lm48312tlx National Semiconductor Corporation, lm48312tlx Datasheet - Page 14

• Manufacturer Part Number: lm48312tlx
• Description: 2.6w, Ultra-low Emi, Filterless, Mono Class D Audio Power Amplifier With E2s
• Manufacturer: National Semiconductor Corporation

www.national.com
Audio Amplifier Input Capacitor Selection
Input capacitors may be required for some applications, or
when the audio source is single-ended. Input capacitors block
the DC component of the audio signal, eliminating any conflict
between the DC component of the audio source and the bias
voltage of the LM48312. The input capacitors create a high-
pass filter with the input resistors R
high pass filter is found using equation (1) below.
Where R
trical Characteristics table.
The input capacitors can also be used to remove low fre-
quency content from the audio signal. Small speakers cannot
reproduce, and may even be damaged by low frequencies.
High pass filtering the audio signal helps protect the speakers.
When the LM48312 is using a single-ended source, power
supply noise on the ground is seen as an input signal. Setting
the high-pass filter point above the power supply noise fre-
quencies, 217Hz in a GSM phone, for example, filters out the
noise such that it is not amplified and heard on the output.
Capacitors with a tolerance of 10% or better are recommend-
ed for impedance matching and improved CMRR and PSRR.
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 LM48312 and the load results in lower output
power and decreased efficiency. Higher trace resistance be-
tween the supply and the LM48312 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
IN
is the value of the input resistor given in the Elec-
f = 1 / 2
π
R
IN
C
IN
IN
. The -3dB point of the
(1)
14
Single-Ended Audio Amplifier Configuration
The LM48312 is compatible with single-ended sources. When
configured for single-ended inputs, input capacitors must be
used to block and DC component at the input of the device.
Figure 4 shows the typical single-ended applications circuit.
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 of 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. In 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 LM48312 and the speaker increases,
the amount of EMI radiation increases due to the output wires
or traces acting as antennas become more efficient with
length. Ferrite chip inductors places close to the LM48312
outputs may be needed to reduce EMI radiation.
FIGURE 4. Single-Ended Input Configuration
DD
in each case. From an EMI stand-
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