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 LM4850,

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.

P

(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.

P

(3)

The LM4850 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 LM4850 does

not require heatsinking. The power dissipation from Equation

3 must not be greater than the power dissipation predicted

by Equation 4:

P

θ

JA

(4)

For the package MTC14,

θ

age MUA10A,

θ

JA

= 194˚C/W. T

JMAX

= 150˚C for the

LM4850. Depending on the ambient temperature, T

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 decreased, the load

impedance increased, or the ambient temperature 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 ap-

proximately 55˚C for package MTC14. This assumes the

device operates at maximum power dissipation 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, operation at higher ambi-

ent temperatures is possible. Refer to the Typical Perfor-

mance Characteristics curves for power dissipation infor-

mation 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

affects the LM4850’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 LM4850. The selection of

bypass capacitors, especially C

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

LM4850 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

ensure correct shutdown operation. By switching the SHUT-

DOWN pin to V

minimized in idle mode. Whereas the device will be disabled

with shutdown voltages less than V

be greater than the typical value of 44µA. 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,

R

the SHUTDOWN pin will not float, thus preventing unwanted

state changes.

HP-IN FUNCTION

The LM4850 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.

Figure 2 shows the implementation of the LM4850’s head-

phone control. The voltage divider formed by R

sets the voltage at HP-IN to be approximately 50mV with no

headphones plugged into the system. This logic-low voltage

at the HP-IN pin enables the BTL mode

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

headphone jack’s contact pin is disconnected from the signal

pin. This also interrupts the voltage divider set up by the

resistors R

HP-IN pin, switching the LM4850 out of BTL mode and into

SE mode. The amplifier then drives the headphones, whose

impedance is in parallel with resistors R

tors R

capability since the typical impedance of headphones is

32

Ω.

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