POWER DISSIPATION

Whether the power amplifier is bridged or single-ended,

power dissipation is a major concern when designing the

amplifier. Equation 1 states the maximum power dissipation

point for a single-ended amplifier operating at a given supply

voltage and driving a specified 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. Equation 2 states the maximum

power dissipation point for a bridge amplifier operating at the

same given conditions.

P

Bridge Mode

(2)

Since the LM4863 is a dual channel power amplifier, the

maximum internal power dissipation is 2 times that of Equa-

tion 1 or Equation 2 depending on the mode of operation.

Even with this substantial increase in power dissipation, the

LM4863 does not require heatsinking. The power dissipation

from Equation 2, assuming a 5V power supply and an 8

Ω

load, must not be greater than the power dissipation that re-

sults from Equation 3:

P

(3)

For packages M16A and MTA20,

θ

package N16A,

θ

JA

= 63˚C/W. T

JMAX

= 150˚C for the

LM4863. Depending on the ambient temperature, T

system 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 de-

creased, the load impedance increased, or the ambient tem-

perature reduced. For the typical application of a 5V power

supply, with an 8

Ω bridged load, the maximum ambient tem-

perature possible without violating the maximum junction

temperature is approximately 48˚C provided that device op-

eration is around the maximum power dissipation point and

assuming surface mount packaging. Internal power dissipa-

tion 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 Perfor-

mance Characteristics curves for power dissipation infor-

mation for different 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 filtering. This does not elimi-

nate the need for bypassing the supply nodes of the

LM4863. The selection of bypass capacitors, especially C

is thus dependent upon desired PSRR 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

LM4863 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

the supply V

switching the shutdown pin to V

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

will be disabled with shutdown pin voltages less than V

the idle current may be greater than the typical value of

0.7 µA. In either case, the shutdown pin should be tied to a

definite voltage 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 LM4863. This scheme guarantees that

the shutdown pin will not float, thus preventing unwanted

state changes.

HP-IN FUNCTION

The LM4863 possesses a headphone control pin that turns

off the amplifiers which drive +OutA and +OutB so that

single-ended operation 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 LM4863’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-IN pin (pin 16) to be approximately 50 mV when there

are no headphones plugged into the system. This logic-low

voltage at the HP-IN pin enables the LM4863 and places it in

bridged mode operation. Resistor R4 limits the amount of

current flowing out of the HP-IN pin when the voltage at that

pin goes below ground resulting from the music coming from

the headphone amplifier. The output coupling capacitors pro-

tect the headphones by blocking the amplifier’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-IN threshold is

set at 4V, even in an ideal situation, the output swing cannot

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-IN 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 resistors R2 and R3. Resistors R2 and R3

have negligible effect on output drive capability since the

typical impedance of headphones are 32

Ω. Also shown in

Figure 2 are the electrical connections for the headphone

jack and plug. A 3-wire plug consists of a Tip, Ring and

Sleave, where the Tip and Ring are signal carrying conduc-

tors and the Sleave is the common ground return. One con-

trol pin contact for each headphone jack is sufficient to indi-

cate to control inputs that the user has inserted a plug into a

jack and that another mode of operation is desired.

The LM4863 can be used to drive both a pair of bridged 8

Ω

speakers and a pair of 32

Ω headphones without using the

HP-IN pin. In this case the HP-IN would not be connected to

the headphone jack but to a microprocessor or a switch. By

enabling the HP-IN pin, the 8

Ω speakers can be muted.

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