LM4782TABD/NOPB National Semiconductor, LM4782TABD/NOPB Datasheet - Page 19

• Manufacturer Part Number: LM4782TABD/NOPB
• Manufacturer: National Semiconductor

Specifications of LM4782TABD/NOPB

• Lead Free Status / Rohs Status: Compliant

Application Information
amplifier than the equivalent impedance on the two amplifi-
ers in parallel will reduce total IC package power dissipation
reducing the heat sink size requirement.
BI-AMP AND TRI-AMP APPLICATIONS
Bi-amping is the practice of using two different amplifiers to
power the individual drivers in a speaker enclosure. For
example, a two-way speaker enclosure might have a tweeter
and a subwoofer. One amplifier would drive the tweeter and
another would drive the subwoofer. One advantage is that
the gain of each amplifier can be adjusted for the different
driver sensitivities. Another advantage is the crossover can
be designed before the amplifier stages with low cost op
amps instead of large passive components. With the cross-
over before the amplifier stages no power is wasted in the
passive crossover as each individual amplifier provides the
correct frequencies for the driver. Tri-Amping is using three
different amplifier stages in the same way bi-amping is done.
Bi-amping can also be done on a three-way speaker design
by using one amplifier for the subwoofer and another for the
midrange and tweeter.
The LM4782 is perfectly suited for bi-amp or tri-amp appli-
cations with it’s three amplifiers. Two of the amplifiers can be
configured for bridge or parallel mode to drive a subwoofer
with the third amplifier driving the tweeter or tweeter and
midrange. An example would be to use a 4Ω subwoofer and
8Ω tweeter/midrange with the LM4782 in parallel and single-
ended modes. Each amplifier would see an 8Ω load but the
subwoofer would have twice the output power as the
tweeter/midrange. The gain of each amplifier may also be
adjusted for the desired response. Using the LM4782 in a
tri-amp configuration would allow the gain of each amplifier
to be adjusted to achieve the desired speaker response.
SINGLE-SUPPLY AMPLIFIER APPLICATION
The typical application of the LM4782 is a split supply am-
plifier. But as shown in Figure 4, the LM4782 can also be
used in a single power supply configuration. This involves
using some external components to create a half-supply bias
which is used as the reference for the inputs and outputs.
Thus, the signal will swing around half-supply much like it
swings around ground in a split-supply application. Along
with proper circuit biasing, a few other considerations must
be accounted for to take advantage of all of the LM4782
functions, like the mute function.
The LM4782 possesses a mute and standby function with
internal logic gates that are half-supply referenced. Thus, to
enable either the Mute or Standby function, the voltage at
these pins must be a minimum of 2.5V above half-supply. In
single-supply systems, devices such as microprocessors
and simple logic circuits used to control the mute and
standby functions, are usually referenced to ground, not
half-supply. Thus, to use these devices to control the logic
circuitry of the LM4782, a “level shifter,” like the one shown in
Figure 6, must be employed. A level shifter is not needed in
a split-supply configuration since ground is also half-supply.
(Continued)
19
When the voltage at the Logic Input node is 0V, the 2N3904
is “off” and thus resistor R
the supply. This enables the mute or standby function. When
the Logic Input is 5V, the 2N3904 is “on” and consequently,
the voltage at the collector is essentially 0V. This will disable
the mute or standby function, and thus the amplifier will be in
its normal mode of operation. R
an RC time constant that reduces transients when the mute
or standby functions are enabled or disabled. Additionally,
R
the LM4782 which insures device reliability. Refer to the
Mute Mode and Standby Mode sections in the Application
Information section for a more detailed description of these
functions.
CLICKS AND POPS
In the typical application of the LM4782 as a split-supply
audio power amplifier, the IC exhibits excellent “click” and
“pop” performance when utilizing the mute and standby
modes. In addition, the device employs Under-Voltage Pro-
tection, which eliminates unwanted power-up and power-
down transients. The basis for these functions are a stable
and constant half-supply potential. In a split-supply applica-
tion, ground is the stable half-supply potential. But in a
single-supply application, the half-supply needs to charge up
at the same rate as the supply rail, V
of attaining a clickless and popless turn-on more challeng-
ing. Any uneven charging of the amplifier inputs will result in
output clicks and pops due to the differential input topology
of the LM4782.
To achieve a transient free power-up and power-down, the
voltage seen at the input terminals should be ideally the
same. Such a signal will be common-mode in nature, and
will be rejected by the LM4782. In Figure 4, the resistor R
serves to keep the inputs at the same potential by limiting the
voltage difference possible between the two nodes. This
should significantly reduce any type of turn-on pop, due to an
uneven charging of the amplifier inputs. This charging is
based on a specific application loading and thus, the system
designer may need to adjust these values for optimal perfor-
mance.
As shown in Figure 4, the resistors labeled R
the LM4782 off the half-supply node at the emitter of the
2N3904. But due to the input and output coupling capacitors
in the circuit, along with the negative feedback, there are two
different values of R
resistors bring up the inputs at the same rate resulting in a
popless turn-on. Adjusting these resistors values slightly
may reduce pops resulting from power supplies that ramp
extremely quick or exhibit overshoot during system turn-on.
shift
limits the current supplied by the internal logic gates of
FIGURE 6. Level Shift Circuit
BI
, namely 10kΩ and 200kΩ. These
c
pulls up mute or standby input to
shift
, along with C
CC
. This makes the task
BI
help bias up
shift
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