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'Class D Amplifiers Deliver Higher Power Efficiency'
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As business development manager, high performance analog (HPA), Texas Instruments India, Apoorva Awasthy analyses key market trends and customer requirements to build strategic liaisons with customers and partners. He has been a part of R&D and Indian electronics industry for about 12 years. Awasthy speaks to Shweta Dhadiwal of EFY Bureau on the latest development in audio power amplifier ICs for consumer electronics.
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Tuesday, September 29, 2009:
Why are “audio power amplifiers” important in a circuit with audio?
Awasthy: Audio amplifiers are required in most of the consumer equipments that have to control the audio output. For example, in a television set, when you increase the volume, the audio output increases. This is driven by the audio amplifier. To what extent the volume can go up depends on how many watts of power the audio amplifier can supply. So we categorise the systems as 10W output or 20W output based on the amplifier used in the system.
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Then what role do the speakers play? And how is their wattage connected?
Awasthy: The wattage of the speakers needs to match with the maximum wattage of the amplifier, since the entire load has to be borne by the speakers. For example, if we have designed a 20W system, then we need a 20W speaker (minimum). Plus, speakers should have the characteristics that prevent distortion in the audio signal. Overall, the speaker should not be fed by a signal that is higher in power than the capacity of the speaker.
Can you elaborate on the SpeakerGuard that is incorporated in the TPA family of audio amplifiers?
Awasthy: Speakers are basically inductive loads. They are badly affected if DC signal is supplied to them (they can even catch fire). Audio is a variable AC signal, still some amount of DC gets added when the signal is fed to the speaker. The typical set-up of audio system is audio processor (audio system on chip), audio power amplifier and a speaker. Between the audio processor and the amplifier, a designer makes sure that no DC signal passes from the audio processor to the amplifier. For this a capacitor is used. Capacitor blocks the DC and allows only the AC signal to pass. In manufacturing after the system is assembled, in case of a faulty capacitor, DC signal passes through the circuit on its first power-up.
This incurs a rise in cost by damaging the speakers. This is a serious issue in manufacturing. To avoid this, Texas Instruments (TI) uses the “DC detect” feature in its TPA family. In case of a faulty capacitor (or if there is a short circuit at the input), the DC detect will ensure that there is DC signal present and the audio amplifier will go into “shut-down” mode. The amplifier will not amplify any signal since it is shut down and hence DC is restricted from the speakers controlling the damage. Hence it is called SpeakerGuard.
The scenario will rarely occur when the equipment is tested and is in the field. But at the time of manufacture, this will definitely help in preventing damage to the speakers.
Why are Class D amplifiers used?
Awasthy: Power amplifiers are classified in different classes. Every class has its own efficiency and use. At high watt and space constrained systems (10W or 15W), the Class D amplifiers are used. Even in mobile phones for 1-2W audio output, Class D is used because of its small-form factor and efficiency. When the thin flat screen televisions came into the market, the size of the circuit was a major constraint. The Class AB amplifier that is widely used in older designs needs a heat sink which is very difficult to fit in the new design maintaining a small-form factor. Depending on the wattage, the heat sink increases in size to keep the part relatively cool because high wattage will give higher heat dissipation.
The Class D amplifiers are highly efficient with efficiency of 90 per cent. They dissipate very less heat. This reduces the need for heat sink. The low heat dissipation that occurs in Class D is taken care of by the copper in the printed circuit board (PCB). Thus the overall form factor is reduced due to the reduction of heat sink. Also the power requirement of the Class D amplifiers is less in comparison to Class AB. Thus for liquid crystal display (LCD), flat screen applications use the Class D audio power amplifier ICs. The Class D amplifiers are widely used in consumer electronics for their higher efficiency and low-form factor.
What is the need of EMI suppression?
Awasthy: The Class D amplifiers generate pulse width modulation (PWM) that results in internal switching. Switching in the circuit leads to electro-magnetic interference (EMI) and electro-magnetic compatibility (EMC). It is necessary to suppress EMI within the set limit, otherwise it starts interfering with the other components. For this, LC filter is used. Because of wattage involved, the size of inductive coil becomes significant. These inductors also add to the cost of the overall system.
In TPA 3110D2, advance EMI suppression technique is used, where inexpensive ferrite bead filters are used in place of the inductive coil. It also makes the design much smaller, reduces the cost of the system and improves the overall cost versus performance graph. The inductors could cost between 7-14 cents whereas the ferrite beads could cost 1-3 cents. In consumer electronics, the aim is to make it as cost effective as possible.
Why go for 15W when 22W audio power amplifiers are already available in the market?
Awasthy: Each type of power amplifier is targeted at different applications. Because of the requirement of a thin-form factor in LCD, flat screens, DVD players etc., Class D amplifier emerges as the suitable choice. Since theses products are thin, the mechanical withstand capacity for the vibrations is low. A higher wattage output produces vibrations which cause rattling of the television set. So you don't have high wattage flat screens. TI offers high wattage power amplifiers up to 300W that are more suitable for home theatre systems and CRT TVs where the size of the system is big.
What are the other design parameters that an engineer needs to consider while designing the audio section of his system?
Awasthy: Output power limiting is an important consideration while designing a system. Suppose a designer wants to restrict the output audio power to 10W even if he has the capacity of 15W power amplifier, he can do it by controlling the output power limit. By a power limiting resistor, the output audio power can be controlled. This situation arises if you are using low power speakers. The threshold limit can be changed by changing the value of the resistor.
TI's TPA provides on-board power limit feature to control the output power. TI also provides the supporting documentation and evaluation board for designing a circuit.
Could you elucidate upon the auto recovery feature?
Awasthy: One of the biggest challenges in an amplifier design is heat dissipation. In a Class D amplifier where there is no external heat sink, the copper in the PCB is itself used for heat dissipation. For some reason if the part is at a higher temperature -- more than 150 °C (as rated for thermal shut-down), it will go into a shut down mode. After the temperature cools down, the part will come up automatically. This is called auto recovery. This feature has been introduced to avoid the thermal run-away of the circuit. This is quite useful in avoiding damage to the IC.
Similarly in the case of over current due to short circuit on the output stage, the part will go into shut down mode and when the condition becomes normal it will auto-recover.
What are the applications that the TPA family is best suited for?
Awasthy: The TPA family is very much suitable for flat panel display, home theatre box, DVD player and CRT TV. You will save cost because of the external circuit which is simplified. The DC detect feature is something that the designers are always looking for. Most of the features are designed from customers' feedback and timely requirement.
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