EUA5202
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 (see Figure56 and Figure57), the load is
driven from the primary amplifier output for each
channel (OUT+, terminals 22 and 3).
In SE mode the gain is set by the RF and RI resistors and
is shown in equation 9. Since the inverting amplifier is
not used to mirror the voltage swing on the load, the
factor of 2, from equation 5, is not included.
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 57. 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
8.
R
R
F
SE Gain =
-------------------------------------- (9)
−
I
The output coupling capacitor required in single-supply
SE mode also places additional constraints on the
selection of other components in the amplifier circuit.
The rules described earlier still hold with the addition of
the following relationship (see equation 10):
1
1
1
--------------- (10)
≤
<
R C
× 25 kΩ
C
C R
L C
B
I I
1
fC
------------------------------------ (8)
=
2 π RLCC
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.
DS5202 Ver 1.6 May. 2005
20
EUTECH [ EUTECH MICROELECTRONICS INC ]
