LM4916
1.5V, Mono 85mW BTL Output, 14mW Stereo Headphone
Audio Amplifier
General Description
The unity gain stable LM4916 is both a mono differential
output (for bridge-tied loads or BTL) audio power amplifier
and a Single Ended (SE) stereo headphone amplifier. Oper-
ating on a single 1.5V supply, the mono BTL mode delivers
85mW into an 8Ωload at 1% THD+N. In Single Ended stereo
headphone mode, the amplifier delivers 14mW per channel
into a 16Ωload at 1% THD+N.
With the LM4916 packaged in the MM and LLP packages,
the customer benefits include low profile and small size.
These packages minimize PCB area and maximizes output
power.
The LM4916 features circuitry that reduces output transients
("clicks" and "pops") during device turn-on and turn-off, an
externally controlled, low-power consumption, active-low
shutdown mode, and thermal shutdown. Boomer audio
power amplifiers are designed specifically to use few exter-
nal components and provide high quality output power in a
surface mount package.
Key Specifications
nMono-BTL output power
n(R
L
=8Ω,V
DD
= 1.5V, THD+N = 1%) 85mW (typ)
nStereo Headphone output power
n(R
L
=16Ω,V
DD
= 1.5V, THD+N = 1%) 14mW (typ)
nMicropower shutdown current 0.02µA (typ)
nSupply voltage operating range 0.9V <V
DD
<2.5V
nPSRR 1kHz, V
DD
= 1.5V 66dB (typ)
Features
nSingle-cell 0.9V to 2.5V battery operation
nBTL mode for mono speaker
nSingle ended headphone operation with coupling
capacitors
nUnity-gain stable
n"Click and pop" suppression circuitry
nActive low micropower shutdown
nLow current, active-low mute mode
nThermal shutdown protection circuitry
Applications
nPortable one-cell audio products
nPortable one-cell electronic devices
Typical Application
Boomer®is a registered trademark of National Semiconductor Corporation.
20048701
FIGURE 1. Block Diagram
July 2006
LM4916 1.5V, Mono 85mW BTL Output, 14mW Stereo Headphone Audio Amplifier
© 2006 National Semiconductor Corporation DS200487 www.national.com
Connection Diagrams
MSOP Package MSOP Marking
20048702
Top View
Order Number LM4916MM
See NS Package Number MUB10A for MSOP
200487F9
Z - Plant Code
X - Date Code
T - Die Traceability
G - Boomer Family
A9 - LM4916MM
LD Package LLP Marking
20048752
Top View
Order Number LM4916LD
See NS Package Number LDA10A
200487G0
Z - Plant Code
XY - Date Code
T - Die Traceability
Bottom Line - Part Number
LM4916
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Typical Connections
20048703
FIGURE 2. Typical Single Ended Output Configuration Circuit
20048705
FIGURE 3. Typical BTL Speaker Configuration Circuit
LM4916
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Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage 3.6V
Storage Temperature −65˚C to +150˚C
Input Voltage −0.3V to V
DD
+0.3V
Power Dissipation (Note 2) Internally limited
ESD Susceptibility(Note 3) 2000V
ESD Susceptibility (Note 4) 200V
Junction Temperature 150˚C
Solder Information
Small Outline Package Vapor
Phase (60sec) 215˚C
Infrared (15 sec) 220˚C
See AN-450 “Surface Mounting and their Effects on
Product Reliablilty” for other methods of soldering
surface mount devices.
Thermal Resistance
θ
JA
(typ) MUB10A 175˚C/W
θ
JA
(typ) LDA10A 73˚C/W
Operating Ratings
Temperature Range
T
MIN
≤T
A
≤T
MAX
−40˚C ≤T
A
≤85˚C
Supply Voltage (Note 10) 0.9V ≤V
DD
≤2.5V
Electrical Characteristics for the LM4916 (Notes 1, 5)
The following specifications apply for the circuit shown in Figure 4 operating with V
DD
= 1. 5V, unless otherwise
specified. Limits apply for T
A
= 25˚C.
Symbol Parameter Conditions LM4916 Units
(Limits)
Typical Limit
(Note 6) (Note 7)
V
DD
Supply Voltage (Notes 10, 11) 0.9 V (min)
2.5 V (max)
I
DD
Quiescent Power Supply Current V
IN
= 0V, I
O
= 0A, R
L
=∞(Note 8) 1.0 1.4 mA (max)
I
SD
Shutdown Current V
SHUTDOWN
= GND 0.02 µA (max)
V
OS
Output Offset Voltage BTL 5 50 mV (max)
P
O
Output Power (Note 9)
f = 1kHz
R
L
=8ΩBTL, THD+N = 1% 85 70 mW (min)
R
L
=16ΩSE, THD+N = 1% 14 mW
THD+N Total Harmonic Distortion + Noise R
L
=8Ω, BTL, P
O
= 25mW, f = 1kHz 0.1 0.5 %
R
L
=16Ω, SE, P
O
= 5mW, f = 1kHz 0.2
V
NO
Output Voltage Noise 20Hz to 20kHz, A-weighted 10 µV
RMS
I
MUTE
Mute Current V
MUTE
=0,SE 15 µA
Crosstalk R
L
=16Ω, SE 55 dB (min)
PSRR Power Supply Rejection Ratio
V
RIPPLE
= 200mV
P-P
C
BYPASS
= 4.7µF, R
L
=8Ω
f = 1kHz, BTL
62 dB
V
RIPPLE
= 200mV
P-P
sine wave
C
BYPASS
= 4.7µF, R
L
=16Ω
f = 1kHz, SE
66 dB (min)
V
IH
Control Logic High 0.7 V (min)
V
IL
Control Logic Low 0.3 V (max)
LM4916
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Note 1: 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 2: The maximum power dissipation is dictated by TJMAX,θJA, and the ambient temperature TAand must be derated at elevated temperatures. The maximum
allowable power dissipation is PDMAX =(T
JMAX −T
A)/θJA. For the LM4916, TJMAX = 150˚C. For the θJAs, please see the Application Information section or the
Absolute Maximum Ratings section.
Note 3: Human body model, 100pF discharged through a 1.5kΩresistor.
Note 4: Machine model, 220pF–240pF discharged through all pins.
Note 5: All voltages are measured with respect to the ground (GND) pins unless otherwise specified.
Note 6: Typicals are measured at 25˚C and represent the parametric norm.
Note 7: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis.
Note 8: The quiescent power supply current depends on the offset voltage when a practical load is connected to the amplifier.
Note 9: Output power is measured at the device terminals.
Note 10: When operating on a power supply voltage of 0.9V, the LM4916 willl not function below 0˚C. At a power supply voltage of 1V or greater, the LM4916 will
operate down to -40˚C.
Note 11: Ripple on power supply line should not exceed 400mVpp.
LM4916
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Typical Performance Characteristics
THD+N vs Frequency
V
DD
= 1.5V, P
O
= 5mW, R
L
=16Ω
BW <80kHz, Single Ended Output
THD+N
vs Frequency
V
DD
= 1.5V, R
L
=8Ω,P
O
= 25mW
BTL Output, A
V
=-1
200487C2 200487D5
THD+N
vs Frequency
V
DD
= 1.2V, P
O
= 5mW
R
L
=16Ω, Single Ended Output, A
V
=-1
THD+N
vs Frequency
V
DD
= 1.2V, R
L
=8Ω,P
O
= 25mW
BTL Output, A
V
=-1
200487D0 200487D6
THD+N
vs Output Power
V
DD
= 1.5V, R
L
=16Ω, f = 1kHz
Single Ended Output, A
V
=-1
THD+N
vs Output Power
V
DD
= 1.5V, R
L
=8Ω, f = 1kHz
BTL Output, A
V
=-1
200487D7 200487D8
LM4916
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Typical Performance Characteristics (Continued)
THD+N
vs Output Power
V
DD
= 1.2V, R
L
=16Ω, f = 1kHz
Single Ended Output, A
V
=-1
THD+N
vs Output Power
V
DD
= 1.2V, R
L
=8Ω, f = 1kHz
BTL Output, A
V
=-1
200487E0 200487D9
Output Power
vs Supply Voltage
f = 1kHz, R
L
=16Ω,
Single Ended Output, A
V
=-1
Output Power
vs Supply Voltage
f = 1kHz, R
L
=8Ω,
BTL Output, A
V
=-1
200487G2 200487G1
Output Power
vs Load Resistance
V
DD
= 1.5V, f = 1kHz
Single Ended Output, A
V
=-1
Output Power
vs Load Resistance
V
DD
= 1.5V, f = 1kHz
BTL Output, A
V
=-1
200487E5 200487E6
LM4916
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Typical Performance Characteristics (Continued)
Output Power
vs Load Resistance
V
DD
= 1.2V, f = 1kHz
Single Ended Output, A
V
=-1
Output Power
vs Load Resistance
V
DD
= 1.2V, f = 1kHz
BTL Output, A
V
=-1
200487E3 200487E4
Power Dissipation
vs Output Power
f = 1kHz, THD+N <1%, A
V
=-1
Single Ended Output, Both Channels
Power Dissipation
vs Output Power
f = 1kHz, THD+N <1%
BTL Output, A
V
=-1
200487F6 200487F5
Channel Separation
R
L
=16Ω,P
O
= 5mW
Single Ended Output, A
V
=-1
Power Supply Rejection Ratio
V
DD
= 1.5V, V
RIPPLE
= 200mV
PP
R
L
=16Ω, Single Ended Output
Input Terminated into 10Ω
200487D4 200487C6
LM4916
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Typical Performance Characteristics (Continued)
Power Supply Rejection Ratio
V
DD
= 1.5V, V
RIPPLE
= 200mV
PP
R
L
=8Ω, BTL
Input Terminated into 10Ω
Power Supply Rejection Ratio
V
DD
= 1.2V, V
RIPPLE
= 200mV
PP
R
L
=16Ω, Single Ended Output
Input Terminated into 10Ω
200487C8 200487C5
Power Supply Rejection Ratio
V
DD
= 1.2V, V
RIPPLE
= 200mV
PP
R
L
=8Ω, BTL
Input Terminated into 10Ω
Frequency Response
vs Input Capacitor Size
V
DD
= 1.5V, R
L
=16Ω
AV = -1, BW <80kHz, Single Ended Output
200487C7 200487F8
Frequency Response
vs Input Capacitor Size
V
DD
= 1.5V, R
L
=8Ω
AV = -1, BW <80kHz, BTL Output
Open Loop Frequency Response
VDD = 1.5V, No load
200487C4 200487B8
LM4916
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Typical Performance Characteristics (Continued)
Supply Voltage
vs Supply Current
Clipping Voltage
vs Supply Voltage
200487F1 200487E2
Noise Floor
VDD = 1.5V, Single Ended Output
16Ω, 80kHz Bandwith
Noise Floor
V
DD
= 1.5V, BTL Output
8Ω, 80kHz Bandwith
200487B7 200487C3
Shutdown Hystresis Voltage
V
DD
= 1.5V
Power Derating Curve
V
DD
= 1.5V
200487E1 200487F4
LM4916
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Typical Performance Characteristics (Continued)
Mute Attenuation
vs Load Resistance
Shutdown Current
Distribution
200487F2 200487F7
Application Information
SINGLE ENDED (SE) CONFIGURATION EXPLANATION
As shown in Figure 2, the LM4916 has two operational
amplifiers internally, which have externally configurable gain.
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
A
VD
= -(R
f
/R
i
)
When the LM4916 operates in Single Ended mode, coupling
capacitors are used on each output (VoA and VoB) and the
SE/BTL pin (Pin 8) is connected to ground. These output
coupling capacitors blocks the half supply voltage to which
the output amplifiers are typically biased and couples the
audio signal to the headphones or other single-ended (SE)
loads. The signal return to circuit ground is through the
headphone jack’s sleeve.
BRIDGED (BTL) CONFIGURATION EXPLANATION
As shown in Figure 3, the LM4916 has two internal opera-
tional amplifiers. The first amplifier’s gain is externally con-
figurable, while the second amplifier should be externally
fixed in a unity-gain, inverting configuration. The closed-loop
gain of the first amplifier is set by selecting the ratio of R
f
to
R
i
while the second amplifier’s gain should be fixed by the
two external 20kΩresistors. Figure 3 shows that the output
of amplifier one serves as the input to amplifier two which
results in both amplifiers producing signals identical in mag-
nitude, but out of phase by 180˚. Consequently, the differen-
tial gain for the IC is
A
VD
= 2 *(R
f
/R
i
).
By driving the load differentially through outputs Vo1 and
Vo2, an amplifier configuration commonly referred to as
"bridged mode" is established. Bridged mode operation is
different from the classical single-ended amplifier configura-
tion where one side of the load is connected to ground. A
bridge amplifier design has a few distinct advantages over
the single-ended configuration. It provides a differential drive
to the load, thus doubling output swing for a specified supply
voltage. Four times the output power is possible as com-
pared 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. In order to choose
an amplifier’s closed-loop gain without causing excessive
clipping, please refer to the Audio Power Amplifier Design
section.
A bridge configuration, such as the one used in LM4916,
also creates a second advantage over single-ended amplifi-
ers. Since the differential outputs, Vo1 and Vo2, are biased
at half-supply, no net DC voltage exists across the load. This
eliminates the need for an output coupling capacitor which is
required in a single supply, single-ended amplifier configura-
tion.
MODE SELECT DETAIL
The LM4916 can be configured in either Single Ended or
BTL mode (see Figure 2 and Figure 3). The default state of
the LM4916 at power up is single ended. During initial power
up or return from shutdown, the LM4916 must detect the
correct mode of operation by sensing the status of the
SE/BTL pin. When the bias voltage of the part ramps up to
60mV (as seen on the Bypass pin), an internal comparator
detects the status of SE/BTL; and at 10mV, latches that
value in place. Ramp up of the bias voltage will proceed at a
different rate from this point on depending upon operating
mode. BTL mode will ramp up about 11 times faster than
Single Ended mode. Shutdown is not a valid command
during this time period (T
WU
) and should not enabled to
ensure a proper power on reset (POR) signal. In addition,
the slew rate of V
DD
must be greater than 2.5V/ms to ensure
reliable POR. Recommended power up timing is shown in
Figure 5 along with proper usage of Shutdown and Mute.
The mode-select circuit is suspended during C
B
discharge
time. The circuit shown in Figure 4 presents an applications
solution to the problem of using different supply voltages
with different turn-on times in a system with the LM4916.
This circuit shows the LM4916 with a 25-50kΩ. Pull-up re-
sistor connected from the shutdown pin to V
DD
. The shut-
LM4916
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Application Information (Continued)
down pin of the LM4916 is also being driven by an open
drain output of an external microcontroller on a separate
supply. This circuit ensures that shutdown is disabled when
powering up the LM4916 by either allowing shutdown to be
high before the LM4916 powers on (the microcontroller pow-
ers up first) or allows shutdown to ramp up with V
DD
(the
LM4916 powers up first). This will ensure the LM4916 pow-
ers up properly and enters the correct mode of operation.
Please note that the SE/BTL pin (Pin 8) should be tied to
GND for Single Ended mode, and to V
DD
for BTL mode.
20048753
FIGURE 4. Recommended Circuit for Different Supply Turn-On Timing
LM4916
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Application Information (Continued)
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful amplifier, whether the amplifier is bridged or
single-ended. A direct consequence of the increased power
delivered to the load by a bridge amplifier is an increase in
internal power dissipation. Since the LM4916 has two opera-
tional amplifiers in one package, the maximum internal
power dissipation is 4 times that of a single-ended amplifier.
The maximum power dissipation for a given BTL application
can be derived from the power dissipation graphs or from
Equation 1.
P
DMAX
= 4*(V
DD
)
2
/(2π
2
R
L
) (1)
When operating in Single Ended mode, Equation 2 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
) (2)
Since the LM4916 has two operational amplifiers in one
package, the maximum internal power dissipation point is
twice that of the number that results from Equation 2. From
Equation 2, assuming a 1.5V power supply and a 16Ωload,
the maximum power dissipation point is 7mW per amplifier.
Thus the maximum package dissipation point is 14mW.
The maximum power dissipation point obtained from either
Equations 1, 2 must not be greater than the power dissipa-
tion that results from Equation 3:
P
DMAX
=(T
JMAX
-T
A
)/θ
JA
(3)
For package MUB10A, θ
JA
= 175˚C/W. T
JMAX
= 150˚C for
the LM4916. 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 1 or 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 1.5V power supply, with a 16Ω
load, the maximum ambient temperature possible without
violating the maximum junction temperature is approximately
146˚C provided that device operation is around the maxi-
mum 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 dissi-
pation information for lower output powers.
20048754
FIGURE 5. Turn-On, Shutdown, and Mute Timing for Cap-Coupled Mode
LM4916
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Application Information (Continued)
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATIONS
The LM4916’s exposed-DAP (die attach paddle) package
(LD) provides a low thermal resistance between the die and
the PCB to which the part is mounted and soldered. This
allows rapid heat transfer from the die to the surrounding
PCB copper traces, ground plane, and surrounding air.
The LD package should have its DAP soldered to a copper
pad on the PCB. The DAP’s PCB copper pad may be con-
nected to a large plane of continuous unbroken copper. This
plane forms a thermal mass, heat sink, and radiation area.
Further detailed and specific information concerning PCB
layout, fabrication, and mounting an LD (LLP) package is
available from National Semiconductor’s Package Engineer-
ing Group under application note AN1187.
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 em-
ploy a battery (or 1.5V regulator) with 10µF tantalum or
electrolytic capacitor and a ceramic bypass capacitor that
aid in supply stability. This does not eliminate the need for
bypassing the supply nodes of the LM4916. A bypass ca-
pacitor value in the range of 0.1µF to 1µF is recommended.
MICRO POWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the
LM4916’s shutdown function. Activate micro-power shut-
down by applying a logic-low voltage to the SHUTDOWN
pin. When active, the LM4916’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 0.02µA (typ) shutdown current is achieved by ap-
plying 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. 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.
MUTE
When in single ended mode, the LM4916 also features a
mute function that enables extremely fast turn-on/turn-off
with a minimum of output pop and click with a low current
consumption (≤20µA, typical). 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. Providing a logic low signal on the
MUTE pin enables mute mode. Threshold voltages and ac-
tivation techniques match those given for the shutdown func-
tion as well. Mute may not appear to function when the
LM4916 is used to drive high impedance loads. This is
because the LM4916 relies on a typical headphone load
(16-32Ω) to reduce input signal feed-through through the
input and feedback resistors. Mute attenuation can thus be
calculated by the following formula:
Mute Attenuation (dB) = 20Log[R
L
/(R
i
+R
F
)]
Parallel load resistance may be necessary to achieve satis-
factory mute levels when the application load is known to be
high impedance. The mute function, described above, is not
necessary when the LM4916 is operating in BTL mode since
the shutdown function operates quickly in BTL mode with
less power consumption than mute. In these modes, the
Mute signal is equivalent to the Shutdown signal. Mute may
be enabled during shutdown transitions, but should not be
toggled for a brief period immediately after exiting or entering
shutdown. These brief time periods are labeled X1 (time
after returning from shutdown) and X2 (time after entering
shutdown) and are shown in the timing diagram given in
Figure 5. X1 occurs immediately following a return from
shutdown (TWU) and lasts 40ms±25%. X2 occurs after the
part is placed in shutdown and the decay of the bias voltage
has occurred (2.2*250k*CB) and lasts for 100ms±25%. The
timing of these transition periods relative to X1 and X2 is also
shown in Figure 5. While in single ended mode, mute should
not be toggled during these time periods, but may be toggled
during the shutdown transitions or any other time the part is
in normal operation. Failure to operate mute correctly may
result in much higher click and pop values or failure of the
device to mute at all.
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 LM4916 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity. The LM4916 is unity-gain stable that gives the designer
maximum system flexibility. The LM4916 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 complete
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 bandwidth is dictated by the
choice of external components shown in Figures 2 and 3.
The input coupling capacitor, C
i
, forms a first order high pass
filter that limits low frequency response. 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
LM4916
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Application Information (Continued)
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 quiescent DC voltage. This charge
comes from the output via the feedback. Thus, by minimizing
the capacitor size based on necessary low frequency re-
sponse, turn-on time can be minimized. A small value of C
i
(in the range of 0.1µF to 0.47µF), is recommended.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consid-
eration should be paid to value of C
B
, the capacitor con-
nected to the BYPASS pin. Since C
B
determines how fast
the LM4916 settles to quiescent operation, its value is critical
when minimizing turn-on pops. The slower the LM4916’s
outputs ramp to their quiescent DC voltage (nominally V
DD
/
2), the smaller the turn-on pop. Choosing C
B
equal to 4.7µF
along with a small value of C
i
(in the range of 0.1µF to
0.47µF), produces a click-less and pop-less shutdown func-
tion. As discussed above, choosing C
i
no larger than neces-
sary for the desired bandwidth helps minimize clicks and
pops. This ensures that output transients are eliminated
when power is first applied or the LM4916 resumes opera-
tion after shutdown.
Minimizing External Components
Operating the LM4916 at higher gain settings can minimize
the use of external components. For instance, a BTL con-
figuration with a gain setting greater than 8V/V (A
V
>8)
makes the output capacitor C
O
unnecessary. For the Single
Ended configuration, a gain setting greater than 4V/V (A
V
>
4) eliminates the need for output capacitor C
O2
and output
resistor R
O
, on each output channel.
If the LM4916 is operating with a lower gain setting (A
V
<4),
external components can be further minimized only in Single
Ended mode. For each channel, output capacitor (C
O2
) and
output resistor (R
O
) can be eliminated. These components
need to be compensated for by adding a 7.5kΩresistor (R
C
)
between the input pin and ground pin on each channel
(between Pin 1 and GND, and between Pin 5 and GND).
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4916 contains circuitry that eliminates turn-on and
shutdown transients ("clicks and pops"). For this discussion,
turn-on refers to either applying the power supply voltage or
when the micro-power shutdown mode is deactivated.
As the V
DD
/2 voltage present at the BYPASS pin ramps to its
final value, the LM4916’s internal amplifiers are configured
as unity gain buffers. An internal current source charges the
capacitor connected between the BYPASS pin and GND in a
controlled, linear manner. Ideally, the input and outputs track
the voltage applied to the BYPASS pin. The gain of the
internal amplifiers remains unity until the voltage on the
bypass pin reaches V
DD
/2. As soon as the voltage on the
bypass pin is stable, the device becomes fully operational
and the amplifier outputs are reconnected to their respective
output pins. Although the BYPASS pin current cannot be
modified, changing the size of C
B
alters the device’s turn-on
time. There is a linear relationship between the size of C
B
and the turn-on time. Here are some typical turn-on times for
various values of C
B
:
Single-Ended
C
B
(µF) T
ON
0.1 117ms
0.22 179ms
0.47 310ms
1.0 552ms
2.2 1.14s
4.7 2.4s
BTL
C
B
(µF) T
ON
(ms)
0.1 72
0.22 79
0.47 89
1.0 112
2.2 163
4.7 283
In order to eliminate "clicks and pops", all capacitors must be
discharged before turn-on. Rapidly switching V
DD
may not
allow the capacitors to fully discharge, which may cause
"clicks and pops".
AUDIO POWER AMPLIFIER DESIGN
A 25mW/32ΩAudio Amplifier
Given:
Power Output 10mWrms
Load Impedance 16Ω
Input Level 0.4Vrms
Input Impedance 20kΩ
A designer must first choose a mode of operation (SE or
BTL) and 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.
1.5V is a standard voltage in most applications, it is chosen
for the supply rail. Extra supply voltage creates headroom
that allows the LM4916 to reproduce peak in excess of
10mW 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.
(4)
From Equation 4, the minimum AV is 1; use A
V
= 1. Since the
desired input impedance is 20k, and with a A
V
gain of 1, a
ratio of 1:1 results from Equation 1 for R
f
to R. The values
are chosen with R
i
= 20k and R
f
= 20k. 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.
LM4916
www.national.com15
Application Information (Continued)
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, fH, and the differential gain, A
V
. With
an AV
V
= 1 and f
H
= 100kHz, the resulting GBWP = 100kHz
which is much smaller than the LM4916 GBWP of 3MHz.
This example displays that if a designer has a need to design
an amplifier with higher differential gain, the LM4916 can still
be used without running into bandwidth limitations.
LM4916
www.national.com 16
Revision History
Rev Date Description
1.0 7/11/03 Re-released the D/S to the WEB.
1.1 7/25/06 Deleted the RL labels on curves E5, E6, E3, and E4, per Allan S., then re-released the D/S to the WEB.
LM4916
www.national.com17
Physical Dimensions inches (millimeters)
unless otherwise noted
MSOP Package
Order Number LM4916MM
NS Package Number MUB10A
LD Package
Order Number LM4916LD
NS Package Number LDA10A
LM4916
www.national.com 18
Notes
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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NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS
WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR
CORPORATION. As used herein:
1. Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the body, or
(b) support or sustain life, and whose failure to perform when
properly used in accordance with instructions for use
provided in the labeling, can be reasonably expected to result
in a significant injury to the user.
2. A critical component is any component of a life support
device or system whose failure to perform can be reasonably
expected to cause the failure of the life support device or
system, or to affect its safety or effectiveness.
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and Materials of Interest Specification (CSP-9-111S2) for regulatory environmental compliance. Details may be found at:
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www.national.com
LM4916 1.5V, Mono 85mW BTL Output, 14mW Stereo Headphone Audio Amplifier
