Application Information

BRIDGE CONFIGURATION EXPLANATION

As shown in

Figure 1, the LM4862 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 10 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

an amplifier configuration commonly referred to as “bridged

mode” is established. Bridged mode operation is different

from the classical single-ended amplifier configuration 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. 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 LM4862,

also creates a second advantage over single-ended amplifi-

ers. Since the differential outputs, V

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

lC power dissipation and also permanent loudspeaker dam-

age.

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

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 LM4862 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 LM4862 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 package M08A,

θ

θ

on the ambient temperature, T

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 decreased, the load impedance in-

creased, or the ambient temperature reduced. For the typical

application of a 5V power supply, with an 8

Ω load, the maxi-

mum ambient temperature possible without violating the

maximum junction temperature is approximately 44˚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 PSSR 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 supply nodes of the

LM4862. The selection of bypass capacitors, especially C

is thus dependant upon desired PSSR 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

LM4862 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.

The trigger point between a logic low and logic high level is

typically half supply. It is best to switch between ground and

supply to provide maximum device performance. By switch-

ing the shutdown pin to V

draw will be minimized in idle mode. While the device will be

disabled with shutdown pin voltages less than V

current may be greater than the typical value of 0.7 µA. In ei-

ther case, the shutdown pin should be tied to a definite volt-

age because leaving the pin floating may result in an un-

wanted shutdown condition.

In many applications, a microcontroller or microprocessor

output is used to control the shutdown circuitry which pro-

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

tion is to use a single-pole, single-throw switch that when

closed, is connected to ground and enables the amplifier. If

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

Ω will

disable the LM4862. There are no soft pull-down resistors in-

side the LM4862, so a definite shutdown pin voltage must be

applied externally, or the internal logic gate will be left float-

ing which could disable the amplifier unexpectedly.

AUTOMATIC SWITCHING CIRCUIT

As shown in

Figure 2, the LM4862 and the LM4880 can be

set up to automatically switch on and off depending on

whether headphones are plugged in. The LM4880 is used to

drive a stereo single ended load, while the LM4862 drives a

bridged internal speaker.

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