LM4873MTEX National Semiconductor, LM4873MTEX Datasheet - Page 13

• Manufacturer Part Number: LM4873MTEX
• Description: IC,Audio Amplifier,DUAL,TSSOP,20PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM4873MTEX

• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Application Information
LM4863 PIN CONFIGURATION COMPATIBILITY
The LM4873’s pin configuration simplifies the process of
upgrading systems that use the LM4863. Except for its four
MUX function pins, the LM4873’s pin configuration matches
the LM4863’s pin configuration. If the LM4873’s MUX func-
tionality is not needed when replacing an LM4863, connect
the MUX CTRL pin to either V
Table 1, grounding the MUX CTRL pin selects stereo input 1
(–IN A1 and –IN B1), whereas applying V
CTRL pin selects stereo input 2 (–IN A2 and –IN B2).
STEREO-INPUT MULTIPLEXER (STEREO MUX)
Typical LM4873 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 1 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 2 or Figure 3. 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.
DD
or ground. As shown in
DD
to the MUX
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micro SMD PACKAGE PCB MOUNTING
CONSIDERATIONS
PCB layout specifications unique to the LM4873’s micro
SMD package are found in National Semiconductor’s
AN1112.
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATIONS
The LM4873’s exposed-DAP (die attach paddle) packages
(MTE, MTE-1, LQ) provide a low thermal resistance between
the die and the PCB to which the part is mounted and
soldered. This 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 thermal design. Failing to optimize thermal design
may compromise the LM4873’s high power performance and
activate unwanted, though necessary, thermal shutdown
protection.
The MTE, MTE-1, and LQ packages must have their DAPs
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 32(4x8) ( (MTE), 40(4x10) (MTE-1), or 6(3x2) (LQ)
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 LM4873 should be
5in
The last two area recommendations apply for 25˚C ambient
temperature. Increase the area to compensate for ambient
temperatures above 25˚C. In systems using cooling fans, the
LM4873MTE can take advantage of forced air cooling. With
an air flow rate of 450 linear-feet per minute and a 2.5in
exposed copper or 5.0in
the LM4873MTE can continuously drive a 3Ω load to full
2
(min) for the same supply voltage and load resistance.
FIGURE 1. Input MUX Example
2
inner layer copper plane heatsink,
2
(min) area is
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