LM4867MT National Semiconductor, LM4867MT Datasheet - Page 12

• Manufacturer Part Number: LM4867MT
• Description: Output-Transient-Free Dual 2.1W Audio Amplifier Plus No Coupling Capacitor Stereo Headphone Function
• Manufacturer: National Semiconductor

www.national.com
Application Information
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
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 4
not placed on the same PCB layer as the LM4867 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
LM4867MTE 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 LM4867MTE can continuously drive a 3
power. The LM4867LQ 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 LM4867’s thermal
shutdown protection. The LM4867’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 3
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, 0.1 trace resistance reduces
the output power dissipated by a 4 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.
2
(min) for the same supply voltage and load resistance.
2
inner layer copper plane heatsink,
load. Heatsink areas
AND 4
(Continued)
2
(min) area is
load to full
LOADS
2
12
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 4 , the LM4867 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 20k
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-
tical in magnitude, but 180˚ out of phase. Taking advantage
of this phase difference, a load is placed between -OUTA
and +OUTA and driven differentially (’commonly referred to
as bridge mode’). This results in a differential gain of
Bridge mode amplifiers are different from single-ended am-
plifiers that drive loads connected between a single amplifi-
er’s output and ground. For a given supply voltage, bridge
mode has a distinct advantage over the single-ended con-
figuration: its differential output doubles the voltage swing
across the load. This produces four times the output power
when compared to a single-ended amplifier under the same
conditions. This increase in attainable output power as-
sumes that the amplifier is not current limited or that the
output signal is not clipped. To ensure minimum output sig-
nal clipping when choosing an amplifier’s closed-loop gain,
refer to the Audio Power Amplifier Design section.
A bridge amplifier design has a few distinct advantages over
the single-ended configuration, as it provides differential
drive to the load, thus doubling the output swing for a speci-
fied supply voltage. Four times the output power is possible
as compared to a single-ended amplifier under the same
conditions. This increase in attainable output power as-
sumes that the amplifier is not current limited or clipped. In
order to choose an amplifier’s closed-loop gain without caus-
ing excessive clipping, please refer to the Audio Power
Amplifier Design section.
Another advantage of the differential bridge output is no net
DC voltage across the load. This is accomplished by biasing
channel A’s and channel B’s outputs at half-supply. This
eliminates the coupling capacitor that single supply,
single-ended amplifiers require. Eliminating an output cou-
pling capacitor in a single-ended configuration forces a
single-supply amplifier’s half-supply bias voltage across the
load. This increases internal IC power dissipation and may
permanently damage loads such as speakers.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful single-ended or bridged amplifier. Equation (2)
states the maximum power dissipation point for a
single-ended amplifier operating at a given supply voltage
and driving a specified output load.
However, a direct consequence of the increased power de-
livered to the load by a bridge amplifier is higher internal
power dissipation for the same conditions.
The LM4867 has two operational amplifiers per channel. The
maximum internal power dissipation per channel operating in
f
and R
i
set the closed-loop gain of Amp1A, whereas two
P
DMAX
resistors set Amp2A’s gain at -1. The LM4867
= (V
A
DD
VD
)
2
/(2
= 2
2
*
R
(R
L
):
f
/R
Single-Ended
i
)
(1)
(2)