LM4834-MS

Manufacturer Part NumberLM4834-MS
ManufacturerNational Semiconductor
LM4834-MS datasheet
 


Specifications of LM4834-MS

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Application Information
amplifiers are set at a gain of 1.4 (2.9dB). The attenuation of
the amplifiers increase until 0V is reached. The attenuator
range is from 2.9dB (pin22 = 4V) to -75dB (pin22 = 0V). Any
DC voltage greater than 4V will result in a gain of 2.9dB.
When the mode pin is given a logic low, the Line/HP amplifier
will be fixed at a gain of 2.9dB regardless of the voltage of
pin 22. Refer to the Typical Performance Characteristics
for detailed information of the attenuation characteristics of
the DC Volume Control pin.
MICROPHONE PREAMPLIFIER
The microphone preamplifier is intended to amplify low-level
signals. The mic input can be directly connected to a micro-
phone network or to low level signal inputs. The mic amplifier
has enough output capability to drive a 1kΩ load. A power
supply buffer is included for microphones which require ex-
ternal biasing.
POWER DISSIPATION
Power dissipation is a major concern when using any power
amplifier and must be thoroughly understood to ensure a
successful design. Equation 1 states the maximum power
dissipation point for a bridged amplifier operating at a given
supply voltage and driving a specified load.
2
2
P
= 4(V
)
/(2π
DMAX
DD
Along with the bridged amplifier, the LM4834 also incorpo-
rates two single-ended amplifiers. Equation 2 states the
maximum power dissipation point for a single-ended ampli-
fier operating at a given supply voltage and driving a speci-
fied load.
2
2
P
= (V
)
/(2π
DMAX
DD
Even with the power dissipation of the bridged amplifier
andthe two single-ended amplifiers, the LM4834 does not
require heatsinking. The power dissipation from the three
amplifiers, must not be greater than the package power
dissipation that results from Equation 3:
)/ θ
P
= (T
− T
DMAX
JMAX
A
For the LM4834 SSOP package, θ
JA
150˚C. Depending on the ambient temperature, T
system surroundings, Equation 3 can be used to find the
maximum internal power dissipation supported by the IC
packaging. If the result of Equation 1 and 2 is greater than
that of Equation 3, then either the supply voltage must be
decreased, the load impedance increased, or the ambient
temperature reduced. For the typical application of a 5V
power supply, with an 8Ω bridged load and 32Ω single ended
loads, the maximum ambient temperature possible without
violating the maximum junction temperature is approximately
82˚C provided that device operation is around the maximum
power dissipation points. Power dissipation is a function of
output power and thus, if typical operation is not around the
maximum power dissipation point, the ambient temperature
can be increased. Refer to the Typical Performance Char-
acteristics curvesfor power dissipation information for differ-
ent output powers.
GROUNDING
In order to achieve the best possible performance, there are
certain grounding techniques to be followed. All input refer-
ence grounds should be tied with their respective source
grounds and brought back to the power supply ground sepa-
rately from the output load ground returns. Bringing the
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ground returns for the output loads back to the supply sepa-
(Continued)
rately will keep large signal currents from interfering with the
stable AC input ground references.
LAYOUT
As stated in the Grounding section, placement of ground
return lines is imperative in maintaining the highest level of
system performance. It is not only important to route the
correct ground return lines together, but also to be aware of
where the ground return lines are routed with respect to each
other. The output load ground returns should be physically
located as far as possible from low signal level lines and their
ground return lines. Critical signal lines are those relating to
the microphone amplifier section, since these lines generally
work at very low signal levels.
POWER SUPPLY BYPASSING
As with any power 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. The effect of a larger half supply bypass capacitor
is improved PSRR due to increased half-supply stability.
Typical applications employ a 5 volt regulator with 10 µF and
a 0.1 µF bypass capacitors which aid in supply stability, but
do not eliminate the need for bypassing the supply nodes of
the LM4834. The selection of bypass capacitors, especially
R
) (1)
L
C
, is thus dependant upon desired PSRR requirements,
B
click and pop performance as explained in the section,
Proper Selection of External Components, system cost,
and size constraints. It is also recommended to decouple
each of the V
R
) (2)
PROPER SELECTION OF EXTERNAL COMPONENTS
L
Proper selection of external components in applications us-
ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4834 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
(3)
ity.
JA
= 95˚C/W and T
=
The LM4834’s bridged amplifier should be used in low gain
JMAX
, of the
configurations to minimize THD+N values, and maximize the
A
signal to noise ratio. Low gain configurations require large
input signals to obtain a given output power. Input signals
equal to or greater than 1Vrms are available from sources
such as audio codecs.
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. Both the input coupling capacitor, C
the output coupling capacitor form first order high pass filters
which limit low frequency response given in Equations 4 and
5.
These values should be chosen based on required fre-
quency response.
Selection of Input and Output Capacitor Size
Large input and output capacitors are both expensive and
space hungry for portable designs. Clearly, a certain sized
capacitor is needed to couple in low frequencies without
severe attenuation. In many cases the speakers used in
portable systems, whether internal or external, have little
12
pins with a 0.1µF capacitor to ground.
DD
f
= 1/(2πR
C
) (4)
IC
i
i
f
= 1/(2πR
C
) (5)
OC
L
O
, and
I