typical operation is not around the maximum power dissipa-
tion point, the ambient temperature can be increased. Refer
to the Typical Performance Characteristics curves for
power dissipation information for different output powers.
Application Information
EXPOSED-DAP MOUNTING CONSIDERATIONS
The exposed-DAP (die attach pad) must be tied to ground.
The exposed-DAP of the LM4835MTE requires special at-
tention to thermal design. If thermal design issues are not
properly addressed, an LM4835MTE driving 4Ω will go into
thermal shutdown.
LAYOUT
As stated in the Grounding section, placement of ground re-
turn lines is imperative in maintaining the highest level of
system performance. It is not only important to route the cor-
rect 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.
The exposed-DAP on the bottom of the LM4835MTE should
be soldered down to a copper plane on the circuit board. The
copper plane will conduct heat away from the exposed-DAP.
If the copper plane is not on the top surface of the circuit
board, 20 to 30 vias of 0.010 inches or smaller in diameter
should be used to thermally couple the exposed-DAP to the
plane. For good thermal conduction, the vias must be
plated-through and solder-filled.
3Ω and 4Ω Layout Considerations
With low impedance loads, the output power at the loads is
heavily dependent on trace resistance from the output pins
of the LM4835. Traces from the output of the LM4835MTE to
the load or load connectors should be as wide as practical.
Any resistance in the output traces will reduce the power de-
livered to the load. For example, with a 4Ω load and 0.1Ω of
trace resistance in each output, output power at the load
drops from 2W to 1.8W.
The copper plane used to conduct heat away from the
exposed-DAP should be as large as practical. If the plane is
on the same side of the circuit board as the exposed-DAP, 2
in2 is the minimum for 5V operation into 4Ω. If the heat sink
plane is buried or not on the same side as the exposed-DAP,
5in2 is the minimum for 5V operation into 4Ω. If the ambient
temperature is higher than 25˚C, a larger copper plane or
forced-air cooling may be required to keep the LM4835MTE
junction temperature below the thermal shutdown tempera-
ture (150˚C). See the power derating curve for the
LM4835MTE for derating information.
Output power is also dependent on supply regulation. To
keep the supply voltage from sagging under full output con-
ditions, the supply traces should be as wide as practical.
The LM4835MTE requires forced-air cooling when operating
into 3Ω.
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
ground returns for the output loads back to the supply sepa-
rately will keep large signal currents from interfering with the
stable AC input ground references. The exposed-DAP of the
LM4835MTE package must be tied to ground.
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 single-ended amplifier operating at a
given supply voltage and driving a specified load.
PDMAX = (VDD)2/(2π 2RL) (1)
However, a direct consequence of the increased power de-
livered to the load by a bridged amplifier is an increase in in-
ternal power dissipation. Equation 2 states the maximum
power dissipation point for a bridged amplifier operating at a
given supply voltage and driving a specified load.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is criti-
cal for low noise performance and high power supply rejec-
tion. 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 applica-
tions employ a 5 volt regulator with 10 µF and a 0.1 µF by-
pass capacitors which aid in supply stability, but do not elimi-
nate the need for bypassing the supply nodes of the
PDMAX = 4(VDD)2/(2π 2RL) (2)
Since theLM4835 is a stereo power amplifier, the maximum
internal power dissipation is two times that of Equation 1 or
Equation 2 depending on the mode of operation. Even with
the power dissipation of the stereo amplifiers, the LM4835
does not require heatsinking. The power dissipation from the
amplifiers, must not be greater than the package power dis-
sipation that results from Equation 3:
LM4835. The selection of bypass capacitors, especially C B
,
is thus dependant upon desired PSRR requirements, 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
VDD pins with a 0.1µF capacitor to ground.
PDMAX = (TJMAX − TA)/ θ
(3)
JA
For the LM4835 TSSOP package, θJA = 80˚C/W and TJMAX
= 150˚C. Depending on the ambient temperature, T A, of the
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 de-
creased, the load impedance increased, or the ambient tem-
perature reduced. For the typical application of a 5V power
supply, with an 8Ω bridged loads, the maximum ambient
temperature possible without violating the maximum junction
temperature is approximately 48˚C provided that device op-
eration is around the maximum power dissipation points.
Power dissipation is a function of output power and thus, if
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 LM4835 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
The LM4835’s bridged amplifier should be used in low gain
configurations to minimize THD+N values, and maximize the
signal to noise ratio. Low gain configurations require large in-
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