LM48413

SNAS460B – NOVEMBER 2008 – REVISED MARCH 2013

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PCB LAYOUT GUIDELINES

As output power increases, interconnect resistance (PCB traces and wires) between the amplifier, load and

power supply 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 between 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 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. 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 VDD in each case. From an 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 micros-strip layout

techniques are all useful in preventing unwanted interference.

As the distance from the LM48413 and the speaker increases, the amount of EMI radiation increases due to the

output wires or traces acting as antennas. The EMI output spectrums of LM48413 evaluation board connected

with different speaker cable lengths to an 8

Ω 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 devices at a much higher frequency than the accepted

audio range (20Hz – 22kHz). Alternately switching the output voltage between VDD and GND allows the LM48413

to operate 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 19).

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 19. This measurement

technique also applies to measurements such as PSRR, CMRR, and output power.

Figure 19. THD+N Measurement Test Setup

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