Application Information

BRIDGE CONFIGURATION EXPLANATION

As shown in

Figure 1, the LM4864 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 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. Four times the output power is possible as

compared to a single-ended amplifier under the same condi-

tions. 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 LM4864,

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. If an output coupling capacitor is not used in a

single-ended configuration, the half-supply bias across the

load would result in both increased internal lC power dissipa-

tion as well as permanent loudspeaker damage.

POWER DISSIPATION

Power dissipation is a major concern when designing a suc-

cessful amplifier, whether the amplifier is bridged or

single-ended. Equation 1 states the maximum power dissi-

pation point for a bridge amplifier operating at a given supply

voltage and driving a specified output load.

P

Single-Ended (1)

However, a direct consequence of the increased power de-

livered to the load by a bridge amplifier is an increase in in-

ternal power dissipation point for a bridge amplifier operating

at the same conditions.

P

Bridge Mode (2)

Since the LM4864 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 LM4864 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 2 must not be greater than the power dissipa-

tion that results from Equation 3:

P

(3)

For package MUA08A,

θ

θ

T

temperature, T

be used to find the maximum internal power dissipation sup-

ported by the IC packaging. If the result of Equation 2 is

greater than that of Equation 3, then either the supply volt-

age must be decreased, the load impedance increased, the

ambient temperature reduced, or the

θ

sinking. In many cases larger traces near the output, V

and Gnd pins can be used to lower the

θ

of copper provide a form of heatsinking allowing a higher

power dissipation. 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 44˚C provided that device operation is

around the maximum power dissipation point and assuming

surface mount packaging. Internal power dissipation is a

function of output power. If typical operation is not around the

maximum power dissipation point, the ambient temperature

can be increased. Refer to the Typical Performance Char-

acteristics curves for power dissipation information for

lower output 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. The

effect of a larger half supply bypass capacitor is improved

PSRR due to increased half-supply stability. Typical applica-

tions 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 LM4864. The

selection of bypass capacitors, especially C

dent upon desired PSRR requirements, click and pop perfor-

mance as explained in the section, Proper Selection of Ex-

ternal Components, system cost, and size constraints.

SHUTDOWN FUNCTION

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

LM4864 contains a shutdown pin to externally turn off the

amplifier’s bias circuitry. This 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 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 into shutdown. Another solu-

tion is to use a single-pole, single-throw switch in conjunction

with an external pull-up resistor. When the switch is closed,

the shutdown pin is connected to ground and enables the

amplifier. If the switch is open, then the external pull-up re-

sistor will disable the LM4864. This scheme guarantees that

the shutdown pin will not float, thus preventing unwanted

state changes.

www.national.com

7