SLOA292 May 2020 TAS5760LD
3 TV 2.1CH Typical Application Design
TAS5760LD is very popular in the TV application, which integrates a stereo 20Watt class D amplifier with the line driver. Smart TV SOC can output inter-IC sound (I2S) and analog signals from the digital-to-analog converter (DAC) to the TAS5760LD class D amplifier that includes hardware and software inter-integrated circuit (I2C) control modes, integrated line driver, and a wide power supply operating range to enable use in a multitude of applications. Therefore, smart TV can adapt TAS5760LD to drive the twitter speaker and use the integrated line driver to drive external active woofer.
Typical circuit diagrams highlight the required external passive components and system level connections for proper operation. Figure 2 shows the stereo mixer circuit to mono line driver output. The design requirement is to make 20 to approximately 200Hz frequency response flat within ±3dB range. The feedback network is around 1.5 V/V. In a general case, the line driver port output impedance is designed to 1 kΩ.
If the typical circuit follows the suggested power sequence as shown in Figure 3, there is no pop noise due to DirectPath architecture, which makes sure the output voltages are centered at zero volts with the capability to swing to the positive rail or negative rail.
In the TV system design, 3.3 V power of the line driver sometime tied to SOC's power supply. So, it is not able to do the power sequence control on line driver and SOC side. However, the SOC DAC output generated the larger transient while the SOC 3.3 V turned off, which resulted in the TAS5760LD line-in receive the unexpected transient.
The slightly voltage transient can be observed on the TAS5760 line out if TAS5760LD AVDD turns off and the design uses the line out to drive the external active woofer speaker. (>150Watt)
In the above design scenario, the output impedance design and the feedback network resistor design are critical factor here to minimize the voltage transient from SOC output. The design challenging is need to make sure the lower frequency 20 to approximately 200HZ within ±3dB range during selecting the proper passive component values.
This application report, with TINA simulation and real PCB design cases, studies the relation between 1 kΩ and 270 Ω output impedance on pop noise improvement. Secondly, this application report discusses the approach by increasing the feedback network resistor values to improve the pop noise. The pop noise was improved by increasing the feedback network resistor 2 times value.
Figure 2. Subwoofer Line Driver Stereo to Mono Design Resistor R5 and resitor R6 are parallel, their effective resistance Reff can be determined from Equation 1. Here is the example for Rlineout 1 kΩ and Rwoofer 33 kΩ case, the equivalent resistor is 1 kΩ.
Table 1. Design Parameters
| Parameter | Example |
|---|---|
| Low Power Supply | 3.3 V |
| High Power Supply | 12 V |
| Host Processor | I2S and I2C |
| Output Filters | Inductor- Capacitor Low Pass Filter |
| Twitter Speakers | 4 Ω or 8 Ω |
| External Active Woofer Speaker Input Impedance | 33 kΩ |
| TAS5760LD Line Out | Mono |
| TV Output Impedance | 1 kΩ |
| Line Out Driver Gain | 1.5 V/V |
| Line Out Driver Frequency Response | 20 to approximately 200Hz ± 3dB |
For the line driver power off, the SOC output usually has the DC level changed, which results in the voltage transient in the line driver input side. Before transient voltage happens, the line driver needs to maintain 3.3 V power supply and keep (DR_MUTE)̅ as low level. This allows the TAS5760LD analog amp output impedance stay at 100 mΩ. The low impedance can effectively reduce the pop noise.
The power down sequence is discussed in Section 3.1. The design needs to keep the line driver power at 3.3 V when turning down the SOC power 3.3 V.