Datasheet

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APPLICATION INFORMATION
BRIDGE-TIED LOAD
Power 
V
(RMS)
2
R
L
V
(RMS)

V
O(PP)
2 2

(1)
R
L
2x V
O(PP)
V
O(PP)
-V
O(PP)
V
DD
V
DD
f
c

1
2 R
L
C
C
(2)
TPA321
SLOS312C – JUNE 2000 – REVISED JUNE 2004
Figure 23 shows a linear audio power amplifier (APA) in a BTL configuration. The TPA321 BTL amplifier consists
of two linear amplifiers driving both ends of the load. There are several potential benefits to this differential drive
configuration but power to the load should be initially considered. 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 × V
O(PP)
into the power equation, where
voltage is squared, yields 4× the output power from the same supply rail and load impedance (see Equation 1 ).
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 single-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 .
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, eliminating 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.
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