LM4854 datasheet(21/29 Pages) NSC

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Application Information (Continued)

power, higher ambient temperatures are allowed as output

power or duty cycle decreases. 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. 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 con-

nections to the ground pin(s), supply pin and amplifier output

pins. External, solder attached SMT heatsinks such as the

Thermalloy 7106D can also improve power dissipation.

When adding a heat sink, the

JA is the sum of

JC,

CS, and

SA.(

JC is the junction-to-case thermal impedance,

CS is

the case-to-sink thermal impedance, and

SA is the sink-to-

ambient thermal impedance.) Refer to the Typical Perfor-

mance Characteristics curves for power dissipation informa-

tion 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. Applications that employ a 5V regulator typically

use a 10µF in parallel with a 0.1µF filter capacitors to stabi-

lize 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

LM4854’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 LM4854’s power supply pin and

ground as short as possible. Connecting a 1µF capacitor,

CB, between the BYPASS pin and ground improves the

internal bias voltage’s stability and improves the amplifier’s

PSRR. The PSRR improvements increase as the bypass 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 capacitor values,

especially CB, depends on desired PSRR requirements,

click and pop performance (as explained in the section,

Proper Selection of External Components), system cost, and

size constraints.

STANDBY

The LM4854 features a low-power, fast turn-on standby

mode. Applying a logic-low to the STANDBY pin act actives

the standby mode. When this mode is active, the power

supply current decreases to a nominal value of 30µA and the

amplifier outputs are muted. Fast turn-on is assured be-

cause all bias points remain at the same voltage as when the

part is in fully active operation. The LM4854 returns to fully

active operation in 100µs (typ) after the input voltage on the

STANDBY pin switches from a logic low to a logic high.

MICRO-POWER SHUTDOWN

The LM4854 features an active-low micro-power shutdown

mode. When active, the LM4854’s micro-power shutdown

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

supply current. The logic threshold is typically V

DD/2. The

low 0.1µA 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.

CONTROLLING STANDBY AND MICROPOWER SHUT-

DOWN

There are a few methods to control standby or micro-power

shutdown. 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

STANDBY or SHUTDOWN pin and V

DD and the SPST

switch between the STANDBY or SHUTDOWN pin and

GND. Select normal amplifier operation by opening the

switch. Closing the switch applies GND to the STANDBY or

SHUTDOWN pins, activating micro-power shutdown. The

switch and resistor guarantee that the STANDBY or SHUT-

DOWN pins 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 STANDBY or SHUTDOWN pin.

HEADPHONE (SINGLE-ENDED) AMPLIFIER OPERATION

Previous single-supply amplifiers that were designed to drive

both BTL and SE loads used a SE (or headphone) "sense"

input. This input typically required two external resistors to

bias the sense input to a preset voltage that selected BTL

operation.

The LM4854 has a unique headphone sense circuit that

eliminates the external resistors. The amplifier has an inter-

nal comparator that monitors the voltage present on the

R-OUT pin. It compares this voltage against the voltage on

the HP-SENSE pin. When these voltages are equal, BTL

mode is selected and AMP3 is shutdown and its output has

a very high impedance. When the comparator’s input signals

are different, (a typical

∆V of 200mV), the comparator’s

output switches and activates the SE (headphone) mode.

AMP3 changes from shutdown state to an active state and,

along with AMP1, drives a stereo load. AMP2 drives the

headphone jack sleeve.

Figure 3 shows the suggested headphone jack electrical

connections. The jack is designed to mate with a three-wire

plug. The plug’s tip should carry a stereo signal’s left-

channel information. The ring adjacent to the tip should each

carry the right-channel signal and the ring furthest from the

tip provides the return to AMP2. A switch can replace the

headphone jack contact pin. When the switch shorts the

HP-SENSE pin to R-OUT, the bridge-connected speaker is

driven by AMP1 and AMP2. AMP3 is shutdown, its output in

a high-impedance state. When the switch opens, the

LM4854 operates in SE stereo mode. If headphone drive is

not needed, short the HP-SENSE pin to the R-OUT pin.

The LM4854’s unique headphone sense circuit requires a

dual switch headphone jack. A five-terminal headphone jack,

such as the Switchcraft 35RAPC4BH3, is shown in Figure 2.

For applications that require an SPDIF interface in the stereo

headphone jack, use a Foxconn 2F1138-TJ-TR.

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