NSC LM4880 Dual 250 mw audio power amplifier with shutdown mode Datasheet

LM4880
Dual 250 mW Audio Power Amplifier with Shutdown
Mode
General Description
The LM4880 is a dual audio power amplifier capable of delivering typically 250 mW per channel of continuous average
power to an 8Ω load with 0.1% (THD) using a 5V power supply.
Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal amount of
external components using surface mount packaging.
Since the LM4880 does not require bootstrap capacitors or
snubber networks, it is optimally suited for low-power portable systems.
The LM4880 features an externally controlled, low-power
consumption shutdown mode, as well as an internal thermal
shutdown protection mechanism.
The unity-gain stable LM4880 can be configured by external
gain-setting resistors.
Key Specifications
n THD at 1 kHz at 85 mW continuous average output
power into 32Ω: 0.1% (typ)
n Output power at 10% THD + N at 1 kHz into 8Ω:
325 mW (typ)
n Shutdown Current: 0.7 µA (typ)
Features
n No bootstrap capacitors or snubber circuits are
necessary
n Small Outline (SO) and DIP packaging
n Unity-gain stable
n External gain configuration capability
Applications
n Headphone Amplifier
n Personal Computers
n CD-ROM Players
n THD at 1 kHz at 200 mW continuous average output
power into 8Ω: 0.1% (max)
Typical Application
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*Refer to the Application Information section for information concerning proper selection of the input and output coupling capacitors.
FIGURE 1. Typical Audio Amplifier Application Circuit
Boomer ® is a registered trademark of National Semiconductor Corporation.
© 1999 National Semiconductor Corporation
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LM4880 Boomer Audio Power Amplifier Series Dual 250 mW Audio Power Amplifier with
Shutdown Mode
November 1995
Connection Diagram
Small Outline and
DIP Packages
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Top View
Order Number LM4880M or LM4880N
See NS Package Number M08A for SO
or NS Package Number N08E for DIP
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Absolute Maximum Ratings (Note 2)
See AN-450 “Surface Mounting and their Effects on
Product Reliability” for other methods of soldering surface
mount devices.
Thermal Resistance
37˚C/W
θJC (DIP)
107˚C/W
θJA (DIP)
35˚C/W
θJC (SO)
170˚C/W
θJA (SO)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage
Storage Temperature
Input Voltage
Power Dissipation (Note 3)
ESD Susceptibility (Note 4)
ESD Susceptibility (Note 5)
Junction Temperature
Soldering Information
Small Outline Package
Vapor Phase (60 sec.)
Infrared (15 sec.)
6.0V
−65˚C to +150˚C
−0.3V to VDD + 0.3V
Internally limited
3500V
250V
150˚C
Operating Ratings
Temperature Range
TMIN≤TA≤TMAX
Supply Voltage
−40˚C≤TA≤+85˚C
2.7V≤VDD≤5.5V
215˚C
220˚C
Electrical Characteristics (Notes 1, 2)
The following specifications apply for VDD = 5V unless otherwise specified. Limits apply for TA = 25˚C.
Symbol
VDD
Parameter
Conditions
LM4880
Units
(Limits)
Typical
Limit
(Note 6)
(Note 7)
Supply Voltage
2.7
V (min)
5.5
V (max)
IDD
Quiescent Power Supply Current
VIN = 0V, IO = 0A
3.6
6.0
mA
(max)
ISD
Shutdown Current
VPIN5 = VDD
0.7
5
µA
(max)
VOS
Output Offset Voltage
VIN = 0V
5
50
mV
(max)
PO
Output Power
THD = 0.1% (max); f = 1 kHz;
RL = 8Ω
250
200
mW
(min)
RL = 32Ω
THD+N = 10%; f = 1 kHz
RL = 8Ω
THD+N
PSRR
Total Harmonic Distortion+Noise
Power Supply Rejection Ratio
RL = 32Ω
RL = 8Ω, PO = 200 mW;
RL = 32Ω, PO = 75 mW;
f = 1 kHz
CB = 1.0 µF,
VRIPPLE = 200 mVrms, f = 100 Hz
85
mW
325
mW
110
mW
0.03
%
0.02
%
50
dB
Note 1: All voltages are measured with respect to the ground pin, unless otherwise specified.
Note 2: Absolute Maximum Ratings indicate limits beyond which damage 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 3: The maximum power dissipation must be derated at elevated temperatures and is dictated by TJMAX, θJA, and the ambient temperature TA. The maximum
allowable power dissipation is PDMAX = (TJMAX − TA)/θJA or the number given in the Absolute Maximum Ratings, whichever is lower. For the LM4880, TJMAX = 150˚C,
and the typical junction-to-ambient thermal resistance is 170˚C/W for package M08A and 107˚C/W for package N08E.
Note 4: Human body model, 100 pF discharged through a 1.5 kΩ resistor.
Note 5: Machine model, 220 pF–240 pF discharged through all pins.
Note 6: Typicals are measured at 25˚C and represent the parametric norm.
Note 7: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).
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Automatic Shutdown Circuit
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FIGURE 2. Automatic Shutdown Circuit
Automatic Switching Circuit
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FIGURE 3. Automatic Switching Circuit
External Components Description
(Figure 1)
Components
Functional Description
1.
Ri
Inverting input resistance which sets the closed-loop gain in conjunction with RF. This resistor
also forms a high pass filter with Ci at fc = 1/(2πRiCi).
2.
Ci
Input coupling capacitor which blocks the DC voltage at the amplifier’s input terminals. Also
creates a high pass filter with Ri at fc = 1/(2πRiCi). Refer to the section, Proper Selection of
External Components, for an explanation of how to determine the value of Ci.
3.
RF
Feedback resistance which sets closed-loop gain in conjunction with Ri.
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External Components Description
(Figure 1) (Continued)
Components
Functional Description
4.
CS
Supply bypass capacitor which provides power supply filtering. Refer to the Application
Information section for proper placement and selection of the supply bypass capacitor.
5.
CB
Bypass pin capacitor which provides half-supply filtering. Refer to the section, Proper
Selection of External Components, for information concerning proper placement and
selection of CB.
6.
Co
Output coupling capacitor which blocks the DC voltage at the amplifier’s output. Forms a high
pass filter with RL at fo = 1/(2πRLCo).
Typical Performance Characteristics
THD + N vs Output Power
THD + N vs Output Power
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THD + N vs Output Power
THD + N vs Output Power
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THD + N vs Output Power
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THD + N vs Frequency
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THD + N vs Output Power
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THD + N vs Frequency
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THD + N vs Frequency
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Typical Performance Characteristics
(Continued)
Output Power vs
Load Resistance
THD + N vs Frequency
Output Power vs
Load Resistance
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Output Power vs
Supply Voltage
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Output Power vs
Supply Voltage
Output Power vs
Supply Voltage
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Clipping Voltage vs
Supply Voltage
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Clipping Voltage vs
Supply Voltage
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Power Dissipation vs
Output Power
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Typical Performance Characteristics
(Continued)
Output Attenuation in
Shutdown Mode
Channel Separation
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Power Supply
Rejection Ratio
Noise Floor
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Open Loop
Frequency Response
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Frequency Response vs
Output Capacitor Size
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Supply Current vs
Supply Voltage
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Frequency Response vs
Output Capacitor Size
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Frequency Response vs
Input Capacitor Size
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Typical Performance Characteristics
Typical Application
Frequency Response
(Continued)
Typical Application
Frequency Response
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For the LM4880 surface mount package, θJA = 170˚ C/W
and TJMAX = 150˚C. Depending on the ambient temperature,
TA, of the system surroundings, Equation (2) can be used to
find the maximum internal power dissipation supported by
the IC packaging. If the result of Equation (1) is greater than
that of Equation (2), then either the supply voltage must be
decreased, the load impedance increased, or the ambient
temperature reduced. For the typical application of a 5V
power supply, with an 8Ω load, the maximum ambient temperature possible without violating the maximum junction
temperature is approximately 96˚C provided that device operation is around the maximum power dissipation point.
Power dissipation is a function of output power and thus, if
typical operation is not around the maximum power dissipation point, the ambient temperature may be increased accordingly. Refer to the Typical Performance Characteristics curves for power dissipation information for lower output
powers.
Application Information
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4880 contains a shutdown pin to externally turn off the
amplifier’s bias circuitry. This shutdown feature turns the amplifier off when a logic high is placed on the shutdown pin.
The trigger point between a logic low and logic high level is
typically half supply. It is best to switch between ground and
the supply to provide maximum device performance. By
switching the shutdown pin to VDD, the LM4880 supply current draw will be minimized in idle mode. While the device
will be disabled with shutdown pin voltages less than VDD,
the idle current may be greater than the typical value of 0.7
µA. In either case, the shutdown pin should be tied to a definite voltage because leaving the pin floating may result in an
unwanted shutdown condition.
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry which provides a quick, smooth transition into shutdown. Another solution is to use a single-pole, single-throw switch in conjunction
with an external pull-up resistor. When the switch is closed,
the shutdown pin is connected to ground and enables the
amplifier. If the switch is open, then the external pull-up resistor will disable the LM4880. This scheme guarantees that
the shutdown pin will not float which will prevent unwanted
state changes.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply rejection. The capacitor location on both the bypass and power
supply pins should be as close to the device as possible. As
displayed in the Typical Performance Characteristics section, the effect of a larger half supply bypass capacitor is improved low frequency PSRR due to increased half-supply
stability. Typical applications employ a 5V regulator with
10 µF and a 0.1 µF bypass capacitors which aid in supply
stability, but do not eliminate the need for bypassing the supply nodes of the LM4880. The selection of bypass capacitors, especially CB, is thus dependant upon desired low frequency PSRR, click and pop performance as explained in
the section, Proper Selection of External Components
section, system cost, and size constraints.
POWER DISSIPATION
Power dissipation is a major concern when using any power
amplifier and must be thoroughly understood to ensure a
successful design. Equation (1) states the maximum power
dissipation point for a single-ended amplifier operating at a
given supply voltage and driving a specified output load.
PDMAX = (VDD)2/(2π2RL)
(1)
AUTOMATIC SHUTDOWN CIRCUIT
As shown in Figure 2, the LM4880 can be set up to automatically shutdown when a load is not connected. This circuit is
based upon a single control pin common in many headphone jacks. This control pin forms a normally closed switch
with one of the output pins. The output of this circuit (the voltage on pin 5 of the LM4880) has two states based on the
state of the switch. When the switch is open, signifying that
headphones are inserted, the LM4880 should be enabled.
When the switch is closed, the LM4880 should be off to minimize power consumption.
Since the LM4880 has two operational amplifiers in one
package, the maximum internal power dissipation point is
twice that of the number which results from Equation (1).
Even with the large internal power dissipation, the LM4880
does not require heat sinking over a large range of ambient
temperatures. From Equation (1), assuming a 5V power supply and an 8Ω load, the maximum power dissipation point is
158 mW per amplifier. Thus the maximum package dissipation point is 317 mW. The maximum power dissipation point
obtained must not be greater than the power dissipation that
results from Equation (2):
(2)
PDMAX = (TJMAX-TA)/θJA
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Power Derating Curve
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Application Information
Selection of Input and Output Capacitor Size
Large input and output capacitors are both expensive and
space hungry for portable designs. Clearly a certain sized
capacitor is needed to couple in low frequencies without severe attenuation. But in many cases the transducers used in
portable systems, whether internal or external, have little
ability to reproduce signals below 100 Hz–150 Hz. Thus using large input and output capacitors may not increase system performance.
(Continued)
The operation of this circuit is rather simple. With the switch
closed, Rp and Ro form a resistor divider which produces a
gate voltage of less than 5 mV. This gate voltage keeps the
NMOS inverter off and Rsd pulls the shutdown pin of the
LM4880 to the supply voltage. This places the LM4880 in
shutdown mode which reduces the supply current to 0.7 µA
typically. When the switch is open, the opposite condition is
produced. Resistor Rp pulls the gate of the NMOS high
which turns on the inverter and produces a logic low signal
on the shutdown pin of the LM4880. This state enables the
LM4880 and places the amplifier in its normal mode of operation.
This type of circuit is clearly valuable in portable products
where battery life is critical, but is also benefical for power
conscious designs such as “Green PC’s”.
In addition to system cost and size, click and pop performance is effected by the size of the input coupling capacitor,
Ci. A larger input coupling capacitor requires more charge to
reach its quiescent DC voltage (normally 1/2 VDD.) This
charge comes from the output via the feedback and is apt to
create pops upon device enable. Thus, by minimizing the capacitor size based on necessary low frequency response,
turn-on pops can be minimized.
Besides minimizing the input and output capacitor sizes,
careful consideration should be paid to the bypass capacitor
size. The bypass capacitor, CB, is the most critical component to minimize turn-on pops since it determines how fast
the LM4880 turns on. The slower the LM4880’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),
should produce a virtually clickless and popless shutdown
function. While the device will function properly, (no oscillations or motorboating), with CB equal to 0.1 µF, the device
will be much more susceptible to turn-on clicks and pops.
Thus, a value of CB equal to 1.0 µF or larger is recommended in all but the most cost sensitive designs.
AUTOMATIC SWITCHING CIRCUIT
A circuit closely related to the Automatic Shutdown Circuit
is the Automatic Switching Circuit of Figure 3. The Automatic Switching Circuit utilizes both the input and output of
the NMOS inverter to toggle the states of two different audio
power amplifiers. The LM4880 is used to drive stereo single
ended loads, while the LM4861 drives bridged internal
speakers.
In this application, the LM4880 and LM4861 are never on at
the same time. When the switch inside the headphone jack
is open, the LM4880 is enabled and the LM4861 is disabled
since the NMOS inverter is on. If a headphone jack is not
present, it is assumed that the internal speakers should be
on and thus the voltage on the LM4861 shutdown pin is low
and the voltage at the LM4880 pin is high. This results in the
LM4880 being shutdown and the LM4861 being enabled.
Only one channel of this circuit is shown in Figure 3 to keep
the drawing simple but the typical application would a
LM4880 driving a stereo external headphone jack and two
LM4861’s driving the internal stereo speakers. If only one internal speaker is required, a single LM4861 can be used as
a summer to mix the left and right inputs into a single mono
channel.
AUDIO POWER AMPLIFIER DESIGN
Design a Dual 200 mW/8Ω Audio Amplifier
Given:
Power Output: 200 mWrms
Load Impedance: 8Ω
Input Level: 1 Vrms (max)
Input Impedance: 20 kΩ
Bandwidth: 100 Hz–20 kHz ± 0.50 dB
A designer must first determine the needed supply rail to obtain the specified output power. Calculating the required supply rail involves knowing two parameters, Vopeak and also the
dropout voltage. As shown in the Typical Performance
Curves, the dropout voltage is typically 0.5V. Vopeak can be
determined from Equation (3).
PROPER SELECTION OF EXTERNAL COMPONENTS
Selection of external components when using integrated
power amplifiers is critical to optimize device and system
performance. While the LM4880 is tolerant of external component combinations, care must be exercised when choosing component values.
The LM4880 is unity-gain stable which gives a designer
maximum system flexibility. The LM4880 should be used in
low gain configurations to minimize THD + N values, and
maximize the signal to noise ratio. Low gain configurations
require large input signals to obtain a given output power. Input signals equal to or greater than 1 Vrms are available
from sources such as audio codecs. Please refer to the section, Audio Power Amplifier Design, for a more complete
explanation of proper gain selection.
Besides gain, one of the major design considerations is the
closed-loop bandwidth of the amplifier. To a large extent, the
bandwidth is dictated by the choice of external components
shown in Figure 1. Both the input coupling capacitor, Ci, and
the output coupling capacitor, Co, form first order high pass
filters which limit low frequency response. These values
should be chosen based on needed frequency response for
a few distinct reasons.
(3)
For 200 mW of output power into an 8Ω load, the required
Vopeak is 1.79V. Since this is a single supply application, the
minimum supply voltage is twice the sum of Vopeak and Vod.
Since 5V is a standard supply voltage in most applications, it
is chosen for the supply rail. Extra supply voltage creates
headroom that allows the LM4880 to reproduce peaks in excess of 200 mW without clipping the signal. At this time, the
designer must make sure that the power supply choice along
with the output impedance does not violate the conditions
explained in the Power Dissipation section. Remember that
the maximum power dissipation value from Equation (1)
must be multiplied by two since there are two independent
amplifiers inside the package.
Once the power dissipation equations have been addressed,
the required gain can be determined from Equation (4).
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Application Information
taken into consideration. The combination of two single order
filters at the same frequency forms a second order response.
This results in a signal which is down 0.34 dB at five times
away from the single order filter −3 dB point. Thus, a frequency of 20 Hz is used in the following equations to ensure
that the response if better than 0.5 dB down at 100 Hz.
Ci ≥ 1/(2π*20kΩ*20Hz) = 0.397 µF; use 0.39 µF
Co ≥ 1/(2π*8Ω*20Hz) = 995 µF; use 1000 µF
(Continued)
(4)
(5)
AV = −RF/Ri
From Equation (4), the minimum gain is:: AV = −1.26
Since the desired input impedance was 20 kΩ, and with a
gain of −1.26, a value of 27 kΩ is designated for Rf, assuming 5% tolerance resistors. This combination results in a
nominal gain of −1.35. The final design step is to address the
bandwidth requirements which must be stated as a pair of
−3 dB frequency points. Five times away from a −3 dB point
is 0.17 dB down from passband response assuming a single
pole roll-off. As stated in the External Components section,
both Ri in conjunction with Ci, and Co with RL, create first order high pass filters. Thus to obtain the desired frequency
low response of 100 Hz within ± 0.5 dB, both poles must be
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The high frequency pole is determined by the product of the
desired high frequency pole, fH, and the closed-loop gain,
AV. With a closed-loop gain magnitude of 1.35 and fH = 100
kHz, the resulting GBWP = 135 kHz which is much smaller
than the LM4880 GBWP of 12.5 MHz. This figure displays
that if a designer has a need top design an amplifier with a
higher gain, the LM4880 can still be used without running
into bandwidth limitations.
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Physical Dimensions
inches (millimeters) unless otherwise noted
8-Lead (0.150" Wide) Molded Small Outline Package, JEDEC
Order Number LM4880M
NS Package Number M08A
8-Lead (0.300" Wide) Molded Dual-In-Line Package
Order Number LM4880N
NS Package Number N08E
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LM4880 Boomer Audio Power Amplifier Series Dual 250 mW Audio Power Amplifier with
Shutdown Mode
Notes
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