POWER DISSIPATION

Power dissipation is a major concern when designing a

successful amplifier, whether the amplifier is bridged or

single-ended. A direct consequence of the increased power

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

internal power dissipation. Since the LM4919 has two opera-

tional amplifiers in one package, the maximum internal

power dissipation is 4 times that of a single-ended amplifier.

The maximum power dissipation for a given BTL application

can be derived from the power dissipation graphs or from

Equation 1.

P

(1)

When operating in Single Ended mode, Equation 2 states

the maximum power dissipation point for a single-ended

amplifier operating at a given supply voltage and driving a

specified output load.

P

(2)

Since the LM4919 has two operational amplifiers in one

package, the maximum internal power dissipation point is

twice that of the number that results from Equation 2. From

Equation 2, assuming a 1.5V power supply and a 16

Ω load,

the maximum power dissipation point is 7mW per amplifier.

Thus the maximum package dissipation point is 14mW.

The maximum power dissipation point obtained from either

Equations 1, 2 must not be greater than the power dissipa-

tion that results from Equation 3:

P

θ

JA

(3)

For package MUB10A,

θ

the LM4919. Depending on the ambient temperature, T

the system surroundings, Equation 3 can be used to find the

maximum internal power dissipation supported by the IC

packaging. If the result of Equation 1 or 2 is greater than that

of Equation 3, then either the supply voltage must be de-

creased, the load impedance increased or T

the typical application of a 1.5V power supply, with a 16

Ω

load, the maximum ambient temperature possible without

violating the maximum junction temperature is approximately

146˚C provided that device operation is around the maxi-

mum power dissipation point. Thus, for typical applications,

power dissipation is not an issue. Power dissipation is a

function of output power and thus, if typical operation is not

around the maximum power dissipation point, the ambient

temperature may be increased accordingly. Refer to the

Typical Performance Characteristics curves for power dissi-

pation information for lower output powers.

POWER SUPPLY BYPASSING

As with any amplifier, proper supply bypassing is important

for low noise performance and high power supply rejection.

The capacitor location on the power supply pins should be

as close to the device as possible. Typical applications em-

ploy a battery (or 1.5V regulator) with 10µF tantalum or

electrolytic capacitor and a ceramic bypass capacitor that

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

bypassing the supply nodes of the LM4919. A bypass ca-

pacitor value in the range of 0.1µF to 1µF is recommended.

MICRO POWER SHUTDOWN

The voltage applied to the SHUTDOWN pin controls the

LM4919’s shutdown function. Activate micro-power shut-

down by applying a logic-low voltage to the SHUTDOWN

pin. When active, the LM4919’s micro-power shutdown fea-

ture turns off the amplifier’s bias circuitry, reducing the sup-

ply current. The trigger point varies depending on supply

voltage and is shown in the Shutdown Hysteresis Voltage

graphs in the Typical Performance Characteristics section.

The low 0.02µA (typ) shutdown current is achieved by ap-

plying a voltage that is as near as ground as possible to the

SHUTDOWN pin. A voltage that is higher than ground may

increase the shutdown current. There are a few ways to

control the micro-power shutdown. These include using a

single-pole, single-throw switch, a microprocessor, or a mi-

crocontroller. When using a switch, connect an external

100k

Ω pull-up resistor between the SHUTDOWN pin and

V

ground. Select normal amplifier operation by opening the

switch. Closing the switch connects the SHUTDOWN pin to

ground, activating micro-power shutdown. The switch and

resistor guarantee that the SHUTDOWN pin will not float.

This prevents unwanted state changes. In a system with a

microprocessor or 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.

MUTE

When in single ended mode, the LM4919 also features a

mute function that enables extremely fast turn-on/turn-off

with a minimum of output pop and click with a low current

consumption (

≤20µA, typical). The mute function leaves the

outputs at their bias level, thus resulting in higher power

consumption than shutdown mode, but also provides much

faster turn on/off times. Providing a logic low signal on the

MUTE pin enables mute mode. Threshold voltages and ac-

tivation techniques match those given for the shutdown func-

tion as well. Mute may not appear to function when the

LM4919 is used to drive high impedance loads. This is

because the LM4919 relies on a typical headphone load

(16-32

Ω) to reduce input signal feed-through through the

input and feedback resistors. Mute attenuation can thus be

calculated by the following formula:

Mute Attenuation (dB) = 20Log[R

Parallel load resistance may be necessary to achieve satis-

factory mute levels when the application load is known to be

high impedance. The mute function, described above, is not

necessary when the LM4919 is operating in BTL mode since

the shutdown function operates quickly in BTL mode with

less power consumption than mute. In these modes, the

Mute signal is equivalent to the Shutdown signal. Mute may

be enabled during shutdown transitions, but should not be

toggled for a brief period immediately after exiting or entering

shutdown. These brief time periods are labeled X1 (time

after returning from shutdown) and X2 (time after entering

shutdown) and are shown in the timing diagram given in

Figure 5. X1 occurs immediately following a return from

part is placed in shutdown and the decay of the bias voltage

timing of these transition periods relative to X1 and X2 is also

shown in Figure 5. While in single ended mode, mute should

not be toggled during these time periods, but may be toggled

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