EUA6019
the part is in the fade-off mode. The gain is set to the
highest level, and the output is at VDD when the amplifier
is shut down.
is due to the high pass filter network created with the
speaker impedance and the coupling capacitance and is
calculated with equation 2.
The power-up sequence is different from the shutdown
sequence and the voltage on the pin does not change the
power-up sequence. Upon a power-up condition, the
EUA6019 begins in the lowest gain setting and steps
up 2 dB every 2 clock cycles until the final value is
reached as determined by the dc voltage applied to the
VOLUME, SEDIFF , and SEMAX pins.
1
fC =
----------------------------------(2)
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 28 show a Class-AB audio power amplifier (APA)
in a BTL configuration. The EUA6019 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 1)
Figure 29. Single-Ended configuration and
Frequency Response
2
V
V
(rms)
O(PP)
V(rms)
=
Power
=
------(1)
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.
R
L
2 2
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 when the
terminal is held high. This puts the negative
SE/BTL
outputs in a high-impedance state, and reduces the
amplifier’s gain to 1V/V.
Figure 28.Bridge-Tied Load configuration
Input MUX 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 29.
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
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.
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
DS6019 Ver 1.1 May. 2007
16
EUTECH [ EUTECH MICROELECTRONICS INC ]
