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LM4835
Stereo 2W Audio Power Amplifiers
with DC Volume Control and Selectable Gain
General Description
The LM4835 is a monolithic integrated circuit that provides
DC volume control, and stereo bridged audio power amplifiers capable of producing 2W into 4Ω (Note 1) with less than
1.0%THD or 2.2W into 3Ω (Note 2) with less than 1.0
%
THD.
Boomer
®
audio integrated circuits were designed specifically
to provide high quality audio while requiring a minimum
amount of external components. The LM4835 incorporates a
DC volume control, stereo bridged audio power amplifiers
and a selectable gain or bass boost, making it optimally
suited for multimedia monitors, portable radios, desktop, and
portable computer applications.
The LM4835 features an externally controlled, low-power
consumption shutdown mode, and both a power amplifier
and headphone mute for maximum system flexibility and
performance.
Note 1: When properly mounted to the circuit board, the LM4835MTE will
deliver 2W into 4Ω. TheLM4835MTwill deliver 1.1W into 8Ω. See the Application Information section for LM4835MTE usage information.
Note 2: An LM4835MTE which has been properly mounted to the circuit
board and forced-air cooled will deliver 2.2W into 3Ω.
Key Specifications
n POat 1%THD+N
into 3Ω (LM4835MTE) 2.2W(typ)
into 4Ω (LM4835MTE) 2.0W(typ)
into 8Ω (LM4835) 1.1W(typ)
n Single-ended mode - THD+N
at 85mW into 32Ω
1.0%(typ)
n Shutdown current 0.7µA(typ)
Features
n PC98 Compliant
n DC Volume Control Interface
n System Beep Detect
n Stereo switchable bridged/single-ended power amplifiers
n Selectable internal/external gain and bass boost
configurable
n “Click and pop” suppression circuitry
n Thermal shutdown protection circuitry
Applications
n Portable and Desktop Computers
n Multimedia Monitors
n Portable Radios, PDAs, and Portable TVs
Block Diagram Connection Diagram
Boomer®is a registered trademark of NationalSemiconductor Corporation.
DS100139-1
FIGURE 1. LM4835 Block Diagram
TSSOP Package
DS100139-2
Top View
Order Number LM4835MT
See NS Package Number MTC28 for TSSOP
Order Number LM4835MTE
See NS Package Number MXA28A for Exposed DAP
TSSOP
March 1999
LM4835 Stereo 2W Audio Power Amplifiers with DC Volume Control and Selectable Gain
© 1999 National Semiconductor Corporation DS100139 www.national.com
Page 2
Absolute Maximum Ratings (Note 10)
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 12) 2000V
ESD Susceptibility (Note 13) 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)— MTC28 20˚C/W
θ
JA
(typ)— MTC28 80˚C/W
θ
JC
(typ)— MXA28A 2˚C/W
θ
JA
(typ)— MXA28A (Note 4) 41˚C/W
θ
JA
(typ)— MXA28A (Note 3) 54˚C/W
θ
JA
(typ)— MXA28A (Note 5) 59˚C/W
θ
JA
(typ)— MXA28A (Note 6) 93˚C/W
Operating Ratings
Temperature Range
T
MIN
≤ TA≤T
MAX
−40˚C ≤TA ≤ 85˚C
Supply Voltage 2.7V≤ V
DD
≤ 5.5V
Electrical Characteristics for Entire IC
(Notes 7, 10)
The following specifications apply for V
DD
= 5V unless otherwise noted. Limits apply for TA= 25˚C.
Symbol Parameter Conditions
LM4835
Units
(Limits)
Typical
(Note 14)
Limit
(Note 15)
V
DD
Supply Voltage 2.7 V (min)
5.5 V (max)
I
DD
Quiescent Power Supply Current VIN= 0V, IO= 0A 15 30 mA (max)
I
SD
Shutdown Current V
pin 2=VDD
0.7 2.0 µA (max)
V
IH
Headphone Sense High Input Voltage 4 V (min)
V
IL
Headphone Sense Low Input Voltage 0.8 V (max)
Electrical Characteristics for Volume Attenuators
(Notes 7, 10)
The following specifications apply for VDD= 5V. Limits apply for TA= 25˚C.
Symbol Parameter Conditions
LM4835
Units
(Limits)
Typical
(Note 14)
Limit
(Note 15)
C
RANGE
Attenuator Range Gain with V
pin 7
=5V 0
±
0.5 dB (max)
Attenuation with V
pin 7
= 0V -81 -80 dB (min)
A
M
Mute Attenuation V
pin 5
= 5V, Bridged Mode -88 -80 dB (min)
V
pin 5
= 5V, Single-Ended Mode -88 -80 dB (min)
Electrical Characteristics for Single-Ended Mode Operation
(Notes 7, 10)
The following specifications apply for VDD= 5V. Limits apply for TA= 25˚C.
Symbol Parameter Conditions
LM4835
Units
(Limits)
Typical
(Note 14)
Limit
(Note 15)
P
O
Output Power THD = 1.0%; f = 1kHz; RL=32Ω 85 mW
THD=10%;f=1kHz; R
L
=32Ω 95 mW
THD+N Total Harmonic Distortion+Noise V
OUT
=1V
RMS
, f=1kHz, RL= 10kΩ,
A
VD
=1
0.065
%
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Electrical Characteristics for Single-Ended Mode Operation (Continued)
(Notes 7, 10)
The following specifications apply for VDD= 5V. Limits apply for TA= 25˚C.
Symbol Parameter Conditions
LM4835
Units
(Limits)
Typical
(Note 14)
Limit
(Note 15)
PSRR Power Supply Rejection Ratio C
B
= 1.0 µF, f =120 Hz,
V
RIPPLE
= 200 mVrms
58 dB
SNR Signal to Noise Ratio P
OUT
=75 mW, RL=32Ω, A-Wtd
Filter
102 dB
X
talk
Channel Separation f=1kHz, CB= 1.0 µF 65 dB
Electrical Characteristics for Bridged Mode Operation
(Notes 7, 10)
The following specifications apply for VDD= 5V, unless otherwise noted. Limits apply for TA= 25˚C.
Symbol Parameter Conditions
LM4835
Units
(Limits)
Typical
(Note 14)
Limit
(Note 15)
V
OS
Output Offset Voltage VIN= 0V 5 30 mV (max)
P
O
Output Power THD+N=1.0%; f=1kHz; RL=3Ω
(Note 8)
2.2 W
THD+N=1.0%; f=1kHz; R
L
=4Ω
(Note 9)
2W
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 mVrms; RL=8Ω
74 dB
SNR Signal to Noise Ratio V
DD
= 5V, P
OUT
= 1.1W, RL=8Ω,
A-Wtd Filter
93 dB
X
talk
Channel Separation f=1kHz, CB= 1.0 µF 70 dB
Note 3: The θJAgiven is for an MXA28A package whose exposed-DAP is soldered to an exposed 2in2piece of 1 ounce printed circuit board copper.
Note 4: The θ
JA
given is for an MXA28A package whose exposed-DAP is soldered to a 2in2piece of 1 ounce printed circuit board copper on a bottom side layer
through 21 8mil vias.
Note 5: The θ
JA
given is for an MXA28A package whose exposed-DAP is soldered to an exposed 1in2piece of 1 ounce printed circuit board copper.
Note 6: The θ
JA
given is for an MXA28A package whose exposed-DAP is not soldered to any copper.
Note 7: All voltages are measured with respect to the ground pins, unless otherwise specified. All specifications are tested using the typical application as shown
in
Figure 1
.
Note 8: When driving 3Ω loads from a 5V supply the LM4835MTE must be mounted to the circuit board and forced-air cooled.
Note 9: When driving 4Ω loads from a 5V supply the LM4835MTE must be mounted to the circuit board.
Note 10:
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 which
guarantee specific performance limits. This assumes that the device is within the Operating Ratings. Marshall Chiu feels there are better ways to obtain ″More Wattage in the Cottage.″ Specifications are not guaranteed for parameters where no limit is given, however, the typical value is a good indication of device performance.
Note 11: 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 LM4835MT,T
JMAX
= 150˚C, and the typical junction-to-ambient thermal resistance, when board
mounted, is 80˚C/W assuming the MTC28 package.
Note 12: Human body model, 100 pF discharged through a 1.5 kΩ resistor.
Note 13: Machine Model, 220 pF–240 pF discharged through all pins.
Note 14: Typicals are measured at 25˚C and represent the parametric norm.
Note 15: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).
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Typical Application
Truth Table for Logic Inputs
(Note 16)
Mute Mode HP Sense DC Vol. Control Bridged Output Single-Ended Output
0 0 0 Fixed Level Vol. Fixed _
0 0 1 Fixed Level Muted Vol. Fixed
0 1 0 Adjustable Vol. Changes _
0 1 1 Adjustable Muted Vol. Changes
1 X X _ Muted Muted
Note 16: If system beep is detected on the Beep In pin (pin 11), the system beep will be passed through the bridged amplifier regardless of the logic of the Mute
and HP sense pins.
DS100139-3
FIGURE 2. Typical Application Circuit
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Typical Performance Characteristics
MTE Specific Characteristics
Note 17: These curves show the thermal dissipation ability of the LM4835MTE at different ambient temperatures given these conditions:
500LFPM + 2in
2
: The part is soldered to a 2in2, 1 oz. copper plane with 500 linear feet per minute of forced-air flow across it.
2in
2
on bottom: The part is soldered to a 2in2, 1oz. copper plane that is on the bottom side of the PC board through 21 8 mil vias.
2in
2
: The part is soldered to a 2in2, 1oz. copper plane.
1in
2
: The part is soldered to a 1in2, 1oz. copper plane.
Not Attached: The part is not soldered down and is not forced-air cooled.
Non-MTE Specific Characteristics
LM4835MTE
THD+N vs Output Power
DS100139-70
LM4835MTE
THD+N vs Frequency
DS100139-71
LM4835MTE
THD+N vs Output Power
DS100139-72
LM4835MTE
THD+N vs Frequency
DS100139-73
LM4835MTE
Power Dissipation vs Output Power
DS100139-65
LM4835MTE(Note 17)
Power Derating Curve
DS100139-64
THD+N vs Frequency
DS100139-57
THD+N vs Frequency
DS100139-58
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Non-MTE Specific Characteristics (Continued)
THD+N vs Frequency
DS100139-14
THD+N vs Frequency
DS100139-15
THD+N vs Frequency
DS100139-16
THD+N vs Frequency
DS100139-17
THD+N vs Frequency
DS100139-18
THD+N vs Frequency
DS100139-19
THD+N vs Frequency
DS100139-20
THD+N vs Frequency
DS100139-21
THD+N vs Frequency
DS100139-22
THD+N vs Output Power
DS100139-24
THD+N vs Output Power
DS100139-25
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Non-MTE Specific Characteristics (Continued)
THD+N vs Output Power
DS100139-26
THD+N vs Output Power
DS100139-27
THD+N vs Output Power
DS100139-28
THD+N vs Output Power
DS100139-29
THD+N vs Output Power
DS100139-30
THD+N vs Output Power
DS100139-31
THD+N vs Output Power
DS100139-32
THD+N vs Output Power
DS100139-33
THD+N vs Output Power
DS100139-34
THD+N vs Output Voltage
Docking Station Pins
DS100139-59
THD+N vs Output Voltage
Docking Station Pins
DS100139-60
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Non-MTE Specific Characteristics (Continued)
Output Power vs
Load Resistance
DS100139-62
Output Power vs
Load Resistance
DS100139-6
Output Power vs
Load Resistance
DS100139-7
Power Supply
Rejection Ratio
DS100139-38
Dropout Voltage
DS100139-53
Output Power vs
Load Resistance
DS100139-8
Noise Floor
DS100139-41
Noise Floor
DS100139-42
Volume Control
Characteristics
DS100139-10
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Non-MTE Specific Characteristics (Continued)
Power Dissipation vs
Output Power
DS100139-51
Power Dissipation vs
Output Power
DS100139-52
External Gain/
Bass Boost
Characteristics
DS100139-61
Power Derating Curve
DS100139-63
Crosstalk
DS100139-49
Crosstalk
DS100139-50
Output Power
vs Supply voltage
DS100139-54
Output Power
vs Supply Voltage
DS100139-56
Supply Current
vs Supply Voltage
DS100139-9
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Application Information
EXPOSED-DAP MOUNTING CONSIDERATIONS
The exposed-DAP (die attach pad) must be tied to ground.
The exposed-DAP of the LM4835MTE requires special attention to thermal design. If thermal design issues are not
properly addressed, an LM4835MTE driving 4Ω will go into
thermal shutdown.
The exposed-DAP on the bottom of the LM4835MTE should
be soldered down to a copper plane on the circuit board. The
copper plane will conduct heat away from the exposed-DAP.
If the copper plane is not on the top surface of the circuit
board, 20 to 30 vias of 0.010 inches or smaller in diameter
should be used to thermally couple the exposed-DAP to the
plane. For good thermal conduction, the vias must be
plated-through and solder-filled.
The copper plane used to conduct heat away from the
exposed-DAP should be as large as practical. If the plane is
on the same side of the circuit board as the exposed-DAP, 2
in
2
is the minimum for 5V operation into 4Ω. If the heat sink
plane is buried or not on the same side as the exposed-DAP,
5in
2
is the minimum for 5V operation into 4Ω. If the ambient
temperature is higher than 25˚C, a larger copper plane or
forced-air cooling may be required to keep the LM4835MTE
junction temperature below the thermal shutdown temperature (150˚C). See the power derating curve for the
LM4835MTE for derating information.
The LM4835MTE requires forced-air cooling when operating
into 3Ω.
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 single-ended amplifier operating at a
given supply voltage and driving a specified load.
P
DMAX
=(VDD)2/(2π2RL) (1)
However, a direct consequence of the increased power delivered to the load by a bridged amplifier is an increase in internal power dissipation. Equation 2 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) (2)
Since theLM4835 is a stereo power amplifier, the maximum
internal power dissipation is two times that of Equation 1 or
Equation 2 depending on the mode of operation. Even with
the power dissipation of the stereo amplifiers, the LM4835
does not require heatsinking. The power dissipation from the
amplifiers, must not be greater than the package power dissipation that results from Equation 3:
P
DMAX
=(T
JMAX−TA
)/ θJA(3)
For the LM4835 TSSOP package, θ
JA
= 80˚C/W and T
JMAX
= 150˚C. Depending on the ambient temperature, TA,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 loads, the maximum ambient
temperature possible without violating the maximum junction
temperature is approximately 48˚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 Characteristics curves for
power dissipation information for different output powers.
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 groundreturn lines are routed with respect to each
other. The output load ground returns should be physically
located as faras possible from low signal level lines and their
ground return lines.
3Ω and 4Ω Layout Considerations
With low impedance loads, the output power at the loads is
heavily dependent on trace resistance from the output pins
of the LM4835. Tracesfrom the output of the LM4835MTE to
the load or load connectors should be as wide as practical.
Any resistance in the output traces will reduce the power delivered to the load. For example, with a 4Ω load and 0.1Ω of
trace resistance in each output, output power at the load
drops from 2W to 1.8W.
Output power is also dependent on supply regulation. To
keep the supply voltage from sagging under full output conditions, the supply traces should be as wide as practical.
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. The exposed-DAP of the
LM4835MTE package must be tied to ground.
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 shouldbe 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
LM4835. 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 LM4835 is tolerant of
external component combinations, consideration to component values must be used to maximize overall system quality.
The LM4835’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 in-
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Application Information (Continued)
put 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 3
. 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,
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 LM4835 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 thedevice is turning on, the amplifiers are internally muted. 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 mute
mode until the bypass pin has reached its half supply voltage, 1/2 V
DD
. As soon as the bypass node is stable, the de-
vice 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 2 ms
0.1 µF 20 ms
0.22 µF 42 ms
0.47 µF 84 ms
1.0 µF 200 ms
C
B
T
ON
4.7 µF 1sec
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 to
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 resistor R7 should be added as shown in Figure 4. The
recommended value for R7 is between 150Ω to 1kΩ.To
achieve virtually clickless and popless performance R7 =
150Ω,C
O
= 220µF, and CB= 1.0µF 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 R7 will increase current consumption.
DOCKING STATION
In an application such as a notebook computer,docking station or line level outputs may be required. Pin 9 and Pin 13
can drive loads greater than 1kΩ rail to rail. These pins are
tied to the output of the input op-amp to drive powered
speakers and other high impedance loads. Output coupling
capacitors need to be placed in series with the load. The recommended values of the capacitors are between 0.33µF to
1.0µF with the positive side of the capacitors toward the IC.
The outputs of the docking station pins cannot be attenuated
with the DC volume control. However the gain of the outputs
can be configured by adjusting the feedback and input resistors for the input op-amp. The input op-amp is in an inverting
configuration where the gain is:
R
F/Ri
=-A
v
Note that by adjusting the gain of the input op-amp the overall gain of the output amplifiers are also affected. Although
the single endedoutputs of the output amplifiers can be used
to drive line level outputs, it is recommended to use Pins 9
and 13 to achieve better performance.
DS100139-5
FIGURE 3. Resistor for Varying Output Loads
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Application Information (Continued)
BEEP DETECT FUNCTION
The Beep Detect pin (pin 11)is a mono input that detects the
presence of a beep signal. When a signal greater than
2.5V
P-P
(or 1/2 VDD) is present at pin 11, the Beep Detect circuitry will enable the bridged amplifiers. Beep in signals less
than 2.5V
P-P
(or 1/2 VDD) will not trigger the Beep Detect circuitry. When triggered, the Beep Detect circuitry will enable
the bridged amplifiers regardless of the state of the mute,
mode, or HP sense pins. As shown in the Fig. 2, a 200kΩ resistor is placed in series with the input capacitor. This 200kΩ
resistor can be changed to vary the amplitude of the beep in
signal. Higher values of the resistor will reduce the amplifier
gain and attenuate the beep in signal. These resistors are required in order for the beep signal to pass to the output. The
Beep Detect pin will not pass the beep signal to the output.
In cases where system beeps are required when the system
is in a suspended mode, the LM4835 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
LM4835 contains a shutdown pin to externally turn off the
bias circuitry.The LM4835 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 LM4835 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 0.7 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 LM4835. This scheme prevents
the shutdown pin from floating.
MODE FUNCTION
The LM4835 was designed to operate in two modes. In
mode 0 (lineout mode),where the mode pin (pin 4) is given a
logic level low, the attenuation of the outputs are fixed at
unity gain. In mode 1 (adjustable mode), where the mode pin
is given alogic level high, the attenuation of the amplifier outputs is controlled through the DC voltage at pin 7.
MUTE FUNCTION
By placing a logic level high on the mute pin (pin5), the outputs of the amplifiers and pins 9 and 13 will be muted. The
beep in signal will be output even if the LM4835 is muted.
The mute pin must not be floated.
HP SENSE FUNCTION
The LM4835 possesses a headphone sense pin (pin 21) 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 4
shows the implementation of the LM4835’s head-
phone control function using a single-supply. The voltage di-
vider of R1, R2, and R4 sets the voltage at the HP sense pin
(pin 21) to be approximately 50 mV when there are no headphones plugged into the system. This logic-low voltage at
the HP sense pin enables the bridged power amplifiers. 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.
Since the threshold of the HP sense pin is set at 4V ( or 80
%
V
DD
), the output swing cannot cause false triggering.
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, and R4. 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, whose impedance is in parallel with resistor R2 and
R3. Also shown in
Figure 4
are the electrical connections for
the headphone jack and plug. A3-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 LM4835 can be used to drive both a bridged 8Ω internal
speaker and a pair of 32Ω speakers without using the HP
sense circuit. In this case the HP sense is controlled by a microprocessor or a switch.
GAIN SELECT FUNCTION (Bass Boost)
External gain/internal gain can be toggled by changing the
logic at pin 3. A logic high will switch the power amplifiers to
external gain mode. In external gain mode the gain of the
amplifier is set by the external resistors. Whereas the internal gain mode sets the amplifiers to unity gain.
In some cases a designer may want to improve the low frequency response of the bridged amplifier or incorporate a
bass boost feature.This bass 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
.
DS100139-4
FIGURE 4. Headphone Sensing Circuit
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Page 13
Application Information (Continued)
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
=
20kΩ. The low frequency boost formulas assume that C
O
,
C
i,fIC,fOC
allow the appropriate low frequency response.
The bass boost feature is enabled/disable by toggling the
logic at pin 3.
DC VOLUME CONTROL
The DC voltage at the DC Volume Control pin (pin 7) determines the attenuation of output of the amplifiers. If the DC
potential of pin 7 is at 4V (or 80%V
DD
) the internal amplifiers
are set at unity gain. The attenuator range is from 0dB (pin7
=80%V
DD
) to -81dB (pin7 = 0V). Any DC voltage greater
than 4V (or 80%V
DD
) will result in a gain of unity. When the
mode pin is given a logic low,amplifiers will be fixed at a gain
of unity regardless of the voltage of pin 7. Refer to the Typi-
cal Performance Characteristics for detailed information of
the attenuation characteristics of the DC Volume Control pin.
DS100139-11
FIGURE 5. Low Frequency Enhancement
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Page 14
Physical Dimensions inches (millimeters) unless otherwise noted
TSSOP Package
Order Number LM4835MT
NS Package Number MTC28 for TSSOP
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Page 15
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
Exposed-DAP TSSOP Package
Order Number LM4835MTE
NS Package Number MXA28A for Exposed-DAP TSSOP
www.national.com15
Page 16
Notes
LIFE SUPPORT POLICY
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 OF NATIONAL
SEMICONDUCTOR CORPORATION. As used herein:
1. Life support devices or systems are devices or
systems which, (a) are intended for surgical implant
into the body, or (b) support or sustain life, and
whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.
2. A critical component is any component of a life
support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
safety or effectiveness.
National Semiconductor
Corporation
Americas
Tel: 1-800-272-9959
Fax: 1-800-737-7018
Email: support@nsc.com
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Japan Ltd.
Tel: 81-3-5639-7560
Fax: 81-3-5639-7507
www.national.com
LM4835 Stereo 2W Audio Power Amplifiers with DC Volume Control and Selectable Gain
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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