Flat panel audio design - where only the screen is flat, not the audio

One of the basic design decisions that needs to be made for many products is the format of the audio signal - digital or analog. A few years ago the TV audio signal chain was primarily analog-based, but now, a digital audio data path is much more common. Both digital and analog paths have advantages and disadvantages.

HDTV audio transmissions are a compressed digital signal, so newer TVs tend to use a digital path, keeping the audio signal 100 percent digital from the RF transmitter to the speaker. The analog signal is more susceptible to noise, so it must be routed carefully, run differentially or shielded to avoid degradation, whereas the digital signal has a high tolerance to noise. Most TV sound processors have both analog and digital outputs, so this is generally not an issue. The analog-based system would not need an ADC to mux in auxiliary inputs. Muxing in the digital domain is very simple, if signals are already digital.

Adding effects, like speaker equalization, midnight mode or third party algorithms like SRS TruSurround, is much simpler in the digital domain. With the digital signal path this can even be done as the final step on the production line, enabling a single PCB to be used across many platforms.

Design Issues
Designing audio amplifiers for flat panel TVs involves several challenging and juxtaposed issues. Engineering is usually fraught with making trade-offs and there’s no better example than in flat panel design. How does one design a 20 watt (W) stereo audio amp in a slim-line DLP, plasma or Liquid Crystal Display (LCD) chassis and still dissipate the heat generated? How does one support the 2-layer, size-constrained board space and still meet the electromagnetic interference (EMI) testing requirements? How does one cost effectively design printed circuit boards (PCBs) for various screen sizes and power levels and still meet tight deadlines?

Fortunately, engineers do not need to tackle these obstacles by themselves. Many of these challenges have been overcome or workarounds have been developed. New devices are now available to help solve these issues through integrating functionality. One way to address these challenges and speed design time is with the latest Texas Instruments audio class-D amplifier product offerings, the TPA3100D2 and TPA3101D2.

The TPA3100D2 and TPA3101D2 are stereo class-D amplifiers designed for the flat panel market. They are pin-for-pin compatible and drive stereo 20W and 10W, respectively. New integrated features include improved pop/click control, adaptive dynamic range control and enhancements to reduce EMI. These products overcome issues in previous class-D generation devices and speed the design process.

Output Wattage

The first consideration in an audio amplifier design is to determine the output wattage needed. This is directly proportional to screen size as explained below. Other factors to consider are speaker impedance, Total Harmonic Distortion (THD), amplifier voltage and amplifier efficiency. All these factors are interrelated. For example, higher speaker impedance gives better efficiency and thermal performance, but requires a higher amplifier voltage or produces clipping at a lower output power. Understanding the trade-offs in these factors and achieving a balance is the key to a good design.

The TPA3100D2 is highly efficient -- capable of 92 percent efficiency at full output power, 20Wx2. At this efficiency, 1.74W needs to be dissipated at full power. This power is easily dissipated from the package by soldering the thermal pad on the bottom of the package to the PCB. A ground plane in the PCB serves as the heat sink, eliminating the need for an external heat sink. These properties make the TPA3100D2 ideal for flat panel Plasma and LCD TVs greater than 32 inches in size. These televisions require higher audio power, because the end user is seated further away from the speakers.

The TPA3101D2 is a cost-effective 10Wx2 solution for smaller LCD televisions ranging from 23 to 32 inches wide. The TPA3101D2 is a pin-to-pin compatible device to the TPA3100D2 with all the same features. The efficiency is 87 percent for 10Wx2.

Because the TPA3100D2 and TPA3101D2 have exactly the same pin-out, package dimensions and external components, this allows the same board design to be used for different products -- saving design and manufacturing time and inventory costs.

Thermal Performance and Maximum Output Power

Thermal performance is another important consideration when designing audio solutions for TVs. Heat dissipation is a critical issue in flat panel TVs. Too much heat build-up can distort the colors of an LCD screen, but over-design can inflate the build of materials (BOM) cost. In addition, as a class-D amplifier heats up, it becomes less efficient and will output less audio power. It is critical that the thermal considerations be taken into account early in the design process.

Different thermal conditions and requirements can impact the maximum output power of the TPA3100D2 and TPA3101D2. The following explains the factors involved and presents a systematic method to determine the device maximum output power. Note that the information is a theoretical estimation; actual thermal measurements are still required.

Obtain the efficiency of the device from data sheet Figures 13 or 14 (shown below as Figure 1) at the operating VCC and RL -- Efficiency determines how much supply power is converted to actual audio power. A higher efficiency means less energy is dissipated, as heat and consequently more output power is delivered to the load. A higher VCC and lower RL results in a lower efficiency.

Figure 1: TPA3100D2 Efficiency versus Output Power Graph

Obtain Derating Factor from the TYPICAL DISSIPATION RATINGS in the datasheet -- The reciprocal of the Derating Factor, RJA, is more commonly used in thermal analysis. RJA is the temperature rise (in oC) per Watt dissipated in the device. RJA is determined by many factors and the Derating Factor in the datasheet is simulated for a standard characterization board (JEDEC, 4-layer). Proper PCB layout and soldering of thermal pad is crucial to obtaining a low RJA.

Measure the ambient temperature -- This is the temperature of the air surrounding the device during normal system operation (i.e. when the TV is powered on), denoted TA.

Obtain maximum junction temperature (TJmax) or maximum case temperature (TCmax) allowed -- Combined with the RJA, this is used to calculate the maximum allowable power dissipation, PD.

Use the following equations to determine the device maximum output power for a given ambient temperature, TA:

If your internal specification requires the case temperature to remain below a given temperature, assume TCmax is slightly less than TJmax. A good rule-of-thumb is to assume TJmax is 10 oC higher than TCmax. The calculations can be re-run to maintain the case temperature within specification.

Using the above method, TPA3100D2 at VCC=18V, RL=8, TA=55 oC and TJmax=150 oC, we obtain a maximum output power of 20W per channel.

If we increase the ambient temperature to 65 oC, the maximum output power will be reduced to 18W per channel.

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