The LM4817 has two operational amplifiers per channel. The

maximum internal power dissipation per channel operating in

the bridge mode is four times that of a single-ended ampli-

fier. From Equation (3), assuming a 5V power supply and an

8

Ω load, the maximum single channel power dissipation is

0.633W or 1.27W for stereo operation.

P

(3)

The LM4817’s power dissipation is twice that given by Equa-

tion (2) or Equation (3) when operating in the single-ended

mode or bridge mode, respectively. Twice the maximum

power dissipation point given by Equation (3) must not ex-

ceed the power dissipation given by Equation (4):

P

θ

JA

(4)

The LM4817’s T

to a DAP pad that expands to a copper area of 2in

, the LM4817’s

θ

ture T

power dissipation supported by the IC packaging. Rearrang-

ing Equation (4) and substituting P

Equation (5). This equation gives the maximum ambient

temperature that still allows maximum stereo power dissipa-

tion without violating the LM4817’s maximum junction tem-

perature.

T

θ

JA

(5)

For a typical application with a 5V power supply and an 8

Ω

load, the maximum ambient temperature that allows maxi-

mum stereo power dissipation without exceeding the maxi-

mum junction temperature is approximately 98˚C for the MH

package.

T

θ

(6)

Equation (6) gives the maximum junction temperature T

J-

MAX

. If the result violates the LM4817’s 150˚C, reduce the

maximum junction temperature by reducing the power sup-

ply voltage or increasing the load resistance. Further allow-

ance should be made for increased ambient temperatures.

The above examples assume that a device is a surface

mount part operating around the maximum power dissipation

point. Since internal power dissipation is a function of output

power, higher ambient temperatures are allowed as output

power or duty cycle decreases.

If twice the value given by Equation (3) exceeds the result of

Equation (4), then decrease the supply voltage, increase the

load impedance, or reduce the ambient temperature. If these

measures are insufficient, a heat sink can be added to

reduce

θ

copper area around the package, with connections to the

ground pin(s), supply pin and amplifier output pins. External,

solder attached SMT heatsinks such as the Thermalloy

7106D can also improve power dissipation. When adding a

heat sink, the

θ

θ

θ

θ

θ

junction−to−case thermal impedance,

thermal impedance, and

θ

impedance.) Refer to the Typical Performance Characteris-

tics curves for power dissipation information at lower output

power levels.

OUTPUT VOLTAGE LIMITER

The LM4817’s adjustable output voltage limiter can be used

to set a maximum and minimum output voltage swing mag-

nitude. The voltage applied to the V

the amount voltage limit magnitude.

Without the limiter’s influence (V

maximum BTL output swing is nominally

2xV

DD

When the limiter input voltage is greater than 0V, the BTL

output voltage swing is

V

For any given value of V

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is

critical for low noise performance and high power supply

rejection. Applications that employ a 5V regulator typically

use a 10µF in parallel with a 0.1µF filter capacitors to stabi-

lize the regulator’s output, reduce noise on the supply line,

and improve the supply’s transient response. However, their

presence does not eliminate the need for a local 1.0µF

tantalum bypass capacitance connected between the

LM4817’s supply pins and ground. Do not substitute a ce-

ramic capacitor for the tantalum. Doing so may cause oscil-

lation in the output signal. Keep the length of leads and

traces that connect capacitors between the LM4817’s power

supply pin and ground as short as possible. Connecting a

1µF capacitor, C

improves the internal bias voltage’s stability and improves

the amplifier’s PSRR. The PSRR improvements increase as

the bypass pin capacitor value increases. Too large, how-

ever, increases turn-on time and can compromise amplifier’s

click and pop performance. The selection of bypass capaci-

tor values, especially C

ments, click and pop performance (as explained in the sec-

tion, Proper Selection of External Components), system

cost, and size constraints.

MICRO-POWER SHUTDOWN

The voltage applied to the SHUTDOWN pin controls the

LM4817’s shutdown function. Activate micro-power shut-

down by applying V

the LM4817’s micro-power shutdown feature turns off the

amplifier’s bias circuitry, reducing the supply current. The

logic threshold is typically V

shutdown current is achieved by applying a voltage that is as

near as V

thrat is less than V

There are a few ways to control the micro-power shutdown.

These include using a single-pole, single-throw switch, a

microprocessor, or a microcontroller. When using a switch,

connect an external 10k

Ω pull-up resistor between the

SHUTDOWN pin and V

SHUTDOWN pin and ground. Select normal amplifier opera-

tion by closing the switch. Opening the switch connects the

SHUTDOWN pin to V

ing micro-power shutdown. The switch and resistor guaran-

tee that the SHUTDOWN pin will not float. This prevents

unwanted state changes. In a system with a microprocessor

or a microcontroller, use a digital output to apply the control

voltage to the SHUTDOWN pin. Driving the SHUTDOWN pin

with active circuitry eliminates the pull up resistor.

TABLE 1. LOGIC LEVEL TRUTH TABLE FOR SHUT-

DOWN OPERATION

SHUTDOWN

OPERATIONAL MODE

Low

Full power, stereo BTL

amplifiers

High

Micro-power Shutdown

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