Application Information

BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 2, the LM4906 has two internal opera-

tional amplifiers. The first amplifier’s gain is either 6dB or

12dB depending on the gain select input (Low = 6dB, High =

12dB). The second amplifier’s gain is fixed by the two inter-

nal 20k

Ω resistors. Figure 2 shows that the output of ampli-

fier 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 LM4906,

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 LM4906 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 LM4906. It is

especially effective when connected to V

output pins. Refer to the application information on the

LM4906 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.

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 the power supply pin should be as

close to the device as possible. Typical applications employ

a 5V regulator with 10µF tantalum or electrolytic capacitor

and a ceramic bypass capacitor which aid in supply stability.

This does not eliminate the need for bypassing the supply

nodes of the LM4906.

TURNING ON THE LM4906

The power supply must first be applied before the application

of an input signal to the device and the ramp time to V

DD

must be less than 4ms, otherwise the wake-up time of the

device will be affected. After applying V

turn-on after an initial minimum threshold input signal of

7mV

An input signal of less than 7mV

output voltage. Once the device is turned on, the input signal

can go below the 7mV

however, SHUTDOWN or V

DD

is cycled, the minimum

threshold requirement for the input signal must first be met

again, with V

SHUTDOWN FUNCTION

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

LM4906 contains shutdown circuitry that is used to turn off

the amplifier’s bias circuitry. The device is placed into shut-

down mode by toggling the Shutdown pin Low/ground. The

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

the Supply Current vs Shutdown Voltage graphs in the Typi-

cal 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-

vides a quick, smooth transition to shutdown. Another solu-

tion is to use a single-throw switch in conjunction with an

external pull-up resistor (or pull-down, depending on shut-

down high or low application). This scheme guarantees that

the shutdown pin will not float, thus preventing unwanted

state changes.

SELECTION OF INPUT CAPACITOR SIZE

Large input capacitors are both expensive and space hungry

for portable designs. Clearly, a certain sized capacitor is

needed to couple in low frequencies without severe attenu-

ation. But in many cases the speakers used in portable

systems, whether internal or external, have little ability to

reproduce signals below 100Hz to 150Hz. Thus, using a

large input capacitor may not increase actual system perfor-

mance.

In addition to system cost and size, click and pop perfor-

mance is effected by the size of the input coupling capacitor,

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