LM4912
Stereo 40mW Low Noise Headphone Amplifier
General Description
Key Specifications
The LM4912 is an stereo audio power amplifier capable of
delivering 40mW per channel of continuous average power
into a 16Ω load or 25mW per channel into a 32Ω load at 1%
THD form a 3V power supply.
Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal amount of
external components. Since the LM4912 does not require
bootstrap capacitors or snubber networks, it is optimally suited for low-power portable systems.
The LM4912 features a low-power consumption shutdown
mode and a power mute mode that allows for faster turn on
time with less than 1mV voltage change at outputs on release.
Additionally, the LM4912 features an internal thermal shutdown protection mechanism.
The LM4912 is unity gain stable and may be configured with
external gain-setting resistors.
65dB (typ)
■ PSRR at 217 Hz and 1kHz
■ Output Power at 1kHz with VDD = 2.4V, 1% THD 25mW
+N into a 16Ω load
(typ)
40mW
+N into a 16Ω load
(typ)
1.0µA (max)
■ Shutdown Current
■ Output Voltage change on release from Shutdown 1mV
VDD = 2.4V, RL = 16Ω
(max)
10µV (typ)
■ Output Noise, 20Hz to 20kHz, A-weighted
■ Output Power at 1kHz with VDD = 3V, 1% THD
Features
■
■
■
■
External gain-setting capability
Available in space-saving MSOP package
Ultra low current shutdown mode
Mute mode allows fast turn-on (10ms) with less than 1mV
change on outputs
■ 2.0V - 5.5V operation
■ Ultra low noise
■ Operation at low supply voltages
Applications
■ Portable CD players
■ PDAs
■ Portable electronics devices
Typical Application
20048180
FIGURE 1. Typical Capacitive Coupled Output Configuration Circuit
Boomer® is a registered trademark of National Semiconductor Corporation.
© 2008 National Semiconductor Corporation
200481
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LM4912 Stereo 40mW Low Noise Headphone Amplifier
June 17, 2008
LM4912
Connection Diagrams
MSOP Package
20048179
Top View
Order Number LM4912MM
See NS Package Number MUB10A
MSOP Package
200481a9
Top View
G-Boomer Family
A3 - LM4912MM
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2
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)
6.0V
−65°C to +150°C
-0.3V to VDD + 0.3V
Internally Limited
2000V
250V
Electrical Characteristics VDD = 5.0V
150°C
θJC (MSOP)
56°C/W
θJA (MSOP)
190°C/W
Operating Ratings
Temperature Range
TMIN ≤ TA ≤ TMAX
−40°C ≤ T A ≤ 85°C
2.0V ≤ VDD ≤ 5.5V
Supply Voltage (VDD)
(Notes 1, 2)
The following specifications apply for VDD = 5.0V, RL = 16Ω, CO = 100µ, and CB = 4.7µF unless otherwise specified. Limits apply
to TA = 25°C.
Symbol
Parameter
Conditions
LM4912
Units
(Limits)
Typ
(Note 6)
Limit
(Note 7)
2
5
0.1
2.0
µA (max)
2
5
mA (max)
IDD
Quiescent Power Supply Current
VIN = 0V, IO = 0A
mA (max)
ISD
Shutdown Current
VSHUTDOWN = GND
IM
Mute Current
VMUTE = VDD
VSDIH
Shutdown Voltage Input High
1.8
V
VSDIL
Shutdown Voltage Input Low
0.4
V
VMIH
Mute Voltage Input High
1.8
V
VMIL
Mute Voltage Input Low
0.4
V
PO
Output Power
R = 16Ω
145
mW
R = 32Ω
80
VNO
Output Noise Voltage
BW = 20 Hz to 20kHz, A-weighted
10
µV
PSRR
Power Supply Rejection Ratio
VRIPPLE = 200mV sine p-p
65
dB
AM
Mute Attenuation
f = 1kHz
85
dB
THD = 1%; f = 1kHz
Electrical Characteristics VDD = 3.0V
(Notes 1, 2)
The following specifications apply for VDD = 3.0V, RL = 16Ω, CO = 100µF, and CB = 4.7µF unless otherwise specified. Limits apply
to TA = 25°C.
Symbol
Parameter
Conditions
LM4912
Typ
(Note 6)
Limit
(Note 7)
Units
(Limits)
IDD
Quiescent Power Supply Current
VIN = 0V, IO = 0A
1.5
3
mA (max)
ISD
Shutdown Current
VSHUTDOWN = GND
0.1
2.0
µA (max)
IM
Mute Current
VMUTE = VDD
1.5
3
mA (max)
PO
Output Power
VNO
Output Noise Voltage
BW = 20 Hz to 20kHz, A-weighted
10
µV
PSRR
Power Supply Rejection Ratio
VRIPPLE = 200mV sine p-p
65
dB
AM
Mute Attenuation
f = 1kHz
80
dB
THD = 1%; f = 1kHz
3
R = 16Ω
40
R = 32Ω
25
mW
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LM4912
Junction Temperature
Thermal Resistance
Absolute Maximum Ratings (Note 2)
LM4912
Electrical Characteristics VDD = 2.4V
(Notes 1, 2)
The following specifications apply for VDD = 2.4V, RL = 16Ω, CO = 100µF, and CB = 4.7µF unless otherwise specified. Limits apply
to TA = 25°C.
Symbol
Parameter
Conditions
LM4912
Typ
(Note 6)
Limit
(Note 7)
Units
(Limits)
IDD
Quiescent Power Supply Current
VIN = 0V, IO = 0A
1.5
3
mA (max)
ISD
Shutdown Current
VSHUTDOWN = GND
0.1
2.0
µA (max)
IM
Mute Current
VMUTE = VDD
1.5
3
mA (max)
THD = 1%; f = 1kHz
PO
Output Power
R = 16Ω
25
R = 32Ω
12
mW
VNO
Output Noise Voltage
BW = 20 Hz to 20kHz, A-weighted
10
µV
PSRR
Power Supply Rejection Ratio
VRIPPLE = 200mV sine p-p
65
dB
TWU
Wake Up Time from Shutdown
2
s
VOSD
Output Voltage Change on
Release from Shutdown
TUM
Time to Un-Mute
AM
Mute Attenuation
0.01
f = 1kHz
80
1
mV (max)
0.02
s (max)
db
Note 1: All voltages are measured with respect to the GND pin unless otherwise specified.
Note 2: : 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 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 Absolute Maximum Ratings, whichever is lower. For the LM4912, see
power derating currents for more information.
Note 4: Human body model, 100pF discharged through a 1.5kΩ resistor.
Note 5: Machine Model, 220pF-240pF 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).
Note 8: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis.
Note 9: 10Ω Terminated input.
External Components Description
See (Figure 1)
Components
Functional Description
1.
RI
2.
CI
Input coupling capacitor which blocks the DC voltage at the amplifier's input terminals. Also creates a high-pass
3.
Rf
Feedback resistance which sets the closed-loop gain in conjunction with Ri.
4.
CS
Supply bypass capacitor which provides power supply filtering. Refer to the Power Supply Bypassing section for
information concerning 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 Proper
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
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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).
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.
fo = 1/(2πRLCo)
4
LM4912
Typical Performance Characteristics
THD+N vs Frequency
THD+N vs Frequency
20048184
20048185
THD+N vs Frequency
THD+N vs Frequency
20048186
20048187
THD+N vs Frequency
THD+N vs Frequency
20048188
20048189
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LM4912
THD+N vs Output Power
THD+N vs Output Power
20048190
20048191
THD+N vs Output Power
THD+N vs Output Power
20048108
20048107
THD+N vs Output Power
THD+N vs Output Power
20048110
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20048109
6
LM4912
Output Resistance vs Load Resistance
Output Power vs Supply Voltage
20048113
20048112
Output Power vs Supply Voltage
Output Power vs Load Resistance
20048196
20048111
Power Dissipation vs. Output Power
Power Dissipation vs. Output Power
20048198
20048199
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LM4912
Power Dissipation vs Output Power
Channel Separation
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Channel Separation
Channel Separation
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20048117
Channel Separation
Channel Separation
20048119
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20048118
8
LM4912
Channel Separation
Mute Attenuation
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20048116
Power Supply Rejection Ratio
Power Supply Rejection Ratio
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Power Supply Rejection Ratio
Power Supply Rejection Ratio
20048120
20048123
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LM4912
Power Supply Rejection Ratio
Power Supply Rejection Ratio
20048121
20048122
Frequency Response vs
Input Capacitor Size
Open Loop Frequency Response
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20048127
Supply Voltage vs
Supply Current
Clipping Voltage vs
Supply Voltage
20048124
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20048125
10
Shutdown Hysteresis Voltage, Vdd=5V
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Shutdown Hysteresis Voltage, Vdd=3V
Power Derating Curve
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LM4912
Noise Floor
LM4912
maximum junction temperature is approximately 144°C provided that device operation is around the maximum power
dissipation point. Thus, for typical applications, power dissipation is not an issue. 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
AMPLIFIER CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4912 has three operational amplifiers internally. Two of the amplifier's have externally configurable gain while the other amplifier is internally fixed at the
bias point acting as a unity-gain buffer. The closed-loop gain
of the two configurable amplifiers is set by selecting the ratio
of Rf to Ri. Consequently, the gain for each channel of the IC
is
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is important
for low noise performance and high power supply rejection.
The capacitor location on the power supply pins should be as
close to the device as possible.
Typical applications employ a 3V regulator with 10mF tantalum or electrolytic capacitor and a ceramic bypass capacitor
which aid in supply stability. This does not eliminate the need
for bypassing the supply nodes of the LM4912. A bypass capacitor value in the range of 0.1µF to 1µF is recommended
for CS.
AVD = -(Rf / Ri)
By driving the loads through outputs VoA and VoB with VoC
acting as a buffered bias voltage the LM4912 does not require
output coupling capacitors. The classical single-ended amplifier configuration where one side of the load is connected
to ground requires large, expensive output coupling capacitors.
A configuration, such as the one used in the LM4912, has a
major advantage over single supply, single-ended amplifiers.
Since the outputs VoA, VoB, and VoC are all biased at 1/2
VDD, no net DC voltage exists across each load. This eliminates the need for output coupling capacitors which are required in a single-supply, single-ended amplifier configuration. Without output coupling capacitors in a typical singlesupply, single-ended amplifier, the bias voltage is placed
across the load resulting in both increased internal IC power
dissipation and possible loudspeaker damage.
MICRO POWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the
LM4912's shutdown function. Activate micro-power shutdown
by applying a logic-low voltage to the SHUTDOWN pin. When
active, the LM4912's micro-power shutdown feature turns off
the amplifier's bias circuitry, reducing the supply current. The
trigger point varies depending on supply voltage and is shown
in the Shutdown Hysteresis Voltage graphs in the Typical
Performance Characteristics section. The low 0.1µA(typ)
shutdown current is achieved by applying a voltage that is as
near as ground as possible to the SHUTDOWN pin. A voltage
that is higher than ground may increase the shutdown current.
There are a few ways to control the micro-power shutdown.
These include using a single-pole, single-throw switch, a microprocessor, or a microcontroller. When using a switch,
connect an external 100kΩ pull-up resistor between the
SHUTDOWN pin and VDD. Connect the switch between the
SHUTDOWN pin and ground. Select normal amplifier operation by opening the switch. Closing the switch connects the
SHUTDOWN pin to ground, activating micro-power shutdown.
The switch and resistor guarantee that the SHUTDOWN pin
will not float. This prevents unwanted state changes. In a system with a microprocessor or microcontroller, use a digital
output to apply the control voltage to the SHUTDOWN pin.
Driving the SHUTDOWN pin with active circuitry eliminates
the pull-up resistor.
Shutdown enable/disable times are controlled by a combination of CB and VDD. Larger values of CB results in longer turn
on/off times from Shutdown. Smaller Vdd values also increase turn on/off time for a given value of CB. Longer shutdown times also improve the LM4912's resistance to click and
pop upon entering or returning from shutdown. For a 2.4V
supply and CB = 4.7µF, the LM4912 requires about 2 seconds
to enter or return from shutdown. This longer shutdown time
enables the LM4912 to have virtually zero pop and click transients upon entering or release from shutdown.
Smaller values of CB will decrease turn-on time, but at the cost
of increased pop and click and reduced PSRR. Since shutdown enable/disable times increase dramatically as supply
voltage gets below 2.2V, this reduced turn-on time may be
desirable if extreme low supply voltage levels are used as this
would offset increases in turn-on time caused by the lower
supply voltage.
POWER DISSIPATION
Power dissipation is a major concern when using any power
amplifier and must be thoroughly understood to ensure a successful design. When operating in capacitor-coupled mode,
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)
Since the LM4912 has two operational amplifiers in one package, the maximum internal power dissipation point is twice
that of the number which results from Equation 1. From Equation 1, assuming a 3V power supply and a 32Ω load, the
maximum power dissipation point is 14mW per amplifier.
Thus the maximum package dissipation point is 28mW.
The maximum power dissipation point obtained from Equation 1 must not be greater than the power dissipation that
results from Equation 2:
PDMAX = (TJMAX - TA) / θJA
(2)
For package MUB10A, θJA = 190°C/W. TJMAX = 150°C for the
LM4912. 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 TA reduced. For the typical
application of a 3V power supply, with an 32Ω load, the maximum ambient temperature possible without violating the
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AUDIO POWER AMPLIFIER DESIGN
A 25mW/32Ω Audio Amplifier
Given:
Power Output
25mWrms
Load Impedance
Input Level
32Ω
1Vrms
Input Impedance
20kΩ
A designer must first determine the minimum supply rail to
obtain the specified output power. By extrapolating from the
Output Power vs Supply Voltage graphs in the Typical Performance Characteristics section, the supply rail can be
easily found.
3V is a standard voltage in most applications, it is chosen for
the supply rail. Extra supply voltage creates headroom that
allows the LM4912 to reproduce peak in excess of 25mW
without producing audible distortion. 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.
Once the power dissipation equations have been addressed,
the required gain can be determined from Equation 2.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications using
integrated power amplifiers is critical to optimize device and
system performance. While the LM4912 is tolerant of external
component combinations, consideration to component values
must be used to maximize overall system quality.
The LM4912 is unity-gain stable which gives the designer
maximum system flexibility. The LM4912 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 1Vrms are available from sources
such as audio codecs. Very large values should not be used
for the gain-setting resistors. Values for Ri and Rf should be
less than 1MΩ. Please refer to the section, Audio Power
Amplifier Design, for a more complete explanation of proper
gain selection
Besides gain, one of the major considerations is the closedloop bandwidth of the amplifier. To a large extent, the bandwidth is dictated by the choice of external components shown
in Figures 2 and 3. The input coupling capacitor, Ci, forms a
first order high pass filter which limits low frequency response.
This value should be chosen based on needed frequency response and turn-on time.
(3)
From Equation 4, the minimum AV is 0.89; use AV = 1. Since
the desired input impedance is 20kΩ, and with a AV gain of 1,
a ratio of 1:1 results from Equation 1 for Rf to Ri. The values
are chosen with Ri = 20kΩ and Rf = 20kΩ. 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.
fL = 100Hz/5 = 20Hz
fH = 20kHz * 5 = 100kHz
SELECTION OF INPUT CAPACITOR SIZE
Amplifying the lowest audio frequencies requires a high value
input coupling capacitor, Ci. A high value capacitor can be
expensive and may compromise space efficiency in portable
designs. In many cases, however, the headphones used in
portable systems have little ability to reproduce signals below
60Hz. Applications using headphones with this limited frequency response reap little improvement by using a high
value input capacitor.
In addition to system cost and size, turn on time is affected by
the size of the input coupling capacitor Ci. A larger input coupling capacitor requires more charge to reach its quiescent
DC voltage. This charge comes from the output via the feedback Thus, by minimizing the capacitor size based on nec-
As stated in the External Components section, Ri in conjunction with Ci creates a
Ci ≥ 1 / (2π * 20kΩ * 20Hz) = 0.397µF; use 0.39µF.
The high frequency pole is determined by the product of the
desired frequency pole, fH, and the differential gain, AV. With
an AV = 1 and fH = 100kHz, the resulting GBWP = 100kHz
which is much smaller than the LM4912 GBWP of 10MHz.
This figure displays that is a designer has a need to design
an amplifier with higher differential gain, the LM4912 can still
be used without running into bandwidth limitations.
Revision History
Rev
Date
1.0
7/15/05
Fixed spelling typos.
Description
1.01
06/16/08
Fixed a typo.
13
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LM4912
essary low frequency response, turn-on time can be minimized. A small value of Ci (in the range of 0.1µF to 0.39µF),
is recommended.
MUTE
When in C-CUPL mode, the LM4912 also features a mute
function that is independent of load impedance and enables
extremely fast turn-on/turn-off with a minimum of output pop
and click. The mute function leaves the outputs at their bias
level, thus resulting in higher power consumption than shutdown mode, but also provides much faster turn on/off times.
Mute mode is enabled by providing a logic high signal on the
MUTE pin in the opposite manner as the shutdown function
described above. Threshold voltages and activation techniques match those given for the shutdown function as well.
Additionally, Mute should not be enabled during shutdown or
while entering or returning from shutdown. This is not a valid
operation condition and may result in much higher pop and
click values.
LM4912
Physical Dimensions inches (millimeters) unless otherwise noted
MSOP
Order Number LM4912MM
NS Package Number MUB10A
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LM4912
Notes
15
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LM4912 Stereo 40mW Low Noise Headphone Amplifier
Notes
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