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LM4766 Overture
™
Audio Power Amplifier Series Dual 40W Audio Power
Amplifier with Mute
General Description
The LM4766 is a stereo audio amplifier capable of delivering
typically 40W per channel with the non-isolated "T" package
and 30W per channel with the isolated "TF" package of
continuous average output power into an 8Ω load with less
than 0.1% (THD+N).
The performance of the LM4766, utilizing its Self Peak Instantaneous Temperature (˚Ke) (SPiKe
™
) Protection Circuitry, places it in a class above discrete and hybrid amplifiers by providing an inherently, dynamically protected Safe
Operating Area (SOA). SPiKe Protection means that these
parts are safeguarded at the output against overvoltage,
undervoltage, overloads, including thermal runaway and instantaneous temperature peaks.
Each amplifier within the LM4766 has an independent
smooth transition fade-in/out mute that minimizes output
pops. The IC’s extremely low noise floor at 2µV and its
extremely low THD+N value of 0.06% at the rated power
make the LM4766 optimum for high-end stereo TVs or minicomponent systems.
Key Specifications
n THD+N at 1kHz at 2 x 30W continuous average output
power into 8Ω: 0.1% (max)
n THD+N at 1kHz at continuous average output power of
2 x 30W into 8Ω: 0.009% (typ)
Features
n SPiKe Protection
n Minimal amount of external components necessary
n Quiet fade-in/out mute mode
n Non-Isolated 15-lead TO-220 package
n Wide Supply Range 20V - 78V
Applications
n High-end stereo TVs
n Component stereo
n Compact stereo
Connection Diagram
Plastic Package
10092802
Top View
Non-Isolated TO-220 Package
Order Number LM4766T
See NS Package Number TA15A
Isolated TO-220 Package
Order Number LM4766TF
See NS Package Number TF15B
SPiKe™Protection and Overture™are trademarks of National Semiconductor Corporation.
July 2003
LM4766 Overture
™
Audio Power Amplifier Series
Dual 40W Audio Power Amplifier with Mute
© 2003 National Semiconductor Corporation DS100928 www.national.com
Page 2
Typical Application
Note: Numbers in parentheses represent pinout for amplifier B.
*
Optional component dependent upon specific design requirements.
10092801
FIGURE 1. Typical Audio Amplifier Application Circuit
LM4766
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Page 3
Absolute Maximum Ratings (Notes 4,
5)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage |V
CC
|+|VEE|
(No Input) 78V
Supply Voltage |V
CC
|+|VEE|
(with Input) 74V
Common Mode Input Voltage (V
CC
or VEE) and
|V
CC
|+|VEE| ≤
60V
Differential Input Voltage 60V
Output Current Internally Limited
Power Dissipation (Note 6) 62.5W
ESD Susceptability (Note 7) 3000V
Junction Temperature (Note 8) 150˚C
Thermal Resistance
Non-Isolated T-Package
θ
JC
1˚C/W
Isolated TF-Package
θ
JC
2˚C/W
Soldering Information
T and TF Packages 260˚C
Storage Temperature −40˚C to +150˚C
Operating Ratings (Notes 4, 5)
Temperature Range
T
MIN
≤ TA≤ T
MAX
−20˚C ≤ TA≤
+85˚C
Supply Voltage |V
CC
|+|VEE| (Note
1) 20V to 60V
Electrical Characteristics (Notes 4, 5)
The following specifications apply for VCC= +30V, VEE= −30V, I
MUTE
= −0.5mA with RL=8Ω unless otherwise specified. Lim-
its apply for T
A
= 25˚C.
Symbol Parameter Conditions LM4766 Units
(Limits)
Typical Limit
(Note 9) (Note 10)
|V
CC
| + Power Supply Voltage GND − VEE≥ 9V 18 20 V (min)
|V
EE
| (Note 11) 60 V (max)
P
O
Output Power T Package, VCC=±30V,
THD+N = 0.1% (max),
f = 1kHz, f = 20kHz
40 30 W/ch (min)
TF Package, V
CC
=±26V(Note 13),
THD+N = 0.1% (max),
30 25 W/ch (min)
(Notes 3, 13) (Continuous Average) f = 1kHz, f = 20kHz
THD+N Total Harmonic Distortion
Plus Noise
T Package
30W/ch, R
L
=8Ω, 20Hz ≤ f ≤ 20kHz,
A
V
= 26dB
0.06 %
TF Package
25W/ch, R
L
=8Ω, 20Hz ≤ f ≤ 20kHz,
A
V
= 26dB
0.06 %
X
talk
Channel Separation f = 1kHz, VO= 10.9Vrms 60 dB
SR
(Note 3)
Slew Rate V
IN
= 1.2Vrms, t
rise
= 2ns 9 5 V/µs (min)
I
total
Total Quiescent Power Both Amplifiers VCM= 0V, 48 100 mA (max)
(Note 2) Supply Current V
O
= 0V, IO= 0mA
V
OS
(Note 2)
Input Offset Voltage V
CM
= 0V, IO= 0mA 1 10 mV (max)
I
B
Input Bias Current VCM= 0V, IO= 0mA 0.2 1 µA (max)
I
OS
Input Offset Current VCM= 0V, IO= 0mA 0.01 0.2 µA (max)
I
O
Output Current Limit |VCC|=|VEE| = 10V, tON= 10ms, 4 3 Apk (min)
V
O
=0V
V
OD
Output Dropout Voltage |VCC–VO|, VCC= 20V, IO= +100mA 1.5 4 V (max)
(Note 2) (Note 12) |V
O–VEE
|, VEE= −20V, IO= −100mA 2.5 4 V (max)
LM4766
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Page 4
Electrical Characteristics (Notes 4, 5) (Continued)
The following specifications apply for VCC= +30V, VEE= −30V, I
MUTE
= −0.5mA with RL=8Ω unless otherwise specified. Lim-
its apply for T
A
= 25˚C.
Symbol Parameter Conditions LM4766 Units
(Limits)
Typical Limit
(Note 9) (Note 10)
PSRR Power Supply Rejection Ratio V
CC
= 30V to 10V, VEE= −30V, 125 85 dB (min)
(Note 2) V
CM
= 0V, IO= 0mA
V
CC
= 30V, VEE= −30V to −10V 110 85 dB (min)
V
CM
= 0V, IO= 0mA
CMRR Common Mode Rejection Ratio V
CC
= 50V to 10V, VEE= −10V to −50V, 110 75 dB (min)
(Note 2) V
CM
= 20V to −20V, IO= 0mA
A
VOL
(Note 2)
Open Loop Voltage Gain R
L
=2kΩ, ∆ VO= 40V 115 80 dB (min)
GBWP Gain Bandwidth Product f
O
= 100kHz, VIN= 50mVrms 8 2 MHz (min)
e
IN
Input Noise IHF — A Weighting Filter 2.0 8 µV (max)
(Note 3) R
IN
= 600Ω (Input Referred)
SNR Signal-to-Noise Ratio P
O
= 1W, A—Weighted, 98 dB
Measured at 1kHz, R
S
=25Ω
P
O
= 25W, A—Weighted 112 dB
Measured at 1kHz, R
S
=25Ω
A
M
Mute Attenuation Pin 6,11 at 2.5V 115 80 dB (min)
Note 1: Operation is guaranteed up to 60V, however, distortion may be introduced from SPiKe Protection Circuitry if proper thermal considerations are not taken
into account. Refer to the Application Information section for a complete explanation.
Note 2: DC Electrical Test; Refer to Test Circuit
#
1.
Note 3: AC Electrical Test; Refer to Test Circuit
#
2.
Note 4: All voltages are measured with respect to the GND pins (5, 10), unless otherwise specified.
Note 5: 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. Specifications are not guaranteed for parameters where no limit
is given, however, the typical value is a good indication of device performance.
Note 6: For operating at case temperatures above 25˚C, the device must be derated based on a 150˚C maximum junction temperature and a thermal resistance
of θ
JC
= 1˚C/W (junction to case) for the T package. Refer to the section Determining the Correct Heat Sink in the Application Information section.
Note 7: Human body model, 100pF discharged through a 1.5kΩ resistor.
Note 8: The operating junction temperature maximum is 150˚C, however, the instantaneous Safe Operating Area temperature is 250˚C.
Note 9: Typicals are measured at 25˚C and represent the parametric norm.
Note 10: Limits are guarantees that all parts are tested in production to meet the stated values.
Note 11: V
EE
must have at least −9V at its pin with reference to ground in order for the under-voltage protection circuitry to be disabled. In addition, the voltage
differential between V
CC
and VEEmust be greater than 14V.
Note 12: The output dropout voltage, V
OD
, is the supply voltage minus the clipping voltage. Refer to the Clipping Voltage vs. Supply Voltage graph in the Typical
Performance Characteristics section.
Note 13: When using the isolated package (TF), the θ
JC
is 2˚C/W verses 1˚C/W for the non-isolated package (T). This increased thermal resistance from junction
to case requires a lower supply voltage for decreased power dissipation within the package. Voltages higher than
±
26V maybe used but will require a heat sink with
less than 1˚C/W thermal resistance to avoid activating thermal shutdown during normal operation.
LM4766
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Test Circuit#1 (Note 2) (DC Electrical Test Circuit)
10092803
Test Circuit#2 (Note 3) (AC Electrical Test Circuit)
10092804
LM4766
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Bridged Amplifier Application Circuit
10092805
FIGURE 2. Bridged Amplifier Application Circuit
LM4766
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Single Supply Application Circuit
Note:*Optional components dependent upon specific design requirements.
Auxiliary Amplifier Application Circuit
10092806
FIGURE 3. Single Supply Amplifier Application Circuit
10092807
FIGURE 4. Special Audio Amplifier Application Circuit
LM4766
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Page 8
Equivalent Schematic
(excluding active protection circuitry)
LM4766 (One Channel Only)
10092808
LM4766
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Page 9
External Components Description
Components Functional Description
1R
B
Prevents currents from entering the amplifier’s non-inverting input which may be passed through to the load
upon power down of the system due to the low input impedance of the circuitry when the undervoltage
circuitry is off. This phenomenon occurs when the supply voltages are below 1.5V.
2R
i
Inverting input resistance to provide AC gain in conjunction with Rf.
3R
f
Feedback resistance to provide AC gain in conjunction with Ri.
4C
i
(Note 14)
Feedback capacitor which ensures unity gain at DC. Also creates a highpass filter with R
i
at fC= 1/(2πRiCi).
5C
S
Provides power supply filtering and bypassing. Refer to the Supply Bypassing application section for proper
placement and selection of bypass capacitors.
6R
V
(Note 14)
Acts as a volume control by setting the input voltage level.
7R
IN
(Note 14)
Sets the amplifier’s input terminals DC bias point when C
IN
is present in the circuit. Also works with CINto
create a highpass filter at f
C
= 1/(2πRINCIN). Refer to Figure 4.
8C
IN
(Note 14)
Input capacitor which blocks the input signal’s DC offsets from being passed onto the amplifier’s inputs.
9R
SN
(Note 14)
Works with C
SN
to stabilize the output stage by creating a pole that reduces high frequency instabilities.
10 C
SN
(Note 14)
Works with R
SN
to stabilize the output stage by creating a pole that reduces high frequency instabilities.
The pole is set at f
C
= 1/(2πRSNCSN). Refer to Figure 4.
11 L (Note 14) Provides high impedance at high frequencies so that R may decouple a highly capacitive load and reduce
the Q of the series resonant circuit. Also provides a low impedance at low frequencies to short out R and
pass audio signals to the load. Refer to Figure 4.
12 R (Note 14)
13 R
A
Provides DC voltage biasing for the transistor Q1 in single supply operation.
14 C
A
Provides bias filtering for single supply operation.
15 R
INP
(Note 14)
Limits the voltage difference between the amplifier’s inputs for single supply operation. Refer to the Clicks
and Pops application section for a more detailed explanation of the function of R
INP
.
16 R
BI
Provides input bias current for single supply operation. Refer to the Clicks and Pops application section for
a more detailed explanation of the function of R
BI
.
17 R
E
Establishes a fixed DC current for the transistor Q1 in single supply operation. This resistor stabilizes the
half-supply point along with C
A
.
18 R
M
Mute resistance set up to allow 0.5mA to be drawn from pin 6 or 11 to turn the muting function off.
→
R
M
is calculated using: RM≤ (|VEE| − 2.6V)/l where l ≥ 0.5mA. Refer to the Mute Attenuation vs Mute
Current curves in the Typical Performance Characteristics section.
19 C
M
Mute capacitance set up to create a large time constant for turn-on and turn-off muting.
20 S
1
Mute switch that mutes the music going into the amplifier when opened.
Note 14: Optional components dependent upon specific design requirements.
LM4766
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Typical Performance Characteristics
THD+N vs Frequency THD+N vs Frequency
10092855 10092856
THD+N vs Output Power THD+N vs Output Power
10092858 10092857
THD+N vs Distribution THD+N vs Distribution
10092872 10092873
LM4766
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Page 11
Typical Performance Characteristics (Continued)
Channel Separation vs
Frequency
Clipping Voltage vs
Supply Voltage
10092810
10092868
Output Power vs
Load Resustance
Output Power vs
Supply Voltage
10092874
10092878
Power Dissipation vs
Output Power
Power Dissipation vs
Output Power
10092876 10092877
LM4766
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Typical Performance Characteristics (Continued)
Max Heatsink Thermal Resistance (˚C/W)
at the Specified Ambient Temperature (˚C)
10092875
Note: The maximum heatsink thermal resistance values,
θ
SA
, in the table above were calculated using a θCS=
0.2˚C/W due to thermal compound.
Safe Area
SPiKe Protection
Response
10092859
10092860
Pulse Power Limit Pulse Power Limit
10092863
10092864
LM4766
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Typical Performance Characteristics (Continued)
Pulse Response Large Signal Response
10092866
10092887
Power Supply
Rejection Ratio
Common-Mode
Rejection Ratio
10092888 10092889
Open Loop
Frequency Response
Supply Current vs
Case Temperature
10092890
10092865
LM4766
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Typical Performance Characteristics (Continued)
Input Bias Current vs
Case Temperature
Mute Attenuation vs
Mute Current (per Amplifier)
10092867
10092885
Mute Attenuation vs
Mute Current (per Amplifier)
Output Power/Channel
vs Supply Voltage
f = 1kHz, R
L
=4Ω, 80kHz BW
10092886
10092891
Output Power/Channel
vs Supply Voltage
f = 1kHz, R
L
=6Ω, 80kHz BW
Output Power/Channel
vs Supply Voltage
f = 1kHz, RL=8Ω, 80kHz BW
10092892 10092893
LM4766
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Page 15
Application Information
MUTE MODE
The muting function of the LM4766 allows the user to mute
the music going into the amplifier by drawing more than
0.5mA out of each mute pin on the device. This is accomplished as shown in the Typical Application Circuit where the
resistor R
M
is chosen with reference to your negative supply
voltage and is used in conjunction with a switch. The switch
when opened cuts off the current flow from pin 6 or 11 to
−V
EE
, thus placing the LM4766 into mute mode. Refer to the
Mute Attenuation vs Mute Current curves in the Typical
Performance Characteristics section for values of attenuation per current out of pins 6 or 11. The resistance R
M
is
calculated by the following equation:
R
M
≤ (|−VEE| − 2.6V)/I
pin6
where I
pin6=Ipin11
≥ 0.5mA.
Both pins 6 and 11 can be tied together so that only one
resistor and capacitor are required for the mute function. The
mute resistance must be chosen such that greater than 1mA
is pulled through the resistor R
M
so that each amplifier is fully
pulled out of mute mode. Taking into account supply line
fluctuations, it is a good idea to pull out 1mA per mute pin or
2 mA total if both pins are tied together.
UNDER-VOLTAGE PROTECTION
Upon system power-up, the under-voltage protection circuitry allows the power supplies and their corresponding
capacitors to come up close to their full values before turning
on the LM4766 such that no DC output spikes occur. Upon
turn-off, the output of the LM4766 is brought to ground
before the power supplies such that no transients occur at
power-down.
OVER-VOLTAGE PROTECTION
The LM4766 contains over-voltage protection circuitry that
limits the output current to approximately 4.0A
PK
while also
providing voltage clamping, though not through internal
clamping diodes. The clamping effect is quite the same,
however, the output transistors are designed to work alternately by sinking large current spikes.
SPiKe PROTECTION
The LM4766 is protected from instantaneous peaktemperature stressing of the power transistor array. The Safe
Operating graph in the Typical Performance Characteris-
tics section shows the area of device operation where
SPiKe Protection Circuitry is not enabled. The waveform to
the right of the SOA graph exemplifies how the dynamic
protection will cause waveform distortion when enabled.
Please refer to AN-898 for more detailed information.
THERMAL PROTECTION
The LM4766 has a sophisticated thermal protection scheme
to prevent long-term thermal stress of the device. When the
temperature on the die reaches 165˚C, the LM4766 shuts
down. It starts operating again when the die temperature
drops to about 155˚C, but if the temperature again begins to
rise, shutdown will occur again at 165˚C. Therefore, the
device is allowed to heat up to a relatively high temperature
if the fault condition is temporary, but a sustained fault will
cause the device to cycle in a Schmitt Trigger fashion between the thermal shutdown temperature limits of 165˚C and
155˚C. This greatly reduces the stress imposed on the IC by
thermal cycling, which in turn improves its reliability under
sustained fault conditions.
Since the die temperature is directly dependent upon the
heat sink used, the heat sink should be chosen such that
thermal shutdown will not be reached during normal operation. Using the best heat sink possible within the cost and
space constraints of the system will improve the long-term
reliability of any power semiconductor device, as discussed
in the Determining the Correct Heat Sink Section.
DETERMlNlNG MAXIMUM POWER DISSIPATION
Power dissipation within the integrated circuit package is a
very important parameter requiring a thorough understanding if optimum power output is to be obtained. An incorrect
maximum power dissipation calculation may result in inadequate heat sinking causing thermal shutdown and thus
limiting the output power.
Equation (1) exemplifies the theoretical maximum power
dissipation point of each amplifier where V
CC
is the total
supply voltage.
P
DMAX=VCC
2
/2π2R
L
(1)
Thus by knowing the total supply voltage and rated output
load, the maximum power dissipation point can be calculated. The package dissipation is twice the number which
results from Equation (1) since there are two amplifiers in
each LM4766. Refer to the graphs of Power Dissipation
versus Output Power in the Typical Performance Charac-
teristics section which show the actual full range of power
dissipation not just the maximum theoretical point that results from Equation (1).
DETERMINING THE CORRECT HEAT SINK
The choice of a heat sink for a high-power audio amplifier is
made entirely to keep the die temperature at a level such
that the thermal protection circuitry does not operate under
normal circumstances.
The thermal resistance from the die (junction) to the outside
air (ambient) is a combination of three thermal resistances,
θ
JC
, θCS, and θSA. In addition, the thermal resistance, θ
JC
(junction to case), of the LM4766T is 1˚C/W. Using Thermalloy Thermacote thermal compound, the thermal resistance,
θ
CS
(case to sink), is about 0.2˚C/W. Since convection heat
flow (power dissipation) is analogous to current flow, thermal
resistance is analogous to electrical resistance, and temperature drops are analogous to voltage drops, the power
dissipation out of the LM4766 is equal to the following:
P
DMAX
=(T
JMAX−TAMB
)/θ
JA
(2)
where T
JMAX
= 150˚C, T
AMB
is the system ambient tempera-
ture and θ
JA
= θJC+ θCS+ θSA.
10092852
Once the maximum package power dissipation has been
calculated using Equation (1), the maximum thermal resistance, θ
SA
, (heat sink to ambient) in ˚C/W for a heat sink can
be calculated. This calculation is made using Equation (3)
which is derived by solving for θ
SA
in Equation (2).
θ
SA
= [(T
JMAX−TAMB
)−P
DMAX(θJC+θCS
)]/P
DMAX
(3)
LM4766
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Page 16
Application Information (Continued)
Again it must be noted that the value of θ
SA
is dependent
upon the system designer’s amplifier requirements. If the
ambient temperature that the audio amplifier is to be working
under is higher than 25˚C, then the thermal resistance for the
heat sink, given all other things are equal, will need to be
smaller.
SUPPLY BYPASSING
The LM4766 has excellent power supply rejection and does
not require a regulated supply. However, to improve system
performance as well as eliminate possible oscillations, the
LM4766 should have its supply leads bypassed with lowinductance capacitors having short leads that are located
close to the package terminals. Inadequate power supply
bypassing will manifest itself by a low frequency oscillation
known as “motorboating” or by high frequency instabilities.
These instabilities can be eliminated through multiple bypassing utilizing a large tantalum or electrolytic capacitor
(10µF or larger) which is used to absorb low frequency
variations and a small ceramic capacitor (0.1µF) to prevent
any high frequency feedback through the power supply lines.
If adequate bypassing is not provided, the current in the
supply leads which is a rectified component of the load
current may be fed back into internal circuitry. This signal
causes distortion at high frequencies requiring that the supplies be bypassed at the package terminals with an electrolytic capacitor of 470µF or more.
BRIDGED AMPLIFIER APPLICATION
The LM4766 has two operational amplifiers internally, allowing for a few different amplifier configurations. One of these
configurations is referred to as “bridged mode” and involves
driving the load differentially through the LM4766’s outputs.
This configuration is shown in Figure 2. 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 distinct advantage over the
single-ended configuration, as it provides differential drive to
the load, thus doubling output swing for a specified supply
voltage. Consequently, theoretically 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.
A direct consequence of the increased power delivered to
the load by a bridge amplifier is an increase in internal power
dissipation. For each operational amplifier in a bridge configuration, the internal power dissipation will increase by a
factor of two over the single ended dissipation. Thus, for an
audio power amplifier such as the LM4766, which has two
operational amplifiers in one package, the package dissipation will increase by a factor of four. To calculate the
LM4766’s maximum power dissipation point for a bridged
load, multiply Equation (1) by a factor of four.
This value of P
DMAX
can be used to calculate the correct size
heat sink for a bridged amplifier application. Since the internal dissipation for a given power supply and load is increased by using bridged-mode, the heatsink’s θ
SA
will have
to decrease accordingly as shown by Equation (3). Refer to
the section, Determining the Correct Heat Sink, for a more
detailed discussion of proper heat sinking for a given application.
SINGLE-SUPPLY AMPLIFIER APPLICATION
The typical application of the LM4766 is a split supply amplifier. But as shown in Figure 3, the LM4766 can also be
used in a single power supply configuration. This involves
using some external components to create a half-supply bias
which is used as the reference for the inputs and outputs.
Thus, the signal will swing around half-supply much like it
swings around ground in a split-supply application. Along
with proper circuit biasing, a few other considerations must
be accounted for to take advantage of all of the LM4766
functions, like the mute function.
CLICKS AND POPS
In the typical application of the LM4766 as a split-supply
audio power amplifier, the IC exhibits excellent “click” and
“pop” performance when utilizing the mute and standby
modes. In addition, the device employs Under-Voltage Protection, which eliminates unwanted power-up and powerdown transients. The basis for these functions are a stable
and constant half-supply potential. In a split-supply application, ground is the stable half-supply potential. But in a
single-supply application, the half-supply needs to charge up
just like the supply rail, V
CC
. This makes the task of attaining
a clickless and popless turn-on more challenging. Any uneven charging of the amplifier inputs will result in output
clicks and pops due to the differential input topology of the
LM4766.
To achieve a transient free power-up and power-down, the
voltage seen at the input terminals should be ideally the
same. Such a signal will be common-mode in nature, and
will be rejected by the LM4766. In Figure 3, the resistor R
INP
serves to keep the inputs at the same potential by limiting the
voltage difference possible between the two nodes. This
should significantly reduce any type of turn-on pop, due to an
uneven charging of the amplifier inputs. This charging is
based on a specific application loading and thus, the system
designer may need to adjust these values for optimal performance.
As shown in Figure 3, the resistors labeled R
BI
help bias up
the LM4766 off the half-supply node at the emitter of the
2N3904. But due to the input and output coupling capacitors
in the circuit, along with the negative feedback, there are two
different values of R
BI
, namely 10kΩ and 200kΩ. These
resistors bring up the inputs at the same rate resulting in a
popless turn-on. Adjusting these resistors values slightly
may reduce pops resulting from power supplies that ramp
extremely quick or exhibit overshoot during system turn-on.
AUDIO POWER AMPLlFIER DESIGN
Design a 30W/8Ω Audio Amplifier
Given:
Power Output 30Wrms
Load Impedance 8Ω
Input Level 1Vrms(max)
Input Impedance 47kΩ
Bandwidth 20Hz−20kHz
±
0.25dB
A designer must first determine the power supply requirements in terms of both voltage and current needed to obtain
the specified output power. V
OPEAK
can be determined from
Equation (4) and I
OPEAK
from Equation (5).
LM4766
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Page 17
Application Information (Continued)
(4)
(5)
To determine the maximum supply voltage the following
conditions must be considered. Add the dropout voltage to
the peak output swing V
OPEAK
, to get the supply rail at a
current of I
OPEAK
. The regulation of the supply determines
the unloaded voltage which is usually about 15% higher. The
supply voltage will also rise 10% during high line conditions.
Therefore the maximum supply voltage is obtained from the
following equation.
Max supplies ≈
±
(V
OPEAK+VOD
) (1 + regulation) (1.1)
For 30W of output power into an 8Ω load, the required
V
OPEAK
is 21.91V. A minimum supply rail of 25.4V results
from adding V
OPEAK
and VOD. With regulation, the maximum
supplies are
±
32V and the required I
OPEAK
is 2.74A from
Equation (5). It should be noted that for a dual 30W amplifier
into an 8Ω load the I
OPEAK
drawn from the supplies is twice
2.74A
PK
or 5.48APK. At this point it is a good idea to check
the Power Output vs Supply Voltage to ensure that the
required output power is obtainable from the device while
maintaining low THD+N. In addition, the designer should
verify that with the required power supply voltage and load
impedance, that the required heatsink value θ
SA
is feasible
given system cost and size constraints. Once the heatsink
issues have been addressed, the required gain can be determined from Equation (6).
(6)
From Equation (6), the minimum A
V
is: AV≥ 15.5.
By selecting a gain of 21, and with a feedback resistor, R
f
=
20kΩ, the value of R
i
follows from Equation (7).
R
i=Rf(AV
− 1) (7)
Thus with R
i
=1kΩ a non-inverting gain of 21 will result.
Since the desired input impedance was 47kΩ, a value of
47kΩ was selected for R
IN
. The final design step is to
address the bandwidth requirements which must be stated
as a pair of −3dB frequency points. Five times away from a
−3dB point is 0.17dB down from passband response which
is better than the required
±
0.25dB specified. This fact re-
sults in a low and high frequency pole of 4Hz and 100kHz
respectively. As stated in the External Components section, R
i
in conjunction with Cicreate a high-pass filter.
C
i
≥ 1/(2π*1kΩ*4Hz) = 39.8µF; use 39µF.
The high frequency pole is determined by the product of the
desired high frequency pole, f
H
, and the gain, AV. With a
A
V
= 21 and fH= 100kHz, the resulting GBWP is 2.1MHz,
which is less than the guaranteed minimum GBWP of the
LM4766 of 8MHz. This will ensure that the high frequency
response of the amplifier will be no worse than 0.17dB down
at 20kHz which is well within the bandwidth requirements of
the design.
LM4766
www.national.com17
Page 18
Physical Dimensions inches (millimeters)
unless otherwise noted
Non-Isolated TO-220 15-Lead Package
Order Number LM4766T
NS Package Number TA15A
LM4766
www.national.com 18
Page 19
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
Isolated TO-220 15-Lead Package
Order Number LM4766TF
NS Package Number TF15B
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.
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Support Center
Email: new.feedback@nsc.com
Tel: 1-800-272-9959
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www.national.com
LM4766 Overture
™
Audio Power Amplifier Series
Dual 40W Audio Power Amplifier with Mute
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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