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Designers are currently experimenting with low-power audio amplifiers with negative swing performance to achieve higher audio performance than traditional single-ended amplifiers, and a zero-volt-biased audio amplifier directly drives multiple speakers through a negative swing switch. An analog switch with a negative swing function and a zero-volt offset audio amplifier allow multiple speakers to share the source or transmit multiple sources to a single speaker. The attractive value of both devices is reflected when the end user turns to a full-featured “telephone + MP3†player combination.
Portable product designers have traditionally relied almost exclusively on standard amplifiers with an output bias of V cc /2. This configuration is ideal for use with standard analog switches, which do not create clipping when the signal passes, maintaining high audio performance. An ac coupling capacitor is placed between the switch and the speaker to remove the dc bias voltage on the audio signal.
The new low-power, Class-D, zero-bias output audio amplifier/negative swing switch combination eliminates the need for AC-coupling capacitors, giving designers a number of system benefits while still providing greater speaker comfort. Power requirements. These benefits include lower cost, less board space, simplified bill of materials, and the ability to eliminate the main source of “POP†noise.
Advantages of a negative swing system
In an AC-coupled system, "POP" noise can be generated when switching from one source to another, or during power up of the amplifier. Since POP noise is generated when the capacitor voltage is abrupt, a low-impedance channel is used to allow current to pass before the charge on the capacitor reaches a steady state. At the moment of charging the capacitor, current spikes appear on the speaker, causing "POP" noise. One of the main advantages of the negative swing system is the ability to significantly reduce the "POP" noise and clicks associated with audio signal line routing.
Negative swing analog switches can reduce "POP" noise in several ways. The first and most important way is to remove the AC coupling capacitor. Figure 1 shows a conventional V cc /2 biased audio channel using an analog switch, where an ac coupling capacitor is required to change its bias voltage from V cc /2 to zero volts before the audio signal reaches the speaker. This type of application is common, making it easy to share a single source in a device with a single speaker.
Figure 1: Standard V cc /2 biased audio channel with AC coupling capacitors.
The second common application is to use a bidirectional characteristic of an analog switch to enable a single amplifier to drive multiple speakers. By connecting the source to a common port, you can use the switch to transmit audio signals between two different modes of output speakers, such as headphones and hands-free mode.
Figure 2 shows the same system configuration with a negative swing audio amplifier and a negative swing analog switch. Since the amplifier's audio output is already biased at zero volts, the ac coupling capacitor is no longer needed. The analog switch can pass the negative input signal, so there is no need to change the output bias of the amplifier to prevent clipping. Eliminating the ac coupling capacitor eliminates the charge and discharge effects of the capacitor and the current spikes that produce POP noise, effectively eliminating the main source of POP noise in the audio system.
Figure 2: Negative swing audio system without ac coupling capacitors.
Removing the coupling capacitor not only improves audio performance and quality, but also saves board space (coupling capacitors are generally bulky, mostly between 100 and 20μF). In addition to the negative polarity signal, some analog switches also have built-in termination resistors on unused ports to further enhance the noise suppression performance of the product by releasing charge accumulation due to parasitic lines or board capacitance on unselected ports.
Removing the capacitor is a major factor in reducing noise, and adding embedded termination resistors also helps to enhance POP noise rejection. In addition to the above pure audio applications, the negative swing feature also improves the design performance of multiple signal sharing.
The FSA2380 shown in Figure 3 is used to switch between full-speed USB, audio, and UART data. Because these switches can be powered from a ground-referenced standard power supply, they can handle not only negative swing audio signals, but also standard signals referenced to ground /V cc /2. This feature makes these products very versatile, allowing designers to easily share a common port for different types of signals.
Figure 3: The analog switch FSA2380 with negative swing performance can multiplex multiple signals.
A second advantage of the negative swing audio system is that it allows the designer to increase the voltage swing of the audio path over a specific supply voltage range. Since the system power supply is often limited to the maximum voltage that the battery can provide, the margin of variation is very small when using a standard audio signal with a V cc /2 bias. Negative swing amplifiers often use an internal charge pump that doubles the amplifier's output range and sets the center of the swing to zero volts.
For example, in a standard V cc /2 bias system, the output signal amplitude must be between zero volts and V cc . However, in a zero-volt bias system, the peak-to-peak value of the output amplitude can be effectively improved, and the system designer can easily transmit the zero-volt bias signal to the shared speaker using an analog switch with negative swing performance. Provides a louder volume. The amplitude of the audio signal can exceed 1V of the maximum battery voltage, and the total swing is 5.25V.
An important feature to consider when choosing a negative swing analog switch is the ability of the device to pass a negative voltage when using a ground-referenced single-ended supply. For example, Fairchild Semiconductor's negative swing devices are optimized for low power (typically less than 1μA) and do not include a charge pump, which can significantly save system power, which is a tight budget for power designers. The words are very important.
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