LM4981 National Semiconductor, LM4981 Datasheet - Page 12

• Manufacturer Part Number: LM4981
• Description: 80mW Stereo Headphone Amplifier
• Manufacturer: National Semiconductor

www.national.com
Application Information
DIGITAL VOLUME CONTROL
The LM4981’s gain is controlled by the signals applied to the
CLOCK and UP/DN inputs. An external clock is required to
drive the CLOCK pin. At each rising edge of the clock signal,
the gain will either increase or decrease by a 3dB step
depending on the logic voltage level applied to the UP/DN
pin. A logic high voltage level applied to the UP/DN pin
causes the gain to increase by 3dB at each rising edge of the
clock signal. Conversely, a logic low voltage level applied to
the UP/DN pin causes the gain to decrease 3dB at each
rising edge of the clock signal. For both the CLOCK and
UP/DN inputs, the trigger point is 1.4V minimum for a logic
high level, and 0.4V maximum for a logic low level.
There are 16 discrete gain settings ranging from +12dB
maximum to −33dB minimum. Upon device power on, the
amplifier’s gain is set to a default value of 0dB. However,
when coming out of shutdown mode, the LM4981 will revert
back to its previous gain setting.
The LM4981’s CLOCK and UP/DN pins should be de-
bounced in order to avoid unwanted state changes during
transitions between V
operation of the digital volume control. A microcontroller or
microprocessor output is recommended to drive the CLOCK
and UP/DN pins.
ELIMINATING THE OUTPUT COUPLING CAPACITOR
The LM4981 features a low noise inverting charge pump that
generates an internal negative supply voltage. This allows
the outputs of the LM4981 to be biased about GND instead
of a nominal DC voltage, like traditional headphone amplifi-
ers. Because there is no DC component, the large DC
blocking capacitors (typically 220µF) are not necessary. The
coupling capacitors are replaced by two, small ceramic
charge pump capacitors, saving board space and cost.
Eliminating the output coupling capacitors also improves low
frequency response. In traditional headphone amplifiers, the
headphone impedance and the output capacitor form a high
pass filter that not only blocks the DC component of the
output, but also attenuates low frequencies, impacting the
bass response. Because the LM4981 does not require the
output coupling capacitors, the low frequency response of
the device is not degraded by external components.
In addition to eliminating the output coupling capacitors, the
ground referenced output nearly doubles the available dy-
namic range of the LM4981 when compared to a traditional
headphone amplifier operating from the same supply volt-
age.
FIGURE 2. Timing Diagram
IL
and V
IH
. This will ensure correct
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12
SUPPLY VOLTAGE SEQUENCING
It is a good general practice to first apply the supply voltage
to a CMOS device before any other signal or supply on other
pins. This is also true for the LM4891 audio amplifier which is
a CMOS device.
Before applying any signal to the inputs or shutdown pins of
the LM4891, it is important to apply a supply voltage to the
V
be applied to the shutdown pins (see MICRO POWER
SHUTDOWN) and input pins.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection. Applications that employ a 3V power supply typi-
cally use a 4.7µF capacitor in parallel with a 0.1µF ceramic
filter capacitor to stabilize the power supply’s output, reduce
noise on the supply line, and improve the supply’s transient
response. Keep the length of leads and traces that connect
capacitors between the LM4981’s power supply pin and
ground as short as possible.
POWER DISSIPATION
Power dissipation is a major concern when using any power
amplifier and must be thoroughly understood to ensure a
successful design. Equation 1 states the maximum power
dissipation point for a single-ended amplifier operating at a
given supply voltage and driving a specified output load.
Since the LM4981 has two operational amplifiers in one
package, the maximum internal power dissipation point is
twice that of the number which results from Equation 1. Even
with the large internal power dissipation, the LM4981 does
not require heat sinking over a large range of ambient tem-
perature. From Equation 1, assuming a 5V power supply and
a 32Ω load, the maximum power dissipation point is 40mW
per amplifier. Thus the maximum package dissipation point
is 80mW. The maximum power dissipation point obtained
must not be greater than the power dissipation predicted by
Equation 2:
For a given ambient temperature, T
roundings, Equation 2 can be used to find the maximum
internal power dissipation supported by the IC packaging. If
the result of Equation 1 is greater than that of Equation 2,
then either the supply voltage must be decreased, the load
impedance increased, or T
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4981 contains shutdown circuitry that is used to turn off
the amplifier’s bias circuitry. In addition, the LM4981 con-
tains a Shutdown Mode pin, allowing the designer to desig-
nate whether the part will be driven into shutdown with a high
level logic signal or a low level logic signal. This allows the
designer maximum flexibility in device use, as the Shutdown
Mode pin may simply be tied permanently to either V
GND to set the LM4981 as either a "shutdown-high" device
or a "shutdown-low" device, respectively. The device may
then be placed into shutdown mode by toggling the Shut-
DD
pins. After the device has been powered, signals may
P
P
DMAX
DMAX
= (T
= (V
A
JMAX
DD
reduced.
)
2
− T
/ (2π
A
) / θ
A
2
, of the system sur-
R
L
JA
)
DD
(1)
(2)
or