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This article will discuss the issue of providing high-power and high-quality audio for music-enabled mobile phones and how supercapacitors can overcome these problems. This supercapacitor can also achieve high-power LED flash photography without sacrificing the ultra-thin shape of the phone.Before discussing the issue, let's talk about the supercapacitor and its role in power management. The supercapacitor fills the power gap between the battery and the normal capacitor. It provides a higher trigger power than the battery and can store more energy than a normal capacitor. Supercapacitors can provide the required trigger power for peak power events (such as GSM/GPRS RF burst transmission, GPS data reading, music playback, flash photography, and video playback) and then accept battery recharging. Benefits include extended talk time, extended battery life, brighter flash and better music quality. Designers can also save space and cost because they only need to consider the battery and power circuits that meet the average power consumption, without having to worry about peak loads.
Audio quality and power issues in current music phone design
Current mobile phones typically use a Class D audio amplifier. These amplifiers use two pairs of FETs in an H-bridge circuit to control the speaker coils. The configuration is shown in Figure 1. When Q1&Q4 is turned on and Q2&Q3 is turned off, the speaker coil is driven in one direction, Q1&Q4 is turned off and Q2&Q3 is turned on to drive the coil in the opposite direction. The power supply for this circuit is typically a 3.6V battery. A cell phone with stereo audio has a pair of amplifiers and speakers. For 8Ω speakers, the maximum audio power = 3.6V2/8Ω = 1.6W, or 3.2W in stereo. Battery current at peak stereo audio power = 3.2W / 3.6V = 0.9A. Therefore, audio playback in this case may be affected by power limitations, distortion, and interference.
Problem 1: The battery cannot meet the peak power requirements of wireless data transmission and audio amplifiers at the same time, resulting in distortion.
When users listen to music on a GSM/GPRS/EDGE phone, the phone battery will not be able to provide peak audio current and peak RF transmit power in response to network access. The network periodically accesses the phone to track which cell the phone is on and determines the transmit power that the phone should use. During this network access, the audio amplifier power supply may drop when the phone responds, and the user will hear a "click". However, the battery can easily provide an average audio current of approximately 100mA to 200mA.
Problem 2: Audible noise/beep is produced when the peak battery current exceeds 1A, which produces significant ripple on the audio amplifier supply voltage.
If the total impedance of the battery pack + connector + PCB trace is equal to 150mΩ, then the peak current of 1A will produce 150mV ripple on the supply voltage and the peak current of 1.8A will produce 270mV ripple. This ripple in the supply voltage will introduce audio noise to the listener. The peak current of GSM/GPRS/EDGE is as high as 1.8A, so audible noise is also generated, and the user will hear a 217Hz beep during a call.
Question 3: Limited audio power and worst bass response in CDMA, GSM & 3G handsets.
Regardless of the model phone, the audio capabilities and quality depend on the audio amplifier's output power and the speaker's impedance. In a typical handset configuration, both Class D amplifiers drive a pair of 8Ω speakers from a 3.6V supply from the battery. As described above, the maximum audio power at this time is 3.2 W, and the peak battery current is 0.9 A. The result, whether provided through the phone's internal speakers or through externally connected speakers/earphones, will be shallow, low-power audio performance with very limited bass response.
Figure 1: Typical configuration of a Class D amplifier.
Improve the design of music phone with super capacitor
Figure 2 shows another circuit scheme using a supercapacitor that solves all of the above problems and provides four times the peak audio power. The CAP-XX HS206 is a 0.55F, 85mΩ dual-cell supercapacitor that provides peak power and a battery that provides average power. The boost converter charges the supercapacitor to 5V. The result is as follows:
The peak power of the stereo phone is increased to 2 x 5V2/8Ω = 6.25W, which is close to twice the power. In addition, because supercapacitors can provide very high peak currents, designers can use 4Ω speakers to boost peak audio power to 12.5W, or four times the original power.
The 0.55F, 85mΩ supercapacitor produces only 200mV ripple when it provides 12.5W peak power for 10msec and peak battery power is 1.8W (0.5A 3.6V).
At present, only the average audio current of 150mA to 300mA can be supplied to charge the supercapacitor. It can also provide peak RF power in response to network access without sacrificing audio power. Therefore, users will not hear "K" when responding to network access. Humming."
In addition, ripple on the battery voltage due to RF emissions is not reflected on the audio amplifier. These ripples have been filtered out by the line regulator circuit and supercapacitor of the boost converter, thus completely eliminating the 217Hz click.
Figure 2: Class D amplifier architecture with supercapacitors.
Test Results
In order to test the audio improvement brought by the super capacitor, we built a test device. In this test setup we built the circuit shown in Figures 1 and 2, which uses a pair of TPA2023D1 to provide a stereo audio channel:
In the absence of a supercapacitor (Figure 1), we connected the audio amplifier to a 3.7V lithium-ion battery and drive a pair of 8Ω speakers.
In the case of a supercapacitor (Figure 2), we connect the battery to the input of the buck-boost converter and limit the input current to 250mA and the output to 5V. A 0.55F, 85mΩ ESR supercapacitor is then connected across the output of the buck-boost converter and connected to the power supply input of the audio amplifier shown in Figure 2. It also drives two pairs of 8Ω speakers, each pair of speakers across each audio amplifier, which reduces the output impedance by half, doubling the total speaker power. Under such a device, we tested the following aspects: a high-power bass burst that appears as a bass beat; a network access when listening to music, we describe it as a 1KHz tone, with the goal of bringing a supercapacitor The improvement effect is more obvious.
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