LM4949TLEVAL National Semiconductor, LM4949TLEVAL Datasheet - Page 38

• Manufacturer Part Number: LM4949TLEVAL
• Description: BOARD EVALUATION LM4949TL
• Manufacturer: National Semiconductor
• Series: Boomer®r

Specifications of LM4949TLEVAL

• Amplifier Type: Class D
• Output Type: 2-Channel (Stereo) with Stereo Headphones
• Max Output Power X Channels @ Load: 2.5W x 2 @ 4 Ohm; 190mW x 2 @ 16 Ohm
• Voltage - Supply: 2.7 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Board Type: Fully Populated
• Utilized Ic / Part: LM4949
• Lead Free Status / RoHS Status: Not applicable / Not applicable

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Differential Audio Amplifier Configuration
As logic supply voltages continue to shrink, system designers
increasingly turn to differential signal handling to preserve
signal to noise ratio with decreasing voltage swing. The
LM4949 can be configured as a fully differential amplifier,
amplifying the difference between the two inputs. The advan-
tage of the differential architecture is any signal component
that is common to both inputs is rejected, improving common-
mode rejection (CMRR) and increasing the SNR of the am-
plifier by 6dB over a single-ended architecture. The improved
CMRR and SNR of a differential amplifier reduce sensitivity
to ground offset related noise injection, especially important
in noisy applications such as cellular phones. Driving the
LM4949 differentially also allows the inputs to be DC coupled,
eliminating two external capacitors per channel. Set bits
L1_INSEL and L2_INSEL = 0 for differential input mode. The
left and right stereo inputs have selectable differential or sin-
gle-ended input modes, while the mono input is always dif-
ferential.
Single-Ended Audio Amplifier Configuration
In single-ended input mode, the audio sources must be ca-
pacitively coupled to the LM4949. With LIN+ ≠ LIN- and RIN
+ ≠ RIN-, the loud speaker gain is 6dB more than in differential
input mode, or when LIN+ = LIN- and RIN+ = RIN-. The head-
phone gain does not change. The mono input channel is not
affected by L1_INSEL and L2_INSEL, and is always config-
ured as a differential input.
Power Dissipation and Efficiency
The major benefit of Class D amplifiers is increased efficiency
versus Class AB. The efficiency of the LM4949 speaker am-
plifiers is attributed to the output transistors’ region of opera-
tion. The Class D output stage acts as current steering
switches, consuming negligible amounts of power compared
to their Class AB counterparts. Most of the power loss asso-
ciated with the output stage is due to the IR loss of the
MOSFET on-resistance, along with the switching losses due
to gate charge.
The maximum power dissipation per headphone channel in
Capacitor-Coupled mode is given by:
In OCL mode, the maximum power dissipation per head-
phone channel increases due to the use of a third amplifier as
a buffer. The power dissipation is given by:
PROPER SELECTION OF EXTERNAL COMPONENTS
Audio Amplifier Power Supply Bypassing / Filtering
Proper power supply bypassing is critical for low noise per-
formance and high PSRR. Place the supply bypass capaci-
P
P
DMAX
DMAX
= V
= V
DD
DD
2
2
/ 2π
/ π
2
2
R
R
L
L
38
tors as close to the device as possible. Typical applications
employ a voltage regulator with 10µF and 0.1µF bypass ca-
pacitors that increase supply stability. These capacitors do
not eliminate the need for bypassing of the LM4949 supply
pins. A 1µF ceramic capacitor placed close to each supply pin
is recommended.
Bypass Capacitor Selection
The LM4949 generates a V
internally. The BYPASS capacitor, C
THD+N by reducing noise at the BYPASS node. Use a 1µF
capacitor, placed as close to the device as possible for C
Audio Amplifier Input Capacitor Selection
Input capacitors, C
of the LM4949 forms a high pass filter that removes the DC
bias from an incoming signal. The AC-coupling capacitor al-
lows the amplifier to bias the signal to an optimal DC level.
Assuming zero source impedance, the -3dB point of the high
pass filter is given by:
Choose C
of interest. Setting f
sponse of the amplifier. Use capacitors with low voltage co-
efficient dielectrics, such as tantalum or aluminum electrolyt-
ic. Capacitors with high-voltage coefficients, such as
ceramics, may result in increased distortion at low frequen-
cies. Other factors to consider when designing the input filter
include the constraints of the overall system. Although high
fidelity audio requires a flat frequency response between
20Hz and 20kHz, portable devices such as cell phones may
only concentrate on the frequency range of the spoken human
voice (typically 300Hz to 4kHz). In addition, the physical size
of the speakers used in such portable devices limits the low
frequency response; in this case, frequencies below 150Hz
may be filtered out.
PCB LAYOUT GUIDELINES
Minimize trace impedance of the power, ground and all output
traces for optimum performance. Voltage loss due to trace
resistance between the LM4949 and the load results in de-
creased output power and efficiency. Trace resistance be-
tween the power supply and GND of the LM4949 has the
same effect as a poorly regulated supply, increased ripple and
reduced peak output power. Use wide traces for power-sup-
ply inputs and amplifier outputs to minimize losses due to
trace resistance, as well as route heat away from the device.
Proper grounding improves audio performance, minimizes
crosstalk between channels and prevents switching noise
from interfering with the audio signal. Use of power and
ground planes is recommended.
Place all digital components and digital signal traces as far as
possible from analog components and traces. Do not run dig-
ital and analog traces in parallel on the same PCB layer.
IN
such that f
IN
-3dB
, in conjunction with the input impedance
f
-3dB
-3dB
too high affects the low-frequency re-
= 1 / 2πR
is well below the lowest frequency
DD
/2 common-mode bias voltage
IN
C
B
, improves PSRR and
IN
B
.