LM4867 datasheet(14/26 Pages) NSC

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Application Information (Continued)

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

HP-IN FUNCTION

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 7 shows the implementation of the LM4867’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 LM4867 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 7,

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 7 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 LM4867 operates in bridge mode. If headphone drive is

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

Single-Ended Output Power Performance and

Measurement Considerations

The LM4867 delivers clean, low distortion SE output power

into loads that are greater than 10

Ω. As an example, output

power for 16

Ω and 32Ω loads are shown in the Typical

Performance Characteristic curves. For loads less than

Ω, the LM4876 can typically supply 180mW of low distor-

tion power. However, when higher dissipation is desired in

loads less than 10

Ω, a dramatic increase in THD+N may

occur. This is normal operation and does not indicate that

proper functionality has ceased. When a jump from moder-

ate to excessively high distortion is seen, simply reducing

the output voltage swing will restore the clean, low distortion

SE operation.

The dramatic jump in distortion for loads less than 10

Ω

occurs when current limiting circuitry activates. During SE

operation, AMP2A (refer to

Figure 4) drives the headphone

sleeve. An on-board circuit monitors this amplifier’s output

current. The sudden increase in THD+N is caused by the

current limit circuitry forcing AMP2A into a high−impedance

output mode. When this occurs, the output waveform has

discontinuities that produce large amounts of distortion. It

has been observed that as the output power is steadily

increased, the distortion may jump from 5% to greater than

35%. Indeed, 10% THD+N may not actually be achievable.

Using the Single−Ended Output for Line Level

Applications

Some samples of the LM4867 may exhibit small amplitude,

high frequency oscillation when the SE output is connected

to a line-level input. This oscillation can be eliminated by

connecting a 5%, 300

Ω resistor between Amp2A’s output pin

and each amplifier, AMP1A and AMP1B, output.

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