LM4809
LM4809 Dual 105mW Headphone Amplifier with Active-Low Shutdown Mode
Literature Number: SNAS126E
LM4809
Dual 105mW Headphone Amplifier with Active-Low
Shutdown Mode
General Description
The LM4809 is a dual audio power amplifier capable of
delivering 105mW per channel of continuous average power
into a 16Ωload with 0.1% (THD+N) from a 5V power supply.
Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal amount of
external components. Since the LM4809 does not require
bootstrap capacitors or snubber networks, it is optimally
suited for low-power portable systems.
The unity-gain stable LM4809 can be configured by external
gain-setting resistors.
The LM4809 features an externally controlled, active-low,
micropower consumption shutdown mode, as well as an
internal thermal shutdown protection mechanism.
Key Specifications
nTHD+N at 1kHz at 105mW continuous average power
into 16Ω0.1% (typ)
nTHD+N at 1kHz at 70mW continuous average power
into 32Ω0.1% (typ)
nShutdown Current 0.4µA (typ)
Features
nActive-low shutdown mode
n"Click and Pop" reduction circuitry
nLow shutdown current
nLLP, MSOP, and SO surface mount packaging
nNo bootstrap capacitors required
nUnity-gain stable
Applications
nHeadphone Amplifier
nPersonal Computers
nMicrophone Preamplifier
nPDA’s
Typical Application
Boomer®is a registered trademark of National Semiconductor Corporation.
20009001
*Refer to the Application Information Section for information concerning proper selection of the input and output coupling capacitors.
FIGURE 1. Typical Audio Amplifier Application Circuit
April 2004
LM4809 Dual 105mW Headphone Amplifier with Active-Low Shutdown Mode
© 2004 National Semiconductor Corporation DS200090 www.national.com
Connection Diagrams
MSOP Package MSOP Marking
20009002
Top View
Order Number LM4809MM
See NS Package Number MUA08A
20009066
SO Package SO Marking
20009002
Top View
Order Number LM4809MA
See NS Package Number M08A
20009067
LLP Package (LD) LLP (LD) Marking
20009061
Top View
Order Number LM4809LD
See NS Package Number LDA08B
20009068
LLP Package (LQ) LLP (LQ) Marking
20009069
Top View
Order Number LM4809LQ
See NS Package Number LQB08A
20009070
LM4809
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Absolute Maximum Ratings (Note 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage 6.0V
Storage Temperature −65˚C to +150˚C
ESD Susceptibility (Note 4) 3.5kV
ESD Machine model (Note 8) 250V
Junction Temperature (T
J
) 150˚C
Soldering Information (Note 1)
Small Outline Package
Vapor Phase (60 sec.) 215˚C
Infrared (15 sec.) 220˚C
Thermal Resistance
θ
JA
(SO) 170˚C/W
θ
JC
(SO) 35˚C/W
θ
JA
(MSOP) 210˚C/W
θ
JC
(MSOP) 56˚C/W
θ
JA
(LLP) 117˚C/W (Note 9)
θ
JA
(LLP) 150˚C/W (Note 10)
θ
JC
(LLP) 15˚C/W
Operating Ratings
Temperature Range
T
MIN
≤T
A
≤T
MAX
−40˚C ≤T
A
≤85˚C
Supply Voltage (V
CC
) 2.0V ≤V
CC
≤5.5V
Note 1: See AN-450 “Surface Mounting and their Effects on Product Reli-
ability” for other methods of soldering surface mount devices.
Electrical Characteristics V
DD
=5V(Notes 2, 3)
The following specifications apply for V
DD
= 5V unless otherwise specified, limits apply to T
A
= 25˚C.
Symbol Parameter Conditions LM4809 Units
(Limits)
Typ
(Note 5)
Limit
(Note 7)
V
DD
Supply Voltage 2.0 V (min)
5.5 V (max)
I
DD
Supply Current V
IN
= 0V, I
O
= 0A 1.4 3 mA (max)
I
SD
Shutdown Current V
IN
= 0V, V
SHUTDOWN
= GND 0.4 2 µA(max)
V
OS
Output Offset Voltage V
IN
= 0V 4.0 50 mV(max)
P
O
Output Power THD+N = 0.1%, f = 1kHz
R
L
=16Ω105 mW
R
L
=32Ω70 65 mW (min)
THD+N Total Harmonic Distortion P
O
= 50mW, R
L
=32Ω
f = 20Hz to 20kHz
0.3 %
Crosstalk Channel Separation R
L
=32Ω;P
O
= 70mW 70 dB
PSRR Power Supply Rejection Ratio C
B
= 1.0µF; V
RIPPLE
= 200mV
PP
,
f = 1kHz; Input terminated into 50Ω
70 dB
V
SDIH
Shutdown Voltage Input High 0.8 x V
DD
V (min)
V
SDIL
Shutdown Voltage Input Low 0.2 x V
DD
V (max)
Electrical Characteristics V
DD
= 3.3V (Notes 2, 3)
The following specifications apply for V
DD
= 3.3V unless otherwise specified, limits apply to T
A
= 25˚C.
Symbol Parameter Conditions LM4809 Units
(Limits)
Typ
(Note 5)
Limit
(Note 7)
I
DD
Supply Current V
IN
= 0V, I
O
= 0A 1.1 mA
I
SD
Shutdown Current V
IN
= 0V, V
SHUTDOWN
= GND 0.4 µA
V
OS
Output Offset Voltage V
IN
= 0V 4.0 mV
P
O
Output Power THD+N = 0.1%, f = 1kHz
R
L
=16Ω40 mW
R
L
=32Ω28 mW
THD+N Total Harmonic Distortion P
O
= 25mW, R
L
=32Ω
f = 20Hz to 20kHz
0.4 %
Crosstalk Channel Separation R
L
=32Ω;P
O
= 25mW 70 dB
LM4809
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Electrical Characteristics V
DD
= 3.3V (Notes 2, 3) (Continued)
The following specifications apply for V
DD
= 3.3V unless otherwise specified, limits apply to T
A
= 25˚C.
Symbol Parameter Conditions LM4809 Units
(Limits)
Typ
(Note 5)
Limit
(Note 7)
PSRR Power Supply Rejection Ratio C
B
= 1.0µF; V
RIPPLE
= 200mV
PP
,
f = 1kHz; Input terminated into 50Ω
70 dB
V
SDIH
Shutdown Voltage Input High 0.8 x V
DD
V (min)
V
SDIL
Shutdown Voltage Input Low 0.2 x V
DD
V (max)
Electrical Characteristics V
DD
= 2.6V (Notes 2, 3)
The following specifications apply for V
DD
= 2.6V unless otherwise specified, limits apply to T
A
= 25˚C.
Symbol Parameter Conditions LM4809 Units
(Limits)
Typ
(Note 5)
Limit
(Note 7)
I
DD
Supply Current V
IN
= 0V, I
O
= 0A 0.9 mA
I
SD
Shutdown Current V
IN
= 0V, V
SHUTDOWN
= GND 0.2 µA
V
OS
Output Offset Voltage V
IN
= 0V 4.0 mV
P
O
Output Power THD+N = 0.1%, f = 1kHz
R
L
=16Ω20 mW
R
L
=32Ω16 mW
THD+N Total Harmonic Distortion P
O
= 15mW, R
L
=32Ω
f = 20Hz to 20kHz
0.6 %
Crosstalk Channel Separation R
L
=32Ω;P
O
= 15mW 70 dB
PSRR Power Supply Rejection Ratio C
B
= 1.0µF; V
RIPPLE
= 200mV
PP
,
f = 1kHz; Input terminated into 50Ω
70 dB
V
SDIH
Shutdown Voltage Input High 0.8 x V
DD
V (min)
V
SDIL
Shutdown Voltage Input Low 0.2 x V
DD
V (max)
Note 2: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur.
Note 3: All voltages are measured with respect to the ground pin, unless otherwise specified.
Note 4: Human body model, 100pF discharged through a 1.5kΩresistor.
Note 5: Typical specifications are specified at +25OC and represent the most likely parametric norm.
Note 6: Tested limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).
Note 7: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis.
Note 8: Machine Model ESD test is covered by specification EIAJ IC-121-1981. A 200pF cap is charged to the specified voltage, then discharged directly into the
IC with no external series resistor (resistance of discharge path must be under 50Ohms).
Note 9: The given θJA is for an LM4809 packaged in an LDA08B wit the Exposed-Dap soldered to a printed circuit board copper pad with an area equivalent to that
of the Exposed-Dap itself.
Note 10: The given θJA is for an LM4809 packaged in an LDA08B with the Exposed-Dap not soldered to any printed circuit board copper.
External Components Description (Figure 1)
Components Functional Description
1. R
i
The inverting input resistance, along with R
f
, set the closed-loop gain. R
i
, along with C
i
, form a high
pass filter with f
c
= 1/(2πR
i
C
i
).
2. C
i
The input coupling capacitor blocks DC voltage at the amplifier’s input terminals. C
i
, along with R
i
,
create a highpass filter with f
C
= 1/(2πR
i
C
i
). Refer to the section, Selecting Proper External
Components, for an explanation of determining the value of C
i
.
3. R
f
The feedback resistance, along with R
i
, set closed-loop gain.
4. C
S
This is the supply bypass capacitor. It provides power supply filtering. Refer to the Application
Information section for proper placement and selection of the supply bypass capacitor.
5. C
B
This is the BYPASS pin capacitor. It provides half-supply filtering. Refer to the section, Selecting
Proper External Components, for information concerning proper placement and selection of C
B
.
6. C
O
This is the output coupling capacitor. It blocks the DC voltage at the amplifier’s output and forms a high
pass filter with R
L
at f
O
= 1/(2πR
L
C
O
)
LM4809
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Typical Performance Characteristics
THD+N vs Frequency THD+N vs Frequency
20009003 20009004
THD+N vs Frequency THD+N vs Frequency
20009005 20009006
THD+N vs Frequency THD+N vs Frequency
20009007 20009008
LM4809
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Typical Performance Characteristics (Continued)
THD+N vs Frequency THD+N vs Frequency
20009009 20009010
THD+N vs Frequency THD+N vs Frequency
20009011 20009012
THD+N vs Output Power THD+N vs Output Power
20009013 20009014
LM4809
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Typical Performance Characteristics (Continued)
THD+N vs Output Power THD+N vs Output Power
20009015 20009016
THD+N vs Output Power THD+N vs Output Power
20009017 20009018
THD+N vs Output Power THD+N vs Output Power
20009019 20009020
LM4809
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Typical Performance Characteristics (Continued)
THD+N vs Output Power
Output Power vs
Load Resistance
20009021 20009022
Output Power vs
Load Resistance
Output Power vs
Load Resistance
20009023 20009024
Output Power vs
Supply Voltage
Output Power vs
Power Supply
20009025 20009026
LM4809
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Typical Performance Characteristics (Continued)
Output Power vs
Power Supply
Dropout Voltage vs
Supply Voltage
20009027 20009028
Power Dissipation vs
Output Power
Power Dissipation vs
Output Power
20009029 20009030
Power Dissipation vs
Output Power
Channel Separation
20009031 20009033
LM4809
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Typical Performance Characteristics (Continued)
Noise Floor Power Supply Rejection Ratio
20009034
20009035
Open Loop
Frequency Response
Open Loop
Frequency Response
20009050
20009051
Open Loop
Frequency Response
Supply Current vs
Supply Voltage
20009038
20009044
LM4809
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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
DD
/2. When active, the
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 100kΩpull-down resistor between the
SHUTDOWN pin and GND. Connect the switch between the
SHUTDOWN pin and V
DD
. Select normal amplifier operation
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 32Ωload.
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.
P
DMAX
=(V
DD
)
2
/(2π
2
R
L
) (1)
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
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
a32Ωload, 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:
P
DMAX
=(T
JMAX
−T
A
)/θ
JA
(2)
For package MUA08A, θ
JA
= 210˚C/W. T
JMAX
= 150˚C for
the LM4809. Depending on the ambient temperature, T
A
,of
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
A
reduced. For
the typical application of a 5V power supply, with a 32Ωload,
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
B
, between the BYPASS pin and
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
B
, depends
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
LM4809
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Application Information (Continued)
CODECs have outputs of 1V
RMS
(2.83V
P-P
). Please refer to
the Audio Power Amplifier Design section for more infor-
mation on selecting the proper gain.
Input and Output Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input and output coupling capacitors (C
I
and C
O
in Figure 1).
A high value capacitor can be expensive and may compro-
mise space efficiency in portable designs. In many cases,
however, the speakers used in portable systems, whether
internal or external, have little ability to reproduce signals
below 150Hz. Applications using speakers with this limited
frequency response reap little improvement by using high
value input and output capacitors.
Besides affecting system cost and size, C
i
has an effect on
the LM4809’s click and pop performance. The magnitude of
the pop is directly proportional to the input capacitor’s size.
Thus, pops can be minimized by selecting an input capacitor
value that is no higher than necessary to meet the desired
−3dB frequency. Please refer to the Optimizing Click and
Pop Reduction Performance section for a more detailed
discussion on click and pop performance.
As shown in Figure 1, the input resistor, R
I
and the input
capacitor, C
I
, produce a −3dB high pass filter cutoff fre-
quency that is found using Equation (3). In addition, the
output load R
L
, and the output capacitor C
O
, produce a -3db
high pass filter cutoff frequency defined by Equation (4).
f
I-3db
=1/2πR
I
C
I
(3)
f
O-3db
=1/2πR
L
C
O
(4)
Also, careful consideration must be taken in selecting a
certain type of capacitor to be used in the system. Different
types of capacitors (tantalum, electrolytic, ceramic) have
unique performance characteristics and may affect overall
system performance.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consid-
eration should be paid to the value of C
B
, the capacitor
connected to the BYPASS pin. Since C
B
determines how
fast the LM4809 settles to quiescent operation, its value is
critical when minimizing turn-on pops. The slower the
LM4809’s outputs ramp to their quiescent DC voltage (nomi-
nally 1/2 V
DD
), the smaller the turn-on pop. Choosing C
B
equal to 4.7µF along with a small value of C
i
(in the range of
0.1µF to 0.47µF), produces a click-less and pop-less shut-
down function. As discussed above, choosing C
i
no larger
than necessary for the desired bandwith helps minimize
clicks and pops.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4809 contains circuitry that minimizes turn-on and
shutdown transients or “clicks and pop”. For this discussion,
turn-on refers to either applying the power supply voltage or
when the shutdown mode is deactivated. During turn-on, the
LM4809’s internal amplifiers are configured as unity gain
buffers. An internal current source charges up the capacitor
on the BYPASS pin in a controlled, linear manner. The gain
of the internal amplifiers remains unity until the voltage on
the BYPASS pin reaches 1/2 V
DD
. As soon as the voltage on
the BYPASS pin is stable, the device becomes fully opera-
tional. During device turn-on, a transient (pop) is created
from a voltage difference between the input and output of the
amplifier as the voltage on the BYPASS pin reaches 1/2 V
DD
.
For this discussion, the input of the amplifier refers to the
node between R
I
and C
I
. Ideally, the input and output track
the voltage applied to the BYPASS pin. During turn-on, the
buffer-configured amplifier output charges the input capaci-
tor, C
I
, through the input resistor, R
I
. This input resistor
delays the charging time of C
I
thereby causing the voltage
difference between the input and output that results in a
transient (pop). Higher value capacitors need more time to
reach a quiescent DC voltage (usually 1/2 V
DD
) when
charged with a fixed current. Decreasing the value of C
I
and
R
I
will minimize turn-on pops at the expense of the desired
-3dB frequency.
Although the BYPASS pin current cannot be modified,
changing the size of C
B
alters the device’s turn-on time and
the magnitude of “clicks and pops”. Increasing the value of
C
B
reduces the magnitude of turn-on pops. However, this
presents a tradeoff: as the size of C
B
increases, the turn-on
time increases. There is a linear relationship between the
size of C
B
and the turn-on time. Here are some typical
turn-on times for various values of C
B
:
C
B
T
ON
0.1µF 80ms
0.22µF 170ms
0.33µF 270ms
0.47µF 370ms
0.68µF 490ms
1.0µF 920ms
2.2µF 1.8sec
3.3µF 2.8sec
4.7µF 3.4sec
10µF 7.7sec
In order eliminate “clicks and pops”, all capacitors must be
discharged before turn-on. Rapidly switching V
DD
may not
allow the capacitors to fully discharge, which may cause
“clicks and pops”. In a single-ended configuration, the output
is coupled to the load by C
O
. This capacitor usually has a
high value. C
O
discharges through internal 20kΩresistors.
Depending on the size of C
O
, the discharge time constant
can be relatively large. To reduce transients in single-ended
mode, an external 1kΩ–5kΩresistor can be placed in par-
allel with the internal 20kΩresistor. The tradeoff for using
this resistor is increased quiescent current.
AUDIO POWER AMPLIFIER DESIGN
Design a Dual 70mW/32ΩAudio Amplifier
Given:
Power Output 70 mW
Load Impedance 32Ω
Input Level 1 Vrms (max)
Input Impedance 20kΩ
Bandwidth 100 Hz–20 kHz ±0.50dB
The design begins by specifying the minimum supply voltage
necessary to obtain the specified output power. One way to
LM4809
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Application Information (Continued)
find the minimum supply voltage is to use the Output Power
vs Supply Voltage curve in the Typical Performance Char-
acteristics section. Another way, using Equation (5), is to
calculate the peak output voltage necessary to achieve the
desired output power for a given load impedance. To ac-
count for the amplifier’s dropout voltage, two additional volt-
ages, based on the Dropout Voltage vs Supply Voltage in the
Typical Performance Characteristics curves, must be
added to the result obtained by Equation (5). For a single-
ended application, the result is Equation (6).
(5)
V
DD
≥(2V
OPEAK
+(V
OD
TOP +V
OD
BOT)) (6)
The Output Power vs Supply Voltage graph for a 32Ωload
indicates a minimum supply voltage of 4.8V. This is easily
met by the commonly used 5V supply voltage. The additional
voltage creates the benefit of headroom, allowing the
LM4809 to produce peak output power in excess of 70mW
without clipping or other audible distortion. The choice of
supply voltage must also not create a situation that violates
maximum power dissipation as explained above in the
Power Dissipation section. Remember that the maximum
power dissipation point from Equation (1) must be multiplied
by two since there are two independent amplifiers inside the
package. Once the power dissipation equations have been
addressed, the required gain can be determined from Equa-
tion (7).
(7)
Thus, a minimum gain of 1.497 allows the LM4809 to reach
full output swing and maintain low noise and THD+N perfro-
mance. For this example, let A
V
=1.5.
The amplifiers overall gain is set using the input (R
i
) and
feedback (R
f
) resistors. With the desired input impedance
set at 20kΩ, the feedback resistor is found using Equation
(8).
A
V
=R
f
/R
i
(8)
The value of R
f
is 30kΩ.
The last step in this design is setting the amplifier’s −3db
frequency bandwidth. To achieve the desired ±0.25dB pass
band magnitude variation limit, the low frequency response
must extend to at lease one−fifth the lower bandwidth limit
and the high frequency response must extend to at least five
times the upper bandwidth limit. The gain variation for both
response limits is 0.17dB, well within the ±0.25dB desired
limit. The results are an
f
L
= 100Hz/5 = 20Hz (9)
and a
f
H
= 20kHz*5 = 100kHz (10)
As stated in the External Components section, both R
i
in
conjunction with C
i
, and C
o
with R
L
, create first order high-
pass filters. Thus to obtain the desired low frequency re-
sponse of 100Hz within ±0.5dB, both poles must be taken
into consideration. The combination of two single order filters
at the same frequency forms a second order response. This
results in a signal which is down 0.34dB at five times away
from the single order filter −3dB point. Thus, a frequency of
20Hz is used in the following equations to ensure that the
response is better than 0.5dB down at 100Hz.
C
i
≥1/(2π* 20kΩ* 20Hz) = 0.397µF; use 0.39µF.(11)
C
o
≥1/(2π*32Ω* 20Hz) = 249µF; use 330µF. (12)
The high frequency pole is determined by the product of the
desired high frequency pole, f
H
, and the closed-loop gain,
A
V
. With a closed-loop gain of 1.5 and f
H
= 100kHz, the
resulting GBWP = 150kHz which is much smaller than the
LM4809’s GBWP of 900kHz. This figure displays that if a
designer has a need to design an amplifier with a higher
gain, the LM4809 can still be used without running into
bandwidth limitations.
LM4809
www.national.com13
Demonstration Board Schematic
20009057
FIGURE 2. LM4809 Demonstration Board Schematic
LM4809
www.national.com 14
Demonstration Board Layout
20009058
FIGURE 3. Recommended MSOP PC Board Layout
Component-Side Silkscreen
20009059
FIGURE 4. Recommended MSOP PC Board Layout
Component-Side Layout
20009060
FIGURE 5. Recommended MSOP PC Board Layout
Bottom-Side Layout
LM4809
www.national.com15
Demonstration Board Layout (Continued)
20009062
FIGURE 6. Recommended LD PC Board Layout
Component-Side Silkscreen
20009063
FIGURE 7. Recommended LD PC Board Layout
Component-Side Layout
20009064
FIGURE 8. Recommended LD PC Board Layout
Bottom-Side Layout
LM4809
www.national.com 16
Demonstration Board Layout (Continued)
20009076
FIGURE 9. Recommended LQ PC Board Layout
Component-Side Silkscreen
20009074
FIGURE 10. Recommended LQ PC Board Layout
Component-Side Layout
LM4809
www.national.com17
Demonstration Board Layout (Continued)
20009075
FIGURE 11. Recommended LQ PC Board Layout
Bottomt-Side Layout
LM4809
www.national.com 18
Physical Dimensions inches (millimeters) unless otherwise noted
Order Number LM4809MM
NS Package Number MUA08A
Order Number LM4809MA
NS Package Number M08A
LM4809
www.national.com19
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
Order Number LM4809LD
NS Package Number LDA08B
LM4809
www.national.com 20
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
Order Number LM4809LQ
NS Package Number LQB08A
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NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
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www.national.com
LM4809 Dual 105mW Headphone Amplifier with Active-Low Shutdown Mode
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
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