LM4870M [NSC]

1.1W Audio Power Amplifier with Shutdown Mode; 1.1W音频功率放大器关断模式


型号：	LM4870M
厂家：	National Semiconductor
描述：	1.1W Audio Power Amplifier with Shutdown Mode 1.1W音频功率放大器关断模式商用集成电路放大器功率放大器光电二极管
文件：	总8页 (文件大小：263K)
中文：	中文翻译
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October 1997

LM4870

1.1W Audio Power Amplifier with Shutdown Mode

General Description

Key Specifications

The LM4870 is a bridge-connected audio power amplifier ca-

pable of delivering 1.1W of continuous average power to an

8Ω load with less than 0.5% THD+N over the audio spectrum

from a 5V power supply.

n THD+N at 1W into 8Ω

0.5% (max)

1.5W (typ)

n Output power into 8Ω at 1kHz

at 10%THD+N

n Shutdown Current

0.6µA (typ)

Boomer audio power amplifiers were designed specifically to

provide high quality output power with a minimal number of

external components. Since the LM4870 does not require

output coupling capacitors, bootstrap capacitors or snubber

networks, it is optimally suited for low-power portable sys-

tems.

Features

n No output coupling capacitors, bootstrap capacitors, or

snubber circuits are necessary

n Small Outline (SOIC) power packaging

n Unity-gain stable

n External gain configuration capability

The LM4870 features an externally controlled, low-power

consumption shutdown mode, as well as an internal thermal

shutdown protection mechanism. It also includes two head-

phone control inputs and a headphone sense output for ex-

ternal monitoring. The LM4870 is unity-gain stable and the

gain is set using external resistors.

Applications

n Personal computers

n Desktop computers

n Low voltage audio system

Typical Application

Connection Diagram

Small Outline Package

DS100094-2

Top View

Order Number LM4870M

See NS Package Number M16A

DS100094-1

FIGURE 1. Typical Audio Amplifier Application Circuit

Boomer^®is a registered trademark of National Semiconductor Corporation.

DS100094

www.national.com

Absolute Maximum Ratings (Note 2)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

Infrared (15 sec.)

Thermal Resistance

220˚C

φ_JC(typ)

φ_JA(typ)

35˚C/W

100˚C/W

See AN-450 “Surface Mounting and their Effects on Product

Reliability” for other methods of soldering surface mount de-

vices.

Supply Voltage

6.0V

−65˚C to +150˚C

−0.3V to V_DD+ 0.3V

Internally limited

3000V

Storage Temperature

Input Voltage

Power Dissipation

Operating Ratings

Temperature Range

ESD Susceptibility (Note 4)

ESD Susceptibility (Note 5)

Junction Temperature

Soldering Information

Small Outline Package

Vapor Phase (60 sec.)

250V

T_MIN≤ T_A≤ T_MAX

−20˚C ≤ T_A≤ +85˚C

2.0V ≤ V_DD≤ 5.5V

150˚C

Supply Voltage

215˚C

Electrical Characteristics

(Notes 1, 2) The following specifications apply for V_DD5V, R_L8Ω unless otherwise specified. Limits apply for T_A25˚C.

Symbol

Parameter

Conditions

LM4870

Units

(Limits)

Typical

Limit

(Note 7)

2.0

(Note 6)

V_DD

Supply Voltage

V (min)

V (max)

mA (max)

µA

5.5

I_DD

Quiescent Power Supply Current

Shutdown Current

V_O0V, I_O0A

7.0

0.4

15.0

V_pin2V_DD

I_SD

V_OS

P_O

Output Offset Voltage

V_IN0V

50.0

1.0

mV (max)

W (min)

Output Power

THD+N 0.5% (max); f 1 kHz

1.1

0.25

P_O1 Wrms; 20 Hz ≤ f ≤ 20 kHz

THD+N

PSRR

V_od

Total Harmonic Distortion + Noise

Power Supply Rejection Ratio

Output Dropout Voltage

V_DD4.9V to 5.1V

V_IN0V to 5V

0.6

2.5

2.8

0.2

1.0

V (max)

HP-SENSE 0V to 4V

V_IH

HP-IN High Input Voltage

HP-IN Low Input Voltage

HP-SENSE High Output Voltage

HP-SENSE Low Output Voltage

V_IL

HP-SENSE 4V to 0V

V_OH

V_OL

I_O500 µA

2.5

0.8

V (min)

V (max)

I_O−500 µA

Note 1: All voltages are measured with respect to the ground pins, unless otherwise specified.

Note 2: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is 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 3: The maximum power dissipation must be derated at elevated temperatures and is dictated by T

, θ , and the ambient temperature T . The maximum

JMAX JA

allowable power dissipation is P

JMAX

− T )/θ or the number given in the Absolute Maximum Ratings, whichever is lower. For the LM4870, T

JA JMAX

DMAX

+150˚C, and the typical junction-to-ambient thermal resistance, when board mounted, is 100˚C/W.

Note 4: Human body model, 100 pF discharged through a 1.5 kΩ resistor.

Note 5: Machine Model, 200 pF–240 pF discharged through all pins.

Note 6: Typicals are measured at 25˚C and represent the parametric norm.

Note 7: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).

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External Components Description (Figure 1)

Components

Functional Description

Inverting input resistance which sets the closed-loop gain in conjunction with R_f. This resistor also

R_i

forms a high pass filter with C_iat f_C1/(2π R_iC_i).

C_i

Input coupling capacitor which blocks DC voltage at the amplifier’s input terminals. Also creates a

highpass filter with R_iat f_C1/(2π R_iC_i).

R_f

Feedback resistance which sets closed-loop gain in conjunction with R_i.

C_S

Supply bypass capacitor which provides power supply filtering. Refer to the Application Information

section for proper placement and selection of supply bypass capacitor.

C_B

Bypass pin capacitor which provides half supply filtering. Refer to Application Information section for

proper placement and selection of bypass capacitor.

Typical Performance Characteristics

THD+N vs Frequency

DS100094-9

DS100094-10

DS100094-11

THD+N vs Output Power

DS100094-12

DS100094-13

DS100094-14

Supply Current vs

Supply Voltage

LM4870 Noise Floor

vs Frequency

Power Derating Curve

DS100094-18

DS100094-17

DS100094-16

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Typical Performance Characteristics (Continued)

Supply Current Distribution

vs Temperature

Power Dissipation

vs Output Power

Output Power vs

Load Resistance

DS100094-19

DS100094-20

DS100094-21

Output Power vs

Supply Voltage

Output Power vs

Supply Voltage

Open Loop

Frequency Response

DS100094-22

DS100094-27

DS100094-23

Power Supply

Rejection Ratio

DS100094-24

Application Information

BRIDGE CONFIGURATION EXPLANATION

By driving the load differentially through outputs V_O1and

_O2, an amplifier configuration commonly referred to as

“bridged mode” is established. Bridged mode operation is

different from the classical single-ended amplifier configura-

tion where one side of its load is connected to ground.

As shown in Figure 1, the LM4870 has two operational am-

plifiers internally, allowing for a few different amplifier con-

figurations. The first amplifier’s gain is externally config-

urable, while the second amplifier is internally fixed in a

unity-gain, inverting configuration. The closed-loop gain of

the first amplifier is set by selecting the ratio of R_fto R _iwhile

the second amplifier’s gain is fixed by the two internal 40 kΩ

resistors. Figure 1 shows that the output of amplifier one

serves as the input to amplifier two which results in both am-

plifiers producing signals identical in magnitude, but out of

phase 180˚. Consequently, the differential gain for the IC is:

A bridge amplifier design has a few distinct advantages over

the single-ended configuration, as it provides differential

drive to the load, thus doubling output swing for a specified

supply voltage. Consequently, four times the output power is

possible as compared to a single-ended amplifier under the

same conditions. This increase in attainable output power

assumes that the amplifier is not current limited or clipped. In

order to choose an amplifier’s closed-loop gain without caus-

(R_f/R_i)

A_vd

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bypass capacitors, especially C_B, is thus dependant upon

desired low frequency THD+N, system cost, and size con-

straints.

Application Information (Continued)

ing excessive clipping which will damage high frequency

transducers used in loudspeaker systems, please refer to

the Audio Power Amplifier Deslgn section.

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the

LM4870 contains a shutdown pin to externally turn off the

amplifier’s bias circuitry. The shutdown feature turns the am-

plifier off when a logic high is placed on the shutdown pin.

Upon going into shutdown, the output is immediately discon-

nected from the speaker. There is a built-in threshold which

produces a drop in quiescent current to 500 µA typically. For

a 5V power supply, this threshold occurs when 2V–3V is ap-

plied to the shutdown pin. A typical quiescent current of

0.6 µA results when the supply voltage is applied to the shut-

down pin. In many applications, a microcontroller or micro-

processor output is used to control the shutdown circuitry

which provides a quick, smooth transition into shutdown. An-

other solution is to use a single-pole, single-throw switch that

when closed, is connected to ground and enables the ampli-

fier. If the switch is open, then a soft pull-up resistor of 47 kΩ

will disable the LM4870. There are no soft pull-down resis-

tors inside the LM4870, so a definite shutdown pin voltage

must be applied externally, or the internal logic gate will be

left floating which could disable the amplifier unexpectedly.

A bridge configuration, such as the one used in Boomer Au-

dio Power Amplifiers, also creates a second advantage over

single-ended amplifiers. Since the differential outputs, V_O1

and V_O2, are biased at half-supply, no net DC voltage exists

across the load. This eliminates the need for an output cou-

pling capacitor which is required in

a single supply,

single-ended amplifier configuration. Without a large output

coupling capacitor in a single supply single-ended amplifier,

the half-supply bias across the load would result in both in-

creased internal IC power dissipation.

POWER DISSIPATION

Power dissipation is a major concern when designing a suc-

cessful amplifier, whether the amplifier is bridged or

single-ended. A direct consequence of the increased power

delivered to the load by a bridge amplifier is an increase in

internal power dissipation. Equation 1 states the maximum

power dissipation point for a bridge amplifier operating at a

given supply voltage and driving a specified output load.

P_DMAX

(V_DD

)

²/(2π²R_L)

(1)

Since the LM4870 has two operational amplifiers in one

package, the maximum internal power dissipation is 4 times

that of a single-ended amplifier. Even with this substantial in-

crease in power dissipation, the LM4870 does not require

heatsinking. From Equation 1, assuming a 5V power supply

and an 8Ω load, the maximum power dissipation point is 625

mW. The maximum power dissipation point obtained from

Equation 1 must not be greater than the power dissipation

that results from Equation 2:

HEADPHONE CONTROL INPUTS

The LM4870 possesses two headphone control inputs that

disable the amplifier and reduce I_DDto less than 1 mA when

either one or both of these inputs have a logic-high voltage

placed on their pins.

Unlike the shutdown function, the headphone control func-

tion does not provide the level of current conservation that is

required for battery powered systems. Since the quiescent

current resulting from the headphone control function is

1000 times more than the shutdown function, the residual

currents in the device may create a pop at the output when

coming out of the headphone control mode. The pop effect

may be eliminated by connecting the headphone sensing

output to the shutdown pin input as shown in Figure 2. This

solution will not only eliminate the output pop, but will also

utilize the full current conservation of the shutdown function

by reducing I_DDto 0.6 µA. The amplifier will then be fully

shutdown. This configuration also allows the designer to use

the control inputs as either two headphone control pins or a

headphone control pin and a shutdown pin where the lowest

level of current consumption is obtained from either function.

P_DMAX(T_JMAX− T _A)/θ_JA

(2)

For the LM4870 surface mount package, θ_JA100˚C/W and

T_JMAX150˚C. Depending on the ambient temperature, T_A,

of the system surroundings, Equation 2 can be used to find

the maximum internal power dissipation supported by the IC

packaging. If the result of Equation 1 is greater than that of

Equation 2, then either the supply voltage must be de-

creased or the load impedance increased. For the typical ap-

plication of a 5V power supply, with an 8Ω load, the maxi-

mum ambient temperature possible without violating the

maximum junction temperature is approximately 88˚C, pro-

vided that device operation is around the maximum power

dissipation point. Power dissipation is a function of output

power and thus, if typical operation is not around the maxi-

mum power dissipation point, the ambient temperature can

be increased. Refer to the Typical Performance Character-

istics curves for power dissipation information for lower out-

put powers.

Figure 3 shows the implementation of the LM4870’s head-

phone control function using a single-supply headphone am-

plifier. The voltage divider of R1 and R2 sets the voltage at

the HP-IN1 pin to be approximately 50 mV when there are

no headphones plugged into the system. This logic-low volt-

age at the HP-IN1 pin enables the LM4870 to amplify AC sig-

nals. Resistor R3 limits the amount of current flowing out of

the HP-IN1 pin when the voltage at that pin goes below

ground resulting from the music coming from the headphone

amplifier. The output coupling cap protects the headphones

by blocking the amplifier’s half-supply DC voltage. The ca-

pacitor also protects the headphone amplifier from the low

voltage set up by resistors R1 and R2 when there aren’t any

headphones plugged into the system. The tricky point to this

setup is that the AC output voltage of the headphone ampli-

fier cannot exceed the 2.0V HP-IN1 voltage threshold when

there aren’t any headphones plugged into the system, as-

suming that R1 and R2 are 100k and 1k, respectively. The

LM4870 may not be fully shutdown when this level is ex-

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is criti-

cal for low noise performance and high power supply rejec-

tion. The capacitor location on both the bypass and power

supply pins should be as close to the device as possible. As

displayed in the Typical Performance CharacterIstIcs sec-

tion, the effect of a larger half-supply bypass capacitor is im-

proved low frequency THD+N due to increased half-supply

stability. Typical applications employ a 5V regulator with

a10 µF tantalum and a 0.1 µF film bypass capacitors which

aid in supply stability, but do not eliminate the need for by-

passing the supply nodes of the LM4870. The selection of

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In addition, the HP-SENSE pin, although it may be con-

nected to the SHUTDOWN pin as shown in Figure 2, may

still be used as a control flag. It is capable of driving the input

to another logic gate or approximately 2 mA without serious

Application Information (Continued)

ceeded momentarily, due to the discharging time constant of

the bias-pin voltage. This time constant is established by the

two 50k resistors (in parallel) with the series bypass capaci-

tor value.

When a set of headphones are plugged into the system, the

contact pin of the headphone jack is disconnected from the

signal pin, interrupting the voltage divider set up by resistors

R1 and R2. Resistor R1 then pulls up the HP-IN1 pin, en-

abling the headphone function and disabling the LM4870

amplifier. The headphone amplifier then drives the head-

phones, whose impedance is in parallel with resistor R2.

Since the typical impedance of headphones are 32Ω, resis-

tor R2 has negligible effect on the output drive capability.

Also shown in Figure 3 are the electrical connections for the

headphone jack and plug. A 3-wire plug consists of a Tip,

Ring, and Sleave, where the Tip and Ring are signal carrying

conductors and the Sleave is the common ground return.

One control pin contact for each headphone jack is sufficient

to indicate to control inputs that the user has inserted a plug

into a jack and that another mode of operation is desired.

DS100094-7

FIGURE 2. HP-SENSE Pin to

SHUTDOWN Pin Connection

For a system implementation where the headphone amplifier

is designed using a split supply, the output coupling cap, C_C

and resistor R2 of Figure 3, can be eliminated. The function-

ality described earlier remains the same, however.

DS100094-8

FIGURE 3. Typical Headphone Control Input Circuitry

AUDIO POWER AMPLIFIER DESIGN

Input Impedance: 20 kΩ

Bandwidth: 100 Hz-20 kHz 0.25 dB

Design a 500 mW/8Ω Audio Amplifier

Given:

A designer must first determine the needed supply rail to ob-

tain the specified output power. The Output Power vs Supply

Voltage graphs in the Typical Performance Characteris-

tics section show how much power the LM4870 will output

given different supply voltage. According to the graph, 4.6V

Power Output: 1.0Wrms

Load Impedance: 8Ω

Input Level: 1 Vrms(max)

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put (R_i) resistor is 1.5:1. If R_iis set to the required input im-

Application Information (Continued)

pedance of 20 kΩ, then R_f30 kΩ. The final design step is

are required for 1.0W of output power. Since 5V rails are

common, it is chosen to be the supply voltage. Supply volt-

age (up to a maximum of 5.5V) above the required 4.6V will

allow the LM4870 to reproduce transient signals at greater

than 1.0W of output power. The extra voltage will also in-

crease the device power dissipation, so the designer must

make sure that the conditions explained in the Power Dissi-

pationsection are not violated.

to address the bandwidth requirements which must be

stated as a pair of −3 dB frequency points. Five times away

from a −3 dB point is 0.17 dB down from passband response

which is better than the required 0.25 dB specified.

f_L100 Hz/5 20 Hz

f_H20 kHz5 100 HzAs stated in the External Compo-

nents section, R_iin conjunction with C_icreate a highpass

filter.

Once the power dissipation issues have been addressed,

the required differential gain can be determined from Equa-

tion 4:

C_i≥ 1/(2π 20 kΩ 20 Hz) 0.397 µF; use 0.39 µF.

The high frequency pole is determined by the product of the

desired high frequency pole, f_H, and the differential gain, A

vd. With a A_vd3 and f_H100 kHz, the resulting GBWP

150 kHz which is much smaller than the LM4870 GBWP of

4 MHz. This figure displays that if a designer has a need to

design an amplifier with

LM4870 can still be used without running into bandwidth

problems.

a higher differential gain, the

2.83; use A_vd

From equation 4, the minimum A_vdis A

Since the amplifier output is bridged, giving a gain of 2, and

the desired system gain is 3, the ratio of feedback (R_f) to in-

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Physical Dimensions inches (millimeters) unless otherwise noted

Small Outline Package (M)

Order Number LM4870M

NS Package Number M16A

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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

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Tel: 1-800-272-9959

Fax: 1-800-737-7018

Email: support@nsc.com

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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.

LM4870M [NSC]

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