LM4675SD NSC [National Semiconductor], LM4675SD Datasheet - Page 11

• Manufacturer Part Number: LM4675SD
• Description: Ultra-Low EMI, Filterless, 2.65W, Mono, Class D Audio Power Amplifier with Spread Spectrum
• Manufacturer: NSC [National Semiconductor]

Application Information
GENERAL AMPLIFIER FUNCTION
The LM4675 features a filterless modulation scheme. The
differential outputs of the device switch at 300kHz from V
to GND. When there is no input signal applied, the two
outputs (V
both outputs in phase. Because the outputs of the LM4675
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 LM4675 outputs changes. For increasing output volt-
ages, the duty cycle of V
V
verse occurs, the duty cycle of V
cycle of V
pulse widths yields the differential output voltage.
SPREAD SPECTRUM MODULATION
The LM4675 features a fitlerless spread spectrum modula-
tion scheme that eliminates the need for output filters, ferrite
beads or chokes. The switching frequency varies by TBD%
about a 300kHz center frequency, reducing the wideband
spectral contend, improving EMI emissions radiated by the
speaker and associated cables and traces. Where a fixed
frequency class D exhibits large amounts of spectral energy
at multiples of the switching frequency, the spread spectrum
architecture of the LM4675 spreads that energy over a larger
bandwidth. The cycle-to-cycle variation of the switching pe-
riod does not affect the audio reproduction of efficiency.
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 LM4675 and in the
transducer load. The amount of power dissipation in the
LM4675 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 LM4675 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 LM4675 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 LM4675 also
offers the possibility of DC input coupling which eliminates
>
O
22kHz) are not useful. The difference between the power
2 decreases. For decreasing output voltages, the con-
O
O
1 and V
1 decreases. The difference between the two
O
2) switch with a 50% duty cycle, with
O
1 increases, while the duty cycle of
O
2 increases while the duty
DD
11
the two external AC coupling, DC blocking capacitors. The
LM4675 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 LM4675 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 LM4675 and the load results is lower output
power and decreased efficiency. Higher trace resistance
between the supply and the LM4675 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
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 LM4675 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 LM4675 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
as close as possible to the LM4675. 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
LM4675. A 4.7µF tantalum capacitor is recommended.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4675 contains shutdown circuitry that reduces current
draw to less than 0.01µA. The trigger point for shutdown is
DD
in each case. From an EMI stand-
S
) location should be
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