Datasheet LM4874MH Datasheet (NSC)

LM4874

2.1W Differential Input, BTL Output Stereo Audio
Amplifier with Selectable Gain and Shutdown

August 2002

LM4874 Boomer 2.1W Differential Input, BTL Output Stereo Audio Amplifier with Selectable Gain

and Shutdown

General Description

The LM4874 features differential stereo inputs, BTL (bridge­tied load) outputs, and four externally selectable fixed gains.
Operating on a single 5V supply, the LM4874 delivers 1.2W,

1.9W, or 2.1W (typ) of output power to an 8Ω,4Ω,or3Ω BTL
load (Note 1), respectively, with less than 1% THD+N. The
LM4874’s gain is selected using two digital inputs. The nomi­nal gain values are 6dB, 10dB, 15.6dB, and 21.6dB.

The LM4874 is designed for notebook and other handheld
portable applications. It delivers high quality output power
from a surface-mount package and requires few external
components.

Other features include an active-low micropower shutdown
mode input and thermal shutdown protection.

Key Specifications

j

BTL output power

R

=3Ω,VDD= 5.0V, and THD+N = 1% 2.1W (typ)

L

j

BTL output power

R

=4Ω,VDD= 5.0V, and THD+N = 1% 1.9W (typ)

L

Connection Diagram

Top View

j

BTL output power

R

=8Ω,VDD= 5.0V, and THD+N = 1% 1.2W (typ)

L

j

Micropower shutdown current 0.1µA (typ)

j

PSRR (@1kHz, VDD= 5V, (Fig.1)) 62dB (typ)

Features

n Fully differential input and output
n Internal gain set: 6dB, 10dB, 15.6dB, and 21.6dB
n Improved ’click and pop’ suppression
n Thermal shutdown protection circuit
n Ultra low current micropower shutdown mode
n 3.0V to 5.5V operation
n Available in space-saving exposed-DAP TSSOP

package

Applications

n Notebook computers
n PDAs
n Portable electronic devices

Order Number LM4874MH

See NS Package Number MXA20A for Exposed-DAP TSSOP

Boomer®is a registered trademark of National Semiconductor Corporation.

© 2002 National Semiconductor Corporation DS200469 www.national.com

20046902

Typical Application

LM4874

FIGURE 1. Typical Audio Amplifier Application Circuit

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20046901

LM4874

Absolute Maximum Ratings (Notes 2,

3)

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 (Note 4) Internally Limited

ESD Susceptibility (Note 5) 2000V

ESD Susceptibility (Note 6) 200V

Junction Temperature 150˚C

Soldering Information

DD

+ 0.3V

Infrared (15 sec.) 220˚C

See AN-450 “Surface Mounting and their Effects on
Product Reliability” for other methods of soldering surface
mount devices.

Thermal Resistance

θ

(typ) MXA20A 2˚C/W

JC

θ

(typ) MXA20A 41˚C/W (Note 7)

JA

Operating Ratings

Temperature Range

≤ TA≤T

T

MIN

MAX

Supply Voltage 3.0 V ≤ V

−40˚C ≤ TA ≤ 85˚C

≤ 5.5V

DD

Small Outline Package

Vapor Phase (60 sec.) 215˚C

Electrical Characteristics for LM4874 (Notes 2, 8)

The following specifications applies to the LM4874 when used in the circuit shown in Figure 1 and operating with VDD=5Vand

= 6dB, unless otherwise specified. Limits apply for TA= 25˚C.

A

V

LM4874

Symbol Parameter Conditions

V

DD

Supply Voltage 3.0

Typical

(Note 8)

Limit

(Notes 9,

10)

5.5

I

DD

I

SD

V

OS

Quiescent Power Supply Current VIN= 0V, IO= 0A, RL=

Shutdown Current V

Output Offset Voltage 7 50 mV (max)

PSRR Output Supply Rejection Ratio V

P

O

Output Power (Note 11) THD+N = 1% (max), f = 1kHz (Note12)

shutdown

DD

= GND 0.1 1.0 µA (max)

= 5V, V

sinewave, C

=8Ω

R

L

=3Ω

R

L

=4Ω

R

L

=8Ω

R

L

RIPPLE

BYPASS

∞

= 200mV

= 0.47µF,

P-P

9.0 12.0 mA (max)

62 dB

2.1

1.9

1.2 1.0

THD+N = 10% (max), f = 1kHz
(Note12)

=3Ω

R

L

=4Ω

R

L

=8Ω

R

L

2.6

2.6

1.5

THD+N Total Harmonic Distortion + Noise 20Hz ≤ f ≤ 20kHz

=4Ω,PO=2W

R

L

=8Ω,PO=1W

R

L

S/N Signal-to-Noise Ratio f = 1kHz, C

= 1.1W, RL=8Ω

P

O

R

IN

Note 1: An LM4874MH that has been properly mounted to a circuit board with a copper heatsink area of at least 2in2will deliver 1.9W into 4Ω or 2.1W into 3Ω.

Note 2: All voltages are measured with respect to the GND pin unless other wise specified.

Note 3: 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 guarantee specific performance limits. Electrical Characteristics state DC and AC electrical specifications under particular test conditions that
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 4: The maximum power dissipation must be derated at elevated temperatures and is dictated by TJMAX, θ
allowable power dissipation is P
currents for more information.

Note 5: Human body model, 100 pF discharged through a 1.5 kΩ resistor.

Input Resistance Pins 5, 7, 9, and 17 See Table 1.

DMAX

=(T

JMAX-TA/θJA

or the number given in Absolute Maximum Ratings, whichever is lower. For the LM4874, see power derating

BYPASS

= 0.47µF,

JA

0.3

0.3

97 dB

, and the ambient temperature, TA. The maximum

Units

(Limits)

V (min)

V (max)

W
W

W (min)

W
W
W

%
%

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Electrical Characteristics for LM4874 (Notes 2, 8) (Continued)

Note 6: Machine Model, 220pF-240pF discharged through all pins.

LM4874

Note 7: The given θ

driving 4Ω loads from a 5V supply, the LM4874MH must be mounted to the circuit board and its exposed-DAP soldered to an exposed 2in

Note 8: Typicals are measured at 25˚C and represent the parametric norm.

Note 9: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).

Note 10: Datasheet minimum and maximum specification limits are guaranteed by design, test, or statistical analysis.

Note 11: Output power is measured at the amplifier’s package pins.

Note 12: When driving 3Ω or 4Ω loads and operating on a 5V supply, the LM4874MH must be mounted to a circuit board that has a minimum of 2in

uninterrupted copper area connected to the MH package’s exposed DAP.

is for an LM4874 packaged in an MXA20A with the exposed-DAP soldered to an exposed 2in2area of 1oz printed circuit board copper. When

JA

External Components Description

See Figure 1.

Components Functional Description

1. C

2. C

The input coupling capacitor blocks DC voltage at the amplifier’s inverting input terminals. Ci, along with the

i

LM4874’s variable input resistance R
inverting and noninverting inputs require a C
EXTERNAL COMPONENTS, for an explanation of determining the value of C

The supply bypass capacitor. Refer to the POWER SUPPLY BYPASSING section for information about

S

(See Table 1), creates a highpass filter with fC= 1/(2πRiCi). Both

1

. Refer to the Application Information section, SELECTING

i

.

i

properly placing, and selecting the value of, this capacitor.

3. C

The capacitor, CB, filters the half-supply voltage present on the BYPASS pin. Refer to the Application

B

Information section, SELECTING EXTERNAL COMPONENTS, for information concerning proper placement
and selecting C

’s value.

B

Typical Performance Characteristics MH Specific Characteristics

2

area of 1oz PCB copper.

2

of exposed,

THD vs Frequency THD vs Frequency

VDD= 5V, RL=4Ω,P

= 1000mW,

OUT

at (from top to bottom at 1kHz):

= 21.6dB, AV= 15.6dB,

A

V

= 10dB, AV= 6dB

A

V

20046998

VDD= 5V, RL=8Ω,P

OUT

= 400mW,

at (from top to bottom at 1kHz):

= 21.6dB, AV= 15.6dB,

A

V

= 10dB, AV= 6dB

A

V

20046997

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Typical Performance Characteristics
MH Specific Characteristics

THD vs Frequency THD vs Frequency

(Continued)

LM4874

VDD= 5V, RL=8Ω,P

OUT

= 400mW,

at (from top to bottom at 1kHz):

= 21.6dB, AV= 15.6dB,

A

V

= 10dB, AV= 6dB

A

V

THD vs Frequency THD vs Frequency

VDD= 5V, RL=3Ω,fIN= 1kHz,

at (from top to bottom at 50mW):

= 21.6dB, AV= 15.6dB,

A

V

= 10dB, AV= 6dB

A

V

20046999

20046944

VDD= 5V, RL=3Ω,fIN= 20Hz,

at (from top to bottom at 50mW):

= 21.6dB, AV= 15.6dB,

A

V

= 10dB, AV= 6dB

A

V

VDD= 5V, RL=3Ω,fIN= 20kHz,

at (from top to bottom at 50mW):

= 21.6dB, AV= 15.6dB,

A

V

= 10dB, AV= 6dB

A

V

20046943

20046945

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Typical Performance Characteristics
MH Specific Characteristics

LM4874

THD vs Output Power THD vs Output Power

(Continued)

VDD= 5V, RL=4Ω,fIN= 20Hz,

at (from top to bottom at 100mW):

= 21.6dB, AV= 15.6dB,

A

V

= 6dB, AV= 10dB

A

V

200469A0

VDD= 5V, RL=4Ω,fIN= 1kHz,

at (from top to bottom at 200mW):

= 21.6dB, AV= 15.6dB,

A

V

= 10dB, AV= 6dB

A

V

THD vs Output Power THD vs Output Power

VDD= 5V, RL=4Ω,fIN= 20kHz,

at (from top to bottom at 200mW):

= 21.6dB, AV= 15.6dB,

A

V

= 10dB, AV= 6dB

A

V

200469A2

VDD= 5V, RL=8Ω,fIN= 20Hz,

at (from top to bottom at 200mW):

= 21.6dB, AV= 15.6dB,

A

V

= 10dB, AV= 6dB

A

V

200469A1

200469A3

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Typical Performance Characteristics
MH Specific Characteristics

THD vs Output Power THD vs Output Power

(Continued)

LM4874

VDD= 5V, RL=8Ω,fIN= 1kHz,

at (from top to bottom at 200mW):

= 21.6dB, AV= 15.6dB,

A

V

= 10dB, AV= 6dB

A

V

200469A4

VDD= 5V, RL=8Ω,fIN= 20kHz,

at (from top to bottom at 200mW):

= 21.6dB, AV= 15.6dB,

A

V

= 10dB, AV= 6dB

A

V

Output Power vs Supply Voltage Output Power vs Supply Voltage

RL=4Ω,fIN= 1kHz,

at (from top to bottom at 4V):

THD+N = 10%, THD+N = 1%

200469C3

RL=8Ω,fIN= 1kHz,

at (from top to bottom at 4V):

THD+N = 10%, THD+N = 1%

200469A5

200469C4

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Typical Performance Characteristics
MH Specific Characteristics

LM4874

PSRR vs Frequency PSRR vs Frequency

(Continued)

V

RIPPLE

VDD= 5V, RL=4Ω,R

= 200mV

, at (from top to bottom at 1kHz):

P-P

= 21.6dB, AV= 15.6dB,

A

V

= 10dB, AV= 6dB

A

V

SOURCE

THD vs Frequency THD vs Frequency

VDD= 3V, RL=4Ω,P

OUT

= 150mW,

at (from top to bottom at 1kHz):

= 21.6dB, AV= 15.6dB,

A

V

= 10dB, AV= 6dB

A

V

=10Ω

200469A8

200469B0

V

RIPPLE

VDD= 5V, RL=8Ω,R

= 200mV

, at (from top to bottom at 1kHz):

P-P

= 21.6dB, AV= 15.6dB,

A

V

= 10dB, AV= 6dB

A

V

VDD= 3V, RL=8Ω,P

SOURCE

OUT

= 150mW,

at (from top to bottom at 1kHz):

= 21.6dB, AV= 15.6dB,

A

V

= 10dB, AV= 6dB

A

V

200469A9

=10Ω

200469B1

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Typical Performance Characteristics
MH Specific Characteristics

THD vs Output Power THD vs Output Power

(Continued)

LM4874

VDD= 3V, RL=4Ω,fIN= 1kHz,

at (from top to bottom at 200mW):

= 21.6dB, AV= 6dB,

A

V

= 15.6dB, AV= 10dB

A

V

200469B3

VDD= 3V, RL=4Ω,fIN= 20Hz,

at (from top to bottom at 100mW):

= 21.6dB, AV= 15.6dB,

A

V

= 6dB, AV= 10dB

A

V

THD vs Output Power THD vs Output Power

VDD= 3V, RL=4Ω,fIN= 20kHz,

at (from top to bottom at 200mW):

= 21.6dB, AV= 15.6dB,

A

V

= 10dB, AV= 6dB

A

V

200469B4

VDD= 3V, RL=8Ω,fIN= 20Hz,

at (from top to bottom at 100mW):

= 21.6dB, AV= 6dB,

A

V

= 15.6dB, AV= 10dB

A

V

200469B2

200469B5

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Typical Performance Characteristics
MH Specific Characteristics

LM4874

THD vs Output Power THD vs Output Power

(Continued)

V

RIPPLE

VDD= 3V, RL=8Ω,fIN= 1kHz,

at (from top to bottom at 200mW):

= 21.6dB, AV= 15.6dB,

A

V

= 6dB, AV= 10dB

A

V

PSRR vs Frequency PSRR vs Frequency

VDD= 3V, RL=4Ω,R

= 200mV

, at (from top to bottom at 1kHz):

P-P

= 21.6dB, AV= 15.6dB,

A

V

= 10dB, AV= 6dB

A

V

SOURCE

=10Ω,

200469B6

200469B8

V

RIPPLE

VDD= 3V, RL=8Ω,fIN= 20kHz,

at (from top to bottom at 200mW):

= 21.6dB, AV= 15.6dB,

A

V

= 10dB, AV= 6dB

A

V

VDD= 3V, RL=8Ω,R

= 200mV

, at (from top to bottom at 1kHz):

P-P

= 21.6dB, AV= 15.6dB,

A

V

= 10dB, AV= 6dB

A

V

SOURCE

=10Ω,

200469B7

200469B9

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Typical Performance Characteristics
MH Specific Characteristics

(Continued)

LM4874

Output Power vs

Load Resistance

fIN= 1kHz, at (from top to bottom at 20Ω):

200469C0

= 5V, THD = 10%; VDD= 5V, THD = 1%;

V

DD

= 3V, THD = 10%; VDD= 3V, THD = 1%

V

DD

Channel-to-Channel gain Mismatch

vs Power Supply Voltage

Channel-to-Channel gain Mismatch

vs Power Supply Voltage

RL=4Ω,fIN= 1kHz,

at (from top to bottom at 4V):

= 21.6dB, AV= 15.6dB,

A

V

= 10dB, AV= 6dB

A

V

Dropout Voltage

vs Power Supply Voltage

200469C1

RL=8Ω,fIN= 1kHz,

at (from top to bottom at 4V):

= 21.6dB, AV= 15.6dB,

A

V

= 10dB, AV= 6dB

A

V

200469C2

RL=8Ω,fIN= 1kHz, both channels driven and loaded

200469C5

at (from top to bottom at 4V):

positive signal swing, negative signal swing

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Typical Performance Characteristics
MH Specific Characteristics

LM4874

(Continued)

Dropout Voltage

vs Power Supply Voltage

200469C6

RL=4Ω,fIN= 1kHz, both channels driven and loaded

at (from top to bottom at 4V):

positive signal swing, negative signal swing

Amplifier Power Dissipation

vs Amplifier Load Dissipation Cross Talk vs Frequency

Amplifier Power Dissipation

vs Amplifier Load Dissipation

VDD= 5V, fIN= 1kHz, at (from top to bottom at 1W):

200469C7

=4Ω,RL=8Ω, single channel driven and loaded

R

L

VDD= 3V, fIN= 1kHz, at (from top to bottom at 0.3W):

200469C8

=4Ω,RL=8Ω, single channel driven and loaded

R

L

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VDD= 5V, RL=8Ω,AV= 6dB,

200469C9

A = Left channel driven, right channel measured;

B = Right channel driven, left channel measured

Typical Performance Characteristics
MH Specific Characteristics

Cross Talk vs Frequency Cross Talk vs Frequency

(Continued)

LM4874

VDD= 5V, RL=8Ω,AV= 10dB,

200469D0

A = Left channel driven, right channel measured;

B = Right channel driven, left channel measured

Cross Talk vs Frequency Cross Talk vs Frequency

200469D2

VDD= 5V, RL=8Ω,AV= 21.6dB,

A = Left channel driven, right channel measured;

B = Right channel driven, left channel measured

VDD= 5V, RL=8Ω,AV= 15.6dB,

200469D1

A = Left channel driven, right channel measured;

B = Right channel driven, left channel measured

200469D3

VDD= 3V, RL=8Ω,AV= 6dB,

A = Left channel driven, right channel measured;

B = Right channel driven, left channel measured

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Typical Performance Characteristics
MH Specific Characteristics

LM4874

Cross Talk vs Frequency Cross Talk vs Frequency

(Continued)

VDD= 3V, RL=8Ω,AV= 10dB,

200469D4

A = Left channel driven, right channel measured;

B = Right channel driven, left channel measured

Cross Talk vs Frequency

VDD= 3V, RL=8Ω,AV= 21.6dB,

A = Left channel driven, right channel measured;

B = Right channel driven, left channel measured

VDD= 3V, RL=8Ω,AV= 15.6dB,

200469D5

A = Left channel driven, right channel measured;

B = Right channel driven, left channel measured

200469D6

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Application Information

PCB LAYOUT AND SUPPLY REGULATION CONSIDERATIONS FOR DRIVING 3W AND 4W LOADS

Power dissipated by a load is a function of the voltage swing
across the load and the load’s impedance. As load imped­ance decreases, load dissipation becomes increasingly de­pendent on the interconnect (PCB trace and wire) resistance
between the amplifier output pins and the load’s connec­tions. Residual trace resistance causes a voltage drop,
which results in power dissipated in the trace and not in the
load as desired. For example, 0.1Ω trace resistance reduces
the output power dissipated by a 4Ω load from 2.1W to 2.0W.
This problem of decreased load dissipation is exacerbated
as load impedance decreases. Therefore, to maintain the
highest load dissipation and widest output voltage swing,
PCB traces that connect the output pins to a load must be as
wide as possible.

Poor power supply regulation also adversely affects maxi­mum output power. A poorly regulated supply’s output volt­age decreases with increasing load current. Reduced supply
voltage causes decreased headroom, output signal clipping,
and reduced output power. Even with tightly regulated sup­plies, trace resistance creates the same effects as poor
supply regulation. Therefore, making the power supply
traces as wide as possible helps maintain full output voltage
swing.

BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 1, each of the LM4874’s stereo channels
consists of two operational amplifiers. The LM4874 can be
used to drive a speaker connected between the two outputs
of each channel’s amplifiers.

Figure 1 shows that the output of Amp1 serves as the input
to Amp2, which results in both amplifiers producing signals
identical in magnitude, but 180˚ out of phase. Taking advan­tage of this phase difference, a load is placed between
OUT+ and OUT- and driven differentially (commonly referred
to as ’bridge mode’). This results in a differential gain of

= 2(RF/RI) (1)

A

VD

Bridge mode is different from single-ended amplifiers that
drive loads connected between a single amplifier’s output
and ground. For a given supply voltage, bridge mode has a
distinct advantage over the single-ended configuration: its
differential output doubles the voltage swing across the load.
This results in four times the output power when compared
to a single-ended amplifier under the same conditions. This
increase in attainable output assumes that the amplifier is
not current limited or the output signal is not clipped. To
ensure minimum output signal clipping when selecting one
of the amplifier’s four closed-loop gains, refer to the Audio
Power Amplifier Design section.

Another advantage of the differential bridge output is no net
DC voltage across the load. This results from biasing OUT+
and OUT- at half-supply. This eliminates the coupling capaci­tor that single supply, single-ended amplifiers require. Elimi­nating an output coupling capacitor in a single-ended con-

figuration forces a single supply amplifier’s half-supply bias
voltage across the load. The current flow created by the
half-supply bias voltage increases internal IC power dissipa­tion and may permanently damage loads such as speakers.

POWER DISSIPATION

Power dissipation is a major concern when designing a
successful bridged or single-ended amplifier. Equation (2)
states the maximum power dissipation point for a single­ended amplifier operating at a given supply voltage and
driving a specified output load.

=(VDD)2/(2π2RL) Single-Ended (2)

P

DMAX

However, a direct consequence of the increased power de­livered to the load by a bridge amplifier is an increase in the
internal power dissipation point for a bridge amplifier oper­ating at the same given conditions.

=4*(VDD)2/(2π2RL) Bridge Mode (3)

P

DMAX

The LM4874 has four operational amplifiers in one package
and the maximum internal power dissipation is four times
that of a single-ended amplifier. From Equation (3), assum­ing a 5V power supply and an 8Ω load, the maximum power
dissipation point is 2W. The maximum power dissipation
point obtained from Equation (3) must not exceed the power
dissipation predicted by Equation (4):

P

DMAX

=(T

JMAX−TA

)/θ

JA

(4)

For the exposed DAP TSSOP package, θJA= 41˚C/W.
T
perature T

= 150˚C for the LM4874. For a given ambient tem-

JAMAX

, Equation (4) can be used to find the maximum

A

internal power dissipation supported by the IC packaging. If
the result of Equation (3) is greater than that of Equation (4),
decrease the supply voltage, increase the load impedance,
or reduce the ambient temperature. For a typical application
with a 5V power supply and an 8Ω load, the maximum
ambient temperature that does not violate the maximum
junction temperature is approximately 68˚C. This further as­sumes that a device is a surface mount part operating
around the maximum power dissipation point. Since internal
power dissipation is a function of output power, higher am­bient temperatures are allowed as output power decreases.
Refer to the Typical Performance Characteristics curves for
power dissipation information at lower output power levels.

BTL GAIN SELECTION

The LM4874 features four fixed, internally set, BTL voltage
gains: 6dB, 10dB, 15.6dB, and 21.6dB. Select one of the
four gains by applying a logic level signal to the GAIN0
(MSB) and GAIN1 (LSB) digital inputs.

The closed-loop gain of the first amplifier is adjustable, hav­ing four different gains, whereas two internal 20kΩ resistors
set the second amplifier’s gain at -1. Table 1 below, shows
the state of the two logic inputs required to select one of the
four gain values.

LM4874

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Application Information (Continued)

LM4874

Table 1. Gain Settings and Input Resistance

GAIN 0 GAIN 1 Selected Gain

Input Resistance

(dB)

0 0 6 90kΩ

0 1 10 70kΩ

1 0 15.6 45kΩ

1 1 21.6 25kΩ

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection. The capacitors connected to the bypass and power
supply pins should be placed as close to the LM4874 as
possible. The capacitor connected between the bypass pin
and ground improves the internal bias voltage’s stability,
producing improved PSRR. The improvements to PSRR
increase as the bypass pin capacitor value increases.

Typical applications employ a 5V regulator with 10µF and a

0.1µF filter capacitors that aid in supply stability. Their pres­ence, however, does not eliminate the need for bypassing
the LM4874’s supply pins. The selection of bypass capacitor
values, especially C

, depends on desired PSRR require-

B

ments, click and pop performance (as explained in theSe-
lecting External Components section), system cost, and
size constraints.

MICRO-POWER SHUTDOWN

The LM4874 features an active-low micro-power shutdown
mode. The voltage applied to the SHUTDOWN pin controls
the LM4874’s shutdown function. Activate micro-power shut­down by applying 0V to the SHUTDOWN pin. The logic
threshold is typically 0.4V for a logic low and 1.5V for a logic
high. When active, the LM4874’s micro-power shutdown
feature turns off the amplifier’s bias circuitry, disables the
internal V

/2 generator, and forces the amplifier outputs

DD

into a high impedance state. The result is greatly reduced
power supply current. The low 0.1µA typical shutdown cur­rent is achieved by applying a voltage to the SHUTDOWN
pin that is as near to GND as possible. A voltage that is
greater than GND may increase the shutdown current.

There are a few methods to control the micro-power shut­down. These include using a single-pole, single-throw switch
(SPST), a microprocessor, or a microcontroller. When using
a switch, connect a 100kΩ pull-down resistor between the
SHUTDOWN pin and GND and the SPST switch between
the SHUTDOWN pin and V

. Select normal amplifier op-

DD

eration by closing the switch. Opening the switch applies
GND to the SHUTDOWN pin, activating micro-power shut­down. The switch and resistor guarantee that the SHUT­DOWN pin will not float. This prevents unwanted state
changes. In a system with a microprocessor or a microcon­troller, use a digital output to apply the active-state voltage to
the SHUTDOWN pin. Driving the SHUTDOWN pin with ac­tive circuitry eliminates the pull-down resistor.

(R

Table 2. Logic Level Truth Table for Shutdown Operation

SHUTDOWN OPERATIONAL

MODE

)

i

High Full Power, stereo

BTL amplifiers

Low Micro-power

Shutdown

SELECTING PROPER EXTERNAL COMPONENTS

Optimizing the LM4874’s performance requires properly se­lecting external components. Though the LM4874 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val­ues. The LM4874 is unity-gain stable, giving a designer
maximum design flexibility. The gain should be set to no
more than a given application requires. This allows the am­plifier to achieve minimum THD+N and maximum signal-to­noise ratio. These parameters are compromised as the
closed-loop gain increases. However, low gain demands
input signals with greater voltage swings to achieve maxi­mum output power. Fortunately, many signal sources such
as audio CODECs have outputs of 1V

RMS

(2.83V

P-P

).
Please refer to the Audio Power Amplifier Design section for
more information on selecting the proper gain.

Input Capacitor Value Selection

Amplifying the lowest audio frequencies requires high value
input coupling capacitors (C

and C3,C4) in Figure 1. A

I,C2

high value capacitor can be expensive and may compromise
space efficiency in portable designs. In many cases, how­ever, the speakers used in portable systems, whether inter­nal or external, have little ability to reproduce signals with
frequencies below 150Hz. Applications using speakers with
this limited frequency response reap little improvement by
using large input capacitor.

Besides effecting system cost and size, CI - C4 can also
affect on the LM4874’s turn-on and turn-off transient (’click
and pop’) performance. When the supply voltage is first
applied, a transient may be created as the charge on the
input capacitor changes from zero to a quiescent state. The
magnitude of the transient is proportional to the value of, and
more importantly, the mismatch between, the capacitors
connected to a given pair of inverting and non-inverting
inputs. The better the match, the less the transient magni­tude.

Higher value capacitors need more time to reach a quiescent
DC voltage (usually V

/2) when charged with a fixed cur-

DD

rent. This fixed current is supplied through amplifiers input
pins. Thus, selecting an input capacitor value that is no
higher than necessary to meet the desired -3dB frequency
will reduce turn-on time and help ensure that transients are
minimized.

The LM4874’s nominal input resistance (R
minimum) and the input capacitor, C

)is25kΩ (20kΩ,

i

, form high pass filter

i

with a -3dB low frequency limit defined by equation (5).

www.national.com 16

= 1/2π(25kΩ)C

f

-3dB

i

(5)

As an example when using a speaker with a low frequency
limit of 150Hz, C

, is 0.047µF. The 0.47µF CIshown in Figure

I

1 allows the LM4874 to drive high efficiency, full range
speaker whose response extends below 30Hz.

LM4874

Application Information (Continued)

Bypass Capacitor Value Selection

Besides optimizing the input capacitor value, careful consid­eration should be paid to value of C
nected between the BYPASS pin and ground. Since C
determines how fast the LM4874 settles to its quiescent
operating state, its value is critical when minimizing turn-on
transients. The slower the LM4874’s outputs ramp to their
quiescent DC voltage (nominally
turn-on transient. Choosing C
small value of C

(in the range of 0.047µF to 0.47µF), pro-

i

equal to 0.47µF along with a

B

duces a transient-free turn-on and shutdown function. As
discussed above, choosing C

no larger than necessary for

i

the desired bandwidth helps minimize turn-on transients.

OPTIMIZING OUTPUT TRANSIENT REDUCTION (CLICK AND POP PERFORMANCE)

The LM4874 contains circuitry to minimize turn-on and shut­down transients or ’clicks and pop’. For this discussion,
turn-on refers to either applying the power supply voltage or
when the shutdown mode is deactivated. While the power
supply voltage is ramping to its final value, the LM4874’s
internal amplifiers are configured as unity gain buffers. An
internal current source changes the voltage of the BYPASS
pin in a controlled, linear manner. Ideally, the amplifier inputs
and outputs track the voltage applied to the BYPASS pin.
The gain of the internal amplifiers remains unity until the
voltage on the bypass pin reaches 1/2 V
voltage on the BYPASS pin is stable, the device becomes
fully operational. Although the bypass pin current can not be
modified, changing the size of C
time and the magnitude of output transients. Increasing the
value of C

reduces the magnitude of turn-on transients.

B

However, this presents a tradeoff: as the size of C
creases, the turn-on time increases. There is a linear rela­tionships between the size of C
time. The table shows some typical turn-on times for various
values of C

:

B

C

B

C

= 0.47µF Ci= 0.33µF

i

0.01µF 110ms 80ms

0.1µF 120ms 90ms

0.22µF 140ms 100ms

0.47µF 170ms 140ms

1.0µF 240ms 210ms

In order to eliminate ’clicks and pops’, all capacitors must be
discharged before turn-on. Rapidly switching V
allow the capacitors to fully discharge, which may cause
’clicks and pops’.

, the capacitor con-

B

1

⁄2VDD), the smaller the

. As soon as the

DD

alters the device’s turn-on

B

+ 2(CI) and the turn-on

B

To n

may not

DD

in-

B

The design begins by specifying the minimum supply voltage
necessary to obtain the desired output power. One way to
find the minimum supply voltage is to use the Output Power
vs Supply Voltage curve in the Typical Performance Char-

B

acteristics section. Another way, using Equation (6), is to
calculate the peak output voltage necessary to achieve the
desired output power for a given load impedance. To ac­count for the amplifier’s dropout voltage, two additional volt­ages, based on the Dropout Voltage vs Supply Voltage in the
Typical Performance Characteristics curves, must be
added to the result obtained by Equation (6). The result is
Equation (7).

(6)

VDD≥ (V

OUTPEAK

+(V

OD

TOP

+V

OD

BOT

)) (7)

The Output Power vs Supply Voltage graph for an 8Ω load
indicates a minimum supply voltage of 4.6V. This is easily
met by the commonly used 5V supply voltage. The additional
voltage creates the benefit of headroom, allowing the
LM4874 to produce peak output power in excess of 1W
without clipping or other audible distortion. The choice of
supply voltage must also not create a situation that violates
of maximum power dissipation as explained above in the
Power Dissipation section.

After satisfying the LM4874’s power dissipation require­ments, the minimum differential gain is found using Equation
(8).

(8)

Thus, a minimum gain of 2.83 allows the LM4874’s to reach
full output swing and maintain low noise and THD+N perfor­mance. For this example, let A
the gain will be set to 10dB (A

= 3. In the example design,

VD

= 3.2) by applying a logic

VD

low to GAIN 0 and a logic high to GAIN 1.

The last step in this design example is setting the amplifier’s

±

-3dB frequency bandwidth. To achieve the desired

0.25dB
pass band magnitude variation limit, the low frequency re­sponse must extend to at least one-fifth the lower bandwidth
limit and the high frequency response must extend to at least
five times the upper bandwidth limit. This extended bandwith
produces a gain variation of -0.17dB at the bandwith’s limits,

±

well within the

0.25dB desired limit. The results are an

AUDIO POWER AMPLIFIER DESIGN

Audio Amplifier Design: Driving 1W into an 8Ω Load

The following are the desired operational parameters:

Power Output: 1 W

Load Impedance: 8Ω

Input Level: 1 V

Input Impedance: 20 kΩ

Bandwidth: 100 Hz−20 kHz

±

0.25 dB

RMS

RMS

and an

= 100Hz/5 = 20Hz (9)

f

L

= 20kHz x 5 = 100kHz (10)

f

H

www.national.com17

Application Information (Continued)

As mentioned in the External Components section, the inter-

LM4874

nal input resistor and C
amplifier’s lower bandpass frequency limit. Find the coupling
capacitor’s value using Equation (11).

The result is (using the minimum RINresistor value to ensure
correct magnitude response at 20Hz)

1/(2π

Use a 0.39µF capacitor, the closest standard value. The
product of the desired high frequency cutoff (100kHz in this
example) and the differential gain, A
per passband response limit. With A
100kHz, the closed-loop gain bandwidth product (GBWP) is

create a high pass filter that sets the

i

= 1/2π(20kΩ)C

f

-3dB

*

20kΩ*20Hz) = 0.398µF (12)

I

, determines the up-

VD

= 3.2 and fH=

VD

(11)

320kHz. This is less than the LM4874’s 3.5MHz GBWP. With
this margin, the amplifier can be used in designs that require
more differential gain while avoiding performance-restricting
bandwidth limitations.

Recommended Printed Circuit Board Layout

Figures 2 through 6 show the recommended four-layer PC
board layout that is optimized for the 20-pin MH-packaged
LM4874 and associated external components. This circuit is
designed for use with an external 5V supply and 3Ω (or
higher) speakers (or load resistors).

This circuit board is easy to use. Apply 5V and ground to the
board’s V
speakers (or load resistors) between the board’s -OUTA and
+OUTA and -OUTB and +OUTB pads. Apply balanced dif­ferential stereo input signals to the input pins labeled ’-INA,’
’+INA,’ ’-INB,’ and ’+INB.’

and GND terminals, respectively. Connect

DD

200469D7

FIGURE 2. Recommended MH PC Board Layout:

Component-Side Silkscreen

200469D8

FIGURE 3. Recommended MH PC Board Layout:

Component-Side Layout

www.national.com 18

Recommended Printed Circuit Board Layout (Continued)

200469D9

FIGURE 4. Recommended MH PC Board Layout:

Upper Inner-Layer Layout

LM4874

200469E0

FIGURE 5. Recommended MH PC Board Layout:

Lower Inner-Layer Layout

200469E1

FIGURE 6. Recommended MH PC Board Layout:

Bottom-Side Layout

www.national.com19

LM4874

www.national.com 20

Physical Dimensions inches (millimeters) unless otherwise noted

LM4874 Boomer 2.1W Differential Input, BTL Output Stereo Audio Amplifier with Selectable Gain

and Shutdown

Exposed-DAP TSSOP Package

Order Number LM4874MH

NS Package Number MXA20A for Exposed-DAP TSSOP

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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

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.

labeling, can be reasonably expected to result in a
significant injury to the user.

National Semiconductor
Corporation

Americas
Email: [e-mail]

www.national.com

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.

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