LM4670
LM4670 Filterless High Efficiency 3W Switching Audio Amplifier
Literature Number: SNAS240B
LM4670
Filterless High Efficiency 3W Switching Audio Amplifier
General Description
The LM4670 is a fully integrated single-supply high efficiency
switching audio amplifier. It features an innovative modulator
that eliminates the LC output filter used with typical switching
amplifiers. Eliminating the output filter reduces external com-
ponent count, simplifies circuit design, and reduces board
area. The LM4670 processes analog inputs with a delta-
sigma modulation technique that lowers output noise and
THD when compared to conventional pulse width modula-
tors.
The LM4670 is designed to meet the demands of mobile
phones and other portable communication devices. Operat-
ing on a single 5V supply, it is capable of driving a 4Ω
speaker load at a continuous average output of 2.3W with
less than 1% THD+N. Its flexible power supply requirements
allow operation from 2.4V to 5.5V.
The LM4670 has high efficiency with speaker loads com-
pared to a typical Class AB amplifier. With a 3.6V supply
driving an 8Ωspeaker, the IC’s efficiency for a 100mW
power level is 77%, reaching 88% at 600mW output power.
The LM4670 features a low-power consumption shutdown
mode. Shutdown may be enabled by driving the Shutdown
pin to a logic low (GND).
The gain of the LM4670 is externally configurable which
allows independent gain control from multiple sources by
summing the signals.
Key Specifications
jEfficiency at 3.6V, 100mW into 8Ωspeaker 77% (typ)
jEfficiency at 3.6V, 600mW into 8Ωspeaker 88% (typ)
jEfficiency at 5V, 1W into 8Ωspeaker 87% (typ)
jQuiescent current, 3.6V supply 4.8mA (typ)
jTotal shutdown power supply current 0.01µA (typ)
jSingle supply range 2.4V to 5.5V
Features
nNo output filter required for inductive loads
nExternally configurable gain
nVery fast turn on time: 1.35ms (typ)
nMinimum external components
n"Click and pop" suppression circuitry
nMicro-power shutdown mode
nShort circuit protection
nAvailable in space-saving microSMD and LLP packages
Applications
nMobile phones
nPDAs
nPortable electronic devices
Typical Application
Boomer®is a registered trademark of National Semiconductor Corporation.
20089901
FIGURE 1. Typical Audio Amplifier Application Circuit
July 2006
LM4670 Filterless High Efficiency 3W Switching Audio Amplifier
© 2006 National Semiconductor Corporation DS200899 www.national.com
Connection Diagrams
9 Bump micro SMD Package micro SMD Marking
20089936
Top View
Order Number LM4670ITL, LM4670ITLX
See NS Package Number TLA09ZZA
200899C6
Top View
X — Date Code
T — Die Traceability
G — Boomer Family
E6 – LM4670ITL
Leadless Leadframe Package (LLP) LLP Marking
20089949
Top View
Order Number LM4670SD
See NS Package Number SDA08A
Contact NSC Sales Office for Availability
20089951
Top View
Z — Plant Code
XY — Date Code
TT — Die Traceability
L4670 — LM4670
LM4670
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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
Voltage at Any Input Pin V
DD
+ 0.3V ≥V≥GND - 0.3V
Power Dissipation (Note 3) Internally Limited
ESD Susceptibility (Note 4) 2.0kV
ESD Susceptibility (Note 5) 200V
Junction Temperature (T
JMAX
) 150˚C
Thermal Resistance
θ
JA
(micro SMD) 220˚C/W
θ
JA
(LLP) 73˚C/W
Soldering Information
See AN-1112 "microSMD Wafers Level Chip Scale
Package."
Operating Ratings (Note 1) (Note 2)
Temperature Range
T
MIN
≤T
A
≤T
MAX
−40˚C ≤T
A
≤85˚C
Supply Voltage (Note 12) 2.4V ≤V
DD
≤5.5V
Electrical Characteristics (Notes 1, 2)
The following specifications apply for A
V
= 2V/V (R
I
= 150kΩ), R
L
=15µH+8Ω+ 15µH unless otherwise specified. Limits ap-
ply for T
A
= 25˚C.
Symbol Parameter Conditions
LM4670 Units
(Limits)
Typical Limit
(Note 6) (Notes 7, 8)
|V
OS
| Differential Output Offset Voltage V
I
= 0V, A
V
= 2V/V,
V
DD
= 2.4V to 5.0V 25 mV (max)
PSRR
GSM
GSM Power Supply Rejection Ratio V
DD
= 2.4V to 5.0V,
Input Referred 64 dB
CMRR
GSM
GSM Common Mode Rejection
Ratio
V
DD
= 2.4V to 5.0V
V
IC
=V
DD
/2 to 0.5V,
V
IC
=V
DD
/2 to V
DD
– 0.8V,
Input Referred
80 dB
|I
IH
| Logic High Input Current V
DD
= 5.0V, V
I
= 5.8V 20 100 µA (max)
|I
IL
| Logic Low Input Current V
DD
= 5.0V, V
I
= –0.3V 1 5 µA (max)
V
IN
= 0V, No Load, V
DD
= 5.0V 7.0 10 mA (max)
I
DD
Quiescent Power Supply Current V
IN
= 0V, No Load, V
DD
= 3.6V 4.8 mA
V
IN
= 0V, No Load, V
DD
= 2.4V 3.8 5 mA (max)
I
SD
Shutdown Current (Note 9) V
SHUTDOWN
=0V
V
DD
= 2.4V to 5.0V 0.01 1 µA (max)
V
SDIH
Shutdown voltage input high 1.0 1.4 V (min)
V
SDIL
Shutdown voltage input low 0.8 0.4 V (max)
R
OSD
Output Impedance V
SHUTDOWN
= 0.4V >100 kΩ
A
V
Gain 300kΩ/R
I
270kΩ/R
I
330kΩ/R
I
V/V (min)
V/V (max)
R
SD
Resistance from Shutdown Pin to
GND 300 kΩ
P
O
Output Power (Notes 11,12)
R
L
=15µH+4Ω+ 15µH,
THD = 10% (max)
f = 1kHz, 22kHz BW
V
DD
=5V
V
DD
= 3.6V
V
DD
= 2.5V
3.0
1.5
675
W
W
mW
R
L
=15µH+4Ω+ 15µH,
THD+N = 1% (max)
f = 1kHz, 22kHz BW
V
DD
= 5V,
V
DD
= 3.6V,
V
DD
= 2.5V,
2.3
1.2
550
W
W
mW
LM4670
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Electrical Characteristics (Notes 1, 2)
The following specifications apply for A
V
= 2V/V (R
I
= 150kΩ), R
L
=15µH+8Ω+ 15µH unless otherwise specified. Limits
apply for T
A
= 25˚C. (Continued)
Symbol Parameter Conditions
LM4670 Units
(Limits)
Typical Limit
(Note 6) (Notes 7, 8)
P
O
Output Power (Note 11)
R
L
=15µH+8Ω+ 15µH,
THD = 10% (max)
f = 1kHz, 22kHz BW
V
DD
=5V
V
DD
= 3.6V
V
DD
= 2.5V
1.65
850
400
W
mW
mW
R
L
=15µH+8Ω+ 15µH,
THD+N = 1% (max)
f = 1kHz, 22kHz BW
V
DD
= 5V,
V
DD
= 3.6V,
V
DD
= 2.5V,
1.35
680
325
600
W
mW (min)
mW
THD+N Total Harmonic Distortion + Noise
V
DD
= 5V, P
O
=1W
RMS
,
f = 1kHz 0.35 %
V
DD
= 3.6V, P
O
= 0.5W
RMS
,
f = 1kHz 0.30 %
V
DD
= 3.6V, P
O
= 0.5W
RMS
,
f = 5kHz 0.30 %
V
DD
= 3.6V, P
O
= 0.5W
RMS
,
f = 10kHz 0.30 %
V
DD
= 3.6V,
V
Ripple
= 200mV
PP
Sine,
f
Ripple
= 217Hz
Inputs to AC GND, C
I
= 0.1µ,
Input Referred
68 dB
PSRR Power Supply Rejection Ratio
V
DD
= 3.6V,
V
Ripple
= 200mV
PP
Sine,
f
Ripple
= 1kHz
Inputs to AC GND, C
I
= 0.1µF
Input Referred
65 dB
V
DD
= 3.6V,
V
Ripple
= 200mV
PP
Sine,
f
Ripple
= 217Hz
f
IN
= 1kHz, P
O
= 10mW
RMS
Input Referred
62 dB
SNR Signal to Noise Ratio V
DD
= 5V, P
O
=1W
RMS
93 dB
e
OUT
Output Noise
V
DD
= 3.6V, f = 20Hz – 20kHz
Inputs to AC GND, C
I
= 0.1µF
No Weighting, Input Referred
85 µV
RMS
V
DD
= 3.6V, Inputs to AC GND
C
I
= 0.1µF, A Weighted
Input Referred
65 µV
RMS
CMRR Common Mode Rejection Ratio V
DD
= 3.6V, V
Ripple
=1V
PP
Sine
f
Ripple
= 217Hz, Input Referred 80 dB
T
WU
Wake-up Time V
DD
= 3.6V 1.35 ms
T
SD
Shutdown Time V
DD
= 3.6V 0.01 ms
LM4670
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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–TA)/θJA or the number given in Absolute Maximum Ratings, whichever is lower. For the LM4670, TJMAX = 150˚C.
The typical θJA is 220˚C/W for the microSMD package and 64˚C/W for the LLP package.
Note 4: Human body model, 100pF discharged through a 1.5kΩresistor.
Note 5: Machine Model, 220pF – 240pF discharged through all pins.
Note 6: Typical specifications are specified at 25˚C and represent the parametric norm.
Note 7: Tested limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).
Note 8: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis.
Note 9: Shutdown current is measured in a normal room environment. Exposure to direct sunlight will increase ISD by a maximum of 2µA. The Shutdown pin should
be driven as close as possible to GND for minimal shutdown current. See the Application Information section under SHUTDOWN FUNCTION for more information.
Note 10: The performance graphs were taken using the Audio Precision AUX-0025 Switching Amplifier Measurement Filter in series with the LC filter on the board.
Note 11: Typical output power numbers are for the LM4670 in the ITL (µSMD) package. In the LLP (SDA) package, the output power will be lower due to higher
resistance seen from the IC output pad to PCB trace. The difference increases with lower impedance loads.
Note 12: The maximum operating voltage for the LM4670 in the SDA (LLP) package when driving 4Ωloads to greater than 10% THD+N is 5.0V.
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. R
I
Gain setting resistor. Differential gain is set by the equation A
V
= 2 * 150kΩ/R
i
(V/V).
LM4670
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Typical Performance Characteristics (Note 10)
THD+N vs Frequency
V
DD
= 2.5V, R
L
=15µH+4Ω+ 15µH
P
OUT
= 375mW, 22kHz BW
THD+N vs Frequency
V
DD
= 3.6V, R
L
=15µH+4Ω+ 15µH
P
OUT
= 750mW, 22kHz BW
20089941 20089943
THD+N vs Frequency
V
DD
= 5V, R
L
=15µH+4Ω+ 15µH
P
OUT
= 1.5W, 22kHz BW
THD+N vs Frequency
V
DD
= 2.5V, R
L
=15µH+8Ω+ 15µH
P
OUT
= 200mW, 22kHz BW
20089945 20089942
THD+N vs Frequency
V
DD
= 3.6V, R
L
=15µH+8Ω+ 15µH
P
OUT
= 500mW, 22kHz BW
THD+N vs Frequency
V
DD
= 5V, R
L
=15µH+8Ω+ 15µH
P
OUT
= 1W, 22kHz BW
20089944 20089946
LM4670
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Typical Performance Characteristics (Note 10) (Continued)
THD+N vs Output Power
R
L
=15µH+4Ω+ 15µH
f = 1kHz, 22kHz BW
THD+N vs Output Power
R
L
=15µH+8Ω+ 15µH
f = 1kHz, 22kHz BW
20089947 20089948
CMRR vs Frequency
V
DD
= 3.6V, R
L
=15µH+8Ω+ 15µH
V
CM
=1V
P-P
Sine Wave, 22kHz BW
PSRR vs Frequency
V
DD
= 3.6V, R
L
=15µH+8Ω+ 15µH
V
CM
= 200mV
P-P
Sine Wave, 22kHz BW
20089910 20089940
Efficiency and Power Dissipation
vs Output Power
R
L
=15µH+4Ω+ 15µH, f = 1kHz, THD <2%
Efficiency and Power Dissipation
vs Output Power
R
L
=15µH+8Ω+ 15µH, f = 1kHz, THD <1%
20089911 20089912
LM4670
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Typical Performance Characteristics (Note 10) (Continued)
Output Power vs Supply Voltage
R
L
=15µH+4Ω+ 15µH, f = 1kHz, 22kHz BW
Output Power vs Supply Voltage
R
L
=15µH+8Ω+ 15µH, f = 1kHz, 22kHz BW
20089914 20089915
Supply Current (RMS) vs Output Power
R
L
=15µH+4Ω+ 15µH, f = 1kHz
Supply Current (RMS) vs Output Power
R
L
=15µH+8Ω+ 15µH, f = 1kHz
20089919 20089920
Shutdwon Threshold
R
L
=15µH+8Ω+ 15µH
Shutdwon Threshold vs Supply Voltage
R
L
=15µH+8Ω+ 15µH
20089918 200899H5
LM4670
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Typical Performance Characteristics (Note 10) (Continued)
Supply Current vs Shutdown Voltage
R
L
=15µH+8Ω+ 15µH
Supply Current vs Supply Voltage
R
L
=15µH+8Ω+ 15µH
20089938 20089922
Supply Current vs Supply Voltage
R
L
= Different µH loads
Differential Gain vs Supply Voltage
R
L
=15µH+8Ω+ 15µH, R
i
= 150kΩ, f = 1kHz
20089939 20089913
LM4670
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Application Information
GENERAL AMPLIFIER FUNCTION
The output signals generated by the LM4670 consist of two,
BTL connected, output signals that pulse momentarily from
near ground potential to V
DD
. The two outputs can pulse
independently with the exception that they both may never
pulse simultaneously as this would result in zero volts across
the BTL load. The minimum width of each pulse is approxi-
mately 350ns. However, pulses on the same output can
occur sequentially, in which case they are concatenated and
appear as a single wider pulse to achieve an effective 100%
duty cycle. This results in maximum audio output power for a
given supply voltage and load impedance. The LM4670 can
achieve much higher efficiencies than class AB amplifiers
while maintaining acceptable THD performance.
The short (350ns) drive pulses emitted at the LM4670 out-
puts means that good efficiency can be obtained with mini-
mal load inductance. The typical transducer load on an audio
amplifier is quite reactive (inductive). For this reason, the
load can act as it’s own filter, so to speak. This "filter-less"
switching amplifier/transducer load combination is much
more attractive economically due to savings in board space
and external component cost by eliminating the need for a
filter.
POWER DISSIPATION AND EFFICIENCY
In general terms, efficiency is considered to be the ratio of
useful work output divided by the total energy required to
produce it with the difference being the power dissipated,
typically, in the IC. The key here is “useful” work. For audio
systems, the energy delivered in the audible bands is con-
sidered useful including the distortion products of the input
signal. Sub-sonic (DC) and super-sonic components
(>22kHz) are not useful. The difference between the power
flowing from the power supply and the audio band power
being transduced is dissipated in the LM4670 and in the
transducer load. The amount of power dissipation in the
LM4670 is very low. This is because the ON resistance of the
switches used to form the output waveforms is typically less
than 0.25Ω. This leaves only the transducer load as a po-
tential "sink" for the small excess of input power over audio
band output power. The LM4670 dissipates only a fraction of
the excess power requiring no additional PCB area or cop-
per plane to act as a heat sink.
DIFFERENTIAL AMPLIFIER EXPLANATION
As logic supply voltages continue to shrink, designers are
increasingly turning to differential analog signal handling to
preserve signal to noise ratios with restricted voltage swing.
The LM4670 is a fully differential amplifier that features
differential input and output stages. A differential amplifier
amplifies the difference between the two input signals. Tra-
ditional audio power amplifiers have typically offered only
single-ended inputs resulting in a 6dB reduction in signal to
noise ratio relative to differential inputs. The LM4670 also
offers the possibility of DC input coupling which eliminates
the two external AC coupling, DC blocking capacitors. The
LM4670 can be used, however, as a single ended input
amplifier while still retaining it’s fully differential benefits. In
fact, completely unrelated signals may be placed on the
input pins. The LM4670 simply amplifies the difference be-
tween the signals. A major benefit of a differential amplifier is
the improved common mode rejection ratio (CMRR) over
single input amplifiers. The common-mode rejection charac-
teristic of the differential amplifier reduces sensitivity to
ground offset related noise injection, especially important in
high noise applications.
PCB LAYOUT CONSIDERATIONS
As output power increases, interconnect resistance (PCB
traces and wires) between the amplifier, load and power
supply create a voltage drop. The voltage loss on the traces
between the LM4670 and the load results is lower output
power and decreased efficiency. Higher trace resistance
between the supply and the LM4670 has the same effect as
a poorly regulated supply, increase ripple on the supply line
also reducing the peak output power. The effects of residual
trace resistance increases as output current increases due
to higher output power, decreased load impedance or both.
To maintain the highest output voltage swing and corre-
sponding 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. While reducing trace resistance, the
use of power planes also creates parasite capacitors that
help to filter the power supply line.
The rising and falling edges are necessarily short in relation
to the minimum pulse width (350ns), having approximately
16ns rise and fall times, typical, depending on parasitic
output capacitance. The inductive nature of the transducer
load can also result in overshoot on one or both edges,
clamped by the parasitic diodes to GND and V
DD
in each
case. From an EMI standpoint, this is an aggressive wave-
form that can radiate or conduct to other components in the
system and cause interference. It is essential to keep the
power and output traces short and well shielded if possible.
Use of ground planes, beads, and micro-strip layout tech-
niques are all useful in preventing unwanted interference.
As the distance from the LM4670 and the speaker increase,
the amount of EMI radiation will increase since the output
wires or traces acting as antenna become more efficient with
length. What is acceptable EMI is highly application specific.
Ferrite chip inductors placed close to the LM4670 may be
needed to reduce EMI radiation. The value of the ferrite chip
is very application specific.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection ratio (PSRR). The capacitor (C
S
) location should be
as close as possible to the LM4670. Typical applications
employ a voltage regulator with a 10µF and a 0.1µF bypass
capacitors that increase supply stability. These capacitors do
not eliminate the need for bypassing on the supply pin of the
LM4670. A 1µF tantalum capacitor is recommended.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4670 contains shutdown circuitry that reduces current
draw to less than 0.01µA. The trigger point for shutdown is
shown as a typical value in the Electrical Characteristics
Tables and in the Shutdown Hysteresis Voltage graphs
found in the Typical Performance Characteristics section.
It is best to switch between ground and supply for minimum
LM4670
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Application Information (Continued)
current usage while in the shutdown state. While the
LM4670 may be disabled with shutdown voltages in between
ground and supply, the idle current will be greater than the
typical 0.01µA value. Increased THD may also be observed
with voltages less than V
DD
on the Shutdown pin when in
PLAY mode.
The LM4670 has an internal resistor connected between
GND and Shutdown pins. The purpose of this resistor is to
eliminate any unwanted state changes when the Shutdown
pin is floating. The LM4670 will enter the shutdown state
when the Shutdown pin is left floating or if not floating, when
the shutdown voltage has crossed the threshold. To mini-
mize the supply current while in the shutdown state, the
Shutdown pin should be driven to GND or left floating. If the
Shutdown pin is not driven to GND, the amount of additional
resistor current due to the internal shutdown resistor can be
found by Equation (1) below.
(V
SD
- GND) / 300kΩ(1)
With only a 0.5V difference, an additional 1.7µA of current
will be drawn while in the shutdown state.
PROPER SELECTION OF EXTERNAL COMPONENTS
The gain of the LM4670 is set by the external resistors, Ri in
Figure 1, The Gain is given by Equation (2) below. Best
THD+N performance is achieved with a gain of 2V/V (6dB).
A
V
=2*150kΩ/R
i
(V/V) (2)
It is recommended that resistors with 1% tolerance or better
be used to set the gain of the LM4670. The Ri resistors
should be placed close to the input pins of the LM4670.
Keeping the input traces close to each other and of the same
length in a high noise environment will aid in noise rejection
due to the good CMRR of the LM4670. Noise coupled onto
input traces which are physically close to each other will be
common mode and easily rejected by the LM4670.
Input capacitors may be needed for some applications or
when the source is single-ended (see Figures 3, 5). Input
capacitors are needed to block any DC voltage at the source
so that the DC voltage seen between the input terminals of
the LM4670 is 0V. Input capacitors create a high-pass filter
with the input resistors, R
i
. The –3dB point of the high-pass
filter is found using Equation (3) below.
f
C
=1/(2πR
i
C
i
) (Hz) (3)
The input capacitors may also be used to remove low audio
frequencies. Small speakers cannot reproduce low bass
frequencies so filtering may be desired . When the LM4670
is using a single-ended source, power supply noise on the
ground is seen as an input signal by the +IN input pin that is
capacitor coupled to ground (See Figures 5 – 7). Setting the
high-pass filter point above the power supply noise frequen-
cies, 217Hz in a GSM phone, for example, will filter out this
noise so it is not amplified and heard on the output. Capaci-
tors with a tolerance of 10% or better are recommended for
impedance matching.
DIFFERENTIAL CIRCUIT CONFIGURATIONS
The LM4670 can be used in many different circuit configu-
rations. The simplest and best performing is the DC coupled,
differential input configuration shown in Figure 2. Equation
(2) above is used to determine the value of the R
i
resistors
for a desired gain.
Input capacitors can be used in a differential configuration as
shown in Figure 3. Equation (3) above is used to determine
the value of the C
i
capacitors for a desired frequency re-
sponse due to the high-pass filter created by C
i
and R
i
.
Equation (2) above is used to determine the value of the R
i
resistors for a desired gain
The LM4670 can be used to amplify more than one audio
source. Figure 4 shows a dual differential input configuration.
The gain for each input can be independently set for maxi-
mum design flexibility using the R
i
resistors for each input
and Equation (2). Input capacitors can be used with one or
more sources as well to have different frequency responses
depending on the source or if a DC voltage needs to be
blocked from a source.
SINGLE-ENDED CIRCUIT CONFIGURATIONS
The LM4670 can also be used with single-ended sources but
input capacitors will be needed to block any DC at the input
terminals. Figure 5 shows the typical single-ended applica-
tion configuration. The equations for Gain, Equation (2), and
frequency response, Equation (3), hold for the single-ended
configuration as shown in Figure 5.
When using more than one single-ended source as shown in
Figure 6, the impedance seen from each input terminal
should be equal. To find the correct values for C
i3
and R
i3
connected to the +IN input pin the equivalent impedance of
all the single-ended sources are calculated. The single-
ended sources are in parallel to each other. The equivalent
capacitor and resistor, C
i3
and R
i3
, are found by calculating
the parallel combination of all C
i
values and then all R
i
val-
ues. Equations (4) and (5) below are for any number of
single-ended sources.
C
i3
=C
i1
+C
i2
+C
in
... (F) (4)
R
i3
= 1 / (1/R
i1
+ 1/R
i2
+ 1/R
in
...) (Ω) (5)
The LM4670 may also use a combination of single-ended
and differential sources. A typical application with one single-
ended source and one differential source is shown in Figure
7. Using the principle of superposition, the external compo-
nent values can be determined with the above equations
corresponding to the configuration.
LM4670
www.national.com11
Application Information (Continued)
20089903
FIGURE 2. Differential input configuration
20089904
FIGURE 3. Differential input configuration with input capacitors
LM4670
www.national.com 12
Application Information (Continued)
20089905
FIGURE 4. Dual differential input configuration
20089906
FIGURE 5. Single-ended input configuration
LM4670
www.national.com13
Application Information (Continued)
20089907
FIGURE 6. Dual single-ended input configuration
20089908
FIGURE 7. Dual input with a single-ended input and a differential input
LM4670
www.national.com 14
Application Information (Continued)
REFERENCE DESIGN BOARD SCHEMATIC
In addition to the minimal parts required for the application
circuit, a measurement filter is provided on the evaluation
circuit board so that conventional audio measurements can
be conveniently made without additional equipment. This is a
balanced input, grounded differential output low pass filter
with a 3dB frequency of approximately 35kHz and an on
board termination resistor of 300Ω(see schematic). Note
that the capacitive load elements are returned to ground.
This is not optimal for common mode rejection purposes, but
due to the independent pulse format at each output there is
a significant amount of high frequency common mode com-
ponent on the outputs. The grounded capacitive filter ele-
ments attenuate this component at the board to reduce the
high frequency CMRR requirement placed on the analysis
instruments.
Even with the grounded filter the audio signal is still differ-
ential, necessitating a differential input on any analysis in-
strument connected to it. Most lab instruments that feature
BNC connectors on their inputs are NOT differential re-
sponding because the ring of the BNC is usually grounded.
The commonly used Audio Precision analyzer is differential,
but its ability to accurately reject fast pulses of 350ns width is
questionable necessitating the on board measurement filter.
When in doubt or when the signal needs to be single-ended,
use an audio signal transformer to convert the differential
output to a single ended output. Depending on the audio
transformer’s characteristics, there may be some attenua-
tion of the audio signal which needs to be taken into account
for correct measurement of performance.
Measurements made at the output of the measurement filter
suffer attenuation relative to the primary, unfiltered outputs
even at audio frequencies. This is due to the resistance of
the inductors interacting with the termination resistor (300Ω)
and is typically about -0.25dB (3%). In other words, the
voltage levels (and corresponding power levels) indicated
through the measurement filter are slightly lower than those
that actually occur at the load placed on the unfiltered out-
puts. This small loss in the filter for measurement gives a
lower output power reading than what is really occurring on
the unfiltered outputs and its load.
20089909
FIGURE 8.
LM4670
www.national.com15
Application Information (Continued)
LM4670 micro SMD BOARD ARTWORK
Composite View Silk Screen
20089932 20089935
Top Layer Internal Layer 1, GND
20089937 20089933
Internal Layer 2, V
DD
Bottom Layer
20089934 20089931
LM4670
www.national.com 16
Application Information (Continued)
LM4670 SDA BOARD ARTWORK
Composite View Silk Screen
20089953 20089956
Top Layer Internal Layer 1, GND
20089957 20089954
Internal Layer 2, V
DD
Bottom Layer
20089955 20089952
LM4670
www.national.com17
Revision History
Rev Date Description
1.0 12/15/04 Initial WEB of the D/S (TL pkg).
1.1 7/06/05 Re-released D/S to the WEB (added the SD package).
1.2 7/13/06 Edited Note 9, then re-released D/S to the WEB.
LM4670
www.national.com 18
Physical Dimensions inches (millimeters) unless otherwise noted
9 Bump micro SMD
Order Number LM4670ITL, LM4670ITLX
NS Package Number TLA09ZZA
X1 = 1.463 X2 = 1.463 X3 = 0.600
LLP
Order Number LM4670SD
NS Package Number SDA08A
LM4670
www.national.com19
Notes
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves
the right at any time without notice to change said circuitry and specifications.
For the most current product information visit us at www.national.com.
LIFE SUPPORT POLICY
NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS
WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR
CORPORATION. As used herein:
1. Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the body, or
(b) support or sustain life, and whose failure to perform when
properly used in accordance with instructions for use
provided in the labeling, can be reasonably expected to result
in a significant injury to the user.
2. A critical component is any component of a life support
device or system whose failure to perform can be reasonably
expected to cause the failure of the life support device or
system, or to affect its safety or effectiveness.
BANNED SUBSTANCE COMPLIANCE
National Semiconductor follows the provisions of the Product Stewardship Guide for Customers (CSP-9-111C2) and Banned Substances
and Materials of Interest Specification (CSP-9-111S2) for regulatory environmental compliance. Details may be found at:
www.national.com/quality/green.
Lead free products are RoHS compliant.
National Semiconductor
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Support Center
Email: new.feedback@nsc.com
Tel: 1-800-272-9959
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Fax: +49 (0) 180-530 85 86
Email: europe.support@nsc.com
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www.national.com
LM4670 Filterless High Efficiency 3W Switching Audio Amplifier
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