LM4674A
LM4674A Filterless 2.5W Stereo Class D Audio Power Amplifier
Literature Number: SNAS366
LM4674A
Filterless 2.5W Stereo Class D Audio Power Amplifier
General Description
The LM4674A is a single supply, high efficiency, 2.5W/
channel, filterless switching audio amplifier. A low noise
PWM architecture eliminates the output filter, reducing ex-
ternal component count, board area consumption, system
cost, and simplifying design.
The LM4674A is designed to meet the demands of mobile
phones and other portable communication devices. Operat-
ing from a single 5V supply, the device is capable of deliv-
ering 2.5W/channel of continuous output power to a 4Ωload
with less than 10% THD+N. Flexible power supply require-
ments allow operation from 2.4V to 5.5V.
The LM4674A features high efficiency compared to conven-
tional Class AB amplifiers. When driving an 8Ωspeaker from
a 3.6V supply, the device features 85% efficiency at P
O
=
500mW. Four gain options are pin selectable through the
GAIN0 and GAIN1 pins.
Output short circuit protection prevents the device from be-
ing damaged during fault conditions. Superior click and pop
suppression eliminates audible transients on power-up/down
and during shutdown. Independent left/right shutdown con-
trols maximizes power savings in mixed mono/stereo appli-
cations.
Key Specifications
jEfficiency at 3.6V, 100mW into 8Ω80% (typ)
jEfficiency at 3.6V, 500mW into 8Ω85% (typ)
jEfficiency at 5V, 1W into 8Ω85% (typ)
jQuiescent Power Supply Current
at 3.6V supply 4mA
jPower Output at V
DD
= 5V,
R
L
=4Ω, THD ≤10% 2.5W (typ)
jPower Output at V
DD
= 5V,
R
L
=8Ω, THD ≤10% 1.5W (typ)
jShutdown current 0.1µA (typ)
Features
nOutput Short Circuit Protection
nStereo Class D operation
nNo output filter required
nLogic selectable gain
nIndependent shutdown control
nMinimum external components
nClick and Pop suppression
nMicro-power shutdown
nAvailable in space-saving 2mm x 2mm x 0.6mm micro
SMD package
Applications
nMobile phones
nPDAs
nLaptops
Boomer®is a registered trademark of National Semiconductor Corporation.
September 2006
LM4674A Filterless 2.5W Stereo Class D Audio Power Amplifier
© 2006 National Semiconductor Corporation DS201961 www.national.com
Typical Application
20196103
FIGURE 1. Typical Audio Amplifier Application Circuit
LM4674A
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Connection Diagrams
TL Package (2mmx2mmx0.6mm)
20196101
Top View
Order Number LM4674ATL
See NS Package Number TL1611A
LM4674ATL Markings
20196125
Top View
XY = 2 digit datecode
TT = Die traceability
G = Boomer Family
I2 = LM4674ATL
LM4674A
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Absolute Maximum Ratings (Notes 1, 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage (Note 1) 6.0V
Storage Temperature −65˚C to +150˚C
Input Voltage –0.3V to V
DD
+0.3V
Power Dissipation (Note 3) Internally Limited
ESD Susceptibility, all other pins (Note 4) 2000V
ESD Susceptibility (Note 5) 200V
Junction Temperature (T
JMAX
) 150˚C
Thermal Resistance
θ
JA
45.7˚C/W
Operating Ratings (Note 1) (Note 2)
Temperature Range
T
MIN
≤T
A
≤T
MAX
−40˚C ≤T
A
≤85˚C
Supply Voltage 2.4V ≤V
DD
≤5.5V
Electrical Characteristics V
DD
= 3.6V (Notes 1, 2)
The following specifications apply for A
V
= 6dB, R
L
=15µH+8Ω+ 15µH, f = 1kHz unless otherwise specified. Limits apply
for T
A
= 25˚C.
Symbol Parameter Conditions
LM4674A Units
(Limits)
Typical Limit
(Note 6) (Note 7)
V
OS
Differential Output Offset Voltage V
IN
=0,V
DD
= 2.4V to 5.0V 5 mV
I
DD
Quiescent Power Supply Current
V
IN
=0,R
L
=∞,
Both channels active, V
DD
= 3.6V 46mA
V
IN
=0,R
L
=∞,
Both channels active, V
DD
=5V 5 7.5 mA
I
SD
Shutdown Current V
SD1
=V
SD2
= GND 0.03 1 µA
V
SDIH
Shutdown Voltage Input High 1.4 V (min)
V
SDIL
Shutdown Voltage Input Low 0.4 V (max)
T
WU
Wake Up Time V
SHUTDOWN
= 0.4V 4.2 ms
A
V
Gain
GAIN0, GAIN1 = GND 6 6 ±0.5 dB
GAIN0 = V
DD
, GAIN1 = GND 12 12 ±0.5 dB
GAIN0 = GND, GAIN1 = V
DD
18 18 ±0.5 dB
GAIN0, GAIN1 = V
DD
24 24 ±0.5 dB
R
IN
Input Resistance
A
V
= 6dB 28 kΩ
A
V
= 12dB 18.75 kΩ
A
V
= 18dB 11.25 kΩ
A
V
= 24dB 6.25 kΩ
LM4674A
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Electrical Characteristics V
DD
= 3.6V (Notes 1, 2)
The following specifications apply for A
V
= 6dB, R
L
=15µH+8Ω+ 15µH, f = 1kHz unless otherwise specified. Limits apply for
TA= 25˚C. (Continued)
Symbol Parameter Conditions
LM4674A Units
(Limits)
Typical Limit
(Note 6) (Note 7)
P
O
Output Power
R
L
=15µH+4Ω+ 15µH, THD = 10%
f = 1kHz, 22kHz BW
V
DD
= 5V 2.5 W
V
DD
= 3.6V 1.2 W
V
DD
= 2.5V 0.530 W
R
L
=15µH+8Ω+ 15µH, THD = 10%
f = 1kHz, 22kHz BW
V
DD
= 5V 1.5 W
V
DD
= 3.6V 0.78 0.6 W
V
DD
= 2.5V 0.350 W
R
L
=15µH+4Ω+ 15µH, THD = 1%
f = 1kHz, 22kHz BW
V
DD
= 5V 1.9 W
V
DD
= 3.6V 1 W
V
DD
= 2.5V 0.430 W
R
L
=15µH+8Ω+ 15µH, THD = 1%
f = 1kHz, 22kHz BW
V
DD
= 5V 1.25 W
V
DD
= 3.6V 0.63 W
V
DD
= 2.5V 0.285 W
THD+N Total Harmonic Distortion P
O
= 500mW, f = 1kHz, RL = 8Ω0.07 %
P
O
= 300mW, f = 1kHz, RL = 8Ω0.05 %
PSRR Power Supply Rejection Ratio
V
RIPPLE
= 200mV
P-P
Sine,
f
Ripple
= 217Hz, Inputs AC GND, C
I
= 1µF, input referred
75 dB
V
RIPPLE
=1V
P-P
Sine,
f
Ripple
= 1kHz, Inputs AC GND, C
I
=
1µF, input referred
75 dB
CMRR Common Mode Rejection Ratio V
RIPPLE
=1V
P-P
f
RIPPLE
= 217Hz 67 dB
ηEfficiency P
O
= 1W, f = 1kHz,
R
L
=8Ω,V
DD
=5V 85 %
Crosstalk P
O
= 500mW, f = 1kHz 84 dB
SNR Signal to Noise Ratio V
DD
= 5V, P
O
=1W 96 dB
e
OS
Output Noise Input referred, A-Weighted Filter 20 µV
Note 1: All voltages are measured with respect to the ground pin, unless otherwise specified.
Note 2: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
functional, but do not guarantee specific performance limits. Electrical Characteristics state DC and AC electrical specifications under particular test conditions which
guarantee specific performance limits. This assumes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters where no limit
is given, however, the typical value is a good indication of device performance.
Note 3: The maximum power dissipation must be derated at elevated temperatures and is dictated by TJMAX,θJA, and the ambient temperature, TA. The maximum
allowable power dissipation is PDMAX =(T
JMAX –T
A)/ θJA or the number given in Absolute Maximum Ratings, whichever is lower. For the LM4674A see power
derating currents for more information.
Note 4: Human body model, 100pF discharged through a 1.5kΩresistor.
Note 5: Machine Model, 220pF–240pF discharged through all pins.
Note 6: Typicals are measured at 25˚C and represent the parametric norm.
Note 7: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).
LM4674A
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External Components Description
(Figure 1)
Components Functional Description
1. C
S
Supply bypass capacitor which provides power supply filtering. Refer to the Power Supply Bypassing
section for information concerning proper placement and selection of the supply bypass capacitor.
2. C
I
Input AC coupling capacitor which blocks the DC voltage at the amplifier’s input terminals.
LM4674A
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Block Diagrams
20196137
LM4674A
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Block Diagrams (Continued)
20196138
Differential Input Configuration
LM4674A
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Typical Performance Characteristics
THD+N vs Output Power
f = 1kHz, A
V
= 24dB, R
L
=8Ω
THD+N vs Output Power
f = 1kHz, A
V
= 6dB, R
L
=8Ω
20196139 20196140
THD+N vs Output Power
f= 1kHz, A
V
= 24dB, R
L
=4Ω
THD+N vs Output Power
f = 1kHz, A
V
= 6dB, R
L
=4Ω
20196141 20196142
THD+N vs Frequency
V
DD
= 2.5V, P
OUT
= 100mW, R
L
=8Ω
THD+N vs Frequency
V
DD
= 3.6V, P
OUT
= 250mW, R
L
=8Ω
20196143 20196144
LM4674A
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Typical Performance Characteristics (Continued)
THD+N vs Frequency
V
DD
= 5V, P
OUT
= 375mW, R
L
=8Ω
THD+N vs Frequency
V
DD
= 2. 5V, P
OUT
= 100mW, R
L
=4Ω
20196145 20196146
THD+N vs Frequency
V
DD
= 3.6V, P
OUT
= 250mW, R
L
=4Ω
THD+N vs Frequency
V
DD
= 5V, P
OUT
= 375mW, R
L
=4Ω
20196147 20196148
Efficiency vs. Output Power
R
L
=4Ω, f = 1kHz
Efficiency vs. Output Power
R
L
=8Ω, f = 1kHz
20196149 20196150
LM4674A
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Typical Performance Characteristics (Continued)
Power Dissipation vs. Output Power
R
L
=4Ω, f = 1kHz
Power Dissipation vs. Output Power
R
L
=8Ω, f = 1kHz
20196151 20196152
Output Power vs. Supply Voltage
R
L
=4Ω, f = 1kHz
Output Power vs. Supply Voltage
R
L
=8Ω, f = 1kHz
20196153 20196154
PSRR vs. Frequency
V
DD
= 3.6V, V
RIPPLE
= 200mV
P-P
,R
L
=8Ω
Crosstalk vs. Frequency
V
DD
= 3.6V, V
RIPPLE
=1V
P-P
,R
L
=8Ω
20196155 20196122
LM4674A
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Typical Performance Characteristics (Continued)
CMRR vs. Frequency
V
DD
= 3.6V, V
CM
=1V
P-P
,R
L
=8Ω
Supply Current vs. Supply Voltage
No Load
20196157 20196158
LM4674A
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Application Information
GENERAL AMPLIFIER FUNCTION
The LM4674A stereo Class D audio power amplifier features
a filterless modulation scheme that reduces external compo-
nent count, conserving board space and reducing system
cost. The outputs of the device transition from V
DD
to GND
with a 300kHz switching frequency. With no signal applied,
the outputs (OUT_A and OUT_B) switch with a 50% duty
cycle, in phase, causing the two outputs to cancel. This
cancellation results in no net voltage across the speaker,
thus there is no current to the load in the idle state.
With the input signal applied, the duty cycle (pulse width) of
the LM4674A outputs changes. For increasing output volt-
age, the duty cycle of OUT_A increases, while the duty cycle
of OUT_B decreases. For decreasing output voltages, the
converse occurs. The difference between the two pulse
widths yields the differential output voltage.
DIFFERENTIAL AMPLIFIER EXPLANATION
As logic supplies continue to shrink, system designers are
increasingly turning to differential analog signal handling to
preserve signal to noise ratios with restricted voltage signs.
The LM4674A features two fully differential amplifiers. A
differential amplifier amplifies the difference between the two
input signals. Traditional audio power amplifiers have typi-
cally offered only single-ended inputs resulting in a 6dB
reduction of SNR relative to differential inputs. The LM4674A
also offers the possibility of DC input coupling which elimi-
nates the input coupling capacitors. A major benefit of the
fully differential amplifier is the improved common mode
rejection ratio (CMRR) over single ended input amplifiers.
The increased CMRR of the differential amplifier reduces
sensitivity to ground offset related noise injection, especially
important in noisy systems.
POWER DISSIPATION AND EFFICIENCY
The major benefit of a Class D amplifier is increased effi-
ciency versus a Class AB. The efficiency of the LM4674A is
attributed to the region of operation of the transistors in the
output stage. The Class D output stage acts as current
steering switches, consuming negligible amounts of power
compared to their Class AB counterparts. Most of the power
loss associated with the output stage is due to the IR loss of
the MOSFET on-resistance, along with switching losses due
to gate charge.
SHUTDOWN FUNCTION
The LM4674A features independent left and right channel
shutdown controls, allowing each channel to be disabled
independently. /SDR controls the right channel, while /SDL
controls the left channel. Driving either low disables the
corresponding channel, reducing supply current to 0.03µA.
It is best to switch between ground and V
DD
for minimum
current consumption while in shutdown. The LM4674A may
be disabled with shutdown voltages in between GND and
V
DD
, the idle current will be greater than the typical 0.03µA
value. Increased THD+N may also be observed when a
voltage of less than V
DD
is applied to /SD_ for logic levels
between GND and V
DD
Bypass /SD_ with a 0.1µF capacitor.
The LM4674A shutdown inputs have internal pulldown resis-
tors. The purpose of these resistors is to eliminate any
unwanted state changes when /SD_ is floating. To minimize
shutdown current, /SD_ should be driven to GND or left
floating. If /SD_ is not driven to GND or floating, an increase
in shutdown supply current will be noticed.
SINGLE-ENDED AUDIO AMPLIFIER CONFIGURATION
The LM4674A is compatible with single-ended sources.
When configured for single-ended inputs, input capacitors
must be used to block and DC component at the input of the
device. Figure 3 shows the typical single-ended applications
circuit.
AUDIO AMPLIFIER POWER SUPPLY
BYPASSING/FILTERING
Proper power supply bypassing is critical for low noise per-
formance and high PSRR. Place the supply bypass capaci-
tor as close to the device as possible. Typical applications
employ a voltage regulator with 10µF and 0.1µF bypass
capacitors that increase supply stability. These capacitors do
not eliminate the need for bypassing of the LM4674A supply
pins. A 1µF capacitor is recommended.
AUDIO AMPLIFIER INPUT CAPACITOR SELECTION
Input capacitors may be required for some applications, or
when the audio source is single-ended. Input capacitors
block the DC component of the audio signal, eliminating any
conflict between the DC component of the audio source and
the bias voltage of the LM4674A. The input capacitors create
a high-pass filter with the input resistors RI. The -3dB point of
the high pass filter is found using Equation 1 below.
f=1/2πR
IN
C
IN
(1)
The values for RI can be found in the EC table for each gain
setting.
The input capacitors can also be used to remove low fre-
quency content from the audio signal. Small speakers can-
not reproduce, and may even be damaged by low frequen-
cies. High pass filtering the audio signal helps protect the
speakers. When the LM4674A is using a single-ended
source, power supply noise on the ground is seen as an
input signal. Setting the high-pass filter point above the
power supply noise frequencies, 217Hz in a GSM phone, for
example, filters out the noise such that it is not amplified and
heard on the output. Capacitors with a tolerance of 10% or
better are recommended for impedance matching and im-
proved CMRR and PSRR.
AUDIO AMPLIFIER GAIN SETTING
The LM4674A features four internally configured gain set-
tings. The device gain is selected through the two logic
inputs, G0 and G1. The gain settings are as shown in the
following table.
G1 G0 GAIN
V/V dB
00 2 6
01 412
10 818
1 1 16 24
LM4674A
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Application Information (Continued)
PCB LAYOUT GUIDELINES
As output power increases, interconnect resistance (PCB
traces and wires) between the amplifier, load and power
supply create a voltage drop. The voltage loss due to the
traces between the LM4674A and the load results in lower
output power and decreased efficiency. Higher trace resis-
tance between the supply and the LM4674A has the same
effect as a poorly regulated supply, increasing ripple on the
supply line, and reducing peak output power. The effects of
residual trace resistance increases as output current in-
creases due to higher output power, decreased load imped-
ance or both. To maintain the highest output voltage swing
and corresponding peak output power, the PCB traces that
connect the output pins to the load and the supply pins to the
power supply should be as wide as possible to minimize
trace resistance.
The use of power and ground planes will give the best
THD+N performance. In addition to reducing trace resis-
tance, the use of power planes creates parasitic capacitors
that help to filter the power supply line.
The inductive nature of the transducer load can also result in
overshoot on one of both edges, clamped by the parasitic
diodes to GND and VDD in each case. From an EMI stand-
point, this is an aggressive waveform that can radiate or
conduct to other components in the system and cause inter-
ference. In is essential to keep the power and output traces
short and well shielded if possible. Use of ground planes
beads and micros-strip layout techniques are all useful in
preventing unwanted interference.
As the distance from the LM4674A and the speaker in-
creases, the amount of EMI radiation increases due to the
output wires or traces acting as antennas become more
efficient with length. Ferrite chip inductors places close to the
LM4674A outputs may be needed to reduce EMI radiation.
LM4674A
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Application Information (Continued)
20196126
Differential Input Configuration
FIGURE 2.
LM4674A
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Application Information (Continued)
20196138
Single-Ended Input Configuration
FIGURE 3.
LM4674A
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Revision Table
Rev Date Description
1.0 9/13/06 Initial WEB release.
LM4674A
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Physical Dimensions inches (millimeters) unless otherwise noted
16 Bump micro SMD
Order Number LM4674ATL
NS Package Number TLA1611A
X1 = 2mm X2 = 2mm X3 = 0.6mm
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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LM4674A Filterless 2.5W Stereo Class D Audio Power Amplifier
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