lm4893ma National Semiconductor Corporation, lm4893ma Datasheet - Page 12

• Manufacturer Part Number: lm4893ma
• Description: 1.1 Watt Audio Power Amplifier
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1 , the LM4893 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 LM4893,
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
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 LM4893 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 LM4893.
It is especially effective when connected to V
the output pins. Refer to the application information on the
LM4893 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
= 2 *(R
DD
JMAX
)
2
/(2π
DMAX
can be determined from the
DMAX
f
/R
2
R
i
. Additional copper foil
)
L
)
and the PC board foil
(1)
DD
, GND, and
f
to R
i
JMAX
while
)
12
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 LM4893. 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.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4893 contains a SHUTDOWN pin to externally turn off the
amplifier’s bias circuitry. This shutdown feature turns the
amplifier off when a logic low is placed on the SHUTDOWN
pin. By switching the SHUTDOWN pin to ground, the
LM4893 supply current draw will be minimized in idle mode.
While the device will be disabled with SHUTDOWN pin
voltages less than 0.4V
than the typical value of 0.1µA. (Idle current is measured
with the SHUTDOWN pin tied to ground).
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry to provide a
quick, smooth transition into shutdown. Another solution 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 which disables the
amplifier. If the switch is open, then the external pull-up
resistor to V
antees that the SHUTDOWN pin will not float thus prevent-
ing unwanted state changes.
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 LM4893 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
The LM4893 is unity-gain stable which gives the designer
maximum system flexibility. The LM4893 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.
Input signals equal to or greater than 1 Vrms are available
from sources such as audio codecs. Please refer to the
section, Audio Power Amplifier Design, for a more com-
plete 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-
width is dictated by the choice of external components
shown in Figure 1 . The input coupling capacitor, C
first order high pass filter which limits low frequency re-
sponse. This value should be chosen based on needed
frequency response for a few distinct reasons.
DD
will enable the LM4893. This scheme guar-
DC
, the idle current may be greater
B
, is dependent upon PSRR
i
, forms a