LM4868 datasheet(16/28 Pages) NSC

16 / 28 page

Application Information (Continued)

pop performance. The selection of bypass capacitor values,

especially C

B, depends on desired PSRR requirements,

click and pop performance (as explained in the section,

Proper Selection of External Components), system cost,

and size constraints.

MICRO−POWER SHUTDOWN

The voltage applied to the SHUTDOWN pin controls the

LM4868’s shutdown function. Activate micro−power shut-

down by applying V

DD to the SHUTDOWN pin. When active,

the LM4868’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 0.7µA typical

shutdown current is achieved by applying a voltage that is as

near as V

DD as possible to the SHUTDOWN pin. A voltage

that is less than V

DD may increase the shutdown current.

Table 1 shows the logic signal levels that activate and deac-

tivate micro−power shutdown and headphone amplifier op-

eration. To ensure that the output signal remains

transient−free, do not cycle the shutdown function

faster than 1Hz.

There are a few ways to control the micro−power shutdown.

These include using a single−pole, single, throw switch, a

microprocessor, or a microcontroller. When using a switch,

connect an external 100k

Ω pull−up resistor between the

SHUTDOWN pin and V

DD. Connect the switch between the

SHUTDOWN pin and ground. Select normal amplifier opera-

tion by closing the switch. Opening the switch connects the

SHUTDOWN pin to V

DD through the pull−up resistor, acti-

vating micro−power shutdown. The switch and resistor guar-

antee 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 control

voltage to the SHUTDOWN pin. Driving the SHUTDOWN pin

with active circuitry eliminates the pull up resistor.

Truth Table for Logic Inputs

SHUTDOWN

HP-IN

MUX CHANNEL

INPUT SELECT

OPERATIONAL MODE (MUX

INPUTCHANNEL #)

Logic Low

= −OUTB signal

Logic Low

Bridged amplifiers (1)

Logic Low

= −OUTB signal

Logic High

Bridged amplifiers (2)

Logic Low

≠ −OUTB signal

Logic Low

Single-ended amplifiers (1)

Logic Low

≠ −OUTB signal

Logic High

Single-ended amplifiers (2)

Logic High

Micro−power shutdown

HEADPHONE (SINGLE-ENDED) AMPLIFIER

OPERATION

An internal pull−up circuit is connected to the HP−IN (pin 20)

headphone amplifier control pin. When this pin is left uncon-

nected, V

DD is applied to the HP−IN. This turns off Amp2B

and switches Amp2A’s input signal from an audio signal to

the V

DD/2 voltage present on pin 14. The result is muted

bridge-connected loads. Quiescent current consumption is

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

Figure 8 shows the implementation of the LM4868’s head-

phone control function. An internal comparator with a nomi-

nal 400mV offset monitors the signal present at the −OUTB

output. It compares this signal against the signal applied to

the HP−IN pin. When these signals are equal, as is the case

when a BTL is connected to the amplifier, the comparator

forces the LM4868 to maintain bridged−amplifier operation.

When the HP−IN pin is externally floated, such as when

headphones are connected to the jack shown in Figure 8,

and internal pull−up forces V

DD on the internal comparator’s

HP−IN inputs. This changes the comparator’s output state

and enables the headphone function: it turns off Amp2B,

switches Amp2A’s input signal from an audio signal to the

DD/2

voltage

present

14,

and

mutes

the

bridge-connected loads. Amp1A and Amp1B drive the head-

phones.

Figure 8 also shows the suggested headphone jack electri-

cal connections. The jack is designed to mate with a

three−wire plug. The plug’s tip and ring should each carry

one of the two stereo output signals, whereas the sleeve

provides the return to Amp2A. A headphone jack with one

control pin contact is sufficient to drive the HP−IN pin when

connecting headphones.

A switch can replace the headphone jack contact pin. When

a switch shorts the HP−IN pin to V

DD, bridge−connected

speakers are muted and Amp1A and Amp2A drive a pair of

headphones. When a switch shorts the HP−IN pin to GND,

the LM4868 operates in bridge mode. If headphone drive is

not needed, short the HP−IN pin to the −OUTB pin.

Figure 7 shows an optional resistor connected between the

amplifier output that drives the headphone jack sleeve and

ground. This resistor provides a ground path that supressed

power supply hum. This hum may occur in applications such

as notebook computers in a shutdown condition and con-

nected to an external powered speaker. The resistor’s 100

Ω

value is a suggested starting point. Its final value must be

determined based on the tradeoff between the amount of

noise suppression that may be needed and minimizing the

additional current drawn by the resistor (25mA for a 100

Ω

resistor and a 5V supply).

ESD Protection

As stated in the Absolute Maximum Ratings, pin 28 on the

MT and MH packages have a maximum ESD susceptibility

rating of 8000V. For higher ESD voltages, the addition of a

PCDN042 dual transil (from California Micro Devices), as

shown in Figure 7, will provide additional protection.

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