LM4873
Dual 2.1W Audio Amplifier Plus Stereo Headphone
Function
General Description
The LM4873 is a dual bridge-connected audio power amplifier which, when connected to a 5V supply, will deliver 2.1W
toa4Ωload (Note 1) or 2.4W to a 3Ω load (Note 2)with less
than 1.0% THD+N. In addition, the headphone input pin allows the amplifiers to operate in single-ended mode to drive
stereo headphones. A Mux Control pin toggles between the
two stereo sets of amplifier inputs, allowing for two selectable amplifier closed-loop responses.
Boomer audio power amplifiers were designed specifically to
provide high quality output power from a surface mount
package while requiring few external components. To simplify audio system design, the LM4873 combines dual bridge
speaker amplifiers and stereo headphone amplifiers on one
chip.
The LM4873 features an externally controlled, low-power
consumption shutdown mode, a stereo headphone amplifier
mode, and thermal shutdown protection. It also utilizes circuitry to reduce “clicks and pops” during device turn-on.
Note 1: An LM4873MTE-1 which has been properly mounted to the circuit
board will deliver 2.1W into 4Ω. The other package options for the LM4873
will deliver 1.1W into 8Ω. See the Application Information section for
LM4873MTE-1 usage information.
Note 2: An LM4873MTE-1 which has been properly mounted to the circuit
board and forced-air cooled will deliver 2.4W into 3Ω.
Key Specifications
n POat 1% THD+N
into 3Ω (LM4873MTE-1) 2.4W(typ)
into 4Ω (LM4873MTE-1) 2.1W(typ)
into 4Ω (LM4873MTE) 1.9W(typ)
into 8Ω (LM4873) 1.1W(typ)
n Single-ended mode - THD+N
at 75mW into 32Ω
0.5%(max)
n Shutdown current 0.7µA(typ)
Features
n Input mux control and two separate inputs per channel
n Stereo headphone amplifier mode
n “Click and pop” suppression circuitry
n Thermal shutdown protection circuitry
n Exposed-DAP TSSOP and TSSOP packaging available
Applications
n Multimedia monitors
n Portable and desktop computers
n Portable audio systems
Typical Application
Boomer®is a registered trademark of National Semiconductor Corporation.
DS100993-31
*
Refer to the section Proper Selection of External Components, for a detailed discussion of CBsize.
FIGURE 1. Typical Audio Amplifier Application Circuit
April 2000
LM4873 Dual 2.1W Audio Amplifier Plus Stereo Headphone Function
© 2000 National Semiconductor Corporation DS100993 www.national.com
Connection Diagram
Connection Diagram
DS100993-30
Top View
Order Number LM4873MTE-1
See NS Package Number MXA28A for Exposed-DAP TSSOP
DS100993-2
Top View
Order Number LM4873MT, LM4873MTE
See NS Package Number MTC20 for TSSOP
See NS Package Number MXA20A for Exposed-DAP TSSOP
LM4873
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Absolute Maximum Ratings (Note 4)
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 (Note 14) Internally limited
ESD Susceptibility (Note 15) 2000V
ESD Susceptibility (Note 16) 200V
Junction Temperature 150˚C
Solder 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 Reliablilty” for other methods of soldering surface
mount devices.
Thermal Resistance
θ
JC
(typ)—M16B 20˚C/W
θ
JA
(typ)—M16B 80˚C/W
θ
JC
(typ)—N16A 20˚C/W
θ
JA
(typ)—N16A 63˚C/W
θ
JC
(typ)—MTC20 20˚C/W
θ
JA
(typ)—MTC20 80˚C/W
θ
JC
(typ)—MXA20A 2˚C/W
θ
JA
(typ)—MXA20A 41˚C/W (Note 5)
θ
JA
(typ)—MXA20A 51˚C/W (Note 6)
θ
JA
(typ)—MXA20A 90˚C/W (Note 7)
θ
JC
(typ)—MXA28A 2˚C/W
θ
JA
(typ)—MXA28A 41˚C/W (Note 8)
θ
JA
(typ)—MXA28A 51˚C/W (Note 9)
θ
JA
(typ)—MXA28A 90˚C/W (Note 10)
Operating Ratings
Temperature Range
T
MIN
≤ TA≤ T
MAX
−40˚C ≤ TA≤ 85˚C
Supply Voltage 2.0V ≤ V
DD
≤ 5.5V
Electrical Characteristics for Entire IC (Notes 3, 4)
The following specifications apply for VDD= 5V unless otherwise noted. Limits apply for TA= 25˚C.
Symbol Parameter Conditions LM4873 Units
(Limits)
Typical Limit
(Note 17) (Note 18)
V
DD
Supply Voltage 2 V (min)
5.5 V (max)
I
DD
Quiescent Power Supply Current VIN= 0V, IO= 0A (Note 19) , HP-IN = 0V 7.5 15 mA (max)
V
IN
= 0V, IO= 0A (Note 19) , HP-IN = 4V 5.8 6 mA (min)
I
SD
Shutdown Current V
PIN1=VDD
0.7 2 µA (min)
V
IH
Headphone High Input Voltage 4 V (min)
V
IL
Headphone Low Input Voltage 0.8 V (max)
Electrical Characteristics for Bridged-Mode Operation (Notes 3, 4)
The following specifications apply for VDD= 5V unless otherwise specified. Limits apply for TA= 25˚C.
Symbol Parameter Conditions LM4873 Units
(Limits)
Typical Limit
(Note
17)
(Note
18)
V
OS
Output Offset Voltage VIN= 0V 5 50 mV (max)
P
O
Output Power (Note 13) THD = 1%, f = 1 kHz
LM4873MTE-1, R
L
=3Ω(Note 11)
2.4 W
LM4873MTE, R
L
=3Ω(Note 11) 2.2 W
LM4873MTE-1, R
L
=4Ω(Note 12) 2.1 W
LM4873MTE, R
L
=4Ω(Note 12) 1.9 W
LM4873, R
L
=8Ω 1.1 1.0 W (min)
THD+N = 10%, f = 1 kHz
LM4873MTE-1, R
L
=3Ω(Note 11) 3.0 W
LM4873MTE-1, R
L
=4Ω(Note 12) 2.6
LM4873, R
L
=8Ω 1.5 W
THD+N = 1%, f = 1 kHz, R
L
=32Ω 0.34 W
LM4873
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Electrical Characteristics for Bridged-Mode Operation (Notes 3, 4) (Continued)
The following specifications apply for VDD= 5V unless otherwise specified. Limits apply for TA= 25˚C.
Symbol Parameter Conditions LM4873 Units
(Limits)
Typical Limit
(Note
17)
(Note
18)
THD+N Total Harmonic Distortion+Noise 20 Hz ≤ f ≤ 20 kHz, A
VD
=2
LM4873MTE-1, R
L
=4Ω,PO=2W
0.3
LM4873, R
L
=8Ω,PO= 1W 0.3 %
PSRR Power Supply Rejection Ratio V
DD
= 5V, V
RIPPLE
= 200 mV
RMS,RL
=8Ω,
C
B
= 1.0 µF
67 dB
X
TALK
Channel Separation f = 1 kHz, CB= 1.0 µF 80 dB
SNR Signal To Noise Ratio V
DD
= 5V, PO= 1.1W, RL=8Ω 97 dB
Electrical Characteristics for Single-Ended Operation (Notes 3, 4)
The following specifications apply for VDD= 5V unless otherwise specified. Limits apply for TA= 25˚C.
Symbol Parameter Conditions LM4873 Units
(Limits)
Typical Limit
(Note
17)
(Note
18)
V
OS
Output Offset Voltage VIN= 0V 5 50 mV (max)
P
O
Output Power THD = 0.5%, f = 1 kHz, RL=32Ω 85 75 mW (min)
THD+N = 1%, f = 1 kHz, R
L
=8Ω 340 mW
THD+N = 10%, f = 1 kHz, R
L
=8Ω 440 mW
THD+N Total Harmonic Distortion+Noise A
V
= −1, PO= 75 mW, 20 Hz ≤ f ≤ 20 kHz,
R
L
=32Ω
0.2 %
PSRR Power Supply Rejection Ratio C
B
= 1.0 µF, V
RIPPLE
= 200 mV
RMS
,
f=1kHz
52 dB
X
TALK
Channel Separation f = 1 kHz, CB= 1.0 µF 60 dB
SNR Signal To Noise Ratio V
DD
= 5V, PO= 340mW, RL=8Ω 94 dB
Note 3: All voltages are measured with respect to the ground pins, 2, 7, and 15, unless otherwise specified.
Note 4: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is func-
tional, but do not guarantee specific performance limits. Electrical Characteristics state DC andAC electrical specifications under particular test conditions which guarantee specific performance limits. This assumes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters where no limit is
given, however, the typical value is a good indication of device performance.
Note 5: The θ
JA
given is for an MXA20A package whose exposed-DAP is soldered to an exposed 2in2piece of 1 ounce printed circuit board copper.
Note 6: The θ
JA
given is for an MXA20A package whose exposed-DAP is soldered to an exposed 1in2piece of 1 ounce printed circuit board copper.
Note 7: The θ
JA
given is for an MXA20A package whose exposed-DAP is not soldered to any copper.
Note 8: The θ
JA
given is for an MXA28A package whose exposed-DAP is soldered to an exposed 2in2piece of 1 ounce printed circuit board copper.
Note 9: 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 10: The θ
JA
given is for an MXA28A package whose exposed-DAP is not soldered to any copper.
Note 11: When driving 3Ω loads from a 5V supply, the LM4873MTE or LM4873MTE-1 must be mounted to the circuit board and forced-air cooled (450 linear-feet
per minute).
Note 12: When driving 4Ω loads from a 5V supply, the LM4873MTE or LM4873MTE-1 must be mounted to the circuit board.
Note 13: Output power is measured at the device terminals.
Note 14: 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 LM4873, T
JMAX
= 150˚C. For the θJAs for different packages, please see the Application Informa-
tion section or the Absolute Maximum Ratings section.
Note 15: Human body model, 100 pF discharged through a 1.5 kΩ resistor.
Note 16: Machine model, 220 pF–240 pF discharged through all pins.
Note 17: Typicals are measured at 25˚C and represent the parametric norm.
Note 18: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).
Note 19: The quiescent power supply current depends on the offset voltage when a practical load is connected to the amplifier.
LM4873
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Truth Table for Logic Inputs
SHUTDOWN HP-IN INPUT
SELECT
LM4873 MODE (INPUT #)
Low Low Low Bridged (1)
Low Low High Bridged (2)
Low High Low Single-Ended (1)
Low High High Single-Ended (2)
High X X Shutdown
LM4873
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External Components Description
(
Figure 1
)
Components Functional Description
1. R
i
Inverting input resistance which sets the closed-loop gain in conjunction with Rf. This resistor also forms a
high pass filter with C
i
at fc= 1/(2πRiCi).
2. C
i
Input coupling capacitor which blocks the DC voltage at the amplifier’s input terminals. Also creates a
highpass 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
.
3. R
f
Feedback resistance which sets the closed-loop gain in conjunction with Ri.
4. 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.
5. 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
.
Typical Performance Characteristics
MTE (20 pin)Specific Characteristics
Note 20: These curves show the thermal dissipation ability of the LM4873MTE at different ambient temperatures given these conditions:
500LFPM + JEDEC board: The part is soldered to a 1S2P 20-lead exposed-DAP TSSOP test board with 500 linear feet per minute of forced-air flow across
it. Board information - copper dimensions: 74x74mm, copper coverage: 100% (buried layer) and 12% (top/bottom layers), 16 vias under the exposed-DAP.
500LFPM + 2.5in
2
: The part is soldered to a 2.5in2, 1 oz. copper plane with 500 linear feet per minute of forced-air flow across it.
2.5in
2
: The part is soldered to a 2.5in2, 1oz. copper plane.
Not Attached: The part is not soldered down and is not forced-air cooled.
LM4873MTE
THD+N vs Output Power
DS100993-33
LM4873MTE
THD+N vs Frequency
DS100993-34
LM4873MTE
THD+N vs Frequency
DS100993-36
LM4873MTE
Power Dissipation vs Power Output
DS100993-90
LM4873MTE(Note 20)
Power Derating Curve
DS100993-95
LM4873
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Typical Performance Characteristics
MTE-1 (28 pin) Specific Characteristics
Note 21: 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
LM4873MTE-1
THD+N vs Output Power
DS100993-97
LM4873MTE-1
THD+N vs Frequency
DS100993-99
LM4873MTE-1
THD+N vs Output Power
DS100993-96
LM4873MTE-1
THD+N vs Frequency
DS100993-98
LM4873MTE-1
Power Dissipation vs Power Output
DS100993-90
LM4873MTE-1(Note 21)
Power Derating Curve
DS100993-A0
THD+N vs Frequency
DS100993-3
THD+N vs Frequency
DS100993-4
THD+N vs Frequency
DS100993-5
LM4873
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Non-MTE Specific Characteristics (Continued)
THD+N vs Output Power
DS100993-6
THD+N vs Output Power
DS100993-7
THD+N vs Output Power
DS100993-8
THD+N vs Output Power
DS100993-87
THD+N vs Frequency
DS100993-89
THD+N vs Output Power
DS100993-86
THD+N vs Frequency
DS100993-88
Output Power vs
Load Resistance
DS100993-84
Power Dissipation vs
Supply Voltage
DS100993-85
Output Power vs
Supply Voltage
DS100993-9
Output Power vs
Supply Voltage
DS100993-10
Output Power vs
Supply Voltage
DS100993-11
LM4873
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Non-MTE Specific Characteristics (Continued)
Output Power vs
Load Resistance
DS100993-12
Output Power vs
Load Resistance
DS100993-13
Power Dissipation vs
Output Power
DS100993-14
Dropout Voltage vs
Supply Voltage
DS100993-15
Power Derating Curve
DS100993-16
Power Dissipation vs
Output Power
DS100993-17
Noise Floor
DS100993-18
Channel Separation
DS100993-19
Channel Separation
DS100993-20
LM4873
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Non-MTE Specific Characteristics (Continued)
Application Information
PIN OUT COMPATIBILITY WITH THE LM4863
The LM4873 pin out was designed to simplify replacing the
LM4863: except for the four bottom pins, which the implement the LM4873’s extra functionality, the LM4873MT/MTE
and LM4863MT/MTE pin outs match.(Note 22)
Note 22: If the LM4873 replaces an LM4863 and the input mux circuitry is
not being used, the LM4873 Mux Control pin must be tied to V
DD
or GND.
INPUT MUX
The has two inputs per channel. The Mux Control pin controls which input is active. As shown in the Truth Table for
Logic Inputs, if the Mux Control is held low,input 1 is active.
If the Mux Control is held high, input 2 is active.
Figure 2
shows an example usage of the Mux Control circuit.
Mux input 1 is connected to a feedback network that increases gain at low frequencies (bass boost). Mux input 2 is
connected to a simple gain circuit. The example circuit has
mux input 1 used to equalize the internal speaker and mux
input 2 used for line-out or headphone driving. In this case,
the Mux Control and HP In pins would be tied together, so
that when the headphone was plugged in, the feedback network would automatically be changed. If the HP In and Mux
Control pins are not connected, the example circuit be used
for user-selectable bass-boost, so that independent of the
HP In state, the user could select bass-boost.
Since the Mux Control switches between the two inverting inputs of the amplifier, thereby changing the input signal
source or the feedback network, an audible click may be
generated during the transition from one mux input to the
other. For example, in the above example circuit, if the two
gains are markedly different, then, when a transition is made
between mux states, a click may be heard as the feedback
network, and therefore the gain, is suddenly changed.
EXPOSED-DAP MOUNTING CONSIDERATIONS
The exposed-DAP package of the LM4873MTE requires
special attention to thermal design. If thermal design issues
are not properly addressed, an LM4873MTE driving 4Ω will
go into thermal shutdown.
The exposed-DAP on the bottom of the LM4873MTE should
be soldered down to a copper pad on the circuit board. Heat
is conducted away from the exposed-DAP by a copper
plane. If the copper plane is not on the top surface of the circuit board, 8 to 10 vias of 0.013 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 pratical. If the plane is
on the same side of the circuit board as the exposed-DAP,
2.5in
2
is the minimum for 5V operation into 4Ω. If the heat
sink plane is buried or not on the same side as the exposedDAP, 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 will be required to keep the
LM4873MTE junction temperature below the thermal shutdown temperature (150˚C). See the power derating curve for
the LM4873MTE for derating information.
The LM4873MTE requires forced-air cooling when operating
into 3Ω. With the part attached to 2.5in
2
of exposed copper,
with a 3Ω load, and with an ambient temperature of 25˚C,
450 linear-feet per minute kept the part out of thermal shutdown. In higher ambient temperatures, higher airflow rates
and/or larger copper areas will be required to keep the part
out of thermal shutdown.
See DEMOBOARD CIRCUIT LAYOUTfor an example of an
exposed-DAP TSSOP circuit board layout.
3Ω &4ΩLAYOUT CONSIDERATIONS
With low impedance loads, the output power at the loads is
heavily dependent on trace resistance from the output pins
of the LM4873. Traces from the output of the LM4873MTE to
the load or load connectors should be as wide as practical.
Any resistance in the output traces will reduce the power de-
Power Supply
Rejection Ratio
DS100993-21
Open Loop
Frequency Response
DS100993-22
Supply Current vs
Supply Voltage
DS100993-23
DS100993-70
FIGURE 2. Input Mux Example
LM4873
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Application Information (Continued)
livered 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 2.1W to 2.0W
Output power is also dependent on supply regulation. To
keep the supply voltage from sagging under full output
power conditions, the supply traces should be as wide as
practical.
BRIDGE CONFIGURATION EXPLANATION
As shown in
Figure 1
, the LM4873 has two pairs of operational amplifiers internally, allowing for a few different amplifier configurations. The first amplifier’s gain is externally configurable, while the second amplifier is internally fixed in a
unity-gain, inverting configuration. The closed-loop gain of
the first amplifier is set by selecting the ratio of R
f
to Riwhile
the second amplifier’s gain is fixed by the two internal 20 kΩ
resistors.
Figure 1
shows that the output of amplifier one
serves as the input to amplifier two which results in both amplifiers producing signals identical in magnitude, but out of
phase 180˚. Consequently, the differential gain for each
channel of the IC is
A
VD
=2*(Rf/Ri)
By driving the load differentially through outputs +OutA and
−OutA or +OutB and −OutB, an amplifier configuration commonly referred to as “bridged mode” is established. Bridged
mode operation is different from the classical single-ended
amplifier configuration where one side of its load is connected to ground.
A bridge amplifier design has a few distinct advantages over
the single-ended configuration, as it provides differential
drive to the load, thus doubling the output swing for a specified supply voltage. Four times the output power is possible
as compared to a single-ended amplifier under the same
conditions. This increase in attainable output power assumes that the amplifier is not current limited or clipped. In
order to choose an amplifier’s closed-loop gain without causing excessive clipping, please refer to the Audio Power Am-
plifier Design section.
A bridge configuration, such as the one used in LM4873,
also creates a second advantage over single-ended amplifiers. Since the differential outputs, +OutA, −OutA, +OutB,
and −OutB, are biased at half-supply, no net DC voltage exists across the load. This eliminates the need for an output
coupling capacitor which is required in a single supply,
single-ended amplifier configuration. If an output coupling
capacitor is not used in a single-ended configuration, the
half-supply bias across the load would result in both increased internal IC power dissipation as well as permanent
loudspeaker damage.
POWER DISSIPATION
Whether the power amplifier is bridged or single-ended,
power dissipation is a major concern when designing the
amplifier. 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): Single-Ended (1)
However, a direct consequence of the increased power delivered to the load by a bridge amplifier is an increase in internal power dissipation. Equation 2 states the maximum
power dissipation point for a bridge amplifier operating at the
same given conditions.
P
DMAX
=4*(VDD)2/(2π2RL): Bridge Mode (2)
Since the LM4873 is a dual channel power amplifier, the
maximum internal power dissipation is 2 times that of Equation 1 or Equation 2 depending on the mode of operation.
Even with this substantial increase in power dissipation, the
LM4873 does not require heatsinking. The power dissipation
from Equation 2, assuming a 5V power supply and an 8Ω
load, must not be greater than the power dissipation that results from Equation 3:
P
DMAX
=(T
JMAX−TA
)/θ
JA
(3)
For packages M16A and MTC20, θ
JA
= 80˚C/W, and for
package N16A, θ
JA
= 63˚C/W. T
JMAX
= 150˚C for the
LM4873. 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 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, 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 point and
assuming surface mount packaging. Internal power dissipation is a function of output power. If typical operation is not
around the maximum power dissipation point, the ambient
temperature can be increased. Refer to the Typical Perfor-
mance Characteristics curves for power dissipation information for different output powers.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply rejection. The capacitor location on both the bypass and power
supply pins should be as close to the device as possible. The
effect of a larger half supply bypass capacitor is improved
PSRR due to increased half-supply stability. Typical applications employ a 5V regulator with 10 µF and a 0.1 µF bypass
capacitors which aid in supply filtering. This does not eliminate the need for bypassing the supply nodes of the
LM4873. The selection of bypass capacitors, especially C
B
,
is thus dependent upon desired PSRR requirements, click
and pop performance as explained in the section, Proper
Selection of External Components, system cost, and size
constraints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4873 contains a shutdown pin to externally turn off the
amplifier’s bias circuitry.This shutdown feature turns the amplifier off 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 LM4873 supply current draw will be minimized in idle mode. 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. In either case, the shutdown pin should be tied to a
definite voltage to avoid unwanted state changes.
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 conjunction
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 re-
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Application Information (Continued)
sistor will disable the LM4873. This scheme guarantees that
the shutdown pin will not float, thus preventing unwanted
state changes.
HP-IN FUNCTION
The LM4873 possesses a headphone control pin that turns
off the amplifiers which drive +OutA and +OutB so that
single-ended operation can occur and a bridged connected
load is muted. Quiescent current consumption is reduced
when the IC is in this single-ended mode.
Figure 3
shows the implementation of the LM4873’s headphone control function using a single-supply headphone amplifier. The voltage divider of R1 and R2 sets the voltage at
the HP-IN pin (pin 16) to be approximately 50 mV when there
are no headphones plugged into the system. This logic-low
voltage at the HP-IN pin enables the LM4873 and places it in
bridged mode operation. Resistor R4 limits the amount of
current flowing out of the HP-IN pin when the voltage at that
pin goes below ground resulting from the music coming from
the headphone amplifier.The output coupling capacitors protect the headphones by blocking the amplifier’s half supply
DC voltage.
When there are no headphones plugged into the system and
the IC is in bridged mode configuration, both loads are essentially at a 0V DC potential. Since the HP-IN threshold is
set at 4V, even in an ideal situation, the output swing cannot
cause a false single-ended trigger.
When a set of headphones are plugged into the system, the
contact pin of the headphone jack is disconnected from the
signal pin, interrupting the voltage divider set up by resistors
R1 and R2. Resistor R1 then pulls up the HP-IN pin, enabling the headphone function. This disables the second
side of the amplifier thus muting the bridged speakers. The
amplifier then drives the headphones, whose impedance is
in parallel with resistors R2 and R3. Resistors R2 and R3
have negligible effect on output drive capability since the
typical impedance of headphones are 32Ω. 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
Sleave, where the Tip and Ring are signal carrying conductors and the Sleave is the common ground return. One control pin contact for each headphone jack is sufficient to indicate to control inputs that the user has inserted a plug into a
jack and that another mode of operation is desired.
The LM4873 can be used to drive both a pair of bridged 8Ω
speakers and a pair of 32Ω headphones without using the
HP-IN pin. In this case the HP-IN would not be connected to
the headphone jack but to a microprocessor or a switch. By
enabling the HP-IN pin, the 8Ω speakers can be muted.
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 LM4873 is tolerant to a
variety of external component combinations, consideration
to component values must be used to maximize overall system quality.
The LM4873 is unity-gain stable, giving the designer maximum system performance. The LM4873 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 1 Vrms are available
from sources such as audio codecs. Please refer to the section, Audio Power Amplifier Design, for a more complete
explanation of proper gain selection.
Besides gain, one of the major considerations is the
closed-loop bandwidth of the amplifier. To a large extent, the
bandwidth is dictated by the choice of external components
shown in
Figure 1
. The input coupling capacitor, Ci, forms a
first order high pass filter which limits low frequency response. This value should be chosen based on needed frequency response for a few distinct reasons.
DS100993-24
FIGURE 3. Headphone Circuit
LM4873
www.national.com 12
Application Information (Continued)
CLICK AND POP CIRCUITRY
The LM4873 contains circuitry to minimize turn-on transients
or “clicks 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, where the gain is set by the external resistors.
Although the bypass pin current source cannot be modified,
the size of C
B
can be changed to alter the device turn-on
time and the amount of “clicks and pops”. By increasing
amount of turn-on pop can be reduced. However, the
tradeoff for using a larger bypass capacitor is an increase in
turn-on time for this device. There is a linear relationship between the size of C
B
and the turn-on time. Here are some
typical turn-on times for a given 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 eliminate “clicks and pops”, all capacitors must be
discharged before turn-on. Rapid on/off switching of the device or the shutdown function may cause the “click and pop”
circuitry to not operate fully, resulting in increased “click and
pop” noise. In a single-ended configuration, the output coupling capacitor, C
O
, is of particular concern. This capacitor
discharges through the internal 20 kΩ resistors. Depending
on the size of C
O
, the time constant can be relatively large.
To reduce transients in single-ended mode, an external
1kΩ–5 kΩresistor can be placed in parallel with the internal
20 kΩ resistor. The tradeoff for using this resistor is an increase in quiescent current.
The value of C
I
will also reflect turn-on pops. Clearly, a cer-
tain size for C
I
is needed to couple in low frequencies without
excessive attenuation. But in many cases, the speakers
used in portable systems, whether integral or external, have
little ability to reproduce signals below 100 Hz to 150 Hz. In
this case, using a large input and output capacitor may not
increase system performance. In most cases, choosing a
small value of C
I
in the range of 0.1 µF to 0.33 µF), along
with C
B
equal to 1.0 µF should produce a virtually clickless
and popless turn-on. In cases where C
I
is larger than
0.33 µF,it may be advantageous to increase the value of C
B
.
Again, it should be understood that increasing the value of
C
B
will reduce the “clicks and pops” at the expense of a
longer device turn-on time.
LM4873
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Application Information (Continued)
NO-LOAD DESIGN CONSIDERATIONS
If the outputs of the LM4873 have a load higher than 10kΩ,
the LM4873 may show a small oscillation at high output levels. To prevent this oscillation, place 5kΩ resistors from the
power outputs to ground.
AUDIO POWER AMPLIFIER DESIGN
Design a 1W/8Ω Bridged Audio Amplifier
Given:
Power Output: 1 Wrms
Load Impedance: 8Ω
Input Level: 1 Vrms
Input Impedance: 20 kΩ
Bandwidth: 100 Hz−20 kHz
±
0.25 dB
A designer must first determine the minimum supply rail to
obtain the specified output power. By extrapolating from the
Output Power vs Supply Voltage graphs in the Typical Per-
formance Characteristics section, the supply rail can be
easily found. A second way to determine the minimum supply rail is to calculate the required V
opeak
using Equation 3
and add the dropout voltage. Using this method, the minimum supply voltage would be (V
opeak
+(2*Vod)), where V
od
is extrapolated from the Dropout Voltage vs Supply Voltage
curve in the Typical Performance Characteristics section.
(4)
Using the Output Power vs Supply Voltage graph for an 8Ω
load, the minimum supply rail is 3.9V.But since 5V is a standard supply voltage in most applications, it is chosen for the
supply rail. Extra supply voltage creates headroom that allows the LM4873 to reproduce peaks in excess of 1W without producing audible distortion. At this time, the designer
must make sure that the power supply choice along with the
output impedance does not violate the conditions explained
in the Power Dissipation section.
Once the power dissipation equations have been addressed,
the required differential gain can be determined from Equation 4.
(5)
R
f/Ri=AVD
/2 (6)
From equation 4, the minimum A
VD
is 2.83; use AVD=3
Since the desired input impedance was 20 kΩ, and with a
A
VD
of 3, a ratio of 1.5:1 of Rfto Riresults in an allocation of
R
i
=20kΩand Rf=30kΩ. The final design step is to ad-
dress the bandwidth requirements which must be stated as a
pair of −3 dB frequency points. Five times away from a pole
gives 0.17 dB down from passband response, which is better
than the required
±
0.25 dB specified.
f
L
= 100 Hz/5 = 20 Hz
f
H
= 20 kHz x 5 = 100 kHz
As stated in the External Components section, R
i
in con-
junction with C
i
create a highpass filter.
Ci≥ 1/(2π*20 kΩ*20 Hz) = 0.397 µF; use 0.33 µF
The high frequency pole is determined by the product of the
desired high frequency pole, f
H
, and the differential gain, A
VD
. With a AVD= 3 and fH= 100 kHz, the resulting GBWP =
150 kHz which is much smaller than the LM4873 GBWP of
3.5 MHz. This figure displays that if a designer has a need to
design an amplifier with a higher differential gain, the
LM4873 can still be used without running into bandwidth
problems.
DEMOBOARD CIRCUIT LAYOUT
The demoboard circuit layout is provided here as an example of a circuit using the LM4873. If an LM4873MTE is
used with this layout, the exposed-DAP is soldered down to
the copper pad beneath the part. Heat is conducted away
from the part by the two large copper pads in the upper corners of the demoboard.
This demoboard provides enough heat dissipation ability to
allow an LM4873MTE to output 1.9W into 4Ω at 25˚C.
DS100993-93
Silk Screen Layer
DS100993-91
Component-side Copper Layers
LM4873
www.national.com 14
Application Information (Continued)
DS100993-92
Solder-side Copper Layers
LM4873
www.national.com15
Physical Dimensions inches (millimeters) unless otherwise noted
20-Lead MOLDED PKG, TSSOP, JEDEC, 4.4mm BODY WIDTH
Order Number LM4873MT
NS Package Number MTC20
LM4873
www.national.com 16
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
20-Lead MOLDED TSSOP, EXPOSED PAD, 6.5x4.4x0.9mm
Order Number LM4873MTE
NS Package Number MXA20A
LM4873
www.national.com17
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
28-Lead MOLDED TSSOP, EXPOSED PAD, 9.7x4.4x0.9mm
Order Number LM4873MTE-1
NS Package Number MXA28A
LM4873
www.national.com 18
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 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
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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Europe
Fax: +49 (0) 180-530 85 86
Email: europe.support@nsc.com
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Response Group
Tel: 65-2544466
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Email: ap.support@nsc.com
National Semiconductor
Japan Ltd.
Tel: 81-3-5639-7560
Fax: 81-3-5639-7507
www.national.com
LM4873 Dual 2.1W Audio Amplifier Plus Stereo Headphone Function
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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