LM4836MTX/NOPB【PDF 4/27 页】2 CHANNEL(S), TONE CONTROL CIRCUIT, PDSO

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Application Information

EXPOSED-DAP PACKAGE PCB MOUNTING

CONSIDERATIONS

The LM4836’s exposed-DAP (die attach paddle) packages

(MTE and 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 4 load.

This high power is achieved through careful consideration of

necessary thermal design. Failing to optimize thermal design

may compromise the LM4836’s high power performance and

activate unwanted, though necessary, thermal shutdown

protection.

The MTE and LQ packages must have their DAPs 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 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

practical copper heat sink area. If the heatsink and amplifier

share the same PCB layer, a nominal 2.5in2 (min) area is

necessary for 5V operation with a 4

load. Heatsink areas

not placed on the same PCB layer as the LM4836 should be

5in

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

The last two area recommendations apply for 25C ambient

temperature. Increase the area to compensate for ambient

temperatures above 25C. In systems using cooling fans, the

LM4836MTE can take advantage of forced air cooling. With

an air flow rate of 450 linear-feet per minute and a 2.5in

exposed copper or 5.0in

2 inner layer copper plane heatsink,

the LM4836MTE can continuously drive a 3

load to full

power. The LM4836LQ achieves the same output power

level without forced air cooling. In all circumstances and

conditions, the junction temperature must be held below

150C to prevent activating the LM4836’s thermal shutdown

protection. The LM4836’s power de-rating curve in the

Typical Performance Characteristics shows the maximum

power dissipation versus temperature. Example PCB layouts

for the exposed-DAP TSSOP and LQ packages are shown in

the Demonstration Board Layout section. Further detailed

and specific information concerning PCB layout, fabrication,

and mounting an LQ (LLP) package is available in National

Semiconductor’s AN1187.

PCB LAYOUT AND SUPPLY REGULATION

CONSIDERATIONS FOR DRIVING 3

AND 4 LOADS

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

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

impedance decreases, load dissipation becomes increas-

ingly dependent on the interconnect (PCB trace and wire)

resistance between the amplifier output pins and the load’s

connections. 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

trace resistance

reduces the output power dissipated by a 4

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

supplies, 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 1, the LM4836 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.) External resistors

f and Ri set the closed-loop gain of Amp1A, whereas two

internal 20k

resistors set Amp2A’s gain at 1. The LM4836

drives a load, such as a speaker, connected between the two

amplifier outputs, OUTA and +OUTA.

Figure 1 shows that Amp1A’s output serves as Amp2A’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 differentially (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

amplifiers that drive loads connected between a single

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

bridge mode has a distinct advantage over the single-ended

configuration: 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

signal 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 capacitor 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

permanently damage loads such as speakers.

POWER DISSIPATION

Power dissipation is a major concern when designing a

successful single-ended or bridged amplifier. Equation (2)

states

the

maximum

power

dissipation

point

for

single-ended amplifier operating at a given supply voltage

and driving a specified output load.

DMAX =(VDD)

2/(2

π2R

Single-Ended

(2)

LM4836

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