Application and discussion of audio systems in mobile phones and PDAs

Audio Power Amplifier Considerations in Portable Products

1) Higher PSRR

It must have a high Power supply rejection ratio (PSRR) to avoid interference from power supply and wiring noise.

2) Fast turn on & off

Having a long standby time is a basic requirement for mobile phones or personal digital assistants. The efficiency of class AB audio amplifiers is about 50% to 60%, and the efficiency of class D audio amplifiers can reach 85% to 90%. Regardless of the type of audio amplifier used, in order to save power consumption, the audio amplifier must enter the standby state when it is not needed. However, when there is a sound, the audio amplifier must immediately enter the power-on state.

3) No "Click & Pop"

"Switching noise" often occurs when the audio amplifier is turned on and off, or when it returns to normal state from standby, or when there is a 217 Hz mobile phone communication signal. Users of mobile phones or personal digital assistants never want to hear annoying noise, so adding a "Click & Pop" elimination circuit to the audio amplifier is a must.

4) Lower operating voltage

In order to increase the battery life, it is often necessary to lower the voltage to 1.8V and still work.

5) Low current consumption and high efficiency

ICs using CMOS technology can reduce current consumption. Sometimes it is necessary to choose a Class D audio amplifier in order to extend the working time of a mobile phone or personal digital assistant.

6) High output power

It has higher output power at the same operating voltage, that is, the closer the swing of the output signal is to Vcc and GND, the higher the output power.

7) Smaller package (uSMD)

As mobile phones and personal digital assistants become smaller and smaller, IC packaging technology becomes more and more important. uSMD is a packaging technology that is commonly used today.

Output power calculation

The single-end amplifier is shown in Figure 1, and its gain is:

Gain = RF/Ri

Rf: feedback resistance, Ri: input resistance

Output power = (VRMS)2/Rload, VRMS = Vpeak /21/2

Therefore, the output power of a single-ended amplifier = (Vpeak)2/2Rload

The bridge-tied (BTL) amplifier is shown in Figure 2. It consists of two single-ended (Single-end) amplifiers with a phase difference of 180, so its gain is:

Gain = 2Rf/Ri

Rf: feedback resistance, Ri: input resistance

Output power = (VRMS)2/Rload, bridge-connected VRMS = 2 Vpeak /21/2

Therefore: Bridge-tied output power = 2 (Vpeak)2/Rload = 4 Single-ended amplifier output power

Figure 2 Bridge-tied amplifier and waveforms acting on the positive and negative terminals of the speaker [page]

Selection of Input and Output Coupling Capacitor Values

As shown in Figure 1, the input resistor and the input coupling capacitor form a high-pass filter. If you want to get a lower frequency response, you need to choose a larger capacitor value. The relationship can be expressed as follows.

fC = 1/2 (RI)(CI)

fC: high-pass filter cutoff frequency, RI: input resistance

CI: Input coupling capacitor value, this capacitor is used to block DC voltage and couple the input signal to the input of the amplifier.

In mobile communication systems, due to volume limitations, even with a larger input coupling capacitor value, the speaker usually cannot display a frequency response below 50Hz. Therefore, assuming the input resistor is 20K, the input coupling capacitor value only needs to be greater than 0.19 F. In this case, 0.22 F is the most appropriate choice.

As for the setting of the output coupling capacitor value, as in Figure 1, if you want to get a better frequency response, you need to choose a larger capacitor value. The relationship can be expressed by the following formula:

fC=1/2(RL)(CO)

fC: high-pass filter cutoff frequency, RL: speaker (headphone) resistance, CO: output coupling capacitor value

For example, when using 32 Ω headphones, if you want a 50 Hz frequency response, you need to choose an output coupling capacitor value of 99 F. In this case, 100 F is the most appropriate choice.

Thermal Considerations

When designing a single-ended amplifier or a bridge-tied (BTL) amplifier, power consumption is one of the main considerations. As the output power to the load is increased, the internal power consumption also increases.

The power consumption of a bridge-tied (BTL) amplifier can be expressed by the following formula:

PDMAX_BTL = 4(VDD)2/(2 2RL)

VDD: power supply voltage applied to the bridge-tied (BTL) amplifier, RL: load resistance

For example, when VDD=5V, RL=8, the power consumption of the bridge-tied amplifier is 634mW. If the load resistance is changed to 32, its internal power consumption is reduced to 158mW.

The power consumption of a single-end amplifier can be expressed by the following formula:

PDMAX_SE = (VDD)2/(2 2RL)

VDD: power supply voltage applied to single-end amplifier, RL: load resistance, that is, the power consumption of single-end amplifier is only one-fourth of that of bridge amplifier. The temperature rise is the sum of all power consumption divided by the thermal resistance (JA) of IC.

Layout Considerations

Designers must follow some basic guidelines when wiring, such as

1) All signal lines should be grounded at a single point as much as possible.

2) To avoid mutual interference between the two signals, parallel routing should be avoided and the wiring should be done in a 90° crossing manner.

3) Digital power supply and grounding should be separated from analog power supply.

4) High-speed digital signal routing should be kept away from analog signal routing and should not be placed under analog components.

Application of 3D enhanced stereo

Most people think that "3D sound" is neither mono nor dual channel. It is an audio processing technology that allows listeners to feel the location of the sound in an unreal environment. This requires great attention to the placement and number of speakers. However, in mobile phones and personal digital assistants, it is impossible to place so many speakers. Therefore, two speakers plus hardware or software have been developed to simulate "3D sound", which is the so-called "3D Enhanced Stereo Sound".

FIG3 is a block diagram of an audio subsystem for 3D enhanced stereophonic sound, which is used in a stereophonic mobile phone or a personal digital assistant. The audio subsystem consists of the following components:

1) The power amplifier section includes a stereo speaker driver, a stereo headphone driver, a mono headphone amplifier (earpiece) and a line out for a hands-free handset (e.g. a car's hands-free handset phone output).

2) Volume control: 32 levels of volume control are available, and the volume of the left, right and mono channels can be controlled independently. [page]

3) Mixer, used to select the relationship between output and input audio sources, can transmit and mix stereo and mono inputs, and divide these inputs into 16 different output modes, so that system design engineers can flexibly transmit mixed mono and stereo audio signals without limiting the signal to be transmitted only to stereo speakers or stereo headphones.

4) Power control and "switching/switching noise" suppression circuit.

5) 3D enhanced stereo uses hardware.

6) Use the I2C compatible interface to control the chip functions.

When sound reaches the left and right ears at different positions, different phase differences will be generated. Using this phase difference principle and hardware method, 3D enhanced stereo sound effects can be simulated. Even if the system is limited in volume or equipment and the left and right speakers must be placed very close, various problems in the positioning of the high and low parts of the stereo can still be improved.

Figure 3 Block diagram of 3D enhanced stereo audio subsystem

As shown in the 3D enhanced stereo block diagram of FIG. 3 , an external resistor and capacitor circuit is used to control the 3D enhanced stereo sound effect, and two independent resistor and capacitor circuits are used to control the stereo speakers and stereo headphones, so that the best 3D enhanced stereo effect can be achieved.

In this resistor and capacitor circuit, the "amount" of 3D enhanced stereo effect is set by the R3D resistor, and they are inversely proportional. The C3D capacitor is used to set the 3dB low-frequency cutoff frequency of the 3D enhanced stereo effect. The 3D enhanced stereo effect can only be seen above the low-frequency cutoff frequency. Increasing the C3D capacitor value will reduce the low-frequency cutoff frequency. The relationship can be expressed by the following formula:

f3D(-3dB)=1/2 (R3D)(C3D)

in conclusion

As mobile phones and personal digital assistants have developed into multifunctional portable devices that can provide a variety of entertainment, manufacturers try to use high-fidelity audio systems and long-life batteries, and make these portable electronic products have stereo speaker amplifiers, a variety of different mixing, and 3D enhanced stereo functions, while also trying to be as light, thin and compact as possible. However, its design scope still does not deviate from the basic principles mentioned above, which is another purpose of this article.