NSC MXF10A

LM4914
1W Monaural, 85mW Stereo Headphone Audio Amplifier
General Description
j Micro-power shutdown supply current
The unity-gain stable LM4914 is both a mono differential
output (for bridge-tied loads, or BTL) audio power amplifier
and a single-ended (SE) stereo headphone amplifier. Operating on a single 5V supply, the mono BTL mode delivers 1W
into an 8Ω load. In SE stereo mode, the amplifier delivers
85mW to 32Ω loads. The LM4914 features circuitry that
suppresses output transients ("clicks and pops").
j PSRR (f = 1kHz)
The LM4914 is designed for notebook and other handheld
portable applications. It delivers high quality output power
from a surface-mount package and requires few external
components. The LM4914 is pin and functionally compatible
with the TPA0253.
Other features include an active-low micro-power shutdown
mode and thermal shutdown protection.
The LM4914 is available in a space efficient 10-lead
exposed-DAP TSSOP package.
Key Specifications
52dB (typ)
52dB (typ)
Features
n
n
n
n
n
n
n
n
Advanced "click and pop" suppression circuitry
Stereo headphone amplifier mode
Low current micro-power shutdown mode
Thermal shutdown protection circuitry
2.5V to 5.5V operation
Unity-gain stable
Gain set with external resistors
Space-saving exposed-DAP TSSOP package
Applications
n PDAs
n Cellular phones
n Handheld portable electronic devices
j BTL output power (RL = 8Ω)
VDD = at 3.0V, THD = 0.1%
VDD = at 5.0V, THD = 0.1%
VDD = at 3.0V, BTL Mode
VDD = at 5.0V, BTL Mode
0.03µA (typ)
330mW (typ)
1W (typ)
j SE output power (RL = 32Ω)
VDD = at 3.0V, THD = 0.1%
VDD = at 5.0V, THD = 0.1%
30mW (typ)
85mW (typ)
Connection Diagram
200634A0
Top View
Order Number LM4914MH
See NS package Number MXF10A
Boomer ® is a registered trademark of National Semiconductor Corporation.
© 2003 National Semiconductor Corporation
DS200634
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LM4914 1W Monaural, 85mW Stereo Headphone Audio Amplifier
March 2003
LM4914
Typical Application
200634C3
FIGURE 1. Typical Audio Amplifier Application Circuit
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2
Infrared (15sec)
(Note 3)
See AN-450 "Surface Mounting
and their Effects on Product
Reliability" for other methods of
soldering surface-mount devices.
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
Thermal Resistance
−65˚C to +150˚C
θJA (typ) - MXF10A
−0.3V to VDD +0.3V
Input Voltage
Power Dissipation (Note 4)
Internally Limited
ESD Susceptibility (Note 5)
2000V
ESD Susceptibility (Note 6)
200V
Junction Temperature
46˚C/W
Operating Ratings
Temperature Range
150˚C
TMIN ≤ TA ≤ TMAX
Solder Information
−40˚C ≤ TA ≤ 85˚C
2.5V ≤ VDD ≤ 5.5V
Supply Voltage
Small Outline Package
Vapor Phase (60sec)
220˚C
215˚C
Electrical Characteristics for Entire Amplifier (VDD = 5V) (Note 3)
The following specifications apply for the circuit shown in Figure 1, unless otherwise specified. Limits apply for TA = 25˚C.
LM4914
Symbol
Parameter
VDD
Supply Voltage
Conditions
IDD
Quiescent Power Supply Current
ISD
Shutdown Quiescent Power Supply VSHUTDOWN = GND
Current
VIN = 0V, IO = 0A, No Load
VOS
Output Offset Voltage
VDD = 5V, CBYPASS = 0.47µF,
Vripple = 200mVp-p 1kHz sine wave
RL = 8Ω
Typical
Limit
(Note 8)
(Notes 9, 10)
Units
(Limits)
2.5
5.5
V (min)
V (max)
4
6.5
mA (max)
0.03
5
µA (max)
7
30
mV (max)
52 BTL
60 SE
PSRR
Power Supply Rejection Ratio
HP-SVIH
HP-SENSE Logic-High Threshold
Voltage
4.5
V (min)
HP-SVIL
HP-SENSE Logic-Low Threshold
Voltage
2.75
V (max)
SDVIH
Shutdown Logic High Threshold
2.0
V (min)
SDVIL
Shutdown Logic LowThreshold
0.8
V (max)
3
dB
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LM4914
Absolute Maximum Ratings
LM4914
Electrical Characteristics: Bridged-Mode Operation (VDD = 5V) (Note 3)
The following specifications apply for the circuit shown in Figure 1, RL = 8Ω, and a measurement bandwidth of 20Hz to
80kHz, unless otherwise specified. Limits apply for TA = 25˚C.
LM4914
Symbol
Parameter
Conditions
PO
Output Power (Note 1, 11)
RL = 8Ω, f = 1kHz,
THD+N = 0.1%, AV = 8dB
THD+N
Total Harmonic Distortion + Noise
RL = 8Ω, f = 1kHz, PO = 1W
VON
Output Voltage Noise
CB = 0.47µF, 20Hz < f < 20kHz
Typical
Limit
(Note 8)
(Notes 9, 10)
1
Units
(Limits)
W (min)
0.33
%
35
µVRMS
Electrical Characteristics: SE Operation (VDD = 5V) (Note 3)
The following specifications apply for the circuit shown in Figure 1 and a measurement bandwidth of 20Hz to 80kHz, unless
otherwise specified. Limits apply for TA = 25˚C.
LM4914
Symbol
Parameter
Conditions
Typical
Limit
(Note 8)
(Notes 9, 10)
Units
(Limits)
PO
Output Power (Note 11)
THD+N = 0.1%, f = 1kHz,
RL = 32Ω, AV = 1.9dB (Note 12)
85
mW (min)
VON
Output Voltage Noise
CB = 0.47µF, 20Hz < f < 20kHz
13
µVRMS
Electrical Characteristics for Entire Amplifier (VDD = 3V) (Note 3)
The following specifications apply for the circuit shown in Figure 1, unless otherwise specified. Limits apply for TA = 25˚C.
LM4914
Symbol
Parameter
VDD
Supply Voltage
Conditions
IDD
Quiescent Power Supply Current
ISD
Shutdown Quiescent Power Supply VSHUTDOWN = GND
Current
VIN = 0V, IO = 0A, No Load
VOS
Output Offset Voltage
VDD = 3V, CBYPASS = 0.47µF,
Vripple = 200mVp-p 1kHz sine wave
RL = 8Ω
Typical
Limit
(Note 8)
(Notes 9, 10)
Units
(Limits)
2.5
5.5
V (min)
V (max)
2.7
5
mA (max)
0.03
4
µA (max)
7
30
mV (max)
52 BTL
60 SE
PSRR
Power Supply Rejection Ratio
HP-SVIH
HP-SENSE Logic-High Threshold
Voltage
2.7
V (min)
HP-SVIL
HP-SENSE Logic-Low Threshold
Voltage
1.65
V (max)
SDVIH
Shutdown Logic High Threshold
2
V (min)
SDVIL
Shutdown Logic LowThreshold
0.8
V (max)
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4
dB
LM4914
Symbol
Parameter
PO
Output Power (Note 11)
THD+N
Total Harmonic Distortion + Noise
VON
Output Voltage Noise
Conditions
THD = 0.1%, f = 1kHz
AV = 14dB, RL = 8Ω
Typical
Limit
(Note 8)
(Notes 9, 10)
Units
(Limits)
330
mW
f = 1kHz
PO = 250mW
0.2
%
CB = 0.47µF, 20Hz < f < 20kHz
29
µVRMS
Electrical Characteristics: SE Operation (VDD = 3V) (Note 3)
The following specifications apply for the circuit shown in Figure 1 and a measurement bandwidth of 20Hz to 80kHz, unless
otherwise specified. Limits apply for TA = 25˚C.
LM4914
Symbol
Parameter
Conditions
Typical
Limit
(Note 8)
(Notes 9, 10)
Units
(Limits)
PO
Output Power (Note 11)
THD+N = 0.1%, f = 1kHz,
RL = 32Ω, AV = 1.9dB (Note 12)
30
mW
VON
Output Voltage Noise
CB = 0.47µF, 20Hz < f < 20kHz
13
µVRMS
Note 1: When operating on a 5VDC, an LM4914MH that has been properly mounted to a circuit board will deliver 1W into 8Ω. See the Application Information
sections for further information concerning PCB layout suggestions to maximize the LM4914MH’s output power into an 8Ω load.
Note 2: All voltages are measured with respect to the GND pin unless otherwise specified.
Note 3: 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 4: 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 LM4914, see power derating
curves for additional information.
Note 5: Human body model, 100pF discharged through a 1.5kΩ resistor.
Note 6: Machine Model, 220pF–240pF discharged through all pins.
Note 7: The given θJA is for an LM4914 packaged in an MH with the Exposed-DAP soldered to an exposed 2in2 area of 1oz printed circuit board copper.
Note 8: Typicals are measured at 25˚C and represent the parametric norm.
Note 9: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).
Note 10: Datasheet minimum and maximum specification limits are guaranteed by design, test, or statistical analysis.
Note 11: Output power is measured at the amplifier’s package pins.
Note 12: See Application Information section "Single-Ended Output Power Performance and Measurement Considerations" for more information.
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LM4914
Electrical Characteristics: Bridged-Mode Operation (VDD = 3V) (Note 3)
The following specifications apply for the circuit shown in Figure 1, unless otherwise specified. Limits apply for TA = 25˚C.
LM4914
External Components Description
(Figure 1)
Components
Functional Description
1.
Ri
This is the input resistor for the inverting input that, along with the 62.5kΩ internal feedback resistor Rf, sets
the first stage’s closed-loop gain. The overall SE gain is AV (SE) = 62.5kΩ / Ri, whereas the overall BTL gain
is AV (BTL) = -125kΩ / Ri. Input resistance Ri and input capacitance Ci form a high pass filter. The filter’s
cutoff frequency is filter’s cutoff frequency is fC = 1 / 2πRiCi.
2.
Ci
This is the input coupling capacitor. It blocks DC voltage at the amplifier’s inverting input. Ci and Ri create a
highpass filter. The filter’s cutoff frequency is fC = 1 / 2πRiCi. Refer to the Application Information section,
SELECTING EXTERNAL COMPONENTS, for an explanation of determining Ci’s value.
3.
CC
This is the output coupling capacitor. Refer to the Application Information section, SELECTING EXTERNAL
COMPONENTS, for an explanation of determining CC’s value.
4.
CS
The supply bypass capacitor. Refer to the POWER SUPPLY BYPASSING section for information about
properly placing, and selecting the value of this capacitor.
5.
CB
This capacitor filters the half-supply voltage present on the BYPASS pin. Refer to the Application Information
section, SELECTING EXTERNAL COMPONENTS, for information about properly placing, and selecting the
value of this capacitor.
Typical Performance Characteristics
THD+N vs Output Power
VDD = 3V, BTL
AV = 8dB, RL = 8Ω
THD+N vs Output Power
VDD = 3V, BTL, f = 1kHz
AV = 8dB, RL = 8Ω
200634C6
200634C4
THD+N vs Output Power
VDD = 5V, BTL, f = 1kHz
AV = 8dB, RL = 8Ω
THD+N vs Output Power
VDD = 3V, Stereo
AV = 1.9dB, RL = 32Ω
200634C8
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200634C9
6
LM4914
Typical Performance Characteristics
(Continued)
THD+N vs Output Power
VDD = 5V, Stereo
AV = 1.9dB, RL = 32Ω
THD+N vs Output Power
VDD = 5V, BTL
AV = 8dB, RL = 8Ω
200634D1
200634D3
THD+N vs Frequency
VDD = 3V, Stereo
AV = 1.9dB, BW = 20Hz to 20kHz
THD+N vs Frequency
VDD = 3V, BTL, RL = 8Ω
AV = 8dB, PO = 250mW
200634C5
200634C7
THD+N vs Frequency
VDD = 5V, BTL
RL = 8Ω, PO = 1W
THD+N vs Frequency
VDD = 5V, Stereo, AV = 1.9dB
200634D0
200634D2
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LM4914
Typical Performance Characteristics
(Continued)
PSRR vs Frequency
VDD = 5V, BTL
AV = 8dB, CB = 0.47µF
Output Voltage Noise
vs Frequency
200634D5
200634D4
PSRR vs Frequency
VDD = 5V, Stereo
AV = 1.9dB, CB = 0.47µF
200634D6
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8
LM4914
Application Information
200634C3
FIGURE 2. Typical Audio Amplifier Application Circuit
Bridge mode amplifiers are different from single-ended amplifiers that drive loads connected between a single amplifier’s output and ground. At any given supply voltage, bridge
mode has a distinct advantage over the single-ended configuration: its differential output doubles the voltage swing
across the load. Theoretically, this produces four times the
output power when compared to a single-ended, capacitively
coupled amplifier under the same conditions. This increase
in attainable output power assumes that an amplifier is not
current limited and that the output signal is not clipped. To
ensure minimum output signal clipping when choosing an
amplifier’s closed-loop gain, refer to the Audio Power Amplifier Design section.
BRIDGE (BTL) OR SINGLE-ENDED (SE)
CONFIGURATION EXPLANATION
As shown in Figure 2, the LM4914 consists of one input
multiplexer (MUX) and two power amplifiers designed to
drive loads that have a minimum impedance of 4Ω. In mono
BTL mode, AMP1 and AMP2 drive a speaker connected
between their outputs. In stereo SE mode, AMP1 and AMP2
each drive a SE load such as stereo headphones.
In mono BTL mode, R1 works with one of AMP1’s internal
62.5kW feedback resistors to set this amplifier’s gain. AMP2
operates unity gain, set by two internal 20kΩ resistors. In
stereo SE modes, R2 and R3 work with AMP1’s and AMP2’s
internal 62.5kΩ feedback resistors to set each amplifier’s
gain. The LM4914 drives a BTL load, such as a speaker,
connected between AMP1’s and AMP2’s outputs. Two SE
loads can also be connected to the LM4914’s outputs, one
driven by AMP1 and the other driven by AMP2.
When the LM4914 operates in BTL mode, AMP1’s output
serves as AMP2’s input through AMP2’s input MUX. This
results in AMP1 and AMP2 producing signals identical in
magnitude, but 180˚ out of phase. Taking advantage of this
phase difference, a load placed between ROUT/M+ and
LOUT/M- is driven differentially (commonly referred to as
"bridge mode"). This results in a differential, or BTL, gain of
AV (BTL) = -2(AV(SE))
AV (SE) = -2(62.5kΩ) / Ri
AV (BTL) = -125kΩ / Ri
(1)
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful single-ended or bridged amplifier. Equation (2)
states the maximum power dissipation point for a singleended amplifier operating at a given supply voltage and
driving a specified output load.
(2)
PDMAX-SE = (VDD)2 / 2π2RL: Single-Ended
However, a direct consequence of the increased power delivered to the load by a bridge amplifier is higher internal
power dissipation for the same conditions. The LM4914 has
two operational amplifiers driving a mono bridge load. The
maximum internal power dissipation operating in the bridge
mode is twice that of a single-ended amplifier. From Equation (3), assuming a 5V power supply and an 8Ω load, the
maximum BTL-mode power dissipation is 158mW.
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LM4914
Application Information
PDMAX-MONOBTL = 2(VDD)2 / 2π2RL:
design. Failing to optimize thermal design may compromise
the LM4914’s high power performance and activate unwanted, though necessary, thermal shutdown protection.
(Continued)
Bridge Mode
(3)
The maximum power dissipation point given by Equation (3)
must not exceed the power dissipation given by Equation
(4):
(4)
PDMAX’ = (TJMAX - TA) / θJA
The MH package must have its DAP soldered to a copper
pad on the PCB. The DAP’s PCB copper pad is connected to
a large plane of continuous unbroken copper. This plane
forms a thermal mass, and heat sink, and radiation area.
Place the heat sink area on either outside plane in the case
of a two-sided PCB, or on an inner layer of a board with more
than two layers. Connecting to a ground plane is permissible. Connect the DAP copper pad to the inner layer or
backside copper heat sink area with 4(2x2) vias. The via
diameter should be 0.012in-0.013in with a 1.27mm pitch.
Ensure efficient thermal conductivity by plating-through and
solder-filling the vias.
Best thermal performance is achieved with the largest practical copper heatsink area. If the heatsink and amplifier share
the same PCB layer, a nominal 2.5in2 (min) area is necessary for 5V operation with an 8Ω load. The heatsink area
should be 5in2 (min) when placed on a layer different from
that used by the LM4914. The last two area recommendations apply for 25˚C ambient temperature. Increase the area
to compensate for ambient temperatures above 25˚C. In all
circumstances and conditions, the junction temperature
must be held below 150˚C to prevent activating the
LM4914’s thermal shutdown protection. The LM4914’s
power de-rating curve in the Typical Performance Characteristics shows the maximum power dissipation versus temperature. An example PCB layout for the LM4914’s exposedDAP package is shown in the Demonstration Board Layout
section.
The LM4914’s TJMAX = 150˚C. In the MH package, the
LM4914’s θJA is 46˚C/W. At any given ambient temperature
TA, use Equation (4) to find the maximum internal power
dissipation supported by the IC packaging. Rearranging
Equation (4) and substituting PDMAX for PDMAX ’ results in
Equation (5). This equation gives the maximum ambient
temperature that still allows maximum mono BTL power
dissipation without violating the LM4914’s maximum junction
temperature.
(5)
TA = TJMAX - PDMAX-MONOBTLθJA
For a typical application with a 5V power supply and an 8Ω
load, the maximum ambient temperature that allows maximum BTL power dissipation without exceeding the maximum
junction temperature is approximately 134˚C for the IBL
package.
(6)
TJMAX = PDMAX-MONOBTLθJA + TA
Equation (6) gives the maximum junction temperature TJMAX. If the result violates the LM4914’s 150˚C TJMAX, reduce
the maximum junction temperature by decreasing the power
supply voltage or increasing the load resistance. Further
allowance should be made for increased ambient temperatures.
The above examples assume that a device is a surface
mount part operating around the maximum power dissipation
point. Since internal power dissipation is a function of output
power, higher ambient temperatures are allowed as output
power or duty cycle decreases. If the result of Equation (3) is
greater than that of Equation (4), then decrease the supply
voltage, increase the load impedance, or reduce the ambient
temperature. If these measures are insufficient, a heat sink
can be added to reduce θJA. The heat sink can be created
using additional copper area around the package, with connections to the ground pin(s), supply pin and amplifier output
pins. External, solder attached SMT heatsinks such as the
Thermalloy 7106D can also improve power dissipation.
When adding a heat sink, the θJA is the sum of θJC, θCS, and
θSA. (θJC is the junction-to-case thermal impedance, θCS is
the case-to-sink thermal impedance, and θSA is the sink-toambient thermal impedance.) Refer to the Typical Performance Characteristics curves for power dissipation information at lower output power levels.
PCB LAYOUT AND SUPPLY REGULATION
CONSIDERATIONS FOR DRIVING 4Ω LOADS
Power dissipated by a load is a function of the voltage swing
across the load and the load’s impedance. As load impedance decreases, load dissipation becomes increasingly dependent on the interconnect (PCB trace and wire) resistance
between the amplifier output pins and the load’s connections. Residual trace resistance causes a voltage drop,
which results in power dissipated in the trace and not in the
load as desired. For example, 0.1Ω trace resistance reduces
the output power dissipated by an 8Ω load from 1W to 0.9W.
This problem of decreased load dissipation is exacerbated
as load impedance decreases. Therefore, to maintain the
highest load dissipation and widest output voltage swing,
PCB traces that connect the output pins to a load must be as
wide as possible.
Poor power supply regulation adversely affects maximum
output power. A poorly regulated supply’s output voltage
decreases with increasing load current. Reduced supply
voltage causes decreased headroom, output signal clipping,
and reduced output power. Even with tightly regulated supplies, trace resistance creates the same effects as poor
supply regulation. Therefore, make the power supply traces
as wide as possible to maintain full output voltage swing.
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATIONS
The LM4914’s exposed-DAP (die attach paddle) package
provides a low thermal resistance between the die and the
PCB to which the part is mounted and soldered. This low
thermal resistance is achieved by soldering the DAP to a
copper pad on the PCB. The copper pad’s dimensions
should match the DAP’s. The copper pad should then connect to a larger copper area. This area can be on the
component side, in an inner layer in a multi-layer board, or
on the board’s back side. This connection from the DAP, to
the DAP pad, and finally to a larger copper area allows rapid
heat transfer away from the die to the surrounding air. The
result is a low voltage audio power amplifier that produces
1W at = 1% THD+N with an 8Ω load. This high power is
achieved through careful consideration of necessary thermal
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POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection. Applications that employ a 5V regulator typically
use a 10µF in parallel with a 0.1µF filter capacitors to stabilize the regulator’s output, reduce noise on the supply line,
and improve the supply’s transient response. However, their
presence does not eliminate the need for a local 0.47µF
tantalum bypass capacitance connected between the
10
(SPST), a microprocessor, or a microcontroller. When using
a switch, connect a 100kΩ pull-up resistor between the
SHUTDOWN pin and VDD and the SPST switch between the
SHUTDOWN pin and GND. Select normal amplifier operation by opening the switch. Closing the switch applies GND
to the SHUTDOWN pin, 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 a microcontroller, use a
digital output to apply the active-state voltage to the SHUTDOWN pin.
(Continued)
LM4914’s supply pins and ground. Do not substitute a ceramic capacitor for the tantalum. Doing so may cause oscillation.
Keep the length of leads and traces that connect capacitors
between the LM4914’s power supply pin and ground as
short as possible. Connecting a 0.47µF capacitor, CB, between the BYPASS pin and ground improves the internal
bias voltage’s stability and improves the amplifier’s PSRR.
The PSRR improvements increase as the bypass pin capacitor value increases. Too large, however, increases
turn-on time and can compromise the amplifier’s click and
pop performance. The selection of bypass capacitor values,
especially CB, depends on desired PSRR requirements,
click and pop performance (as explained in the section,
Proper Selection of External Components), system cost, and
size constraints.
HEADPHONE (SINGLE-ENDED) AMPLIFIER
OPERATION
BTL/SE [Mono (BTL)/Stereo (SE)] Function
Applying a voltage greater than 0.9VDD to the LM4914’s
BTL/SE headphone control pin switches the amplifier’s operation from mono BTL to stereo SE. Applying a voltage less
than 0.55VDD to the LM4914’s BTL/SE headphone control
pin switches the amplifier’s operation from stereo SE to
mono BTL.
MICRO-POWER SHUTDOWN
The LM4914 features an active-low micro-power shutdown
mode. When active, the LM4914’s micro-power shutdown
feature turns off the amplifier’s bias circuitry, reducing the
supply current. The logic threshold is typically VDD/2. The
low 0.03µA typical shutdown current is achieved by applying
a voltage to the SHUTDOWN pin that is as near to GND as
possible. A voltage that is greater than GND may increase
the shutdown current.
There are a few methods to control the micro-power shutdown. These include using a single-pole, single-throw switch
Figure 3 shows how to control the LM4914’s headphone
function using four external resistors and a dual-switch stereo headphone jack. External resistors R4 - R6 provide the
control voltages that are applied through the upper headphone jack switch. R6 and R7 provide a DC return path for
the SE coupling capacitors.
200634A1
FIGURE 3. Headphone Operation and BTL - SE Mode Switching
connection between AMP4’s output and the BTL speaker,
muting it. The output coupling capacitors block the amplifier’s half supply DC voltage, protecting the headphones from
the VDD/2 DC output voltage.
Figure 3 also shows the suggested headphone jack electrical connections. The jack is designed to mate with a threewire plug. The plug’s tip and adjacent ring should carry the
left and right channel stereo signals, respectively. The sleeve
furthest from the tip should carry the ground return. The
Switchcraft 35RAPC4BH3 five-terminal headphone jack
With no headphones connected to the headphone jack, the
R5-R6 voltage divider sets the voltage applied to the BTL/SE
pin (pin 7) at approximately 50mV (comfortably below the
0.55VDD logic-low threshold). This 50mV tells the LM4914 to
select the signal applied to the MONO-IN input and places
the LM4914 in mono BTL operation. When stereo SE operation is desired, both headphone jack switches are opened
with a headphone plug. Opening the lower one allows R5 to
apply VDD to the BTL/SE pin. This switches the amplifier’s
inputs to the stereo signal. Opening the lower one breaks the
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LM4914
Application Information
LM4914
Application Information
headphone jack. For applications that require an SPDIF
interface in the stereo headphone jack, use a Foxconn
2F1138-TJ-TR.
(Continued)
easily satisfies the LM4914’s requirement for a dual switch
200634A2
FIGURE 4. Headphone Circuit
sary for the desired bandwidth helps minimize clicks and
pops. CB’s value should be in the range of 5 times to 7 times
the value of Ci. This ensures that output transients are
eliminated when power is first applied or the LM4914 resumes operation after shutdown.
SELECTING EXTERNAL COMPONENTS
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitors (C1 and C2 in Figure 2). A high
value capacitor can be expensive and may compromise
space efficiency in portable designs. In many cases, however, the speakers used in portable systems, whether internal or external, have little ability to reproduce signals below
150Hz. Applications using speakers with this limited frequency response reap little improvement by using large
input capacitor.
The LM4914’s advanced output transient suppression circuitry has eliminated the need to select the input capacitor’s
value in relation to the BYPASS capacitor’s value as was
necessary in some previous Boomer amplifiers. The value of
C1 and C2 are now strictly determined by the desired low
frequency response.
As shown in Figure 2, the input resistors (R1 and R2) and the
input capacitors (C1 and C2) produce a high pass filter cutoff
frequency that is found using Equation (7).
(7)
fC = 1 / 2πRiCi
As an example, when using a speaker with a low frequency
limit of 150Hz, and input resistances (R2 and R3) of 16.8kΩ,
using Equation (7), input capacitances (C1 and C2) are
0.063µF. The 0.1µF capacitors (C1 and C2) shown in Figure
2 allow the LM4914 to drive high efficiency, full range
speaker whose response extends below 30Hz.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4914 contains circuitry that eliminates turn-on and
shutdown transients ("clicks and pops") and transients that
could occur when switching between BTL speakers and
single-ended headphones. For this discussion, turn-on refers to either applying the power supply voltage or when the
micro-power shutdown mode is deactivated.
As the VDD/2 voltage present at the BYPASS pin ramps to its
final value, the LM4914’s internal amplifiers are configured
as unity gain buffers and are disconnected from the RO/MO+
and LO/MO- pins. 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 VDD/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 CB alters the device’s turn-on
time. There is a linear relationship between the size of CB
and the turn-on time. Here are some typical turn-on times for
various values of CB:
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consideration should be paid to value of CB, the capacitor connected to the BYPASS pin. Since CB determines how fast
the LM4914 settles to quiescent operation, its value is critical
when minimizing turn-on pops. The slower the LM4914’s
outputs ramp to their quiescent DC voltage (nominally VDD/
2), the smaller the turn-on pop. Choosing CB equal to 1.0µF
along with a small value of Ci (in the range of 0.1µF to
0.39µF), produces a click-less and pop-less shutdown function. As discussed above, choosing Ci no larger than neceswww.national.com
CB(µF)
12
TON (ms)
0.01
2
0.1
20
0.22
42
0.47
90
1.0
200
2.2
420
(RESUME HERE- All that is left is to discuss the BTL low
frequency phase shift.)
(Continued)
In order to eliminate "clicks and pops", all capacitors must be
discharged before turn-on. Rapidly switching VDD may not
allow the capacitors to fully discharge, which may cause
"clicks and pops".
(10)
Thus, a minimum gain of 2.83 allows the LM4914’s to reach
full output swing and maintain low noise and THD+N performance. For this example, let AV(BTL) = 3. The amplifier’s
overall gain is set using the input (Ri), the first stage internal
feedback resistor, and the second stage’s fixed gain of 1.25.
With the desired input impedance set at 20kW, the feedback
resistor is found using Equation (11).
(11)
Ri = -125kΩ / AV (BTL)
AUDIO POWER AMPLIFIER DESIGN
Audio Amplifier Design: Driving 1W into an 8Ω Load
Given:
Power Output
Load Impedance
1 Wrms
8Ω
Input Level
1 Vrms
Input Impedance
> 20kΩ
Bandwidth
The value of Ri is 44.2kΩ. The nominal output power is
1.13W.
The last step in this design example is setting the amplifier’s
-3dB frequency bandwidth. To achieve the desired ± 0.25dB
pass band magnitude variation limit, the low frequency response must extend to at least one-fifth the lower bandwidth
limit and the high frequency response must extend to at least
five times the upper bandwidth limit. The gain variation for
both response limits is 0.17dB, well within the ± 0.25dBdesired limit. The results are an
(12)
fL = 100Hz / 5 = 20Hz
and an
(13)
fL = 20kHz x 5 = 100kHz
As mentioned in the SELECTING EXTERNAL COMPONENTS section, Ri and Ci create a highpass filter that sets
the amplifier’s lower bandpass frequency limit. Find the coupling capacitor’s value using Equation (14).
(14)
Ci = 1 / 2πRifL
The result is
1 / 2π x 44.2kΩ x 20Hz = 0.180µF
(15)
Use a 180µF capacitor, the closest standard value.
The product of the desired high frequency cutoff (100kHz in
this example) and the differential gain AV(BTL), determines
the upper passband response limit. With AV(BTL) = 3 and fH
= 100kHz, the closed-loop gain bandwidth product (GBWP)
is 300kHz. This is less than the LM4914’s 3.5MHz GBWP.
With this margin, the amplifier can be used in designs that
require more differential gain while avoiding performance
restricting bandwidth limitations.
100Hz – 20kHz ± 0.25 dB
The design begins by specifying the minimum supply voltage
necessary to obtain the specified output power. One way to
find the minimum supply voltage is to use the Output Power
vs Supply Voltage curve in the Typical Performance Characteristics section. Another way, using Equation (8), is to calculate the peak output voltage necessary to achieve the
desired output power for a given load impedance. To account for the amplifier’s dropout voltage, two additional voltages, based on the Dropout Voltage vs Supply Voltage in the
Typical Performance Characteristics curves, must be added
to the result obtained by Equation (8). The result is Equation
(9).
(8)
(9)
VDD = VOUTPEAK + VODTOP + VODBOT
The Output Power vs. Supply Voltage graph for an 8Ω load
indicates a minimum supply voltage of 4.6V. The commonly
used 5V supply voltage easily meets this. The additional
voltage creates the benefit of headroom, allowing the
LM4914 to produce peak output power in excess of 1W
without clipping or other audible distortion. The choice of
supply voltage must also not create a situation that violates
of maximum power dissipation as explained above in the
Power Dissipation section.
After satisfying the LM4914’s power dissipation requirements, the minimum differential gain needed to achieve 1W
dissipation in an 8Ω load is found using Equation (10).
13
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LM4914
Application Information
LM4914
Application Information
(Continued)
LM4914MH DEMO BOARD ARTWORK
200634D7
200634D9
FIGURE 5. Top Overlay
FIGURE 7. Bottom Layer
200634D8
FIGURE 6. Top Layer
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14
LM4914 1W Monaural, 85mW Stereo Headphone Audio Amplifier
Physical Dimensions
inches (millimeters) unless otherwise noted
TSSOP EXP-DAP
Order Number LM4914MH
NS Package Number MXF10A
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