is from 2.9dB (pin22 = 4V) to -75dB (pin22 = 0V). Any DC

voltage greater than 4V will result in a gain of 2.9dB. When

the mode pin is given a logic low, the Line/HP amplifier will

be fixed at a gain of 2.9dB regardless of the voltage of pin

22. Refer to the Typical Performance Characteristics for

detailed information of the attenuation characteristics of the

DC Volume Control pin.

MICROPHONE PREAMPLIFIER

The microphone preamplifier is intended to amplify low-level

signals. The mic input can be directly connected to a micro-

phone network or to low level signal inputs. The mic amplifier

has enough output capability to drive a 1k

Ω load. A power

supply buffer is included for microphones which require ex-

ternal biasing.

POWER DISSIPATION

Power dissipation is a major concern when using any power

amplifier and must be thoroughly understood to ensure a

successful design. Equation 1 states the maximum power

dissipation point for a bridged amplifier operating at a given

supply voltage and driving a specified load.

P

(1)

Along with the bridged amplifier, the LM4834 also incorpo-

rates two single-ended amplifiers. Equation 2 states the

maximum power dissipation point for a single-ended ampli-

fier operating at a given supply voltage and driving a speci-

fied load.

P

(2)

Even with the power dissipation of the bridged amplifier

andthe two single-ended amplifiers, the LM4834 does not re-

quire heatsinking. The power dissipation from the three am-

plifiers, must not be greater than the package power dissipa-

tion that results from Equation 3:

P

(3)

For the LM4834 SSOP package,

θ

150˚C. 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 1 and 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 and 32Ω single ended loads,

the maximum ambient temperature possible without violating

the maximum junction temperature is approximately 82˚C

provided that device operation is around the maximum

power dissipation points. Power dissipation is a function of

output power and thus, if typical operation is not around the

maximum power dissipation point, the ambient temperature

can be increased. Refer to the Typical Performance Char-

acteristics curvesfor power dissipation information for differ-

ent output powers.

GROUNDING

In order to achieve the best possible performance, there are

certain grounding techniques to be followed. All input refer-

ence grounds should be tied with their respective source

grounds and brought back to the power supply ground sepa-

rately from the output load ground returns. Bringing the

ground returns for the output loads back to the supply sepa-

rately will keep large signal currents from interfering with the

stable AC input ground references.

LAYOUT

As stated in the Grounding section, placement of ground re-

turn lines is imperative in maintaining the highest level of

system performance. It is not only important to route the cor-

rect ground return lines together, but also to be aware of

where the ground return lines are routed with respect to each

other. The output load ground returns should be physically

located as far as possible from low signal level lines and their

ground return lines. Critical signal lines are those relating to

the microphone amplifier section, since these lines generally

work at very low signal levels.

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 5 volt regulator with 10 µF and a 0.1 µF by-

pass capacitors which aid in supply stability, but do not elimi-

nate the need for bypassing the supply nodes of the

LM4834. The selection of bypass capacitors, especially C

is thus dependant upon desired PSRR requirements, click

and pop performance as explained in the section, Proper

Selection of External Components, system cost, and size

constraints. It is also recommended to decouple each of the

V

PROPER SELECTION OF EXTERNAL COMPONENTS

Proper selection of external components in applications us-

ing integrated power amplifiers is critical to optimize device

and system performance. While the LM4834 is tolerant of

external component combinations, consideration to compo-

nent values must be used to maximize overall system qual-

ity.

The LM4834’s bridged amplifier should be used in low gain

configurations to minimize THD+N values, and maximize the

signal to noise ratio. Low gain configurations require large in-

put signals to obtain a given output power. Input signals

equal to or greater than 1Vrms are available from sources

such as audio codecs.

Besides gain, one of the major considerations is the closed-

loop bandwidth of the amplifier. To a large extent, the band-

width is dictated by the choice of external components

shown in

Figure 1. Both the input coupling capacitor, C

the output coupling capacitor form first order high pass filters

which limit low frequency response given in Equations 4 and

5.

f

(4)

f

(5)

These values should be chosen based on required fre-

quency response.

Selection of Input and Output Capacitor Size

Large input and output capacitors are both expensive and

space hungry for portable designs. Clearly, a certain sized

capacitor is needed to couple in low frequencies without se-

vere attenuation. In many cases the speakers used in por-

table systems, whether internal or external, have little ability

to reproduce signals below 100 Hz–150 Hz. In this case, us-

inga large input or output capacitor may not increase system

performance.

In addition to system cost and size, click and pop perfor-

mance is effected by the size of the input coupling capacitor,

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