LM4918LQ National Semiconductor, LM4918LQ Datasheet - Page 14

• Manufacturer Part Number: LM4918LQ
• Description: Stereo Audio Amp with AGC Control
• Manufacturer: National Semiconductor

www.national.com
Application Information
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
shutdown pin. By switching the shutdown pin to V
LM4918 supply current draw will be minimized in idle mode.
While the device will be disabled with shutdown pin voltages
less than 0.8V
typical value of 1µA. (Idle current is measured with the
shutdown pin at V
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
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
V
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
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
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.
DD
) maintains the outputs at their bias level, but with no
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, the idle current may be greater than the
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).
), is characterized by a delayed activa-
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or less), is characterized by the
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and disables the amplifier.
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) is placed on the
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, the
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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 50kΩ 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 50kΩ 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
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
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
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 32Ω.
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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. This enables the head-
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to the LM4918’s HP
. While the LM4918