TPA721

700-mW MONO LOW-VOLTAGE AUDIO POWER AMPLIFIER

SLOS231B – NOVEMBER 1998 – REVISED MARCH 2000

12

POST OFFICE BOX 655303

APPLICATION INFORMATION

bridged-tied load

Figure 23 shows a linear audio power amplifier (APA) in a BTL configuration. The TPA721 BTL amplifier consists

of two linear amplifiers driving both ends of the load. There are several potential benefits to this differential drive

configuration but initially consider power to the load. The differential drive to the speaker means that as one side

is slewing up, the other side is slewing down, and vice versa. This in effect doubles the voltage swing on the

load as compared to a ground referenced load. Plugging 2

squared, yields 4

× the output power from the same supply rail and load impedance (see equation 1).

Power

+

V

(rms)

2

R

L

(1)

V

(rms) +

V

O(PP)

22

RL

2x VO(PP)

VO(PP)

–VO(PP)

VDD

VDD

Figure 23. Bridge-Tied Load Configuration

In a typical portable handheld equipment sound channel operating at 3.3 V, bridging raises the power into an

8-

Ω speaker from a singled-ended (SE, ground reference) limit of 62.5 mW to 250 mW. 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 24. 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 2.

f

(corner) +

1

2

pR

L

C

C

(2)