TPA0132

2-W STEREO AUDIO POWER AMPLIFIER

WITH DC VOLUME CONTROL

SLOS223B – MAY 1999 – REVISED MARCH 2000

21

POST OFFICE BOX 655303

APPLICATION INFORMATION

In a typical computer sound channel operating at 5 V, bridging raises the power into an 8-

Ω speaker from a

singled-ended (SE, ground reference) limit of 250 mW to 1 W. In sound power that is a 6-dB improvement —

which is loudness that can be heard. In addition to increased power there are frequency response concerns.

Consider the single-supply SE configuration shown in Figure 33. A coupling capacitor is required to block the

dc offset voltage from reaching the load. These capacitors can be quite large (approximately 33

µF to 1000 µF)

so they tend to be expensive, heavy, occupy valuable PCB area, and have the additional drawback of limiting

low-frequency performance of the system. This frequency limiting effect is due to the high pass filter network

created with the speaker impedance and the coupling capacitance and is calculated with equation 6.

f

1

2

pR

(6)

For example, a 68-

µF capacitor with an 8-Ω speaker would attenuate low frequencies below 293 Hz. The BTL

configuration cancels the dc offsets, which eliminates the need for the blocking capacitors. Low-frequency

performance is then limited only by the input network and speaker response. Cost and PCB space are also

minimized by eliminating the bulky coupling capacitor.

RL

CC

VO(PP)

VO(PP)

VDD

–3 dB

fc

Figure 33. Single-Ended Configuration and Frequency Response

Increasing power to the load does carry a penalty of increased internal power dissipation. The increased

dissipation is understandable considering that the BTL configuration produces 4

× the output power of the SE

configuration. Internal dissipation versus output power is discussed further in the

crest factor and thermal

considerations section.

single-ended operation

In SE mode (see Figure 32 and Figure 33), the load is driven from the primary amplifier output for each channel

(OUT+, terminals 21 and 4).

The amplifier switches single-ended operation when the SE/BTL terminal is held high. This puts the negative

outputs in a high-impedance state, and reduces the amplifier’s gain to 1 V/V.

BTL amplifier efficiency

Class-AB amplifiers are notoriously inefficient. The primary cause of these inefficiencies is voltage drop across

the output stage transistors. There are two components of the internal voltage drop. One is the headroom or

dc voltage drop that varies inversely to output power. The second component is due to the sinewave nature of

the output. The total voltage drop can be calculated by subtracting the RMS value of the output voltage from

power dissipation of the amplifier.