LM4991LDX National Semiconductor, LM4991LDX Datasheet - Page 11

• Manufacturer Part Number: LM4991LDX
• Manufacturer: National Semiconductor

Specifications of LM4991LDX

• Operational Class: Class-AB
• Audio Amplifier Output Configuration: 1-Channel Mono
• Output Power (typ): 3x1@3OhmW
• Audio Amplifier Function: Speaker
• Total Harmonic Distortion: 0.2@8Ohm@0.5W%
• Single Supply Voltage (typ): 3/5V
• Dual Supply Voltage (typ): Not RequiredV
• Power Supply Requirement: Single
• Rail/rail I/o Type: No
• Power Supply Rejection Ratio: 64dB
• Single Supply Voltage (min): 2.2V
• Single Supply Voltage (max): 5.5V
• Dual Supply Voltage (min): Not RequiredV
• Dual Supply Voltage (max): Not RequiredV
• Operating Temp Range: -40C to 85C
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 8
• Package Type: LLP EP
• Lead Free Status / Rohs Status: Not Compliant

Application Information
LD (LLP) package is available from National Semiconduc-
tor’s Package Engineering Group under application note
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-
pendant 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 2.0W to
1.95W. This problem of decreased load dissipation is exac-
erbated as load impedance decreases. Therefore, to main-
tain 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 1, the LM4991 has two operational
amplifiers internally, allowing for a few different amplifier
configurations. The first amplifier’s gain is externally config-
urable; the second amplifier is internally fixed in a unity-gain,
inverting configuration. The closed-loop gain of the first am-
plifier is set by selecting the ratio of R
amplifier’s gain is fixed. Figure 1 shows that the output of
amplifier one serves as the input to amplifier two, which
results in both amplifiers producing signals identical in mag-
nitude, but 180˚ out of phase. Consequently, the differential
gain for the IC is
By driving the load differentially through outputs Vo1 and
Vo2, an amplifier configuration commonly referred to as
“bridged mode” is established. Bridged mode operation is
different from the classical single-ended amplifier configura-
tion where one side of its load is connected to ground.
A bridge amplifier design has a few distinct advantages over
the single-ended configuration, as it provides differential
drive to the load, thus doubling output swing for a specified
supply voltage. Four times the output power is possible as
compared to a single-ended amplifier under the same con-
ditions. This increase in attainable output power assumes
that the amplifier is not current limited or clipped. In order to
choose an amplifier’s closed-loop gain without causing ex-
cessive clipping, please refer to the Audio Power Amplifier
Design section.
Another advantage of the differential bridge output is no net
DC voltage across load. This results from biasing V
V
eliminates the coupling capacitor that single supply, single-
ended amplifiers require. Eliminating an output coupling ca-
pacitor in a single-ended configuration forces a single supply
O
2 at the same DC voltage, in this case V
A
VD
= 2 *(R
f
/R
i
)
f
to R
i
(Continued)
while the second
DD
/2 . This
O
1 and
11
amplifier’s half-supply bias voltage across the load. The
current flow created by the half-supply bias voltage in-
creases internal IC power dissipation and my permanently
damage loads such as speakers.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful amplifier, whether the amplifier is bridged or
single-ended. A direct consequence of the increased power
delivered to the load by a bridge amplifier is an increase in
internal power dissipation. Equation 1 states the maximum
power dissipation point for a bridge amplifier operating at a
given supply voltage and driving a specified output load.
Since the LM4991 has two operational amplifiers in one
package, the maximum internal power dissipation is 4 times
that of a single-ended ampifier. Even with this substantial
increase in power dissipation, the LM4991 does not require
heatsinking under most operating conditions and output
loading. From Equation 1, assuming a 5V power supply and
an 8Ω load, the maximum power dissipation point is
625 mW. The maximum power dissipation point obtained
from Equation 1 must not be greater than the power dissi-
pation that results from Equation 2:
For the SO package, θ
soldered to a DAP pad that expands to a copper area of
1.0in
150˚C for the LM4991. The θ
some form of heat sinking. The resultant θ
summation of the θ
case of the package (or to the exposed DAP, as is the case
with the LD package), θ
resistance and θ
resistance. By adding additional copper area around the
LM4991, the θ
the SO package. Increasing the copper area around the LD
package from 1.0in
to 46˚C/W. Depending on the ambient temperature, T
the θ
power dissipation supported by the IC packaging. If the
result of Equation 1 is greater than that of Equation 2, then
either the supply voltage must be decreased, the load im-
pedance increased, the θ
perature reduced. For the typical application of a 5V power
supply, with an 8Ω load, and no additional heatsinking, the
maximum ambient temperature possible without violating the
maximum junction temperature is approximately 61˚C pro-
vided that device operation is around the maximum power
dissipation point and assuming surface mount packaging.
For the LD package in a typical application of a 5V power
supply, with a 4Ω load, and 1.0in
the exposed DAP pad, the maximum ambient temperature is
approximately 77˚C providing device operation is around the
maximum power dissipation point. Internal power dissipation
is a function of output power. If typical operation is not
around the maximum power dissipation point, the ambient
temperature can be increased. Refer to the Typical Perfor-
mance Characteristics curves for power dissipation infor-
mation for different output powers and output loading.
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is critical for
low noise performance and high power supply rejection. The
capacitor location on both the bypass and power supply pins
should be as close to the LM4991 as possible. The capacitor
2
JA
, Equation 2 can be used to find the maximum internal
on a PCB, the LM4991’s θ
P
P
DMAX
JA
DMAX
SA
can be reduced from its free air value for
2
JC
= 4*(V
to 2.0in
= (T
is the heat sink to ambient thermal
, θ
JA
CS
CS
JMAX
JA
= 140˚C/W. For the LD package
DD
, and θ
is the case to heat sink thermal
decreased, or the ambient tem-
2
)
JA
area results in a θ
2
–T
/(2π
can be decreased by using
A
2
)/θ
SA
2
copper area soldered to
JA
R
JA
. θ
L
)
is 56˚C/W. T
JC
is the junction to
(2)
(1)
JA
JA
www.national.com
will be the
decrease
JMAX
A
, and
=