Application Information

BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 1 , the LM4861 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 40k

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

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

Audio Power Amplifiers, also creates a second advantage

over single-ended amplifiers. Since the differential outputs,

V

exists across the load. This eliminates the need for an output

coupling capacitor which is required in a single supply,

single-ended amplifier configuration. Without an output cou-

pling capacitor in a single supply, single-ended amplifier, the

half-supply bias across the load would result in both in-

creased internal IC power dissipation and also permanent

loudspeaker damage. An output coupling capacitor forms a

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

as 470µF be used with an 8

Ω load to preserve low frequency

response. This combination does not produce a flat re-

sponse down to 20Hz, but does offer a compromise between

printed circuit board size and system cost, versus low fre-

quency response.

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. 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

increase 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

625mW.The maximum power dissipation point obtained from

Equation 1 must not be greater than the power dissipation

that results from Equation 2:

P

θ

JA

(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

application of a 5V power supply, with an 8

Ω load, the

maximum ambient temperature possible without violating the

maximum junction temperature is approximately 62.5˚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 Charac-

teristics curves for power dissipation information for lower

output powers.

POWER SUPPLY BYPASSING

As with any power 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. As displayed in the Typical Performance Charac-

teristics section, the effect of a larger half supply bypass

capacitor is improved low frequency THD+N due to in-

creased half-supply stability. Typical applications employ a

5V regulator with 10µF and a 0.1µF bypass capacitors which

aid in supply stability, but do not eliminate the need for

bypassing the supply nodes of the LM4861. The selection of

bypass capacitors, especially C

desired low frequency THD+N, system cost, and size con-

straints.

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

amplifier 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 0.6µA

results when the supply voltage is applied to the shutdown

pin. In many applications, a microcontroller or microproces-

sor output is used to control the shutdown circuitry which

provides a quick, smooth transition into shutdown. Another

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

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

plifier. If the switch is open, then a soft pull-up resistor of

47k

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

resistors 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 unex-

pectedly.

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

feedback capacitor creates a low pass filter that eliminates

www.national.com

8