lm4883sqx National Semiconductor Corporation, lm4883sqx Datasheet - Page 14

• Manufacturer Part Number: lm4883sqx
• Description: Dual 2.1w Audio Amplifier Plus Stereo Headphone
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
STEREO-INPUT MULTIPLEXER (STEREO MUX)
Typical LM4883 applications use the MUX to switch between
two stereo input signals. Each stereo channel’s gain can be
tailored to produce the required output signal level. Choos-
ing the input and feedback resistor ratio sets a MUX chan-
nel’s gain. Another configuration uses the MUX to select two
different gains or frequency compensated gains to amplify a
single pair of stereo input signals. Figure 2 shows two differ-
ent feedback networks, Network 1 and Network 2. Network 1
produces increasing gain as the input signal’s frequency
decreases. This can be used to compensate a small, full-
range speaker’s low frequency response roll-off. Network 2
sets the gain for an alternate load such as headphones.
Connecting the MUX CTRL and HP-IN pins together applies
the same control voltage to the MUX pins when connecting
and disconnecting headphones using the headphone jack
shown in Figure 3 or Figure 4. Simultaneously applying the
control voltage automatically selects the amplifier (head-
phone or bridge loads) and switches the gain (MUX channel
selection). Alternatively, leave the control pins independently
accessible. This allows a user to select bass boost as
needed. This alternative user-selectable bass-boost scheme
requires connecting equal ratio resistor feedback networks
to each MUX input channel. The value of the resistor in the
RC network is chosen to give a gain that is necessary to
achieve the desired bass-boost.
Switching between the MUX channels may change the input
signal source or the feedback resistor network. During the
channel switching transition, the average voltage level
present on the internal amplifier’s input may change. This
change can slew at a rate that may produce audible voltage
transients or clicks in the amplifier’s output signal. Using the
MUX to select between two vastly dissimilar gains is a typical
transient-producing situation. As the MUX is switched, an
audible click may occur as the gain suddenly changes.
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATIONS
The LM4883’s SQ exposed-DAP (die attach paddle) pack-
age provides a low thermal resistance between the die and
the PCB to which the part is mounted and soldered. This
FIGURE 2. Input MUX Example
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14
allows rapid heat transfer from the die to the surrounding
PCB copper traces, ground plane and, finally, surrounding
air. The result is a low voltage audio power amplifier that
produces 2.1W at ≤ 1% THD with a 4Ω load. This high power
is achieved through careful consideration of necessary ther-
mal design. Failing to optimize thermal design may compro-
mise the LM4883SQ’s high power performance and activate
unwanted, though necessary, thermal shutdown protection.
The SQ package must have its DAP soldered to a copper
pad on the PCB. The DAP’s PCB copper pad is connected to
a large plane of continuous unbroken copper. This plane
forms a thermal mass and heat sink and radiation area.
Place the heat sink area on either outside plane in the case
of a two-sided PCB, or on an inner layer of a board with more
than two layers. Connect the DAP copper pad to the inner
layer or backside copper heat sink area with 6 (3x2) SQ vias.
The via diameter should be 0.012in–0.013in with a 1.27mm
pitch. Ensure efficient thermal conductivity by plating-
through and solder-filling the vias.
Best thermal performance is achieved with the largest prac-
tical copper heat sink area. If the heatsink and amplifier
share the same PCB layer, a nominal 2.5in
necessary for 5V operation with a 4Ω load. Heatsink areas
not placed on the same PCB layer as the LM4883SQ should
be 5in
tance. The last two area recommendations apply for 25˚C
ambient temperature. Increase the area to compensate for
ambient temperatures above 25˚C. In all circumstances and
conditions, the junction temperature must be held below
150˚C to prevent activating the LM4883SQ’s thermal shut-
down protection. The LM4883SQ’s power de-rating curve in
the Typical Performance Characteristics shows the maxi-
mum power dissipation versus temperature. Example PCB
layouts for the exposed-Dap SQ package is shown in the
Demonstration Board Layout section. Further detailed and
specific information concerning PCB layout, fabrication, and
mounting an SQ package is available from National Semi-
conductor’s AN1187.
PCB LAYOUT AND SUPPLY REGULATION
CONSIDERATIONS FOR DRIVING 3Ω AND 4Ω LOADS
Power dissipated by a load is a function of the voltage swing
across the load and the load’s impedance. As load imped-
ance decreases, load dissipation becomes increasingly de-
pendent on the interconnect (PCB trace and wire) resistance
between the amplifier output pins and the load’s connec-
tions. Residual trace resistance causes a voltage drop,
which results in power dissipated in the trace and not in the
load as desired. For example, 0.1Ω trace resistance reduces
the output power dissipated by a 4Ω load from 2.1W to 2.0W.
This problem of decreased load dissipation is exacerbated
as load impedance decreases. Therefore, to maintain the
highest load dissipation and widest output voltage swing,
PCB traces that connect the output pins to a load must be as
wide as possible.
Poor power supply regulation adversely affects maximum
output power. A poorly regulated supply’s output voltage
decreases with increasing load current. Reduced supply
voltage causes decreased headroom, output signal clipping,
and reduced output power. Even with tightly regulated sup-
plies, trace resistance creates the same effects as poor
supply regulation. Therefore, making the power supply
traces as wide as possible helps maintain full output voltage
swing.
2
(min) for the same supply voltage and load resis-
2
(min) area is