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LM4673TM bảng dữ liệu(PDF) 10 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
tên linh kiện LM4673TM
Giải thích chi tiết về linh kiện  Filterless, 2.65W, Mono, Class D Audio Power Amplifier
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LM4673TM bảng dữ liệu(HTML) 10 Page - National Semiconductor (TI)

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Application Information
GENERAL AMPLIFIER FUNCTION
The LM4673 features a filterless modulation scheme. The
differential outputs of the device switch at 300kHz from V
DD
to GND. When there is no input signal applied, the two
outputs (V
O1 and VO2) switch with a 50% duty cycle, with
both outputs in phase. Because the outputs of the LM4673
are differential, the two signals cancel each other. This re-
sults in no net voltage across the speaker, thus there is no
load current during an idle state, conserving power.
With an input signal applied, the duty cycle (pulse width) of
the LM4673 outputs changes. For increasing output volt-
ages, the duty cycle of V
O1 increases, while the duty cycle of
V
O2 decreases. For decreasing output voltages, the con-
verse occurs, the duty cycle of V
O2 increases while the duty
cycle of V
O1 decreases. The difference between the two
pulse widths yields the differential output voltage.
POWER DISSIPATION AND EFFICIENCY
In general terms, efficiency is considered to be the ratio of
useful work output divided by the total energy required to
produce it with the difference being the power dissipated,
typically, in the IC. The key here is “useful” work. For audio
systems, the energy delivered in the audible bands is con-
sidered useful including the distortion products of the input
signal.
Sub-sonic
(DC)
and
super-sonic
components
(>22kHz) are not useful. The difference between the power
flowing from the power supply and the audio band power
being transduced is dissipated in the LM4673 and in the
transducer load. The amount of power dissipation in the
LM4673 is very low. This is because the ON resistance of the
switches used to form the output waveforms is typically less
than 0.25
Ω. This leaves only the transducer load as a po-
tential "sink" for the small excess of input power over audio
band output power. The LM4673 dissipates only a fraction of
the excess power requiring no additional PCB area or cop-
per plane to act as a heat sink.
DIFFERENTIAL AMPLIFIER EXPLANATION
As logic supply voltages continue to shrink, designers are
increasingly turning to differential analog signal handling to
preserve signal to noise ratios with restricted voltage swing.
The LM4673 is a fully differential amplifier that features
differential input and output stages. A differential amplifier
amplifies the difference between the two input signals. Tra-
ditional audio power amplifiers have typically offered only
single-ended inputs resulting in a 6dB reduction in signal to
noise ratio relative to differential inputs. The LM4673 also
offers the possibility of DC input coupling which eliminates
the two external AC coupling, DC blocking capacitors. The
LM4673 can be used, however, as a single ended input
amplifier while still retaining it’s fully differential benefits. In
fact, completely unrelated signals may be placed on the
input pins. The LM4673 simply amplifies the difference be-
tween the signals. A major benefit of a differential amplifier is
the improved common mode rejection ratio (CMRR) over
single input amplifiers. The common-mode rejection charac-
teristic of the differential amplifier reduces sensitivity to
ground offset related noise injection, especially important in
high noise applications.
PCB LAYOUT CONSIDERATIONS
As output power increases, interconnect resistance (PCB
traces and wires) between the amplifier, load and power
supply create a voltage drop. The voltage loss on the traces
between the LM4673 and the load results is lower output
power and decreased efficiency. Higher trace resistance
between the supply and the LM4673 has the same effect as
a poorly regulated supply, increased ripple on the supply line
also reducing the peak output power. The effects of residual
trace resistance increases as output current increases due
to higher output power, decreased load impedance or both.
To maintain the highest output voltage swing and corre-
sponding peak output power, the PCB traces that connect
the output pins to the load and the supply pins to the power
supply should be as wide as possible to minimize trace
resistance.
The use of power and ground planes will give the best
THD+N performance. While reducing trace resistance, the
use of power planes also creates parasite capacitors that
help to filter the power supply line.
The inductive nature of the transducer load can also result in
overshoot on one or both edges, clamped by the parasitic
diodes to GND and V
DD in each case. From an EMI stand-
point, this is an aggressive waveform that can radiate or
conduct to other components in the system and cause inter-
ference. It is essential to keep the power and output traces
short and well shielded if possible. Use of ground planes,
beads, and micro-strip layout techniques are all useful in
preventing unwanted interference.
As the distance from the LM4673 and the speaker increase,
the amount of EMI radiation will increase since the output
wires or traces acting as antenna become more efficient with
length. What is acceptable EMI is highly application specific.
Ferrite chip inductors placed close to the LM4673 may be
needed to reduce EMI radiation. The value of the ferrite chip
is very application specific.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection ratio (PSRR). The capacitor (C
S) location should be
as close as possible to the LM4673. Typical applications
employ a voltage regulator with a 10µF and a 0.1µF bypass
capacitors that increase supply stability. These capacitors do
not eliminate the need for bypassing on the supply pin of the
LM4673. A 4.7µF tantalum capacitor is recommended.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4673 contains shutdown circuitry that reduces current
draw to less than 0.01µA. The trigger point for shutdown is
shown as a typical value in the Electrical Characteristics
Tables and in the Shutdown Hysteresis Voltage graphs
found in the Typical Performance Characteristics section.
It is best to switch between ground and supply for minimum
current usage while in the shutdown state. While the
LM4673 may be disabled with shutdown voltages in between
ground and supply, the idle current will be greater than the
typical 0.01µA value. Increased THD may also be observed
with voltages less than V
DD on the Shutdown pin when in
PLAY mode.
The LM4673 has an internal resistor connected between
GND and Shutdown pins. The purpose of this resistor is to
eliminate any unwanted state changes when the Shutdown
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10


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