LM2876 [TI]
具有静音功能的高性能 40 瓦音频功率放大器;
| 型号： | LM2876 |
| 厂家： | 
TEXAS INSTRUMENTS |
| 描述： | 具有静音功能的高性能 40 瓦音频功率放大器 放大器 功率放大器 |
| 文件： | 总29页 (文件大小：973K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LM2876
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SNAS088C –AUGUST 1995–REVISED MARCH 2013
LM2876 Overture™ Audio Power Amplifier Series
High-Performance 40W Audio Power Amplifier w/Mute
Check for Samples: LM2876
1
FEATURES
DESCRIPTION
The LM2876 is a high-performance audio power
23
•
40W Continuous Average Output Power Into
8Ω
amplifier capable of delivering 40W of continuous
average power to an 8Ω load with 0.1% THD+N from
20Hz–20kHz.
•
75W Instantaneous Peak Output Power
Capability
The performance of the LM2876, utilizing its Self
Peak Instantaneous Temperature (°Ke) (SPiKe)
protection circuitry, puts it in a class above discrete
and hybrid amplifiers by providing an inherently,
dynamically protected Safe Operating Area (SOA).
SPiKe protection means that these parts are
completely safeguarded at the output against
overvoltage, undervoltage, overloads, including shorts
to the supplies, thermal runaway, and instantaneous
temperature peaks.
•
•
•
Signal-to-Noise Ratio ≥ 95 dB(min)
An Input Mute Function
Output Protection From A Short to Ground Or
to the Supplies Via Internal Current Limiting
Circuitry
•
•
Output Over-Voltage Protection Against
Transients From Inductive Loads
Supply Under-Voltage Protection, Not Allowing
Internal Biasing to Occur When |VEE| + |VCC| ≤
12V, Thus Eliminating Turn-On and Turn-Off
Transients
The LM2876 maintains an excellent signal-to-noise
ratio of greater than 95dB (min) with a typical low
noise floor of 2.0μV. It exhibits extremely low THD+N
values of 0.06% at the rated output into the rated
load over the audio spectrum, and provides excellent
linearity with an IMD (SMPTE) typical rating of
0.004%.
•
•
11-Lead PFM Package
Wide Supply Range 20V - 72V
APPLICATIONS
•
•
•
•
•
Component Stereo
Compact Stereo
Self-Powered Speakers
Surround-Sound Amplifiers
High-End Stereo TVs
1
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
2
3
Overture is a trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 1995–2013, Texas Instruments Incorporated
LM2876
SNAS088C –AUGUST 1995–REVISED MARCH 2013
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Typical Application
* Optional components dependent upon specific design requirements. Refer to the External Components Description
section for a component functional description.
Figure 1. Typical Audio Amplifier Application Circuit
Connection Diagram
†Connect Pin 5 to V+ for Compatibility with LM3886.
*Preliminary: Call your local Texas Instruments' sales rep. or distributor for availability.
Figure 2. Top View
See Package Number NDJ0011B
for Staggered Lead Non-Isolated Package
or NDA0011B for Staggered Lead Isolated Package
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These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
(1) (2)(3)
ABSOLUTE MAXIMUM RATINGS
Supply Voltage |V+|+|V−| (No Signal)
Supply Voltage |V+|+|V−| (Input Signal)
Common Mode Input Voltage
Differential Input Voltage
72V
70V
(V+ or V−) and |V+| + |V−| ≤ 60V
60V
Internally Limited
125W
Output Current
(4)
Power Dissipation
(5)
ESD Susceptibility
3000V
(6)
Junction Temperature
150°C
Soldering Information
Storage Temperature
NDJ Package (10 seconds)
260°C
−40°C to +150°C
1°C/W
θJC
θJA
Thermal Resistance(7)
43°C/W
(1) 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 ensure specific performance limits. Electrical Characteristics state DC and AC electrical
specifications under particular test conditions which ensure specific performance limits. This assumes that the device is within the
Operating Ratings. Specifications are not ensured for parameters where no limit is given, however, the typical value is a good indication
of device performance.
(2) If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and
specifications.
(3) All voltages are measured with respect to the GND pin (pin 7), unless otherwise specified.
(4) For operating at case temperatures above 25°C, the device must be derated based on a 150°C maximum junction temperature and a
thermal resistance of θJC = 1.0 °C/W (junction to case). Refer to the Thermal Resistance figure in the APPLICATION INFORMATION
section under THERMAL CONSIDERATIONS.
(5) Human body model, 100 pF discharged through a 1.5 kΩ resistor.
(6) The operating junction temperature maximum is 150°C, however, the instantaneous Safe Operating Area temperature is 250°C.
(7) The LM2876T package NDJ0011B is a non-isolated package, setting the tab of the device and the heat sink at V− potential when the
LM2876 is directly mounted to the heat sink using only thermal compound. If a mica washer is used in addition to thermal compound,
θCS (case to sink) is increased, but the heat sink will be isolated from V−.
(1) (2)(3)
OPERATING RATINGS
Temperature Range
TMIN ≤ TA ≤ TMAX
−20°C ≤ TA ≤ +85°C
Supply Voltage |V+| + |V−|
20V to 60V
(1) All voltages are measured with respect to the GND pin (pin 7), unless otherwise specified.
(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 ensure specific performance limits. Electrical Characteristics state DC and AC electrical
specifications under particular test conditions which ensure specific performance limits. This assumes that the device is within the
Operating Ratings. Specifications are not ensured for parameters where no limit is given, however, the typical value is a good indication
of device performance.
(3) Operation is specified up to 60V, however, distortion may be introduced from SPiKe Protection Circuitry if proper thermal considerations
are not taken into account. Refer to the THERMAL CONSIDERATIONS section for more information. (See SPiKe Protection Response.)
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(1) (2)
ELECTRICAL CHARACTERISTICS
The following specifications apply for V+ = +30V, V− = −30V, IMUTE = −0.5 mA with RL = 8Ω unless otherwise specified. Limits
apply for TA = 25°C.
Symbol
Parameter
Conditions
LM2876
Units
(Limits)
Typical(3 Limit(4)
)
20
V (min)
V (max)
dB (min)
(5)
|V+| + |V−|
AM
Power Supply Voltage
V
pin7 − V− ≥ 9V
18
60
Pin 8 Open or at 0V, Mute: On Current out of Pin
8 > 0.5 mA, Mute: Off
115
80
Mute Attenuation
(6)
PO
Output Power (Continuous Average)
Instantaneous Peak Output Power
Total Harmonic Distortion Plus Noise
THD + N = 0.1% (max) f = 1 kHz; f = 20 kHz
40
75
25
W (min)
Peak PO
THD + N
W
%
25W, 20 Hz ≤ f ≤ 20 kHz AV = 26 dB
0.06
(6)
(7)
SR
Slew Rate
VIN = 1.2 Vrms, f = 10 kHz, Square-Wave, RL
2 kΩ
=
V/μs (min)
9
5
I+(8)
Total Quiescent Power Supply Current VCM = 0V, Vo = 0V, Io = 0A
24
1
50
10
1
mA (max)
mV (max)
μA (max)
μA (max)
A (min)
(8)
VOS
Input Offset Voltage
Input Bias Current
Input Offset Current
Output Current Limit
VCM = 0V, Io = 0 mA
IB
VCM = 0V, Io = 0 mA
0.2
0.01
4
IOS
Io
VCM = 0V, Io = 0 mA
0.2
3
|V+| = |V−| = 10V, tON = 10 ms, VO = 0V
|V+–VO|, V+ = 20V, Io = +100 mA
|VO–V−|, V− = −20V, Io = −100 mA
1.5
2.5
125
4
V (max)
V (max)
dB (min)
(8)
(9)
Vod
Output Dropout Voltage
4
V+ = 30V to 10V, V− = −30V, VCM = 0V, Io = 0
85
mA
(8)
PSRR
Power Supply Rejection Ratio
Common Mode Rejection Ratio
V+ = 30V, V− = −30V to −10V, VCM = 0V, Io = 0
mA
V+ = 50V to 10V, V− = −10V to −50V, VCM = 20V
to −20V, Io = 0 mA
110
85
dB (min)
(8)
CMRR
110
75
dB (min)
(8)
AVOL
Open Loop Voltage Gain
Gain-Bandwidth Product
|V+| = |V−| = 30V, RL = 2 kΩ, ΔVO = 40V
|V+| = |V−| = 30V, fO = 100 kHz, VIN = 50 mVrms
115
8
80
2
dB (min)
GBWP
MHz (min)
IHF—A Weighting Filter, RIN = 600Ω (Input
Referred)
(6)
eIN
Input Noise
2.0
98
8
μV (max)
dB
PO = 1W, A-Weighted, Measured at 1 kHz, RS
25Ω
=
PO = 25W, A-Weighted, Measured at 1 kHz, RS
= 25Ω
SNR
IMD
Signal-to-Noise Ratio
112
117
dB
Ppk= 75W, A-Weighted, Measured at 1 kHz, RS
= 25Ω
dB
60 Hz, 7 kHz, 4:1 (SMPTE)
60 Hz, 7 kHz, 1:1 (SMPTE)
0.004
0.006
Intermodulation Distortion Test
%
(1) All voltages are measured with respect to the GND pin (pin 7), unless otherwise specified.
(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 ensure specific performance limits. Electrical Characteristics state DC and AC electrical
specifications under particular test conditions which ensure specific performance limits. This assumes that the device is within the
Operating Ratings. Specifications are not ensured for parameters where no limit is given, however, the typical value is a good indication
of device performance.
(3) Typicals are measured at 25°C and represent the parametric norm.
(4) Limits are specified to Texas Instruments' AOQL (Average Outgoing Quality Level).
(5) V− must have at least −9V at its pin with reference to ground in order for the under-voltage protection circuitry to be disabled.
(6) AC Electrical Test; refer to Test Circuit #2 .
(7) The feedback compensation network limits the bandwidth of the closed-loop response and so the slew rate will be reduced due to the
high frequency roll-off. Without feedback compensation, the slew rate is typically larger.
(8) DC Electrical Test; refer to Test Circuit #1 .
(9) The output dropout voltage is the supply voltage minus the clipping voltage. Refer to the Clipping Voltage vs Supply Voltage graph in the
TYPICAL PERFORMANCE CHARACTERISTICS section.
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Test Circuit #1
(DC Electrical Test Circuit)
Figure 3.
Test Circuit #2
(AC Electrical Test Circuit)
Figure 4.
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SINGLE SUPPLY APPLICATION CIRCUIT
*Optional components dependent upon specific design requirements. Refer to the External Components Description
section for a component functional description.
Figure 5. Typical Single Supply Audio Amplifier Application Circuit
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Equivalent Schematic
(excluding active protection circuitry)
Figure 6.
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External Components Description(10)
Components
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Functional Description
1.
2.
3.
4.
5.
RIN
RA
CA
C
Acts as a volume control by setting the voltage level allowed to the amplifier's input terminals.
Provides DC voltage biasing for the single supply operation and bias current for the positive input terminal.
Provides bias filtering.
Provides AC coupling at the input and output of the amplifier for single supply operation.
RB
Prevents currents from entering the amplifier's non-inverting input which may be passed through to the load upon
power-down of the system due to the low input impedance of the circuitry when the under-voltage circuitry is off. This
phenomenon occurs when the supply voltages are below 1.5V.
(1)
6.
CC
Reduces the gain (bandwidth of the amplifier) at high frequencies to avoid quasi-saturation oscillations of the output
transistor. The capacitor also suppresses external electromagnetic switching noise created from fluorescent lamps.
7.
Ri
Inverting input resistance to provide AC Gain in conjunction with Rf1.
(1)
(1)
8.
Ci
Feedback capacitor. Ensures unity gain at DC. Also a low frequency pole (highpass roll-off) at: fc = 1/(2πRi Ci)
Feedback resistance to provide AC Gain in conjunction with Ri.
9.
Rf1
Rf2
10.
At higher frequencies feedback resistance works with Cf to provide lower AC Gain in conjunction with Rf1 and Ri. A high
frequency pole (lowpass roll-off) exists at: fc = [Rf1 Rf2 (s + 1/Rf2Cf)]/[(Rf1 + Rf2)(s + 1/Cf(Rf1 + Rf2))]
11.
12.
Cf(1)
RM
Compensation capacitor that works with Rf1 and Rf2 to reduce the AC Gain at higher frequencies.
Mute resistance set up to allow 0.5 mA to be drawn from pin 8 to turn the muting function off. → RM is calculated
using: RM ≤ (|VEE| − 2.6V)/I8 where I8 ≥ 0.5 mA. Refer to the Mute Attenuation vs Mute Current curves in the TYPICAL
PERFORMANCE CHARACTERISTICS section.
13.
14.
15.
CM
Mute capacitance set up to create a large time constant for turn-on and turn-off muting.
(1)
RSN
CSN
Works with CSN to stabilize the output stage by creating a pole that eliminates high frequency oscillations.
(1)
Works with RSN to stabilize the output stage by creating a pole that eliminates high frequency oscillations. fc =
1/(2πRSNCSN
)
(1)
16.
17.
L
Provides high impedance at high frequecies so that R may decouple a highly capacitive load and reduce the Q of the
series resonant circuit due to capacitive load. Also provides a low impedance at low frequencies to short out R and pass
audio signals to the load.
(1)
R
18.
19.
CS
S1
Provides power supply filtering and bypassing.
Mute switch that mutes the music going into the amplifier when opened.
(10) (Figure 1 and Figure 5)
(1) Optional components dependent upon specific design requirements. Refer to the APPLICATION INFORMATION section for more
information.
OPTIONAL EXTERNAL COMPONENT INTERACTION
Although the optional external components have specific desired functions that are designed to reduce the
bandwidth and eliminate unwanted high frequency oscillations they may cause certain undesirable effects when
they interact. Interaction may occur for components whose reactances are in close proximity to one another. One
example would be the coupling capacitor, CC, and the compensation capacitor, Cf. These two components act as
low impedances to certain frequencies which will couple signals from the input to the output. Please take careful
note of basic amplifier component functionality when designing in these components.
The optional external components shown in Figure 5 and described above are applicable in both single and split
voltage supply configurations.
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TYPICAL PERFORMANCE CHARACTERISTICS
SPiKe
Protection Response
Safe Area
Figure 7.
Figure 8.
Supply Current vs
Supply Voltage
Pulse Thermal
Resistance
Figure 9.
Figure 10.
Pulse Thermal
Resistance
Supply Current vs
Output Voltage
Figure 11.
Figure 12.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
Pulse Power Limit
Pulse Power Limit
Figure 13.
Figure 14.
Supply Current vs
Case Temperature
Pulse Response
Figure 15.
Figure 16.
Input Bias Current vs
Case Temperature
Clipping Voltage vs
Supply Voltage
Figure 17.
Figure 18.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
THD + N
vs
Frequency
THD + N vs
Output Power
Figure 19.
Figure 20.
THD + N vs
Output Power
THD + N Distribution
Figure 21.
Figure 22.
Output Power vs
Load Resistance
THD + N Distribution
Figure 23.
Figure 24.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
Max Heatsink Thermal Resistance (°C/W)
at the Specified Ambient Temperature (°C)
Maximum Power Dissipation
vs
Supply Voltage
Note: The maximum heat sink thermal resistance values, øSA, in the table above were calculated using a øCS = 0.2°C/W due to thermal
compound.
Figure 25.
Power Dissipation vs
Output Power
Power Dissipation vs
Output Power
Figure 26.
Figure 27.
Output Power vs
Supply Voltage
IMD 60 Hz, 4:1
Figure 28.
Figure 29.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
IMD 60 Hz, 7 kHz, 4:1
IMD 60 Hz, 7 kHz, 4:1
Figure 30.
Figure 31.
IMD 60 Hz, 1:1
IMD 60 Hz, 7 kHz 1:1
Figure 32.
Figure 33.
Mute Attenuation
vs Mute Current
IMD 60 Hz, 7 kHz, 1:1
Figure 34.
Figure 35.
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TYPICAL PERFORMANCE CHARACTERISTICS (continued)
Mute Attenuation
vs Mute Current
Large Signal Response
Figure 36.
Figure 37.
Power Supply
Rejection Ratio
Common-Mode
Rejection Ratio
Figure 38.
Figure 39.
Open Loop
Frequency Response
Figure 40.
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APPLICATION INFORMATION
GENERAL FEATURES
Mute Function
The muting function of the LM2876 allows the user to mute the music going into the amplifier by drawing less
than 0.5 mA out of pin 8 of the device. This is accomplished as shown in the Typical Application Circuit where
the resistor RM is chosen with reference to your negative supply voltage and is used in conjuction with a switch.
The switch (when opened) cuts off the current flow from pin 8 to V−, thus placing the LM2876 into mute mode.
Refer to the Mute Attenuation vs Mute Current curves in the TYPICAL PERFORMANCE CHARACTERISTICS
section for values of attenuation per current out of pin 8. The resistance RM is calculated by the following
equation:
RM (|VEE| − 2.6V)/I8
where
•
I8 ≥ 0.5 mA
Under-Voltage Protection
Upon system power-up the under-voltage protection circuitry allows the power supplies and their corresponding
caps to come up close to their full values before turning on the LM2876 such that no DC output spikes occur.
Upon turn-off, the output of the LM2876 is brought to ground before the power supplies such that no transients
occur at power-down.
Over-Voltage Protection
The LM2876 contains overvoltage protection circuitry that limits the output current to approximately 4Apeak while
also providing voltage clamping, though not through internal clamping diodes. The clamping effect is quite the
same, however, the output transistors are designed to work alternately by sinking large current spikes.
SPiKe Protection
The LM2876 is protected from instantaneous peak-temperature stressing by the power transistor array. The Safe
Operating Area graph in the TYPICAL PERFORMANCE CHARACTERISTICS section shows the area of device
operation where the SPiKe Protection Circuitry is not enabled. The waveform to the right of the SOA graph
exemplifies how the dynamic protection will cause waveform distortion when enabled.
Thermal Protection
The LM2876 has a sophisticated thermal protection scheme to prevent long-term thermal stress to the device.
When the temperature on the die reaches 165°C, the LM2876 shuts down. It starts operating again when the die
temperature drops to about 155°C, but if the temperature again begins to rise, shutdown will occur again at
165°C. Therefore the device is allowed to heat up to a relatively high temperature if the fault condition is
temporary, but a sustained fault will cause the device to cycle in a Schmitt Trigger fashion between the thermal
shutdown temperature limits of 165°C and 155°C. This greatly reduces the stress imposed on the IC by thermal
cycling, which in turn improves its reliability under sustained fault conditions.
Since the die temperature is directly dependent upon the heat sink, the heat sink should be chosen as discussed
in the THERMAL CONSIDERATIONS section, such that thermal shutdown will not be reached during normal
operation. Using the best heat sink possible within the cost and space constraints of the system will improve the
long-term reliability of any power semiconductor device.
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THERMAL CONSIDERATIONS
Heat Sinking
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The choice of a heat sink for a high-power audio amplifier is made entirely to keep the die temperature at a level
such that the thermal protection circuitry does not operate under normal circumstances. The heat sink should be
chosen to dissipate the maximum IC power for a given supply voltage and rated load.
With high-power pulses of longer duration than 100 ms, the case temperature will heat up drastically without the
use of a heat sink. Therefore the case temperature, as measured at the center of the package bottom, is entirely
dependent on heat sink design and the mounting of the IC to the heat sink. For the design of a heat sink for your
audio amplifier application refer to the Determining the Correct Heat Sink section.
Since a semiconductor manufacturer has no control over which heat sink is used in a particular amplifier design,
we can only inform the system designer of the parameters and the method needed in the determination of a heat
sink. With this in mind, the system designer must choose his supply voltages, a rated load, a desired output
power level, and know the ambient temperature surrounding the device. These parameters are in addition to
knowing the maximum junction temperature and the thermal resistance of the IC, both of which are provided by
Texas Instruments.
As a benefit to the system designer we have provided Maximum Power Dissipation vs Supply Voltages curves
for various loads in the TYPICAL PERFORMANCE CHARACTERISTICS section, giving an accurate figure for
the maximum thermal resistance required for a particular amplifier design. This data was based on θJC = 1°C/W
and θCS = 0.2°C/W. We also provide a section regarding heat sink determination for any audio amplifier design
where θCS may be a different value. It should be noted that the idea behind dissipating the maximum power
within the IC is to provide the device with a low resistance to convection heat transfer such as a heat sink.
Therefore, it is necessary for the system designer to be conservative in his heat sink calculations. As a rule, the
lower the thermal resistance of the heat sink the higher the amount of power that may be dissipated. This is of
course guided by the cost and size requirements of the system. Convection cooling heat sinks are available
commercially, and their manufacturers should be consulted for ratings.
Proper mounting of the IC is required to minimize the thermal drop between the package and the heat sink. The
heat sink must also have enough metal under the package to conduct heat from the center of the package
bottom to the fins without excessive temperature drop.
A thermal grease such as Wakefield type 120 or Thermalloy Thermacote should be used when mounting the
package to the heat sink. Without this compound, thermal resistance will be no better than 0.5°C/W, and
probably much worse. With the compound, thermal resistance will be 0.2°C/W or less, assuming under 0.005
inch combined flatness runout for the package and heat sink. Proper torquing of the mounting bolts is important
and can be determined from heat sink manufacturer's specification sheets.
Should it be necessary to isolate V− from the heat sink, an insulating washer is required. Hard washers like
beryluum oxide, anodized aluminum and mica require the use of thermal compound on both faces. Two-mil mica
washers are most common, giving about 0.4°C/W interface resistance with the compound.
Silicone-rubber washers are also available. A 0.5°C/W thermal resistance is claimed without thermal compound.
Experience has shown that these rubber washers deteriorate and must be replaced should the IC be
dismounted.
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Determining Maximum Power Dissipation
Power dissipation within the integrated circuit package is a very important parameter requiring a thorough
understanding if optimum power output is to be obtained. An incorrect maximum power dissipation (PD)
calculation may result in inadequate heat sinking, causing thermal shutdown circuitry to operate and limit the
output power.
The following equations can be used to accurately calculate the maximum and average integrated circuit power
dissipation for your amplifier design, given the supply voltage, rated load, and output power. These equations
can be directly applied to the Power Dissipation vs Output Power curves in the TYPICAL PERFORMANCE
CHARACTERISTICS section.
Equation 1 exemplifies the maximum power dissipation of the IC and Equation 2 and Equation 3 exemplify the
average IC power dissipation expressed in different forms.
PDMAX = VCC2/2π2RL
where
•
VCC is the total supply voltage
(1)
(2)
(3)
PDAVE = (VOpk/RL)[VCC/π − VOpk/2]
where
•
VCC is the total supply voltage and VOpk = VCC/π
PDAVE = VCC VOpk/πRL − VOpk2/2RL
where
•
VCC is the total supply voltage
Determining the Correct Heat Sink
Once the maximum IC power dissipation is known for a given supply voltage, rated load, and the desired rated
output power the maximum thermal resistance (in °C/W) of a heat sink can be calculated. This calculation is
made using Equation 4equation (4) and is based on the fact that thermal heat flow parameters are analogous to
electrical current flow properties.
It is also known that typically the thermal resistance, θJC (junction to case), of the LM2876 is 1°C/W and that
using Thermalloy Thermacote thermal compound provides a thermal resistance, θCS (case to heat sink), of about
0.2°C/W as explained in the Heat Sinking section.
Referring to the figure below, it is seen that the thermal resistance from the die (junction) to the outside air
(ambient) is a combination of three thermal resistances, two of which are known, θJC and θCS. Since convection
heat flow (power dissipation) is analogous to current flow, thermal resistance is analogous to electrical
resistance, and temperature drops are analogous to voltage drops, the power dissipation out of the LM2876 is
equal to the following:
PDMAX = (TJmax − TAmb)/θJA
where
•
θJA = θJC + θCS + θSA
But since we know PDMAX, θJC, and θSC for the application and we are looking for θSA, we have the following:
θSA = [(TJmax − TAmb) − PDMAX (θJC + θCS)]/PDMAX
(4)
Again it must be noted that the value of θSA is dependent upon the system designer's amplifier application and its
corresponding parameters as described previously. If the ambient temperature that the audio amplifier is to be
working under is higher than the normal 25°C, then the thermal resistance for the heat sink, given all other things
are equal, will need to be smaller.
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Equation 1 and Equation 4 are the only equations needed in the determination of the maximum heat sink thermal
resistance. This is of course given that the system designer knows the required supply voltages to drive his rated
load at a particular power output level and the parameters provided by the semiconductor manufacturer. These
parameters are the junction to case thermal resistance, θJC, TJmax = 150°C, and the recommended Thermalloy
Thermacote thermal compound resistance, θCS
.
SIGNAL-TO-NOISE RATIO
In the measurement of the signal-to-noise ratio, misinterpretations of the numbers actually measured are
common. One amplifier may sound much quieter than another, but due to improper testing techniques, they
appear equal in measurements. This is often the case when comparing integrated circuit designs to discrete
amplifier designs. Discrete transistor amps often “run out of gain” at high frequencies and therefore have small
bandwidths to noise as indicated below.
Integrated circuits have additional open loop gain allowing additional feedback loop gain in order to lower
harmonic distortion and improve frequency response. It is this additional bandwidth that can lead to erroneous
signal-to-noise measurements if not considered during the measurement process. In the typical example above,
the difference in bandwidth appears small on a log scale but the factor of 10 in bandwidth, (200 kHz to 2 MHz)
can result in a 10 dB theoretical difference in the signal-to-noise ratio (white noise is proportional to the square
root of the bandwidth in a system).
In comparing audio amplifiers it is necessary to measure the magnitude of noise in the audible bandwidth by
using a “weighting” filter (see Note below). A “weighting” filter alters the frequency response in order to
compensate for the average human ear's sensitivity to the frequency spectra. The weighting filters at the same
time provide the bandwidth limiting as discussed in the previous paragraph.
NOTE
CCIR/ARM: A Practical Noise Measurement Method; by Ray Dolby, David Robinson and
Kenneth Gundry, AES Preprint No. 1353 (F-3).
In addition to noise filtering, differing meter types give different noise readings. Meter responses include:
1. RMS reading,
2. average responding,
3. peak reading, and
4. quasi peak reading.
Although theoretical noise analysis is derived using true RMS based calculations, most actual measurements are
taken with ARM (Average Responding Meter) test equipment.
Typical signal-to-noise figures are listed for an A-weighted filter which is commonly used in the measurement of
noise. The shape of all weighting filters is similar, with the peak of the curve usually occurring in the 3 kHz–7 kHz
region as shown below.
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SUPPLY BYPASSING
The LM2876 has excellent power supply rejection and does not require a regulated supply. However, to eliminate
possible oscillations all op amps and power op amps should have their supply leads bypassed with low-
inductance capacitors having short leads and located close to the package terminals. Inadequate power supply
bypassing will manifest itself by a low frequency oscillation known as “motorboating” or by high frequency
instabilities. These instabilities can be eliminated through multiple bypassing utilizing a large tantalum or
electrolytic capacitor (10 μF or larger) which is used to absorb low frequency variations and a small ceramic
capacitor (0.1 μF) to prevent any high frequency feedback through the power supply lines.
If adequate bypassing is not provided the current in the supply leads which is a rectified component of the load
current may be fed back into internal circuitry. This signal causes low distortion at high frequencies requiring that
the supplies be bypassed at the package terminals with an electrolytic capacitor of 470 μF or more.
LEAD INDUCTANCE
Power op amps are sensitive to inductance in the output lead, particularly with heavy capacitive loading.
Feedback to the input should be taken directly from the output terminal, minimizing common inductance with the
load.
Lead inductance can also cause voltage surges on the supplies. With long leads to the power supply, energy is
stored in the lead inductance when the output is shorted. This energy can be dumped back into the supply
bypass capacitors when the short is removed. The magnitude of this transient is reduced by increasing the size
of the bypass capacitor near the IC. With at least a 20 μF local bypass, these voltage surges are important only if
the lead length exceeds a couple feet (> 1 μH lead inductance). Twisting together the supply and ground leads
minimizes the effect.
LAYOUT, GROUND LOOPS AND STABILITY
The LM2876 is designed to be stable when operated at a closed-loop gain of 10 or greater, but as with any other
high-current amplifier, the LM2876 can be made to oscillate under certain conditions. These usually involve
printed circuit board layout or output/input coupling.
When designing a layout, it is important to return the load ground, the output compensation ground, and the low
level (feedback and input) grounds to the circuit board common ground point through separate paths. Otherwise,
large currents flowing along a ground conductor will generate voltages on the conductor which can effectively act
as signals at the input, resulting in high frequency oscillation or excessive distortion. It is advisable to keep the
output compensation components and the 0.1 μF supply decoupling capacitors as close as possible to the
LM2876 to reduce the effects of PCB trace resistance and inductance. For the same reason, the ground return
paths should be as short as possible.
In general, with fast, high-current circuitry, all sorts of problems can arise from improper grounding which again
can be avoided by returning all grounds separately to a common point. Without isolating the ground signals and
returning the grounds to a common point, ground loops may occur.
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“Ground Loop” is the term used to describe situations occurring in ground systems where a difference in potential
exists between two ground points. Ideally a ground is a ground, but unfortunately, in order for this to be true,
ground conductors with zero resistance are necessary. Since real world ground leads possess finite resistance,
currents running through them will cause finite voltage drops to exist. If two ground return lines tie into the same
path at different points there will be a voltage drop between them. The first figure below shows a common ground
example where the positive input ground and the load ground are returned to the supply ground point via the
same wire. The addition of the finite wire resistance, R2, results in a voltage difference between the two points as
shown below.
The load current IL will be much larger than input bias current II, thus V1 will follow the output voltage directly, i.e.
in phase. Therefore the voltage appearing at the non-inverting input is effectively positive feedback and the
circuit may oscillate. If there were only one device to worry about then the values of R1 and R2 would probably be
small enough to be ignored; however, several devices normally comprise a total system. Any ground return of a
separate device, whose output is in phase, can feedback in a similar manner and cause instabilities. Out of
phase ground loops also are troublesome, causing unexpected gain and phase errors.
The solution to most ground loop problems is to always use a single-point ground system, although this is
sometimes impractical. The third figure below is an example of a single-point ground system.
The single-point ground concept should be applied rigorously to all components and all circuits when possible.
Violations of single-point grounding are most common among printed circuit board designs, since the circuit is
surrounded by large ground areas which invite the temptation to run a device to the closest ground spot. As a
final rule, make all ground returns low resistance and low inductance by using large wire and wide traces.
Occasionally, current in the output leads (which function as antennas) can be coupled through the air to the
amplifier input, resulting in high-frequency oscillation. This normally happens when the source impedance is high
or the input leads are long. The problem can be eliminated by placing a small capacitor, CC, (on the order of 50
pF to 500 pF) across the LM2876 input terminals. Refer to the External Components Description(1) section
relating to component interaction with Cf.
(1) (Figure 1 and Figure 5)
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REACTIVE LOADING
It is hard for most power amplifiers to drive highly capacitive loads very effectively and normally results in
oscillations or ringing on the square wave response. If the output of the LM2876 is connected directly to a
capacitor with no series resistance, the square wave response will exhibit ringing if the capacitance is greater
than about 0.2 μF. If highly capacitive loads are expected due to long speaker cables, a method commonly
employed to protect amplifiers from low impedances at high frequencies is to couple to the load through a 10Ω
resistor in parallel with a 0.7 μH inductor. The inductor-resistor combination as shown in the Typical Application
Circuit isolates the feedback amplifier from the load by providing high output impedance at high frequencies thus
allowing the 10Ω resistor to decouple the capacitive load and reduce the Q of the series resonant circuit. The LR
combination also provides low output impedance at low frequencies thus shorting out the 10Ω resistor and
allowing the amplifier to drive the series RC load (large capacitive load due to long speaker cables) directly.
GENERALIZED AUDIO POWER AMPLIFIER DESIGN
The system designer usually knows some of the following parameters when starting an audio amplifier design:
Desired Power Output
Input Impedance
Input Level
Load Impedance
Bandwidth
Maximum Supply Voltage
The power output and load impedance determine the power supply requirements, however, depending upon the
application some system designers may be limited to certain maximum supply voltages. If the designer does
have a power supply limitation, he should choose a practical load impedance which would allow the amplifier to
provide the desired output power, keeping in mind the current limiting capabilities of the device. In any case, the
output signal swing and current are found from (where PO is the average output power):
(5)
(6)
To determine the maximum supply voltage the following parameters must be considered. Add the dropout
voltage (4V for LM2876) to the peak output swing, Vopeak, to get the supply rail value (i.e. ± (Vopeak + Vod) at a
current of Iopeak). The regulation of the supply determines the unloaded voltage, usually about 15% higher.
Supply voltage will also rise 10% during high line conditions. Therefore, the maximum supply voltage is obtained
from the following equation:
Max. supplies
≈ ± (Vopeak + Vod)(1 + regulation)(1.1)
(7)
The input sensitivity and the output power specs determine the minimum required gain as depicted below:
(8)
Normally the gain is set between 20 and 200; for a 40W, 8Ω audio amplifier this results in a sensitivity of 894 mV
and 89 mV, respectively. Although higher gain amplifiers provide greater output power and dynamic headroom
capabilities, there are certain shortcomings that go along with the so called “gain.” The input referred noise floor
is increased and hence the SNR is worse. With the increase in gain, there is also a reduction of the power
bandwidth which results in a decrease in feedback thus not allowing the amplifier to respond quickly enough to
nonlinearities. This decreased ability to respond to nonlinearities increases the THD + N specification.
The desired input impedance is set by RIN. Very high values can cause board layout problems and DC offsets at
the output. The value for the feedback resistance, Rf1, should be chosen to be a relatively large value (10
kΩ–100 kΩ), and the other feedback resistance, Ri, is calculated using standard op amp configuration gain
equations. Most audio amplifiers are designed from the non-inverting amplifier configuration.
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DESIGN A 25W/8Ω AUDIO AMPLIFIER
Given:
Power Output
Load Impedance
Input Level
25W
8Ω
1V(max)
Input Impedance
Bandwidth
100 kΩ
20 Hz–20 kHz ± 0.25 dB
Equation 5 and Equation 6 give:
25W/8Ω
Vopeak = 20.0V
Iopeak = 2.5A
Therefore the supply required is: ±24.0V @ 2.5A
With 15% regulation and high line the final supply voltage is ±30.36V using Equation 7. At this point it is a good
idea to check the Power Output vs Supply Voltage to ensure that the required output power is obtainable from
the device while maintaining low THD + N. It is also good to check the Power Dissipation vs Supply Voltage to
ensure that the device can handle the internal power dissipation. At the same time designing in a relatively
practical sized heat sink with a low thermal resistance is also important. Refer to TYPICAL PERFORMANCE
CHARACTERISTICS graphs and the THERMAL CONSIDERATIONS section for more information.
The minimum gain from Equation 8 is: AV ≥ 14
We select a gain of 15 (Non-Inverting Amplifier); resulting in a sensitivity of 942.8 mV.
Letting RIN equal 100 kΩ gives the required input impedance, however, this would eliminate the “volume control”
unless an additional input impedance was placed in series with the 10 kΩ potentiometer that is depicted in
Figure 1. Adding the additional 100 kΩ resistor would ensure the minimum required input impedance.
For low DC offsets at the output we let Rf1 = 100 kΩ. Solving for Ri (Non-Inverting Amplifier) gives the following:
Ri = Rf1/(AV − 1) = 100k/(15 − 1) = 7.1 kΩ; use 6.8 kΩ
The bandwidth requirement must be stated as a pole, i.e., the 3 dB frequency. Five times away from a pole gives
0.17 dB down, which is better than the required 0.25 dB. Therefore:
fL = 20 Hz/5 = 4 Hz
fH = 20 kHz × 5 = 100 kHz
At this point, it is a good idea to ensure that the Gain-Bandwidth Product for the part will provide the designed
gain out to the upper 3 dB point of 100 kHz. This is why the minimum GBWP of the LM2876 is important.
GBWP ≥ AV × f3 dB = 15 × 100 kHz = 1.5 MHz
GBWP = 2.0 MHz (min) for the LM2876
Solving for the low frequency roll-off capacitor, Ci, we have:
Ci ≥ 1/(2π Ri fL) = 5.9 μF; use 10 μF.
Definition of Terms
Input Offset Voltage: The absolute value of the voltage which must be applied between the input terminals
through two equal resistances to obtain zero output voltage and current.
Input Bias Current: The absolute value of the average of the two input currents with the output voltage and
current at zero.
Input Offset Current: The absolute value of the difference in the two input currents with the output voltage and
current at zero.
Input Common-Mode Voltage Range (or Input Voltage Range): The range of voltages on the input terminals
for which the amplifier is operational. Note that the specifications are not ensured over the full common-mode
voltage range unless specifically stated.
Common-Mode Rejection: The ratio of the input common-mode voltage range to the peak-to-peak change in
input offset voltage over this range.
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Power Supply Rejection: The ratio of the change in input offset voltage to the change in power supply voltages
producing it.
Quiescent Supply Current: The current required from the power supply to operate the amplifier with no load
and the output voltage and current at zero.
Slew Rate: The internally limited rate of change in output voltage with a large amplitude step function applied to
the input.
Class B Amplifier: The most common type of audio power amplifier that consists of two output devices each of
which conducts for 180° of the input cycle. The LM2876 is a Quasi − AB type amplifier.
Crossover Distortion: Distortion caused in the output stage of a class B amplifier. It can result from inadequate
bias current providing a dead zone where the output does not respond to the input as the input cycle goes
through its zero crossing point. Also for ICs an inadequate frequency response of the output PNP device can
cause a turn-on delay giving crossover distortion on the negative going transition through zero crossing at the
higher audio frequencies.
THD + N: Total Harmonic Distortion plus Noise refers to the measurement technique in which the fundamental
component is removed by a bandreject (notch) filter and all remaining energy is measured including harmonics
and noise.
Signal-to-Noise Ratio: The ratio of a system's output signal level to the system's output noise level obtained in
the absence of a signal. The output reference signal is either specified or measured at a specified distortion
level.
Continuous Average Output Power: The minimum sine wave continuous average power output in watts (or
dBW) that can be delivered into the rated load, over the rated bandwidth, at the rated maximum total harmonic
distortion.
Music Power: A measurement of the peak output power capability of an amplifier with either a signal duration
sufficiently short that the amplifier power supply does not sag during the measurement, or when high quality
external power supplies are used. This measurement (an IHF standard) assumes that with normal music
program material the amplifier power supplies will sag insignificantly.
Peak Power: Most commonly referred to as the power output capability of an amplifier that can be delivered to
the load; specified by the part's maximum voltage swing.
Headroom: The margin between an actual signal operating level (usually the power rating of the amplifier with
particular supply voltages, a rated load value, and a rated THD + N figure) and the level just before clipping
distortion occurs, expressed in decibels.
Large Signal Voltage Gain: The ratio of the output voltage swing to the differential input voltage required to
drive the output from zero to either swing limit. The output swing limit is the supply voltage less a specified quasi-
saturation voltage. A pulse of short enough duration to minimize thermal effects is used as a measurement
signal.
Output-Current Limit: The output current with a fixed output voltage and a large input overdrive. The limiting
current drops with time once SPiKe protection circuitry is activated.
Output Saturation Threshold (Clipping Point): The output swing limit for a specified input drive beyond that
required for zero output. It is measured with respect to the supply to which the output is swinging.
Output Resistance: The ratio of the change in output voltage to the change in output current with the output
around zero.
Power Dissipation Rating: The power that can be dissipated for a specified time interval without activating the
protection circuitry. For time intervals in excess of 100 ms, dissipation capability is determined by heat sinking of
the IC package rather than by the IC itself.
Thermal Resistance: The peak, junction-temperature rise, per unit of internal power dissipation (units in °C/W),
above the case temperature as measured at the center of the package bottom.
The DC thermal resistance applies when one output transistor is operating continuously. The AC thermal
resistance applies with the output transistors conducting alternately at a high enough frequency that the peak
capability of neither transistor is exceeded.
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Power Bandwidth: The power bandwidth of an audio amplifier is the frequency range over which the amplifier
voltage gain does not fall below 0.707 of the flat band voltage gain specified for a given load and output power.
Power bandwidth also can be measured by the frequencies at which a specified level of distortion is obtained
while the amplifier delivers a power output 3 dB below the rated output. For example, an amplifier rated at 60W
with ≤ 0.25% THD + N, would make its power bandwidth measured as the difference between the upper and
lower frequencies at which 0.25% distortion was obtained while the amplifier was delivering 30W.
Gain-Bandwidth Product: The Gain-Bandwidth Product is a way of predicting the high-frequency usefulness of
an op amp. The Gain-Bandwidth Product is sometimes called the unity-gain frequency or unity-gain cross
frequency because the open-loop gain characteristic passes through or crosses unity gain at this frequency.
Simply, we have the following relationship: ACL1 × f1 = ACL2 × f2
Assuming that at unity-gain (ACL1 = 1 or (0 dB)) fu = fi = GBWP, then we have the following: GBWP = ACL2 × f2
This says that once fu (GBWP) is known for an amplifier, then the open-loop gain can be found at any frequency.
This is also an excellent equation to determine the 3 dB point of a closed-loop gain, assuming that you know the
GBWP of the device. Refer to the diagram on the following page.
Biamplification: The technique of splitting the audio frequency spectrum into two sections and using individual
power amplifiers to drive a separate woofer and tweeter. Crossover frequencies for the amplifiers usually vary
between 500 Hz and 1600 Hz. “Biamping” has the advantages of allowing smaller power amps to produce a
given sound pressure level and reducing distortion effects prodused by overdrive in one part of the frequency
spectrum affecting the other part.
C.C.I.R./A.R.M.:
Literally: International Radio Consultative Committee
Average Responding Meter
This refers to a weighted noise measurement for a Dolby B type noise reduction system. A filter characteristic is
used that gives a closer correlation of the measurement with the subjective annoyance of noise to the ear.
Measurements made with this filter cannot necessarily be related to unweighted noise measurements by some
fixed conversion factor since the answers obtained will depend on the spectrum of the noise source.
S.P.L.: Sound Pressure Level—usually measured with a microphone/meter combination calibrated to a pressure
level of 0.0002 μBars (approximately the threshold hearing level).
S.P.L. = 20 Log 10P/0.0002 dB
where
•
P is the R.M.S. sound pressure in microbars. (1 Bar = 1 atmosphere = 14.5 lb/in2 = 194 dB S.P.L.).
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REVISION HISTORY
Changes from Revision B (March 2013) to Revision C
Page
•
Changed layout of National Data Sheet to TI format .......................................................................................................... 24
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PACKAGE OPTION ADDENDUM
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12-Apr-2023
PACKAGING INFORMATION
Orderable Device
Status Package Type Package Pins Package
Eco Plan
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
Samples
Drawing
Qty
(1)
(2)
(3)
(4/5)
(6)
LM2876TF/NOPB
ACTIVE
TO-220
NDA
11
20
RoHS-Exempt
& Non-Green
SN
Level-1-NA-UNLIM
-20 to 85
LM2876TF
Samples
(1) The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2) RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of <=1000ppm threshold. Antimony trioxide based
flame retardants must also meet the <=1000ppm threshold requirement.
(3) MSL, Peak Temp. - The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature.
(4) There may be additional marking, which relates to the logo, the lot trace code information, or the environmental category on the device.
(5) Multiple Device Markings will be inside parentheses. Only one Device Marking contained in parentheses and separated by a "~" will appear on a device. If a line is indented then it is a continuation
of the previous line and the two combined represent the entire Device Marking for that device.
(6)
Lead finish/Ball material - Orderable Devices may have multiple material finish options. Finish options are separated by a vertical ruled line. Lead finish/Ball material values may wrap to two
lines if the finish value exceeds the maximum column width.
Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information
provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and
continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals.
TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited information may not be available for release.
In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI to Customer on an annual basis.
Addendum-Page 1
PACKAGE MATERIALS INFORMATION
www.ti.com
5-Jan-2022
TUBE
*All dimensions are nominal
Device
Package Name Package Type
NDA TO-220
Pins
SPQ
L (mm)
W (mm)
T (µm)
B (mm)
LM2876TF/NOPB
11
20
502
37
12700
10.29
Pack Materials-Page 1
MECHANICAL DATA
NDA0011B
TF11B (Rev D)
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