Datasheet LM4766TF Datasheet (NSC)

Page 1
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 8load 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 8: 0.1% (max)
n THD+N at 1kHz at continuous average output power of
2 x 30W into 8: 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
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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)
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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
= 600(Input Referred)
SNR Signal-to-Noise Ratio P
O
= 1W, A—Weighted, 98 dB
Measured at 1kHz, R
S
=25
P
O
= 25W, A—Weighted 112 dB
Measured at 1kHz, R
S
=25
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 1.5kresistor.
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.
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Test Circuit#1 (Note 2) (DC Electrical Test Circuit)
10092803
Test Circuit#2 (Note 3) (AC Electrical Test Circuit)
10092804
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Bridged Amplifier Application Circuit
10092805
FIGURE 2. Bridged Amplifier Application Circuit
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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
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Equivalent Schematic
(excluding active protection circuitry)
LM4766 (One Channel Only)
10092808
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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.
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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
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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
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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
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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
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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
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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)
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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.
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 10kand 200k. 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/8Audio Amplifier
Given:
Power Output 30Wrms
Load Impedance 8
Input Level 1Vrms(max)
Input Impedance 47k
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).
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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 8load, 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 8load 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
=
20k, 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 47k, a value of 47kwas 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*1k*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
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Page 18
Physical Dimensions inches (millimeters)
unless otherwise noted
Non-Isolated TO-220 15-Lead Package
Order Number LM4766T
NS Package Number TA15A
LM4766
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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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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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