lm4995tmx National Semiconductor Corporation, lm4995tmx Datasheet - Page 10

• Manufacturer Part Number: lm4995tmx
• Description: Boomer-r Audio Power Amplifier Series1.3 W Audio Power Amplifier
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4995 has two internal opera-
tional amplifiers. 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
ond amplifier's gain is fixed by the two internal 20kΩ resistors.
Figure 1 shows that the output of amplifier one serves as the
input to amplifier two which results in both amplifiers produc-
ing 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 configuration 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 conditions. This
increase in attainable output power assumes that the ampli-
fier is not current limited or clipped. In order to choose an
amplifier's closed-loop gain without causing excessive clip-
ping, please refer to the Audio Power Amplifier Design
section.
A bridge configuration, such as the one used in LM4995, also
creates a second advantage over single-ended amplifiers.
Since the differential outputs, Vo1 and Vo2, are biased at half-
supply, no net DC voltage exists across the load. This elimi-
nates 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
delivered to the load by a bridge amplifier is an increase in
internal power dissipation. Since the LM4995 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 Equation
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 copper foil, the thermal resistance of the ap-
plication can be reduced from the free air value of θ
ing in higher P
protection circuitry being activated. Additional copper foil can
be added to any of the leads connected to the LM4995. It is
especially effective when connected to V
P
DMAX
DMAX
= 4*(V
A
values without thermal shutdown
VD
JMAX
= 2 *(R
DD
)
2
/(2π
can be determined from the
DMAX
f
/R
2
i
)
R
L
and the PC board foil
)
f
to R
DD
, GND, and the
i
while the sec-
JA
, result-
JMAX
(1)
of
10
output pins. Refer to the application information on the
LM4995 reference design board for an example of good heat
sinking. If T
must be made. These changes can include reduced supply
voltage, higher load impedance, or reduced ambient temper-
ature. Internal power 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 supply rejection. The capac-
itor location on both the bypass and power supply pins should
be as close to the device as possible. A ceramic 0.1μF placed
in parallel with the tantalum 2.2μF bypass (C
aid in supply stability. This does not eliminate the need for
bypassing the power supply pins of the LM4995. The selec-
tion of a bypass capacitor, especially C
PSRR requirements, click and pop performance (as ex-
plained in the section, Proper Selection of External Com-
ponents), system cost, and size constraints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4995 contains shutdown circuitry that is used to turn off the
amplifier's bias circuitry. This shutdown feature turns the am-
plifier off when logic low is placed on the shutdown pin. By
switching the shutdown pin to GND, the LM4995 supply cur-
rent draw will be minimized in idle mode. Idle current is
measured with the shutdown pin connected to GND. The trig-
ger point for shutdown is shown as a typical value in the
Shutdown Hysteresis Voltage graphs in the Typical Perfor-
mance Characteristics section. It is best to switch between
ground and supply for maximum performance. While the de-
vice may be disabled with shutdown voltages in between
ground and supply, the idle current may be greater than the
typical value of 0.01µA. In either case, the shutdown pin
should be tied to a definite voltage to avoid unwanted state
changes.
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry, which pro-
vides a quick, smooth transition to shutdown. Another solution
is to use a single-throw switch in conjunction with an external
pull-up resistor. This scheme guarantees that the shutdown
pin will not float, thus preventing unwanted state changes.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications using
integrated power amplifiers is critical to optimize device and
system performance. While the LM4995 is tolerant of external
component combinations, consideration to component values
must be used to maximize overall system quality.
The LM4995 is unity-gain stable which gives the designer
maximum system flexibility. The LM4995 should be used in
low gain configurations to minimize THD+N values, and max-
imize the signal to noise ratio. Low gain configurations require
large input signals to obtain a given output power. Input sig-
nals 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 complete expla-
nation 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-
JMAX
still exceeds 150°C, then additional changes
B
, is dependent upon
B
) capacitor will