EUA6412
Bridged-Tied Load Versus Single-Ended Mode
For example, a 68µF 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.
Figure 37 show a Class-AB audio power amplifier (APA)
in a BTL configuration. The EUA6412 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×VO(PP) into the power
equation, where voltage is squared, yields 4× the output
power from the same supply rail and load impedance(see
equation 5)
2
V
V
(rms)
O(PP)
V(rms)
=
Power
=
------(5)
R
2 2
L
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 the load is driven from the primary
amplifier output for each channel (OUT+, terminals 21
and 4 ). The amplifier switches single-ended operation
In a typical computer sound channel operating at 5V,
bridging raises the power into an 8-Ω speaker from a
singled-ended (SE, ground reference) limit of 250 mW
to 1W. 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 38.
when the
terminal is held high. This puts the
SE/BTL
negative outputs in a high-impedance state, and reduces
the amplifier’s gain to 1V/V.
Input MUX Operation
The input MUX allows two separate inputs to be applied
to the amplifier. This allow the designer to choose which
input is active independent of the state of the
A coupling capacitor is required to block the dc offset
voltage from reaching the load. These capacitors can be
quite large (approximately 33µF to 1000µF) 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.
terminal. When the
terminal is held
HP/LINE
SE/BTL
high, the headphone inputs are active. When the
terminal is held low, the line BTL inputs are
HP/LINE
active.
1
fC =
----------------------------------(6)
2π R C
L
C
DS6412 Ver 1.0 Dec. 2006
15
EUTECH [ EUTECH MICROELECTRONICS INC ]
