LM4875

SNAS042C – JANUARY 2002 – REVISED MAY 2013

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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. Typical applications employ a 5V regulator with 10µF and a 0.1µF filter capacitors that aid in

supply stability. Their presence, however does not eliminate the need for bypassing the supply nodes of the

and pop performance (as explained in the section, PROPERLY SELECTING 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

control voltage is shown in the TYPICAL PERFORMANCE CHARACTERISTICS section. 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 control 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 desired 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 applied 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 26 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 part-to-part variation is typically ±1dB for a given control voltage.

Figure 26. Typical Part-to-Part Gain Variation as a Function of DC-Vol Control 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

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 CHARACTERISTICS .

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