LM4952 datasheet(13/20 Pages) NSC

13 / 20 page

Application Information (Continued)

power dissipation is a function of output power, higher am-

bient temperatures are allowed as output power or duty

cycle decreases.

If the result of Equation (1) is greater than that of Equation

(2), then decrease the supply voltage, increase the load

impedance, or reduce the ambient temperature. Further,

ensure that speakers rated at a nominal 4

Ω do not fall below

Ω. If these measures are insufficient, a heat sink can be

added to reduce

JA. The heat sink can be created using

additional copper area around the package, with connec-

tions to the ground pins, supply pin and amplifier output pins.

Refer to the Typical Performance Characteristics curves

for power dissipation information at lower output power lev-

els.

POWER SUPPLY VOLTAGE LIMITS

Continuous proper operation is ensured by never exceeding

the voltage applied to any pin, with respect to ground, as

listed in the Absolute Maximum Ratings section.

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 voltage regulator typi-

cally use a 10µF in parallel with a 0.1µF filter capacitors to

stabilize 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 10µF

tantalum bypass capacitance connected between the

LM4952’s supply pins and ground. Do not substitute a ce-

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

lation. Keep the length of leads and traces that connect

capacitors between the LM4952’s power supply pin and

ground as short as possible.

BYPASS PIN BYPASSING

Connecting a 4.7µF capacitor, C

BYPASS, between the BY-

PASS pin and ground improves the internal bias voltage’s

stability and improves the amplifier’s PSRR. The PSRR im-

provements increase as the bypass pin capacitor value in-

creases. Too large, however, increases turn-on time. The

selection of bypass capacitor values, especially C

BYPASS,

depends on desired PSRR requirements, click and pop per-

formance (as explained in the section, SELECTING EXTER-

NAL COMPONENTS), system cost, and size constraints.

MICRO-POWER SHUTDOWN

The LM4952 features an active-low micro-power shutdown

mode. When active, the LM4952’s micro-power shutdown

feature turns off the amplifier’s bias circuitry, reducing the

supply current. The low 55µA typical shutdown current is

achieved by applying a voltage to the SHUTDOWN pin that

is as near to GND as possible. A voltage that is greater than

GND may increase the shutdown current.

There are a few methods to control the micro-power shut-

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

(SPST), a microprocessor, or a microcontroller. Figure 3

shows a simple switch-based circuit that can be used to

control the LM4952’s shutdown fucntion. Select normal am-

plifier operation by closing the switch. Opening the switch

applies GND to the SHUTDOWN pin, activating micro-power

shutdown. The switch and resistor guarantee that the SHUT-

DOWN pin will not float. This prevents unwanted state

changes. In a system with a microprocessor or a microcon-

troller, use a digital output to apply the active-state voltage to

the SHUTDOWN pin.

DC VOLUME CONTROL

The LM4952 has an internal stereo volume control whose

setting is a function of the DC voltage applied to the DC VOL

input pin.

The LM4952 volume control consists of 31 steps that are

individually selected by a variable DC voltage level on the

volume control pin. As shown in Figure 4, the range of the

steps, controlled by the DC voltage, is 20dB to -46dB.

The gain levels are 1dB/step from 20dB to 14dB, 2dB/step

from 14dB to -16dB, 3dB/step from -16dB to -27dB, 4dB/step

from -27db to -31dB, 5dB/step from -31dB to -46dB.

Like all volume controls, the LM4952’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 input 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

200809H3

FIGURE 3. Simple switch and voltage divider

generates shutdown control signal

200809F5

FIGURE 4. Volume control response

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