The via diameter should be 0.012in - 0.013in. Ensure effi-

cient thermal conductivity by plating-through and solder-

filling the vias.

Best thermal performance is achieved with the largest prac-

tical copper heat sink area. In all circumstances and condi-

tions, the junction temperature must be held below 150˚C to

prevent activating the LM4923’s thermal shutdown protec-

tion. The LM4923’s power de-rating curve in the Typical

Performance Characteristics shows the maximum power

dissipation versus temperature. Example PCB layouts are

shown in the Demonstration Board Layout section. Further

detailed and specific information concerning PCB layout,

fabrication, and mounting an LLP package is available from

National Semiconductor’s package Engineering Group un-

der application note AN1187.

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. This problem of decreased load dissipation

is exacerbated 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 amplifer, 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.

P

(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.

P

(3)

Since the LM4923 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 LM4923 does not require additional heatsinking

under most operating conditions and output loading. From

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

Ω load,

the maximum power dissipation point is 625mW. The maxi-

mum power dissipation point obtained from Equation 3 must

not be greater than the power dissipation results from Equa-

tion 4:

P

θ

JA

(4)

The LM4923’s

θ

pending on the ambient temperature, T

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

θ

traces near the output, V

lower the

θ

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 62˚C pro-

vided that device operation is around the maximum power

dissipation point. Recall that internal power dissipation is a

function of output power. If typical operation is not around the

maximum power dissipation point, the LM4923 can operate

at higher ambient temperatures. Refer to the Typical Per-

formance Characteristics curves for power dissipation in-

formation.

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is

critical for low noise performance and high power supply

rejection ratio (PSRR). The capacitor location on both the

bypass and power supply pins should be as close to the

device as possible. A larger half-supply bypass capacitor

improves PSRR because it increases half-supply stability.

Typical applications employ a 5V regulator with 10µF and

0.1µF bypass capacitors that increase supply stability. This,

however, does not eliminate the need for bypassing the

supply nodes of the LM4923. The LM4923 will operate with-

out the bypass capacitor C

crease. A 1µF capacitor is recommended for C

maximizes PSRR performance. Lesser values may be used,

but PSRR decreases at frequencies below 1kHz. The issue

of C

click and pop performance as explained in the section

Proper Selection of External Components.

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the

LM4923 contains shutdown circuitry that is used to turn off

the amplifier’s bias circuitry. The device may then be placed

into shutdown mode by toggling the Shutdown Select pin to

logic low. The trigger point for shutdown is shown as a typical

value in the Supply Current vs Shutdown Voltage graphs in

the Typical Performance Characteristics section. It is best

to switch between ground and supply for maximum perfor-

mance. While the device may be disabled with shutdown

voltages in between ground and supply, the idle current may

be greater than the typical value of 0.1µA. In either case, the

shutdown pin should be tied to a definite voltage to avoid

unwanted state changes.

In many applications, a microcontroller or microprocessor

output is used to control the shutdown circuitry, which pro-

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