Datasheet LM4872 Datasheet (National Semiconductor)

October 2002
LM4872
1 Watt Audio Power Amplifier in micro SMD package
LM4872 1 Watt Audio Power Amplifier micro SMD package

General Description

The LM4872 is a bridge-connected audio power amplifier capable of delivering1Wofcontinuous average power to an 8load with less than .2% (THD) 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 LM4872 does not require output coupling capacitors or bootstrap capacitors. It is op­timally suited for low-power portable applications.
The LM4872 features an externally controlled, low-power consumption shutdown mode, as well as an internal thermal shutdown protection mechanism.
The unity-gain stable LM4872 can be configured by external gain-setting resistors.

Typical Application

Key Specifications

n Power Output at 0.2% THD 1W (typ) n Shutdown Current 0.01µA (typ)

Features

n micro SMD package (see App. note AN-1112) n 5V - 2V operation n No output coupling capacitors or bootstrap capacitors. n Unity-gain stable n External gain configuration capability

Applications

n Cellular Phones n Portable Computers n Low Voltage Audio Systems
10123001

FIGURE 1. Typical Audio Amplifier Application Circuit

Boomer®is a registered trademark of National Semiconductor Corporation.
© 2002 National Semiconductor Corporation DS101230 www.national.com

Connection Diagram

LM4872
8 Bump micro SMD
Top View
Order Number LM4872IBP, LM4872IBPX
See NS Package Number BPA08B6B
10123023
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LM4872

Absolute Maximum Ratings (Note 2)

If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/
Soldering Information
See AN-1112 ’Micro-SMD Wafers Level Chip Scale Package’.
Distributors for availability and specifications.
Supply Voltage 6.0V
Storage Temperature −65˚C to +150˚C
Input Voltage −0.3V to V
DD
+0.3V
Power Dissipation (Note 3) Internally Limited

Operating Ratings

Temperature Range
ESD Susceptibility (Note 4) 2500V
ESD Susceptibility (Note 5) 250V
Supply Voltage 2.0V V
Junction Temperature 150˚C

Electrical Characteristics VDD=5V (Notes 1, 2, 9)

The following specifications apply for V
Symbol Parameter Conditions
V
DD
I
DD
I
SD
V
OS
P
o
Supply Voltage 2.0 V (min)
Quiescent Power Supply Current VIN= 0V, Io= 0A 5.3 7 mA (max)
Shutdown Current V
Output Offset Voltage VIN= 0V 5 50 mV (max)
Output Power THD = 0.2% (max);f=1kHz 1 W
THD+N Total Harmonic Distortion+Noise P
PSRR Power Supply Rejection Ratio V
= 5V and 8Load unless otherwise specified. Limits apply for TA= 25˚C.
DD
PIN1=VDD
= 0.25 Wrms; AVD=2;20Hz
o
f 20 kHz
= 4.9V to 5.1V 65 dB
DD
T
MIN
TA≤ T
MAX
−40˚C TA≤ 85˚C
LM4872
Typical Limit
(Note 6) (Note 7)
5.5 V (max)
0.01 2 µA (max)
0.1 %
5.5V
DD
Units
(Limits)

Electrical Characteristics VDD= 3.3V (Notes 1, 2, 9)

The following specifications apply for V
Symbol Parameter Conditions
V
DD
I
DD
I
SD
V
OS
P
o
Supply Voltage 2.0 V (min)
Quiescent Power Supply Current VIN= 0V, Io= 0A 4 mA (max)
Shutdown Current V
Output Offset Voltage VIN= 0V 5 mV (max)
Output Power THD = 1% (max);f=1kHz .5 .45 W
THD+N Total Harmonic Distortion+Noise P
PSRR Power Supply Rejection Ratio V
= 3.3V and 8Load unless otherwise specified. Limits apply for TA= 25˚C.
DD
PIN1=VDD
= 0.25 Wrms; AVD=2;20Hz
o
f 20 kHz
= 3.2V to 3.4V 65 dB
DD

Electrical Characteristics VDD= 2.6V (Notes 1, 2, 8, 9)

The following specifications apply for V
Symbol Parameter Conditions
V
DD
I
DD
I
SD
Supply Voltage 2.0 V (min)
Quiescent Power Supply Current VIN= 0V, Io= 0A 3.4 mA (max)
Shutdown Current V
= 2.6V and 8Load unless otherwise specified. Limits apply for TA= 25˚C.
DD
PIN1=VDD
LM4872
Typical Limit
(Note 6) (Note 7)
Units
(Limits)
5.5 V (max)
0.01 µA (max)
0.15 %
LM4872
Typical Limit
(Note 6) (Note 7)
Units
(Limits)
5.5 V (max)
0.01 µA (max)
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Electrical Characteristics VDD= 2.6V (Notes 1, 2, 8, 9)
The following specifications apply for V
LM4872
25˚C. (Continued)
= 2.6V and 8Load unless otherwise specified. Limits apply for TA=
DD
Symbol Parameter Conditions
V
OS
P
0
Output Offset Voltage VIN= 0V 5 mV (max)
Output Power ( 8) Output Power ( 4)
THD = 0.3% (max);f=1kHzTHD = 0.5% (max);f=1kHz
THD+N Total Harmonic Distortion+Noise Po= 0.25 Wrms; AVD=2;20Hz
LM4872
Typical Limit
(Note 6) (Note 7)
0.25
0.5
0.25 %
Units
(Limits)
f 20 kHz
PSRR Power Supply Rejection Ratio V
Note 1: All voltages are measured with respect to the ground 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 which 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 T allowable power dissipation is P The typical junction-to-ambient thermal resistance is 150˚C/W.
Note 4: Human body model, 100 pF discharged through a 1.5 kresistor.
Note 5: Machine Model, 220 pF–240 pF 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: Low Voltage Circuit - See Fig. 4
Note 9: Shutdown current is measured in a Normal Room Environment. Exposure to direct sunlight will increase I
DMAX
=(T
)/θJAor the number given in Absolute Maximum Ratings, whichever is lower. For the LM4872, T
JMAX–TA
= 2.5V to 2.7V 65 dB
DD
, θJA, and the ambient temperature TA. The maximum
JMAX
by a maximum of 2µA.
SD
JMAX
= 150˚C.

External Components Description

(Figure 1)
W W
Components Functional Description
1. R
2. C
3. R
4. C
Inverting input resistance which sets the closed-loop gain in conjunction with Rf. This resistor also forms a
i
high pass filter with C
Input coupling capacitor which blocks the DC voltage at the amplifiers input terminals. Also creates a
i
highpass filter with R for an explanation of how to determine the value of C
Feedback resistance which sets the closed-loop gain in conjunction with Ri.
f
Supply bypass capacitor which provides power supply filtering. Refer to the Power Supply Bypassing
S
at fC= 1/(2π RiCi).
i
at fc= 1/(2π RiCi). Refer to the section, Proper Selection of External Components,
i
.
i
section for information concerning proper placement and selection of the supply bypass capacitor.
5. C
Bypass pin capacitor which provides half-supply filtering. Refer to the section, Proper Selection of External
B
Components, for information concerning proper placement and selection of C
.
B
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Typical Performance Characteristics

LM4872
THD+N vs Frequency
at 5V and 8
THD+N vs Frequency
at 2.6V and 8
THD+N vs Frequency
at 3.3V and 8
10123003 10123006
THD+N vs Frequency
at 2.6V and 4
THD+N vs Output Power
@
VDD=5V
10123005 10123004
THD+N vs Output Power
@
VDD= 3.3V
10123007 10123008
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Typical Performance Characteristics (Continued)
LM4872
THD+N vs
Output Power
2.6V at 8
Output Power vs
Supply Voltage
THD+N vs
Output Power
2.6V at 4
10123009 10123010
Output Power vs Load Resistance
Power Derating Curve
10123011 10123012
Power Dissipation vs
Output Power
=5V
V
DD
10123014 10123026
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Typical Performance Characteristics (Continued)
LM4872
Power Dissipation vs
Output Power
= 3.3V
V
DD
Clipping Voltage vs
Supply Voltage
10123027
Power Dissipation vs
Output Power
VDD= 2.6V
10123028
Supply Current vs Shutdown Voltage
10123015
10123020
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Typical Performance Characteristics (Continued)
LM4872
Frequency Response vs
Input Capacitor Size
10123017
Power Supply
Rejection Ratio
Open Loop
Frequency Response Noise Floor
10123018
10123019 10123016
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Application Information

BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 1, the LM4872 has two operational amplifiers internally, allowing for a few different amplifier configurations. The first amplifier’s gain is externally config­urable, while the second amplifier is internally fixed in a unity-gain, inverting configuration. The closed-loop gain of the first amplifier is set by selecting the ratio of R the second amplifier’s gain is fixed by the two internal 10 k resistors. Figure 1 shows that the output of amplifier one serves as the input to amplifier two which results in both amplifiers producing signals identical in magnitude, but out of phase by 180˚. Consequently, the differential gain for the IC is
= 2 *(Rf/Ri)
A
VD
By driving the load differentially through outputs Vo1 and Vo2, an amplifier configuration commonly referred to as “bridged mode” is established. Bridged mode operation is different from the classical single-ended amplifier configura­tion where one side of its load is connected to ground.
A bridge amplifier design has a few distinct advantages over the single-ended configuration, as it provides differential drive to the load, thus doubling output swing for a specified supply voltage. Four times the output power is possible as compared to a single-ended amplifier under the same con­ditions. This increase in attainable output power assumes that the amplifier is not current limited or clipped. In order to choose an amplifier’s closed-loop gain without causing ex­cessive clipping, please refer to the Audio Power Amplifier Design section.
A bridge configuration, such as the one used in LM4872, also creates a second advantage over single-ended amplifi­ers. Since the differential outputs, Vo1 and Vo2, are biased at half-supply, no net DC voltage exists across the load. This eliminates the need for an output coupling capacitor which is required in a single supply, single-ended amplifier configura­tion. Without an output coupling capacitor, the half-supply bias across the load would result in both increased internal IC power dissipation and also possible loudspeaker damage.

POWER DISSIPATION

Power dissipation is a major concern when designing a successful amplifier, whether the amplifier is bridged or single-ended. A direct consequence of the increased power delivered to the load by a bridge amplifier is an increase in internal power dissipation. Since the LM4872 has two opera­tional amplifiers in one package, the maximum internal power dissipation is 4 times that of a single-ended amplifier. The maximum power dissipation for a given application can be derived from the power dissipation graphs or from Equa­tion 1.
= 4*(VDD)2/(2π2RL) (1)
P
DMAX
It is critical that the maximum junction temperature T 150˚C is not exceeded. T power derating curves by using P
can be determined from the
JMAX
and the PC board foil
DMAX
area. By adding additional copper foil, the thermal resistance of the application can be reduced from a free air value of 150˚C/W, resulting in higher P
. Additional copper foil
DMAX
can be added to any of the leads connected to the LM4872. It is especially effective when connected to V the output pins. Refer to the application information on the LM4872 reference design board for an example of good heat sinking. If T
still exceeds 150˚C, then additional
JMAX
changes must be made. These changes can include re-
to Riwhile
f
DD,GND
JMAX
, and
LM4872
duced supply voltage, higher load impedance, or reduced ambient temperature. The National Reference Design board using a 5V supply and an 8 ohm load will run in a 110˚C ambient environement without exceeding T power dissipation is a function of output power. Refer to the Typical Performance Characteristics curves for power dis­sipation information for different output powers and output loading.

POWER SUPPLY BYPASSING

, is dependent upon PSRR require-
B
ments, click and pop performance as explained in the sec­tion, Proper Selection of External Components, system cost, and size constraints.

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the LM4872 contains a shutdown pin to externally turn off the amplifier’s bias circuitry. This shutdown feature turns the amplifier off when a logic high is placed on the shutdown pin. The trigger point between a logic low and logic high level is typically half- supply. It is best to switch between ground and supply to provide maximum device performance. By switch­ing the shutdown pin to V
, the LM4872 supply current
DD
draw will be minimized in idle mode. While the device will be disabled with shutdown pin voltages less than V current may be greater than the typical value of 0.01 µA. In either case, the shutdown pin should be tied to a stable voltage to avoid unwanted state changes.
In many applications, a microcontroller or microprocessor output is used to control the shutdown circuitry which pro­vides a quick, smooth transition into shutdown. Another so­lution is to use a single-pole, single-throw switch in conjunc­tion with an external pull-up resistor. When the switch is closed, the shutdown pin is connected to ground and en­ables the amplifier. If the switch is open, then the external pull-up resistor will disable the LM4872. This scheme guar­antees that the shutdown pin will not float thus preventing unwanted state changes.

PROPER SELECTION OF EXTERNAL COMPONENTS

Proper selection of external components in applications us­ing integrated power amplifiers is critical to optimize device and system performance. While the LM4872 is tolerant of external component combinations, consideration to compo­nent values must be used to maximize overall system qual-
of
ity. The LM4872 is unity-gain stable which gives a designer
maximum system flexibility. The LM4872 should be used in low gain configurations to minimize THD+N values, and maximize the signal to noise ratio. Low gain configurations require large input signals to obtain a given output power. Input signals equal to or greater than 1 Vrms are available from sources such as audio codecs. Please refer to the section, Audio Power Amplifier Design, for a more com­plete explanation of proper gain selection.
Besides gain, one of the major considerations is the closed­loop bandwidth of the amplifier. To a large extent, the band-
JMAX
. Internal
, the idle
DD
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Application Information (Continued)
width is dictated by the choice of external components
LM4872
shown in Figure 1. The input coupling capacitor, C first order high pass filter which limits low frequency re­sponse. This value should be chosen based on needed frequency response for a few distinct reasons.

Selection Of Input Capacitor Size

Large input capacitors are both expensive and space hungry for portable designs. Clearly, a certain sized capacitor is needed to couple in low frequencies without severe attenu­ation. But in many cases the speakers used in portable systems, whether internal or external, have little ability to reproduce signals below 100 Hz to 150 Hz. Thus, using a large input capacitor may not increase actual system perfor­mance.
In addition to system cost and size, click and pop perfor­mance is effected by the size of the input coupling capacitor,
A larger input coupling capacitor requires more charge to
C
i.
reach its quiescent DC voltage (nominally 1/2 V charge comes from the output via the feedback and is apt to create pops upon device enable. Thus, by minimizing the capacitor size based on necessary low frequency response, turn-on pops can be minimized.
Besides minimizing the input capacitor size, careful consid­eration should be paid to the bypass capacitor value. Bypass capacitor, C turn-on pops since it determines how fast the LM4872 turns on. The slower the LM4872’s outputs ramp to their quiescent DC voltage (nominally 1/2 V Choosing C (in the range of 0.1 µF to 0.39 µF), should produce a virtually clickless and popless shutdown function. While the device will function properly, (no oscillations or motorboating), with
equal to 0.1 µF, the device will be much more susceptible
C
B
to turn-on clicks and pops. Thus, a value of C
1.0 µF is recommended in all but the most cost sensitive designs.
, is the most critical component to minimize
B
), the smaller the turn-on pop.
equal to 1.0 µF along with a small value of C
B
DD
, forms a
i
). This
DD
equal to
B

AUDIO POWER AMPLIFIER DESIGN

A 1W/8AUDIO AMPLIFIER
Given:
Power Output 1 Wrms
Load Impedance 8
Input Level 1 Vrms
Input Impedance 20 k
Bandwidth 100 Hz– 20 kHz
±
0.25 dB
A designer must first determine the minimum supply rail to obtain the specified output power. By extrapolating from the Output Power vs Supply Voltage graphs in the Typical Per- formance Characteristics section, the supply rail can be easily found. A second way to determine the minimum sup­ply rail is to calculate the required V
using Equation 2
opeak
and add the output voltage. Using this method, the minimum supply voltage would be (V
and V
V
OD
BOT
age vs Supply Voltage curve in the Typical Performance
are extrapolated from the Dropout Volt-
OD
TOP
opeak
+(V
OD
TOP
+V
OD
)), where
BOT
Characteristics section.
(2)
Using the Output Power vs Supply Voltage graph for an 8 load, the minimum supply rail is 4.6V. But since 5V is a standard voltage in most applications, it is chosen for the supply rail. Extra supply voltage creates headroom that al­lows the LM4872 to reproduce peaks in excess of 1W with-
i
out producing audible distortion. At this time, the designer must make sure that the power supply choice along with the output impedance does not violate the conditions explained in the Power Dissipation section.
Once the power dissipation equations have been addressed, the required differential gain can be determined from Equa­tion 3.
LOW VOLTAGE APPLICATIONS ( BELOW 3.0 V
)
DD
The Lm4872 will function at voltages below 3 volts but this mode of operation requires the addition of a 1kresistor from each of the differential output pins ( pins 8 and 4 ) directly to ground. The addition of the pair of 1kresistors ( R4 & R5 ) assures stable operation below 3 Volt Vdd opera­tion. The addition of the two resistors will however increase the idle current by as much as 5mA. This is because at 0v input both of the outputs of the LM4872’s 2 internal opamps go to 1/2 V
( 2.5 volts for a 5v power supply ), causing
DD
current to flow through the 1K resistors from output to ground. See fig 4.
Jumper options have been included on the reference design, Fig. 4, to accommodate the low voltage application. J2 & J3 connect R4 and R5 to the outputs. J1 operates the shutdown function. J1 must be installed to operate the part. A switch may be installed in place of J1 for easier evaluation of the shutdown function.
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(3)
R
f/Ri=AVD
From Equation 3, the minimum A
/2
is 2.83; use AVD=3.
VD
Since the desired input impedance was 20 k, and with a
impedance of 2, a ratio of 1.5:1 of Rfto Riresults in an
A
VD
allocation of R
=20kΩ and Rf=30kΩ. The final design step
i
is to address the bandwidth requirements which must be stated as a pair of −3 dB frequency points. Five times away from a −3 dB point is 0.17 dB down from passband response
±
which is better than the required
= 100 Hz/5 = 20 Hz
f
L
f
=20kHz*5=100kHz
H
As stated in the External Components section, R junction with C
1/(2π*20 k*20 Hz) = 0.397 µF; use 0.39 µF
C
i
create a highpass filter.
i
0.25 dB specified.
in con-
i
The high frequency pole is determined by the product of the desired frequency pole, f With a A
= 3 and fH= 100 kHz, the resulting GBWP =
VD
, and the differential gain, AVD.
H
150 kHz which is much smaller than the LM4872 GBWP of 4 MHz. This figure displays that if a designer has a need to design an amplifier with a higher differential gain, the LM4872 can still be used without running into bandwidth limitations.
Application Information (Continued)
LM4872

FIGURE 2. Higher Gain Audio Amplifier

The LM4872 is unity-gain stable and requires no external components besides gain-setting resistors, an input coupling capacitor, and proper supply bypassing in the typical appli­cation. However, if a closed-loop differential gain of greater than 10 is required, a feedback capacitor may be needed as shown in Figure 2 to bandwidth limit the amplifier. This feedback capacitor creates a low pass filter that eliminates possible high frequency oscillations. Care should be taken
10123024
when calculating the -3dB frequency in that an incorrect combination of R
and C4will cause rolloff before 20kHz. A
3
typical combination of feedback resistor and capacitor that will not produce audio band high frequency rolloff is R 20kand C
= 25pf. These components result in a -3dB
4
3
point of approximately 320 kHz. It is not recommended that the feedback resistor and capacitor be used to implement a band limiting filter below 100kHZ.
=
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Application Information (Continued)
LM4872
10123029

FIGURE 3. Differential Amplifier Configuration for LM4872

Mono LM4872 Reference Design Board - Assembly Part Number: 980011207-100 Revision: A Bill of Material
Item Part Number Part Description Qty Ref Designator
1 551011208-001 LM4872 Mono Reference Design Board PCB etch 001 1
10 482911183-001 LM4872 Audio AMP micro SMD 8 Bumps 1 U1
20 151911207-001 Cer Cap 0.1uF 50V +80/-20% 1206 1 C1
21 151911207-002 Cer Cap 0.39uF 50V Z5U 20% 1210 1 C2
25 152911207-001 Tant Cap 1uF 16V 10% Size=A 3216 1 C3
30 472911207-001 Res 20K Ohm 1/10W 5% 0805 3 R1, R2, R3
31 472911207-002 Res 1K Ohm 1/10W 5% 0805 2 R4, R5,
35 210007039-002 Jumper Header Vertical Mount 2X1 0.100 3 J1, J2, J3
36 210007582-001 Jumper Shunt 2 position 0.100 3
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Application Information (Continued)
Silk Screen Top Layer
LM4872
10123030
Bottom Layer Inner Layer V
10123032
Inner Layer Ground
10123031
DD
10123033
10123034
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Application Information (Continued)
LM4872

Reference Design Board and PCB Layout Guidelines

10123025

FIGURE 4. Reference Design Board

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.

General Mixed Signal Layout Recommendation

Power and Ground Circuits

For 2 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 have a major impact on low level signal performance. Star trace routing refers to using individual traces to feed power and ground to each circuit or even device. This technique will take require a greater amount of design time but will not increase the final price of the board. The only extra parts required will be some jumpers.

Single-Point Power / Ground Connections

The analog power traces should be connected to the digital traces through a single point (link). A ’Pi-filter’ can be helpful in minimizing High Frequency noise coupling between the analog and digital sections. It is further recommended to put digital and analog power traces over the corresponding digi­tal and analog ground traces to minimize noise coupling.

Placement of Digital and Analog Components

All digital components and high-speed digital signals traces should be located as far away as possible from analog components and circuit traces.

Avoiding Typical Design / Layout Problems

Avoid ground loops or running digital and analog traces parallel to each other (side-by-side) on the same PCB layer. When traces must cross over each other do it at 90 degrees. Running digital and analog traces at 90 degrees to each other from the top to the bottom side as much as possible will minimize capacitive noise coupling and cross talk.
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Physical Dimensions inches (millimeters) unless otherwise noted

LM4872 1 Watt Audio Power Amplifier micro SMD package
Note: Unless otherwise specified.
1. Epoxy coating.
2. 63Sn/37Pb eutectic bump.
3. Recommend non-solder mask defined landing pad.
4. Pin 1 is established by lower left corner with respect to text orientation pins are numbered counterclockwise.
5. Reference JEDEC registration MO-211, variation BC.
8-Bump micro SMD
Order Number LM4872IBP, LM4872IBPX
NS Package Number BPA08B6B
±
X1 = 1.31
0.03 X2 = 1.97±0.03 X3 = 0.850±0.10
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labeling, can be reasonably expected to result in a significant injury to the user.
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Americas Email: new.feedback@nsc.com
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