LM4733TABD National Semiconductor, LM4733TABD Datasheet - Page 16

• Manufacturer Part Number: LM4733TABD
• Description: BOARD EVALUATION LM4733TA
• Manufacturer: National Semiconductor

Specifications of LM4733TABD

• Amplifier Type: Class AB
• Output Type: 3-Channel
• Max Output Power X Channels @ Load: 30W x 3 @ 8 Ohm
• Voltage - Supply: 20 V ~ 64 V, ±10 V ~ 32 V
• Operating Temperature: -20°C ~ 85°C
• Board Type: Fully Populated
• Utilized Ic / Part: LM4733
• Lead Free Status / RoHS Status: Not applicable / Not applicable

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Application Information
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 LM4733 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 LM4733 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 LM4733 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 LM4733 is a split supply am-
plifier. But as shown in Figure 4, the LM4733 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 LM4733
functions, like the mute function.
CLICKS AND POPS
In the typical application of the LM4733 as a split-supply
audio power amplifier, the IC exhibits excellent “click” and
“pop” performance when utilizing the mute mode. In addition,
the device employs Under-Voltage Protection, which elimi-
nates unwanted power-up and power-down transients. The
basis for these functions are a stable and constant half-
supply potential. In a split-supply application, ground is the
stable half-supply potential. But in a single-supply applica-
tion, the half-supply needs to charge up at the same rate as
the supply rail, V
clickless and popless turn-on more challenging. Any uneven
charging of the amplifier inputs will result in output clicks and
pops due to the differential input topology of the LM4733.
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 LM4733. 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 LM4733 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.
CC
. This makes the task of attaining a
BI
, namely 10kΩ and 200kΩ. These
(Continued)
BI
help bias up
INP
16
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components is required to meet
the design targets of an application. The choice of external
component values that will affect gain and low frequency
response are discussed below.
The gain of each amplifier is set by resistors R
non-inverting configuration shown in Figure 1. The gain is
found by Equation (6) below:
For best noise performance, lower values of resistors are
used. A value of 1kΩ is commonly used for R
setting the value of R
the gain should be set no lower than 10V/V and no higher
than 50V/V. Gain settings below 10V/V may experience
instability and using the LM4733 for gains higher than 50V/V
will see an increase in noise and THD.
The combination of R
pass filter. The low frequency response is determined by
these two components. The -3dB point can be found from
Equation (7) shown below:
If an input coupling capacitor is used to block DC from the
inputs as shown in Figure 5, there will be another high pass
filter created with the combination of C
using a input coupling capacitor R
bias point on the amplifier’s input terminal. The resulting
-3dB frequency response due to the combination of C
R
With large values of R
outputs when the inputs are left floating. Decreasing the
value of R
oscillations. If the value of R
C
frequency response.
HIGH PERFORMANCE CONSIDERATIONS
Using low cost electrolytic capacitors in the signal path such
as C
performance. However, electrolytic capacitors are less linear
than other premium capacitors. Higher THD+N performance
may be obtained by using high quality polypropylene capaci-
tors in the signal path. A more cost effective solution may be
the use of smaller value premium capacitors in parallel with
the larger electrolytic capacitors. This will maintain signal
quality in the upper audio band where any degradation is
most noticeable while also coupling in the signals in the
lower audio band for good bass response.
Distortion is introduced as the audio signal approaches the
lower -3dB point, determined as discussed in the section
above. By using larger values of capacitors such that the
-3dB point is well outside of the audio band will reduce this
distortion and improve THD+N performance.
Increasing the value of the large supply bypass capacitors
will improve burst power output. The larger the supply by-
pass capacitors the higher the output pulse current without
supply droop increasing the peak output power. This will also
increase the headroom of the amplifier and reduce THD.
SIGNAL-TO-NOISE RATIO
In the measurement of the signal-to-noise ratio, misinterpre-
tations of the numbers actually measured are common. One
amplifier may sound much quieter than another, but due to
improper testing techniques, they appear equal in measure-
IN
IN
will need to increase in order to maintain the same -3dB
can be found from Equation (8) shown below:
IN
and C
IN
or not letting the inputs float will remove the
i
(see Figures 1 - 5) will result in very good
f
IN
A
f
i
V
= 1 / (2πR
f
= 1 / (2πR
i
IN
= 1 + R
for the desired gain. For the LM4733
with C
oscillations may be observed on the
IN
i
is decreased then the value of
f
(see Figure 1) creates a high
IN
/ R
i
C
C
i
IN
) (Hz)
i
IN
(V/V)
is needed to set the DC
) (Hz)
IN
and R
f
and R
i
and then
IN
. When
i
for the
IN
and
(6)
(7)
(8)