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DESCRIPTION JP2008259104

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DESCRIPTION JP2008259104
The present invention is a single-ended digital amplification device that minimizes characteristic
deterioration with simple measures that can be easily adopted in portable devices. A power
amplification device receives, at its output circuit portion, a pulse width modulated signal of an
audio signal as a control signal at each gate, and has a conductive channel connected in series
between a power supply voltage and a ground. A pair of FETs (output transistors) 712 and 713
of different conductivity types are provided. A low pass filter 715 is connected to the connection
point of the pair of FETs, and the output signal is output to the speaker. A series circuit 90 of a
resistor and a capacitor connected between the ground and the low pass filter 715 is connected
before the low pass filter 715. In the case of stereo reproduction, two sets of these configurations
are provided, and both sets share the same power supply unit. [Selected figure] Figure 9
Power amplification device and audio reproduction device
[0001]
The present invention relates to a single-ended digital amplification device and an audio
reproduction device using the same.
[0002]
A digital amplifier or a class D amplifier outputs digital audio signal output as a PWM (Pulse
Width Modulation) signal and outputs sound from a speaker, and has been commonly used in
recent years.
[0003]
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1
In portable terminals, there are currently few examples used for low power output to the extent
that the earphones are ringed, but the applications for listening to music on portable terminals
are increasing, and the output efficiency is high and for low power consumption. The adoption of
digital amplifiers is expected to increase in the future.
[0004]
As shown in FIG. 2, it is known that the digital amplifier causes a phenomenon called a pumping
phenomenon in which a power supply voltage rises.
The power supply voltage maximum change amount ΔVBUSmax is expressed by the following
equation.
Here, VBUS represents a power supply voltage, fpwm represents a frequency of a PWM digital
signal, Rload represents a load resistance, and Cbus represents a power supply capacitor.
ΔVBUSmax = VBUS / (8 · π · fpwm · Rload · Cbus)
[0005]
The actual power supply voltage change largely changes depending on the signal state (PWM
signal duty ratio etc.). Especially, such phenomenon is caused by the fact that the duty is low
when the capacity of the power supply capacitor is small at low frequency output and low load. It
occurs when it deviates greatly from 50% (eg 25%, 75%), etc.
[0006]
In the output circuit portion of the half bridge type shown in FIG. 1 (a) or the full bridge type
digital amplifier shown in FIG. 1 (b), two or four plural transistors (FETs) are used for differential
operation. I am driving a speaker.
General speaker driving is often performed in this circuit form.
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[0007]
However, it is not possible to use a differential output that requires special connection when
driving an earphone in a portable device, and as shown in FIG. 3, the earphone is connected
using the output circuit portion of a single-ended digital amplification device Is used. In FIG. 3,
LDO (Low Drop Out) is a constant voltage power supply IC, that is, a linear voltage regulator that
operates when the input voltage exceeds a desired output voltage. Further, Cp indicates a power
supply capacitor, L and C indicate a filter inductor and capacitor, Co indicates a coupling
capacitor, and D-amp indicates a digital amplifier. The output of the digital amplifier is a PWM
digital signal and is supplied to the earphone as an analog signal through an LC filter.
[0008]
When the pumping phenomenon occurs in such a circuit system, not only the power supply
voltage rises, but also deterioration of the audio distortion factor and deterioration of the stereo
channel separation (the degree of left / right separation) may remarkably occur.
[0009]
A large enough capacitor is inserted into the power supply capacitor as a countermeasure against
pumping, a technology for performing power supply control as shown in Patent Document 1, or
the output of a digital amplifier as shown in Patent Document 2 is fed back to the input.
Techniques for suppressing distortion have also been proposed.
JP-A-2006-40169 JP-A-2006-279509
[0010]
However, in portable devices, the size of the power supply capacitor can not be made too large
due to size limitations, and it is difficult to adopt current portable devices because of problems
with the circuit configuration and power consumption due to the characteristic improvement by
feedback circuits.
[0011]
The present invention has been made in such a background, and is a single-ended digital
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amplifying device, which is intended to minimize characteristic deterioration with a simple
measure that can be easily adopted in portable devices. There is.
[0012]
A digital amplification device according to the present invention is a single-ended digital
amplification device, which receives a pulse width modulated signal of an audio signal as a
control signal, and has a conductive channel connected in series between a power supply voltage
and a ground. A low pass filter connected to the connection point of a pair of output transistors
of different conductivity types and the pair of output transistors and outputting an output signal
to a speaker, and between the ground and the ground in the front of the low pass filter And a
series circuit of a resistor and a capacitor connected to the
[0013]
Another digital amplifier according to the present invention is a single-ended digital amplifier for
outputting a stereo audio signal, comprising a pair of output circuits for both left and right
channels sharing a power supply voltage, each output circuit comprising Receiving a pulse width
modulated audio signal as a control signal, connecting a pair of output transistors of different
conductivity types having a conductive channel connected in series between the power supply
voltage and the ground, and the pair of output transistors A low-pass filter connected to a point
and outputting an output signal to a speaker, and a series circuit of a resistor and a capacitor
connected between the low-pass filter and the ground at a stage before the low-pass filter. .
[0014]
Thus, in the present invention, by arranging a series circuit of a resistor and a capacitor between
the output transistor and the low pass filter, the electromotive force by the inductor in the low
pass filter is absorbed by the series circuit. The adverse effect of the inductor's electromotive
force can be suppressed.
[0015]
Further, the present invention provides an audio reproduction apparatus adopting the digital
amplification apparatus as described above.
[0016]
According to the present invention, the following effects can be obtained in the digital
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amplification device.
1.
Power supply voltage rise is suppressed.
2.
Audio distortion is suppressed.
3.
When used for stereo reproduction, channel separation can be improved. 4. This eliminates
the need for complicated circuits such as feedback circuits, and can be realized with a slight
increase in circuit size and cost.
[0017]
Hereinafter, preferred embodiments of the present invention will be described in detail with
reference to the drawings.
[0018]
Although the configuration of the digital amplification device of the audio reproduction device
according to the present embodiment is shown in FIG. 9, before describing this configuration, in
order to better understand the present invention, the digital amplification device not adopting
the configuration of the present invention A specific example is given and demonstrated.
[0019]
In order to confirm the above-mentioned pumping phenomenon, when experimental
measurement was performed using a digital amplifier of a certain manufacturer with the
configuration shown in FIG. 3, as shown in the waveform diagram of FIG. The voltage rise could
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be confirmed.
In this case, since the power supply voltage is 1.6 V and the maximum withstand voltage of the
device is 1.8 V, if this phenomenon occurs in this environment, there is a high possibility of the
device being broken.
Also, the power supply IC (LDO) may be weak to the reverse voltage, and in the worst case, the
LDO may be destroyed.
[0020]
Further, as shown in FIG. 5, the distortion rate is considerably deteriorated in the low frequency
band of 500 Hz or less, and is up to less than 13%. The lower two waveforms in FIG. 4 are output
waveforms when a 20 Hz sine wave is input, and it can be seen that the waveform distortion is
large as well. This measurement is obtained by adding the same signal to the stereo left and right
channels, but the difference between the audio outputs of the channels in which either the left or
right signal is set to the non-signal state and the signal is put only on one channel. The left and
right channel separations can be measured.
[0021]
When channel separation is measured in this manner, as shown in the graph of FIG. 6, it becomes
as bad as about 20 dB in the low frequency band as in the distortion factor.
[0022]
Although this measurement was performed under certain conditions, it has been confirmed by
experiments that it becomes worse depending on the measurement conditions such as the value
of the LC filter.
[0023]
It was also confirmed that this phenomenon occurs notably when the power supply capacitor is
small, but it was also found that the capacity of several tens of microfarads (μF) usually used in
the power supply of portable devices is insufficient.
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In the inventor's experiment, the pumping phenomenon could be almost completely suppressed
by putting a capacitor as large as 4700 μF in the power supply.
However, at present, it is difficult to put such a large-capacity capacitor in a portable device in
consideration of the size and the like, and the cost increases, which is not very realistic.
[0024]
In order to improve this, we further examined the circuit and examined the waveform in detail.
[0025]
FIG. 7 shows a configuration example of a digital amplification device (output circuit portion) to
be compared with the digital amplification device of the present embodiment.
This circuit is almost identical to that shown in FIG. 3, but the circuit itself is stereophonic.
[0026]
A digital amplifier (D-amp) 700 has audio signal demodulators for both left and right channels,
and includes a pair of PWM generators 711 for pulse width modulating the demodulated audio
signal, and two sets of output transistors. The two sets of output transistors include a pair of
FETs 712, 713 for the left and right channels respectively. Each pair of FETs receives an audio
signal pulse width modulated signal as a control signal at each gate and is a FET of different
conductivity type having a conductive channel connected in series between the power supply
voltage and the ground. . A low pass filter 715 is connected to the connection point of each pair
of FETs via the resistors RL and RR. Low pass filter 715 is formed of an inductance receiving the
output of digital amplifier 700 and a capacitor whose other end is AC grounded. The outputs of
low pass filter 715 are connected to left and right speakers 717 via coupling capacitors CoL and
CoR, respectively. In the figure, subscripts R and L in the subscript indicate the right channel and
the left channel, respectively. In particular, when the left and right are not distinguished, the
suffix is removed and written. In this circuit, the resistors R (RL, RR) are connected, but these are
resistors that may be inserted for the purpose of improving the sound quality of the digital
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amplifier, and may be omitted. If this resistance is inserted, the power supplied to the speaker
(earphone) will be reduced accordingly.
[0027]
A common LDO 730 is connected to the output transistors of both channels of the digital
amplifier 700 as a power supply unit. Further, a power supply capacitor Cp is provided between
the output terminal of the LDO and the ground.
[0028]
As described above, since the common power supply unit is directly connected to the output
transistors of both channels, if a phenomenon that causes the power supply voltage fluctuation in
one channel occurs, the other channel is easily affected. There is. As a result, channel separation
is considered to be worse as described above.
[0029]
In the circuit of FIG. 7, the output waveform of the digital amplifier 700 also becomes a
rectangular waveform as shown in FIG. 8A by PWM. When this waveform is at the high level, the
upper transistor of the digital amplifier is turned on, and the lower transistor is turned off, so a
current Io (IoL) flows as indicated by an arrow 71. Conversely, when the waveform is low, the
transistor on the upper side of the digital amplifier is off and the transistor on the lower side is
on, so the current Ir (IrL) is in the direction of arrow 72 Flow. When such a PWM waveform is
passed through a filter made of LC, it is smoothed to obtain an audio waveform as shown in FIG.
8 (b).
[0030]
In the pumping phenomenon, when the current is flowing in the direction of the arrow 72
opposite to the arrow 71 at Low level and the voltage level suddenly becomes High, the current
flows in the direction of the arrow 72 as before due to the nature of the inductor L The current
flows as indicated by arrow 72 and is generated by flowing into the power supply through the
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upper transistor of the digital amplifier. As shown in FIG. 8A, when the low level period is long,
an electromotive force generated at L is added to the original power supply voltage, the voltage
rises in the high pulse period, and this flows back to the power supply through the transistor.
[0031]
In order to prevent such a phenomenon, it can be easily considered that one means is to absorb
the current flowing back from the inductor L by increasing the capacity of the power supply
capacitor as described above. As another means, although it is effective to reduce the value of L
to reduce the electromotive force of the inductor, it causes problems in the characteristics of the
LC filter, which is the original role, and lowers the circuit impedance. Because the audio
frequency characteristics may be affected, the value of L can not be reduced significantly.
[0032]
When a resistor R is connected in series to the output as shown in FIG. 7, if the resistance is
increased, the reverse current is also limited by this resistance, so it is possible to reduce the
influence of the pumping phenomenon. However, as mentioned earlier, increasing this resistance
directly affects the audio output level, so it can not be made too large.
[0033]
Based on these studies, as shown in FIG. 9, an output circuit of a circuit system was newly
created that suppresses (absorbs) an electromotive voltage generated in the inductor L. In FIG. 9,
the same elements as those shown in FIG. 7 carry the same reference numerals for which
duplicate descriptions are to be omitted. The output circuit of FIG. 9 adds a series circuit (filter)
90 of a resistor Rc and a capacitor Cc to the ground in each stage before the low pass filter 715
in the circuit of FIG. 7, that is, before the inductor L. It is
[0034]
In the circuit of FIG. 7, when back electromotive force is generated in the inductor L, it becomes a
problem that in the original circuit, a path through which the electromotive current escapes can
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only go around the power supply through the transistor. On the other hand, in the circuit of FIG.
9, the RC filter 90 functions as a relief of the electromotive current by the inductor L.
[0035]
Since the output of a digital amplifier is usually a PWM signal with a frequency of several
hundred KHz or more, adding an RC filter 90 at this point may affect the audio output
characteristics, power consumption, etc. depending on the filter constant. Care should be taken in
selecting the constant of the filter 90. The operating voltage and the PWM frequency can be
taken into consideration and experimentally determined as required.
[0036]
In the study of the inventor, an experiment was conducted by temporarily adding an RC filter 90
to both the left and right channels as R = 22Ω and C = 0.01 uF as shown in FIG. The results are
shown in FIG. 10 and FIG. FIG. 10 shows distortion factor characteristics of the audio signal. In
this graph, the distortion factor at 20 Hz is 0.4% or less, which is a significant improvement as
compared with the original circuit in which the low band distortion factor in FIG. 5 is 10% or
more. (Note that the scale of the vertical axis is different in the graphs of FIGS. 10 and 5 and the
graph of FIG. 10 is shown enlarged in the vertical direction).
[0037]
FIG. 11 shows the measurement results of channel separation. As can be seen from this graph,
channel separation is also improved as compared with the original circuit, and the addition of the
RC filter 90 suppresses the pumping phenomenon to a substantially negligible level. The power
supply voltage was also measured, but even with this measurement, the power supply voltage
rise due to the pumping phenomenon could not be observed.
[0038]
In the measurement result of the channel separation shown in FIG. 11, the characteristic of the
upper graph inputted with the signal substantially represents the frequency characteristic. This is
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10
almost the same characteristic as the original characteristic, and there is almost no deterioration
of the frequency characteristic due to the insertion of the RC filter 90. Also, the audio signal
output level was compared with the original characteristics, but there was no particular
difference in this example.
[0039]
In this example, when the increase in current consumption due to the insertion of the RC filter 90
is examined, an increase of about 10 mA is recognized, and the current consumption and the
distortion factor improvement value are considered to be in a trade-off relationship. Therefore, it
has also been confirmed that the distortion factor is also improved to about several percent while
the increase of the current consumption is suppressed by 2 to 3 mA by adjusting the constant of
the RC filter 90 also in practice. Therefore, it is considered that the output circuit of this
embodiment can be used effectively for portable devices using digital amplifiers.
[0040]
FIG. 12 shows a schematic device configuration of an audio reproduction device adopting the
digital amplification device of the present invention. The audio reproduction device is preferably
a portable terminal such as a portable telephone terminal or a portable music player.
[0041]
The control unit 100 is a unit that controls each unit of the audio reproduction apparatus, and
typically includes a CPU, a ROM, and a RAM, and performs program control of each unit. The
audio reproduction unit 101 is an audio reproduction unit that reproduces a recording medium
such as an MD or a CD. Alternatively, or in addition to this, a broadcast receiving unit that
receives an audio signal by broadcasting, or a data communication unit that receives an audio
signal via a communication network may be provided. The audio decoding unit 103 is a part that
normally decodes an audio signal in a form of compression coding. The decoded audio signal in
the present invention is converted to a PWM signal and input to the digital amplification device
105. The digital amplification device 105 has the above-described output circuit portion, and the
output thereof is supplied to the speaker 107. The audio reproduction apparatus further includes
an operation unit 111 for receiving user operations and data inputs, a display unit 113 for
displaying information to the user, and a power supply unit 115 for supplying power to each unit
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of the apparatus. When the audio reproduction unit 101 performs stereo reproduction, one pair
of the audio decoding unit 103, the digital amplification device 105, and the speaker 107 is
provided.
[0042]
Digital amplifiers are often provided as ICs. Although the RC filter 90 is added as an external
circuit in the circuit of FIG. 6, it is also possible in some cases to incorporate this circuit into a
digital amplifier IC. In this case, if the value of the filter resistance Rc can be set from the outside
of the IC, the user may consider the current consumption and the distortion factor as needed and
select an appropriate value according to the use situation. It becomes possible.
[0043]
By the way, in the audio playback apparatus, in the state where the volume is reduced (the
volume is small), it is difficult for the pumping to occur. Therefore, the resistance Rc built in the
audio processing IC may be used as a variable resistor to automatically detect the abovementioned state and increase the resistance value of the resistor Rc. As a result, the current
consumption can be further reduced when the volume is low.
[0044]
FIG. 13 shows the configuration of an audio reproduction apparatus according to an application
of the present invention. This apparatus includes a CPU 200, a memory 201, an audio processing
IC 210, a low pass filter 715, a speaker 717, and an RC filter 90. In the illustrated example, only
one channel is shown, and the configuration of the other channel is omitted. The audio
processing IC 210 includes a decoder 211 for decompressing compressed audio data, a CPU
interface 212 for interfacing with the CPU 200, a control unit 213 for controlling the audio
processing IC under control of the CPU 200, and a volume for adjusting volume. It includes a
volume control unit 214 including, an effector 215 for setting various sound effects, a digital
amplifier 216 according to the present invention, and a resistor Rc. The resistance Rc of the RC
filter 90 is built in the audio processing IC 210, and the control unit 213 can control its
resistance value. Although the capacitor Cc of the RC filter 90 is externally attached, it may be
incorporated in the IC depending on the capacitance value.
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[0045]
Normally, when playing back compressed music data in the memory 201, music k data is
transferred from the memory 201 to the audio processing IC 210 under the control of the CPU
200, decompressed by the decoder 211, and digitally through the volume control unit 214 and
the effector 215. After being amplified by the amplifier 216, the signal is output to the outside.
[0046]
As described above, the audio processing IC 210 can control both the volume adjustment unit
214 and the resistor Rc for digital amplifier correction from the CPU 200.
However, the resistance value of the resistor Rc can be automatically determined by the volume
setting of the volume adjustment unit 214 without the control from the CPU 200. In this way, the
control algorithm is simpler than separately controlling. As an example of the control method, for
example, a method of dividing the range of the sound volume into three stages of large, medium,
and small can be considered. That is, the control unit 213 determines the current volume range,
and if the volume is "high", the amount of pumping increases, so the control unit 213 lowers the
resistance value (for example, about 33 Ω) and sets the volume In the case of “medium”, the
resistance value is increased (for example, 330Ω), and when the volume is “low”, the
resistance value is further raised to be the settable maximum value. The resistance value
illustrated here also depends on the value of the correction external capacitor Cc. Therefore, the
designer can select a mode with high power consumption but a high sound quality by increasing
the value of the capacitor Cc and a mode with low power consumption but high power can not be
expected by decreasing the capacitor value. It is possible. In addition, when the capacitor Cc is
switchable, both modes can be selected by the user.
[0047]
Although the preferred embodiments of the present invention have been described above,
various modifications and changes can be made other than those mentioned above. For example,
although the circuit configuration of both left and right channels corresponding to stereo
reproduction has been described, the present invention is not limited to stereo reproduction and
may be a circuit configuration of a single channel. The connection order of the resistors and
capacitors of the RC filter 90 may be reversed from that shown in the drawing.
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[0048]
It is a figure which shows the output circuit part of the conventional digital amplifier for the
speaker drive of half bridge type | mold and full bridge type | mold. It is explanatory drawing of
the pumping phenomenon in a digital amplifier. FIG. 5 is a diagram showing an output circuit
portion of a single-ended digital amplification device for connection of earphones. It is a graph
which shows the voltage rise of the power supply voltage of an experiment measurement using a
digital amplifier by the structure shown in FIG. It is a graph which shows the audio distortion
factor characteristic in the experiment measurement of FIG. It is a graph of the channel
separation in the experiment measurement of FIG. It is a figure showing an example of
composition of a digital amplification device (output circuit portion) contrasted with a digital
amplification device of an embodiment of the invention. It is a wave form diagram which shows
the output waveform of a digital amplifier, and the audio waveform corresponding to this. It is a
figure which shows the structure of the digital amplification apparatus of the audio reproduction
apparatus in embodiment of this invention. It is a graph which shows the distortion factor
characteristic of the audio signal in the circuit of FIG. It is a graph which shows the measurement
result of the channel separation in the circuit of FIG. It is a figure which shows the general |
schematic apparatus structure of the audio reproduction apparatus which employ | adopted the
digital amplification apparatus of this invention. It is a figure which shows the structure of the
audio reproduction apparatus regarding the application example of this invention.
Explanation of sign
[0049]
DESCRIPTION OF SYMBOLS 90 ... RC filter, 100 ... Control part, 101 ... Audio reproduction part,
103 ... Audio decoding part, 105 ... Digital amplification apparatus, 107 ... Speaker, 111 ...
Operation part, 113 ... Display part, 115 ... Power supply part, 210 ... Audio Processing IC, 700:
digital amplifier, 711: PWM generator, 715: low pass filter, 717: speaker
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