LM4951A
Wide Voltage Range 1.8 Watt Audio Amplifier With Short
Circuit Protection
General Description
Features
The LM4951A is an audio power amplifier designed for applications with supply voltages ranging from 2.7V up to 9V.
The LM4951A is capable of delivering 1.8W continuous average power with less than 1% THD+N into a bridge connected 8Ω load when operating from a 7.5VDC power supply.
Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal amount of
external components. The LM4951A does not require bootstrap capacitors, or snubber circuits.
The LM4951A features a low-power consumption active-low
shutdown mode. Additionally, the LM4951A features an internal thermal shutdown protection mechanism and short
circuit protection.
The LM4951A contains advanced pop & click circuitry that
eliminates noises which would otherwise occur during turn-on
and turn-off transitions.
The LM4951A is unity-gain stable and can be configured by
external gain-setting resistors.
■ Pop & click circuitry eliminates noise during turn-on and
■
■
■
■
■
■
■
turn-off transitions
Wide supply voltage range: 2.7V to 9V
Low current, active-low shutdown mode
Low quiescent current
Thermal shutdown protection
Short circuit protection
Unity-gain stable
External gain configuration capability
Applications
■
■
■
■
■
Portable devices
Cell phones
Laptop computers
Computer speaker systems
MP3 player speakers
Key Specifications
■ Wide Voltage Range
2.7V to 9V
■ Quiescent Power Supply Current
(VDD = 7.5V)
2.5mA (typ)
■ Power Output BTL at 7.5V,
1.8W (typ)
1% THD
■ Shutdown Current
0.01µA (typ)
■ Fast Turn on Time
25ms (typ)
Boomer® is a registered trademark of National Semiconductor Corporation.
© 2008 National Semiconductor Corporation
300578
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LM4951A Wide Voltage Range 1.8 Watt Audio Amplifier With Short Circuit Protection
September 5, 2008
LM4951A
Typical Application
300578f4
FIGURE 1. Typical Bridge-Tied-Load (BTL) Audio Amplifier Application Circuit
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2
LM4951A
Connection Diagrams
SD Package
30057829
Top View
Order Number LM4951ASD
See NS Package Number SDC10A
SD Package Marking
30057831
Top View
U = Fab site code
Z = Assembly plant code
XY = Date code
TT = Die traceability
4951A = LM4951A
SD = Package code
Ordering Information
Order Number
Package
Package DWG #
MSL Level
Green Status
LM4951ASD
10 Lead LLP
SDC10A
1000 units in Tape and Reel
Transport Media
1
RoH and no Sb/Br
LM4951ASDX
10 Lead LLP
SDC10A
4500 units in Tape and Reel
1
RoH and no Sb/Br
3
Features
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LM4951A
TABLE 1. Pin Name and Function
Pin Number
Name
Function
Type
1
Bypass
½ supply reference voltage bypass output. See sections POWER
SUPPLY BYPASSING and SELECTING EXTERNAL COMPONENTS for
more information.
Analog Output
2
Shutdown
3
CCHG
4
NC
No connection to die. Pin can be connected to any potential.
No Connect
5
VIN
Single-ended signal input pin.
Analog Input
6
VO-
Inverting output of amplifier.
7
GND
Shutdown control active low signal. A logic low voltage will put the
LM4951A into Shutdown mode.
Input capacitor charge to decrease turn on time. See section Selecting A
Value for RC for more information.
Analog Output
Analog Output
Ground connection.
Ground
8
NC
No connection to die. Pin can be connected to any potential.
9
VDD
Power supply.
10
VO+
Non-Inverting output of amplifier.
Exposed DAP
NC
No connect. Pin must be electrically isolated (floating) or connected to
GND.
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Digital Input
No Connect
Power
4
Analog Output
No Connect
θJA (LLP) (Note 3)
Soldering Information
See AN-1187 'Leadless Leadframe
Packaging (LLP).'
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 Rating (Note 4)
ESD Rating (Note 5)
Junction Temperature (TJMAX)
9.5V
−65°C to +150°C
−0.3V to VDD + 0.3V
Internally limited
2000V
200V
150°C
Electrical Characteristics VDD = 7.5V
Operating Ratings
73°C/W
(Notes 1, 2)
Temperature Range
TMIN ≤ TA ≤ TMAX
−40°C ≤ T A ≤ +85°C
2.7V ≤ VDD ≤ 9V
Supply Voltage
(Notes 1, 2)
The following specifications apply for VDD = 7.5V, AV-BTL = 6dB, RL = 8Ω unless otherwise specified. Limits apply for TA = 25°C.
LM4951A
Symbol
Parameter
Conditions
Typical
(Note 6)
Limit
(Note 7)
4.5
Units
(Limits)
IDD
Quiescent Power Supply Current
VIN = 0V, IO = 0A, RL = 8Ω BTL
2.5
ISD
Shutdown Current
VSD = GND (Note 8)
0.01
5
µA (max)
VOS
Output Offset Voltage
5
30
mV (max)
VSDIH
Shutdown Voltage Input High
1.2
V (min)
VSDIL
Shutdown Voltage Input Low
0.4
V (max)
RPULLDOWN
Pull-down Resistor on SD pin
TWU
Wake-up Time
CB = 1.0µF
TSD
Shutdown time
CB = 1.0µF
TSD
Thermal Shutdown Temperature
PO
Output Power
THD+N
Total Harmonic Distortion + Noise
εOS
Output Noise
PSRR
Power Supply Rejection Ratio
mA (max)
75
45
kΩ (min)
25
35
ms (max)
10
ms (max)
170
150
190
°C (min)
°C (max)
THD = 1% (max); f = 1kHz
RL = 8Ω Mono BTL
1.8
1.5
W (min)
PO = 600mWRMS; f = 1kHz
AV-BTL = 6dB
0.07
0.5
% (max)
PO = 600mWRMS; f = 1kHz
AV-BTL = 26dB
0.35
%
10
µV
A-Weighted Filter, Ri = Rf = 20kΩ
Input Referred (Note 9)
VRIPPLE = 200mVp-p, f = 217Hz,
CB = 1.0μF, Input Referred
Electrical Characteristics VDD = 3.3V
66
56
dB (min)
(Notes 1, 2)
The following specifications apply for VDD = 3.3V, AV-BTL = 6dB, RL = 8Ω unless otherwise specified. Limits apply for TA = 25°C.
LM4951A
Symbol
Parameter
Conditions
Typical
(Note 6)
Limit
(Note 7)
Units
(Limits)
IDD
Quiescent Power Supply Current
VIN = 0V, IO = 0A, RL = 8Ω BTL
2.5
4.5
mA (max)
ISD
Shutdown Current
VSHUTDOWN = GND (Note 8)
0.01
2
µA (max)
VOS
Output Offset Voltage
VSDIH
Shutdown Voltage Input High
VSDIL
Shutdown Voltage Input Low
TWU
Wake-up Time
CB = 1.0µF
TSD
Shutdown time
CB = 1.0µF
3
5
30
mV (max)
1.2
V (min)
0.4
V (max)
25
ms
10
ms (max)
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LM4951A
Thermal Resistance
Absolute Maximum Ratings (Notes 1, 2)
LM4951A
LM4951A
Symbol
PO
THD+N
Parameter
Output Power
Total Harmonic Distortion + Noise
εOS
Output Noise
PSRR
Power Supply Rejection Ratio
Conditions
Units
(Limits)
Typical
(Note 6)
Limit
(Note 7)
THD = 1% (max); f = 1kHz
RL = 8Ω Mono BTL
280
230
mW (min)
PO = 100mWRMS = 1kHz
AV-BTL = 6dB
0.07
0.5
% (max)
PO = 100mWRMS; f = 1kHz
AV-BTL = 26dB
0.35
%
10
µV
A-Weighted Filter, Ri = Rf = 20kΩ
Input Referred, (Note 9)
VRIPPLE = 200mVp-p, f = 217Hz,
CB = 1μF, Input Referred
71
61
dB (min)
Note 1: “Absolute Maximum Ratings” indicate limits beyond which damage to the device may occur, including inoperability and degradation of device reliability
and/or performance. Functional operation of the device and/or non-degradation at the s or other conditions beyond those indicated in the Recommended Operating
Conditions is not implied. The Recommended Operating Conditionsindicate conditions at which the device is functional and the device should not be operated
beyond such conditions. All voltages are measured with respect to the ground pin, unless otherwise specified.
Note 2: The Electrical Characteristics tables list guaranteed specifications under the listed Recommended Operating Conditions except as otherwise modified
or specified by the Electrical Characteristics Conditions and/or Notes. Typical specifications are estimations only and are not guaranteed.
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 LM4951A typical application
(shown in Figure 1) with VDD = 7.5V, RL = 8Ω mono-BTL operation the max power dissipation is 1.42W. θJA = 73ºC/W.
Note 4: Human body model, applicable std. JESD22-A114C.
Note 5: Machine model, applicable std. JESD22-A115-A.
Note 6: Typical values represent most likely parametric norms at TA = +25ºC, and at the Recommended Operation Conditions at the time of product
characterization and are not guaranteed.
Note 7: Datasheet min/max specification limits are guaranteed by test or statistical analysis.
Note 8: Shutdown current is measured in a normal room environment. The Shutdown pin should be driven as close as possible to GND for minimum shutdown
current.
Note 9: Noise measurements are dependent on the absolute values of the closed loop gain setting resistors (input and feedback resistors).
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LM4951A
Typical Performance Characteristics
THD+N vs Frequency
VDD = 3.3V, PO = 100mW, AV = 6dB
THD+N vs Frequency
VDD = 3.3V, PO = 100mW, AV = 26dB
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THD+N vs Frequency
VDD = 5V, PO = 400mW, AV = 6dB
THD+N vs Frequency
VDD = 5V, PO = 400mW, AV = 26dB
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THD+N vs Frequency
VDD = 7.5V, PO = 600mW, AV = 6dB
THD+N vs Frequency
VDD = 7.5V, PO = 600mW, AV = 26dB
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LM4951A
THD+N vs Output Power
VDD = 3.3V, f = 1kHz, AV = 6dB
THD+N vs Output Power
VDD = 3.3V, f = 1kHz, AV = 26dB
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THD+N vs Output Power
VDD = 5V, f = 1kHz, AV = 6dB
THD+N vs Output Power
VDD = 5V, f = 1kHz, AV = 26dB
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THD+N vs Output Power
VDD = 7.5V, f = 1kHz, AV = 6dB
THD+N vs Output Power
VDD = 7.5V, f = 1kHz, AV = 26dB
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Power Supply Rejection vs Frequency
VDD = 3.3V, AV = 26dB, VRIPPLE = 200mVP-P
Input Terminated into 10Ω
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Power Supply Rejection vs Frequency
VDD = 5V, AV = 6dB, VRIPPLE = 200mVP-P
Input Terminated into 10Ω
Power Supply Rejection vs Frequency
VDD = 5V, AV = 26dB, VRIPPLE = 200mVP-P
Input Terminated into 10Ω
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Power Supply Rejection vs Frequency
VDD = 7.5V, AV = 6dB, VRIPPLE = 200mVP-P
Input Terminated into 10Ω
Power Supply Rejection vs Frequency
VDD = 7.5V, AV = 26dB, VRIPPLE = 200mVP-P
Input Terminated into 10Ω
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LM4951A
Power Supply Rejection vs Frequency
VDD = 3.3V, AV = 6dB, VRIPPLE = 200mVP-P
Input Terminated into 10Ω
LM4951A
Noise Floor
VDD = 3.3V, AV = 6dB, Ri = Rf = 20kΩ
BW < 80kHz, A-weighted
Noise Floor
VDD = 3V, AV = 26dB, Ri = 20kΩ, Rf = 200kΩ
BW < 80kHz, A-weighted
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Noise Floor
VDD = 5V, AV = 6dB, Ri = Rf = 20kΩ
BW < 80kHz, A-weighted
Noise Floor
VDD = 5V, AV = 26dB, Ri = 20kΩ, Rf = 200kΩ
BW < 80kHz, A-weighted
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Noise Floor
VDD = 7.5V, AV = 6dB, Ri = Rf = 20kΩ
BW < 80kHz, A-weighted
Noise Floor
VDD = 7.5V, AV = 26dB, Ri = 20kΩ, Rf = 200kΩ
BW < 80kHz, A-weighted
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LM4951A
Power Dissipation
vs Output Power
VDD = 3.3V, RL = 8Ω, f = 1kHz
Power Dissipation
vs Output Power
VDD = 7.5V, RL = 8Ω, f = 1kHz
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Supply Current
vs Supply Voltage
RL = 8Ω, VIN = 0V, Rsource = 50Ω
Clipping Voltage vs Supply Voltage
RL = 8Ω,
from top to bottom: Negative Voltage Swing; Positive
Voltage Swing
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Output Power vs Load Resistance
VDD = 3.3V, f = 1kHz
from top to bottom: THD+N = 10%, THD+N = 1%
Output Power vs Supply Voltage
RL = 8Ω,
from top to bottom: THD+N = 10%, THD+N = 1%
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LM4951A
Output Power vs Load Resistance
VDD = 7.5V, f = 1kHz
from top to bottom: THD+N = 10%, THD+N = 1%
Frequency Response vs Input Capacitor Size
RL = 8Ω
from top to bottom: Ci = 1.0µF, Ci = 0.39µF, Ci = 0.039µF
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BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4951A consists of two operational amplifiers that drive a speaker connected between their
outputs. The value of input and feedback resistors determine
the gain of each amplifier. External resistors Ri and Rf set the
closed-loop gain of AMPA, whereas two 20kΩ internal resistors set AMPB's gain to -1. Figure 1 shows that AMPA's output
serves as AMPB's input. This results in both amplifiers producing signals identical in magnitude, but 180° out of phase.
Taking advantage of this phase difference, a load is placed
between AMPA and AMPB and driven differentially (commonly
referred to as "bridge-tied load"). This results in a differential,
or BTL, gain of:
AVD = 2(Rf / Ri)
(V/V)
TA = TJMAX - PDMAX-MONOBTLθJA (°C)
For a typical application with a 7.5V power supply and a BTL
8Ω load, the maximum ambient temperature that allows maximum stereo power dissipation without exceeding the maximum junction temperature is 46°C for the SD package.
TJMAX = PDMAX-MONOBTLθJA + TA (°C)
(1)
The above examples assume that a device is 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 (2) is greater than that of Equation (3),
then decrease the supply voltage, increase the load
impedance, or reduce the ambient temperature. Further, ensure that speakers rated at a nominal 8Ω do not fall below
6Ω. 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 pins, supply pin and amplifier output pins. Refer
to the Typical Performance Characteristics curves for power dissipation information at lower output power levels.
Another advantage of the differential bridge output is no net
DC voltage across the load. This is accomplished by biasing
AMP1's and AMP2's outputs at half-supply. This eliminates
the coupling capacitor that single supply, single-ended amplifiers require. Eliminating an output coupling capacitor in a
typical single-ended configuration forces a single-supply
amplifier's half-supply bias voltage across the load. This increases internal IC power dissipation and may permanently
damage loads such as speakers.
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 voltage 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 1.0µF tantalum
bypass capacitance connected between the LM4951A'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
LM4951A's power supply pin and ground as short as possible.
Connecting a larger capacitor, CBYPASS, 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 CBYPASS, depends on desired PSRR requirements, click and pop performance, system cost, and size constraints.
POWER DISSIPATION
The LM4951A's dissipation when driving a BTL load is given
by Equation (2). For a 7.5V supply and a single 8Ω BTL load,
the dissipation is 1.42W.
(2)
The maximum power dissipation point given by Equation (2)
must not exceed the power dissipation given by Equation (3):
PDMAX = (TJMAX - TA) / θJA
(5)
Equation (5) gives the maximum junction temperature
TJMAX. If the result violates the LM4951A's maximum junction
temperature of 150°C, reduce the maximum junction temperature by reducing the power supply voltage or increasing the
load resistance. Further allowance should be made for increased ambient temperatures.
Bridge mode amplifiers are different from single-ended amplifiers that drive loads connected between a single amplifier's
output and ground. For a given supply voltage, bridge mode
has an 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 amplifier under the same conditions. This increase in attainable output power assumes that
the amplifier is not current limited and that the output signal
is not clipped. Under rare conditions, with unique combinations of high power supply voltage and high closed loop gain
settings, the LM4951A may exhibit low frequency oscillations.
PDMAX-MONOBTL = 4(VDD) 2 / 2π2RL (W)
(4)
(3)
The LM4951A's TJMAX = 150°C. In the SD package, the
LM4951A's θJA is 73°C/W when the metal tab is soldered to
a copper plane of at least 1in2. This plane can be split between
the top and bottom layers of a two-sided PCB. Connect the
two layers together under the tab with an array of vias. At any
given ambient temperature TA, use Equation (3) to find the
maximum internal power dissipation supported by the IC
packaging. Rearranging Equation (3) and substituting
PDMAX for PDMAX' results in Equation (4). This equation gives
the maximum ambient temperature that still allows maximum
MICRO-POWER SHUTDOWN
The LM4951A features an active-low micro-power shutdown
mode. When active, the LM4951A's micro-power shutdown
feature turns off the amplifier's bias circuitry, reducing the
supply current. The low 0.01µA typical shutdown current is
achieved by applying a voltage to the SHUTDOWN pin that
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LM4951A
stereo power dissipation without violating the LM4951A's
maximum junction temperature.
Application Information
LM4951A
is as near to GND as possible. A voltage that is greater than
GND may increase the shutdown current.
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 LM4951A'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 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, there is a delay to prevent undesirable output transients (“click and pops”). After this delay,
the device becomes fully functional.
SELECTING EXTERNAL COMPONENTS
Input Capacitor Value Selection
Two quantities determine the value of the input coupling capacitor: the lowest audio frequency that requires amplification
and desired output transient suppression.
As shown in Figure 1, the input resistor (Ri) and the input capacitor (Ci) create a high-pass filter. The cutoff frequency can
be found using Equation (6).
fc = 1/2πRiCi
(Hz)
(6)
THERMAL SHUTDOWN AND SHORT CIRCUIT
PROTECTION
The LM4951A has thermal shutdown and short circuit protection to fully protect the device. The thermal shutdown
circuit is activated when the die temperature exceeds a safe
temperature. The short circuit protection circuitry senses the
output current. When the output current exceeds the threshold under a short condition, a short will be detected and the
output deactivated until the short condition is removed. If the
output current is lower than the threshold then a short will not
be detected and the outputs will not be deactivated. Under
such conditions the die temperature will increase and, if the
condition persist to raise the die temperature to the thermal
shutdown threshold, initiate a thermal shutdown response.
Once the die cools the outputs will become active.
As an example when using a speaker with a low frequency
limit of 50Hz, Ci, using Equation (6) is 0.159µF with Ri set to
20kΩ. The values for Ci and Ri shown in Figure 1 allow the
LM4951A to drive a high efficiency, full range speaker whose
response extends down to 20Hz.
Selecting Value A For RC
The LM4951A is designed for very fast turn on time. The
CCHG pin allows the input capacitor to charge quickly to improve click/pop performance. RC protects the CCHG pin from
any over/under voltage conditions caused by excessive input
signal or an active input signal when the device is in shutdown. The recommended value for RC is 1kΩ. If the input
signal is less than VDD+0.3V and greater than -0.3V, and if the
input signal is disabled when in shutdown mode, RC may be
shorted out.
RECOMMENDED PRINTED CIRCUIT BOARD LAYOUT
Figures 2–4 show the recommended two-layer PC board layout that is optimized for the SD10A. This circuit is designed
for use with an external 7.5V supply 8Ω (min) speakers.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4951A contains circuitry that eliminates turn-on and
shutdown transients ("clicks and pops"). For this discussion,
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14
LM4951A
Demonstration Board Circuit
30057830
FIGURE 2. Demo Board Circuit
15
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LM4951A
Demonstration Board Layout
30057832
FIGURE 3. Top Silkscreen
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FIGURE 4. Top Layer
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FIGURE 5. Bottom Layer
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LM4951A
Bill Of Materials
TABLE 2. Bill Of Materials
Designator
Value
Tolerance
RIN1
20kΩ
1%
1/8W, 0805 Resistor
Part Description
R1
200kΩ
1%
1/8W, 0805 Resistor
RPULLUP
100kΩ
1%
1/8W, 0805 Resistor
R2
1kΩ
1%
1/8W, 0805 Resistor
R4, R5
0Ω
1%
1/8W, 0805 Resistor
CIN1
0.39μF
10%
Ceramic Capacitor, 25V, Size 1206
CSUPPLY
4.7μF
10%
16V Tantalum Capacitor, Size A
CBYPASS
1μF
10%
16V Tantalum Capacitor, Size A
C1
Not Used
0.100” 1x2 header, vertical mount
U1
Comments
LM4951A, Mono, 1.8W, Audio Amplifier
17
Input, Output, Vdd/GND
Shutdown
SDC10A package
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LM4951A
Revision History
Rev
Date
1.0
08/13/08
Initial release.
1.01
09/05/08
Text edits.
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Description
18
LM4951A
Physical Dimensions inches (millimeters) unless otherwise noted
Order Number LM4951ASD
NS Package Number SDC10A
19
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LM4951A Wide Voltage Range 1.8 Watt Audio Amplifier With Short Circuit Protection
Notes
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