lm48413tlx National Semiconductor Corporation, lm48413tlx Datasheet - Page 13

• Manufacturer Part Number: lm48413tlx
• Description: Ultra Low Emi, Filterless, 1.2w Stereo Class D Audio Power Amplifier With E2s And National 3d Enhancement
• Manufacturer: National Semiconductor Corporation

PCB LAYOUT GUIDELINES
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 due to the traces
between the LM48413 and the load results in lower output
power and decreased efficiency. Higher trace resistance be-
tween the supply and the LM48413 has the same effect as a
poorly regulated supply, increasing ripple on the supply line,
and reducing peak output power. The effects of residual trace
resistance increases as output current increases due to high-
er output power, decreased load impedance or both. To main-
tain 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. In addition to reducing trace resistance, the
use of power planes creates parasitic 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 micros-strip layout techniques are all useful in pre-
venting unwanted interference.
As the distance from the LM48413 and the speaker increases,
the amount of EMI radiation increases due to the output wires
DD
in each case. From an EMI stand-
FIGURE 4. THD+N Measurement Test Setup
13
or traces acting as antennas. The EMI output spectrums of
LM48413 evaluation board connected with different speaker
cable lengths to an 8Ω load were measured (See Typical
Performance Characteristics). Lengths from 3 inches to 12
inches are shown all fall within the limit of the FCC Class B
requirement.
THD+N MEASUREMENT
Class D amplifiers, by design, switch their output power de-
vices at a much higher frequency than the accepted audio
range (20Hz – 22kHz). Alternately switching the output volt-
age between V
at much higher efficiency. However, it also increases the out-
of-band noise. Since THD+N measurement is a bandwidth
limited measurement, it can be significantly affected by out-
of-band noise, resulting in a higher than expected THD+N
measurement. To achieve a more accurate measurement of
THD+N, the test equipment’s input bandwidth must be limited.
The input filter limits the out-of-band noise resulting in a more
relevant THD+N value. A low-pass filter with a cut-off at
28kHz was used in addition to the internal filter of the THD+N
measurement equipment (See Figure 4).
In real applications, the output filters are not necessary since
the speakers will act as low-pass filters blocking the remaining
switching noise and smoothing the output signals. Instead of
connecting the LM48413's BTL outputs to speakers during
measurements, the 28kHz low-pass filter is used as shown in
Figure 4. This measurement technique also applies to mea-
surements such as PSRR, CMRR, and output power.
DD
and GND allows the LM48413 to operate
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