P

Bridge Mode

(3)

The maximum power dissipation point given by Equation (3)

must not exceed the power dissipation given by Equation

(4):

P

θ

JA

(4)

The LM4914’s T

LM4914’s

θ

TA, use Equation (4) to find the maximum internal power

dissipation supported by the IC packaging. Rearranging

Equation (4) and substituting P

Equation (5). This equation gives the maximum ambient

temperature that still allows maximum mono BTL power

dissipation without violating the LM4914’s maximum junction

temperature.

T

θ

JA

(5)

For a typical application with a 5V power supply and an 8

Ω

load, the maximum ambient temperature that allows maxi-

mum BTL power dissipation without exceeding the maximum

junction temperature is approximately 134˚C for the IBL

package.

T

θ

(6)

Equation (6) gives the maximum junction temperature T

J-

MAX.

If the result violates the LM4914’s 150˚C T

the maximum junction temperature by decreasing the power

supply voltage or increasing the load resistance. Further

allowance should be made for increased ambient tempera-

tures.

The above examples assume that a device is a surface

mount part operating around the maximum power dissipation

point. Since internal power dissipation is a function of output

power, higher ambient temperatures are allowed as output

power or duty cycle decreases. If the result of Equation (3) is

greater than that of Equation (4), then decrease the supply

voltage, increase the load impedance, or reduce the ambient

temperature. If these measures are insufficient, a heat sink

can be added to reduce

θ

using additional copper area around the package, with con-

nections to the ground pin(s), supply pin and amplifier output

pins. External, solder attached SMT heatsinks such as the

Thermalloy 7106D can also improve power dissipation.

When adding a heat sink, the

θ

θ

θ

θ

θ

θ

the case-to-sink thermal impedance, and

θ

ambient thermal impedance.) Refer to the Typical Perfor-

mance Characteristics curves for power dissipation informa-

tion at lower output power levels.

EXPOSED-DAP PACKAGE PCB MOUNTING

CONSIDERATIONS

The LM4914’s exposed-DAP (die attach paddle) package

provides a low thermal resistance between the die and the

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

thermal resistance is achieved by soldering the DAP to a

copper pad on the PCB. The copper pad’s dimensions

should match the DAP’s. The copper pad should then con-

nect to a larger copper area. This area can be on the

component side, in an inner layer in a multi-layer board, or

on the board’s back side. This connection from the DAP, to

the DAP pad, and finally to a larger copper area allows rapid

heat transfer away from the die to the surrounding air. The

result is a low voltage audio power amplifier that produces

1W at = 1% THD+N with an 8

Ω load. This high power is

achieved through careful consideration of necessary thermal

design. Failing to optimize thermal design may compromise

the LM4914’s high power performance and activate un-

wanted, though necessary, thermal shutdown protection.

The MH 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. Connecting to a ground plane is permis-

sible. 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 1.27mm pitch.

Ensure efficient thermal conductivity by plating-through and

solder-filling the vias.

Best thermal performance is achieved with the largest prac-

tical copper heatsink area. If the heatsink and amplifier share

the same PCB layer, a nominal 2.5in

sary for 5V operation with an 8

Ω load. The heatsink area

should be 5in

that used by the LM4914. The last two area recommenda-

tions apply for 25˚C ambient temperature. Increase the area

to compensate for ambient temperatures above 25˚C. In all

circumstances and conditions, the junction temperature

must be held below 150˚C to prevent activating the

LM4914’s thermal shutdown protection. The LM4914’s

power de-rating curve in the Typical Performance Character-

istics shows the maximum power dissipation versus tem-

perature. An example PCB layout for the LM4914’s exposed-

DAP package is shown in the Demonstration Board Layout

section.

PCB LAYOUT AND SUPPLY REGULATION

CONSIDERATIONS FOR DRIVING 4

Ω 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 an 8

Ω load from 1W to 0.9W.

This problem of decreased load dissipation is exacerbated

as load impedance decreases. Therefore, to maintain the

highest load dissipation and widest output voltage 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, make the power supply traces

as wide as possible to maintain full output voltage swing.

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is

critical for low noise performance and high power supply

rejection. Applications that employ a 5V regulator typically

use a 10µF in parallel with a 0.1µF filter capacitors to stabi-

lize the regulator’s output, reduce noise on the supply line,

and improve the supply’s transient response. However, their

presence does not eliminate the need for a local 0.47µF

tantalum bypass capacitance connected between the

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