LM4920
Ground-Referenced, Ultra Low Noise, Fixed Gain, 80mW
Stereo Headphone Amplifier
General Description
The LM4920 is a ground referenced, fixed-gain audio power
amplifier capable of delivering 80mW of continuous average
power into a 16Ωsingle-ended load with less than 1%
THD+N from a 3V power supply.
The LM4920 features a new circuit technology that utilizes a
charge pump to generate a negative reference voltage. This
allows the outputs to be biased about ground, thereby elimi-
nating output-coupling capacitors typically used with normal
single-ended loads.
Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal amount of
external components. The LM4920 does not require output
coupling capacitors or bootstrap capacitors, and therefore is
ideally suited for mobile phone and other low voltage appli-
cations where minimal power consumption is a primary re-
quirement.
The LM4920 features a low-power consumption shutdown
mode selectable for either channel separately. This is ac-
complished by driving either the SD_RC (Shutdown Right
Channel) or SD_LC (Shutdown Left Channel) (or both) pins
with logic low, depending on which channel is desired shut-
down. Additionally, the LM4920 features an internal thermal
shutdown protection mechanism.
The LM4920 contains advanced pop & click circuitry that
eliminates noises which would otherwise occur during
turn-on and turn-off transitions.
The LM4920 has an internal fixed gain of 1.5V/V.
Key Specifications
jImproved PSRR at 217Hz 70dB (typ)
jPower Output at V
DD
= 3V,
R
L
=16Ω, THD %1% 80mW (typ)
jShutdown Current 0.01µA (typ)
jInternal Fixed Gain 1.5V/V (typ)
jOperating Voltage 1.6V to 4.2V
Features
nFixed Logic Levels
nGround referenced outputs
nHigh PSRR
nAvailable in space-saving micro SMD package
nUltra low current shutdown mode
nImproved pop & click circuitry eliminates noises during
turn-on and turn-off transitions
nNo output coupling capacitors, snubber networks,
bootstrap capacitors, or gain-setting resistors required
nShutdown either channel independently
Applications
nMobile Phones
nMP3 Players
nPDAs
nPortable electronic devices
nNotebook PCs
Boomer®is a registered trademark of National Semiconductor Corporation.
October 2006
LM4920 Ground-Referenced, Ultra Low Noise, Fixed Gain, 80mW Stereo Headphone Amplifier
© 2006 National Semiconductor Corporation DS201793 www.national.com
Typical Application
201793B8
FIGURE 1. Typical Audio Amplifier Application Circuit
LM4920
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Connection Diagrams
microSMD Package 14 – Bump TL Marking
20179309
Top View
Order Number LM4920TL
See NS Package Number TLE1411A
20179378
Top View
XY – Date Code
TT – Lot Traceability
G – Boomer Family
H4 – LM4920TL
Pin Descriptions
Pin Name Function
A1 R_IN Right Channel Input
A2 SGND Signal Ground
A3 CPV
DD
Charge Pump Power Supply
A4 C
CP+
Positive Terminal - Charge Pump Flying Capacitor
B1 SD_RC Active-Low Shutdown, Right Channel
B2 SD_LC Active-Low Shutdown, Left Channel
B4 PGND Power Ground
C1 L_IN Left Channel Input
C2 R_OUT Right Channel Input
C4 C
CP-
Negative Terminal - Charge Pump Flying Capacitor
D1 +AV
DD
Positive Power Supply - Amplifier
D2 L_OUT Left Channel Output
D3 -AV
DD
Negative Power Supply - Amplifier
D4 V
CP_OUT
Charge Pump Power Output
LM4920
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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 4.5V
Storage Temperature −65˚C to +150˚C
Input Voltage -0.3V to V
DD
+ 0.3V
Power Dissipation (Note 3) Internally Limited
ESD Susceptibility (Note 4) 2000V
ESD Susceptibility (Note 5) 200V
Junction Temperature 150˚C
Thermal Resistance
θ
JA
(typ) TLE1411A (Note 11) 86˚C/W
Operating Ratings
Temperature Range
T
MIN
≤T
A
≤T
MAX
−40˚C ≤T
A
≤85˚C
Supply Voltage (V
DD
) 1.6V ≤V
DD
≤4.2V
Electrical Characteristics V
DD
=3V(Note 1)
The following specifications apply for V
DD
= 3V and 16Ωload unless otherwise specified. Limits apply to T
A
= 25˚C.
Symbol Parameter Conditions LM4920 Units
(Limits)
Typ
(Note 6)
Limit
(Notes 7, 8)
I
DD
Quiescent Power Supply Current
Full Power Mode
V
IN
= 0V, inputs terminated
both channels enabled 7 10 mA (max)
V
IN
= 0V, inputs terminated
one channel enabled 5mA
I
SD
Shutdown Current V
SD_LC
=V
SD_RC
= GND 0.1 1.8 µA (max)
V
OS
Output Offset Voltage R
L
=32Ω,V
IN
= 0V 0.7 5 mV (max)
A
V
Voltage Gain –1.5 V/V
∆A
V
Gain Match 1 %
R
IN
Input Resistance 20 15
25
kΩ(min)
kΩ(max)
P
O
Output Power
THD+N = 1% (max); f = 1kHz,
R
L
=16Ω, one channel 80 mW
THD+N = 1% (max); f = 1kHz,
R
L
=32Ω, one channel 65 mW
THD+N = 1% (max); f = 1kHz,
R
L
=16Ω, (two channels in phase) 43 38 mW (min)
THD+N = 1% (max); f = 1kHz,
R
L
=32Ω, (two channels in phase) 50 45 mW (min)
THD+N Total Harmonic Distortion +
Noise
P
O
= 60mW, f = 1kHz, R
L
=16Ω
single channel
0.04
%
P
O
= 50mW, f = 1kHz, R
L
=32Ω
single channel
0.03
PSRR Power Supply Rejection Ratio
Full Power Mode
V
RIPPLE
= 200mVp-p, Input Referred
f = 217Hz 70
dBf = 1kHz 65
f = 20kHz 50
SNR Signal-to-Noise Ratio
R
L
=32Ω,P
OUT
= 20mW,
(A-weighted)
f = 1kHz, BW = 20Hz to 22kHz
100 dB
V
IH
Shutdown Input Voltage High V
DD
= 1.8V to 4.2V 1.2 V (min)
V
IL
Shutdown Input Voltage Low V
DD
= 1.8V to 4.2V 0.45 V (max)
X
TALK
Crosstalk R
L
=16Ω,P
O
= 1.6mW,
f = 1kHz 60 dB
Z
OUT
Output Impedance
V
SD-LC
=V
SD-RC
= GND
Input Terminated
Input not terminated
50
∞
30 kΩ
LM4920
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Electrical Characteristics V
DD
=3V(Note 1) (Continued)
The following specifications apply for V
DD
= 3V and 16Ωload unless otherwise specified. Limits apply to T
A
= 25˚C.
Symbol Parameter Conditions LM4920 Units
(Limits)
Typ
(Note 6)
Limit
(Notes 7, 8)
Z
OUT
Output Impedance
V
SD-LC
=V
SD-RC
= GND
–500mV ≤V
OUT
≤+500mV
(Note 12)
82kΩ(min)
I
L
Input Leakage ±0.1 nA
Note 1: All voltages are measured with respect to the GND pin unless otherwise specified.
Note 2: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
functional but do not guarantee specific performance limits. Electrical Characteristics state DC and AC electrical specifications under particular test conditions that
guarantee specific performance limits. This assumes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters where no limit
is given; however, the typical value is a good indication of device performance.
Note 3: The maximum power dissipation must be derated at elevated temperatures and is dictated by TJMAX,θJA, and the ambient temperature, TA. The maximum
allowable power dissipation is PDMAX =(T
JMAX -T
A)/θJA or the number given in Absolute Maximum Ratings, whichever is lower. For the LM4920, see power
de-rating currents for more information.
Note 4: Human body model, 100pF discharged through a 1.5kΩresistor.
Note 5: Machine Model, 220pF - 240pF discharged through all pins.
Note 6: Typicals are measured at 25˚C and represent the parametric norm.
Note 7: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).
Note 8: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis.
Note 9: If the product is in shutdown mode and VDD exceeds 4.2V (to a max of 4.5V VDD), then most of the excess current will flow through the ESD protection
circuits. If the source impedance limits the current to a max of 10mA, then the part will be protected. If the part is enabled when VDD is above 4.5V, circuit
performance will be curtailed or the part may be permanently damaged.
Note 10: Human body model, 100pF discharged through a 1.5kΩresistor.
Note 11: θJA value is measured with the device mounted on a PCB with a 3” x 1.5”, 1oz copper heatsink.
Note 12: VOUT refers to signal applied to the LM4920 outputs.
External Components Description (Figure 1)
Components Functional Description
1. C
i
Input coupling capacitor which blocks the DC voltage at the amplifier’s input terminals. Also creates a
high-pass filter with R
i
at f
C
= 1/(2πR
i
C
i
). Refer to the section Proper Selection of External Components, for
an explanation of how to determine the value of C
i
.
2. C
1
Flying capacitor. Low ESR ceramic capacitor (≤100mΩ)
3. C
2
Output capacitor. Low ESR ceramic capacitor (≤100mΩ)
4. C
3
Tantalum capacitor. Supply bypass capacitor which provides power supply filtering. Refer to the Power Supply
Bypassing section for information concerning proper placement and selection of the supply bypass capacitor.
5. C
4
Ceramic capacitor. Supply bypass capacitor which provides power supply filtering. Refer to the Power Supply
Bypassing section for information concerning proper placement and selection of the supply bypass capacitor.
LM4920
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Typical Performance Characteristics
THD+N vs Frequency
V
DD
= 1.6V, R
L
=16Ω,P
O
= 1mW
THD+N vs Frequency
V
DD
= 1.6V, R
L
=32Ω,P
O
= 1mW
20179328 20179329
THD+N vs Frequency
V
DD
= 1.8V, R
L
=16Ω,P
O
= 5mW
THD+N vs Frequency
V
DD
= 1.8V, R
L
=32Ω,P
O
= 5mW
20179330 20179331
THD+N vs Frequency
V
DD
= 3V, R
L
=16Ω,P
O
= 50mW
THD+N vs Frequency
V
DD
= 3V, R
L
=32Ω,P
O
= 50mW
20179332 20179333
LM4920
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Typical Performance Characteristics (Continued)
THD+N vs Frequency
V
DD
= 3.6V, R
L
=16Ω,P
O
= 100mW
THD+N vs Frequency
V
DD
= 3.6V, R
L
=32Ω,P
O
= 100mW
20179334 20179335
THD+N vs Frequency
V
DD
= 4.2V, R
L
=16Ω,P
O
= 150mW
THD+N vs Frequency
V
DD
= 4.2V, R
L
=32Ω,P
O
= 150mW
20179336 20179337
THD+N vs Output Power
V
DD
= 1.6V, R
L
=16Ω,f=1kH
One channel enabled
THD+N vs Output Power
V
DD
= 1.6V, R
L
=32Ω, f = 1kHz
One channel enabled
20179347 20179349
LM4920
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Typical Performance Characteristics (Continued)
THD+N vs Output Power
V
DD
= 1.6V, R
L
=16Ω, f = 1kHz
Two channels in phase
THD+N vs Output Power
V
DD
= 1.6V, R
L
=32Ω, f = 1kHz
Two channels in phase
20179351 20179353
THD+N vs Output Power
V
DD
= 1.8V, R
L
=16Ω, f = 1kHz
One channel enabled
THD+N vs Output Power
V
DD
= 1.8V, R
L
=32Ω, f = 1kHz
One channel enabled
20179359 20179361
THD+N vs Output Power
V
DD
= 1.8V, R
L
=16Ω, f = 1kHz
Two channels in phase
THD+N vs Output Power
V
DD
= 1.8V, R
L
=32Ω, f = 1kHz
Two channels in phase
20179363 20179365
LM4920
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Typical Performance Characteristics (Continued)
THD+N vs Output Power
V
DD
= 3.0V, R
L
=16Ω, f = 1kHz
One channel enabled
THD+N vs Output Power
V
DD
= 3.0V, R
L
=32Ω, f = 1kHz
One channel enabled
201793G2 201793E1
THD+N vs Output Power
V
DD
= 3.0V, R
L
=16Ω, f = 1kHz
Two channels in phase
THD+N vs Output Power
V
DD
= 3.0V, R
L
=32Ω, f = 1kHz
Two channels in phase
201793G4 201793E5
THD+N vs Output Power
V
DD
= 3.6V, R
L
=16Ω, f = 1kHz
One channel enabled
THD+N vs Output Power
V
DD
= 3.6V, R
L
=32Ω, f = 1kHz
One channel enabled
201793F1 201793F3
LM4920
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Typical Performance Characteristics (Continued)
THD+N vs Output Power
V
DD
= 3.6V, R
L
=16Ω, f = 1kHz
Two channels in phase
THD+N vs Output Power
V
DD
= 3.6V, R
L
=32Ω, f = 1kHz
two channels in phase
201793F5 201793F7
THD+N vs Output Power
V
DD
= 4.2V, R
L
=16Ω, f = 1kHz
One channel enabled
THD+N vs Output Power
V
DD
= 4.2V, R
L
=32Ω, f = 1kHz
One channel enabled
20179373 20179380
THD+N vs Output Power
V
DD
= 4.2V, R
L
=16Ω, f = 1kHz
Two channels in phase
THD+N vs Output Power
V
DD
= 4.2V, R
L
=32Ω, f = 1kHz
Two channels in phase
20179382 20179384
LM4920
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Typical Performance Characteristics (Continued)
PSRR vs Frequency
V
DD
= 1.6V, R
L
=16Ω
PSRR vs Frequency
V
DD
= 1.6V, R
L
=32Ω
20179340 20179341
PSRR vs Frequency
V
DD
= 3V, R
L
=16Ω
PSRR vs Frequency
V
DD
= 3V, R
L
=32Ω
20179342 20179343
PSRR vs Frequency
V
DD
= 4.2V, R
L
=16Ω
PSRR vs Frequency
V
DD
= 4.2V, R
L
=32Ω
20179344 20179345
LM4920
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Typical Performance Characteristics (Continued)
Output Power vs Supply Voltage
R
L
=16Ω, one channel
Output Power vs Supply Voltage
R
L
=32Ω, one channel
20179338 20179339
Output Power vs Supply Voltage
R
L
=16Ω, 2 channels in phase
Output Power vs Supply Voltage
R
L
=32Ω, 2 channels in phase
201793G8 201793G9
Supply Current vs Supply Voltage
R
L
=16Ω
20179310
LM4920
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Application Information
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 LM4920 audio amplifier which is
a CMOS device.
Before applying any signal to the inputs or shutdown pins of
the LM4920, it is important to apply a supply voltage to the
V
DD
pins. After the device has been powered, signals may
be applied to the shutdown pins (see MICRO POWER
SHUTDOWN) and input pins.
ELIMINATING THE OUTPUT COUPLING CAPACITOR
The LM4920 features a low noise inverting charge pump that
generates an internal negative supply voltage. This allows
the outputs of the LM4920 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 LM4920 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 LM4920 when compared to a traditional
headphone amplifier operating from the same supply volt-
age.
OUTPUT TRANSIENT (’CLICK AND POPS’)
ELIMINATED
The LM4920 contains advanced circuitry that virtually elimi-
nates output transients (’clicks and pops’). This circuitry
prevents all traces of transients when the supply voltage is
first applied or when the part resumes operation after coming
out of shutdown mode.
AMPLIFIER CONFIGURATION EXPLANATION
As shown in Figure 2, the LM4920 has two internal opera-
tional amplifiers. The two amplifiers have internally config-
ured gain, the closed loop gain is set by selecting the ratio of
R
f
to R
i
. Consequently, the gain for each channel of the IC is
A
V
= -(R
f
/R
i
) = 1.5 V/V
where R
F
= 30kΩand R
i
= 20kΩ.
Since this is an output ground-referenced amplifier, by driv-
ing the headphone through R
OUT
(Pin C2) and L
OUT
(Pin
D2), the LM4920 does not require output coupling capaci-
tors. The typical single-ended amplifier configuration re-
quires large, expensive output capacitors.
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 LM4920 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 large internal power dissipation, the LM4920 does not
require heat sinking over a large range of ambient tempera-
tures. From Equation 1, assuming a 3V power supply and a
16Ωload, the maximum power dissipation point is 28mW per
amplifier. Thus the maximum package dissipation point is
56mW. 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 the micro SMD package, θ
JA
= 105˚C/W. T
JMAX
= 150˚C
for the LM4920. 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 3V power supply, with a 16Ωload,
the maximum ambient temperature possible without violating
the maximum junction temperature is approximately 144˚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.
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 LM4920’s power supply pin and
ground as short as possible.
MICRO POWER SHUTDOWN
The voltage applied to the SD_LC (shutdown left channel)
pin and the SD_RC (shutdown right channel) pin controls the
LM4920’s shutdown function. When active, the LM4920’s
micropower shutdown feature turns off the amplifiers’ bias
circuitry, reducing the supply current. The trigger point is
0.45V for a logic-low level, and 1.2V for logic-high level. The
low 0.01µA (typ) shutdown current is achieved by applying a
voltage that is as near as ground a possible to the SD_LC/
SD_RC pins. 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 100kΩpull-up resistor between the
LM4920
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Application Information (Continued)
SD_LC/SD_RC pins and V
DD
. Connect the switch between
the SD_LC/SD_RC pins and ground. Select normal amplifier
operation by opening the switch. Closing the switch con-
nects the SD_LC/SD_RC pins to ground, activating micro-
power shutdown. The switch and resistor guarantee that the
SD_LC/SD_RC pins will not float. This prevents unwanted
state changes. In a system with a microprocessor or micro-
controller, use a digital output to apply the control voltage to
the SD_LC/SD_RC pins. Driving the SD_LC/SD_RC pins
with active circuitry eliminates the pull-up resistor.
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4920’s performance requires properly se-
lecting external components. Though the LM4920 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val-
ues.
Charge Pump Capacitor Selection
Use low ESR (equivalent series resistance) (<100mΩ) ce-
ramic capacitors with an X7R dielectric for best perfor-
mance. Low ESR capacitors keep the charge pump output
impedance to a minimum, extending the headroom on the
negative supply. Higher ESR capacitors result in reduced
output power from the audio amplifiers.
Charge pump load regulation and output impedance are
affected by the value of the flying capacitor (C1). A larger
valued C1 (up to 3.3uF) improves load regulation and mini-
mizes charge pump output resistance. Beyond 3.3uF, the
switch-on resistance dominates the output impedance for
capacitor values above 2.2uF.
The output ripple is affected by the value and ESR of the
output capacitor (C2). Larger capacitors reduce output ripple
on the negative power supply. Lower ESR capacitors mini-
mize the output ripple and reduce the output impedance of
the charge pump.
The LM4920 charge pump design is optimized for 2.2uF, low
ESR, ceramic, flying, and output capacitors.
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitors (C
i
in Figure 1). A high value ca-
pacitor can be expensive and may compromise space effi-
ciency in portable designs. In many cases, however, the
speakers used in portable systems, whether internal or ex-
ternal, have little ability to reproduce signals below 150Hz.
Applications using speakers with this limited frequency re-
sponse reap little improvement by using high value input and
output capacitors.
Besides affecting system cost and size, C
i
has an effect on
the LM4920’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.
As shown in Figure 1, the internal input resistor, R
i
and the
input capacitor, C
i
, produce a -3dB high pass filter cutoff
frequency that is found using Equation (3). Conventional
headphone amplifiers require output capacitors; Equation (3)
can be used, along with the value of R
L
, to determine to-
wards the value of output capacitor needed to produce a
–3dB high pass filter cutoff frequency.
f
i-3dB
=1/2πR
i
C
i
(3)
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. (See the section entitled Charge Pump
Capacitor Selection.)
LM4920
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Application Information (Continued)
LM4920 micro SMD DEMO BOARD ARTWORK
Top Layer Mid Layer 1
20179305
20179306
Mid Layer 2 Bottom Layer
20179307
20179308
LM4920
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Revision History
Rev Date Description
1.0 03/27/06 Initial release.
1.1 10/18/06 Text edits.
LM4920
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Physical Dimensions inches (millimeters) unless otherwise noted
14 – Bump micro SMD
Order Number LM4920TL
NS Package Number TLE1411A
X1 = 1.970±0.03mm, X2 = 1.970±0.03mm, X3 = 0.600±0.075mm,
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.
For the most current product information visit us at www.national.com.
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Lead free products are RoHS compliant.
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LM4920 Ground-Referenced, Ultra Low Noise, Fixed Gain, 80mW Stereo Headphone Amplifier
