LM1875
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LM1875 20W Audio Power Amplifier
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1FEATURES DESCRIPTION
The LM1875 is a monolithic power amplifier offering
2• Up to 30 Watts Output Power very low distortion and high quality performance for
• AVO Typically 90 dB consumer audio applications.
• Low Distortion: 0.015%, 1 kHz, 20 W The LM1875 delivers 20 watts into a 4Ωor 8Ωload
• Wide Power Bandwidth: 70 kHz on ±25V supplies. Using an 8Ωload and ±30V
• Protection for AC and DC Short Circuits to supplies, over 30 watts of power may be delivered.
The amplifier is designed to operate with a minimum
Ground of external components. Device overload protection
• Thermal Protection with Parole Circuit consists of both internal current limit and thermal
• High Current Capability: 4A shutdown.
• Wide Supply Range 16V-60V The LM1875 design takes advantage of advanced
• Internal Output Protection Diodes circuit techniques and processing to achieve
extremely low distortion levels even at high output
• 94 dB Ripple Rejection power levels. Other outstanding features include high
• Plastic Power Package TO-220 gain, fast slew rate and a wide power bandwidth,
large output voltage swing, high current capability,
APPLICATIONS and a very wide supply range. The amplifier is
• High Performance Audio Systems internally compensated and stable for gains of 10 or
greater.
• Bridge Amplifiers
• Stereo Phonographs
• Servo Amplifiers
• Instrument Systems
Connection Diagram
Figure 1. NDH0005D, KC0005A,
NEB0005E, NEB0005B,
Front View
1Please 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.
2All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date. Copyright © 2004, Texas Instruments Incorporated
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
LM1875
SNAS524A –MAY 2004–REVISED MAY 2004
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Typical Applications
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.
Absolute Maximum Ratings(1)
Supply Voltage 60V
Input Voltage −VEE to VCC
Storage Temperature −65°C to + 150°C
Junction Temperature 150°C
(Soldering, 10 seconds) 260°C
Lead Temperature θJC 3°C
θJA 73°C
(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.
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Electrical Characteristics
VCC=+25V, −VEE=−25V, TAMBIENT=25°C, RL=8Ω, AV=20 (26 dB), fo=1 kHz, unless otherwise specified.
Parameter Conditions Typical Tested Limits Units
Supply Current POUT=0W 70 100 mA
Output Power(1) THD=1% 25 W
THD(1) POUT=20W, fo=1 kHz 0.015 %
POUT=20W, fo=20 kHz 0.05 0.4 %
POUT=20W, RL=4Ω, fo=1 kHz 0.022 %
POUT=20W, RL=4Ω, fo=20 kHz 0.07 0.6 %
Offset Voltage ±1 ±15 mV
Input Bias Current ±0.2 ±2 μA
Input Offset Current 0 ±0.5 μA
Gain-Bandwidth Product fo=20 kHz 5.5 MHz
Open Loop Gain DC 90 dB
PSRR VCC, 1 kHz, 1 Vrms 95 52 dB
VEE, 1 kHz, 1 Vrms 83 52 dB
Max Slew Rate 20W, 8Ω, 70 kHz BW 8 V/μs
Current Limit VOUT = VSUPPLY −10V 4 3 A
Equivalent Input Noise Voltage RS=600Ω, CCIR 3 μVrms
(1) Assumes the use of a heat sink having a thermal resistance of 1°C/W and no insulator with an ambient temperature of 25°C. Because
the output limiting circuitry has a negative temperature coefficient, the maximum output power delivered to a 4Ωload may be slightly
reduced when the tab temperature exceeds 55°C.
Typical Applications For Single Supply Operation
Figure 2.
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Typical Performance Characteristics
THD vs Power Output THD vs Frequency
Figure 3. Figure 4.
Power Output vs Supply Supply Current vs Supply
Voltage Voltage
Figure 5. Figure 6.
PSRR vs Frequency Device Dissipation vs Ambient Temperature†
†φINTERFACE = 1°C/W.
See Application Hints.
Figure 7. Figure 8.
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Typical Performance Characteristics (continued)
Power Dissipation vs Power Output Power Dissipation vs Power Output
Figure 9. Figure 10.
IOUT vs VOUT-Current Limit/
Safe Operating Area Boundary Open Loop Gain and Phase vs Frequency
Thermal shutdown with infinite heat sink
Thermal shutdown with 1°C/W heat sink
Figure 11. Figure 12.
Input Bias Current vs Supply Voltage
Figure 13.
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APPLICATION HINTS
STABILITY
The LM1875 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 LM1875 can be made to oscillate under certain conditions. These usually involve
printed circuit board layout or output/input coupling.
Proper layout of the printed circuit board is very important. While the LM1875 will be stable when installed in a
board similar to the ones shown in this data sheet, it is sometimes necessary to modify the layout somewhat to
suit the physical requirements of a particular application. When designing a different 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 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 LM1875 to reduce the effects of PCB trace
resistance and inductance. For the same reason, the ground return paths for these components should be as
short as possible.
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 (on the order of 50 pF to
500 pF) across the circuit input.
Most power amplifiers do not drive highly capacitive loads well, and the LM1875 is no exception. If the output of
the LM1875 is connected directly to a capacitor with no series resistance, the square wave response will exhibit
ringing if the capacitance is greater than about 0.1 μF. The amplifier can typically drive load capacitances up to 2
μF or so without oscillating, but this is not recommended. If highly capacitive loads are expected, a resistor (at
least 1Ω) should be placed in series with the output of the LM1875. A method commonly employed to protect
amplifiers from low impedances at high frequencies is to couple to the load through a 10Ωresistor in parallel with
a 5 μH inductor.
DISTORTION
The preceding suggestions regarding circuit board grounding techniques will also help to prevent excessive
distortion levels in audio applications. For low THD, it is also necessary to keep the power supply traces and
wires separated from the traces and wires connected to the inputs of the LM1875. This prevents the power
supply currents, which are large and nonlinear, from inductively coupling to the LM1875 inputs. Power supply
wires should be twisted together and separated from the circuit board. Where these wires are soldered to the
board, they should be perpendicular to the plane of the board at least to a distance of a couple of inches. With a
proper physical layout, THD levels at 20 kHz with 10W output to an 8Ωload should be less than 0.05%, and less
than 0.02% at 1 kHz.
CURRENT LIMIT AND SAFE OPERATING AREA (SOA) PROTECTION
A power amplifier's output transistors can be damaged by excessive applied voltage, current flow, or power
dissipation. The voltage applied to the amplifier is limited by the design of the external power supply, while the
maximum current passed by the output devices is usually limited by internal circuitry to some fixed value. Short-
term power dissipation is usually not limited in monolithic audio power amplifiers, and this can be a problem
when driving reactive loads, which may draw large currents while high voltages appear on the output transistors.
The LM1875 not only limits current to around 4A, but also reduces the value of the limit current when an output
transistor has a high voltage across it.
When driving nonlinear reactive loads such as motors or loudspeakers with built-in protection relays, there is a
possibility that an amplifier output will be connected to a load whose terminal voltage may attempt to swing
beyond the power supply voltages applied to the amplifier. This can cause degradation of the output transistors
or catastrophic failure of the whole circuit. The standard protection for this type of failure mechanism is a pair of
diodes connected between the output of the amplifier and the supply rails. These are part of the internal circuitry
of the LM1875, and needn't be added externally when standard reactive loads are driven.
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THERMAL PROTECTION
The LM1875 has a sophisticated thermal protection scheme to prevent long-term thermal stress to the device.
When the temperature on the die reaches 170°C, the LM1875 shuts down. It starts operating again when the die
temperature drops to about 145°C, but if the temperature again begins to rise, shutdown will occur at only 150°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 limit the maximum die temperature to a lower value. This greatly reduces the stresses
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 for thermal
resistance low enough 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.
POWER DISSIPATION AND HEAT SINKING
The LM1875 must always be operated with a heat sink, even when it is not required to drive a load. The
maximum idling current of the device is 100 mA, so that on a 60V power supply an unloaded LM1875 must
dissipate 6W of power. The 54°C/W junction-to-ambient thermal resistance of a TO-220 package would cause
the die temperature to rise 324°C above ambient, so the thermal protection circuitry will shut the amplifier down if
operation without a heat sink is attempted.
In order to determine the appropriate heat sink for a given application, the power dissipation of the LM1875 in
that application must be known. When the load is resistive, the maximum average power that the IC will be
required to dissipate is approximately:
where
• VSis the total power supply voltage across the LM1875
• RLis the load resistance
• PQis the quiescent power dissipation of the amplifier
The above equation is only an approximation which assumes an “ideal” class B output stage and constant power
dissipation in all other parts of the circuit. The curves of “Power Dissipation vs Power Output” give a better
representation of the behavior of the LM1875 with various power supply voltages and resistive loads. As an
example, if the LM1875 is operated on a 50V power supply with a resistive load of 8Ω, it can develop up to 19W
of internal power dissipation. If the die temperature is to remain below 150°C for ambient temperatures up to
70°C, the total junction-to-ambient thermal resistance must be less than
Using θJC=2°C/W, the sum of the case-to-heat-sink interface thermal resistance and the heat-sink-to-ambient
thermal resistance must be less than 2.2°C/W. The case-to-heat-sink thermal resistance of the TO-220 package
varies with the mounting method used. A metal-to-metal interface will be about 1°C/W if lubricated, and about
1.2°C/W if dry.
If a mica insulator is used, the thermal resistance will be about 1.6°C/W lubricated and 3.4°C/W dry. For this
example, we assume a lubricated mica insulator between the LM1875 and the heat sink. The heat sink thermal
resistance must then be less than
4.2°C/W−2°C/W−1.6°C/W=0.6°C/W.
This is a rather large heat sink and may not be practical in some applications. If a smaller heat sink is required
for reasons of size or cost, there are two alternatives. [EM00001]The maximum ambient operating temperature
can be reduced to 50°C (122°F), resulting in a 1.6°C/W heat sink, or the heat sink can be isolated from the
chassis so the mica washer is not needed. This will change the required heat sink to a 1.2°C/W unit if the case-
to-heat-sink interface is lubricated.
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NOTE
When using a single supply, maximum transfer of heat away from the LM1875 can be
achieved by mounting the device directly to the heat sink (tab is at ground potential); this
avoids the use of a mica or other type insulator.
The thermal requirements can become more difficult when an amplifier is driving a reactive load. For a given
magnitude of load impedance, a higher degree of reactance will cause a higher level of power dissipation within
the amplifier. As a general rule, the power dissipation of an amplifier driving a 60° reactive load (usually
considered to be a worst-case loudspeaker load) will be roughly that of the same amplifier driving the resistive
part of that load. For example, a loudspeaker may at some frequency have an impedance with a magnitude of
8Ωand a phase angle of 60°. The real part of this load will then be 4Ω, and the amplifier power dissipation will
roughly follow the curve of power dissipation with a 4Ωload.
Component Layouts
Figure 14. Split Supply
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PACKAGE OPTION ADDENDUM
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Addendum-Page 1
PACKAGING INFORMATION
Orderable Device Status
(1)
Package Type Package
Drawing Pins Package
Qty Eco Plan
(2)
Lead finish/
Ball material
(6)
MSL Peak Temp
(3)
Op Temp (°C) Device Marking
(4/5)
Samples
LM1875T ACTIVE TO-220 NDH 5 45 Non-RoHS &
Non-Green Call TI Call TI 0 to 70 LM1875T
LM1875T/LB02 ACTIVE TO-220 NEB 5 45 Non-RoHS &
Non-Green Call TI Call TI 0 to 70 LM1875T
LM1875T/LB03 ACTIVE TO-220 NDH 5 45 Non-RoHS &
Non-Green Call TI Call TI 0 to 70 LM1875T
LM1875T/LB05 ACTIVE TO-220 NEB 5 45 Non-RoHS &
Non-Green Call TI Call TI 0 to 70 LM1875T
LM1875T/LF02 ACTIVE TO-220 NEB 5 45 RoHS & Green SN Level-1-NA-UNLIM 0 to 70 LM1875T
LM1875T/LF03 ACTIVE TO-220 NDH 5 45 RoHS & Green SN Level-1-NA-UNLIM 0 to 70 LM1875T
LM1875T/LF05 ACTIVE TO-220 NEB 5 45 RoHS & Green SN Level-1-NA-UNLIM 0 to 70 LM1875T
LM1875T/NOPB ACTIVE TO-220 NDH 5 45 RoHS & Green SN Level-1-NA-UNLIM 0 to 70 LM1875T
(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.
PACKAGE OPTION ADDENDUM
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Addendum-Page 2
(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.
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MECHANICAL DATA
NEB0005B
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MECHANICAL DATA
NEB0005E
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MECHANICAL DATA
NDH0005D
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