Datasheet

LM4911, LM4911Q
SNAS152M –MAY 2004–REVISED JUNE 2011 www.ti.com
MICRO POWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the LM4911/LM4911Q's shutdown function. Activate micro-
power shutdown by applying a logic-low voltage to the SHUTDOWN pin. When active, the LM4911/LM4911Q's
micro-power shutdown feature turns off the amplifier's bias circuitry, reducing the supply current. The trigger point
varies depending on supply voltage and is shown in the Shutdown Hysteresis Voltage graphs in the Typical
Performance Characteristics section. The low 0.1µA(typ) shutdown current is achieved by applying a voltage that
is as near as ground as possible to the SHUTDOWN pin. A voltage that is higher than ground may increase the
shutdown current. 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 100kΩ
pull-up resistor between the SHUTDOWN pin and V
DD
. Connect the switch between the SHUTDOWN pin and
ground. Select normal amplifier operation by opening the switch. Closing the switch connects the SHUTDOWN
pin to ground, activating micro-power shutdown.
The switch and resistor specifies that the SHUTDOWN pin will not float. This prevents unwanted state changes.
In a system with a microprocessor or 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.
Shutdown enable/disable times are controlled by a combination of C
B
and V
DD
. Larger values of C
B
results in
longer turn on/off times from Shutdown. Smaller V
DD
values also increase turn on/off time for a given value of C
B
.
Longer shutdown times also improve the LM4911/LM4911Q's resistance to click and pop upon entering or
returning from shutdown. For a 2.4V supply and C
B
= 4.7µF, the LM4911/LM4911Q requires about 2 seconds to
enter or return from shutdown. This longer shutdown time enables the LM4911/LM4911Q to have virtually zero
pop and click transients upon entering or release from shutdown.
Smaller values of C
B
will decrease turn-on time, but at the cost of increased pop and click and reduced PSRR.
Since shutdown enable/disable times increase dramatically as supply voltage gets below 2.2V, this reduced turn-
on time may be desirable if extreme low supply voltage levels are used as this would offset increases in turn-on
time caused by the lower supply voltage. This technique is not recommended for OCL mode since shutdown
enable/disable times are very fast (0.5s) independent of supply voltage.
When in cap-coupled mode, some restrictions on the usage of Mute are in effect when entering or returning from
shutdown. These restrictions require Mute not be toggled immediately following a return or entrance to shutdown
for a brief period. These periods are shown as X1 and X2 and are discussed in greater detail in the Mute section
as well as shown in Figure 63.
MUTE
When in C-CUPL mode, the LM4911/LM4911Q also features a mute function that enables extremely fast turn-
on/turn-off with a minimum of output pop and click with a low current consumption (≤ 100µA). The mute function
leaves the outputs at their bias level, thus resulting in higher power consumption than shutdown mode, but also
provides much faster turn on/off times. Mute mode is enabled by providing a logic high signal on the MUTE pin in
the opposite manner as the shutdown function described above. Threshold voltages and activation techniques
match those given for the shutdown function as well.
Mute may not appear to function when the LM4911/LM4911Q is used to drive high impedance loads. This is
because the LM4911/LM4911Q relies on a typical headphone load (16-32Ω) to reduce input signal feedthrough
through the input and feedback resistors. Mute attenuation can thus be calculated by the following formula:
Mute Attenuation (dB) = 20Log(R
L
/ (R
i
+R
F
)
Parallel load resistance may be necessary to achieve satisfactory Mute levels when the application load is known
to be high impedance.
The mute function is not necessary when the LM4911/LM4911Q is operating in OCL mode since the shutdown
function operates quickly in OCL mode with less power consumption than mute.
Mute may be enabled during shutdown transitions, but should not be toggled for a brief period immediately after
exiting or entering shutdown. These brief time periods are labeled X1 (time after returning from shutdown) and
X2 (time after entering shutdown) and are shown in the timing diagram given in Figure 63. X1 occurs
immediately following a return from shutdown (T
WU
) and lasts 40ms±25%. X2 occurs after the part is placed in
shutdown and the decay of the bias voltage has occurred (2.2*400k*C
B
for cap-coupled and 2.2*100k*C
B
for
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