lm4809mmx National Semiconductor Corporation, lm4809mmx Datasheet - Page 11

• Manufacturer Part Number: lm4809mmx
• Description: Dual 105mw Headphone Amplifier With Active-low Shutdown Mode
• Manufacturer: National Semiconductor Corporation

Application Information
MICRO-POWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the
LM4809’s shutdown function. Activate micro-power shut-
down by applying a logic low voltage to the SHUTDOWN pin.
The logic threshold is typically V
LM4809’s micro-power shutdown feature turns off the ampli-
fier’s bias circuitry, reducing the supply current. The low
0.4µA typical shutdown current is achieved by applying a
voltage that is as near as GND as possible to the SHUT-
DOWN pin. A voltage that is above GND 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-down resistor between the
SHUTDOWN pin and GND. Connect the switch between the
SHUTDOWN pin and V
by closing the switch. Opening the switch connects the
SHUTDOWN pin to GND through the pull-down resistor,
activating micro-power shutdown. The switch and resistor
guarantee that the SHUTDOWN pin will not float. This pre-
vents unwanted state changes. In a system with a micropro-
cessor or a microcontroller, use a digital output to apply the
control voltage to the SHUTDOWN pin. Driving the SHUT-
DOWN pin with active circuitry eliminates the pull-down re-
sistor.
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATION
The LM4809’s exposed-Dap (die attach paddle) package
(LD or LQ) provides a low thermal resistance between the
die and the PCB to which the part is mounted and soldered.
This allows rapid heat transfer from the die to the surround-
ing PCB copper traces, ground plane, and surrounding air.
The LD or LQ package should have its DAP soldered to a
copper pad on the PCB. The DAP’s PCB copper pad may be
connected to a large plane of continuous unbroken copper.
This plane forms a thermal mass, heat sink, and radiation
area.
However, since the LM4809 is designed for headphone ap-
plications, connecting a copper plane to the DAP’s PCB
copper pad is not required. The LM4809’s Power Dissipation
vs Output Power Curve in the Typical Performance Char-
acteristics shows that the maximum power dissipated is just
45mW per amplifier with a 5V power supply and a 32Ω load.
Further detailed and specific information concerning PCB
layout, fabrication, and mounting an LD or LQ package is
available from National Semiconductor’s Package Engineer-
ing Group under application note AN1187.
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 LM4809 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
P
DMAX
DD
= (V
. Select normal amplifier operation
DD
)
2
/ (2π
DD
/2. When active, the
2
R
L
)
(1)
11
with the large internal power dissipation, the LM4809 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 that results
from Equation 2:
For package MUA08A, θ
the LM4809. Depending on the ambient temperature, T
the system surroundings, 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 de-
creased, the load impedance increased or T
the typical application of a 5V power supply, with a 32Ω load,
the maximum ambient temperature possible without violating
the maximum junction temperature is approximately 133.2˚C
provided that device operation is around the maximum
power dissipation point. Power dissipation is a function of
output power and thus, if typical operation is not around the
maximum power dissipation point, the ambient temperature
may be increased accordingly. Refer to the Typical Perfor-
mance Characteristics curves for power dissipation infor-
mation for lower output powers.
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 5V regulator typically
use a 10µF in parallel with a 0.1µF filter capacitors to stabi-
lize the regulator’s output, reduce noise on the supply line,
and improve the supply’s transient response. However, their
presence does not eliminate the need for a local 1.0µF
tantalum bypass capacitance connected between the
LM4809’s supply pins and ground. Keep the length of leads
and traces that connect capacitors between the LM4809’s
power supply pin and ground as short as possible. Connect-
ing a 4.7µF capacitor, C
ground improves the internal bias voltage’s stability and
improves the amplifier’s PSRR. The PSRR improvements
increase as the bypass pin capacitor value increases. Too
large, however, increases the amplifier’s turn-on time. The
selection of bypass capacitor values, especially C
on desired PSRR requirements, click and pop performance
(as explained in the section, Selecting Proper External
Components), system cost, and size constraints.
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4809’s performance requires properly se-
lecting external components. Though the LM4809 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val-
ues.
The LM4809 is unity-gain stable, giving a designer maximum
design flexibility. The gain should be set to no more than a
given application requires. This allows the amplifier to
achieve minimum THD+N and maximum signal-to-noise ra-
tio. These parameters are compromised as the closed-loop
gain increases. However, low gain demands input signals
with greater voltage swings to achieve maximum output
power. Fortunately, many signal sources such as audio
P
DMAX
= (T
JA
B
, between the BYPASS pin and
JMAX
= 210˚C/W. T
− T
A
) / θ
JA
JMAX
A
reduced. For
= 150˚C for
www.national.com
B
, depends
A
, of
(2)