LM4838ITL datasheet(15/36 Pages) NSC

15 / 36 page

Application Information

EXPOSED-DAP PACKAGE PCB MOUNTING

CONSIDERATIONS

The LM4838’s exposed-DAP (die attach paddle) packages

(MTE, LQ) provide 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 2.1W 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 LM4838’s high power performance and activate

unwanted, though necessary, thermal shutdown protection.

The MTE and LQ packages must have their exposed DAPs

soldered to a grounded copper pad on the PCB. The DAP’s

PCB copper pad is connected to a large grounded plane of

continuous unbroken copper. This plane forms a thermal

mass 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 32(4x8) (MTE) or 6(3x2) (LQ) 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 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 LM4838 MTE and

LQ packages should be 5in

2 (min) for the same supply

voltage and load resistance. The last two area recommen-

dations apply for 25˚C ambient temperature. Increase the

area to compensate for ambient temperatures above 25˚C.

In systems using cooling fans, the LM4838MTE can take

advantage of forced air cooling. With an air flow rate of 450

linear-feet per minute and a 2.5in

2 exposed copper or 5.0in2

inner layer copper plane heatsink, the LM4838MTE can

continuously drive a 3

Ω load to full power. The LM4838LQ

achieves the same output power level without forced air

cooling. In all circumstances and conditions, the junction

temperature must be held below 150˚C to prevent activating

the LM4838’s thermal shutdown protection. The LM4838’s

power de-rating curve in the Typical Performance Charac-

teristics shows the maximum power dissipation versus tem-

perature. Example PCB layouts for the exposed-DAP

TSSOP and LQ packages are shown in the Demonstration

Board Layout section. Further detailed and specific infor-

mation concerning PCB layout, fabrication, and mounting an

LQ (LLP) package is available in National Semiconductor’s

AN1187.

The micro SMD package (LM4838ITL) thermals work in a

similar way to the LQ and MTE packages in that a thermal

plane increases the heat transfer from the die. The thermal

plane can be any electrical potential but needs to be below

the package to aid in the spreading the heat from the die out

to surrounding PCB areas to reduce the thermal resistance

of the micro SMD package. The thermal plane is most effec-

tive when placed on the top or first internal PCB layers. The

traces connecting the bumps also contribute to spreading

heat away from the die. The same recommendations for the

size of the thermal plane as given above apply for the ITL

package, namely 2.5in

2 minimum for top layer thermal plane

and 5in

2 minimum for internal or bottom layers.

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 2.1W to 2.0W.

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, making the power supply

traces as wide as possible helps maintain full output voltage

swing.

BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 2, the LM4838 output stage consists of

two pairs of operational amplifiers, forming a two-channel

(channel A and channel B) stereo amplifier. (Though the

following discusses channel A, it applies equally to channel

B.)

Figure 2 shows that the first amplifier’s negative (-) output

serves as the second amplifier’s input. This results in both

amplifiers producing signals identical in magnitude, but 180˚

out of phase. Taking advantage of this phase difference, a

load is placed between −OUTA and +OUTA and driven dif-

ferentially (commonly referred to as “bridge mode”). This

results in a differential gain of

VD =2 * (Rf/R i)

(1)

Bridge mode amplifiers are different from single-ended am-

plifiers that drive loads connected between a single amplifi-

er’s output and ground. For a given supply voltage, bridge

mode has a distinct advantage over the single-ended con-

figuration: its differential output doubles the voltage

swing across the load. This produces four times the output

power when compared to a single-ended amplifier under the

same conditions. This increase in attainable output power

assumes that the amplifier is not current limited or that the

output signal is not clipped. To ensure minimum output sig-

nal clipping when choosing an amplifier’s closed-loop gain,

refer to the Audio Power Amplifier Design section.

Another advantage of the differential bridge output is no net

DC voltage across the load. This is accomplished by biasing

channel A’s and channel B’s outputs at half-supply. This

eliminates the coupling capacitor that single supply, single-

ended amplifiers require. Eliminating an output coupling ca-

pacitor in a single-ended configuration forces a single-supply

amplifier’s half-supply bias voltage across the load. This

increases internal IC power dissipation and may perma-

nently damage loads such as speakers.

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