LM4980SDX/NOPB National Semiconductor, LM4980SDX/NOPB Datasheet - Page 14

• Manufacturer Part Number: LM4980SDX/NOPB
• Description: IC AMP AUDIO .042W STER AB 10LLP
• Manufacturer: National Semiconductor
• Series: Boomer®, PowerWise®r
• Type: Class ABr

Specifications of LM4980SDX/NOPB

• Output Type: Headphones, 2-Channel (Stereo)
• Max Output Power X Channels @ Load: 42mW x 2 @ 16 Ohm
• Voltage - Supply: 1.5 V ~ 3.3 V
• Features: Depop, Shutdown, Thermal Protection
• Mounting Type: Surface Mount
• Package / Case: 10-LLP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM4980SDX

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Application Information
tivating shutdown. As C
transient magnitude can also increase. This increase can be
mitigated by a corresponding increase in C
minimum starting point when selecting C
6.8µF when using 220µF output coupling capacitors.
SELECTING THE INPUT CAPACITOR VALUE
Amplifiying the lowest audio frequencies requires a relatively
high value input coupling capacitor, (C
value capacitor can be expensive and may compromise
space efficiency in portable designs. In many cases, how-
ever, the headphones used in portable systems have limited
ability to reproduce signals below 60Hz. Applications using
headphones with this limited frequency response reap little
improvement by using a high value input capacitor. A small
value of Ci (in the range of 0.1µF to 1.0µF), is recommended.
DRIVING POWERED SPEAKERS
Though the LM4980 is design primarily to drive headphones,
in many cases, it may be called on to act as a line level driver
when powered speakers or other devices may be connected
to the amplifier outputs. For powered speakers or other
devices with typical input resistances (10kΩ) that are signifi-
cantly higher than the typical headphone resistance (32Ω),
the output transients may not sufficiently suppressed when
using the Figure 2 circuit. If this is anticipated, a minor
modification of an additional resistor (a nominal value of
1kΩ) between each output and ground in the Figure 2 circuit
is needed to ensure that the output transient suppression is
not compromised. This reduces both the load resistance
seen by the LM4980 and the magnitude of power-on and
shutdown output transients.
POWER DISSIPATION
Power dissipation has to be evaluated and considered when
designing a successful amplifier. A direct consequence of the
power delivered to a load an amplifier is internal power
dissipation. The maximum per-amplifier power dissipation
for a given application can be derived from the power dissi-
pation graphs or from Equation 2.
It is critical that the maximum junction temperature T
150˚C is not exceeded. Since the typical application is for
headphone operation (16Ω impedance) using a 3.0V supply
the maximum power dissipation is less than 29mW. There-
fore, in the case of this headphone amplifier, the power
dissipation is not a major concern.
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is important
for low noise performance and high power supply rejection.
The capacitor location on the power supply pins should be
as close to the device as possible. Typical applications em-
ploy a 3.0V regulator with 10µF tantalum or electrolytic ca-
pacitor and a ceramic bypass capacitor which aid in supply
stability. This does not eliminate the need for local power
supply bypassing connected as close as possible to the
LM4980’s supply pin. A power supply bypass capacitor value
in the range of 1.0µF to 10µF is recommended.
PDMAX = V
COUPLING
DD
2
/ 2πR
’s value increases, output
IN
LOAD
in Figure 2). A high
(Continued)
MIDCAP
MIDCAP
’s value is
’s value. A
JMAX
(2)
of
14
MICRO POWER SHUTDOWN
The voltage applied to the shutdown (SHDN) pin controls the
LM4980’s shutdown function. Activate micro-power shut-
down by applying a logic-low voltage to the SHDN pin. When
active, the LM4980’s micro-power shutdown feature turns off
the amplifier’s bias circuitry, reducing the supply current. The
trigger point is 0.4V (max) for a logic-low level, and 1.4V
(min) for a logic-high level. The low 0.1µA (typ) shutdown
current is achieved by applying a voltage that is as near as
ground as possible to the SHDN 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
SHDN pin and GND. Connect the switch between the SHDN
pin and V
switch. Opening the switch connects the SHDN pin to
ground, activating micro-power shutdown. The switch and
resistor guarantee that the SHDN pin will not float. This
prevents unwanted state changes. In a system with a micro-
processor or microcontroller, use a digital output to apply the
control voltage to the SHDN pin. Driving the SHDN pin with
active circuitry eliminates the pull-up resistor.
SUGGESTED PCB SCHEMATIC
Figure 5 is the schematic for the suggested PCB Layout.
This schematic and its associated PCB provide both a lean
tested layout and platform that can be used to verify the
LM4980’s outstanding audio performance.
Suggested PCB Design and Layout
Figures 6 through 9 show a suggested PCB layout for a
headphone amplifier circuit using the LM4980 .
PCB Layout Guidelines
This section provides practical guidelines for mixed signal
PCB layout that involves various digital/analog power and
ground traces. Designers should note that these are only
"rule-of-thumb" recommendations and the actual results will
depend heavily on the final layout.
MINIMIZING THD+N
PCB trace impedance on the power, ground, and all output
traces should be minimized to achieve optimal THD perfor-
mance. Therefore, use PCB traces that are as wide as
possible for these connections. As the gain of the amplifier is
increased, the trace impedance will have an ever increasing
adverse affect on THD performance. At unity-gain (0dB) the
parasitic trace impedance effect on THD performance is
reduced but still a negative factor in the THD performance of
the LM4980 in a given application.
GENERAL MIXED SIGNAL LAYOUT
RECOMMENDATION
Power and Ground Circuits
For two layer mixed signal design, it is important to isolate
the digital power and ground trace paths from the analog
power and ground trace paths. Star trace routing techniques
(bringing individual traces back to a central point rather than
daisy chaining traces together in a serial manner) can
greatly enhance low level signal performance. Star trace
DD
. Select normal amplifier operation by closing the