Application Information

EXPOSED-DAP PACKAGE PCB MOUNTING CONSIDERATION

The LM4900’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 from the die to the surrounding PCB cop-

per traces, ground plane, and surrounding air. This allows

the LM4900LD to operate at higher output power levels in

higher ambient temperatures than the MM package. Failing

to optimize thermal design may compromise the 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, 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 2 vias. The via

diameter should be 0.012in - 0.013in with a 1.27mm pitch.

Ensure efficient thermal conductivity by plating through the

vias.

Best thermal performance is achieved with the largest prac-

tical heat sink area. The power derating curve in the Typical

Performance Characteristics shows the maximum power

dissipation versus temperature for several different areas of

heat sink area. Placing the majority of the heat sink area on

another plane is preferred as heat is best dissipated through

the bottom of the chip. Further detailed and specific informa-

tion concerning PCB layout, fabrication, and mounting an LD

(LLP) package is available from National Semiconductor’s

Package Engineering Group under application note AN1187.

BRIDGE CONFIGURATION EXPLANATION

As shown in

Figure 1, the LM4900 has two operational

amplifiers internally, allowing for a few different amplifier

configurations. The first amplifier’s gain is externally config-

urable, while the second amplifier is internally fixed in a

unity-gain, inverting configuration. The closed-loop gain of

the first amplifier is set by selecting the ratio of R

the second amplifier’s gain is fixed by the two internal 10 k

Ω

resistors.

Figure 1 shows that the output of amplifier one

serves as the input to amplifier two which results in both

amplifiers producing signals identical in magnitude, but out

of phase 180˚. Consequently, the differential gain for the IC

is

A

By driving the load differentially through outputs V

an amplifier configuration commonly referred to as “bridged

mode” is established. Bridged mode operation is different

from the classical single-ended amplifier configuration where

one side of its load is connected to ground.

A bridge amplifier design has a few distinct advantages over

the single-ended configuration, as it provides differential

drive to the load, thus doubling output swing for a specified

supply voltage. Four times the output power is possible as

compared to a single-ended amplifier under the same con-

ditions. This increase in attainable output power assumes

that the amplifier is not current limited or clipped. In order to

choose an amplifier’s closed-loop gain without causing ex-

cessive clipping, please refer to the Audio Power Amplifier

Design section.

A bridge configuration, such as the one used in LM4900,

also creates a second advantage over single-ended amplifi-

ers. Since the differential outputs, V

half-supply, no net DC voltage exists across the load. This

eliminates the need for an output coupling capacitor which is

required in a single supply, single-ended amplifier configura-

tion. If an output coupling capacitor is not used in a

single-ended configuration, the half-supply bias across the

load would result in both increased internal lC power dissi-

pation as well as permanent loudspeaker damage.

POWER DISSIPATION

Power dissipation is a major concern when designing a

successful amplifier, whether the amplifier is bridged or

single-ended. Equation 1 states the maximum power dissi-

pation point for a bridge amplifier operating at a given supply

voltage and driving a specified output load.

P

Single-Ended (1)

However, a direct consequence of the increased power de-

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

internal power dissipation point for a bridge amplifier oper-

ating at the same conditions.

P

Bridge Mode (2)

Since the LM4900 has two operational amplifiers in one

package, the maximum internal power dissipation is 4 times

that of a single-ended amplifier. Even with this substantial

increase in power dissipation, the LM4900 does not require

heatsinking. From Equation 1, assuming a 5V power supply

and an 8

Ω load, the maximum power dissipation point is

625 mW. The maximum power dissipation point obtained

from Equation 2 must not be greater than the power dissi-

pation that results from Equation 3:

P

(3)

For package MUA08A,

θ

the LM4900. Depending on the ambient temperature, T

the system surroundings, Equation 3 can be used to find the

maximum internal power dissipation supported by the IC

packaging. If the result of Equation 2 is greater than that of

Equation 3, then either the supply voltage must be de-

creased, the load impedance increased, the ambient tem-

perature reduced, or the

θ

many cases larger traces near the output, V

pins can be used to lower the

θ

provide a form of heatsinking allowing a higher power dissi-

pation. 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 ap-

proximately 30˚C provided that device operation is around

the maximum power dissipation point. Internal power dissi-

pation is a function of output power. If typical operation is not

around the maximum power dissipation point, the ambient

temperature can be increased. Refer to the Typical Perfor-

mance Characteristics curves for power dissipation infor-

mation for lower output powers.

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