NSC LM4884MH 2.1w differential input, btl output stereo audio amplifier with rf suppression and shutdown Datasheet

LM4884
2.1W Differential Input, BTL Output Stereo Audio
Amplifier with RF Suppression and Shutdown
General Description
j BTL output power
The LM4884 features differential stereo inputs, BTL (bridgetied load) outputs, and four externally selectable fixed gains.
The LM4884 also features proprietary internal circuitry that
suppresses the coupling of RF signals into the chip. This is
important because certain types of RF signals (such as
GSM) can couple into audio amplifiers in such a way that
part of the signal is heard through the speaker. The RF
suppression circuitry in the LM4884 makes it well-suited for
notebook and portable applications in which strong RF signals generated by an antenna from a PC wireless card or a
cellular phone may couple audibly into the amplifier.
Operating on a single 5V supply, the LM4884 delivers 1.2W,
1.9W, or 2.1W (typ) of output power to an 8Ω, 4Ω, or 3Ω BTL
load (Note 1), respectively, with less than 1% THD+N. The
LM4884’s gain is selected using two digital inputs. The nominal gain values are 6dB, 10dB, 15.6dB, and 21.6dB.
Other features of the LM4884 are that it delivers high-quality
output power from a surface-mount package, requires few
external components, has an active-low micropower shutdown mode input, and has thermal shutdown protection.
Key Specifications
RL = 8Ω, VDD = 5.0V, and THD+N = 1%
j Micropower shutdown current
j PSRR ( @ 1kHz, VDD = 5V, (Fig.1))
1.2W (typ)
0.1µA (typ)
62dB (typ)
Features
n Improved RF immunity, by up to 28dB over previous
designs (such as the LM4874) in selected applications.
n Fully differential input and output
n Internal gain set: 6dB, 10dB, 15.6dB, and 21.6dB
n Improved "click and pop" suppression
n Thermal shutdown protection circuit
n Ultra low current micropower shutdown mode
n 3.0V to 5.5V operation
n Available in space-saving exposed-DAP TSSOP
package
Applications
n Notebook computers
n PDAs
n Portable electronic devices
j BTL output power
RL = 3Ω, VDD = 5.0V, and THD+N = 1%
2.1W (typ)
j BTL output power
RL = 4Ω, VDD = 5.0V, and THD+N = 1%
1.9W (typ)
Connection Diagram
Top View
20116302
Order Number LM4884MH
See NS Package Number MXA20A for Exposed-DAP TSSOP
Boomer ® is a registered trademark of National Semiconductor Corporation.
© 2005 National Semiconductor Corporation
DS201163
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LM4884 Boomer 2.1W Differential Input, BTL Output Stereo Audio Amplifier with RF Suppression
and Shutdown
April 2005
LM4884
Typical Application
20116301
FIGURE 1. Typical Audio Amplifier Application Circuit
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2
Infrared (15 sec.)
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.
6.0V
Thermal Resistance
-65˚C to + 150˚C
θJC (typ) MXA20A
2˚C/W
−0.3V to VDD + 0.3V
θJA (typ) MXA20A
41˚C/W (Note 7)
Supply Voltage
Storage Temperature
Input Voltage
220˚C
Power Dissipation (Note 4)
Internally Limited
ESD Susceptibility (Note 5)
2000V
ESD Susceptibility (Note 6)
200V
Junction Temperature
Operating Ratings
150˚C
Soldering Information
Small Outline Package
Vapor Phase (60 sec.)
Temperature Range
TMIN ≤ TA ≤TMAX
−40˚C ≤ TA ≤ 85˚C
Supply Voltage
3.0 V ≤ VDD ≤ 5.5V
215˚C
Electrical Characteristics for LM4884 (Notes 2, 8)
The following specifications applies to the LM4884 when used in the circuit shown in Figure 1 and operating with VDD = 5V and
AV = 6dB, unless otherwise specified. Limits apply for TA = 25˚C.
LM4884
Symbol
Parameter
Conditions
Typical
(Note 8)
Limit
(Notes 9,
10)
Units
(Limits)
3.0
5.5
V (min)
V (max)
mA (max)
VDD
Supply Voltage
IDD
Quiescent Power Supply Current
VIN = 0V, IO = 0A, RL = ∞
9.0
12.0
ISD
Shutdown Current
Vshutdown = GND
0.1
1.0
µA (max)
VOS
Output Offset Voltage
7
50
mV (max)
PSRR
Output Supply Rejection Ratio
VDD = 5V, VRIPPLE = 200mVP-P
sinewave, CBYPASS = 0.47µF,
RL = 8Ω
PO
Output Power (Note 11)
THD+N
Total Harmonic Distortion + Noise
62
dB
THD+N = 1% (max), f = 1kHz (Note12)
RL = 3Ω
RL = 4Ω
RL = 8Ω
2.1
1.9
1.2
W
W
W (min)
THD+N = 10% (max), f = 1kHz
(Note12)
RL = 3Ω
RL = 4Ω
RL = 8Ω
2.6
2.3
1.5
W
W
W
20Hz ≤ f ≤ 20kHz
RL = 4Ω, PO = 2W
RL = 8Ω, PO = 1W
0.3
0.3
%
%
97
dB
S/N
Signal-to-Noise Ratio
f = 1kHz, CBYPASS = 0.47µF,
PO = 1.1W, RL = 8Ω
RIN
Input Resistance
Pins 5, 7, 9, and 17
3
1.0
See Table 1.
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LM4884
Absolute Maximum Ratings (Notes 2, 3)
LM4884
Electrical Characteristics for LM4884 (Notes 2, 8)
(Continued)
Note 1: An LM4884MH that has been properly mounted to a circuit board with a copper heatsink area of at least 2in2 will deliver 1.9W into 4Ω or 2.1W into 3Ω.
Note 2: All voltages are measured with respect to the GND pin unless other wise 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 that
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 LM4884, see power derating
currents for more information.
Note 5: Human body model, 100 pF discharged through a 1.5 kΩ resistor.
Note 6: Machine Model, 220pF-240pF discharged through all pins.
Note 7: The given θJA is for an LM4884 packaged in an MXA20A with the exposed-DAP soldered to an exposed 2in2 area of 1oz printed circuit board copper. When
driving 4Ω loads from a 5V supply, the LM4884MH must be mounted to the circuit board and its exposed-DAP soldered to an exposed 2in2 area of 1oz PCB 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: When driving 3Ω or 4Ω loads and operating on a 5V supply, the LM4884MH must be mounted to a circuit board that has a minimum of 2in2 of exposed,
uninterrupted copper area connected to the MH package’s exposed DAP.
External Components Description
See Figure 1.
Components
Functional Description
1.
Ci
The input coupling capacitor blocks DC voltage at the amplifier’s inverting input terminals. Ci, along with the
LM4884’s variable input resistance R1 (See Table 1), creates a highpass filter with fC = 1/(2πRiCi). Both
inverting and noninverting inputs require a Ci. Refer to the Application Information section, SELECTING
EXTERNAL COMPONENTS, for an explanation of determining the value of Ci.
2.
CS
The supply bypass capacitor. Refer to the POWER SUPPLY BYPASSING section for information about
properly placing, and selecting the value of, this capacitor.
3.
CB
The capacitor, CB, filters the half-supply voltage present on the BYPASS pin. Refer to the Application
Information section, SELECTING EXTERNAL COMPONENTS, for information concerning proper placement
and selecting CB’s value.
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LM4884
Typical Performance Characteristics
THD vs Frequency
THD vs Frequency
20116398
20116397
VDD = 5V, RL = 4Ω, POUT = 1000mW,
at (from top to bottom at 1kHz):
AV = 21.6dB, AV = 15.6dB,
AV = 10dB, AV = 6dB
VDD = 5V, RL = 8Ω, POUT = 400mW,
at (from top to bottom at 1kHz):
AV = 21.6dB, AV = 15.6dB,
AV = 10dB, AV = 6dB
THD vs Frequency
THD vs Output Power
20116399
20116343
VDD = 5V, RL = 8Ω, POUT = 400mW,
at (from top to bottom at 1kHz):
AV = 21.6dB, AV = 15.6dB,
AV = 10dB, AV = 6dB
VDD = 5V, RL = 3Ω, fIN = 20Hz,
at (from top to bottom at 50mW):
AV = 21.6dB, AV = 15.6dB,
AV = 10dB, AV = 6dB
5
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LM4884
Typical Performance Characteristics
(Continued)
THD vs Output Power
THD vs Output Power
20116344
20116345
VDD = 5V, RL = 3Ω, fIN = 1kHz,
at (from top to bottom at 50mW):
AV = 21.6dB, AV = 15.6dB,
AV = 10dB, AV = 6dB
VDD = 5V, RL = 3Ω, fIN = 20kHz,
at (from top to bottom at 50mW):
AV = 21.6dB, AV = 15.6dB,
AV = 10dB, AV = 6dB
THD vs Output Power
THD vs Output Power
201163A0
201163A1
VDD = 5V, RL = 4Ω, fIN = 20Hz,
at (from top to bottom at 100mW):
AV = 21.6dB, AV = 15.6dB,
AV = 6dB, AV = 10dB
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VDD = 5V, RL = 4Ω, fIN = 1kHz,
at (from top to bottom at 200mW):
AV = 21.6dB, AV = 15.6dB,
AV = 10dB, AV = 6dB
6
LM4884
Typical Performance Characteristics
(Continued)
THD vs Output Power
THD vs Output Power
201163A2
201163A3
VDD = 5V, RL = 4Ω, fIN = 20kHz,
at (from top to bottom at 200mW):
AV = 21.6dB, AV = 15.6dB,
AV = 10dB, AV = 6dB
VDD = 5V, RL = 8Ω, fIN = 20Hz,
at (from top to bottom at 200mW):
AV = 21.6dB, AV = 15.6dB,
AV = 10dB, AV = 6dB
THD vs Output Power
THD vs Output Power
201163A4
201163A5
VDD = 5V, RL = 8Ω, fIN = 1kHz,
at (from top to bottom at 200mW):
AV = 21.6dB, AV = 15.6dB,
AV = 10dB, AV = 6dB
VDD = 5V, RL = 8Ω, fIN = 20kHz,
at (from top to bottom at 200mW):
AV = 21.6dB, AV = 15.6dB,
AV = 10dB, AV = 6dB
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LM4884
Typical Performance Characteristics
(Continued)
Output Power vs Supply Voltage
Output Power vs Supply Voltage
201163C3
201163C4
RL = 4Ω, fIN = 1kHz,
at (from top to bottom at 4V):
THD+N = 10%, THD+N = 1%
RL = 8Ω, fIN = 1kHz,
at (from top to bottom at 4V):
THD+N = 10%, THD+N = 1%
PSRR vs Frequency
PSRR vs Frequency
201163A8
VRIPPLE
201163A9
VDD = 5V, RL = 4Ω, RSOURCE = 10Ω
= 200mVP-P, at (from top to bottom at 1kHz):
AV = 21.6dB, AV = 15.6dB,
AV = 10dB, AV = 6dB
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VRIPPLE
8
VDD = 5V, RL = 8Ω, RSOURCE = 10Ω
= 200mVP-P, at (from top to bottom at 1kHz):
AV = 21.6dB, AV = 15.6dB,
AV = 10dB, AV = 6dB
LM4884
Typical Performance Characteristics
(Continued)
THD vs Frequency
THD vs Frequency
201163B0
201163B1
VDD = 3V, RL = 4Ω, POUT = 150mW,
at (from top to bottom at 1kHz):
AV = 21.6dB, AV = 15.6dB,
AV = 10dB, AV = 6dB
VDD = 3V, RL = 8Ω, POUT = 150mW,
at (from top to bottom at 1kHz):
AV = 21.6dB, AV = 15.6dB,
AV = 10dB, AV = 6dB
THD vs Output Power
THD vs Output Power
201163B3
201163B2
VDD = 3V, RL = 4Ω, fIN = 1kHz,
at (from top to bottom at 200mW):
AV = 21.6dB, AV = 6dB,
AV = 15.6dB, AV = 10dB
VDD = 3V, RL = 4Ω, fIN = 20Hz,
at (from top to bottom at 100mW):
AV = 21.6dB, AV = 15.6dB,
AV = 6dB, AV = 10dB
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LM4884
Typical Performance Characteristics
(Continued)
THD vs Output Power
THD vs Output Power
201163B4
201163B5
VDD = 3V, RL = 4Ω, fIN = 20kHz,
at (from top to bottom at 200mW):
AV = 21.6dB, AV = 15.6dB,
AV = 10dB, AV = 6dB
VDD = 3V, RL = 8Ω, fIN = 20Hz,
at (from top to bottom at 100mW):
AV = 21.6dB, AV = 6dB,
AV = 15.6dB, AV = 10dB
THD vs Output Power
THD vs Output Power
201163B6
201163B7
VDD = 3V, RL = 8Ω, fIN = 1kHz,
at (from top to bottom at 200mW):
AV = 21.6dB, AV = 15.6dB,
AV = 6dB, AV = 10dB
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VDD = 3V, RL = 8Ω, fIN = 20kHz,
at (from top to bottom at 200mW):
AV = 21.6dB, AV = 15.6dB,
AV = 10dB, AV = 6dB
10
(Continued)
PSRR vs Frequency
PSRR vs Frequency
201163B8
VRIPPLE
LM4884
Typical Performance Characteristics
201163B9
VDD = 3V, RL = 4Ω, RSOURCE = 10Ω,
= 200mVP-P, at (from top to bottom at 1kHz):
AV = 21.6dB, AV = 15.6dB,
AV = 10dB, AV = 6dB
VRIPPLE
Output Power vs
Load Resistance
VDD = 3V, RL = 8Ω, RSOURCE = 10Ω,
= 200mVP-P, at (from top to bottom at 1kHz):
AV = 21.6dB, AV = 15.6dB,
AV = 10dB, AV = 6dB
Channel-to-Channel gain Mismatch
vs Power Supply Voltage
201163C0
fIN = 1kHz, at (from top to bottom at 20Ω):
VDD = 5V, THD = 10%; VDD = 5V, THD = 1%;
VDD = 3V, THD = 10%; VDD = 3V, THD = 1%
201163C1
RL = 4Ω, fIN = 1kHz,
at (from top to bottom at 4V):
AV = 21.6dB, AV = 15.6dB,
AV = 10dB, AV = 6dB
11
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LM4884
Typical Performance Characteristics
(Continued)
Channel-to-Channel gain Mismatch
vs Power Supply Voltage
Dropout Voltage
vs Power Supply Voltage
201163C5
RL = 8Ω, fIN = 1kHz, both channels driven and loaded
at (from top to bottom at 4V):
positive signal swing, negative signal swing
201163C2
RL = 8Ω, fIN = 1kHz,
at (from top to bottom at 4V):
AV = 21.6dB, AV = 15.6dB,
AV = 10dB, AV = 6dB
Dropout Voltage
vs Power Supply Voltage
Amplifier Power Dissipation
vs Amplifier Load Dissipation
201163C6
201163C7
RL = 4Ω, fIN = 1kHz, both channels driven and loaded
at (from top to bottom at 4V):
positive signal swing, negative signal swing
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VDD = 5V, fIN = 1kHz, at (from top to bottom at 1W):
RL = 4Ω, RL = 8Ω, single channel driven and loaded
12
LM4884
Typical Performance Characteristics
(Continued)
Amplifier Power Dissipation
vs Amplifier Load Dissipation
Cross Talk vs Frequency
201163C8
201163C9
VDD = 3V, fIN = 1kHz, at (from top to bottom at 0.3W):
RL = 4Ω, RL = 8Ω, single channel driven and loaded
VDD = 5V, RL = 8Ω, AV = 6dB,
A = Left channel driven, right channel measured;
B = Right channel driven, left channel measured
Cross Talk vs Frequency
Cross Talk vs Frequency
201163D0
201163D1
VDD = 5V, RL = 8Ω, AV = 10dB,
A = Left channel driven, right channel measured;
B = Right channel driven, left channel measured
VDD = 5V, RL = 8Ω, AV = 15.6dB,
A = Left channel driven, right channel measured;
B = Right channel driven, left channel measured
13
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LM4884
Typical Performance Characteristics
(Continued)
Cross Talk vs Frequency
Cross Talk vs Frequency
201163D2
201163D3
VDD = 5V, RL = 8Ω, AV = 21.6dB,
A = Left channel driven, right channel measured;
B = Right channel driven, left channel measured
VDD = 3V, RL = 8Ω, AV = 6dB,
A = Left channel driven, right channel measured;
B = Right channel driven, left channel measured
Cross Talk vs Frequency
Cross Talk vs Frequency
201163D4
201163D5
VDD = 3V, RL = 8Ω, AV = 10dB,
A = Left channel driven, right channel measured;
B = Right channel driven, left channel measured
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VDD = 3V, RL = 8Ω, AV = 15.6dB,
A = Left channel driven, right channel measured;
B = Right channel driven, left channel measured
14
LM4884
Typical Performance Characteristics
(Continued)
Cross Talk vs Frequency
201163D6
VDD = 3V, RL = 8Ω, AV = 21.6dB,
A = Left channel driven, right channel measured;
B = Right channel driven, left channel measured
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, each of the LM4884’s stereo channels
consists of two operational amplifiers. The LM4884 can be
used to drive a speaker connected between the two outputs
of each channel’s amplifiers.
Figure 1 shows that the output of Amp1 serves as the input
to Amp2, which 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
OUT+ and OUT- and driven differentially (commonly referred
to as "bridge mode"). This results in a differential gain of
Application Information
OPTIMIZING RF IMMUNITY
The internal circuitry of the LM4884 suppresses the amount
of RF signal that is coupled into the chip. However, certain
external factors, such as output trace length, output trace
orientation, distance between the chip and the antenna,
antenna strength, speaker type, and type of RF signal, may
affect the RF immunity of the LM4884. In general, the RF
immunity of the LM4884 is application specific. Nevertheless, optimal RF immunity can be achieved by using short
output traces and increasing the distance between the
LM4884 and the antenna.
AVD = 2(RF/RI)
PCB LAYOUT AND SUPPLY REGULATION
CONSIDERATIONS FOR DRIVING 3W AND 4W 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 a 4Ω load from 2.1W to 2.0W.
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 also 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, making the power supply
traces as wide as possible helps maintain full output voltage
swing.
(1)
Bridge mode is 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 a
distinct advantage over the single-ended configuration: its
differential output doubles the voltage swing across the load.
This results in four times the output power when compared
to a single-ended amplifier under the same conditions. This
increase in attainable output assumes that the amplifier is
not current limited or the output signal is not clipped. To
ensure minimum output signal clipping when selecting one
of the amplifier’s four closed-loop gains, refer to the Audio
Power Amplifier Design section.
Another advantage of the differential bridge output is no net
DC voltage across the load. This results from biasing OUT+
and OUT- at half-supply. This eliminates the coupling capacitor that single supply, single-ended amplifiers require. Eliminating an output coupling capacitor in a single-ended configuration forces a single supply amplifier’s half-supply bias
voltage across the load. The current flow created by the
half-supply bias voltage increases internal IC power dissipation and may permanently damage loads such as speakers.
15
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LM4884
Application Information
Table 1. Gain Settings and Input Resistance
(Continued)
GAIN 0 GAIN 1
POWER DISSIPATION
Power dissipation is a major concern when
successful bridged or single-ended amplifier.
states the maximum power dissipation point
ended amplifier operating at a given supply
driving a specified output load.
2
2
PDMAX = (VDD) /(2π RL)
designing a
Equation (2)
for a singlevoltage and
Single-Ended
0
0
6
90kΩ
0
1
10
70kΩ
1
0
15.6
45kΩ
1
1
21.6
25kΩ
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection. The capacitors connected to the bypass and power
supply pins should be placed as close to the LM4884 as
possible. The capacitor connected between the bypass pin
and ground improves the internal bias voltage’s stability,
producing improved PSRR. The improvements to PSRR
increase as the bypass pin capacitor value increases.
Typical applications employ a 5V regulator with 10µF and a
0.1µF filter capacitors that aid in supply stability. Their presence, however, does not eliminate the need for bypassing
the LM4884’s supply pins. The selection of bypass capacitor
values, especially CB, depends on desired PSRR requirements, click and pop performance (as explained in theSelecting External Components section), system cost, and
size constraints.
Bridge Mode
(3)
The LM4884 has four operational amplifiers in one package
and the maximum internal power dissipation is four times
that of a single-ended amplifier. From Equation (3), assuming a 5V power supply and an 8Ω load, the maximum power
dissipation point is 2W. The maximum power dissipation
point obtained from Equation (3) must not exceed the power
dissipation predicted by Equation (4):
PDMAX = (TJMAX − TA)/θJA
(4)
MICRO-POWER SHUTDOWN
The LM4884 features an active-low micro-power shutdown
mode. The voltage applied to the SHUTDOWN pin controls
the LM4884’s shutdown function. Activate micro-power shutdown by applying 0V to the SHUTDOWN pin. The logic
threshold is typically 0.4V for a logic low and 1.5V for a logic
high. When active, the LM4884’s micro-power shutdown
feature turns off the amplifier’s bias circuitry, disables the
internal VDD/2 generator, and forces the amplifier outputs
into a high impedance state. The result is greatly reduced
power supply current. The low 0.1µA 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
(SPST), a microprocessor, or a microcontroller. When using
a switch, connect a 100kΩ pull-down resistor between the
SHUTDOWN pin and GND and the SPST switch between
the SHUTDOWN pin and VDD. Select normal amplifier operation by closing the switch. Opening 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. Driving the SHUTDOWN pin with active circuitry eliminates the pull-down resistor.
For the exposed DAP TSSOP package, θJA= 41˚C/W.
TJAMAX = 150˚C for the LM4884. For a given ambient temperature TA, Equation (4) can be used to find the maximum
internal power dissipation supported by the IC packaging. If
the result of Equation (3) is greater than that of Equation (4),
decrease the supply voltage, increase the load impedance,
or reduce the ambient temperature. For a typical application
with a 5V power supply and an 8Ω load, the maximum
ambient temperature that does not violate the maximum
junction temperature is approximately 68˚C. This further assumes 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 decreases.
Refer to the Typical Performance Characteristics curves for
power dissipation information at lower output power levels.
BTL GAIN SELECTION
The LM4884 features four fixed, internally set, BTL voltage
gains: 6dB, 10dB, 15.6dB, and 21.6dB. Select one of the
four gains by applying a logic level signal to the GAIN0
(MSB) and GAIN1 (LSB) digital inputs.
The closed-loop gain of the first amplifier is adjustable, having four different gains, whereas two internal 20kΩ resistors
set the second amplifier’s gain at -1. Table 1 below, shows
the state of the two logic inputs required to select one of the
four gain values.
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Input Resistance
(Ri)
(2)
However, a direct consequence of the increased power delivered to the load by a bridge amplifier is an increase in the
internal power dissipation point for a bridge amplifier operating at the same given conditions.
PDMAX = 4 * (VDD)2/(2π2RL)
Selected Gain
(dB)
16
As an example when using a speaker with a low frequency
limit of 150Hz, CI, is 0.047µF. The 0.47µF CI shown in Figure
1 allows the LM4884 to drive high efficiency, full range
speaker whose response extends below 30Hz.
(Continued)
Table 2. Logic Level Truth Table for Shutdown
Operation
SHUTDOWN
OPERATIONAL
MODE
High
Full Power, stereo
BTL amplifiers
Low
Micro-power
Shutdown
Bypass Capacitor Value Selection
Besides optimizing the input capacitor value, careful consideration should be paid to value of CB, the capacitor connected between the BYPASS pin and ground. Since CB
determines how fast the LM4884 settles to its quiescent
operating state, its value is critical when minimizing turn-on
transients. The slower the LM4884’s outputs ramp to their
quiescent DC voltage (nominally 1⁄2 VDD), the smaller the
turn-on transient. Choosing CB equal to 0.47µF along with a
small value of Ci (in the range of 0.047µF to 0.47µF), produces a transient-free turn-on and shutdown function. As
discussed above, choosing Ci no larger than necessary for
the desired bandwidth helps minimize turn-on transients.
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4884’s performance requires properly selecting external components. Though the LM4884 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component values. The LM4884 is unity-gain stable, giving a designer
maximum design flexibility. The gain should be set to no
more than a given application requires. This allows the amplifier to achieve minimum THD+N and maximum signal-tonoise ratio. These parameters are compromised as the
closed-loop gain increases. However, low gain demands
input signals with greater voltage swings to achieve maximum output power. Fortunately, many signal sources such
as audio CODECs have outputs of 1VRMS (2.83VP-P).
Please refer to the Audio Power Amplifier Design section for
more information on selecting the proper gain.
OPTIMIZING OUTPUT TRANSIENT REDUCTION (CLICK
AND POP PERFORMANCE)
The LM4884 contains circuitry to minimize turn-on and shutdown transients or ’clicks and pop’. For this discussion,
turn-on refers to either applying the power supply voltage or
when the shutdown mode is deactivated. While the power
supply voltage is ramping to its final value, the LM4884’s
internal amplifiers are configured as unity gain buffers. An
internal current source changes the voltage of the BYPASS
pin in a controlled, linear manner. Ideally, the amplifier inputs
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 1/2 VDD. As soon as the
voltage on the BYPASS pin is stable, the device becomes
fully operational. Although the bypass pin current can not be
modified, changing the size of CB alters the device’s turn-on
time and the magnitude of output transients. Increasing the
value of CB reduces the magnitude of turn-on transients.
However, this presents a tradeoff: as the size of CB increases, the turn-on time increases. There is a linear relationships between the size of CB + 2(CI) and the turn-on
time. The table shows some typical turn-on times for various
values of CB:
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitors (CI, C2 and C3, C4) in Figure 1. 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 with
frequencies below 150Hz. Applications using speakers with
this limited frequency response reap little improvement by
using large input capacitor.
Besides effecting system cost and size, CI - C4 can also
affect on the LM4884’s turn-on and turn-off transient ("click
and pop") performance. When the supply voltage is first
applied, a transient may be created as the charge on the
input capacitor changes from zero to a quiescent state. The
magnitude of the transient is proportional to the value of, and
more importantly, the mismatch between, the capacitors
connected to a given pair of inverting and non-inverting
inputs. The better the match, the less the transient magnitude.
Higher value capacitors need more time to reach a quiescent
DC voltage (usually VDD/2) when charged with a fixed current. This fixed current is supplied through amplifiers input
pins. Thus, selecting an input capacitor value that is no
higher than necessary to meet the desired -3dB frequency
will reduce turn-on time and help ensure that transients are
minimized.
The LM4884’s nominal input resistance (Ri) is 25kΩ (20kΩ,
minimum) and the input capacitor, Ci, form high pass filter
with a -3dB low frequency limit defined by equation (5).
f-3dB = 1/2π(25kΩ)Ci
CB
Ton
Ci = 0.47µF
Ci = 0.33µF
0.01µF
110ms
80ms
0.1µF
120ms
90ms
0.22µF
140ms
100ms
0.47µF
170ms
140ms
1.0µF
240ms
210ms
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’.
AUDIO POWER AMPLIFIER DESIGN
Audio Amplifier Design: Driving 1W into an 8Ω Load
The following are the desired operational parameters:
(5)
Power Output:
Load Impedance:
17
1 WRMS
8Ω
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LM4884
Application Information
LM4884
Application Information
Input Level:
1 VRMS
Input Impedance:
Bandwidth:
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. This extended bandwith
produces a gain variation of -0.17dB at the bandwith’s limits,
well within the ± 0.25dB desired limit. The results are an
(Continued)
20 kΩ
100 Hz−20 kHz ± 0.25 dB
The design begins by specifying the minimum supply voltage
necessary to obtain the desired 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 (6), 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 (6). The result is
Equation (7).
fL = 100Hz/5 = 20Hz
(9)
fH = 20kHz x 5 = 100kHz
(10)
and an
As mentioned in the External Components section, the internal input resistor and Ci create a high pass filter that sets the
amplifier’s lower bandpass frequency limit. Find the coupling
capacitor’s value using Equation (11).
f-3dB = 1/2π(20kΩ)CI
(6)
VDD ≥ (VOUTPEAK+ (VODTOP + VODBOT))
(11)
The result is (using the minimum RIN resistor value to ensure
correct magnitude response at 20Hz)
(7)
1/(2π*20kΩ*20Hz) = 0.398µF
The Output Power vs Supply Voltage graph for an 8Ω load
indicates a minimum supply voltage of 4.6V. This is easily
met by the commonly used 5V supply voltage. The additional
voltage creates the benefit of headroom, allowing the
LM4884 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.
(12)
Use a 0.39µF capacitor, the closest standard value. The
product of the desired high frequency cutoff (100kHz in this
example) and the differential gain, AVD, determines the upper passband response limit. With AVD = 3.2 and fH =
100kHz, the closed-loop gain bandwidth product (GBWP) is
320kHz. This is less than the LM4884’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.
After satisfying the LM4884’s power dissipation requirements, the minimum differential gain is found using Equation
(8).
Recommended Printed Circuit
Board Layout
Figures 2 through 6 show the recommended four-layer PC
board layout that is optimized for the 20-pin MH-packaged
LM4884 and associated external components. This circuit is
designed for use with an external 5V supply and 3Ω (or
higher) speakers (or load resistors).
This circuit board is easy to use. Apply 5V and ground to the
board’s VDD and GND terminals, respectively. Connect
speakers (or load resistors) between the board’s -OUTA and
+OUTA and -OUTB and +OUTB pads. Apply balanced differential stereo input signals to the input pins labeled "-INA,"
"+INA," "-INB," and "+INB."
(8)
Thus, a minimum gain of 2.83 allows the LM4884’s to reach
full output swing and maintain low noise and THD+N performance. For this example, let AVD = 3. In the example design,
the gain will be set to 10dB (AVD = 3.2) by applying a logic
low to GAIN 0 and a logic high to GAIN 1.
The last step in this design example is setting the amplifier’s
-3dB frequency bandwidth. To achieve the desired ± 0.25dB
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18
LM4884
Recommended Printed Circuit Board Layout
(Continued)
20116304
FIGURE 2. Recommended MH PC Board Layout:
Component-Side Silkscreen
20116305
FIGURE 3. Recommended MH PC Board Layout:
Component-Side Layout
19
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LM4884
Recommended Printed Circuit Board Layout
(Continued)
20116306
FIGURE 4. Recommended MH PC Board Layout:
Upper Inner-Layer Layout
20116307
FIGURE 5. Recommended MH PC Board Layout:
Lower Inner-Layer Layout
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20
LM4884
Recommended Printed Circuit Board Layout
(Continued)
20116308
FIGURE 6. Recommended MH PC Board Layout:
Bottom-Side Layout
21
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LM4884 Boomer 2.1W Differential Input, BTL Output Stereo Audio Amplifier with RF Suppression
and Shutdown
Physical Dimensions
inches (millimeters) unless otherwise noted
Exposed-DAP TSSOP Package
Order Number LM4884MH
NS Package Number MXA20A for Exposed-DAP TSSOP
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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(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
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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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