LM4868MT National Semiconductor, LM4868MT Datasheet - Page 14

• Manufacturer Part Number: LM4868MT
• Description: Output-Transient-Free Dual 2.1W Audio Amplifier Plus No Coupling Capacitor Stereo Headphone Function
• Manufacturer: National Semiconductor

www.national.com
Application Information
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.
PIN OUT COMPATIBILITY WITH THE LM4863
The LM4868 pin out was designed to simplify replacing the
LM4863: except for the four Pins(-IN A
and NC) that implement the LM4868’s extra functionality, the
LM4868MT/MTE and LM4863MT/MTE pin outs match.
(Note 19)
Note 19: If the LM4868 replaces an LM4863 and the input MUX circuitry is
not being used, the LM4868 MUX CTRL pin must be tied to V
the unused MUX inputs must be connected to GND.
EXPOSED-DAP MOUNTING CONSIDERATIONS
The LM4868’s exposed-DAP (die attach paddle) packages
(MTE and 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 surround-
ing PCB copper area heatsink, copper traces, ground plane,
and finally, surrounding air. The result is a low voltage audio
power amplifier that produces 2.4W dissipation in a 4Ω load
at ≤ 1% THD+N and over 3W in a 3Ω load at 10% THD+N.
This high power is achieved through careful consideration of
necessary thermal design. Failing to optimize thermal design
may compromise the LM4868’s high power performance and
activate unwanted, though necessary, thermal shutdown
protection.
The MTE and LQ packages must have their DAPs soldered
to a copper pad on the PCB. The DAP’s PCB copper pad is
then, ideally, connected to a large plane of continuous un-
broken copper. This plane forms a thermal mass, heat sink,
and radiation area. Place the heat sink area on either outside
plane in the case of a two-sided or multi-layer PCB. (The
heat sink area can also be placed on an inner layer of a
multi-layer board. The thermal resistance, however, will be
higher.) Connect the DAP copper pad to the inner layer or
backside copper heat sink area with 32 (4 X 8) (MTE) or 6 (3
X 2) (LQ) vias. The via diameter should be 0.012in - 0.013in
amplifier and, with the additional NC switch, changes
FIGURE 6. As configured, connecting headphones to
this jack automatically selects the stereo headphone
MUX channels (Network 2 in Figure 5 )
2
, MUX CTRL, -IN B
(Continued)
DD
or GND and
20026739
2
,
14
with a 1.27mm pitch. Ensure efficient thermal conductivity by
plugging and tenting the vias with plating and solder mask,
respectively.
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 LM4868 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
LM4868MTE 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 LM4868MTE can continuously drive a 3Ω load to full
power. The LM4868LQ achieves the same output power
level without forced-air cooling. In all circumstances and
under all conditions, the junction temperature must be held
below 150˚C to prevent activating the LM4868’s thermal
shutdown protection. The LM4868’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 TSSOP and LQ packages are
shown in the Demonstration Board Layout section. Further
detailed and specific information concerning PCB layout and
fabrication and mounting an LQ (LLP) is found in National
Semiconductor’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.
The 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.
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 4, the LM4868 consists of two pairs of
operational amplifiers, forming a two-channel (channel A and
channel B) stereo amplifier. (Though the following discusses
channel A, it applies equally to channel B.) External resistors
R
internal 20kΩ resistors set Amp2A’s gain at -1. The LM4868
drives a load, such as a speaker, connected between the two
amplifier outputs, -OUTA and +OUTA.
Figure 4 shows that Amp1A’s output serves as Amp2A’s
input. This results in both amplifiers producing signals iden-
f
2
and R
(min) for the same supply voltage and load resistance.
i
set the closed-loop gain of Amp1A, whereas two
2
inner layer copper plane heatsink,
2
(min) area is
2