LM4918 datasheet(14/17 Pages) NSC

14 / 17 page

Application Information (Continued)

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the

LM4918 contains a shutdown pin to externally turn off the

amplifier’s bias circuitry. This shutdown feature turns the

amplifier off when a logic high (0.8 x V

DD) is placed on the

shutdown pin. By switching the shutdown pin to V

DD, the

LM4918 supply current draw will be minimized in idle mode.

While the device will be disabled with shutdown pin voltages

less than 0.8V

DD, the idle current may be greater than the

typical value of 1µA. (Idle current is measured with the

shutdown pin at V

DD).

In many applications, a microcontroller or microprocessor

output is used to control the shutdown circuitry to provide a

quick, smooth transition into shutdown. Another solution is to

use a single-pole, single-throw switch in conjunction with an

external pull-down resistor. When the switch is closed, the

shutdown pin is connected to V

DD and disables the amplifier.

If the switch is open, then the external pull-down resistor will

enable the LM4918. This scheme guarantees that the shut-

down pin will not float thus preventing unwanted state

changes.

STANDBY

When in MODE 0, activating Standby with a logic high (0.8 x

DD) maintains the outputs at their bias level, but with no

signal. This allows for extremely fast (less than 1ms) turn

off/on of the audio outputs with a minimum current draw

while in Standby mode (1mA typ).

When in MODE 1, enabling/disabling Standby is also depen-

dant upon the input signal level and the input threshold

setting. In this mode, Standby is not enabled until both a

logic high is placed on the Standby pin and the input signal

level has remained below the set input threshold level for a

period of time determined by Chold (typically a few sec-

onds). This delay time set by Cstdby prevents the LM4918

from inadvertently triggering Standby mode during quiet pas-

sages in music. Returning from Standby while in MODE 0 is

also dependant upon both a logic low on the Standby pin and

the input signal level being above the set threshold. In this

case, however, return from Standby is immediate and does

not require the input to remain above the threshold level for

any amount of time.

STANDBY MODE SELECT

The LM4918 has two user-selectable modes for standby

usage. The first mode (MODE 0), valid when MODE is equal

to logic low (0.2 x V

DD or less), is characterized by the

immediate activation of Standby mode (outputs remain at

bias point, with no signal) when a logic high is applied to the

Standby pin, and the immediate return from Standby upon a

logic low on the Standby pin.

The second mode (MODE 1), valid when MODE is equal to

logic high (0.8 x V

DD), is characterized by a delayed activa-

tion of Standby mode. In this case Standby is delayed until

both a logic high is placed on the Standby pin and the input

signal has maintained a value below the input threshold (as

determined by the resistance on Input Vth) for a given

amount of time (determined by Cstdby). Returning from

Standby also requires both a logic high on the Standby pin

and a sufficiently large input signal to overcome the set input

threshold. No hold time applies to return from Standby –

deactivation is immediate upon both conditions being met.

INPUT THRESHOLD DETECT

The LM4819 has an input threshold detect function that

works in conjunction with the Standby mode and is only

enabled when the LM4918 is in MODE 1 (see STANDBY

MODE SELECT). This threshold level may be set anywhere

from 14mV to 50mV (typical peak values) by adjusting the

resistance placed from Input Vth to GND. Typical values

have about 50k

Ω for a 14mVpk level and 15kΩ for a 50mVpk

level. Care should be taken not to set the threshold values

too low, as the part may cycle in and out of shutdown if any

spurious noise was present on the inputs. For this reason,

threshold values less than 14mVpk with corresponding re-

sistance values greater than 50k

Ω are not recommended.

This threshold value sets the peak input level that must be

exceeded for the part to return from Standby when in MODE

1. It also is the level which the input must remain below (for

an amount of time determined by Cstdby) for the part to

enter Standby when in MODE 1.

HP SENSE FUNCTION

Applying a voltage greater than 0.8V

DD to the LM4918’s HP

Sense pin turns off the second amplifier on each channel,

muting a bridged-connected load. Quiescent current con-

sumption is reduced when the IC is in this single-ended

mode.

Figure 2 shows the implementation of the LM4918’s head-

phone sense function. With no headphones connected to the

headphone jack, the R14-R17 voltage divider sets the volt-

age applied to the HP Sense pin (pin 30) at approximately

50mV. This 50mV enables the second amplifiers, placing the

LM4918 in bridged mode operation. The output coupling

capacitor blocks the amplifier’s half supply DC voltage, pro-

tecting the headphones.

The HP Sense threshold is set at 0.8V

DD. While the LM4918

operates in bridged mode, the DC potential across the load

is essentially 0V. Therefore, even in an ideal situation, the

output swing cannot cause a false single-ended trigger. Con-

necting headphones to the headphone jack disconnects the

headphone jack contact pin from VoR− and allows R14 to

pull the HP Sense pin up to V

DD. This enables the head-

phone function, turns off the second amplifiers, and mutes

the bridged speaker. The amplifier then drives the head-

phones, whose impedance is in parallel with resistor R17

and R18. These resistors have negligible effect on the

LM4918’s output drive capability since the typical impedance

of headphones is 32

Ω.

Figure 2 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 should

carry the ground return. A headphone jack with one control

pin contact is sufficient to drive the HP Sense pin when

connecting headphones. A microprocessor or a switch can

replace the headphone jack contact pin. When a micropro-

cessor or switch applies a voltage greater than 4V to the

HP-IN pin, a bridge-connected speaker is muted and the first

set of amplifiers drive a pair of headphones.

PEAK DETECT

The LM4918 also incorporates a peak-detect function to

properly monitor output level in conjunction with the AGC.

This peak detect function may be set for positive or negative

detect, based on the users preference. On the demo board,

shown in Figure 2, both circuits are available for testing. The

normal, positive peak detect, is engaged by inserting jump-

ers at S1 and S3, while removing jumpers at S2 and S4. The

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