Portable Audio Power Amplifier Solution System

First, single-ended (SE) output vs. Bridge (BTL) output

There are two common output structure, SE (Single-ended) mode and the BTL (Bridge-Tied-Load) mode, see Figure -1.

Portable Audio Power Amplifier Solution System

Figure -1 single-ended mode and bridge mode

In portable systems, the common DC power supply voltage, (+1.8 V, +2.5 V, +3.3 V) is usually no more than +5.0 V, if a single-ended structure, the output of the peak - the maximum peak voltage is only 5.0 Vp-p V, in fact the output level as the upper and lower voltage drop of the pipe, without being subject to clipping, Vp-p maximum is only about 4.5V, so a portable system RMS Audio Power Amplifier Solution = Audio Power Amplifier Portable System solution = 1.59V, all added to the RL = 4ohm load, output power portable audio power amplifier system solutions = 0.63W. Therefore, single-ended structure, not the output power of 2W.

If the BTL output structure, Vp-p, you can reach 8.0V, RMS Audio Power Amplifier Portable System Portable System Solution = Audio Power Amplifier Solution = 2.828V, added to the RL = 4ohm load, the power output of portable audio system amplifier solution = 2.0W.

Therefore to VCC = 5.0V Output 2W or so under the conditions of power, but to use bridge output structure. Therefore, the structure is often used to drive single-ended headphones, while BTL structure used to drive the speakers. See below -2 AA4002 typical application schematic.

Portable Audio Power Amplifier Solution System

Typical application diagram Figure -2 AA4002

From the image above to see the drivers headset, also requires a large electrolytic capacitor, its role is

① partition DC reference voltage Vbias (1/2VCC). If there is no blocking, flow through the DC voltage will be directly behind the speaker coil, cone end balance of the bias, Vbias is too large or even damage the coils.

② AC coupling the audio signal, load it with speakers form the first-order high-pass filter, shown in Figure -1. By the classical formula (1) known, the size of the impact of low-frequency capacitance cut-off frequency fc the Department.

Portable Audio Power Amplifier Solution system Equation (1)

The greater the capacitance Co, the lower the cutoff frequency fc, which means a lower frequency can be coupled to the load, shown in Figure -3.

Portable Audio Power Amplifier Solution System

Figure -3 frequency response under different coupling capacitors (RL = 16ohm)

BTL structure is without coupling capacitors, saving system cost and PCB space savings, improved low frequency response.

Moreover, BTL structure of output, it can effectively suppress common mode noise. The same output power conditions, the bridge model is smaller than the noise single-ended mode, the noise, see below -4 contrast, channel 1 (Blue) - across the load, channel 2 (green) - Power . This is because the same impact will appear in the output structure'+','-' both ends of the bridge, through the load, will cancel each other out, not the speaker acting, can not hear "POP" sound, such a structure for the electricity, power-down noise, operation noise suppression are very good.

In fact, about how much output power usually need to specify the conditions, such as power supply voltage [VCC], the output load [RL], harmonic distortion [THD + N]. Only after these conditions are established, the output power makes sense. In the product specifications book, usually the following chart, the output power vs. The supply voltage, see Figure -5, output power vs. Load, Figure -6, and output power vs. Harmonic distortion + noise, Figure -7.

Portable Audio Power Amplifier Solution System

Figure -5 AA4002 Po vs. Supply voltage with different THD + N

Portable Audio Power Amplifier Solution System

Figure -6 AA4002 Output Power vs. Load Resistor

Portable Audio Power Amplifier Solution Systems

Figure -7 AA4002 THD + N vs. Output Power

2, Bass Boost

For low-power speaker, due to size restrictions, they are usually very poor low frequency response, while the human ear has chosen to reflect the low frequency is not sensitive to the music. Electronic circuit on the need to find ways to solve this problem. Bass boost, its implementation is in the feedback loop, by increasing the capacitance, to achieve the low frequency part of the gain is greater than the passband gain, equivalent to loop the system adds a pole, a zero, in a typical application of AA4002 circuit, the capacitor C2, C11 is the capacitance of this

So it's ideal transfer function, portable audio power amplifier system solution formula (x)

Pole, portable audio power amplifier system solutions

0:00, portable audio power amplifier system solutions

Suppose R1 = R2 = R3 = 20K, C2 = 0.068uF, you can calculate fZERO = 234Hz, fPOLE = 117Hz, Bode plots are as follows. The amplitude and phase of this paragraph only for bass boost, if BTL output, the gain will be doubled (6dB), if the signal input from the PIN10/12 feet, after two RP, then the output, input phase close 0DEG, concrete can refer to the AA4002 Design tools.

Portable Audio Power Amplifier Solution System

Figure -8 Bass boost part of the amplitude and phase curves

Adjust the size of the value of its content, you can adjust the gain and the gain of the size of the inflection point position. Figure -9 actually measured for the AA4002 bass enhancement curve.

Portable Audio Power Amplifier Solution System

Figure -9 AA4002 capacity values of different bass boost curve

3, POP noise

POP noise is in the audio system, power, instantaneous power, and power stability, the various actions brought by the transient impact of the explosion that formed. This is the user does not want to hear, system designers always try to avoid it. Bridge structure mentioned above can be effectively inhibited the POP noise. In addition, there are other commonly used methods.

1. For as shown in Figure -1 single-ended structure, reducing the capacity of the value of Co can "POP" impact to smaller, narrower pulse width of the impact of its energy spectrum are mostly in high frequency, the same can be reduced to heard the noise. In Figure -10, the capacitor Co were 10uF, 47uF, 100uF, 220uF impact of the situation in the switch machine. Can see that when the Co is reduced to a certain value, and then reduce its value by the noise suppression effect is caused by a small, but according to the formula (1) Because the capacitance of Jianxiao brought the higher the cutoff frequency fc is indeed obvious, see Figure -2 above. Therefore, designers need to weigh to make a compromise choice. Usually good quality headset itself has a better low frequency response, for AA4002, recommended to choose the coupling capacitor 100uF.

Portable Audio Power Amplifier Solution System

Figure -10 under different coupling capacitors "POP" sound of impact

2. Another AA4002 has a pin22, Vref, it is the internal dc reference voltage, that is, the work order to the internal circuit, the bias voltage must be established. Usually in the application of an external bypass electrolytic capacitors on the ground, play a role in filtering noise. Its voltage is about equal to 1/2Vdd. Increase the capacity of the capacitor value, but also can reduce the "POP" noise. When the chip or from the standby power switch to the working state, the DC bias voltage has been established gradually increased from 0V on Vbias filter capacitor charge, after a certain time to build up time, the voltage rises to 1/2Vdd, chips can , the output of the audio signal is based on the DC voltage up and down. Similarly, when the chip power-down or standby state, the filter capacitor discharge bias began to decline from 1/2Vdd until 0V. Experimental results show that the chip power, power-down of the "POP" sound is caused by bias voltage transition moment, the plan -11 is the simulation results, red-Vbias voltage; Blue Line - single-ended mode load (after the coupling capacitor, Figure -1 the left, Co = 220uF, RL = 16ohm). If Vbias transition of slow, POP impact will be smaller, see Figure -12. We see the impact rate dropped, POP Sound also smaller. Vbias up, slows down, that increase the reference voltage transition delay. Assume that the current charging and discharging filter capacitor is constant, this process is simplified into a first-order RC model. Delay by the classical formula (2), from 10% → 90% of the time required.

Therefore, increasing Vbias filter capacitor DC voltage reference can slow down the rise and fall action, play a reduced "POP" sound effect. Figure -13 is AA4002 increased capacitance, the reference voltage slows the comparison chart. Blue Line - the supply voltage Vdd, the Green Line-Vbias voltage (assuming Vdd = 5.0V, Vbias = 2.5V).

Portable Audio Power Amplifier Solution System

Portable Audio Power Amplifier Solution System

Figure -13 Vbias under different bypass capacitor voltage (above, Cbias = 1.0uF. Under the plan, Cbias = 2.2uF)

Note that this filter capacitor is too large, the chip will establish over time the voices of people feel "a long time," no output. Capacitance is too large will also make an important indicator of the audio system - harmonic distortion + noise (THD + N) worse, do not explain in detail the reasons here, please refer to the corresponding value when the data manual to make a compromise choice.

3. AA4002 also provides two very useful functions MUTE, SHUTDOWN. MUTE effective, the mechanism is greatly attenuated incoming signal input; the STB when work is off the Vbias bias circuit, which is the audio circuit static part of the most power to further reduce quiescent current, reducing system standby power consumption to extend battery life. For AA4002, the standby mode quiescent current ICc = 0.7uA (Typ.).

Can see from the above discussion, alone STB, as Vbias transient, will inevitably lead to "POP" sound. If this feet, the proper use of a certain order, you can effectively inhibit the switch machine noise, shown in Figure -14. Chip power, the first to MUTE, STB effective until the power supply stabilized, the first release of STB, and then release the MUTE. When the power-down operation, in preparation for power-down before making MUTE effective, after making STB effective until Vdd becomes 0V. This is because the operation is usually caused by MUTE "POP" sound is far less than the STB operation the "POP" sound.

Portable Audio Power Amplifier Solution System

Figure -14 power, power-down and the STB when the correct timing MUTE

Figure -14 is prone to misunderstanding, STB MUTE operation of the whole role of being covered, is not without STB, it can also suppress the noise? Yes, regardless of the state of STB, only use MUTE executed in the order in accordance with Figure -14, indeed can inhibit the "POP" sound. Note that the chip in the power process (from 0V → Vdd), the power only to a value less than the voltage Vdd, Vbias will jump from 0V to 1/2Vdd, this time the power has not yet stable, Vdd will control the load through the output driver can not produce a forecast, the impact of random noise. If Vbias not been established at this time, still 0V, this impact will be minimal, at least in Figure -14 can inhibit the operation of the power transient caused by the impact of "POP" sound, such as power supply stabilized, Vbias is also the impact of only from 0V → 1/2Vdd, rather than 0V → Vdd power due to jump. But in fact the market some amplifiers, the DC reference input and output DC voltage reference are two separate, STB effective, the output Vbias not jump. MUTE is valid even if the state, but only the MUTE input is grounded, the output Vbias impact will still pass through the coupling capacitor Co to the load. Whatever the case, from the perspective of noise suppression, we always want to output Vbias is slowly changing, it is best to keep fixed, and is always 0V. Therefore reduce the POP sound, is to prevent the DC transients.

So for the POP noise, more difficult to address is the chip power, power-down occurs POP sound. The actual system is also true, 當 Vdd Moyouzhihou, 可能 mean that the whole system also has Shi Qu Le Dian Yuan, MCU Yijing Bunenggongzuo, I / O Zhuangtai loss of control, Wu Fa complete the Caozuo Figure -14. However, there are still some ways to solve this problem.

Suppose there is a delay circuit unit, the power, the need to go through after a fixed delay, Vbias began to rise slowly until the stable, from low to high increases in delay time tpLH; when the chip power-down, then difficult to further delay time to start to decline, but if you will discharge the equivalent resistance increases, and still can be dropped from high to low delay tpHL increased to achieve better inhibitory effect.

Portable Audio Power Amplifier Solution system Equation (3)

Portable Audio Power Amplifier Solution system Equation (4)

Figure -15.

Portable Audio Power Amplifier Solution System

Figure -15 change in timing delay circuit Vbias

Conclusion: Although several measures mentioned above, but are not isolated problems encountered in practical application, to find produce "POP" sound of the main reasons, needs to integrate, to select the most targeted, most economical solution .

4, volume control

Audio power amplifier for driving speaker, volume control is a very useful feature. Common methods of linear volume control on the number of two ways. The figure for the standard linear, logarithmic decay curve, because the vertical axis for the gain to dB that has been done on the number of operations, so left for the linear attenuation, while a few are shown here on the attenuation.

Portable Audio Power Amplifier Solution System

Portable Audio Power Amplifier Solution System

Figure -16 volume attenuation methods: linear (above), log (below)

AA4002 PIN7 by changing the DC level to achieve the volume attenuation, Figure -17 actual volume for the AA4002 attenuation curve for the piecewise linear attenuation, the attenuation of each stage of which is somewhat different.

Portable Audio Power Amplifier Solution System

Figure -17 AA4002 volume attenuation

0dB to-6dB, 1dB/step

-6dB to-36dB, 2dB/step

-36dB to-47dB, 3dB/step

-47db to-51dB, 4dB/step

-51dB to-66dB, 5dB/step

From-66dB up to-78dB, 12dB/step.

Conclusion, portable system for audio power amplifiers, BCD Design Release now have two ---- AA4001, AA4002. This two pin-pin compatible, which is a 1Wx2CHs stereo power amplifier AA4001, AA4002 is 2Wx2CHs power amplifier, using TSSOP-28-PAD package with heat sink bottom.

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