of a single-ended amplifier. However, even with this substan-

tial increase in power dissipation, the LM4875 does not

require heatsinking. From Equation (2), assuming a 5V

power supply and an 8

Ω load, the maximum power dissipa-

tion point is 633 mW. The maximum power dissipation point

obtained from Equation (2) must not be greater than the

power dissipation that results from Equation (3):

P

θ

JA

(3)

For the SO package,

θ

has a 190˚C/W

θ

given ambient temperature T

find the maximum internal power dissipation supported by

the IC packaging. If the result of Equation (2) is greater than

that of Equation (3), then either decrease the supply voltage,

increase the load impedance, or reduce the ambient tem-

perature. For a typical application using the SO packaged

LM4875, a 5V power supply, and an 8

Ω load, the maximum

ambient temperature that does not violate the maximum

junction temperature is approximately 55˚C. The maximum

ambient temperature for the MSOP package with the same

conditions is approximately 30˚C. These results further as-

sume 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 decreases. Refer

to the Typical Performance Characteristics curves for

power dissipation information at lower 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 capacitors connected to the bypass and power

supply pins should be placed as close to the LM4875 as

possible. The capacitor connected between the bypass pin

and ground improves the internal bias voltage’s stability,

producing improved PSRR. The improvements to PSRR

increase as the bypass pin capacitor value increases. Typi-

cal applications employ a 5V regulator with 10µF and a

0.1µF filter capacitors that aid in supply stability. Their pres-

ence, however does not eliminate the need for bypassing the

supply nodes of the LM4875. The selection of bypass ca-

pacitor values, especially C

requirements, click and pop performance (as explained in

the section, Proper Selection of External Components),

system cost, and size constraints.

DC VOLTAGE VOLUME CONTROL

The LM4875’s internal volume control is controlled by the DC

voltage applied its DC Vol/SD pin (pin 4). The volume con-

trol’s input range is from GND to V

typical volume response versus input control voltage is

shown in the Typical Performance Characteristicssection.

The DC Vol/SD pin also functions as the control pin for the

LM4875’s micropower shutdown feature. See the Mute and

Shutdown Function section for more information.

Like all volume controls, the Lm4875’s internal volume con-

trol is set while listening to an amplified signal that is applied

to an external speaker. The actual voltage applied to the DC

Vol/SD pin is a result of the volume a listener desires. As

such, the volume control is designed for use in a feedback

system that includes human ears and preferences. This

feedback system operates quite well without the need for

accurate gain. The user simply sets the volume to the de-

sired level as determined by their ear, without regard to the

actual DC voltage that produces the volume. Therefore, the

accuracy of the volume control is not critical, as long as

volume changes monotonically and step size is small

enough to reach a desired volume that is not too loud or too

soft. Since gain accuracy is not critical, there may be a

volume variation from part-to-part even with the same ap-

plied DC control voltage. The gain of a given LM4875 can be

set with a fixed external voltage, but another LM4875 may

require a different control voltage to achieve the same gain.

Figure 2 is a curve showing the volume variation of seven

typical LM4875s as the voltage applied to the DC Vol/SD pin

is varied. For gains between -20dB and +16dB, the typical

voltage.

MUTE AND SHUTDOWN FUNCTION

The LM4875’s mute and shutdown functions are controlled

through the DC Vol/SD pin. Mute is activated by applying a

voltage in the range of 500mV to 1V. A typical attenuation of

75dB is achieved is while mute is active. The LM4875’s

micropower shutdown mode turns off the amplifier’s bias

circuitry. The micropower shutdown mode is activated by

applying less than 300mV

shutdown is active, they supply current is reduced to 0.7µA

(typ). A degree of uncertainty exists when the voltage applied

to the DC Vol/SD pin is in the range of 300mV to 500mV. The

LM4875 can be in mute, still fully powered, or in micropower

shutdown and fully muted. In mute mode, the LM4875 draws

the typical quiescent supply current. The DC Vol/SD pin

should be tied to GND for best shutdown mode performance.

As the DC Vol/SD is increased above 0.5V the amplifier will

follow the attenuation curve in Typical Performance Char-

acteristics.

HP-Sense FUNCTION

Applying a voltage between 4V and V

HP-Sense headphone control pin turns off Amp2 and mutes

a bridged-connected load. Quiescent current consumption is

reduced when the IC is in this single-ended mode.

Figure 3 shows the implementation of the LM4875’s head-

phone control function. With no headphones connected to

the headphone jack, the R1-R2 voltage divider sets the

voltage applied to the HP-Sense pin (pin 3) at approximately

50mV. This 50mV enables the LM4875 and places it in

bridged mode operation.

10104248

FIGURE 2. Typical part-to-part gain variation as a

function of DC-Vol control voltage

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