EUA6027A
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 30.
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 4.
Midrail Bypass Capacitor, (CBYP
)
The midrail bypass capacitor, CBYP, the most critical
capacitor and serves several important functions. During
start-up or recovery from shutdown mode, CBYP
determines the rate at which the amplifier starts up. The
second function is to reduce noise produced by the power
supply caused by coupling into the output drive signal.
This noise is from the midrail generation circuit internal to
the amplifier, which appears as degraded PSRR and
THD+N.
Bypass capacitor, CBYP, values of 0.47µF to 1µF ceramic
or tantalum low-ESR capacitors are recommended for the
best THD and noise performance.
Using Low- ESR Capacitors
Low- ESR capacitors are recommended throughout this
applications section. A real (as opposed to ideal) capacitor
can be modeled simply as a resistor in series with an ideal
capacitor. The voltage drop across this resistor minimizes
the beneficial effects of the capacitor in the circuit. The
lower the equivalent value of this resistance the more the
real capacitor behaves like an ideal capacitor.
1
fC =
----------------------------------(4)
2π R C
L
C
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.
Bridged-Tied Load Versus Single-Ended Mode
Figure 29 show a Class-AB audio power amplifier (APA)
in a BTL configuration. The EUA6027A 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 3)
2
Figure 30. Single-Ended configuration and
Frequency Response
V
V
(rms)
O(PP)
V(rms)
=
Power
=
------(3)
R
L
2 2
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.
Figure 29.Bridge-Tied Load configuration
DS6027A Ver 1.1 June 2008
12
EUTECH [ EUTECH MICROELECTRONICS INC ]
