Datasheet LM4834MSX, LM4834MS Datasheet (NSC)

LM4834

1.75W Audio Power Amplifier with DC Volume Control
and Microphone Preamp

General Description

The LM4834 is a monolithic integrated circuit that provides
DC volume control, and a bridged audio power amplifier ca­pable of producing 1.75W into 4Ω with less than 1.0

%
(THD). In addition, the headphone/lineout amplifier is ca­pable of driving 70 mW into 32Ω with less than 0.1%(THD).
The LM4834 incorporates a volume control and an input mi­crophone preamp stage capable of drivinga1kΩload im-
pedance.

Boomer

®

audio integrated circuits were designed specifically
to provide high quality audio while requiring a minimum
amount of external components in surface mount packaging.
The LM4834 incorporates a DC volume control, a bridged
audio power amplifier and a microphone preamp stage,
making it optimally suited for multimedia monitors and desk­top computer applications.

The LM4834 features an externally controlled, low-power
consumption shutdown mode, and both a power amplifier
and headphone mute for maximum system flexibility and
performance.

Key Specifications

n THD at 1.1W continuous average

output power into 8Ω at 1 kHz 0.5%(max)

n Output Power into 4Ω at 1.0

%

THD+N

1.75W(typ)

n THD at 70mW continuous average

output power into 32Ω at 1 kHz

0.1%(typ)

n Shutdown Current 1.0µA(max)
n Supply Current 17.5mA(typ)

Features

n PC98 Compliant
n “Click and Pop” suppression circuitry
n Stereo line level outputs with mono input capability for

system beeps

n Microphone preamp with buffered power supply
n DC Volume Control Interface
n Thermal shutdown protection circuitry

Applications

n Multimedia Monitors
n Desktop and Portable Computers

Block Diagram Connection Diagram

Boomer®is a registered trademark of NationalSemiconductor Corporation.

DS100015-1

FIGURE 1. LM4834 Block Diagram

SSOP Package

DS100015-2

Top View

Order Number LM4834MS

See NS Package Number MSA028CB for SSOP

November 1997

LM4834 1.75W Audio Power Amplifier with DC Volume Control and Microphone Preamp

© 1997 National Semiconductor Corporation DS100015 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

DD

+0.3V
Power Dissipation Internally limited
ESD Susceptibility (Note 4) 2000V
Pin 5 1500V
ESD Susceptibility (Note 5) 200V
Junction Temperature 150˚C
Soldering Information

Small Outline Package

Vapor Phase (60 sec.) 215˚C

Infrared (15 sec.) 220˚C

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

θ

JC

(typ)— MSA028CB 29˚C/W

θ

JA

(typ)— MSA028CB 95˚C/W

Operating Ratings

Temperature Range

T

MIN

≤ TA≤T

MAX

−40˚C ≤TA ≤ 85˚C

Supply Voltage 4.5 ≤ V

DD

≤ 5.5V

Electrical Characteristics for Entire IC

(Notes 1, 2)
The following specifications apply for VDD= 5V unless otherwise noted. Limits apply for TA= 25˚C.

Symbol Parameter Conditions

LM4834

Units

(Limits)

Typical

(Note 6)

Limit

(Note 7)

V

DD

Supply Voltage 4.5 V (min)

5.5 V (max)

I

DD

Quiescent Power Supply Current VIN= 0V, IO= 0A 17.5 26 mA (max)

I

SD

Shutdown Current V

pin13=VDD

0.6 2.0 µA (max)

Electrical Characteristics for Volume Attenuators

(Notes 1, 2)
The following specifications apply for VDD= 5V. Limits apply for TA= 25˚C.

Symbol Parameter Conditions

LM4834

Units

(Limits)

Typical

(Note 6)

Limit

(Note 7)

C

RANGE

Attenuator Range Gain with V

pin 22

= 5V 2.6 3.65 dB (max)

Attenuation with V

pin 22

= 0V -75 -88 dB (min)

A

M

Mute Attenuation V

pin 15

= 5V, Sum Out -92 -105 dB (max)

V

pin 15

= 5V, Line Out/Headphone

Amp

-92 -105 dB (max)

Electrical Characteristics for Microphone Preamp and Power Supply

(Notes 1, 2)
The following specifications apply forV

DD

= 5V unless otherwise noted. Limits apply for TA= 25˚C.

Symbol Parameter Conditions

LM4834

Units

(Limits)

Typical

(Note 6)

Limit

(Note 7)

V

OS

Offset Voltage VIN= 0V 0.9 mV

SNR Signal to Noise Ratio V

DD

= 5V, RL=1k,f=1kHz, V

OUT

=

4.7V, A-Wtd Filter

123 dB

V

SWING

Output Voltage Swing f = 1 kHz, THD<1.0%,RL=1kΩ 4.72 V

E

NO

Input Referred Noise A-Weighted Filter 1.2 µV

PSRR Power Supply Rejection Ratio f = 120 Hz, V

RIPPLE

= 200 mVrms,

C

B

=1µF

28 dB

V

S

Mic Power Supply RL=1kΩ, Bias In = 2.5V 2.5 2.5 V (min)

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Electrical Characteristics for Line/Headphone Amplifier

(Notes 1, 2)
The following specifications apply for VDD= 5V. Limits apply for TA= 25˚C.

Symbol Parameter Conditions

LM4834

Units

(Limits)

Typical

(Note 6)

Limit

(Note 7)

P

O

Output Power THD = 0.1%; f = 1kHz; RL=32Ω 70 mW

THD=10%;f=1kHz; R

L

=32Ω 95 mW

THD+N Total Harmonic Distortion+Noise V

OUT

=4V

P-P

,20Hz<f<20 kHz,

R

L

= 10kΩ,AVD=−1

0.05

%

PSRR Power Supply Rejection Ratio C

B

= 1.0 µF, f =120 Hz,

V

RIPPLE

= 200 mVrms

30 dB

SNR Signal to Noise Ratio V

DD

=5V, P

OUT

=75mW, RL=32Ω,

A-Wtd Filter

102 dB

Electrical Characteristics for Bridged Speaker Amplifer

(Notes 1, 2)
The following specifications apply for VDD= 5V, unless otherwise noted. Limits apply for TA= 25˚C.

Symbol Parameter Conditions

LM4834

Units

(Limits)

Typical

(Note 6)

Limit

(Note 7)

V

OS

Output Offset Voltage VIN= 0V 5 30 mV (max)

P

O

Output Power THD = 0.5%(max);f = 1 kHz;

R

L

=8Ω

1.1 1.0 W (min)

THD+N = 10%;f = 1 kHz; R

L

=8Ω 1.5 W

THD+N Total Harmonic Distortion+Noise P

O

= 1W, 20 Hz<f<20 kHz,

R

L

=8Ω,AVD=2

0.3

%

P

O

= 340 mW, RL=32Ω 1.0

%

PSRR Power Supply Rejection Ratio C

B

= 1.0 µF, f = 120 Hz,V

RIPPLE

=

200 mVmrs

58 dB

SNR Signal to Noise Ratio V

DD

= 5V, P

OUT

= 1.1W, RL=8Ω,

A-Wtd Filter

93 dB

Note 1: All voltages are measured with respect to the ground pins, unlessotherwise specified. All specifications are tested using the typical application as shown in

Figure 1

.

Note 2:

Absolute Maximum Ratings

indicate limits beyond which damage to the device may occur.

Operating Ratings

indicate conditionsfor which the device is func-

tional, but do not guarantee specific performance limits.

Electrical Characteristics

state DC andAC electrical specifications under particular test conditions which guar­antee specific performance limits. This assumes that the deviceis 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

JMAX

, θJA, and the ambient temperature TA. The maximum

allowable power dissipation is P

DMAX

=(T

JMAX−TA

)/θJA.For the LM4834MS, T

JMAX

= 150˚C, and the typical junction-to-ambient thermal resistance, when board

mounted, is 95˚C/W assuming the MSA028CB package.

Note 4: Human body model, 100 pF discharged through a 1.5 kΩ resistor.
Note 5: Machine Model, 220 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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Typical Application

Truth Table for Logic Inputs

Mode Mute HP Sense DC Vol. Control Line/HP Left Line/HP Right Speaker Out

0 0 0 Adjustable Fixed Level Fixed Level Vol. Changes
0 0 1 Adjustable Fixed Level Fixed Level Muted
0 1 X _ Fixed Level Fixed Level Muted
1 0 0 Adjustable Vol. Changes Vol. Changes Vol. Changes
1 0 1 Adjustable Vol. Changes Vol. Changes Muted
1 1 X _ Muted Muted Muted

External Components Description

Figure 2

Components. Functional Description

1. C

i

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

i

at fc= 1/(2πRiCi). Refer to the section, Proper Selection of External

Components, for an explanation of how to determine the value of C

i

.

2. C

S

Supply bypass capacitor which provides power supply filtering. Refer to the Power Supply Bypassing
section for information concerning proper placement and selection of the supply bypass capacitor.

3. C

B

Bypass pin capacitor which provides half-supply filtering. Refer to the section, Proper Selection of
External Components, for information concerning proper placement and selection of C

B

.

DS100015-3

FIGURE 2. Typical Application Circuit

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External Components Description

Figure 2

(Continued)

Components. Functional Description

4. C

O

Output coupling capacitor which blocks the DC voltage at the amplifiers output. Forms a high pass filter
with R

L

at fo= 1/(2πRLCO).

5. R

S

Summing resistor that combines the right and left line level outputs into the mono input of the bridged
amplifier. The two summing resistors in parallel determine the value of the input resistance of the bridged
amplifier.

6. R

LFE

Resistor for the bridged power amplifier in series with RFat high frequencies. Used in conjunction with
C

LFE

to increase closed-loop gain at low frequencies.

7. R

F

Feedback resistor which sets the closed-loop gain in conjunction with the equivalent RSfor the bridged
power amplifier.

8. R

M1

Resistor in series with Microphone supply pin and the microphone for biasing differential input
microphones.

9. R

M2

Resistor in series with reference ground and the microphone used for biasing differential input
microphones.

Typical Performance Characteristics

THD+N vs Frequency
Bridged Power Amp

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THD+N vs Frequency
Bridged Power Amp

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THD+N vs Frequency
Bridge Power Amp

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THD+N vs Frequency
Line Out/HP Amplifiers

DS100015-7

THD+N vs Frequency
Line Out/HP Amplifiers

DS100015-8

THD+N vs Frequency
Line Out/HP Amplifiers

DS100015-9

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Typical Performance Characteristics (Continued)

THD+N vs Output Power
Bridged Power Amp

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THD+N vs Output Power
Bridged Power Amp

DS100015-10

THD+N vs Output Power
Bridged Power Amp

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THD+N vs Output Power
Line Out/HP Amplifiers

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THD+N vs Output Power
Line Out/HP Amplifiers

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THD+N vs Output Power
Line Out/HP Amplifiers

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Output Power vs
Load Resistance
Bridged Power Amp

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Output Power vs
Load Resistance
Line Out/HP Amplifiers

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Volume Control
Characteristics

DS100015-25

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Typical Performance Characteristics (Continued)

Noise Floor
Bridged Power Amp

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Noise Floor
Line Out/HP Amp

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Noise Floor
Mic Preamp

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Power Supply
Rejection Ratio
Bridged Power Amp

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Power Supply
Rejection Ratio
Line Out/HP Amplifiers

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Power Supply
Rejection Ratio
Mic Preamp

DS100015-24

Power Dissipation vs
Output Power
Bridged Power Amp

DS100015-18

Power Dissipation vs
Output Power
Line Out/HP Amplifiers

DS100015-26

Low Frequency Enhancement
Characteristics
Bridged Power Amp

DS100015-27

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Typical Performance Characteristics (Continued)

Power Derating Curve

DS100015-28

Open Loop
Frequency Response
Bridged Power Amp

DS100015-29

Crosstalk
Line Out/HP Amplifiers

DS100015-30

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Application Information

BEEP IN FUNCTION

The Beep In pin (pin 14) is a mono input, for system beeps,
that is mixed into the left and right input. This Beep In pin will
allow an input signal to pass through to the Sum Out and
Line/HP output pins. The minimum potential for the input of
the Beep In signal is 300mV. Beep in signals less than
300mV

P-P

will not pass through to the output. The beep in
circuitry provides left-right signal isolation to prevent
crosstalk at the summed input. As shown in the Fig. 2, it is
required that a resistor and capacitor is placed in series with
the Beep In pin and the node tied to V

DD

through a 100kΩ re­sistor. The recommended value for the input resistor is be­tween 120kΩ to 10kΩ and the input capacitor is between
.22Fµ and .47µF. The input resistor can be changed to vary
the amplitude of the beep in signal. Higher values of the in­put resistor will reduce the amplifier gain and attenuate the
beep in signal. In cases where system beeps are required
when the system is in a suspended mode, the LM4834 must
be brought out of shutdown before the beep in signal is input.

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the
LM4834 contains a shutdown pin to externally turn off the
bias circuitry.The LM4834 will shutdown when a logic high is
placed on the shutdown pin. The trigger point between a
logic low and logic high level is typically half supply.It is best
to switch between ground and the supply V

DD

to provide
maximum device performance. By switching the shutdown
pin to V

DD

, the LM4834 supply current draw will be mini­mized. While the device will be disabled with shutdown pin
voltages less than V

DD

, the idle current may be greater than
the typical value of 0.6 µA.The shutdown pin should not be
floated, since this may result in an unwanted shutdown con­dition.

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 in conjuction
with an external pull-up resistor. When the switch is
closed,the shutdown pin is connected to ground and enables
the amplifier. If the switch is open, then the external pull-up
resistor will shutdown the LM4834. This scheme prevents
the shutdown pin from floating.

MODE FUNCTION

The LM4834 was designed to operate in two modes. In
mode 0 (lineout mode),where the mode pin (pin 17) is given
a logic level low, the attenuation at the Line/HP outputs are
fixed at a gain of 1.4. In mode 1 (headphone mode), where
the mode pin is given a logic level high, the attenuation of the
Line/HP outputs is controlled through the DC voltage at pin

22. The signal levels of the Left and Right Sum Out pins are
always controlled by the DC potential at pin 22 regardless of
the mode of the IC. In mode 0, pin 5 and pin 24 are line out
drivers. In mode 1, pin 5 and pin 24 are headphone drivers.

MUTE FUNCTION

By placing a logic level high on the mute pin (pin 15), the
Right and Left Sum Out pins will be muted. If the LM4834 is
in the headphone mode, the HP/Line out pins as well as the
Sum Out pins are muted. The mute pin must not be floated.

HP SENSE FUNCTION

The LM4834 possesses a headphone sense pin (pin 16) that
mutes the bridged amplifier, when given a logic high, so that
headphone or line out operation can occur while the bridged
connected load will be muted.

Figure 3

shows the implementation of the LM4834’s head­phone control function using a single-supply.The voltage di­vider of R1, R2, R4, and R5 sets the voltage at the HPsense
pin (pin 16) to be approximately 50 mV when there are no
headphones plugged into the system. This logic-low voltage
at the HP sense pin enables bridged power amplifier.Resis­tor R4 limits the amount of current flowing out of the HP
sense pin when the voltage at that pin goes below ground re­sulting from the music coming from the headphone amplifier.
Resistor R1, R4, and R5 form a resistor divider that prevents
false triggering of the HP sense pin when the voltage at the
output swings near the rail, since V

IH

is about 2.5V.

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, R2, R4, and R5. Resistor R1 then pulls up the HP sense
pin, enabling the headphone function and disabling the
bridged amplifier. The headphone amplifier then drives the
headphones, whoseimpedance is in parallel with resistor R2
and R3. 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 Sleeve, where the Tip and Ring are signal
carrying conductors and the Sleeve is the common ground
return. One control pin contact for each headphone jack is
sufficient to indicate that the user has inserted a plug into a
jack and that another mode of operation is desired.

The LM4834 can be used to drive both a bridged 8Ω internal
speaker and a pair of 32Ω speakers without using the HP
sense pin. In this case the HP sense is controlled by a micro­processor or a switch.

DC VOLUME CONTROL

The DC voltage at the DC VolumeControl pin (pin 22) deter­mines the attenuation of the Sum Out and Line/HP amplifi­ers. If the DC potential of pin 22 is at 4V the internal ampli­fiers are set at a gain of 1.4 (2.9dB). The attenuation of the
amplifiers increase until 0V is reached. The attenuator range

DS100015-31

FIGURE 3. Headphone Input Circuit

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Application Information (Continued)

is from 2.9dB (pin22 = 4V) to -75dB (pin22 = 0V). Any DC
voltage greater than 4V will result in a gain of 2.9dB. When
the mode pin is given a logic low, the Line/HP amplifier will
be fixed at a gain of 2.9dB regardless of the voltage of pin

22. Refer to the Typical Performance Characteristics for
detailed information of the attenuation characteristics of the
DC Volume Control pin.

MICROPHONE PREAMPLIFIER

The microphone preamplifier is intended to amplify low-level
signals. The mic input can be directly connected to a micro­phone network or to low level signal inputs. The mic amplifier
has enough output capability to drive a 1kΩ load. A power
supply buffer is included for microphones which require ex­ternal biasing.

POWER DISSIPATION

Power dissipation is a major concern when using any power
amplifier and must be thoroughly understood to ensure a
successful design. Equation 1 states the maximum power
dissipation point for a bridged amplifier operating at a given
supply voltage and driving a specified load.

P

DMAX

= 4(VDD)2/(2π2RL) (1)

Along with the bridged amplifier, the LM4834 also incorpo­rates two single-ended amplifiers. Equation 2 states the
maximum power dissipation point for a single-ended ampli­fier operating at a given supply voltage and driving a speci­fied load.

P

DMAX

=(VDD)2/(2π2RL) (2)

Even with the power dissipation of the bridged amplifier
andthe two single-ended amplifiers, the LM4834 does not re­quire heatsinking. The power dissipation from the three am­plifiers, must not be greater than the package power dissipa­tion that results from Equation 3:

P

DMAX

=(T

JMAX−TA

)/ θJA(3)

For the LM4834 SSOP package, θ

JA

= 95˚C/W and T

JMAX

=

150˚C. Depending on the ambient temperature, T

A

,ofthe
system surroundings, Equation 3 can be used to find the
maximum internal power dissipation supported by the IC
packaging. If the result of Equation 1 and 2 is greater than
that of Equation 3, then either the supply voltage must be de­creased, the load impedance increased, or the ambient tem­perature reduced. For the typical application of a 5V power
supply,with an 8Ω bridged load and 32Ω single ended loads,
the maximum ambient temperature possible without violating
the maximum junction temperature is approximately 82˚C
provided that device operation is around the maximum
power dissipation points. 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 Typical Performance Char-
acteristics curvesfor power dissipation information for differ­ent output powers.

GROUNDING

In order to achieve the best possible performance, there are
certain grounding techniques to be followed. All input refer­ence grounds should be tied with their respective source
grounds and brought back to the power supply ground sepa­rately from the output load ground returns. Bringing the
ground returns for the output loads back to the supply sepa­rately will keep large signal currents from interfering with the
stable AC input ground references.

LAYOUT

As stated in the Grounding section, placement of ground re­turn lines is imperative in maintaining the highest level of
system performance. It is not only important to route the cor­rect ground return lines together, but also to be aware of
where the ground return lines are routed with respect to each
other. The output load ground returns should be physically
located as far as possible from low signal level lines and their
ground return lines. Critical signal lines are those relating to
the microphone amplifier section, since these lines generally
work at very low signal levels.

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. The
effect of a larger half supply bypass capacitor is improved
PSRR due to increased half-supply stability. Typical applica­tions employ a 5 volt regulator with 10 µF and a 0.1 µF by­pass capacitors which aid in supply stability, but do not elimi­nate the need for bypassing the supply nodes of the
LM4834. The selection of bypass capacitors, especially C

B

,
is thus dependant upon desired PSRR requirements, click
and pop performance as explained in the section, Proper
Selection of External Components, system cost, and size
constraints. It is also recommended to decouple each of the
V

DD

pins with a 0.1µF capacitor to ground.

PROPER SELECTION OF EXTERNAL COMPONENTS

Proper selection of external components in applications us­ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4834 is tolerant of
external component combinations, consideration to compo­nent values must be used to maximize overall system qual­ity.

The LM4834’s bridged amplifier should be used in low gain
configurations to minimize THD+N values, and maximize the
signal to noise ratio. Low gain configurations require large in­put signals to obtain a given output power. Input signals
equal to or greater than 1Vrms are available from sources
such as audio codecs.

Besides gain, one of the major considerations is the closed­loop bandwidth of the amplifier. To a large extent, the band­width is dictated by the choice of external components
shown in

Figure 1

. Both the input coupling capacitor, CI, and
the output coupling capacitor form first order high pass filters
which limit low frequency response given in Equations 4 and

5.

f

IC

= 1/(2πRiCi) (4)

f

OC

= 1/(2πRLCO) (5)

These values should be chosen based on required fre­quency response.

Selection of Input and Output Capacitor Size

Large input and output capacitors are both expensive and
space hungry for portable designs. Clearly, a certain sized
capacitor is needed to couple in low frequencies without se­vere attenuation. In many cases the speakers used in por­table systems, whether internal or external, have little ability
to reproduce signals below 100 Hz–150 Hz. In this case, us­inga large input or output capacitor may not increase system
performance.

In addition to system cost and size, click and pop perfor­mance is effected by the size of the input coupling capacitor,

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Application Information (Continued)

C

i

. A larger input coupling capacitor requires more charge to

reach its quiescent DC voltage (nominally 1/2 V

DD

.) This
charge comes from the output through the feedback and is
apt to create pops once the device is enabled. By minimizing
the capacitor size based on necessary low frequency re­sponse, turn-on pops can be minimized.

CLICK AND POP CIRCUITRY

The LM4834 contains circuitry to minimize turn-on transients
or “click and pops”. In this case, turn-on refers to either
power supply turn-on or the device coming out of shutdown
mode. When the device is turning on, the amplifiers are inter­nally configured as unity gain buffers. An internal current
source ramps up the voltage of the bypass pin. Both the in­puts and outputs ideally track the voltage at the bypass pin.
The device will remain in buffer mode until the bypass pin
has reached its half supply voltage, 1/2 V

DD

. As soon as the
bypass node is stable, the device will become fully opera­tional.

Although the bypass pin current source cannot be modified,
the size of the bypass capacitor,C

B

, can be changed to alter
the device turn-on time and the amount of “click and pop”. By
increasing C

B

, the amount of turn-on pop can be reduced.
However, the trade-off for using a larger bypass capacitor is
an increase in the turn-on time for the device. Reducing C

B

will decrease turn-on time and increase “click and pop”.
There is a linear relationship between the size of C

B

and the
turn-on time. Here are some typical turn-on times for differ­ent values of C

B

:

C

B

T

ON

0.01 µF 20 ms

0.1 µF 200 ms

0.22 µF 420 ms

0.47 µF 840 ms

1.0 µF 2 sec

In order to eliminate “click and pop”, all capacitors must be
discharged before turn-on. Rapid on/off switching of the de­vice or shutdown function may cause the “click and pop” cir­cuitry to not operate fully, resulting in increased “click and
pop” noise.

In systems where the line out and headphone jack are the
same, the output coupling cap, C

O

, is of particular concern.

C

O

is chosen for a desired cutoff frequency with a headphone
load. This desired cutoff frequency will change when the
headphone load is replaced by a high impedance line out
load(powered speakers). The input impedance of head­phones are typically between 32Ω and 64Ω. Whereas, the
input impedance of powered speakers can vary from 1kΩ
top 100kΩ. As the RC time constant of the load and the out­put coupling capacitor increases, the turn off transients are
increased.

To improve click and pop performance in this situation, exter­nal resistors R6 and R7 should be added. The recom­mended value for R6 is between 150Ω to 1kΩ. The recom­mended value for R7 is between 100Ω to 500Ω. To achieve
virtually clickless and popless performance R6 = 150Ω,R7=
100Ω,C

O

= 220µF, and CB= 0.47µF should be used. Lower
values of R6 will result in better click and pop performance.
However, it should be understood that lower resistance val­ues of R6 will increase quiescent current.

LOW FREQUENCY ENHANCEMENT

In some cases a designer may want to improve the low fre­quency response of the bridged amplifier.This low frequency
boost can be useful in systems where speakers are housed
in small enclosures. A resistor, R

LFE

, and a capacitor, C

LFE

,
in parallel, can be placed in series with the feedback resistor
of the bridged amplifier as seen in

Figure 5

.

At low frequencies the capacitor will be virtually an open cir­cuit. At high frequencies the capacitor will be virtually a short
circuit. As a result of this, the gain of the bridge amplifier is
increased at low frequencies. A first order pole is formed with
a corner frequency at:

f

c

= 1/(2πR

LFECLFE

)

The resulting low frequency differential gain of this bridged
amplifier becomes:

2(R

f+RLFE

)/Ri=A

vd

With RF= 20kΩ,R

LFE

= 20kΩ, and C

LFE

= 0.068 µF, a first
order pole is formed with a corner frequency of 120 Hz. At
low frequencies the differential gain will be 4, assuming R

S

=

20k. The low frequency boost formulas assume that C

O,Ci

,

f

IC,fOC

allow the appropriate low frequency response.

DS100015-33

FIGURE 4. Resistors for Varying Output Loads

DS100015-32

FIGURE 5. Low Frequency Enhancement

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Physical Dimensions inches (millimeters) unless otherwise noted

LIFE SUPPORT POLICY

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SSOP Package

Order Number LM4834MS

NS Package Number MSA028CB for SSOP

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