lm4891mm National Semiconductor Corporation, lm4891mm Datasheet - Page 9

• Manufacturer Part Number: lm4891mm
• Description: 1 Watt Audio Power Amplifier
• Manufacturer: National Semiconductor Corporation

Typical Performance
Characteristics
Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4891 has two operational
amplifiers internally, allowing for a few different amplifier
configurations. The first amplifier’s gain is externally config-
urable, while the second amplifier is internally fixed in a
unity-gain, inverting configuration. The closed-loop gain of
the first amplifier is set by selecting the ratio of R
the second amplifier’s gain is fixed by the two internal 20 kΩ
resistors. 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 magnitude, but out
of phase by 180˚. 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 the 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.
A bridge configuration, such as the one used in LM4891,
also creates a second advantage over single-ended amplifi-
ers. Since the differential outputs, Vo1 and Vo2, are biased
at half-supply, no net DC voltage exists across the load. This
eliminates the need for an output coupling capacitor which is
required in a single supply, single-ended amplifier configura-
tion. Without an output coupling capacitor, the half-supply
bias across the load would result in both increased internal
IC power dissipation and also possible loudspeaker damage.
A
Noise Floor
VD
= 2 *(R
(Continued)
f
/R
i
)
20007456
f
to R
i
while
9
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATIONS FOR THE LM4891LD
The LM4891LD’s exposed-DAP (die attach paddle) package
(LD) provides a low thermal resistance between the die and
the PCB to which the part is mounted and soldered. The
LM4891LD package should have its DAP soldered to the
grounded copper pad (heatsink) under the LM4891LD (the
NC pins, no connect, and ground pins should also be directly
connected to this copper pad-heatsink area). The area of the
copper pad (heatsink) can be determined from the LD Power
Derating graph. If the multiple layer copper heatsink areas
are used, then these inner layer or backside copper heatsink
areas should be connected to each other with 4 (2 x 2) vias.
The diameter for these vias should be between 0.013 inches
and 0.02 inches with a 0.050inch pitch-spacing. Ensure
efficient thermal conductivity by plating through and solder-
filling the vias. Further detailed information concerning PCB
layout, fabrication, and mounting an LLP package is avail-
able from National Semiconductor’s Package Engineering
Group under application note AN1187.
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. Since the LM4891 has two opera-
tional amplifiers in one package, the maximum internal
power dissipation is 4 times that of a single-ended amplifier.
The maximum power dissipation for a given application can
be derived from the power dissipation graphs or from Equa-
tion 1.
It is critical that the maximum junction temperature (T
of 150˚C is not exceeded. T
power derating curves by using P
area. By adding additional copper foil, the thermal resistance
of the application can be reduced from a free air value of
150˚C/W, resulting in higher P
can be added to any of the leads connected to the LM4891.
It is especially effective when connected to V
the output pins. Refer to the application information on the
LM4891 reference design board for an example of good heat
sinking. If T
changes must be made. These changes can include re-
duced supply voltage, higher load impedance, or reduced
ambient temperature. Internal power dissipation is a function
of output power. Refer to the Typical Performance Charac-
teristics curves for power dissipation information for differ-
ent 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 device as possible. Typical appli-
cations employ a 5V regulator with 10 µF tantalum or elec-
trolytic capacitor and a ceramic bypass capacitor which aid
in supply stability. This does not eliminate the need for
bypassing the supply nodes of the LM4891. The selection of
a bypass capacitor, especially C
requirements, click and pop performance (as explained in
the section, Proper Selection of External Components),
system cost, and size constraints.
P
JMAX
DMAX
still exceeds 150˚C, then additional
= 4*(V
DD
JMAX
)
2
/(2π
DMAX
B
can be determined from the
DMAX
, is dependent upon PSRR
2
R
. Additional copper foil
L
)
and the PC board foil
(1)
DD
, GND, and
www.national.com
JMAX
)