LM4929
Stereo 40mW Low Noise Headphone Amplifier
with OCL Output
General Description
The LM4929 is an stereo audio power amplifier capable of
delivering 40mW per channel of continuous average power
into a 16Ωload or 25mW per channel into a 32Ωload at 1%
THD+N from 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 LM4929 does not require
bootstrap capacitors or snubber networks, it is optimally
suited for low-power portable systems. The LM4929 is con-
figured for OCL (Output Capacitor-Less) outputs, operating
with no DC blocking capacitors on the outputs.
The LM4929 features a low-power consumption shutdown
mode with a faster turn on time. Additionally, the LM4929
features an internal thermal shutdown protection mecha-
nism.
The LM4929 is unity gain stable and may be configured with
external gain-setting resistors.
Key Specifications
jPSRR at 217Hz and 1kHz 65dB (typ)
jOutput Power at 1kHz with V
DD
= 2.4V,
1% THD+N into a 16Ωload 25mW (typ)
jOutput Power at 1kHz with V
DD
= 3V,
1% THD+N into a 16Ωload 40mW (typ)
jShutdown current 2.0µA (max)
jOutput Voltage change on release
from Shutdown V
DD
= 2.4V, R
L
=16Ω1mV (max)
Features
nOCL outputs — No DC Blocking Capacitors
nExternal gain-setting capability
nAvailable in space-saving MSOP package
nUltra low current shutdown mode
n2V - 5.5V operation
nUltra low noise
Applications
nPortable CD players
nPDAs
nPortable electronics devices
Block Diagram
Boomer®is a registered trademark of National Semiconductor Corporation.
20132441
FIGURE 1. Block Diagram
December 2004
LM4929 Stereo 40mW Low Noise Headphone Amplifier with OCL Output
© 2004 National Semiconductor Corporation DS201324 www.national.com
Typical Application
Connection Diagram
MSOP Package
20132428
Top View (Note 10)
Order Number LM4929MM
See NS Package Number MUB10A
20132481
FIGURE 2. Typical OCL Output Configuration Circuit
LM4929
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Absolute Maximum Ratings (Note 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage 6.0V
Storage Temperature −65˚C to +150˚C
Input Voltage -0.3V to V
DD
+ 0.3V
Power Dissipation (Note 3) Internally Limited
ESD Susceptibility (Note 4) 2000V
ESD Susceptibility (Note 5) 200V
Junction Temperature 150˚C
Thermal Resistance
θ
JC
(MSOP) 56˚C/W
θ
JA
(MSOP) 190˚C/W
Operating Ratings
Temperature Range
T
MIN
≤T
A
≤T
MAX
−40˚C ≤T
A
≤85˚C
Supply Voltage 2V ≤V
DD
≤5.5V
Electrical Characteristics V
DD
=5V(Notes 1, 2)
The following specifications apply for V
DD
= 5V, R
L
=16Ω, and C
B
= 4.7µF unless otherwise specified. Limits apply to T
A
=
25˚C. Pin 3 connected to GND.
Symbol Parameter Conditions LM4929 Units
(Limits)
Typ
(Note 6)
Limit
(Note 7)
I
DD
Quiescent Power Supply Current V
IN
= 0V, I
O
= 0A 2 5 mA (max)
I
SD
Shutdown Current V
SHUTDOWN
= GND 0.1 2.0 µA(max)
V
SDIH
Shutdown Voltage Input High 1.8 V
V
SDIL
Shutdown Voltage Input Low 0.4 V
P
O
Output Power
THD=1%;f=1kHZ
mWR
L
=16Ω80
R
L
=32Ω80
V
NO
Output Noise Voltage BW = 20Hz to 20kHz, A-weighted 10 µV
PSRR Power Supply Rejection Ratio V
RIPPLE
= 200mV sine p-p 65 dB
Electrical Characteristics V
DD
= 3.0V (Notes 1, 2)
The following specifications apply for V
DD
= 3.0V, R
L
=16Ω, and C
B
= 4.7µF unless otherwise specified. Limits apply to T
A
=
25˚C. Pin 3 connected to GND.
Symbol Parameter Conditions LM4929 Units
(Limits)
Typ
(Note 6)
Limit
(Note 7)
I
DD
Quiescent Power Supply Current V
IN
= 0V, I
O
= 0A 1.5 3.5 mA (max)
I
SD
Shutdown Current V
SHUTDOWN
= GND 0.1 2.0 µA(max)
P
O
Output Power
THD = 1%; f = 1kHz
mWR=16Ω40
R=32Ω25
V
NO
Output Noise Voltage BW = 20 Hz to 20kHz, A-weighted 10 µV
PSRR Power Supply Rejection Ratio V
RIPPLE
= 200mV sine p-p 65 dB
Electrical Characteristics V
DD
= 2.4V (Notes 1, 2)
The following specifications apply for V
DD
= 2.4V, R
L
=16Ω, and C
B
= 4.7µF unless otherwise specified. Limits apply to T
A
=
25˚C. Pin 3 connected to GND.
Symbol Parameter Conditions LM4929 Units
(Limits)
Typ
(Note 6)
Limit
(Note 7)
I
DD
Quiescent Power Supply Current V
IN
= 0V, I
O
= 0A 1.5 3 mA (max)
I
SD
Shutdown Current V
SHUTDOWN
= GND 0.1 2.0 µA(max)
P
O
Output Power
THD = 1%; f = 1kHz
mWR=16Ω25
R=32Ω12
LM4929
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Electrical Characteristics V
DD
= 2.4V (Notes 1, 2) (Continued)
The following specifications apply for V
DD
= 2.4V, R
L
=16Ω, and C
B
= 4.7µF unless otherwise specified. Limits apply to T
A
=
25˚C. Pin 3 connected to GND.
Symbol Parameter Conditions LM4929 Units
(Limits)
Typ
(Note 6)
Limit
(Note 7)
V
NO
Output Noise Voltage BW = 20 Hz to 20kHz, A-weighted 10 µV
PSRR Power Supply Rejection Ratio V
RIPPLE
= 200mV sine p-p 65 dB
T
WU
Wake Up Time from Shutdown OCL 0.5 s
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 =(T
JMAX -T
A)/ θJA or the number given in Absolute Maximum Ratings, whichever is lower. For the LM4929, see
power derating currents for more information.
Note 4: Human body model, 100pF discharged through a 1.5kΩresistor.
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: 10ΩTerminated input.
Note 10: Pin 3 (NC) should be connected to GND for proper part operation.
External Components Description (Figure 2)
Components Functional Description
1. R
I
Inverting input resistance which sets the closed-loop gain in conjunction with R
f
. This resistor also forms a
high-pass filter with C
i
at f
c
= 1/(2πR
i
C
i
).
2. C
I
Input coupling capacitor which blocks the DC voltage at the amplifier’s input terminals. Also creates a
high-pass filter with R
i
at f
c
= 1/(2πR
i
C
i
). Refer to the section Proper Selection of External Components, for
an explanation of how to determine the value of C
i
.
3. R
f
Feedback resistance which sets the closed-loop gain in conjunction with R
i
.
4. C
S
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. C
B
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 C
B
LM4929
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Typical Performance Characteristics
THD+N vs Frequency THD+N vs Frequency
20132482 20132483
THD+N vs Frequency THD+N vs Frequency
20132406 20132403
THD+N vs Frequency THD+N vs Frequency
20132405 20132404
LM4929
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Typical Performance Characteristics (Continued)
THD+N vs Output Power THD+N vs Output Power
20132492 20132493
Output Power vs Load Resistance Output Power vs Supply Voltage
20132413 20132494
Output Power vs Supply Voltage Output Power vs Load Resistance
20132495 20132497
LM4929
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Typical Performance Characteristics (Continued)
Power Supply Rejection Ratio Power Supply Rejection Ratio
201324A5 201324A6
Frequency Response vs
Input Capacitor Size Open Loop Frequency Response
20132426 201324A7
Supply Voltage vs
Supply Current
Clipping Voltage vs
Supply Voltage
20132424 20132425
LM4929
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Typical Performance Characteristics (Continued)
Shutdown Hysteresis Voltage, V
DD
= 5V Shutdown Hysteresis Voltage, V
DD
=3V
201324B1 201324B2
Power Dissipation vs Output Power
V
DD
=5V
Power Dissipation vs Output Power
V
DD
=3V
20132401 20132402
Power Dissipation vs Output Power
V
DD
= 2.4V
THD+N vs Output Power
V
DD
= 3V, R
L
=32Ω
20132414 20132415
LM4929
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Typical Performance Characteristics (Continued)
THD+N vs Output Power
V
DD
= 2.4V, R
L
=32Ω
THD+N vs Output Power
V
DD
= 3V, R
L
=16Ω
20132438 20132439
THD+N vs Output Power
V
DD
= 2.4V, R
L
=16Ω
Power Derating Curve
20132440 20132429
LM4929
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Application Information
AMPLIFIER CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4929 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 select-
ing the ratio of R
f
to R
i
. Consequently, the gain for each
channel of the IC is
A
VD
= -(R
f
/R
i
)
By driving the loads through outputs V
o
A and V
o
B with V
o
C
acting as a buffered bias voltage the LM4929 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 cou-
pling capacitors.
A configuration such as the one used in the LM4929 has a
major advantage over single supply, single-ended amplifiers.
Since the outputs V
o
A, V
o
B, and V
o
C are all biased at 1/2
V
DD
, no net DC voltage exists across each load. This elimi-
nates the need for output coupling capacitors which are
required in a single-supply, single-ended amplifier configura-
tion. Without output coupling capacitors in a typical single-
supply, single-ended amplifier, the bias voltage is placed
across the load resulting in both increased internal IC power
dissipation and possible loudspeaker damage.
The LM4929 eliminates these output coupling capacitors by
running in OCL mode. Unless shorted to ground, VoC is
internally configured to apply a 1/2 V
DD
bias voltage to a
stereo headphone jack’s sleeve. This voltage matches the
bias voltage present on VoA and VoB outputs that drive the
headphones. The headphones operate in a manner similar
to a bridge-tied load (BTL). Because the same DC voltage is
applied to both headphone speaker terminals this results in
no net DC current flow through the speaker. AC current flows
through a headphone speaker as an audio signal’s output
amplitude increases on the speaker’s terminal.
The headphone jack’s sleeve is not connected to circuit
ground when used in OCL mode. Using the headphone
output jack as a line-level output will place the LM4929’s 1/2
V
DD
bias voltage on a plug’s sleeve connection. This pre-
sents no difficulty when the external equipment uses capaci-
tively coupled inputs. For the very small minority of equip-
ment that is DC coupled, the LM4929 monitors the current
supplied by the amplifier that drives the headphone jack’s
sleeve. If this current exceeds 500mAPK, the amplifier is
shutdown, protecting the LM4929 and the external equip-
ment.
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.
P
DMAX
=(V
DD
)
2
/(2π
2
R
L
) (1)
Since the LM4929 has three operational amplifiers in one
package, the maximum power dissipation increases due to
the use of the third amplifier as a buffer and is given in
Equation 2:
P
DMAX
= 4(V
DD
)
2
/(2π
2
R
L
) (2)
The maximum power dissipation point obtained from Equa-
tion 2 must not be greater than the power dissipation that
results from Equation 3:
P
DMAX
=(T
JMAX
-T
A
)/θ
JA
(3)
For package MUB10A, θ
JA
= 190˚C/W. T
JMAX
= 150˚C for
the LM4929. Depending on the ambient temperature, T
A
,of
the system surroundings, Equation 3 can be used to find the
maximum internal power dissipation supported by the IC
packaging. If the result of Equation 2 is greater than that of
Equation 3, then either the supply voltage must be de-
creased, the load impedance increased or T
A
reduced. For
the typical application of a 3V power supply, with a 32Ωload,
the maximum ambient temperature possible without violating
the 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 Perfor-
mance Characteristics curves for power dissipation informa-
tion for lower output powers.
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 tanta-
lum 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 LM4929. A bypass
capacitor value in the range of 0.1µF to 1µF is recommended
for C
S
.
MICRO POWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the
LM4929’s shutdown function. Activate micro-power shut-
down by applying a logic-low voltage to the SHUTDOWN
pin. When active, the LM4929’s micro-power shutdown fea-
ture turns off the amplifier’s bias circuitry, reducing the sup-
ply 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 apply-
ing 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 mi-
crocontroller. When using a switch, connect an external
100kΩpull-up resistor between the SHUTDOWN pin and
V
DD
. 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.
LM4929
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Application Information (Continued)
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 digi-
tal output to apply the control voltage to the SHUTDOWN
pin. Driving the SHUTDOWN pin with active circuitry elimi-
nates the pull-up resistor.
Shutdown enable/disable times are controlled by a combina-
tion of C
B
and V
DD
. Larger values of C
B
results in longer turn
on/off times from Shutdown. Smaller V
DD
values also in-
crease turn on/off time for a given value of C
B
. Longer
shutdown times also improve the LM4929’s resistance to
click and pop upon entering or returning from shutdown. For
a 2.4V supply and C
B
= 4.7µF, the LM4929 requires about 2
seconds to enter or return from shutdown. This longer shut-
down time enables the LM4929 to have virtually zero pop
and click transients upon entering or release from shutdown.
Smaller values of C
B
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. This technique is not recom-
mended for OCL mode since shutdown enable/disable times
are very fast (0.5s) independent of supply voltage.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications us-
ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4929 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
The LM4929 is unity-gain stable which gives the designer
maximum system flexibility. The LM4929 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 1V
rms
are available
from sources such as audio codecs. Very large values
should not be used for the gain-setting resistors. Values for
R
i
and R
f
should be less than 1MΩ. Please refer to the
section, Audio Power Amplifier Design, for a more com-
plete explanation of proper gain selection
Besides gain, one of the major considerations is the closed-
loop bandwidth of the amplifier. To a large extent, the band-
width is dictated by the choice of external components
shown in Figure 2. The input coupling capacitor, C
i
, forms a
first order high pass filter which limits low frequency re-
sponse. This value should be chosen based on needed
frequency response and turn-on time.
SELECTION OF INPUT CAPACITOR SIZE
Amplifying the lowest audio frequencies requires a high
value input coupling capacitor, C
i
. A high value capacitor can
be expensive and may compromise space efficiency in por-
table 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 C
i
. A larger input
coupling capacitor requires more charge to reach its quies-
cent DC voltage. This charge comes from the output via the
feedback Thus, by minimizing the capacitor size based on
necessary low frequency response, turn-on time can be
minimized. A small value of C
i
(in the range of 0.1µF to
0.39µF), is recommended.
AUDIO POWER AMPLIFIER DESIGN
A 25mW/32ΩAUDIO AMPLIFIER
Given:
Power Output 25mWrms
Load Impedance 32Ω
Input Level 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 Per-
formance 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 LM4929 to reproduce peak in excess of 25mW
without producing audible distortion. At this time, the de-
signer 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.
(4)
From Equation 4, the minimum A
V
is 0.89; use A
V
= 1. Since
the desired input impedance is 20kΩ, and with a A
V
gain of
1, a ratio of 1:1 results from Equation 1 for R
f
to R
i
. The
values are chosen with R
i
= 20kΩand R
f
= 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 speci-
fied.
f
L
= 100Hz/5 = 20Hz
f
H
= 20kHz*5=100kHz
As stated in the External Components section, R
i
in con-
junction with C
i
creates a
C
i
≥1/(2π* 20kΩ* 20Hz) = 0.397µF; use 0.39µF.
The high frequency pole is determined by the product of the
desired frequency pole, f
H
, and the differential gain, A
V
. With
an A
V
= 1 and f
H
= 100kHz, the resulting GBWP = 100kHz
which is much smaller than the LM4929 GBWP of 10MHz.
This figure displays that is a designer has a need to design
an amplifier with higher differential gain, the LM4929 can still
be used without running into bandwidth limitations.
Figure 3 shows an optional resistor connected between the
amplifier output that drives the headphone jack sleeve and
ground. This resistor provides a ground path that supressed
power supply hum. Thishum may occur in applications such
as notebook computers in a shutdown condition and con-
nected to an external powered speaker. The resistor’s 100Ω
value is a suggested starting point. Its final value must be
LM4929
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Application Information (Continued)
determined based on the tradeoff between the amount of
noise suppression that may be needed and minimizing the
additional current drawn by the resistor (25mA for a 100Ω
resistor and a 5V supply).
ESD PROTECTION
As stated in the Absolute Maximum Ratings, the LM4929
has a maximum ESD susceptibility rating of 2000V. For
higher ESD voltages, the addition of a PCDN042 dual transil
(from California Micro Devices), as shown in Figure 3, will
provide additional protection.
201324B4
FIGURE 3. The PCDN042 provides additional ESD protection beyond the 2000V shown in the
Absolute Maximum Ratings for the V
O
C output
LM4929
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Physical Dimensions inches (millimeters) unless otherwise noted
MSOP
Order Number LM4929MM
NS Package Number MUB10A
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.
For the most current product information visit us at www.national.com.
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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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Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain no ‘‘Banned
Substances’’ as defined in CSP-9-111S2.
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LM4929 Stereo 40mW Low Noise Headphone Amplifier with OCL Output
