for its excellent shielding performance because the inductor coil is completely surrounded by the core except for the two narrow slots used to traverse the wire.
Capacitor Selection: One of the most important parameters in evaluating high-frequency chip capacitors is Q (quality factor), or the associated equivalent series resistance (ESR).
Simply put, ESR is a measure of all series and parallel losses in a capacitor at a given frequency. In theory, the "ideal" capacitor will have an ESR of 0 Ω and is purely reactive, with no real (resistive) component. The current flowing through the capacitor will just exceed the voltage across the capacitor by 90° at all frequencies. But in reality, the capacitor will always show a certain degree of ESR.
Figure 6: Package size of the TOKO (A7503HY-270M) inductor. |
The quality factor Q is a dimensionless value that is equal to the quotient of the capacitance reactance and the parasitic resistance (ESR) of the capacitor.
Since both reactance and resistance change with frequency, the Q value will vary greatly with frequency. The reactance of the capacitor fluctuates greatly with changes in frequency or capacitance, and therefore causes a significant change in Q.
Metal film capacitors maintain high temperature, frequency and voltage stability. In common audio systems, it is highly recommended to replace the ceramic capacitor with a metal film capacitor. At the same time, when using capacitors, another parameter called “rated voltage†must also be considered to ensure that the capacitor has no fault expectation during its useful life.
Rated voltage: The rated voltage of the capacitor is calculated by:
In order to obtain better output signals and overall performance from the amplifier, the output filter design is undoubtedly a crucial factor, but power supply filtering will also be an important issue worthy of attention.
Power supply filtering in a Class D amplifier has two purposes.
1. Isolate the Class D amplifier from power supply noise.
2. Bypass the high frequency noise. In a Class D amplifier device, there are at least two sets of power supplies, namely analog input and control (AVDD) and output transistor drive (PVDD).
Figure 7: Block diagram of the internal circuit of the LM4681. |
In order to achieve decoupling capacitors, we must consider the peak switching current to get a minimum capacitance. The minimum effective capacitance for the peak switch current can be calculated as:
Where: for the period, D MAX is the maximum duty cycle, and V RIPPLE is the ripple voltage.
ESR causes ripple voltage in most cases. The maximum V RIPPLE generated by ESR and I PEAK is:
From the above equation we noticed that ESR will have an effect on the effective capacitance of the capacitor. It is recommended to connect two or more capacitors in parallel to reduce the ESR for different frequency ranges. Two different types of capacitors are usually used. Generally, electrolytic capacitors or tantalum capacitors with higher capacitance values ​​are used for low frequency filters (less than 10 kHz), and a small capacitor ceramic capacitor with parallel capacitance is used for high frequency filtering (> 300kHz).
For Class D audio amplifiers, National Semiconductor has introduced the 10W mono Class D LM4668 and LM4680. These products require only a small number of external components, giving engineers an easy and complete solution for audio products. The LM4668 and LM4680 feature a balanced, floating modulator design to eliminate substrate noise. The balanced modulator's PWM output is used to drive the gate of the LM4668 or LM4680's H-bridge configuration output power MOSFET. The pulse sequence will be applied to an output LC filter to eliminate unwanted high frequency signals. The modulators of the LM4668 and LM4680 have a nominal switching frequency of approximately 450 kHz.
The application block diagram of the LM4680 (see Figure 5) is given below, and the value of the LC output filter has been determined (Table 2).
The relationship between the derived component and the cutoff frequency (F c ) is:
The values ​​of the two inductors of the filter are equal to:
The values ​​of the three capacitors are equal to:
It is recommended to use a second-order LC output filter with the above specified parameter values ​​for the output filtering of the LM4680 (for an 8Ω load); a nominal cutoff frequency of 47kHz can be obtained. It ensures that the attenuation at 20kHz is much less than 3dB.
Table 1: The inductance and capacitance values ​​corresponding to specific f c and R L are listed in the table. |
Advice on inductance
When driving the load to maximum power dissipation, the output filter inductor must have a maximum current rating that is higher than the amplifier's maximum output current. Therefore, when delivering 10W of output power to an 8Ω load, the maximum output current may be around 1.1A (RMS), so the inductor should be rated for at least 1.2A (RMS) to prevent any saturation of the inductor. Shielded inductors are recommended to better suppress EMI. For example, TOKO (A7503HY-270M) inductor (see Figure 6).
The LM4668 and LM4680 can be used in a variety of applications, including LCD displays, televisions, computer sound cards, multimedia speakers, and broadcast systems. Both devices are capable of delivering 6W BTL output power (0.2% THD) to an 8Ω load or 10W output power (less than 10% THD) to an 8Ω load.
The LM4668 and LM4680 provide short-circuit protection, thermal protection, and overmodulation protection. As such, they are ideal for implementing high quality and high stability Class D audio amplifiers in your system. The LM4668 is available in TSSOP-20 and LLP-14 packages, and the LM4680 is available in LLP-14 packages.
Table 2: Values ​​of the LC output filter of the LM4680. |
The BTL output 10W Class D stereo device LM4681 based on the same Class D algorithm will be available soon. In addition to the 10W stereo channel that drives an 8Ω load, the LM4681 offers a stereo headphone driver and an I2C/SPI optional control interface for shutdown and 32-level volume control. It has a wealth of features for many applications. The internal circuit block diagram of the LM4681 is shown in Figure 7.
Because the I2C and SPI control interfaces are widely used in different types of systems, it will facilitate engineers to design on the system hardware and build the system with fewer external components. The LM4681 is capable of delivering 10W of output power per channel (less than 10% THD) to an 8Ω load, or 80mW per channel (<0.5% THD) to a 32Ω headphone. When driving the speakers, the volume control range is +30dB to -48dB; when the headphones are driven, the volume control range is +13dB to -65dB. The device is available in an LLP-48 package.
Summary of this article
In this digital world, many systems and products have been digitized, and Class D amplifiers are common audio amplification techniques based on this principle. It provides higher efficiency and is therefore suitable for a slimmer design and saves more power. In this latest electronics industry, National's Class D audio amplifiers are perfect for providing high quality and stability for systems in a wide range of applications.
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