lm4664 National Semiconductor Corporation, lm4664 Datasheet - Page 9

• Manufacturer Part Number: lm4664
• Description: Filterless High Efficiency 1.1w Switching Audio Amplifier
• Manufacturer: National Semiconductor Corporation

Application Information
GENERAL AMPLIFIER FUNCTION
The output signals generated by the LM4664 consist of two,
BTL connected, output signals that pulse momentarily from
near ground potential to V
independently with the exception that they both may never
pulse simultaneously as this would result in zero volts across
the BTL load. The minimum width of each pulse is approxi-
mately 160ns. However, pulses on the same output can
occur sequentially, in which case they are concatenated and
appear as a single wider pulse to achieve an effective 100%
duty cycle. This results in maximum audio output power for a
given supply voltage and load impedance. The LM4664 can
achieve much higher efficiencies than class AB amplifiers
while maintaining acceptable THD performance.
The short (160ns) drive pulses emitted at the LM4664 out-
puts means that good efficiency can be obtained with mini-
mal load inductance. The typical transducer load on an audio
amplifier is quite reactive (inductive). For this reason, the
load can act as it’s own filter, so to speak. This "filter-less"
switching amplifier/transducer load combination is much
more attractive economically due to savings in board space
and external component cost by eliminating the need for a
filter.
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
(
flowing from the power supply and the audio band power
being transduced is dissipated in the LM4664 and in the
transducer load. The amount of power dissipation in the
LM4664 is very low. This is because the ON resistance of the
switches used to form the output waveforms is typically less
than 0.25Ω. This leaves only the transducer load as a po-
tential "sink" for the small excess of input power over audio
band output power. The LM4664 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 LM4664 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 LM4664 also
offers the possibility of DC input coupling which eliminates
the two external AC coupling, DC blocking capacitors. The
LM4664 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 LM4664 simply amplifies the difference be-
tween the signals. A major benefit of a differential amplifier is
the improved common mode rejection ratio (CMRR) over
>
22kHz) are not useful. The difference between the power
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. The two outputs can pulse
9
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 LM4664 and the load results is lower output
power and decreased efficiency. Higher trace resistance
between the supply and the LM4664 has the same effect as
a poorly regulated supply, increase 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 rising and falling edges are necessarily short in relation
to the minimum pulse width (160ns), having approximately
2ns rise and fall times, typical, depending on parasitic output
capacitance. 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
EMI standpoint, this is an aggressive waveform that can
radiate or conduct to other components in the system and
cause interference. 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.
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
as close as possible to the LM4664. 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
LM4664. A 1µF tantalum capacitor is recommended.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4664 contains shutdown circuitry that reduces current
draw to less than 0.4µA. 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
LM4664 may be disabled with shutdown voltages in between
ground and supply, the idle current will be greater than the
typical 0.4µA value. Increased THD may also be observed
with voltages less than V
PLAY mode.
The LM4664 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
pin is floating. The LM4664 will enter the shutdown state
when the Shutdown pin is left floating or if not floating, when
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on the Shutdown pin when in
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in each case. From an
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