Datasheet

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SLOS386A − NOVEMBER 2001 − REVISED APRIL 2007
19
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APPLICATION INFORMATION
bridged-tied load versus single-ended mode
Figure 31 shows a Class-AB audio power amplifier (APA) in a BTL configuration. The TPA6017A2 BTL amplifier
consists of two Class-AB 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 × 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 3).
Power +
V
(rms)
2
R
L
(3)
V
(rms)
+
V
O(PP)
22
Ǹ
R
L
2x V
O(PP)
V
O(PP)
−V
O(PP)
V
DD
V
DD
Figure 31. Bridge-Tied Load Configuration
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 32. 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 4.
f
c
+
1
2p R
L
C
C
(4)