LM1876T_技术文档

February 1998

™

LM1876 Overture Audio Power Amplifier Series

Dual 20W Audio Power Amplifier with Mute and Standby

Modes

General Description

Key Specifications

The LM1876 is a stereo audio amplifier capable of delivering

typically 20W per channel of continuous average output

power into a 4Ω or 8Ω load with less than 0.1% (THD + N).

j

THD+N at 1 kHz at 2 x 15W continuous average

output power into 4Ω or 8Ω:

THD+N at 1 kHz at continuous average

output power of 2 x 20W into 8Ω:

Standby current:

0.1% (max)

Each amplifier has an independent smooth transition fade-in/

out mute and a power conserving standby mode which can

be controlled by external logic.

0.009% (typ)

4.2 mA (typ)

The performance of the LM1876, utilizing its Self Peak In-

™

stantaneous Temperature (˚Ke) (SPiKe ) Protection Cir-

Features

cuitry, places it in a class above discrete and hybrid amplifi-

ers by providing an inherently, dynamically protected Safe

Operating Area (SOA). SPiKe Protection means that these

parts are safeguarded at the output against overvoltage, un-

dervoltage, overloads, including thermal runaway and in-

stantaneous temperature peaks.

n SPiKe Protection

n Minimal amount of external components necessary

n Quiet fade-in/out mute mode

n Standby-mode

n Isolated 15-lead TO-220 package

n Non-Isolated 15-lead TO-220 package

Applications

n High-end stereo TVs

n Component stereo

n Compact stereo

Typical Application

Connection Diagram

Plastic Package

DS012072-2

Top View

Isolated Package

Order Number LM1876TF

See NS Package Number TF15B

Non-Isolated Package

Order Number LM1876T

See NS Package Number TA15A

DS012072-1

FIGURE 1. Typical Audio Amplifier Application Circuit

Note: Numbers in parentheses represent pinout for amplifier B.

*

Optional component dependent upon specific design requirements.

™

SPiKe Protection and Overture are trademarks of National Semiconductor Corporation.

DS012072

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Absolute Maximum Ratings (Notes 4, 5)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

Thermal Resistance

Isolated TF-Package

θ_JC

2˚C/W

1˚C/W

Non-Isolated T-Package

θ_JC

Supply Voltage |V_CC| + |V_EE

(No Input)

|

Soldering Information

TF Package (10 sec.)

Storage Temperature

64V

260˚C

Supply Voltage |V_CC| + |V_EE

(with Input)

|

−40˚C to +150˚C

64V

(V_CCor V_EE) and

|V_CC| + |V_EE| ≤ 54V

54V

Common Mode Input Voltage

Operating Ratings (Notes 4, 5)

Differential Input Voltage

Output Current

Temperature Range

Internally Limited

62.5W

T_MIN≤ T_A≤ T_MAX

−20˚C ≤ T_A≤ +85˚C

Power Dissipation (Note 6)

ESD Susceptability (Note 7)

Junction Temperature (Note 8)

Supply Voltage |V_CC| + |V_EE| (Note 1)

20V to 64V

2000V

150˚C

Electrical Characteristics (Notes 4, 5)

=

The following specifications apply for V_CC+22V, V_EE−22V with R_L8Ω unless otherwise specified. Limits apply for T_A

25˚C.

Symbol

Parameter

Conditions

LM1876

Units

(Limits)

Typical

Limit

(Note 10)

20

(Note 9)

|V_CC| +

Power Supply Voltage

(Note 11)

GND − V_EE≥ 9V

V (min)

V (max)

|V_EE

|

64

=

THD + N 0.1% (max),

P_O

Output Power

=

1 kHz

(Note 3)

THD + N

X_talk

(Continuous Average)

f

=

|V_CC

|

|V_EE

|

22V, R_L8Ω

20

22

15

W/ch (min)

=

|

20V, R_L4Ω (Note 13)

W/ch (min)

=

Total Harmonic Distortion

Plus Noise

15 W/ch, R_L8Ω

0.08

0.1

%

=

| 20V

15 W/ch, R_L4Ω, |V_CC

|

|V_EE

=

20 Hz ≤ f ≤ 20 kHz, A_V26 dB

=

Channel Separation

Slew Rate

f

1 kHz, V_O10.9 Vrms

80

18

dB

=

SR

V_IN1.414 Vrms, t_rise2 ns

12

V/µs (min)

(Note 3)

=

Both Amplifiers V_CM0V,

I_total

Total Quiescent Power

Supply Current

=

(Note 2)

V_O0V, I_O0 mA

Standby: Off

50

4.2

2.0

80

6

mA (max)

mV (max)

Standby: On

=

V_OS

Input Offset Voltage

V_CM0V, I_O0 mA

15

(Note 2)

=

I_B

Input Bias Current

Input Offset Current

Output Current Limit

V_CM0V, I_O0 mA

0.2

0.002

3.5

0.5

0.2

2.9

µA (max)

Apk (min)

= =

V_CM0V, I_O0 mA

I_OS

I_O

=

= =

10V, t_ON10 ms,

|V_CC

|

|V_EE

V_O0V

|V_CC–V_O|, V_CC20V, I_O+100 mA

|

=

V_OD

Output Dropout Voltage

(Note 12)

1.8

2.5

115

2.3

3.2

85

V (max)

dB (min)

= =

|V_O–V_EE|, V_EE−20V, I_O−100 mA

(Note 2)

PSRR

(Note 2)

= =

V_CC25V to 10V, V_EE−25V,

Power Supply Rejection Ratio

=

V_CM0V, I_O0 mA

=

V_CC25V, V_EE−25V to −10V

110

85

80

dB (min)

=

V_CM0V, I_O0 mA

= =

Common Mode Rejection Ratio V_CC35V to 10V, V_EE−10V to −35V,

CMRR

= =

V_CM10V to −10V, I_O0 mA

(Note 2)

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2

Electrical Characteristics (Notes 4, 5) (Continued)

=

The following specifications apply for V_CC+22V, V_EE−22V with R_L8Ω unless otherwise specified. Limits apply for T_A

25˚C.

Symbol

Parameter

Conditions

LM1876

Units

(Limits)

Typical

Limit

(Note 10)

90

(Note 9)

110

=

A_VOL

Open Loop Voltage Gain

R_L2 kΩ, ∆ V_O20 V

dB (min)

(Note 2)

=

GBWP

e_IN

Gain Bandwidth Product

Input Noise

f_O100 kHz, V_IN50 mVrms

7.5

2.0

5

8

MHz (min)

µV (max)

IHF — A Weighting Filter

=

R_IN600Ω (Input Referred)

(Note 3)

SNR

=

Signal-to-Noise Ratio

P_O1W, A — Weighted,

98

dB

=

Measured at 1 kHz, R_S25Ω

=

P_O15W, A — Weighted

108

115

=

Measured at 1 kHz, R_S25Ω

A_M

Mute Attenuation

Pin 6,11 at 2.5V

80

dB (min)

Standby

V_IL

V_IH

Standby Low Input Voltage

Standby High Input Voltage

Not in Standby Mode

In Standby Mode

0.8

2.5

V (max)

V (min)

2.0

Mute pin

V_IL

Mute Low Input Voltage

Mute High Input Voltage

Outputs Not Muted

Outputs Muted

0.8

2.5

V (max)

V (min)

V_IH

Note 1: Operation is guaranteed up to 64V, however, distortion may be introduced from SPiKe Protection Circuitry if proper thermal considerations are not taken into

account. Refer to the Application Information section for a complete explanation.

Note 2: DC Electrical Test; Refer to Test Circuit #1.

Note 3: AC Electrical Test; Refer to Test Circuit #2.

Note 4: All voltages are measured with respect to the GND pins (5, 10), unless otherwise specified.

Note 5: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is func-

tional, but do not guarantee specific performance limits. Electrical Characteristics state DC and AC electrical specifications under particular test conditions which guar-

antee 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 6: 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

=

1˚C/W for the T package. Refer to the section Determining the Correct Heat Sink in the Application In-

θ

2˚C/W (junction to case) for the TF package and θ

JC

formation section.

JC

Note 7: Human body model, 100 pF discharged through a 1.5 kΩ resistor.

Note 8: The operating junction temperature maximum is 150˚C, however, the instantaneous Safe Operating Area temperature is 250˚C.

Note 9: Typicals are measured at 25˚C and represent the parametric norm.

Note 10: Limits are guarantees that all parts are tested in production to meet the stated values.

Note 11:

V

must have at least −9V at its pin with reference to ground in order for the under-voltage protection circuitry to be disabled. In addition, the voltage dif-

EE

ferential between V

and V must be greater than 14V.

EE

CC

Note 12: The output dropout voltage, V , is the supply voltage minus the clipping voltage. Refer to the Clipping Voltage vs. Supply Voltage graph in the Typical Per-

OD

formance Characteristics section.

±

Note 13: For a 4Ω load, and with 20V supplies, the LM1876 can deliver typically 22W of continuous average output power with less than 0.1% (THD + N). With

±

supplies above 20V, the LM1876 cannot deliver more than 22W into a 4Ω due to current limiting of the output transistors. Thus, increasing the power supply above

±

20V will only increase the internal power dissipation, not the possible output power. Increased power dissipation will require a larger heat sink as explained in the

Application Information section.

3

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#

Test Circuit 1 (Note 2) (DC Electrical Test Circuit)

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#

Test Circuit 2 (Note 3) (AC Electrical Test Circuit)

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Bridged Amplifier Application Circuit

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FIGURE 2. Bridged Amplifier Application Circuit

Single Supply Application Circuit

DS012072-6

FIGURE 3. Single Supply Amplifier Application Circuit

*

Note: Optional components dependent upon specific design requirements.

5

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Auxiliary Amplifier Application Circuit

DS012072-7

FIGURE 4. Special Audio Amplifier Application Circuit

Equivalent Schematic ^{(excluding active protection circuitry)}

LM1876 (per Amp)

DS012072-8

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External Components Description

Components

Functional Description

1

R_B

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

undervoltage circuitry is off. This phenomenon occurs when the supply voltages are below 1.5V.

2

3

4

R_i

R_f

Inverting input resistance to provide AC gain in conjunction with R_f.

Feedback resistance to provide AC gain in conjunction with R_i.

=

C_i

Feedback capacitor which ensures unity gain at DC. Also creates a highpass filter with R_iat f_C

(Note 14)

1/(2πR_iC_i).

5

6

C_S

Provides power supply filtering and bypassing. Refer to the Supply Bypassing application section for

proper placement and selection of bypass capacitors.

R_V

Acts as a volume control by setting the input voltage level.

(Note 14)

7

R_IN

Sets the amplifier’s input terminals DC bias point when C_INis present in the circuit. Also works with C_INto

=

(Note 14)

create a highpass filter at f_C1/(2πR_INC_IN). Refer to Figure 4.

8

C_IN

(Note 14)

Input capacitor which blocks the input signal’s DC offsets from being passed onto the amplifier’s inputs.

Works with C_SNto stabilize the output stage by creating a pole that reduces high frequency instabilities.

Works with R_SNto stabilize the output stage by creating a pole that reduces high frequency instabilities.

9

R_SN

(Note 14)

10

C_SN

=

(Note 14)

The pole is set at f_C1/(2πR_SNC_SN). Refer to Figure 4.

11

12

L (Note 14)

Provides high impedance at high frequencies so that R may decouple a highly capacitive load and reduce

the Q of the series resonant circuit. Also provides a low impedance at low frequencies to short out R and

pass audio signals to the load. Refer to Figure 4.

R (Note 14)

13

14

15

R_A

C_A

Provides DC voltage biasing for the transistor Q1 in single supply operation.

Provides bias filtering for single supply operation.

R_INP

Limits the voltage difference between the amplifier’s inputs for single supply operation. Refer to the Clicks

(Note 14)

and Pops application section for a more detailed explanation of the function of R_INP

Provides input bias current for single supply operation. Refer to the Clicks and Pops application section

for a more detailed explanation of the function of R_BI

.

16

17

R_BI

.

R_E

Establishes a fixed DC current for the transistor Q1 in single supply operation. This resistor stabilizes the

half-supply point along with C_A.

Note 14: Optional components dependent upon specific design requirements.

Typical Performance Characteristics

THD + N vs Frequency

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Typical Performance Characteristics (Continued)

THD + N vs

Output Power

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THD + N vs

Output Power

DS012072-19

DS012072-20

DS012072-21

Clipping Voltage vs

Supply Voltage

Clipping Voltage vs

Supply Voltage

Clipping Voltage vs

Supply Voltage

DS012072-22

DS012072-23

DS012072-24

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Typical Performance Characteristics (Continued)

Output Power vs

Load Resistance

Power Dissipation vs

Output Power

Power Dissipation vs

Output Power

DS012072-26

DS012072-25

DS012072-27

Output Power vs

Supply Voltage

Output Mute vs

Mute Pin Voltage

Output Mute vs

Mute Pin Voltage

DS012072-28

DS012072-29

DS012072-30

Channel Separation vs

Frequency

Large Signal Response

Pulse Response

DS012072-32

DS012072-33

DS012072-31

9

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Typical Performance Characteristics (Continued)

Power Supply

Rejection Ratio

Common-Mode

Rejection Ratio

Open Loop

Frequency Response

DS012072-34

DS012072-35

DS012072-36

DS012072-39

DS012072-42

Safe Area

SPiKe Protection

Response

Supply Current vs

Supply Voltage

DS012072-37

DS012072-38

Pulse Thermal

Resistance

Pulse Thermal

Resistance

Supply Current vs

Output Voltage

DS012072-40

DS012072-41

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Typical Performance Characteristics (Continued)

Pulse Power Limit

Supply Current vs

Case Temperature

DS012072-43

DS012072-44

DS012072-45

Supply Current (I_CC) vs

Standby Pin Voltage

Supply Current (I_EE) vs

Standby Pin Voltage

Input Bias Current vs

Case Temperature

DS012072-47

DS012072-46

DS012072-48

11

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Application Information

MUTE MODE

tween 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.

By placing a logic-high voltage on the mute pins, the signal

going into the amplifiers will be muted. If the mute pins are

left floating or connected to a logic-low voltage, the amplifi-

ers will be in a non-muted state. There are two mute pins,

one for each amplifier, so that one channel can be muted

without muting the other if the application requires such a

configuration. Refer to the Typical Performance Character-

istics section for curves concerning Mute Attenuation vs

Mute Pin Voltage.

Since the die temperature is directly dependent upon the

heat sink used, the heat sink should be chosen such that

thermal shutdown will not be reached during normal opera-

tion. 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, as discussed

in the Determining the Correct Heat Sink Section.

STANDBY MODE

DETERMlNlNG MAXIMUM POWER DISSIPATION

The standby mode of the LM1876 allows the user to drasti-

cally reduce power consumption when the amplifiers are

idle. By placing a logic-high voltage on the standby pins, the

amplifiers will go into Standby Mode. In this mode, the cur-

rent drawn from the V_CCsupply is typically less than 10 µA

total for both amplifiers. The current drawn from the V_EEsup-

ply is typically 4.2 mA. Clearly, there is a significant reduction

in idle power consumption when using the standby mode.

There are two Standby pins, so that one channel can be put

in standby mode without putting the other amplifier in

standby if the application requires such flexibility. Refer to

the Typical Performance Characteristics section for

curves showing Supply Current vs. Standby Pin Voltage for

both supplies.

Power dissipation within the integrated circuit package is a

very important parameter requiring a thorough understand-

ing if optimum power output is to be obtained. An incorrect

maximum power dissipation calculation may result in inad-

equate heat sinking causing thermal shutdown and thus lim-

iting the output power.

Equation (1) exemplifies the theoretical maximum power dis-

sipation point of each amplifier where V_CCis the total supply

voltage.

P_DMAXV_CC2/2π²R_L

(1)

=

Thus by knowing the total supply voltage and rated output

load, the maximum power dissipation point can be calcu-

lated. The package dissipation is twice the number which re-

sults from equation (1) since there are two amplifiers in each

LM1876. Refer to the graphs of Power Dissipation versus

Output Power in the Typical Performance Characteristics

section which show the actual full range of power dissipation

not just the maximum theoretical point that results from

equation (1).

UNDER-VOLTAGE PROTECTION

Upon system power-up, the under-voltage protection cir-

cuitry allows the power supplies and their corresponding ca-

pacitors to come up close to their full values before turning

on the LM1876 such that no DC output spikes occur. Upon

turn-off, the output of the LM1876 is brought to ground be-

fore the power supplies such that no transients occur at

power-down.

DETERMINING THE CORRECT HEAT SINK

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.

OVER-VOLTAGE PROTECTION

The LM1876 contains over-voltage protection circuitry that

limits the output current to approximately 3.5 Apk 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 alter-

nately by sinking large current spikes.

The thermal resistance from the die (junction) to the outside

air (ambient) is a combination of three thermal resistances,

θ_JC, θ_CS, and θ_SA. In addition, the thermal resistance, θ_JC

(junction to case), of the LM1876TF is 2˚C/W and the

LM1876T is 1˚C/W. Using Thermalloy Thermacote thermal

compound, the thermal resistance, θ_CS(case to sink), is

about 0.2˚C/W. Since convection heat flow (power dissipa-

tion) 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 LM1876 is equal to the following:

SPiKe PROTECTION

The

LM1876

is

protected

from

instantaneous

peak-temperature stressing of the power transistor array.

The Safe Operating graph in the Typical Performance

Characteristics section shows the area of device operation

where SPiKe Protection Circuitry is not enabled. The wave-

form to the right of the SOA graph exemplifies how the dy-

namic protection will cause waveform distortion when en-

abled.

=

P_DMAX(T_JMAX−T_AMB)/θ_JA

(2)

=

where T_JMAX150˚C, T_AMBis the system ambient tempera-

=

ture and θ_JAθ_JC+ θ_CS+ θ_SA

.

Once the maximum package power dissipation has been

calculated using equation (1), the maximum thermal resis-

tance, θ_SA, (heat sink to ambient) in ˚C/W for a heat sink can

be calculated. This calculation is made using equation (3)

which is derived by solving for θ_SAin equation (2).

THERMAL PROTECTION

The LM1876 has a sophisticated thermal protection scheme

to prevent long-term thermal stress of the device. When the

temperature on the die reaches 165˚C, the LM1876 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 de-

vice 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 be-

=

θ_SA[(T_JMAX−T_AMB)−P_DMAX(θ_JC+θ_CS)]/P_DMAX(3)

Again it must be noted that the value of θ_SAis dependent

upon the system designer’s amplifier requirements. If the

ambient temperature that the audio amplifier is to be working

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12

SINGLE-SUPPLY AMPLIFIER APPLICATION

Application Information (Continued)

The typical application of the LM1876 is a split supply ampli-

fier. But as shown in Figure 3, the LM1876 can also be used

in a single power supply configuration. This involves using

under is higher than 25˚C, then the thermal resistance for the

heat sink, given all other things are equal, will need to be

smaller.

some external components to create

a half-supply bias

which is used as the reference for the inputs and outputs.

Thus, the signal will swing around half-supply much like it

swings around ground in a split-supply application. Along

with proper circuit biasing, a few other considerations must

be accounted for to take advantage of all of the LM1876

functions.

SUPPLY BYPASSING

The LM1876 has excellent power supply rejection and does

not require a regulated supply. However, to improve system

performance as well as eliminate possible oscillations, the

LM1876 should have its supply leads bypassed with

low-inductance capacitors having short leads that are lo-

cated close to the package terminals. Inadequate power

supply bypassing will manifest itself by a low frequency oscil-

lation known as “motorboating” or by high frequency insta-

bilities. 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.

The LM1876 possesses a mute and standby function with in-

ternal logic gates that are half-supply referenced. Thus, to

enable either the Mute or Standby function, the voltage at

these pins must be a minimum of 2.5V above half-supply. In

single-supply systems, devices such as microprocessors

and simple logic circuits used to control the mute and

standby functions, are usually referenced to ground, not

half-supply. Thus, to use these devices to control the logic

circuitry of the LM1876, a “level shifter,” like the one shown in

Figure 5, must be employed. A level shifter is not needed in

a split-supply configuration since ground is also half-supply.

If adequate bypassing is not provided, the current in the sup-

ply leads which is a rectified component of the load current

may be fed back into internal circuitry. This signal causes

distortion at high frequencies requiring that the supplies be

bypassed at the package terminals with an electrolytic ca-

pacitor of 470 µF or more.

BRIDGED AMPLIFIER APPLICATION

The LM1876 has two operational amplifiers internally, allow-

ing for a few different amplifier configurations. One of these

configurations is referred to as “bridged mode” and involves

driving the load differentially through the LM1876’s outputs.

This configuration is shown in Figure 2. Bridged mode op-

eration is different from the classical single-ended amplifier

configuration where one side of its load is connected to

ground.

DS012072-12

FIGURE 5. Level Shift Circuit

When the voltage at the Logic Input node is 0V, the 2N3904

is “off” and thus resistor R_cpulls up mute or standby input to

the supply. This enables the mute or standby function. When

the Logic Input is 5V, the 2N3904 is “on” and consequently,

the voltage at the collector is essentially 0V. This will disable

the mute or standby function, and thus the amplifier will be in

its normal mode of operation. R_shift, along with C_shift, creates

an RC time constant that reduces transients when the mute

or standby functions are enabled or disabled. Additionally,

A bridge amplifier design has a distinct advantage over the

single-ended configuration, as it provides differential drive to

the load, thus doubling output swing for a specified supply

voltage. Consequently, theoretically 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.

A direct consequence of the increased power delivered to

the load by a bridge amplifier is an increase in internal power

dissipation. For each operational amplifier in a bridge con-

figuration, the internal power dissipation will increase by a

factor of two over the single ended dissipation. Thus, for an

audio power amplifier such as the LM1876, which has two

operational amplifiers in one package, the package dissipa-

R

_shiftlimits the current supplied by the internal logic gates of

the LM1876 which insures device reliability. Refer to the

Mute Mode and Standby Mode sections in the Application

Information section for a more detailed description of these

functions.

CLICKS AND POPS

tion will increase by

a factor of four. To calculate the

In the typical application of the LM1876 as a split-supply au-

dio power amplifier, the IC exhibits excellent “click” and “pop”

performance when utilizing the mute and standby modes. In

addition, the device employs Under-Voltage Protection,

which eliminates unwanted power-up and power-down tran-

sients. The basis for these functions are a stable and con-

LM1876’s maximum power dissipation point for a bridged

load, multiply equation (1) by a factor of four.

This value of P_DMAXcan be used to calculate the correct size

heat sink for a bridged amplifier application. Since the inter-

nal dissipation for a given power supply and load is in-

creased by using bridged-mode, the heatsink’s θ_SAwill have

to decrease accordingly as shown by equation (3). Refer to

the section, Determining the Correct Heat Sink, for a more

detailed discussion of proper heat sinking for a given appli-

cation.

stant half-supply potential. In

ground is the stable half-supply potential. But in

a split-supply application,

a

single-supply application, the half-supply needs to charge up

just like the supply rail, V_CC. This makes the task of attaining

a clickless and popless turn-on more challenging. Any un-

even charging of the amplifier inputs will result in output

clicks and pops due to the differential input topology of the

LM1876.

13

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loaded voltage which is usually about 15% higher. The sup-

ply voltage will also rise 10% during high line conditions.

Therefore the maximum supply voltage is obtained from the

following equation.

Application Information (Continued)

To achieve a transient free power-up and power-down, the

voltage seen at the input terminals should be ideally the

same. Such a signal will be common-mode in nature, and

will be rejected by the LM1876. In Figure 3, the resistor R_INP

serves to keep the inputs at the same potential by limiting the

voltage difference possible between the two nodes. This

should significantly reduce any type of turn-on pop, due to an

uneven charging of the amplifier inputs. This charging is

based on a specific application loading and thus, the system

designer may need to adjust these values for optimal perfor-

mance.

±

Max supplies ≈ (V_OPEAK+ V_OD) (1 + regulation) (1.1)

For 15W of output power into an 8Ω load, the required

V_OPEAKis 15.49V. A minimum supply rail of 20.5V results

from adding V_OPEAKand V_OD. With regulation, the maximum

±

supplies are 26V and the required I_OPEAKis 1.94A from

equation (5). It should be noted that for a dual 15W amplifier

into an 8Ω load the I_OPEAKdrawn from the supplies is twice

1.94 Apk or 3.88 Apk. At this point it is a good idea to check

the Power Output vs Supply Voltage to ensure that the re-

quired output power is obtainable from the device while

maintaining low THD+N. In addition, the designer should

verify that with the required power supply voltage and load

impedance, that the required heatsink value θ_SAis feasible

given system cost and size constraints. Once the heatsink

issues have been addressed, the required gain can be deter-

mined from Equation (6).

As shown in Figure 3, the resistors labeled R_BIhelp bias up

the LM1876 off the half-supply node at the emitter of the

2N3904. But due to the input and output coupling capacitors

in the circuit, along with the negative feedback, there are two

different values of R_BI, namely 10 kΩ and 200 kΩ. These re-

sistors bring up the inputs at the same rate resulting in a pop-

less turn-on. Adjusting these resistors values slightly may re-

duce pops resulting from power supplies that ramp

extremely quick or exhibit overshoot during system turn-on.

(6)

AUDIO POWER AMPLlFIER DESIGN

Design a 15W/8Ω Audio Amplifier

Given:

From equation 6, the minimum A_Vis: A_V≥ 11.

=

By selecting a gain of 21, and with a feedback resistor, R_f

20 kΩ, the value of R_ifollows from equation (7).

=

R_iR_f(A_V− 1)

(7)

Power Output

Load Impedance

Input Level

15 Wrms

8Ω

=

Thus with R_i1 kΩ a non-inverting gain of 21 will result.

Since the desired input impedance was 47 kΩ, a value of 47

kΩ was selected for R_IN. The final design step 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 bet-

1 Vrms(max)

47 kΩ

Input Impedance

Bandwidth

20 Hz−20 kHz

±

0.25 dB

±

ter than the required 0.25 dB specified. This fact results in

A designer must first determine the power supply require-

ments in terms of both voltage and current needed to obtain

the specified output power. V_OPEAKcan be determined from

equation (4) and I_OPEAKfrom equation (5).

a low and high frequency pole of 4 Hz and 100 kHz respec-

tively. As stated in the External Components section, R_iin

conjunction with C_icreate a high-pass filter.

=

*

C_i≥ 1/(2π 1 kΩ 4 Hz) 39.8 µF;

use 39 µF.

The high frequency pole is determined by the product of the

desired high frequency pole, f_H, and the gain, A_V. With a

(4)

(5)

=

A_V21 and f_H100 kHz, the resulting GBWP is 2.1 MHz,

which is less than the guaranteed minimum GBWP of the

LM1876 of 5 MHz. This will ensure that the high frequency

response of the amplifier will be no worse than 0.17 dB down

at 20 kHz which is well within the bandwidth requirements of

the design.

To determine the maximum supply voltage the following con-

ditions must be considered. Add the dropout voltage to the

peak output swing V_OPEAK, to get the supply rail at a current

of I_OPEAK. The regulation of the supply determines the un-

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14

Physical Dimensions inches (millimeters) unless otherwise noted

Isolated TO-220 15-Lead Package

Order Number LM1876TF

NS Package Number TF15B

15

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Physical Dimensions inches (millimeters) unless otherwise noted (Continued)

Non-Isolated TO-220 15-Lead Package

Order Number LM1876T

NS Package Number TA15A

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NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT

DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL

COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices or

systems which, (a) are intended for surgical implant

into the body, or (b) support or sustain life, and

whose failure to perform when properly used in

accordance with instructions for use provided in the

labeling, can be reasonably expected to result in a

significant injury to the user.

2. A critical component is any component of a life

support device or system whose failure to perform

can be reasonably expected to cause the failure of

the life support device or system, or to affect its

safety or effectiveness.

National Semiconductor

Corporation

Americas

Tel: 1-800-272-9959

Fax: 1-800-737-7018

Email: support@nsc.com

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Response Group

Tel: 65-2544466

Fax: 65-2504466

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National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.


型号：	LM1876T
厂家：	National Semiconductor
描述：	Overture⑩ Audio Power Amplifier Series Dual 20W Audio Power Amplifier with Mute and Standby Modes Overture⑩音频功率放大器系列双20W音频功率放大器静音和待机模式放大器功率放大器
文件：	总16页 (文件大小：596K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LM1876T [NSC]

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