LM48411_08 Datasheet, PDF (13/22 Pages) National Semiconductor (TI) – Ultra-Low EMI, Filterless, 2.5W, Stereo, Class D Audio Power Amplifier with E2S

Application Information
GENERAL AMPLIFIER FUNCTION
The LM48411 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 V_O2) switch with a 50% duty cycle, with both outputs
in phase. Because the outputs of the LM48411 are differen-
tial, the two signals cancel each other. This results in no net
voltage across the speaker, thus there is no load current dur-
ing an idle state, conserving power.
With an input signal applied, the duty cycle (pulse width) of
the LM48411 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 converse
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.
SPREAD SPECTRUM MODULATION
The LM48411 features a fitlerless spread spectrum modula-
tion scheme that eliminates the need for output filters, ferrite
beads or chokes. The switching frequency varies by ±30%
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 fre-
quency class D exhibits large amounts of spectral energy at
multiples of the switching frequency, the spread spectrum ar-
chitecture of the LM48411 spreads that energy over a larger
bandwidth. The cycle-to-cycle variation of the switching peri-
od does not affect the audio reproduction of efficiency.
ENHANCED EMISSIONS SUPPRESSION SYSTEM (E²S)
The LM48411 features National’s patent-pending E²S system
that reduces EMI, while maintaining high quality audio repro-
duction and efficiency. The E²S system features a synchro-
nizable oscillator with selectable spread spectrum, and
advanced edge rate control (ERC). The LM48411 ERC great-
ly reduces the high frequency components of the output
square waves by controlling the output rise and fall times,
slowing the transitions to reduce RF emissions, while maxi-
mizing THD+N and efficiency performance.
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 pro-
duce it with the difference being the power dissipated, typi-
cally, 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 be-
ing transduced is dissipated in the LM48411 and in the trans-
ducer load. The amount of power dissipation in the LM48411
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 potential
"sink" for the small excess of input power over audio band
output power. The LM48411 dissipates only a fraction of the
excess power requiring no additional PCB area or copper
plane to act as a heat sink.
DIFFERENTIAL AMPLIFIER EXPLANATION
As logic supply voltages continue to shrink, designers are in-
creasingly turning to differential analog signal handling to
preserve signal to noise ratios with restricted voltage swing.
The LM48411 is a fully differential amplifier that features dif-
ferential input and output stages. A differential amplifier am-
plifies the difference between the two input signals. Tradition-
al 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 LM48411 also offers
the possibility of DC input coupling which eliminates the two
external AC coupling, DC blocking capacitors. The LM48411
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
LM48411 simply amplifies the difference between the signals.
A major benefit of a differential amplifier is the improved com-
mon mode rejection ratio (CMRR) over single input amplifiers.
The common-mode rejection characteristic of the differential
amplifier reduces sensitivity to ground offset related noise in-
jection, 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 sup-
ply create a voltage drop. The voltage loss on the traces
between the LM48411 and the load results is lower output
power and decreased efficiency. Higher trace resistance be-
tween the supply and the LM48411 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 corresponding
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 fil-
ter 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 pre-
venting unwanted interference.
As the distance from the LM48411 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 LM48411 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 crit-
ical for low noise performance and high power supply rejec-
tion ratio (PSRR). The capacitor (C_S) location should be as
close as possible to the LM48411. Typical applications em-
ploy 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
LM48411. A 4.7µF tantalum capacitor is recommended.
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