Application Information

BRIDGE CONFIGURATION EXPLANATION

As shown in

Figure 1 , the LM4861 has two operational am-

plifiers internally, allowing for a few different amplifier con-

figurations. 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 40 k

Ω

resistors.

Figure 1 shows that the output of amplifier one

serves as the input to amplifier two which results in both am-

plifiers 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

V

“bridged mode” is established. Bridged mode operation is

different from the classical single-ended amplifier configura-

tion 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. Consequently, four times the output power is

possible as 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 clipped. In

order to choose an amplifier’s closed-loop gain without caus-

ing excessive clipping which will damage high frequency

transducers used in loudspeaker systems, please refer to

the Audio Power Amplifier Design section.

A bridge configuration, such as the one used in Boomer Au-

dio Power Amplifiers, also creates a second advantage over

single-ended amplifiers. Since the differential outputs, V

O1

and V

across the load. This eliminates the need for an output cou-

pling capacitor which is required in a single supply, single-

ended amplifier configuration. Without an output coupling ca-

pacitor in a single supply, single-ended amplifier, the half-

supply bias across the load would result in both increased

internal IC power dissipation and also permanent loud-

speaker damage. An output coupling capacitor forms a high

pass filter with the load requiring that a large value such as

470 µF be used with an 8

Ω load to preserve low frequency

response. This combination does not produce a flat re-

sponse down to 20 Hz, but does offer a compromise be-

tween printed circuit board size and system cost, versus low

frequency response.

POWER DISSIPATION

Power dissipation is a major concern when designing a suc-

cessful amplifier, whether the amplifier is bridged or single-

ended. A direct consequence of the increased power deliv-

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

internal power dissipation. Equation 1 states the maximum

power dissipation point for a bridge amplifier operating at a

given supply voltage and driving a specified output load.

P

(1)

Since the LM4861 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 in-

crease in power dissipation, the LM4861 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 1 must not be greater than the power dissipa-

tion that results from Equation 2:

P

(2)

For the LM4861 surface mount package,

θ

T

of the system surroundings, Equation 2 can be used to find

the maximum internal power dissipation supported by the IC

packaging. If the result of Equation 1 is greater than that of

Equation 2, then either the supply voltage must be de-

creased or the load impedance increased. For the typical ap-

plication of a 5V power supply, with an 8

Ω load, the maxi-

mum ambient temperature possible without violating the

maximum junction temperature is approximately 62.5˚C pro-

vided that device operation is around the maximum power

dissipation point. Power dissipation is a function of output

power and thus, if typical operation is not around the maxi-

mum power dissipation point, the ambient temperature can

be increased. Refer to the Typical Performance Character-

istics curves for power dissipation information for lower out-

put powers.

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is criti-

cal for low noise performance and high power supply rejec-

tion. The capacitor location on both the bypass and power

supply pins should be as close to the device as possible. As

displayed in the Typical Performance Characteristics sec-

tion, the effect of a larger half supply bypass capacitor is im-

proved low frequency THD + N due to increased half-supply

stability. Typical applications employ a 5V regulator with

10 µF and a 0.1 µF bypass capacitors which aid in supply

stability, but do not eliminate the need for bypassing the sup-

ply nodes of the LM4861. The selection of bypass capaci-

tors, especially C

quency THD + N, system cost, and size constraints.

SHUTDOWN FUNCTION

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

LM4861 contains a shutdown pin to externally turn off the

amplifier’s bias circuitry. The shutdown feature turns the am-

plifier off when a logic high is placed on the shutdown pin.

Upon going into shutdown, the output is immediately discon-

nected from the speaker. A typical quiescent current of

0.6 µA results when the supply voltage is applied to the shut-

down pin. In many applications, a microcontroller or micro-

processor output is used to control the shutdown circuitry

which provides a quick, smooth transition into shutdown. An-

other solution is to use a single-pole, single-throw switch that

when closed, is connected to ground and enables the ampli-

fier. If the switch is open, then a soft pull-up resistor of 47 k

Ω

will disable the LM4861. There are no soft pull-down resis-

tors inside the LM4861, so a definite shutdown pin voltage

must be applied externally, or the internal logic gate will be

left floating which could disable the amplifier unexpectedly.

HIGHER GAIN AUDIO AMPLIFIER

The LM4861 is unity-gain stable and requires no external

components besides gain-setting resistors, an input coupling

capacitor, and proper supply bypassing in the typical appli-

cation. However, if a closed-loop differential gain of greater

than 10 is required, a feedback capacitor may be needed, as

shown in

Figure 2, to bandwidth limit the amplifier. This feed-

back capacitor creates a low pass filter that eliminates pos-

sible high frequency oscillations. Care should be taken when

calculating the −3 dB frequency in that an incorrect combina-

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