A bridge configuration, such as the one used in LM4865,

also creates a second advantage over single-ended amplifi-

ers. Since the differential outputs, V

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. 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 IC 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 single-ended 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 given conditions.

P

Bridge Mode

(2)

Since the LM4865 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 LM4865 does not require

heatsinking. From

Equation (1), assuming a 5V power sup-

ply and an 8

Ω load, the maximum power dissipation point is

633 mW. The maximum power dissipation point obtained

from

Equation (2) must not be greater than the power dissi-

pation that results from

Equation (3):

P

(3)

For package M08A,

θ

JA

= 150˚C/W, and for package

MUA08A,

θ

Depending on the ambient temperature, T

surroundings,

Equation (3) can be used to find the maximum

internal power dissipation supported by the IC packaging. If

the result of

Equation (2) is greater than that of Equation (3),

then either the supply voltage must be decreased, the load

impedance increased, or the ambient temperature reduced.

For the typical application of a 5V power supply, with an 8

Ω

load, the maximum temperature possible without violating

the maximum junction temperature is approximately 55˚C

provided that device operation is around the maximum

power dissipation point and assuming surface mount pack-

aging. 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 Characteristics 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-stability. Typical applications

employ a 5V regulator with 10 µF and a 0.1 µF bypass ca-

pacitors which aid in supply stability, but do not eliminate the

need for bypassing the supply nodes of the LM4865. The se-

lection of bypass capacitors, especially C

dent upon desired PSRR requirements, click and pop perfor-

mance 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

LM4865 contains a DC Vol/SD pin. The DC Vol/SD pin al-

lows the LM4865 to externally turn off the amplifier’s bias cir-

cuitry. The shutdown feature turns the amplifier off when the

DC Vol/SD pin is brought below 0.3 volts. When the DC

Vol/SD pin is between 0.3V to 0.5V, the LM4865 will be ei-

ther be in shutdown or mute mode. In mute mode the current

drawn will be that of the quiescent supply current. The DC

Vol/SD pin should be tied to GND supply rail for best perfor-

mance if the LM4865 is to go into shutdown mode. As the DC

Vol/SD is increased above 0.5V the amplifier will follow the

attenuation and gain curve in Typical Performance Charac-

teristics.

HP-Sense FUNCTION

The LM4865 possesses a headphone control pin that turns

off the amplifier which drives +Vo2 so that single-ended op-

eration can occur and a bridged connected load is muted.

Quiescent current consumption is reduced when the IC is in

this single-ended mode.

Figure 2 shows the implementation of the LM4865’s head-

phone control function using a single-supply headphone am-

plifier. The voltage divider of R1 and R2 sets the voltage at

the HP-Sense pin (pin 3) to be approximately 50 mV when

there are no headphones plugged into the system. This

logic-low voltage at the HP-Sense pin enables the LM4865

and places it in bridged mode operation. The output coupling

capacitors protect the headphones by blocking the amplifi-

er’s half supply DC voltage.

When there are no headphones plugged into the system and

the IC is in bridged mode configuration, both loads are es-

sentially at a 0V DC potential. Since the HP-Sense threshold

is set at 4V, even in an ideal situation, the output swing can-

not cause a false single-ended trigger.

When a set of headphones are plugged into the system, the

contact pin of the headphone jack is disconnected from the

signal pin, interrupting the voltage divider set up by resistors

R1 and R2. Resistor R1 then pulls up the HP-Sense pin, en-

abling the headphone function. This disables the second

side of the amplifier thus muting the bridged speakers. The

amplifier then drives the headphones, whose impedance is

in parallel with resistor R2. Resistor R2 has negligible effect

on output drive capability since the typical impedance of

headphones are 32

Ω.

The LM4865 can be used to drive both a bridged 8

Ω speaker

anda32

Ω headphone without using the HP-Sense pin. In

this case the HP-Sense would not be connected to the head-

phone jack but to a microprocessor or a switch. By enabling

the HP-Sense pin, the 8

Ω speaker can be muted.

PROPER SELECTION OF EXTERNAL COMPONENTS

Proper selection of external components in applications us-

ing integrated power amplifiers is critical to optimize device

and system performance. While the LM4865 is tolerant to a

variety of external component combinations, consideration

to component values must be used to maximize overall sys-

tem quality.

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