LM4834
LM4834 1.75W Audio Power Amplifier with DC Volume Control and Microphone
Preamp
Literature Number: SNAS004A
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 4Ωwith less than 1.0%
(THD). In addition, the headphone/lineout amplifier is ca-
pable of driving 70 mW into 32Ωwith less than 0.1%(THD).
The LM4834 incorporates a volume control and an input
microphone preamp stage capable of drivinga1kΩload
impedance.
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 desk-
top 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
nTHD at 1.1W continuous average output power into 8Ω
at 1kHz 0.5% (max)
nOutput Power into 4Ωat 1.0% THD+N 1.75W (typ)
nTHD at 70mW continuous average output power into
32Ωat 1kHz 0.1% (typ)
nShutdown Current 1.0µA (max)
nSupply Current 17.5mA (typ)
Features
nPC98 Compliant
n“Click and Pop” suppression circuitry
nStereo line level outputs with mono input capability for
system beeps
nMicrophone preamp with buffered power supply
nDC Volume Control Interface
nThermal shutdown protection circuitry
Applications
nMultimedia Monitors
nDesktop and Portable Computers
Block Diagram Connection Diagram
SSOP Package
10001502
Top View
Order Number LM4834MS
See NS Package Number MSA028CB for SSOP
Boomer®is a registered trademark of NationalSemiconductor Corporation.
10001501
FIGURE 1. LM4834 Block Diagram
August 2000
LM4834 1.75W Audio Power Amplifier with DC Volume Control and Microphone Preamp
© 2004 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
≤T
A
≤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 V
DD
= 5V unless otherwise noted. Limits apply for T
A
= 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 V
IN
= 0V, I
O
= 0A 17.5 26 mA (max)
I
SD
Shutdown Current V
pin13
=V
DD
0.6 2.0 µA (max)
Electrical Characteristics for Volume Attenuators
(Notes 1, 2)
The following specifications apply for V
DD
= 5V. Limits apply for T
A
= 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 T
A
= 25˚C.
Symbol Parameter Conditions
LM4834 Units
(Limits)
Typical
(Note 6)
Limit
(Note 7)
V
OS
Offset Voltage V
IN
= 0V 0.9 mV
SNR Signal to Noise Ratio V
DD
= 5V, R
L
=1k,f=1kHz, V
OUT
=
4.7V
, A-Wtd Filter
123 dB
V
SWING
Output Voltage Swing f = 1 kHz, THD <1.0%, R
L
=1kΩ4.72 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
LM4834
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Electrical Characteristics for Microphone Preamp and Power
Supply (Continued)
(Notes 1, 2)
The following specifications apply forV
DD
= 5V unless otherwise noted. Limits apply for T
A
= 25˚C.
Symbol Parameter Conditions
LM4834 Units
(Limits)
Typical
(Note 6)
Limit
(Note 7)
V
S
Mic Power Supply R
L
=1kΩ, Bias In = 2.5V 2.5 2.5 V (min)
Electrical Characteristics for Line/Headphone Amplifier
(Notes 1, 2)
The following specifications apply for V
DD
= 5V. Limits apply for T
A
= 25˚C.
Symbol Parameter Conditions
LM4834 Units
(Limits)
Typical
(Note 6)
Limit
(Note 7)
P
O
Output Power THD = 0.1%; f = 1kHz; R
L
=32Ω70 mW
THD=10%;f=1kHz; R
L
=32Ω95 mW
THD+N Total Harmonic Distortion+Noise V
OUT
=4V
P-P
,20Hz<f<20 kHz,
R
L
= 10kΩ,A
VD
=−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, R
L
=32Ω,
A-Wtd Filter
102 dB
Electrical Characteristics for Bridged Speaker Amplifer
(Notes 1, 2)
The following specifications apply for V
DD
= 5V, unless otherwise noted. Limits apply for T
A
= 25˚C.
Symbol Parameter Conditions
LM4834 Units
(Limits)
Typical
(Note 6)
Limit
(Note 7)
V
OS
Output Offset Voltage V
IN
= 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
=8Ω1.5 W
THD+N Total Harmonic Distortion+Noise P
O
= 1W, 20 Hz<f<20 kHz,
R
L
=8Ω,A
VD
=2
0.3 %
P
O
= 340 mW, R
L
=32Ω1.0 %
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, R
L
=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
functional, but do not guarantee specific performance limits. Electrical Characteristicsstate DC and AC 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 TJMAX,θJA, and the ambient temperature TA. The maximum
allowable power dissipation is PDMAX =(T
JMAX −T
A)/θJA.For the LM4834MS, TJMAX = 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 kΩresistor.
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).
LM4834
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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
10001503
FIGURE 2. Typical Application Circuit
LM4834
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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 f
c
= 1/(2πR
i
C
i
). 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
.
4. C
O
Output coupling capacitor which blocks the DC voltage at the amplifiers output. Forms a high pass filter
with R
L
at f
o
= 1/(2πR
L
C
O
).
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 R
F
at 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 R
S
for 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
THD+N vs Frequency
Bridged Power Amp
10001505 10001504
LM4834
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Typical Performance Characteristics (Continued)
THD+N vs Frequency
Bridge Power Amp
THD+N vs Frequency
Line Out/HP Amplifiers
10001506 10001507
THD+N vs Frequency
Line Out/HP Amplifiers
THD+N vs Frequency
Line Out/HP Amplifiers
10001508 10001509
THD+N vs Output Power
Bridged Power Amp
THD+N vs Output Power
Bridged Power Amp
10001511 10001510
LM4834
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Typical Performance Characteristics (Continued)
THD+N vs Output Power
Bridged Power Amp
THD+N vs Output Power
Line Out/HP Amplifiers
10001512 10001513
THD+N vs Output Power
Line Out/HP Amplifiers
THD+N vs Output Power
Line Out/HP Amplifiers
10001514 10001515
Output Power vs
Load Resistance
Bridged Power Amp
Output Power vs
Load Resistance
Line Out/HP Amplifiers
10001516 10001517
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Typical Performance Characteristics (Continued)
Volume Control
Characteristics
Noise Floor
Bridged Power Amp
10001525
10001519
Noise Floor
Line Out/HP Amp
Noise Floor
Mic Preamp
10001520 10001521
Power Supply
Rejection Ratio
Bridged Power Amp
Power Supply
Rejection Ratio
Line Out/HP Amplifiers
10001522 10001523
LM4834
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Typical Performance Characteristics (Continued)
Power Supply
Rejection Ratio
Mic Preamp
Power Dissipation vs
Output Power
Bridged Power Amp
10001524 10001518
Power Dissipation vs
Output Power
Line Out/HP Amplifiers
Low Frequency Enhancement
Characteristics
Bridged Power Amp
10001526 10001527
Power Derating Curve
Open Loop
Frequency Response
Bridged Power Amp
10001528 10001529
LM4834
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Typical Performance Characteristics (Continued)
Crosstalk
Line Out/HP Amplifiers
10001530
LM4834
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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 120kΩto 10kΩand 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 mini-
mized. 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 pro-
vides a quick, smooth transition into shutdown. Another so-
lution is to use a single-pole, single-throw switch in conjuc-
tion 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 head-
phone control function using a single-supply.The voltage
divider of R1, R2, R4, and R5 sets the voltage at the HP
sense pin (pin 16) to be approximately 50 mV when there are
no headphones plugged into the system. This logic-low volt-
age 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 ampli-
fier. 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 connec-
tions 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 8Ωinternal
speaker and a pair of 32Ωspeakers 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 Volume Control 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
10001531
FIGURE 3. Headphone Input Circuit
LM4834
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Application Information (Continued)
amplifiers are set at a gain of 1.4 (2.9dB). The attenuation of
the amplifiers increase until 0V is reached. The attenuator
range 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 micro-
phone network or to low level signal inputs. The mic amplifier
has enough output capability to drive a 1kΩload. A power
supply buffer is included for microphones which require ex-
ternal 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(V
DD
)
2
/(2π
2
R
L
) (1)
Along with the bridged amplifier, the LM4834 also incorpo-
rates two single-ended amplifiers. Equation 2 states the
maximum power dissipation point for a single-ended ampli-
fier operating at a given supply voltage and driving a speci-
fied load.
P
DMAX
=(V
DD
)
2
/(2π
2
R
L
) (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
−T
A
)/ θ
JA
(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 8Ωbridged load and 32Ωsingle 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 differ-
ent output powers.
GROUNDING
In order to achieve the best possible performance, there are
certain grounding techniques to be followed. All input refer-
ence grounds should be tied with their respective source
grounds and brought back to the power supply ground sepa-
rately from the output load ground returns. Bringing the
ground returns for the output loads back to the supply sepa-
rately 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 us-
ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4834 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
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 closed-
loop bandwidth of the amplifier. To a large extent, the band-
width is dictated by the choice of external components
shown in Figure 1. Both the input coupling capacitor, C
I
, 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/(2πR
i
C
i
) (4)
f
OC
= 1/(2πR
L
C
O
) (5)
These values should be chosen based on required fre-
quency 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
LM4834
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Application Information (Continued)
ability to reproduce signals below 100 Hz–150 Hz. In this
case, usinga large input or output capacitor may not in-
crease system performance.
In addition to system cost and size, click and pop perfor-
mance is effected by the size of the input coupling capacitor,
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 re-
sponse, 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 opera-
tional.
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 differ-
ent 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 head-
phones are typically between 32Ωand 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, ex-
ternal resistors R6 and R7 should be added. The recom-
mended value for R6 is between 150Ωto 1kΩ. The recom-
mended value for R7 is between 100Ωto 500Ω. To achieve
virtually clickless and popless performance R6 = 150Ω,R7=
100Ω,C
O
= 220µF, and C
B
= 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 val-
ues 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 fre-
quency boost can be useful in systems where speakers are
housed in small enclosures. A resistor, R
LFE
, and a capaci-
tor, C
LFE
, in parallel, can be placed in series with the feed-
back 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 ampli-
fier is increased at low frequencies. A first order pole is
formed with a corner frequency at:
f
c
= 1/(2πR
LFE
C
LFE
)
The resulting low frequency differential gain of this bridged
amplifier becomes:
2(R
f
+R
LFE
)/R
i
=A
vd
With R
F
= 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
,C
i
,
f
IC
,f
OC
allow the appropriate low frequency response.
10001533
FIGURE 4. Resistors for Varying Output Loads
10001532
FIGURE 5. Low Frequency Enhancement
LM4834
www.national.com13
Physical Dimensions inches (millimeters) unless otherwise noted
SSOP Package
Order Number LM4834MS
NS Package Number MSA028CB for SSOP
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.
For the most current product information visit us at www.national.com.
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NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS
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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.
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LM4834 1.75W Audio Power Amplifier with DC Volume Control and Microphone Preamp
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