Datasheet LM4870M Datasheet (NSC)

LM4870

1.1W Audio Power Amplifier with Shutdown Mode

LM4870 1.1W Audio Power Amplifier with Shutdown Mode

October 1997

General Description

The LM4870 is a bridge-connected audio power amplifier ca­pable 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.

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 sys­tems.

The LM4870 features an externally controlled, low-power
consumption shutdown mode, as well as an internal thermal
shutdown protection mechanism. It also includes two head­phone control inputs and a headphone sense output for ex­ternal monitoring. The LM4870 is unity-gain stable and the
gain is set using external resistors.

Key Specifications

n THD+N at 1W into 8Ω 0.5%(max)
n Output power into 8Ω at 1kHz

at 10%THD+N

n Shutdown Current 0.6µA (typ)

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

Applications

n Personal computers
n Desktop computers
n Low voltage audio system

Typical Application Connection Diagram

Small Outline Package

1.5W (typ)

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.

© 1999 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.

Supply Voltage 6.0V
Storage Temperature −65˚C to +150˚C
Input Voltage −0.3V to V
Power Dissipation Internally limited
ESD Susceptibility (Note 4) 3000V
ESD Susceptibility (Note 5) 250V
Junction Temperature 150˚C
Soldering Information

Small Outline Package

Vapor Phase (60 sec.) 215˚C

DD

+ 0.3V

Infrared (15 sec.) 220˚C

Thermal Resistance

(typ) 35˚C/W

φ

JC

(typ) 100˚C/W

φ

JA

See AN-450

“Surface Mounting and their Effects on Product
Reliability” for other methods of soldering surface mount de­vices.

Operating Ratings

Temperature Range

≤ TA≤ T

T

MIN

MAX

Supply Voltage 2.0V ≤ V

−20˚C ≤ TA≤ +85˚C
≤ 5.5V

DD

Electrical Characteristics

(Notes 1, 2) The following specifications apply for V

=

DD

Symbol Parameter Conditions LM4870 Units

V

I

DD

I

SD

V
P
THD+N Total Harmonic Distortion + Noise P
PSRR Power Supply Rejection Ratio V
V
V
V
V
V

Supply Voltage 2.0 V (min)

DD

Quiescent Power Supply Current V
Shutdown Current V
Output Offset Voltage V

OS

Output Power THD+N=0.5%(max); f=1 kHz 1.1 1.0 W (min)

O

Output Dropout Voltage V

od

HP-IN High Input Voltage HP-SENSE=0V to 4V 2.5 V

IH

HP-IN Low Input Voltage HP-SENSE=4V to 0V 2.5 V

IL

HP-SENSE High Output Voltage I

OH

HP-SENSE Low Output Voltage I

OL

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 guar­antee specific performance limits. This assumes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters where no limit is
given, however, the typical value is a good indication of device performance.

Note 3: The maximum power dissipation must be derated at elevated temperatures and is dictated by T
allowable power dissipation is P
+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).

DMAX

=

(T

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

JMAX−TA

=

O
pin2

=

IN

=

O

=

DD

=

IN

=

500 µA 2.8 2.5 V (min)

O

=

−500 µA 0.2 0.8 V (max)

O

5V, R

=

8Ω unless otherwise specified. Limits apply for T

L

Typical Limit

=

A

(Limits)

25˚C.

(Note 6) (Note 7)

5.5 V (max)

=

0V, I

0A 7.0 15.0 mA (max)

O

=

V

DD

0.4 µA

0V 10 50.0 mV (max)

1 Wrms; 20 Hz ≤ f ≤ 20 kHz 0.25

%

4.9V to 5.1V 60 dB

0V to 5V 0.6 1.0 V (max)

, θJA, and the ambient temperature TA. The maximum

JMAX

JMAX

=

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External Components Description (

Figure 1

)

Components Functional Description

1. R

2. C

3. R

4. C

5. C

i

i

f

S

B

Inverting input resistance which sets the closed-loop gain in conjunction with Rf. This resistor also
forms a high pass filter with C

at f

1/(2π R

i

C

iCi

).

=

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

at f

1/(2π R

i

C

iCi

).

=
Feedback resistance which sets closed-loop gain in conjunction with Ri.
Supply bypass capacitor which provides power supply filtering. Refer to the Application Information

section for proper placement and selection of supply bypass capacitor.
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

THD+N vs Output Power

Supply Current vs
Supply Voltage

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THD+N vs Frequency

THD+N vs Output Power

Power Derating Curve

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THD+N vs Frequency

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THD+N vs Output Power

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LM4870 Noise Floor
vs Frequency

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DS100094-18

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Typical Performance Characteristics (Continued)

Supply Current Distribution
vs Temperature

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Output Power vs
Supply Voltage

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Power Supply
Rejection Ratio

Power Dissipation
vs Output Power

Output Power vs
Supply Voltage

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Output Power vs
Load Resistance

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Open Loop
Frequency Response

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Application Information

BRIDGE CONFIGURATION EXPLANATION

Figure 1

As shown in
plifiers internally, allowing for a few different amplifier con­figurations. The first amplifier’s gain is externally config­urable, while the second amplifier is internally fixed in a
unity-gain, inverting configuration. The closed-loop gain of
the first amplifier is set by selecting the ratio of R
the second amplifier’s gain is fixed by the two internal 40 kΩ
resistors.

Figure 1

serves as the input to amplifier two which results in both am­plifiers producing signals identical in magnitude, but out of
phase 180˚. Consequently, the differential gain for the IC is:

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, the LM4870 has two operational am-

to Riwhile

f

shows that the output of amplifier one

=

*

A

(Rf/Ri)

2

vd

By driving the load differentially through outputs V
V

, an amplifier configuration commonly referred to as

O2

“bridged mode” is established. Bridged mode operation is

and

O1

different from the classical single-ended amplifier configura­tion where one side of its load is connected to ground.

A bridge amplifier design has a few distinct advantages over
the single-ended configuration, as it provides differential
drive to the load, thus doubling output swing for a specified
supply voltage. 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-

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.

A bridge configuration, such as the one used in Boomer Au­dio Power Amplifiers, also creates a second advantage over
single-ended amplifiers. Since the differential outputs, V
and VO2, are biased at half-supply, no net DC voltage exists
across the load. This eliminates the need for an output cou­pling capacitor which is required in a single supply,
single-ended amplifier configuration. Without a large output
coupling capacitor in a single supply single-ended amplifier,
the half-supply bias across the load would result in both in­creased internal IC power dissipation.

POWER DISSIPATION

Power dissipation is a major concern when designing a suc­cessful amplifier, whether the amplifier is bridged or
single-ended. A direct consequence of the increased power
delivered to the load by a bridge amplifier is an increase in
internal power dissipation. Equation 1 states the maximum
power dissipation point for a bridge amplifier operating at a
given supply voltage and driving a specified output load.

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 in­crease 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:

For the LM4870 surface mount package, θ

=

T

150˚C. Depending on the ambient temperature, T

JMAX

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 de­creased or the load impedance increased. For the typical ap­plication of a 5V power supply, with an 8Ω load, the maxi­mum ambient temperature possible without violating the
maximum junction temperature is approximately 88˚C, pro­vided 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 maxi­mum power dissipation point, the ambient temperature can
be increased. Refer to the TypicalPerformance Character-
istics curves for power dissipation information for lower out­put powers.

POWER SUPPLY BYPASSING

As with any power amplifier,proper supply bypassing is criti­cal for low noise performance and high power supply rejec­tion. 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 sec­tion, the effect of a larger half-supply bypass capacitor is im­proved 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 by­passing the supply nodes of the LM4870. The selection of

P

DMAX

P

DMAX

=

*

(VDD)2/(2π2RL)(1)

4

=

(T

JMAX−TA

)/θ

(2)

JA

=

100˚C/W and

JA

bypass capacitors, especially C
desired low frequency THD+N, system cost, and size con-

, is thus dependant upon

B

straints.

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the
LM4870 contains a shutdown pin to externally turn off the

O1

amplifier’s bias circuitry. The shutdown feature turns the am­plifier off when a logic high is placed on the shutdown pin.
Upon going into shutdown, the output is immediately discon­nected 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 ap­plied to the shutdown pin. A typical quiescent current of

0.6 µA results when the supply voltage is applied to the shut­down pin. In many applications, a microcontroller or micro­processor output is used to control the shutdown circuitry
which provides a quick, smooth transition into shutdown. An­other solution is to use a single-pole, single-throw switch that
when closed, is connected to ground and enables the ampli­fier.If the switch is open, then a soft pull-up resistor of 47 kΩ
will disable the LM4870. There are no soft pull-down resis­tors 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.

HEADPHONE CONTROL INPUTS

The LM4870 possesses two headphone control inputs that
disable the amplifier and reduce I
either one or both of these inputs have a logic-high voltage

to less than 1 mA when

DD

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

A

coming out of the headphone control mode. The pop effect

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

,

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
shutdown. This configuration also allows the designer to use

to 0.6 µA. The amplifier will then be fully

DD

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.

Figure 3

shows the implementation of the LM4870’s head­phone control function using a single-supply headphone am­plifier. 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 volt­age at the HP-IN1 pin enables the LM4870 to amplifyAC sig­nals. 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 ca­pacitor 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 ampli­fier cannot exceed the 2.0V HP-IN1 voltage threshold when
there aren’t any headphones plugged into the system, as­suming that R1 and R2 are 100k and 1k, respectively. The
LM4870 may not be fully shutdown when this level is ex-

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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 capaci­tor 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, en­abling the headphone function and disabling the LM4870
amplifier. The headphone amplifier then drives the head­phones, whose impedance is in parallel with resistor R2.
Since the typical impedance of headphones are 32Ω, resis­tor R2 has negligible effect on the output drive capability.
Also shown in
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.

For a system implementation where the headphone amplifier
is designed using a split supply, the output coupling cap, C
and resistor R2 of
ality described earlier remains the same, however.

Figure 3

are the electrical connections for the

Figure 3

, can be eliminated. The function-

In addition, the HP-SENSE pin, although it may be con­nected to the SHUTDOWN pin as shown in
still be used as a control flag. It is capable of driving the input
to another logic gate or approximately 2 mA without serious
loading.

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FIGURE 2. HP-SENSE Pin to
SHUTDOWN Pin Connection

C

Figure 2

, may

FIGURE 3. Typical Headphone Control Input Circuitry

AUDIO POWER AMPLIFIER DESIGN
Design a 500 mW/8Ω Audio Amplifier

Given:
Power Output: 1.0Wrms
Load Impedance: 8Ω
Input Level: 1 Vrms(max)

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DS100094-8

Input Impedance: 20 kΩ

±

Bandwidth: 100 Hz-20 kHz

0.25 dB

A designer must first determine the needed supply rail to ob­tain 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

Application Information (Continued)

are required for 1.0W of output power. Since 5V rails are
common, it is chosen to be the supply voltage. Supply volt­age (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 in­crease the device power dissipation, so the designer must
make sure that the conditions explained in the Power Dissi-
pationsection are not violated.

Once the power dissipation issues have been addressed,
the required differential gain can be determined from Equa­tion 4:

From equation 4, the minimum Avdis A

3.
Since the amplifier output is bridged, giving a gain of 2, and

the desired system gain is 3, the ratio of feedback (R

=

vd

2.83; use A

)toin-

f

vd

put (R

) resistor is 1.5:1. If Riis set to the required input im-

i

pedance of 20 kΩ, then R
to address the bandwidth requirements which must be

=

30 kΩ. The final design step is

f

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

=

f

100 Hz/5=20 Hz

=

20 kHz5=100 HzAs stated in the External Compo-

f

H

nents section, R

C

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

i

L

in conjunction with Cicreate a highpass

i

±

0.25 dB specified.

filter.

The high frequency pole is determined by the product of the
desired high frequency pole, f

vd

. With a A

150 kHz which is much smaller than the LM4870 GBWP of

=

3 and f

vd

, and the differential gain, A

H

=

100 kHz, the resulting GBWP

H

4 MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the

=

LM4870 can still be used without running into bandwidth
problems.

=

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Physical Dimensions inches (millimeters) unless otherwise noted

Small Outline Package (M)

Order Number LM4870M

NS Package Number M16A

LM4870 1.1W Audio Power Amplifier with Shutdown Mode

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