LM4898LD datasheet(12/17 Pages) NSC

12 / 17 page

Application Information (Continued)

amplifier is not current limited or clipped. In order to choose

an amplifier’s closed-loop gain without causing excess clip-

ping, please refer to the Audio Power Amplifier Design sec-

tion.

A bridged configuration, such as the one used in theLM4898,

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

assumes that the input resistor pair and the feedback resis-

tor pair are properly matched (see Proper Selection of Ex-

ternal Components). BTL configuration eliminates the output

coupling capacitor required in single supply, single-ended

amplifier configurations. If an output coupling capacitor is not

used in a single-ended output configuration, the half-supply

bias across the load would result in both increased internal

IC power dissipation as well as permanent loudspeaker

damage. Further advantages of bridged mode operation

specific to fully differential amplifiers like the LM4898 include

increased power supply rejection ratio, common-mode noise

reduction, and click and pop reduction.

EXPOSED-DAP PACKAGE PCB MOUNTING

CONSIDERATIONS

The LM4898’s exposed-DAP (die attach paddle) package

(LD) provides a low thermal resistance between the die and

the PCB to which the part is mounted and soldered. This

allows rapid heat transfer from the die to the surrounding

PCB copper traces, ground plane and, finally, surrounding

air. The result is a low voltage audio power amplifier that

produces 1.4W at

≤1% THD with a 4Ω load. This high power

is achieved through careful consideration of necessary ther-

mal design. Failing to optimize thermal design may compro-

mise the LM4898’s high power performance and activate

unwanted, though necessary, thermal shutdown protection.

The LD package must have its DAP soldered to a copper

pad on the PCB. The DAP’s PCB copper pad is connected to

a large plane of continuous unbroken copper. This plane

forms a thermal mass and heat sink and radiation area.

Place the heat sink area on either outside plane in the case

of a two-sided PCB, or on an inner layer of a board with more

than two layers. Connect the DAP copper pad to the inner

layer or backside copper heat sink area with 4 (2x2) vias.

The via diameter should be 0.012in - 0.013in with a 0.050in

pitch. Ensure efficient thermal conductivity by plating through

and solder-filling the vias.

Best thermal performance is achieved with the largest prac-

tical copper heat sink area. If the heatsink and amplifier

share the same PCB layer, a nominal 2.5in

2 (min) area is

necessary for 5V operation with a 4

Ω load. Heatsink areas

not placed on the same PCB layer as the LM4898 should be

5in

2 (min) for the same supply voltage and load resistance.

The last two area recommendations apply for 25˚C ambient

temperature. In all circumstances and conditions, the junc-

tion temperature must be held below 150˚C to prevent acti-

vating the LM4898’s thermal shutdown protection. The

LM4898’s power derating curve in the Typical Performance

Characteristics shows the maximum power dissipation ver-

sus temperature. Further detailed and specific information

concerning PCB layout, fabrication, and mounting an LLP

package is available from National Semiconductor’s pack-

age Engineering Group under application note AN-1187.

PCB LAYOUT AND SUPPLY REGULATION

CONSIDERATIONS FOR DRIVING 3

Ω AND 4Ω LOADS

Power dissipated by a load is a function of the voltage swing

across the load and the load’s impedance. As load imped-

ance decreases, load dissipation becomes increasingly de-

pendent on the interconnect (PCB trace and wire) resistance

between the amplifier output pins and the load’s connec-

tions. Residual trace resistance causes a voltage drop,

which results in power dissipated in the trace and not in the

load as desired. For example, 0.1

Ω trace resistance reduces

the output power dissipated by a 4

Ω load from 1.4W

to1.37W. This problem of decreased load dissipation is ex-

acerbated as load impedance decreases. Therefore, to

maintain the highest load dissipation and widest output volt-

age swing, PCB traces that connect the output pins to a load

must be as wide as possible.

Poor power supply regulation adversely affects maximum

output power. A poorly regulated supply’s output voltage

decreases with increasing load current. Reduced supply

voltage causes decreased headroom, output signal clipping,

and reduced output power. Even with tightly regulated sup-

plies, trace resistance creates the same effects as poor

supply regulation. Therefore, making the power supply

traces as wide as possible helps maintain full output voltage

swing.

POWER DISSIPATION

Power dissipation is a major concern when designing a

successful amplifier, whether the amplifier is bridged or

single-ended. Equation 2 states the maximum power dissi-

pation point for a single-ended amplifier operating at a given

supply voltage and driving a specified output load.

DMAX=(VDD)

2 /(2

π2R

L) Single-Ended

(2)

However, a direct consequence of the increased power de-

livered to the load by a bridge amplifier is an increase in

internal power dissipation versus a single-ended amplifier

operating at the same conditions.

DMAX = 4*(VDD)

2/(2

π2R

L) Bridge Mode

(3)

Since the LM4898 has bridged outputs, the maximum inter-

nal power dissipation is 4 times that of a single-ended am-

plifier. Even with this substantial increase in power dissipa-

tion, the LM4898 does not require additional heatsinking

under most operating conditions and output loading. From

Equation 3, assuming a 5V power supply and an 8. load,the

maximum power dissipation point is 625mW. The maximum

power dissipation point obtained from Equation 3 must not

be greater than the power dissipation results from Equa-

tion4:

DMAX =(TJMAX -TA)/

(4)

The LM4898’s

JA in an MUA10A package is 190˚C/W.

Depending on the ambient temperature, T

A, of the system

surroundings, Equation 4 can be used to find the maximum

internal power dissipation supported by the IC packaging. If

the result of Equation 3 is greater than that of Equation 4,

then either the supply voltage must be decreased, the load

impedance increased, the ambient temperature reduced, or

the

reduced with heatsinking. In many cases, larger

traces near the output, V

DD, and GND pins can be used to

lower the

JA. The larger areas of copper provide a form of

heatsinking allowing higher power dissipation. For the typical

application of a 5V power supply, with an 8

Ω load, the

maximum ambient temperature possible without violating the

maximum junction temperature is approximately 30˚C pro-

vided that device operation is around the maximum power

dissipation point. Recall that internal power dissipation is a

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