LM4835 datasheet(10/16 Pages) NSC

10 / 16 page

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 4

Ω will go into

thermal shutdown.

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.

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

2 is the minimum for 5V operation into 4

Ω. If the heat sink

plane is buried or not on the same side as the exposed-DAP,

5in

2 is the minimum for 5V operation into 4

Ω. 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.

The LM4835MTE requires forced-air cooling when operating

into 3

Ω.

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.

DMAX =(VDD)

2/(2

π 2R

(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.

DMAX = 4(VDD)

2/(2

π 2R

(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:

DMAX =(TJMAX −TA)/ θ JA

(3)

For the LM4835 TSSOP package,

JA = 80˚C/W and TJMAX

= 150˚C. Depending on the ambient temperature, T

A,ofthe

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 8

Ω 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

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.

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.

Ω 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 4

Ω load and 0.1Ω of

trace resistance in each output, output power at the load

drops from 2W to 1.8W.

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.

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 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

LM4835. The selection of bypass capacitors, especially C

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

DD pins with a 0.1µF capacitor to ground.

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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