If the result of Equation (3) is greater than that of Equation

(4), 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

3

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

added to reduce

θ

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 1.0µF

tantalum bypass capacitance connected between the

LM4940’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 LM4940’s power supply pin and

ground as short as possible. Connecting a 10µF capacitor,

C

the internal bias voltage’s stability and improves the amplifi-

er’s PSRR. The PSRR improvements increase as the by-

pass pin capacitor value increases. Too large, however,

increases turn-on time and can compromise the amplifier’s

click and pop performance. The selection of bypass capaci-

tor values, especially C

requirements, click and pop performance (as explained in

the section, SELECTING EXTERNAL COMPONENTS),

system cost, and size constraints.

MICRO-POWER SHUTDOWN

The LM4940 features an active-low micro-power shutdown

mode. When active, the LM4940’s micro-power shutdown

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

supply current. The low 40µA 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. When using

a switch, connect a 100k

Ω pull-up resistor between the

SHUTDOWN pin and V

SHUTDOWN pin and GND. Select normal amplifier opera-

tion by opening the switch. Closing the switch applies GND

to the SHUTDOWN pin, 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 a microcontroller, use a

digital output to apply the active-state voltage to the SHUT-

DOWN pin.

SELECTING EXTERNAL COMPONENTS

Input Capacitor Value Selection

Two quantities determine the value of the input coupling

capacitor: the lowest audio frequency that requires amplifi-

cation and desired output transient suppression.

As shown in Figure 3, the input resistor (R

capacitor (C

that is found using Equation (7).

f

πR

(5)

As an example when using a speaker with a low frequency

limit of 50Hz, C

C

efficiency, full range speaker whose response extends below

30Hz.

Output Coupling Capacitor Value Selection

The capacitors C

put DC bias voltage and couple the output AC signal to the

amplifier loads also determine low frequency response.

These capacitors, combined with their respective loads cre-

ate a highpass filter cutoff frequency. The frequency is also

given by Equation (6).

Using the same conditions as above, with a 4

Ω speaker,

C

Bypass Capacitor Value

Besides minimizing the input capacitor size, careful consid-

eration should be paid to value of C

connected to the BYPASS pin. Since C

how fast the LM4940 settles to quiescent operation, its value

is critical when minimizing turn-on pops. The slower the

LM4940’s outputs ramp to their quiescent DC voltage (nomi-

nally V

equal to 10µF along with a small value of C

0.1µF to 0.39µF), produces a click-less and pop-less shut-

down function. As discussed above, choosing C

than necessary for the desired bandwidth helps minimize

clicks and pops.

OPTIMIZING CLICK AND POP REDUCTION

PERFORMANCE

The LM4940 contains circuitry that eliminates turn-on and

shutdown transients ("clicks and pops"). For this discussion,

turn-on refers to either applying the power supply voltage or

when the micro-power shutdown mode is deactivated.

As the V

final value, the LM4940’s internal amplifiers are configured

as unity gain buffers and are disconnected from the AMP

A

and AMP

pacitor connected between the BYPASS pin and GND in a

controlled manner. Ideally, the input and outputs track the

voltage applied to the BYPASS pin. The gain of the internal

amplifiers remains unity until the voltage applied to the BY-

PASS pin.

The gain of the internal amplifiers remains unity until the

voltage on the bypass pin reaches V

voltage on the bypass pin is stable, the device becomes fully

operational and the amplifier outputs are reconnected to

their respective output pins. Although the BYPASS pin cur-

rent cannot be modified, changing the size of C

the device’s turn-on time. Here are some typical turn-on

times for various values of C

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