LM4877 National Semiconductor, LM4877 Datasheet - Page 8

• Manufacturer Part Number: LM4877
• Description: 1 Watt Audio Power Amplifier in micro SMD package with Shutdown Logic Low
• Manufacturer: National Semiconductor

www.national.com
Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1 , the LM4877 has two operational am-
plifiers internally, allowing for a few different amplifier con-
figurations. 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 10 k
resistors. Figure 1 shows that the output of amplifier one
serves as the input to amplifier two which results in both am-
plifiers 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 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 condi-
tions. 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 LM4877,
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.
POWER DISSIPATION
Power dissipation is a major concern when designing a suc-
cessful amplifier, whether the amplifier is bridged or single-
ended. A direct consequence of the increased power deliv-
ered to the load by a bridge amplifier is an increase in
internal power dissipation. Since the LM4877 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
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 LM4877.
It is especially effective when connected to V
the output pins. Refer to the application information on the
LM4877 reference design board for an example of good heat
sinking. If T
changes must be made. These changes can include re-
P
JMAX
DMAX
still exceeds 150˚C, then additional
= 4*(V
A
VD
JMAX
= 2 *(R
DD
)
2
/(2
can be determined from the
DMAX
DMAX
f
/R
2
R
i
. Additional copper foil
)
L
)
and the PC board foil
(1)
DD
f
, G
to R
ND
JMAX
i
, and
while
of
8
duced supply voltage, higher load impedance, or reduced
ambient temperature. The National Reference Design board
using a 5V supply and an 8 ohm load will run in a 110˚C am-
bient environement without exceeding T
dissipation is a function of output power. Refer to the Typical
Performance Characteristics curves for power dissipation
information 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 device as possible. Typical applica-
tions employ a 5V regulator with 10 µF Tantalum or electro-
lytic capacitor and a 0.1 µF bypass capacitor which aid in
supply stability. This does not eliminate the need for bypass-
ing the supply nodes of the LM4877. The selection of a by-
pass capacitor, especially C
quirements, click and pop performance as explained in the
section Proper Selection of External Components, sys-
tem cost, and size constraints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4877 contains a shutdown pin to externally turn off the
amplifier’s bias circuitry. This shutdown feature turns the am-
plifier off when a logic low is placed on the shutdown pin. By
switching the shutdown pin to ground, the LM4877 supply
current draw will be minimized in idle mode. While the device
will be disabled with shutdown pin voltages less than
0.4V
value of 0.01 µA.
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry which pro-
vides a quick, smooth transition into shutdown. Another solu-
tion is to use a single-pole, single-throw switch in conjunction
with an external pull-up resistor. When the switch is closed,
the shutdown pin is connected to ground and disables the
amplifier. If the switch is open, then the external pull-up re-
sistor will enable the LM4877. This scheme guarantees that
the shutdown pin will not float thus preventing unwanted
state changes. Another way to operate the shutdown circuit
is with a pulldown resistor (R
circuit on Figure 3. J1 operates the shutdown function. J1
must be installed to operate the part. A switch may be in-
stalled in place of J1 for easier evaluation of the shutdown
function.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications us-
ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4877 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
The LM4877 is unity-gain stable which gives a designer
maximum system flexibility. The LM4877 should be used in
low gain configurations to minimize THD+N values, and
maximize the signal to noise ratio. Low gain configurations
require large input signals to obtain a given output power. In-
put signals equal to or greater than 1 Vrms are available
from sources such as audio codecs. Please refer to the sec-
tion, Audio Power Amplifier Design, for a more complete
explanation of proper gain selection.
Besides gain, one of the major considerations is the closed-
loop bandwidth of the amplifier. To a large extent, the band-
DC
, the idle current may be greater than the typical
B
1
, is dependent upon PSRR re-
), as shown in the applications
JMAX
. Internal power