NSC LM4873

LM4873
Dual 2.1W Audio Amplifier Plus Stereo Headphone
Function
General Description
Key Specifications
The LM4873 is a dual bridge-connected audio power amplifier which, when connected to a 5V supply, will deliver 2.1W
to a 4Ω load (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 when
driving stereo headphones. A MUX control pin allows selection between the two stereo sets of amplifier inputs. The
MUX control can also be used to select two different
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.
j PO at 1% THD+N
LM4873LQ, 3Ω, 4Ω loads
2.4W(typ), 2.1W(typ)
LM4873MTE-1, 3Ω, 4Ω loads 2.4W(typ), 2.1W(typ)
LM4873IBL, 8Ω load
1.1W(typ)
LM4873MTE, 4Ω
1.9W(typ)
LM4873, 8Ω
1.1W(typ)
j Single-ended mode THD+N
0.5%(max)
at 75mW into 32Ω
j Shutdown current
0.7µA(typ)
j Supply voltage range
2V to 5.5V
Features
Note 1: An LM4873MTE-1, LM4873MTE, or LM4873LQ that has been
properly mounted to a 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 sections for further information concerning the
LM4873MTE-1, LM4873MTE, and the LM4873LQ.
Input mux control and two separate inputs per channel
Stereo headphone amplifier mode
“Click and pop” suppression circuitry
Thermal shutdown protection circuitry
PCB area-saving micro SMD and thin micro SMD
packages
n TSSOP and exposed-DAP TSSOP and LLP packages
Note 2: An LM4873MTE-1, LM4873MTE, or LM4873LQ that has been properly mounted to a circuit board and forced-air cooled will deliver 2.4W into 3Ω.
Applications
n
n
n
n
n
n Multimedia monitors
n Portable and desktop computers
n Portable audio systems
Connection Diagrams
10099330
Top View
Order Number LM4873MTE-1
See NS Package Number MXA28A for Exposed-DAP
TSSOP
10099302
Top View
Order Number LM4873MT, LM4873MTE
See NS Package Number MTC20 for TSSOP
See NS Package Number MXA20A for Exposed-DAP
TSSOP
Boomer ® is a registered trademark of National Semiconductor Corporation.
© 2002 National Semiconductor Corporation
DS100993
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LM4873 Dual 2.1W Audio Amplifier Plus Stereo Headphone Function
October 2002
LM4873
Connection Diagrams
(Continued)
10099353
Top View
(Bump-side down)
Order Number LM4873IBL, LM4873ITL
See NS Package Number BLA20AAB for micro SMD
See NS Package Number TLA20AAA
10099338
Top View
Order Number LM4873LQ
See NS Package Number LQA24A for Exposed-DAP LLP
micro SMD Marking
10099357
10099328
Top View
XY - Date Code
TT - Die Traceability
G - Boomer Family
I - LM4873IBL
Top View
XY - Date Code
TT - Die Traceability
G - Boomer Family
B2 - LM4873ITL
LM4873IBP Pin Designations
Pin (Bump) Number
Pin (Bump) Function
Pin (Bump) Number
A1
-IN A1
C3
VDD
A2
-IN A2
C4
+IN B
A3
-IN B2
D1
+OUT A
A4
-IN B1
D2
GND
B1
-OUT A
D3
GND
B2
GND
D4
+OUT B
B3
GND
E1
MUX CTRL
B4
-OUT B
E2
SHUTDOWN
C1
+IN A
E3
HP-IN
C2
VDD
E4
BYPASS
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Pin (Bump) Function
LM4873
Typical Application
10099331
Note: Pin out shown for the 28-pin Exposed-DAP TSSOP package. Refer to the Connection Diagrams for the pin out of the 20-pin Exposed-DAP TSSOP,
Exposed-DAP LLP, and micro SMD packages.
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LM4873
Absolute Maximum Ratings
(Note 3)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage
θJC (typ) — MTC20
20˚C/W
θJA (typ) — MTC20
80˚C/W
θJC (typ) — MXA20A
2˚C/W
θJA (typ) — MXA20A
41˚C/W (Note 7)
6.0V
θJA (typ) — MXA20A
51˚C/W (Note 8)
−65˚C to +150˚C
θJA (typ) — MXA20A
90˚C/W (Note 9)
−0.3V to VDD
+0.3V
θJC (typ) — MXA28A
2˚C/W
θJA (typ) — MXA28A
41˚C/W (Note 10)
Power Dissipation (Note 4)
Internally limited
θJA (typ) — MXA28A
51˚C/W (Note 11)
ESD Susceptibility (Note 5)
2000V
θJA (typ) — MXA28A
90˚C/W (Note 12)
ESD Susceptibility (Note 6)
200V
θJC (typ) — LQA24A
3.0˚C/W
θJA (typ) — LQA24A
42˚C/W (Note 13)
θJA (typ) — micro SMD
60˚C/W (Note 14)
Storage Temperature
Input Voltage
Junction Temperature
150˚C
Solder Information
Small Outline Package
Vapor Phase (60 sec.)
215˚C
Infrared (15 sec.)
220˚C
Operating Ratings
Temperature Range
See AN-450 “Surface Mounting and their Effects on
Product Reliablilty” for other methods of soldering
surface mount devices.
TMIN ≤ TA ≤ TMAX
−40˚C ≤ TA ≤ 85˚C
2.0V ≤ VDD ≤ 5.5V
Supply Voltage
Thermal Resistance
Electrical Characteristics (Notes 3, 15)
The following specifications apply for VDD = 5V unless otherwise noted. Limits apply for TA = 25˚C.
Symbol
Parameter
VDD
Supply Voltage
IDD
Quiescent Power Supply Current
Conditions
LM4873
Typical
Limit
(Note 16)
(Note 17)
2
Units
(Limits)
V (min)
5.5
V (max)
VIN = 0V, IO = 0A (Note 18) , HP-IN = 0V
7.5
15
mA (max)
VIN = 0V, IO = 0A (Note 18) , HP-IN = 4V
5.8
6
mA (min)
VDD applied to the SHUTDOWN pin
0.7
2
µA (max)
ISD
Shutdown Current
VIH
Headphone High Input Voltage
4
V (min)
VIL
Headphone Low Input Voltage
0.8
V (max)
Electrical Characteristics for Bridged-Mode Operation (Notes 3, 15)
The following specifications apply for VDD = 5V unless otherwise specified. Limits apply for TA = 25˚C.
Symbol
Parameter
Conditions
LM4873
Typical
Limit
Units
(Limits)
(Note 16) (Note 17)
VOS
Output Offset Voltage
VIN = 0V
PO
Output Power (Note 19)
THD+N = 1%, f = 1kHz (Note 20)
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5
50
mV (max)
LM4873MTE-1, RL = 3Ω
2.4
W
LM4873MTE, RL = 3Ω
2.2
W
LM4873LQ, RL = 3Ω
2.2
W
LM4873MTE-1, RL = 4Ω
2.1
W
LM4873MTE, RL = 4Ω
1.9
W
LM4873LQ, RL = 4Ω
1.9
W
LM4873MT, RL = 4Ω
1.9
W
LM4873, RL = 8Ω
1.1
4
1.0
W (min)
LM4873
Electrical Characteristics for Bridged-Mode Operation (Notes 3, 15)
(Continued)
The following specifications apply for VDD = 5V unless otherwise specified. Limits apply for TA = 25˚C.
Symbol
Parameter
Conditions
LM4873
Typical
Units
(Limits)
Limit
(Note 16) (Note 17)
THD+N = 10%, f = 1kHz (Note 20)
THD+N
Total Harmonic Distortion+Noise
LM4873MTE-1, RL = 3Ω
3.0
LM4873LQ, RL = 3Ω
3.0
W
W
LM4873MTE-1, RL = 4Ω
2.6
W
LM4873LQ, RL = 4Ω
2.6
W
LM4873, RL = 8Ω
1.5
W
THD+N = 1%, f = 1kHz, RL = 32Ω
0.34
W
20Hz ≤ f ≤ 20kHz, AVD = 2
LM4873MTE-1, RL = 4Ω, PO = 2W
0.3
LM4873LQ, RL = 4Ω, PO = 2W
LM4873, RL = 8Ω, PO = 1W
PSRR
Power Supply Rejection Ratio
VDD = 5V, VRIPPLE = 200mVRMS, RL = 8Ω,
CB = 1.0µF
0.3
%
67
dB
XTALK
Channel Separation
f = 1kHz, CB = 1.0µF
80
dB
SNR
Signal To Noise Ratio
VDD = 5V, PO = 1.1W, RL = 8Ω
97
dB
Electrical Characteristics for Single-Ended Operation (Notes 3, 15)
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 16)
(Note 17)
VOS
Output Offset Voltage
VIN = 0V
5
50
mV (max)
PO
Output Power
THD+N = 0.5%, f = 1 kHz, RL = 32Ω
85
75
mW (min)
THD+N = 1%, f = 1 kHz, RL = 8Ω
340
mW
THD+N = 10%, f = 1 kHz, RL = 8Ω
440
mW
THD+N
Total Harmonic Distortion+Noise
AV = −1, PO = 75mW, 20Hz ≤ f ≤ 20kHz,
RL = 32Ω
0.2
%
PSRR
Power Supply Rejection Ratio
CB = 1.0µF, VRIPPLE = 200mV
f = 1kHz
52
dB
RMS,
XTALK
Channel Separation
f = 1kHz, CB = 1.0µF
60
dB
SNR
Signal To Noise Ratio
VDD = 5V, PO = 340mW, RL = 8Ω
94
dB
Note 3: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
functional, but do not guarantee specific performance limits. Electrical Characteristics state DC and AC electrical specifications under particular test conditions which
guarantee specific performance limits. This assumes that the device operates within the Operating Ratings. Specifications are not guaranteed for parameters where
no limit is given. The typical value however, is a good indication of device performance.
Note 4: The maximum power dissipation must be derated at elevated temperatures and is dictated by TJMAX, θ JA, and the ambient temperature TA. The maximum
allowable power dissipation is PDMAX = (TJMAX − T A)/θJA. For the LM4873, TJMAX = 150˚C. For the θJAs for different packages, please see the Application
Information section or the Absolute Maximum Ratings section.
Note 5: Human body model, 100 pF discharged through a 1.5 kΩ resistor.
Note 6: Machine model, 220 pF–240 pF discharged through all pins.
Note 7: The given θJA is for an LM4873 packaged in an MXA20A with the Exposed-DAP soldered to an exposed 2in2 area of 1oz printed circuit board copper.
Note 8: The given θJA is for an LM4873 packaged in an MXA20A with the Exposed-DAP soldered to an exposed 1in2 area of 1oz printed circuit board copper.
Note 9: The given θJA is for an LM4873 packaged in an MXA20A with the Exposed-DAP not soldered to printed circuit board copper.
Note 10: The given θJA is for an LM4873 packaged in an MXA28A with the Exposed-DAP soldered to an exposed 2in2 area of 1oz printed circuit board copper.
Note 11: The given θJA is for an LM4873 packaged in an MXA28A with the Exposed-DAP soldered to an exposed 1in2 area of 1oz printed circuit board copper.
Note 12: The given θJA is for an LM4873 packaged in an MXA28A with the Exposed-DAP not soldered to printed circuit board copper.
Note 13: The given θJA is for an LM4873 packaged in an LQA24A with the Exposed-DAP soldered to an exposed 2in2 area of 1oz printed circuit board copper.
Note 14: The θJA is specified for an LM4873 packaged in a BLA20AAB or TLA20AAA with their four ground connections soldered to a 3in2, 1oz copper plane.
Note 15: All voltages are measured with respect to the ground (GND) pins, unless otherwise specified.
Note 16: Typicals are specified at 25˚C and represent the parametric norm.
Note 17: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis.
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LM4873
Electrical Characteristics for Single-Ended Operation (Notes 3, 15)
(Continued)
Note 18: The quiescent power supply current depends on the offset voltage when a practical load is connected to the amplifier.
Note 19: Output power is measured at the device terminals.
Note 20: When driving 3Ω or 4Ω loads and operating on a 5V supply, the LM4873LQ must be mounted to a circuit board that has a minimum of 2.5in2 of exposed,
uninterrupted copper area connected to the LLP package’s exposed DAP.
Typical Performance Characteristics
MTE (20-pin) and LQ (24-pin) Specific Characteristics
LM4873MTE, LM4873LQ
THD+N vs Output Power
LM4873MTE, LM4873LQ
THD+N vs Frequency
10099334
10099333
LM4873MTE, LM4873LQ
THD+N vs Output Power
LM4873MTE, LM4873LQ
Power Dissipation vs Power Output
10099390
10099336
LM4873MTE (Note 21)
Power Derating Curve
LM4873LQ
Power Derating Curve
10099395
10099356
Note 21: This curve shows the LM4873MTE’s and the LM4873LQ’s thermal dissipation ability 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.
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500LFPM +
2.5in2:
The part is soldered to a
2.5in2,
LM4873
Typical Performance Characteristics
MTE (20-pin) and LQ (24-pin) Specific Characteristics
(Continued)
1 oz. copper plane with 500 linear feet per minute of forced-air flow across it.
2.5in2: 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.
Typical Performance Characteristics
MTE-1 (28 pin) Specific Characteristics
LM4873MTE-1
THD+N vs Output Power
LM4873MTE-1
THD+N vs Frequency
10099399
10099397
LM4873MTE-1
THD+N vs Output Power
LM4873MTE-1
THD+N vs Frequency
10099398
10099396
LM4873MTE-1
Power Dissipation vs Power Output
LM4873MTE-1 (Note 22)
Power Derating Curve
10099390
100993A0
Note 22: This curve shows the LM4835MTE-1’s thermal dissipation ability at different ambient temperatures given these conditions:
500LFPM + 2in2: The part is soldered to a 2in2, 1 oz. copper plane with 500 linear feet per minute of forced-air flow across it.
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LM4873
Typical Performance Characteristics
MTE-1 (28 pin) Specific Characteristics
2in2on
bottom: The part is soldered to a
2in2,
(Continued)
1oz. copper plane that is on the bottom side of the PC board through 21 8 mil vias.
2in2: The part is soldered to a 2in2, 1oz. copper plane.
1in2: The part is soldered to a 1in2, 1oz. copper plane.
Not Attached: The part is not soldered down and is not forced-air cooled.
Typical Performance Characteristics
THD+N vs Frequency
THD+N vs Frequency
10099303
10099304
THD+N vs Frequency
THD+N vs Output Power
10099305
10099306
THD+N vs Output Power
THD+N vs Output Power
10099307
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10099308
8
LM4873
Typical Performance Characteristics
(Continued)
THD+N vs Output Power
THD+N vs Frequency
10099387
10099389
THD+N vs Output Power
THD+N vs Frequency
10099386
10099388
Output Power vs
Load Resistance
Power Dissipation vs
Supply Voltage
10099384
10099385
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LM4873
Typical Performance Characteristics
(Continued)
Output Power vs
Supply Voltage
Output Power vs
Supply Voltage
10099309
10099310
Output Power vs
Supply Voltage
Output Power vs
Load Resistance
10099312
10099311
Output Power vs
Load Resistance
LM4873IBL Stereo Output Power
vs Power Dissipation
10099313
10099355
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LM4873
Typical Performance Characteristics
(Continued)
Power Dissipation vs
Output Power
Dropout Voltage vs
Supply Voltage
10099314
10099315
Power Derating Curve
Power Dissipation vs
Output Power
10099317
10099316
Noise Floor
Channel Separation
10099319
10099318
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LM4873
Typical Performance Characteristics
(Continued)
Power Supply
Rejection Ratio
Channel Separation
10099320
10099321
Open Loop
Frequency Response
Supply Current vs
Supply Voltage
10099323
10099322
External Components Description
(Refer to Figure 1.)
Components
Functional Description
1.
Ri
The inverting input resistance, along with Rf, set the closed-loop gain. Ri, along with Ci, form a high pass
filter with fc = 1/(2πRiCi).
2.
Ci
The input coupling capacitor blocks DC voltage at the amplifier’s input terminals. Ci, along with Ri, create a
highpass filter with fc = 1/(2πRiCi). Refer to the section, SELECTING PROPER EXTERNAL
COMPONENTS, for an explanation of determining the value of Ci.
3.
Rf
The feedback resistance, along with Ri, set the closed-loop gain.
4.
Cs
The supply bypass capacitor. Refer to the POWER SUPPLY BYPASSING section for information about
properly placing, and selecting the value of, this capacitor.
5.
CB
The capacitor, CB, filters the half-supply voltage present on the BYPASS pin. Refer to the SELECTING
PROPER EXTERNAL COMPONENTS section for information concerning proper placement and selecting
CB’s value.
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LM4873
Application Information
LM4863 PIN CONFIGURATION COMPATIBILITY
The LM4873’s pin configuration simplifies the process of
upgrading systems that use the LM4863. Except for its four
MUX function pins, the LM4873’s pin configuration matches
the LM4863’s pin configuration. If the LM4873’s MUX functionality is not needed when replacing an LM4863, connect
the MUX CTRL pin to either VDD or ground. As shown in
Table 1, grounding the MUX CTRL pin selects stereo input 1
(–IN A1 and –IN B1), whereas applying VDD to the MUX
CTRL pin selects stereo input 2 (–IN A2 and –IN B2).
STEREO-INPUT MULTIPLEXER (STEREO MUX)
Typical LM4873 applications use the MUX to switch between
two stereo input signals. Each stereo channel’s gain can be
tailored to produce the required output signal level. Choosing the input and feedback resistor ratio sets a MUX channel’s gain. Another configuration uses the MUX to select two
different gains or frequency compensated gains to amplify a
single pair of stereo input signals. Figure 1 shows two different feedback networks, Network 1 and Network 2. Network 1
produces increasing gain as the input signal’s frequency
decreases. This can be used to compensate a small, fullrange speaker’s low frequency response roll-off. Network 2
sets the gain for an alternate load such as headphones.
Connecting the MUX CTRL and HP-IN pins together applies
the same control voltage to the MUX pins when connecting
and disconnecting headphones using the headphone jack
shown in Figure 2 or Figure 3. Simultaneously applying the
control voltage automatically selects the amplifier (headphone or bridge loads) and switches the gain (MUX channel
selection). Alternatively, leave the control pins independently
accessible. This allows a user to select bass boost as
needed. This alternative user-selectable bass-boost scheme
requires connecting equal ratio resistor feedback networks
to each MUX input channel. The value of the resistor in the
RC network is chosen to give a gain that is necessary to
achieve the desired bass-boost.
Switching between the MUX channels may change the input
signal source or the feedback resistor network. During the
channel switching transition, the average voltage level
present on the internal amplifier’s input may change. This
change can slew at a rate that may produce audible voltage
transients or clicks in the amplifier’s output signal. Using the
MUX to select between two vastly dissimilar gains is a typical
transient-producing situation. As the MUX is switched, an
audible click may occur as the gain suddenly changes.
10099370
FIGURE 1. Input MUX Example
micro SMD PACKAGE PCB MOUNTING
CONSIDERATIONS
PCB layout specifications unique to the LM4873’s micro
SMD package are found in National Semiconductor’s
AN1112.
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATIONS
The LM4873’s exposed-DAP (die attach paddle) packages
(MTE, MTE-1, LQ) provide a low thermal resistance between
the die and the PCB to which the part is mounted and
soldered. This allows rapid heat transfer from the die to the
surrounding PCB copper traces, ground plane and, finally,
surrounding air. The result is a low voltage audio power
amplifier that produces 2.1W at ≤ 1% THD with a 4Ω load.
This high power is achieved through careful consideration of
necessary thermal design. Failing to optimize thermal design
may compromise the LM4873’s high power performance and
activate unwanted, though necessary, thermal shutdown
protection.
The MTE, MTE-1, and LQ packages must have their DAPs
soldered to a copper pad on the PCB. The DAP’s PCB
copper pad is connected to a large plane of continuous
unbroken copper. This plane forms a thermal mass and heat
sink and radiation area. Place the heat sink area on either
outside plane in the case of a two-sided PCB, or on an inner
layer of a board with more than two layers. Connect the DAP
copper pad to the inner layer or backside copper heat sink
area with 32(4x8) ( (MTE), 40(4x10) (MTE-1), or 6(3x2) (LQ)
vias. The via diameter should be 0.012in–0.013in with a
1.27mm pitch. Ensure efficient thermal conductivity by
plating-through and solder-filling the vias.
Best thermal performance is achieved with the largest practical copper heat sink area. If the heatsink and amplifier
share the same PCB layer, a nominal 2.5in2 (min) area is
necessary for 5V operation with a 4Ω load. Heatsink areas
not placed on the same PCB layer as the LM4873 should be
5in2 (min) for the same supply voltage and load resistance.
The last two area recommendations apply for 25˚C ambient
temperature. Increase the area to compensate for ambient
temperatures above 25˚C. In systems using cooling fans, the
LM4873MTE can take advantage of forced air cooling. With
an air flow rate of 450 linear-feet per minute and a 2.5in2
exposed copper or 5.0in2 inner layer copper plane heatsink,
the LM4873MTE can continuously drive a 3Ω load to full
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LM4873
Application Information
between the amplifier output pins and the load’s connections. Residual trace resistance causes a voltage drop,
which results in power dissipated in the trace and not in the
load as desired. For example, 0.1Ω trace resistance reduces
the output power dissipated by a 4Ω load from 2.1W to 2.0W.
This problem of decreased load dissipation is exacerbated
as load impedance decreases. Therefore, to maintain the
highest load dissipation and widest output voltage swing,
PCB traces that connect the output pins to a load must be as
wide as possible.
Poor power supply regulation adversely affects maximum
output power. A poorly regulated supply’s output voltage
decreases with increasing load current. Reduced supply
voltage causes decreased headroom, output signal clipping,
and reduced output power. Even with tightly regulated supplies, trace resistance creates the same effects as poor
supply regulation. Therefore, making the power supply
traces as wide as possible helps maintain full output voltage
swing.
(Continued)
power. The LM4873LQ achieves the same output power
level without forced air cooling. In all circumstances and
conditions, the junction temperature must be held below
150˚C to prevent activating the LM4873’s thermal shutdown
protection. The LM4873’s power de-rating curve in the Typical Performance Characteristics shows the maximum
power dissipation versus temperature. Example PCB layouts
for the exposed-DAP TSSOP and LQ packages are shown in
the Demonstration Board Layout section. Further detailed
and specific information concerning PCB layout, fabrication,
and mounting an LQ (LLP) package is available from National Semiconductor’s AN1187.
PCB LAYOUT AND SUPPLY REGULATION
CONSIDERATIONS FOR DRIVING 3Ω AND 4Ω LOADS
Power dissipated by a load is a function of the voltage swing
across the load and the load’s impedance. As load impedance decreases, load dissipation becomes increasingly dependent on the interconnect (PCB trace and wire) resistance
10099331
* Refer to the section Selecting Proper External Components, for a detailed discussion of CB size.
FIGURE 2. Typical Audio Amplifier Application Circuit
Pin out shown for the 28-pin Expoased-DAP TSSOP package. Refer to the Connection Diagrams for the pin out of the
20-pin Exposed-DAP TSSOP, Exposed-DAP LLP, and micro SMD package.
Figure 2 shows that Amp1A’s output serves as Amp2A’s
input. This results in both amplifiers producing signals identical in magnitude, but 180˚ out of phase. Taking advantage
of this phase difference, a load is placed between −OUTA
and +OUTA and driven differentially (commonly referred to
as “bridge mode”). This results in a differential gain of
(1)
AVD = 2 * (Rf/R i)
Bridge mode amplifiers are different from single-ended amplifiers that drive loads connected between a single amplifier’s output and ground. For a given supply voltage, bridge
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 2, the LM4873 consists of two pairs of
operational amplifiers, forming a two-channel (channel A and
channel B) stereo amplifier. (Though the following discusses
channel A, it applies equally to channel B.) External resistors
Rf and Ri set the closed-loop gain of Amp1A, whereas two
internal 20kΩ resistors set Amp2A’s gain at −1. The LM4873
drives a load, such as a speaker, connected between the two
amplifier outputs, −OUTA and +OUTA.
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Equation (6) gives the maximum junction temperature
TJMAX. If the result violates the LM4873’s 150˚C, reduce the
maximum junction temperature by reducing the power supply voltage or increasing the load resistance. Further allowance should be made for increased ambient temperatures.
The above examples assume that a device is a surface
mount part operating around the maximum power dissipation
point. Since internal power dissipation is a function of output
power, higher ambient temperatures are allowed as output
power or duty cycle decreases.
(Continued)
mode has a distinct advantage over the single-ended configuration: its differential output doubles the voltage swing
across the load. This produces four times the output power
when 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 that the
output signal is not clipped. To ensure minimum output signal clipping when choosing an amplifier’s closed-loop gain,
refer to the Audio Power Amplifier Design section.
Another advantage of the differential bridge output is no net
DC voltage across the load. This is accomplished by biasing
channel A’s and channel B’s outputs at half-supply. This
eliminates the coupling capacitor that single supply, singleended amplifiers require. Eliminating an output coupling capacitor in a single-ended configuration forces a single-supply
amplifier’s half-supply bias voltage across the load. This
increases internal IC power dissipation and may permanently damage loads such as speakers.
If the result of Equation (2) is greater than that of Equation
(3), then decrease the supply voltage, increase the load
impedance, or reduce the ambient temperature. If these
measures are insufficient, a heat sink can be added to
reduce θJA. The heat sink can be created using additional
copper area around the package, with connections to the
ground pin(s), supply pin and amplifier output pins. External,
solder attached SMT heatsinks such as the Thermalloy
7106D can also improve power dissipation. When adding a
heat sink, the θJA is the sum of θJC, θCS, and θSA. (θJC is the
junction-to-case thermal impedance, θCS is the case-to-sink
thermal impedance, and θSA is the sink-to-ambient thermal
impedance.) Refer to the Typical Performance Characteristics curves for power dissipation information at lower output power levels.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful single-ended or bridged amplifier. Equation (2)
states the maximum power dissipation point for a singleended amplifier operating at a given supply voltage and
driving a specified output load.
(2)
PDMAX = (VDD)2/(2π2RL) Single-Ended
However, a direct consequence of the increased power delivered to the load by a bridge amplifier is higher internal
power dissipation for the same conditions.
The LM4873 has two operational amplifiers per channel. The
maximum internal power dissipation per channel operating in
the bridge mode is four times that of a single-ended amplifier. From Equation (3), assuming a 5V power supply and a
4Ω load, the maximum single channel power dissipation is
1.27W or 2.54W for stereo operation.
(3)
PDMAX = 4 * (VDD)2/(2π2RL) Bridge Mode
The LM4873’s power dissipation is twice that given by Equation (2) or Equation (3) when operating in the single-ended
mode or bridge mode, respectively. Twice the maximum
power dissipation point given by Equation (3) must not exceed the power dissipation given by Equation (4):
(4)
PDMAX' = (TJMAX − TA)/θJA
The LM4873’s TJMAX = 150˚C. In the LQ package soldered
to a DAP pad that expands to a copper area of 5in2 on a
PCB, the LM4873’s θJA is 20˚C/W. In the MTE and MTE-1
packages soldered to a DAP pad that expands to a copper
area of 2in2 on a PCB, the LM4873’s θJA is 41˚C/W. At any
given ambient temperature TA, use Equation (4) to find the
maximum internal power dissipation supported by the IC
packaging. Rearranging Equation (4) and substituting PDMAX
for PDMAX' results in Equation (5). This equation gives the
maximum ambient temperature that still allows maximum
stereo power dissipation without violating the LM4873’s
maximum junction temperature.
(5)
TA = TJMAX – 2*PDMAX θJA
For a typical application with a 5V power supply and an 4Ω
load, the maximum ambient temperature that allows maximum stereo power dissipation without exceeding the maximum junction temperature is approximately 99˚C for the LQ
package and 45˚C for the MTE and MTE-1 packages.
(6)
TJMAX = PDMAX θJA + TA
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection. Applications that employ a 5V regulator typically
use a 10 µF in parallel with a 0.1 µF filter capacitors to
stabilize the regulator’s output, reduce noise on the supply
line, and improve the supply’s transient response. However,
their presence does not eliminate the need for a local 1.0 µF
tantalum bypass capacitance connected between the
LM4873’s supply pins and ground. Do not substitute a ceramic capacitor for the tantalum. Doing so may cause oscillation. Keep the length of leads and traces that connect
capacitors between the LM4873’s power supply pin and
ground as short as possible. Connecting a 1µF capacitor,
CB, between the BYPASS pin and ground improves the
internal bias voltage’s stability and improves the amplifier’s
PSRR. The PSRR improvements increase as the bypass pin
capacitor value increases. Too large, however, increases
turn-on time and can compromise amplifier’s click and pop
performance. The selection of bypass capacitor values, especially CB, depends on desired PSRR requirements, click
and pop performance (as explained in the section, Selecting
Proper External Components), system cost, and size constraints.
MICRO-POWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the
LM4873’s shutdown function. Activate micro-power shutdown by applying VDD to the SHUTDOWN pin. When active,
the LM4873’s micro-power shutdown feature turns off the
amplifier’s bias circuitry, reducing the supply current. The
logic threshold is typically VDD/2. The low 0.7 µA typical
shutdown current is achieved by applying a voltage that is as
near as VDD as possible to the SHUTDOWN pin. A voltage
that is less than VDD may increase the shutdown current.
Table 1 shows the logic signal levels that activate and deactivate micro-power shutdown and headphone amplifier operation.
There are a few ways to control the micro-power shutdown.
These include using a single-pole, single-throw switch, a
15
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LM4873
Application Information
LM4873
Application Information
ing micro-power shutdown. The switch and resistor guarantee that the SHUTDOWN pin will not float. This prevents
unwanted state changes. In a system with a microprocessor
or a microcontroller, use a digital output to apply the control
voltage to the SHUTDOWN pin. Driving the SHUTDOWN pin
with active circuitry eliminates the pull up resistor.
(Continued)
microprocessor, or a microcontroller. When using a switch,
connect an external 10kΩ pull-up resistor between the
SHUTDOWN pin and VDD. Connect the switch between the
SHUTDOWN pin and ground. Select normal amplifier operation by closing the switch. Opening the switch connects the
SHUTDOWN pin to VDD through the pull-up resistor, activat-
TABLE 1. Logic Level Truth Table for SHUTDOWN, HP-IN, and MUX Operation
SHUTDOWN
PIN
HP-INPIN
MUX CHANNEL
SELECT PIN
OPERATIONAL MODE
(MUX INPUT CHANNEL #)
Logic Low
Logic Low
Logic Low
Logic Low
Bridged Amplifiers (1)
Logic Low
Logic High
Bridged Amplifiers (2)
Logic Low
Logic High
Logic Low
Single-Ended Amplifiers (1)
Logic Low
Logic High
Logic High
Single-Ended Amplifiers (2)
Logic High
X
X
Micro-Power Shutdown
HP-IN FUNCTION
Applying a voltage between 4V and VDD to the LM4873’s
HP-IN headphone control pin turns off Amp2A and Amp2B,
muting a bridged-connected load. 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. With no headphones connected to
the headphone jack, the R1-R2 voltage divider sets the
voltage applied to the HP-IN pin (pin 16) at approximately
50mV. This 50mV enables Amp1B and Amp2B, placing the
LM4873 in bridged mode operation. The output coupling
capacitor blocks the amplifier’s half supply DC voltage, protecting the headphones.
The HP-IN threshold is set at 4V. While the LM4873 operates
in bridged mode, the DC potential across the load is essentially 0V. Therefore, even in an ideal situation, the output
swing cannot cause a false single-ended trigger. Connecting
headphones to the headphone jack disconnects the headphone jack contact pin from −OUTA and allows R1 to pull the
HP Sense pin up to VDD. This enables the headphone function, turns off Amp2A and Amp2B, and mutes the bridged
speaker. The amplifier then drives the headphones, whose
impedance is in parallel with resistor R2 and R3. These
resistors have negligible effect on the LM4873’s output drive
capability since the typical impedance of headphones is
32Ω.
Figure 3 also shows the suggested headphone jack electrical connections. The jack is designed to mate with a threewire plug. The plug’s tip and ring should each carry one of
the two stereo output signals, whereas the sleeve should
carry the ground return. A headphone jack with one control
pin contact is sufficient to drive the HP-IN pin when connecting headphones.
A microprocessor or a switch can replace the headphone
jack contact pin. When a microprocessor or switch applies a
voltage greater than 4V to the HP-IN pin, a bridge-connected
speaker is muted and Amp1A and Amp2A drive a pair of
headphones.
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10099324
FIGURE 3. Headphone Circuit
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4873’s performance requires properly selecting external components. Though the LM4873 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component values.
The LM4873 is unity-gain stable, giving a designer maximum
design flexibility. The gain should be set to no more than a
given application requires. This allows the amplifier to
achieve minimum THD+N and maximum signal-to-noise ratio. These parameters are compromised as the closed-loop
gain increases. However, low gain demands input signals
with greater voltage swings to achieve maximum output
power. Fortunately, many signal sources such as audio
CODECs have outputs of 1VRMS (2.83VP-P). Please refer to
the Audio Power Amplifier Design section for more information on selecting the proper gain.
16
the magnitude of “clicks and pops”. Increasing the value of
CB reduces the magnitude of turn-on pops. However, this
presents a tradeoff: as the size of CB increases, the turn-on
time increases. There is a linear relationship between the
size of CB and the turn-on time. Here are some typical
turn-on times for various values of CB:
(Continued)
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitor (Ci in Figure 2). A high value capacitor can be expensive and may compromise space efficiency
in portable designs. In many cases, however, the speakers
used in portable systems, whether internal or external, have
little ability to reproduce signals below 150 Hz. Applications
using speakers with this limited frequency response reap
little improvement by using large input capacitor.
Besides effecting system cost and size, Ci has an affect on
the LM4873’s click and pop performance. When the supply
voltage is first applied, a transient (pop) is created as the
charge on the input capacitor changes from zero to a quiescent state. The magnitude of the pop is directly proportional
to the input capacitor’s size. Higher value capacitors need
more time to reach a quiescent DC voltage (usually VDD/2)
when charged with a fixed current. The amplifier’s output
charges the input capacitor through the feedback resistor,
Rf. Thus, pops can be minimized by selecting an input
capacitor value that is no higher than necessary to meet the
desired −3dB frequency.
A shown in Figure 2, the input resistor (RI) and the input
capacitor, CI produce a −3dB high pass filter cutoff frequency
that is found using Equation (7).
CB
TON
0.01µF
20ms
0.1µF
200ms
0.22µF
440ms
0.47µF
940ms
1.0µF
2sec
In order eliminate “clicks and pops”, all capacitors must be
discharged before turn-on. Rapidly switching VDD may not
allow the capacitors to fully discharge, which may cause
“clicks and pops”. In a single-ended configuration, the output
is coupled to the load by COUT. This capacitor usually has a
high value. COUT discharges through internal 20kΩ resistors.
Depending on the size of COUT, the discharge time constant
can be relatively large. To reduce transients in single-ended
mode, an external 1kΩ–5kΩ resistor can be placed in parallel with the internal 20kΩ resistor. The tradeoff for using
this resistor is increased quiescent current.
NO LOAD STABILITY
The LM4873 may exhibit low level oscillation when the load
resistance is greater than 10kΩ. This oscillation only occurs
as the output signal swings near the supply voltages. Prevent this oscillation by connecting a 5kΩ between the output
pins and ground.
(7)
As an example when using a speaker with a low frequency
limit of 150Hz, Ci, using Equation (4) is 0.063µF. The 1.0µF
Ci shown in Figure 2 allows the LM4873 to drive high efficiency, full range speaker whose response extends below
30Hz.
AUDIO POWER AMPLIFIER DESIGN
Audio Amplifier Design: Driving 1W into an 8Ω Load
The following are the desired operational parameters:
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consideration should be paid to value of CB, the capacitor connected to the BYPASS pin. Since CB determines how fast
the LM4873 settles to quiescent operation, its value is critical
when minimizing turn-on pops. The slower the LM4873’s
outputs ramp to their quiescent DC voltage (nominally 1/2
VDD), the smaller the turn-on pop. Choosing CB equal to
1.0 µF along with a small value of Ci (in the range of 0.1 µF
to 0.39 µF), produces a click-less and pop-less shutdown
function. As discussed above, choosing Ci no larger than
necessary for the desired bandwith helps minimize clicks
and pops.
Power Output:
Load Impedance:
Input Level:
Input Impedance:
Bandwidth:
1WRMS
8Ω
1Vrms
20kΩ
100Hz−20kHz ± 0.25dB
The design begins by specifying the minimum supply voltage
necessary to obtain the specified output power. One way to
find the minimum supply voltage is to use the Output Power
vs Supply Voltage curve in the Typical Performance Characteristics section. Another way, using Equation (8), is to
calculate the peak output voltage necessary to achieve the
desired output power for a given load impedance. To account for the amplifier’s dropout voltage, two additional voltages, based on the Dropout Voltage vs Supply Voltage in the
Typical Performance Characteristics curves, must be
added to the result obtained by Equation (8). The result in
Equation (9).
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4873 contains circuitry that minimizes turn-on and
shutdown transients or “clicks and pop”. For this discussion,
turn-on refers to either applying the power supply voltage or
when the shutdown mode is deactivated. While the power
supply is ramping to its final value, the LM4873’s internal
amplifiers are configured as unity gain buffers. An internal
current source changes the voltage of the BYPASS pin in a
controlled, linear manner. Ideally, the input and outputs track
the voltage applied to the BYPASS pin. The gain of the
internal amplifiers remains unity until the voltage on the
bypass pin reaches 1/2 VDD . As soon as the voltage on the
bypass pin is stable, the device becomes fully operational.
Although the BYPASS pin current cannot be modified,
changing the size of CB alters the device’s turn-on time and
(8)
(9)
VDD ≥ (VOUTPEAK + (VODTOP + VODBOT))
The Output Power vs Supply Voltage graph for an 8Ω load
indicates a minimum supply voltage of 4.6V. This is easily
met by the commonly used 5V supply voltage. The additional
voltage creates the benefit of headroom, allowing the
LM4873 to produce peak output power in excess of 1W
17
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LM4873
Application Information
LM4873
Application Information
These circuit boards are easy to use. Apply 5V and ground to
the board’s VDD and GND pads, respectively. Connect 4Ω
speakers between the board’s −OUTA and +OUTA and
OUTB and +OUTB pads.
(Continued)
without clipping or other audible distortion. The choice of
supply voltage must also not create a situation that violates
maximum power dissipation as explained above in the
Power Dissipation section.
After satisfying the LM4873’s power dissipation requirements, the minimum differential gain needed to achieve 1W
dissipation in an 8Ω load is found using Equation (10).
(10)
Thus, a minimum gain of 2.83 allows the LM4873’s to reach
full output swing and maintain low noise and THD+N performance. For this example, let AVD = 3.
The amplifier’s overall gain is set using the input (Ri) and
feedback (Rf) resistors. With the desired input impedance
set at 20kΩ, the feedback resistor is found using Equation
(11).
(11)
Rf/Ri = AVD/2
The value of Rf is 30kΩ.
The last step in this design example is setting the amplifier’s
−3dB frequency bandwidth. To achieve the desired ± 0.25dB
pass band magnitude variation limit, the low frequency response must extend to at least one-fifth the lower bandwidth
limit and the high frequency response must extend to at least
five times the upper bandwidth limit. The gain variation for
both response limits is 0.17dB, well within the ± 0.25dB
desired limit. The results are an
fL = 100Hz/5 = 20Hz
and an
fH = 20kHz*5 = 100kHz.
As mentioned in the External Components section, Ri and
Ci create a highpass filter that sets the amplifier’s lower
bandpass frequency limit. Find the coupling capacitor’s
value using Equation (12).
(12)
Ci ≥ 1/(2πRifL)
The result is
1/(2π*20kΩ*20Hz) = 0.398µF.
Use a 0.39µF capacitor, the closest standard value.
The product of the desired high frequency cutoff (100kHz in
this example) and the differential gain, AVD, determines the
upper passband response limit. With AVD = 3 and fH =
100kHz, the closed-loop gain bandwidth product (GBWP) is
300kHz. This is less than the LM4873’s 3.5MHz GBWP. With
this margin, the amplifier can be used in designs that require
more differential gain while avoiding performance-restricting
bandwidth limitations.
10099393
FIGURE 4. Recommended MTE PC Board Layout:
Component-Side Silkscreen
10099391
FIGURE 5. Recommended MTE PC Board Layout:
Component-Side Layout
RECOMMENDED PRINTED CIRCUIT BOARD LAYOUT
Figures 4 through 6 show the recommended two-layer PC
board layout that is optimized for the 20-pin MTE-packaged
LM4873 and associated external components. Figures 7
through 11 show the recommended four-layer PC board
layout that is optimized for the 24-pin LQ-packaged LM4873
and associated external components. Figures 12 through 16
show the recommended four-layer PC board layout that is
optimized for the 20-pin micro SMD-packaged LM4873 and
associated external components. These circuits are designed for use with an external 5V supply and 4Ω speakers.
10099392
FIGURE 6. Recommended MTE PC Board Layout:
Bottom-Side Layout
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18
LM4873
Application Information
(Continued)
10099340
10099339
Figure 8. Recommended LQ PC Board Layout:
Component-Side Layout
Figure 7. Recommended LQ PC Board Layout:
Component-Side Silkscreen
10099342
10099341
Figure 10. Recommended LQ PC Board Layout:
Lower Inner-Layer Layout
Figure 9. Recommended LQ PC Board Layout:
Upper Inner-Layer Layout
10099344
Figure 12. Recommended 20-pin micro SMD PC Board
Layout:
Component-Side Silkscreen
10099343
Figure 11. Recommended LQ PC Board Layout:
Bottom-Side Layout
19
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LM4873
Application Information
(Continued)
10099346
Figure 14. Recommended 20-pin micro SMD PC Board
Layout:
Upper Inner-Layer Layout
10099345
Figure 13. Recommended 20-pin micro SMD PC Board
Layout:
Component-Side Layout
10099347
Figure 15. Recommended 20-pin micro SMD PC Board
Layout:
Lower Inner-Layer Layout
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10099348
Figure 16. Recommended 20-pin micro SMD PC Board
Layout:
Bottom-Side Layout
20
LM4873
Physical Dimensions
inches (millimeters) unless otherwise noted
20-Bump micro SMD
Order Number LM4873IBL
NS Package Number BLA20AAB
X1 = 1.996 ± 0.03 X2 = 2.492 ± 0.03 X3 = 0.945 ± 0.10
20-Bump micro SMD
Order Number LM4873ITL
NS Package Number TLA20AAA
X1 = 1.996 ± 0.03 X2 = 2.492 ± 0.03 X3 = 0.600 ± 0.075
21
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LM4873
Physical Dimensions
inches (millimeters) unless otherwise noted (Continued)
20-Lead MOLDED PKG, TSSOP, JEDEC, 4.4mm BODY WIDTH
Order Number LM4873MT
NS Package Number MTC20
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22
LM4873
Physical Dimensions
inches (millimeters) unless otherwise noted (Continued)
24-Lead MOLDED PKG, Leadless Leadframe Package LLP
Order Number LM4873LQ
NS Package Number LQA24A
23
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LM4873
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
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24
LM4873 Dual 2.1W Audio Amplifier Plus Stereo Headphone Function
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
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