LM4858MM datasheet(12/17 Pages) NSC

12 / 17 page

Application Information (Continued)

amplifier’s closed-loop gain without causing excessive out-

put signal clipping, please refer to the Audio Power Ampli-

fier Design section.

A bridge configuration, such as the one used in LM4858,

also creates a second advantage over single-ended amplifi-

ers. Since the differential outputs, BTL OUT- and BTL OUT+,

are biased at half-supply, no net DC voltage exists across

the load. This eliminates the need for the output coupling

capacitor that a single supply, single-ended amplifier con-

figuration requires. Eliminating an output coupling capacitor

in a single-ended configuration forces the half-supply bias

voltage across the load. This increases internal IC power

dissipation and may cause permanent loudspeaker damage.

POWER DISSIPATION

Whether the power amplifier is bridged or single-ended,

power dissipation is a major concern when designing the

amplifier. Equation 2 states the maximum power dissipation

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

voltage and driving a specified load.

DMAX =(VDD)

2 /(2

π2 R

L): Single-Ended

(2)

However, a direct consequence of the increased power de-

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

internal power dissipation. Equation 3 states the maximum

power dissipation point for a bridge amplifier operating at the

same given conditions.

DMAX =4x(VDD)

2 /(2

π2 R

L): Bridge Mode

(3)

The LM4858 is designed to drive either two single-ended

loads simultaneously or one mono bridged-tied load. In SE

mode, the maximum internal power dissipation is 2 times

that of Equation 2. In BTL mode, the maximum internal

power dissipation is the result of Equation 3. Even with this

substantial increase in power dissipation, the LM4858 does

not require heatsinking. The power dissipation from Equation

3 must not be greater than the power dissipation predicted

by Equation 4:

DMAX =(TJMAX -TA)/

(4)

For the package MUB10A,

JA = 194˚C/W. TJMAX = 150˚C

for the LM4858. Depending on the ambient temperature, T

of the surroundings, Equation 4 can be used to find the

maximum internal power dissipation supported by the IC

packaging. If the result of Equation 3 is greater than that of

Equation 4, 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, and an 8

Ω bridged load, the maximum ambient

temperature possible without violating the maximum junction

temperature is approximately 27˚C for package MUB10A.

This assumes the device operates at maximum power dis-

sipation and uses surface mount packaging. Internal power

dissipation is a function of output power. If typical operation

is not around the maximum power dissipation point, opera-

tion at higher ambient temperatures is possible. Refer to the

Typical Performance Characteristics curves for power dis-

sipation information for different output power levels.

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. The value of the pin bypass capacitor, C

B, directly

affects the LM4858’s half-supply voltage stability and PSRR.

The stability and supply rejection increase as the bypass

capacitor’s value increases Typical applications employ a 5V

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

aid in supply filtering. This does not eliminate the need for

bypassing the supply nodes of the LM4858. The selection of

bypass capacitors, especially C

B, is thus dependent upon

desired PSRR requirements, click and pop performance,

system cost, and size constraints.

SHUTDOWN FUNCTION

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

LM4858 features amplifier bias circuitry shutdown. This shut-

down function is activated by applying a logic high to the

SHUTDOWN pin. The trigger point is 2.0V minimum for a

logic high level, and 0.8V maximum for a logic low level. It is

best to switch between ground and the supply, V

DD,to

ensure correct shutdown operation. By switching the SHUT-

DOWN pin to V

DD, the LM4858 supply current draw will be

minimized in idle mode. Whereas the device will be disabled

with shutdown voltages less than V

DD, the idle current may

be greater than the typical value of 18µA. In either case, the

SHUTDOWN pin should be tied to a fixed voltage to avoid

unwanted state changes.

In many applications, a microcontroller or microprocessor

output is used to control the shutdown circuitry. This provides

a quick, smooth shutdown transition. Another solution 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, the external pull-up resistor,

PU2 will disable the LM4858. This scheme guarantees that

the SHUTDOWN pin will not float, thus preventing unwanted

state changes.

HP-IN FUNCTION

The LM4858 features a headphone control pin, HP-IN, that

enables the switching between BTL and SE modes. A logic-

low to HP-IN activates the BTL mode, while a logic-high

activates the SE mode. The trigger point is 2.0V minimum for

a logic high level and 0.8V maximum for a logic low level. A

microcontroller or microprocessor output should be used to

control the headphone sense circuitry. This provides a quick,

smooth shutdown transition. Another solution is to use a

single-pole, single-throw switch in conjunction with an exter-

nal pull-up resistor. When the switch is closed, the HP-IN pin

is connected to the ground and activates BTL mode. If the

switch is open, the external pull-up resistor, R

PU1, will acti-

vate SE mode. This scheme guarantees that the HP-IN pin

will not float, thus preventing unwanted state changes.

PROPER SELECTION OF EXTERNAL COMPONENTS

Proper selection of external components in applications us-

ing integrated power amplifiers is critical for optimum device

and system performance. While the LM4858 is tolerant to a

variety of external component combinations, consideration

must be given to the external component values that maxi-

mize overall system quality.

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