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February 1998
LM1876 Overture
Dual 20W Audio Power Amplifier with Mute and Standby Modes
LM1876
Overture
™
Audio Power Amplifier Series
Dual 20W Audio Power Amplifier with Mute and Standby
Modes
General Description
The LM1876 is a stereo audio amplifier capable of delivering
typically 20W per channel of continuous average output
power into a 4Ω or 8Ω load with less than 0.1%(THD + N).
Each amplifierhasanindependent smooth transition fade-in/
out mute and a power conserving standby mode which can
be controlled by external logic.
The performance of the LM1876, utilizing its Self Peak Instantaneous Temperature (˚Ke) (SPiKe
cuitry, 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.
™
) Protection Cir-
Typical Application
Key Specifications
j
THD+N at 1 kHz at 2 x 15W continuous average
output power into 4Ω or 8Ω: 0.1%(max)
j
THD+N at 1 kHz at continuous average
output power of 2 x 20W into 8Ω: 0.009%(typ)
j
Standby current: 4.2 mA (typ)
Features
n SPiKe Protection
n Minimal amount of external components necessary
n Quiet fade-in/out mute mode
n Standby-mode
n Isolated 15-lead TO-220 package
n Non-Isolated 15-lead TO-220 package
Applications
n High-end stereo TVs
n Component stereo
n Compact stereo
Connection Diagram
Plastic Package
™
Audio Power Amplifier Series
DS012072-2
Top View
Isolated Package
Order Number LM1876TF
DS012072-1
FIGURE 1. Typical Audio Amplifier Application Circuit
Note: Numbers in parentheses represent pinout for amplifier B.
*
Optional component dependent upon specific design requirements.
SPiKe™Protection and Overture™are trademarks of National Semiconductor Corporation.
© 1999 National Semiconductor Corporation DS012072 www.national.com
See NS Package Number TF15B
Non-Isolated Package
Order Number LM1876T
See NS Package Number TA15A
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
(No Input) 64V
Supply Voltage |V
(with Input) 64V
Common Mode Input Voltage (V
Differential Input Voltage 54V
Output Current Internally Limited
Power Dissipation (Note 6) 62.5W
ESD Susceptability (Note 7) 2000V
Junction Temperature (Note 8) 150˚C
|+|VEE|
CC
|+|VEE|
CC
or VEE) and
CC
|+|VEE| ≤ 54V
|V
CC
Thermal Resistance
Isolated TF-Package
θ
JC
2˚C/W
Non-Isolated T-Package
θ
JC
1˚C/W
Soldering Information
TF Package (10 sec.) 260˚C
Storage Temperature −40˚C to +150˚C
Operating Ratings (Notes 4, 5)
Temperature Range
≤ TA≤ T
T
MIN
Supply Voltage |V
MAX
|+|VEE| (Note 1) 20V to 64V
CC
−20˚C ≤ TA≤ +85˚C
Electrical Characteristics (Notes 4, 5)
The following specifications apply for V
25˚C.
CC
=
+22V, V
Symbol Parameter Conditions LM1876 Units
| + Power Supply Voltage GND − VEE≥ 9V 20 V (min)
|V
CC
|V
| (Note 11) 64 V (max)
EE
P
O
Output Power THD + N=0.1%(max),
(Note 3) (Continuous Average) f=1 kHz
THD + N Total Harmonic Distortion 15 W/ch, R
Plus Noise 15 W/ch, R
X
SR
talk
Channel Separation f=1 kHz, V
Slew Rate V
(Note 3)
I
total
Total Quiescent Power Both Amplifiers V
(Note 2) Supply Current V
V
OS
(Note 2)
I
B
I
OS
I
O
V
OD
Input Offset Voltage V
Input Bias Current V
Input Offset Current V
Output Current Limit |VCC|=|VEE|=10V, t
Output Dropout Voltage |VCC–VO|, V
(Note 2) (Note 12) |V
PSRR Power Supply Rejection Ratio V
(Note 2) V
CMRR Common Mode Rejection Ratio V
(Note 2) V
=
EE
−22V with R
=
8Ω unless otherwise specified. Limits apply for T
L
Typical Limit
(Limits)
(Note 9) (Note 10)
|V
|=|VEE|=22V, R
CC
|V
|=|VEE|=20V, R
CC
=
8Ω 0.08
L
=
4Ω,|V
L
20 Hz ≤ f ≤ 20 kHz, A
=
O
=
1.414 Vrms, t
IN
=
=
0V, I
O
0mA
O
=
8Ω 20 15 W/ch (min)
L
=
4Ω (Note 13) 22 15 W/ch (min)
L
|=|VEE|=20V 0.1
CC
=
26 dB
V
10.9 Vrms 80 dB
=
2 ns 18 12 V/µs (min)
rise
=
0V,
CM
Standby: Off 50 80 mA (max)
Standby: On 4.2 6 mA (max)
=
=
0V, I
CM
CM
CM
V
O
CC
CM
V
CC
V
CM
CC
CM
=
=
=
O–VEE
=
=
=
=
=
=
0V
0 mA 2.0 15 mV (max)
O
=
0V, I
0 mA 0.2 0.5 µA (max)
O
=
0V, I
0 mA 0.002 0.2 µA (max)
O
=
CC
=
|, V
EE
25V to 10V, V
=
0V, I
O
25V, V
EE
=
0V, I
O
35V to 10V, V
10V to −10V, I
=
10 ms, 3.5 2.9 Apk (min)
ON
=
20V, I
−20V, I
+100 mA 1.8 2.3 V (max)
O
=
−100 mA 2.5 3.2 V (max)
O
=
−25V, 115 85 dB (min)
EE
0mA
=
−25V to −10V 110 85 dB (min)
0mA
=
−10V to −35V, 110 80 dB (min)
EE
=
0mA
O
=
A
%
%
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Electrical Characteristics (Notes 4, 5) (Continued)
The following specifications apply for V
25˚C.
CC
=
+22V, V
Symbol Parameter Conditions LM1876 Units
A
VOL
(Note 2)
Open Loop Voltage Gain R
GBWP Gain Bandwidth Product f
e
IN
Input Noise IHF— A Weighting Filter 2.0 8 µV (max)
(Note 3) R
SNR Signal-to-Noise Ratio P
A
M
Mute Attenuation Pin 6,11 at 2.5V 115 80 dB (min)
Standby
Pin
V
V
Standby Low Input Voltage Not in Standby Mode 0.8 V (max)
IL
Standby High Input Voltage In Standby Mode 2.0 2.5 V (min)
IH
Mute pin
V
V
Note 1: Operation is guaranteed up to 64V,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
Note 3: AC Electrical Test; Refer to Test Circuit
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 func-
tional, but do not guarantee specific performance limits. Electrical Characteristics state DC andAC electrical specifications under particular test conditions which guarantee specific performance limits. This assumes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters where no limit is
given, however, the typical value is a good indication of device performance.
Note 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
formation section.
Note 7: Human body model, 100 pF discharged through a 1.5 kΩ 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
ferential between V
Note 12: The output dropout voltage, V
formance Characteristics section.
Note 13: Fora4Ωload, and with
supplies above
±
20V will only increase the internal power dissipation, not the possible output power. Increased power dissipation will require a larger heat sink as explained in the
Application Information section.
Mute Low Input Voltage Outputs Not Muted 0.8 V (max)
IL
Mute High Input Voltage Outputs Muted 2.0 2.5 V (min)
IH
#
1.
#
2.
=
2˚C/W (junction to case) for the TF package and θ
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 dif-
EE
and VEEmust be greater than 14V.
CC
±
20V,the LM1876 cannot deliver more than 22W into a 4Ω due to current limiting of the output transistors. Thus, increasing the power supply above
, is the supply voltage minus the clipping voltage. Refer to the Clipping Voltage vs. Supply Voltage graph in the Typical Per-
OD
±
20V supplies, the LM1876 can deliver typically 22W of continuous average output power with less than 0.1%(THD + N). With
JC
=
EE
−22V with R
=
8Ω unless otherwise specified. Limits apply for T
L
Typical Limit
(Limits)
(Note 9) (Note 10)
=
2kΩ,∆V
L
=
100 kHz, V
O
=
600Ω (Input Referred)
IN
=
1W, A — Weighted, 98 dB
O
Measured at 1 kHz, R
=
P
15W, A — Weighted 108 dB
O
Measured at 1 kHz, R
=
1˚C/W for the T package. Refer to the section Determining the Correct Heat Sink in the Application In-
=
20 V 110 90 dB (min)
O
=
50 mVrms 7.5 5 MHz (min)
IN
=
25Ω
S
=
25Ω
S
=
A
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Test Circuit#1 (Note 2) (DC Electrical Test Circuit)
Test Circuit#2 (Note 3) (AC Electrical Test Circuit)
DS012072-3
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DS012072-4
Bridged Amplifier Application Circuit
FIGURE 2. Bridged Amplifier Application Circuit
Single Supply Application Circuit
DS012072-5
FIGURE 3. Single Supply Amplifier Application Circuit
Note:*Optional components dependent upon specific design requirements.
DS012072-6
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Auxiliary Amplifier Application Circuit
FIGURE 4. Special Audio Amplifier Application Circuit
Equivalent Schematic
(excluding active protection circuitry)
LM1876 (per Amp)
DS012072-7
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DS012072-8
External Components Description
Components Functional Description
1R
B
2R
i
3R
f
4C
i
(Note 14)
5C
S
6R
V
(Note 14)
7R
IN
(Note 14)
8C
IN
(Note 14)
9R
SN
(Note 14)
10 C
SN
(Note 14)
11 L (Note 14) Provides high impedance at high frequencies so that R may decouple a highly capacitive load and reduce
12 R (Note 14)
13 R
A
14 C
A
15 R
INP
(Note 14)
16 R
BI
17 R
E
Note 14: Optional components dependent upon specific design requirements.
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.
Inverting input resistance to provide AC gain in conjunction with Rf.
Feedback resistance to provide AC gain in conjunction with Ri.
Feedback capacitor which ensures unity gain at DC. Also creates a highpass filter with R
).
1/(2πR
iCi
=
at f
i
C
Provides power supply filtering and bypassing. Refer to the Supply Bypassing application section for
proper placement and selection of bypass capacitors.
Acts as a volume control by setting the input voltage level.
Sets the amplifier’s input terminals DC bias point when C
create a highpass filter at f
=
1/(2πR
C
INCIN
). Refer to
is present in the circuit. Also works with CINto
IN
Figure 4
.
Input capacitor which blocks the input signal’s DC offsets from being passed onto the amplifier’s inputs.
Works with C
Works with R
The pole is set at f
to stabilize the output stage by creating a pole that reduces high frequency instabilities.
SN
to stabilize the output stage by creating a pole that reduces high frequency instabilities.
SN
=
1/(2πR
C
SNCSN
). Refer to
Figure 4
.
the Q of the series resonant circuit. Also provides a low impedance at low frequencies to short out R and
Figure 4
pass audio signals to the load. Refer to
.
Provides DC voltage biasing for the transistor Q1 in single supply operation.
Provides bias filtering for single supply operation.
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
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
Establishes a fixed DC current for the transistor Q1 in single supply operation. This resistor stabilizes the
half-supply point along with C
.
A
Typical Performance Characteristics
THD+NvsFrequency
DS012072-13
THD+NvsFrequency
DS012072-14
THD+NvsFrequency
DS012072-15
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Typical Performance Characteristics (Continued)
THD+Nvs
Output Power
THD+Nvs
Output Power
Clipping Voltage vs
Supply Voltage
DS012072-16
DS012072-19
THD+Nvs
Output Power
THD+Nvs
Output Power
Clipping Voltage vs
Supply Voltage
DS012072-17
DS012072-20
THD+Nvs
Output Power
DS012072-18
THD+Nvs
Output Power
DS012072-21
Clipping Voltage vs
Supply Voltage
DS012072-22
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DS012072-23
DS012072-24
Typical Performance Characteristics (Continued)
Output Power vs
Load Resistance
Output Power vs
Supply Voltage
Channel Separation vs
Frequency
DS012072-25
DS012072-28
Power Dissipation vs
Output Power
Output Mute vs
Mute Pin Voltage
Pulse Response
DS012072-26
DS012072-29
Power Dissipation vs
Output Power
DS012072-27
Output Mute vs
Mute Pin Voltage
DS012072-30
Large Signal Response
DS012072-31
DS012072-32
DS012072-33
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Typical Performance Characteristics (Continued)
Power Supply
Rejection Ratio
Safe Area
Pulse Thermal
Resistance
DS012072-34
DS012072-37
Common-Mode
Rejection Ratio
SPiKe Protection
Response
Pulse Thermal
Resistance
DS012072-35
DS012072-38
Open Loop
Frequency Response
DS012072-36
Supply Current vs
Supply Voltage
DS012072-39
Supply Current vs
Output Voltage
DS012072-40
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DS012072-41
DS012072-42
Typical Performance Characteristics (Continued)
Pulse Power Limit
Supply Current (ICC)vs
Standby Pin Voltage
DS012072-43
DS012072-46
Pulse Power Limit
Supply Current (IEE)vs
Standby Pin Voltage
DS012072-44
DS012072-47
Supply Current vs
Case Temperature
DS012072-45
Input Bias Current vs
Case Temperature
DS012072-48
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Application Information
MUTE MODE
By placing a logic-high voltage on the mute pins, the signal
going into the amplifiers will be muted. If the mute pins are
left floating or connected to a logic-low voltage, the amplifiers will be in a non-muted state. There are two mute pins,
one for each amplifier, so that one channel can be muted
without muting the other if the application requires such a
configuration. Refer to the TypicalPerformance Character-
istics section for curves concerning Mute Attenuation vs
Mute Pin Voltage.
STANDBY MODE
The standby mode of the LM1876 allows the user to drastically reduce power consumption when the amplifiers are
idle. By placing a logic-high voltage on the standby pins, the
amplifiers will go into Standby Mode. In this mode, the current drawn from the V
total for both amplifiers. The current drawn from the V
ply is typically 4.2 mA. Clearly,there is a significant reduction
in idle power consumption when using the standby mode.
There are two Standby pins, so that one channel can be put
in standby mode without putting the other amplifier in
standby if the application requires such flexibility. Refer to
the Typical Performance Characteristics section for
curves showing Supply Current vs. Standby Pin Voltage for
both supplies.
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 LM1876 such that no DC output spikes occur. Upon
turn-off, the output of the LM1876 is brought to ground before the power supplies such that no transients occur at
power-down.
OVER-VOLTAGE PROTECTION
The LM1876 contains over-voltage protection circuitry that
limits the output current to approximately 3.5 Apk 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 LM1876 is protected from instantaneous
peak-temperature stressing of the power transistor array.
The Safe Operating graph in the Typical Performance
Characteristics 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.
THERMAL PROTECTION
The LM1876 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 LM1876 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 be-
supply is typically less than 10 µA
CC
sup-
EE
tween 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
voltage.
=
P
DMAX
V
CC
2/2π2R
is the total supply
CC
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
LM1876. Refer to the graphs of Power Dissipation versus
Output Power in the Typical Performance Characteristics
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
, θCS, and θSA. In addition, the thermal resistance, θ
JC
(junction to case), of the LM1876TF is 2˚C/W and the
LM1876T is 1˚C/W. Using Thermalloy Thermacote thermal
compound, the thermal resistance, θ
about 0.2˚C/W. Since convection heat flow (power dissipa-
(case to sink), is
CS
tion) 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 LM1876 is equal to the following:
=
where T
ture and θ
JMAX
JA
P
=
150˚C, T
=
θ
JC
(T
DMAX
AMB
+ θCS+ θSA.
JMAX−TAMB
is the system ambient tempera-
)/θ
JA
(2)
Once the maximum package power dissipation has been
calculated using equation (1), the maximum thermal resistance, θ
be calculated. This calculation is made using equation (3)
which is derived by solving for θ
Again it must be noted that the value of θ
upon the system designer’s amplifier requirements. If the
, (heat sink to ambient) in ˚C/W for a heat sink can
SA
in equation (2).
=
θ
[(T
SA
JMAX−TAMB
SA
)−P
DMAX(θJC+θCS
)]/P
DMAX
is dependent
SA
(3)
ambient temperature that the audio amplifier is to be working
JC
www.national.com 12
Application Information (Continued)
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 LM1876 has excellent power supply rejection and does
not require a regulated supply. However, to improve system
performance as well as eliminate possible oscillations, the
LM1876 should have its supply leads bypassed with
low-inductance 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
(10 µF or larger) which is used to absorb low frequency
variations and a small ceramic capacitor (0.1 µF) 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 470 µF or more.
BRIDGED AMPLIFIER APPLICATION
The LM1876 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 LM1876’s outputs.
This configuration is shown in
eration 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 LM1876, which has two
operational amplifiers in one package, the package dissipation will increase by a factor of four. To calculate the
LM1876’s maximum power dissipation point for a bridged
load, multiply equation (1) by a factor of four.
This value of P
heat sink for a bridged amplifier application. Since the inter-
can be used to calculate the correct size
DMAX
nal dissipation for a given power supply and load is increased by using bridged-mode, the heatsink’s θ
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.
Figure 2
. Bridged mode op-
will have
SA
SINGLE-SUPPLYAMPLIFIER APPLICATION
The typical application of the LM1876 is a split supply amplifier. But as shown in
Figure 3
, the LM1876 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 LM1876
functions.
The LM1876 possesses a mute and standby function with internal logic gates that are half-supply referenced. Thus, to
enable either the Mute or Standby function, the voltage at
these pins must be a minimum of 2.5V above half-supply. In
single-supply systems, devices such as microprocessors
and simple logic circuits used to control the mute and
standby functions, are usually referenced to ground, not
half-supply. Thus, to use these devices to control the logic
circuitry of the LM1876, a “level shifter,” like the one shown in
Figure 5
, must be employed. A level shifter is not needed in
a split-supply configuration since ground is also half-supply.
DS012072-12
FIGURE 5. Level Shift Circuit
When the voltage at the Logic Input node is 0V, the 2N3904
is “off” and thus resistor R
the supply.This enables the mute or standby function. When
pulls up mute or standby input to
c
the Logic Input is 5V, the 2N3904 is “on” and consequently,
the voltage at the collector is essentially 0V.This will disable
the mute or standby function, and thus the amplifier will be in
its normal mode of operation. R
an RC time constant that reduces transients when the mute
, along with C
shift
shift
, creates
or standby functions are enabled or disabled. Additionally,
R
limits the current supplied by the internal logic gates of
shift
the LM1876 which insures device reliability. Refer to the
Mute Mode and Standby Mode sections in the Application
Information section for a more detailed description of these
functions.
CLICKS AND POPS
In the typical application of the LM1876 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 power-down 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
a clickless and popless turn-on more challenging. Any un-
. This makes the task of attaining
CC
even charging of the amplifier inputs will result in output
clicks and pops due to the differential input topology of the
LM1876.
www.national.com13
Application Information (Continued)
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 LM1876. In
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 RBIhelp bias up
the LM1876 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
sistors bring up the inputs at the same rate resulting in a pop-
, namely 10 kΩ and 200 kΩ. These re-
BI
less 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 15W/8Ω Audio Amplifier
Given:
Power Output 15 Wrms
Load Impedance 8Ω
Input Level 1 Vrms(max)
Input Impedance 47 kΩ
Bandwidth 20 Hz−20 kHz
A designer must first determine the power supply requirements in terms of both voltage and current needed to obtain
the specified output power. V
equation (4) and I
OPEAK
To determine the maximum supply voltage the following conditions must be considered. Add the dropout voltage to the
peak output swing V
of I
. The regulation of the supply determines the un-
OPEAK
OPEAK
Figure 3
, the resistor R
±
0.25 dB
can be determined from
OPEAK
from equation (5).
, to get the supply rail at a current
INP
(4)
(5)
loaded 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 15W of output power into an 8Ω load, the required
V
is 15.49V. A minimum supply rail of 20.5V results
OPEAK
from adding V
supplies are
equation (5). It should be noted that for a dual 15W amplifier
into an 8Ω load the I
1.94 Apk or 3.88 Apk. At this point it is a good idea to check
and VOD. With regulation, the maximum
OPEAK
±
26V and the required I
drawn from the supplies is twice
OPEAK
OPEAK
is 1.94A from
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
given system cost and size constraints. Once the heatsink
is feasible
SA
issues have been addressed, the required gain can be determined from Equation (6).
(6)
From equation 6, the minimum A
By selecting a gain of 21, and with a feedback resistor, R
20 kΩ, the value of R
Thus with R
Since the desired input impedance was 47 kΩ, a value of 47
i
kΩ was selected for R
the bandwidth requirements which must be stated as a pair
follows from equation (7).
i
=
R
=
i
1kΩa non-inverting gain of 21 will result.
. The final design step is to address
IN
is: AV≥ 11.
V
− 1) (7)
R
f(AV
f
of −3 dB frequency points. Five times away from a −3 dB
point is 0.17 dB down from passband response which is better than the required
±
0.25 dB specified. This fact results in
a low and high frequency pole of 4 Hz and 100 kHz respectively.As stated in the External Components section, R
conjunction with C
≥ 1/(2π*1kΩ*4 Hz)=39.8 µF; use 39 µF.
C
i
create a high-pass filter.
i
i
The high frequency pole is determined by the product of the
desired high frequency pole, f
=
A
21 and f
V
which is less than the guaranteed minimum GBWP of the
=
100 kHz, the resulting GBWP is 2.1 MHz,
H
, and the gain, AV. With a
H
LM1876 of 5 MHz. This will ensure that the high frequency
response of the amplifier will be no worse than 0.17 dB down
at 20 kHz which is well within the bandwidth requirements of
the design.
=
in
www.national.com 14
Physical Dimensions inches (millimeters) unless otherwise noted
Isolated TO-220 15-Lead Package
Order Number LM1876TF
NS Package Number TF15B
www.national.com15
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
Audio Power Amplifier Series
™
LM1876 Overture
Non-Isolated TO-220 15-Lead Package
Order Number LM1876T
NS Package Number TA15A
Dual 20W Audio Power Amplifier with Mute and Standby Modes
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Tel: 1-800-272-9959
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www.national.com
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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