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PDF ( 数据手册 , 数据表 ) L4733TA

零件编号 L4733TA
描述 3 Channel 30W Audio Power Amplifier with Mute
制造商 National Semiconductor
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L4733TA 数据手册, 描述, 功能
September 2003
LM4733
3 Channel 30W Audio Power Amplifier with Mute
General Description
The LM4733 is a three channel audio amplifier capable of
typically delivering 30W per channel of continuous average
output power into a 4or 8load with less than 10%
THD+N from 20Hz - 20kHz.
The LM4733 has short circuit protection and a thermal shut
down feature that is activated when the die temperature
exceeds 150˚C. The LM4733 also has an under voltage lock
out feature for click and pop free power on and off.
Each amplifier of the LM4733 has an independent smooth
transition fade-in/out mute.
The LM4733 has a wide operating supply range from ±10V
- ±32V allowing for lower cost unregulated power supplies to
be used.
The LM4733 amplifiers can easily be configured for bridge or
parallel operation for higher power and bi-amp solutions
Key Specifications
j Output Power/Channel at 10% THD+N,
1kHz into 4or 8
j THD+N at 3 x 1W into 8, 1kHz
j Mute Attenuation
j PSRR
j Slew Rate
30W (typ)
0.03% (typ)
110dB (typ)
85dB (typ)
9V/µs (typ)
Features
n Low external component count
n Quiet fade-in/out mute mode
n Wide supply range: 20V - 64V
Applications
n Audio amplifier for component stereo
n Audio amplifier for compact stereo
n Audio amplifier for self-powered speakers
n Audio amplifier for high-end and HD TVs
Typical Application
200794B5
FIGURE 1. Typical Audio Amplifier Application Circuit
© 2003 National Semiconductor Corporation DS200794
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L4733TA pdf, 数据表
External Components Description
(Figures 1-5)
Components
1 RB
2 Ri
3 Rf
4 Rf2
(Note 14)
5 Cf
(Note 14)
6 CC
(Note 14)
7 Ci
(Note 14)
8 CS
9 RV
(Note 14)
10 RIN
(Note 14)
11 CIN
(Note 14)
12 RSN
(Note 14)
13 CSN
(Note 14)
14 L (Note 14)
15 R (Note 14)
16 RA
17 CA
18 RINP
(Note 14)
19 RBI
20 RE
21 RM
22 CM
23 S1
24 ROUT
Functional Description
Prevents current from entering the amplifier’s non-inverting input. This current may pass through to the load
during system power down, because of the amplifier’s low input impedance when the undervoltage circuitry
is off. This phenomenon occurs when the V+ and V- supply voltages are below 1.5V.
Inverting input resistance. Along with Rf, sets AC gain.
Feedback resistance. Along with Ri, sets AC gain.
Feedback resistance. Works with Cf and Rf creating a lowpass filter that lowers AC gain at high
frequencies. The -3dB point of the pole occurs when: (Rf - Ri)/2 = Rf // [1/(2πfcCf) + Rf2] for the
Non-Inverting configuration shown in Figure 5.
Compensation capacitor. Works with Rf and Rf2 to reduce AC gain at higher frequencies.
Compensation capacitor. Reduces the gain at higher frequencies to avoid quasi-saturation oscillations of the
output transistor. Also suppresses external electromagnetic switching noise created from fluorescent lamps.
Feedback capacitor which ensures unity gain at DC. Along with Ri also creates a highpass filter at fc =
1/(2πRiCi).
Provides power supply filtering and bypassing. Refer to the Supply Bypassing application section for proper
placement and selection of bypass capacitors.
Acts as a volume control by setting the input voltage level.
Sets the amplifier’s input terminals DC bias point when CIN is present in the circuit. Also works with CIN to
create a highpass filter at fC = 1/(2πRINCIN). If the value of RIN is too large, oscillations may be observed on
the outputs when the inputs are floating. Recommended values are 10kto 47k. Refer to Figure 5.
Input capacitor. Prevents the input signal’s DC offsets from being passed onto the amplifier’s inputs.
Works with CSN to stabilize the output stage by creating a pole that reduces high frequency instabilities.
Works with RSN to stabilize the output stage by creating a pole that reduces high frequency instabilities. The
pole is set at fC = 1/(2πRSNCSN). Refer to Figure 5.
Provides high impedance at high frequencies so that R may decouple a highly capacitive load and reduce
the Q of the series resonant circuit. Also provides a low impedance at low frequencies to short out R and
pass audio signals to the load. Refer to Figure 5.
Provides DC voltage biasing for the transistor Q1 in single supply operation.
Provides bias filtering for single supply operation.
Limits the voltage difference between the amplifier’s inputs for single supply operation. Refer to the Clicks
and Pops application section for a more detailed explanation of the function of RINP.
Provides input bias current for single supply operation. Refer to the Clicks and Pops application section for
a more detailed explanation of the function of RBI.
Establishes a fixed DC current for the transistor Q1 in single supply operation. This resistor stabilizes the
half-supply point along with CA.
Mute resistance set up to allow 0.5mA to be drawn from each MUTE pin to turn the muting function off.
RM is calculated using: RM (|VEE| − 2.6V)/l where l 0.5mA. Refer to the Mute Attenuation vs Mute
Current curves in the Typical Performance Characteristics section.
Mute capacitance set up to create a large time constant for turn-on and turn-off muting.
Mute switch. When open or switched to GND, the amplifier will be in mute mode.
Reduces current flow between outputs that are caused by Gain or DC offset differences between the
amplifiers.
Note 14: Optional components dependent upon specific design requirements.
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L4733TA equivalent, schematic
Application Information (Continued)
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 4subwoofer and
8tweeter/midrange with the LM4733 in parallel and single-
ended modes. Each amplifier would see an 8load 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, VCC. This makes the task of attaining a
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 RINP
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 RBI help bias up
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 RBI, namely 10kand 200k. These
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.
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 Rf and Ri for the
non-inverting configuration shown in Figure 1. The gain is
found by Equation (6) below:
AV = 1 + Rf / Ri (V/V)
(6)
For best noise performance, lower values of resistors are
used. A value of 1kis commonly used for Ri and then
setting the value of Rf for the desired gain. For the LM4733
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 Ri with Ci (see Figure 1) creates a high
pass filter. The low frequency response is determined by
these two components. The -3dB point can be found from
Equation (7) shown below:
fi = 1 / (2πRiCi) (Hz)
(7)
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 CIN and RIN. When
using a input coupling capacitor RIN is needed to set the DC
bias point on the amplifier’s input terminal. The resulting
-3dB frequency response due to the combination of CIN and
RIN can be found from Equation (8) shown below:
fIN = 1 / (2πRINCIN) (Hz)
(8)
With large values of RIN oscillations may be observed on the
outputs when the inputs are left floating. Decreasing the
value of RIN or not letting the inputs float will remove the
oscillations. If the value of RIN is decreased then the value of
CIN will need to increase in order to maintain the same -3dB
frequency response.
HIGH PERFORMANCE CONSIDERATIONS
Using low cost electrolytic capacitors in the signal path such
as CIN and Ci (see Figures 1 - 5) will result in very good
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-
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