Application Information

BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 1, the LM4907 has two internal opera-

tional amplifiers. The first amplifier’s gain is externally con-

figurable, 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 20k

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 by 180˚. Consequently, the differential gain for the

IC is

A

By driving the load differentially through outputs Vo1 and

Vo2, an amplifier configuration commonly referred to as

“bridged mode” is established. Bridged mode operation is

different from the classical single-ended amplifier configura-

tion where one side of the 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 LM4907,

also creates a second advantage over single-ended amplifi-

ers. Since the differential outputs, Vo1 and Vo2, are biased

at 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. Without an output coupling capacitor, the half-supply

bias across the load would result in both increased internal

IC power dissipation and also possible loudspeaker damage.

POWER DISSIPATION

Power dissipation is a major concern when designing a

successful amplifier, whether the amplifier is bridged or

single-ended. A direct consequence of the increased power

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

internal power dissipation. Since the LM4907 has two opera-

tional amplifiers in one package, the maximum internal

power dissipation is 4 times that of a single-ended amplifier.

The maximum power dissipation for a given application can

be derived from the power dissipation graphs or from Equa-

tion 1.

P

(1)

It is critical that the maximum junction temperature T

150˚C is not exceeded. T

power derating curves by using P

area. By adding copper foil, the thermal resistance of the

application can be reduced from the free air value of

θ

resulting in higher P

protection circuitry being activated. Additional copper foil can

be added to any of the leads connected to the LM4907. It is

especially effective when connected to V

output pins. Refer to the application information on the

LM4907 reference design board for an example of good heat

sinking. If T

JMAX

still exceeds 150˚C, then additional

changes must be made. These changes can include re-

duced supply voltage, higher load impedance, or reduced

ambient temperature. Internal power dissipation is a function

of output power. Refer to the Typical Performance Charac-

teristics curves for power dissipation information for differ-

ent output powers and output loading.

EXPOSED-DAP MOUNTING CONSIDERATIONS

The LM4907’s exposed-DAP (die attach paddle) packages

(LD) 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 area heatsink, copper traces, ground plane, and

finally, surrounding air. The result is a low voltage audio

power amplifier that produces 1.07W dissipation in an 8

Ω

load at

≤ 1% THD+N. This power is achieved through careful

consideration of necessary thermal design. Failing to opti-

mize thermal design may compromise the LM4907’s perfor-

mance and activate unwanted, though necessary, thermal

shutdown protection.

The LM4907LD must have its DAP soldered to a copper pad

on the PCB. The DAP’s PCB copper pad is then, ideally,

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 or multi-layer PCB. (The heat sink area

can also be placed on an inner layer of a multi-layer board.

The thermal resistance, however, will be higher.) 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 ther-

mal conductivity by plugging and tenting the vias with plating

and solder mask, respectively.

POWER SUPPLY BYPASSING

As with any amplifier, proper supply bypassing is critical for

low noise performance and high power supply rejection. The

capacitor location on both the bypass and power supply pins

should be as close to the device as possible. Typical appli-

cations employ a 5V regulator with 10 µF tantalum or elec-

trolytic capacitor and a ceramic bypass capacitor which aid

in supply stability. This does not eliminate the need for

bypassing the supply nodes of the LM4907. The selection of

a bypass capacitor, especially C

requirements, click and pop performance (as explained in

the section, Proper Selection of External Components),

system cost, and size constraints.

SHUTDOWN FUNCTION

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

LM4907 contains shutdown circuitry that is used to turn off

the amplifier’s bias circuitry. This shutdown feature turns the

amplifier off when logic low is placed on the shutdown pin.

By switching the shutdown pin to GND, the LM4907 supply

current draw will be minimized in idle mode. Idle current is

measured with the shutdown pin connected to GND. The

trigger point for shutdown is shown as a typical value in the

Shutdown Hysteresis Voltage graphs in the Typical Perfor-

mance Characteristics section. It is best to switch between

ground and supply for maximum performance. While the

device may be disabled with shutdown voltages in between

ground and supply, the idle current may be greater than the

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