JPH11146301A - Power supply circuit for video signal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the power supply circuit capable of obtaing a high quality video by digital signal processing. SOLUTION: A control circuit 22 of the power supply circuit 1 controls a switching element 11 so that it is turned off for a video signal period and turned on for a horizontal blanking period. The energizing period of the switching element 11 is set to a period in response to a counted value of a counter 45. A comparator 411 (412 ) compares a sampling voltage Vs corresponding to a DC output voltage VOUT with an upper limit voltage Vref1 and lower limit voltage Vref2 to control the counted value depending on the comparison result. Since no analog processing is conducted, the operation is made stable. Since the switching element 11 is energized within the horizontal blanking period, no noise intrudes into the video signal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply circuit used for an electronic device handling video signals, and more particularly to a power supply circuit for obtaining a high-quality image.

[0002]

2. Description of the Related Art In general, switching type power supply circuits are classified into three types: a boost type for increasing an input voltage, a step-down type for decreasing an input voltage, and a polarity inversion type for inverting the polarity of an input voltage. Power supply circuits of the type are also widely used in electronic devices. Among the switching power supply circuits of the prior art, a step-down power supply circuit is indicated by reference numeral 101 in FIG.

The power supply circuit 101 has a p-channel MOS
It has a switching element 111 composed of a transistor, an inductance element 112 connected in series to the switching element 111, and a control circuit 115 for operating the switching element 111. When the control circuit 115 makes the switching element 111 conductive, The inductance element 112 is connected to the input side power supply 118, and the input voltage _Vb is applied to the inductance element 112.

At this time, the diode element 114 connected between the ground voltage line and the drain terminal of the switching element 111 is reverse-biased, and the input-side power supply 11
The current i ₁ supplied from 8 is the inductance element 112
, And supplied to the load and the smoothing capacitor 113.

The control circuit 115 controls the switching element 11
When 1 is changed from conduction to interruption, the electromotive force generated in the inductance element 112 causes the diode 114 to be forward-biased, and the magnetic energy stored in the inductance element 112 is converted into a current i ₂ and supplied to the load and the smoothing capacitor 113. Is done.

When the magnetic energy stored in the inductance element 112 disappears from this state, the smoothing capacitor 113 discharges, and a current is supplied to the load.
When 11 is turned on again, power is supplied from the input side power supply 118 to the load and the smoothing capacitor 113.

As described above, the control circuit 115 causes the switching element 111 to repeat conduction and interruption, thereby supplying power to the load. The control circuit 115 detects a DC output voltage appearing on the smoothing capacitor 113. By controlling the conduction period of the switching element 111, the DC output voltage is stabilized at a predetermined value.

However, in the above-described power supply circuit 101 of the related art including the power supply circuit of the boost type and the polarity inversion type, an analog feedback loop is formed using an amplifier such as an operational amplifier, and the DC output voltage is set to a predetermined value. The conduction period of the switching element 111 is changed so as to be stable, and the time constant setting of the feedback loop is troublesome so that the feedback loop operates normally and malfunctions such as oscillation do not occur. .

In a switching type power supply circuit,
Noise is generated when the switching element repeats conduction and cutoff. In particular, when the switching power supply circuit supplies power to the electronic circuit that handles the video signal, the noise enters the video signal included in the video signal. However, there is a problem that image quality is deteriorated.

[0010]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned disadvantages of the prior art, and has an object to prevent noise accompanying a switching operation from being superimposed on a video signal of a video signal. An object of the present invention is to provide a power supply circuit suitable for an electronic circuit that handles video signals.

[0011]

According to a first aspect of the present invention, there is provided a video signal including a video signal period and a horizontal blanking period, wherein a switching element is cut off during the video signal period. A control circuit for conducting the switching element during at least the horizontal blanking period, configured to convert a current flowing through the switching element into a DC output voltage and supply the DC output voltage to an electronic circuit that handles the video signal. A power supply circuit, wherein the control circuit sets a conduction period of the switching element during the horizontal blanking period to a time corresponding to a count value stored in a counter, and the DC output voltage And a comparator for comparing a sample voltage corresponding to a predetermined reference voltage with the DC output based on a comparison result of the comparator. As pressure becomes a predetermined value, and having a counter operation circuit for operating the count value.

According to a second aspect of the present invention, in the power supply circuit according to the first aspect, the reference voltage includes a lower limit voltage and an upper limit voltage, and the comparator compares the sample voltage with the lower limit voltage and the upper limit voltage. In comparison with the counter operation circuit,
When the sample voltage is outside the range of the voltage constituted by the lower limit voltage and the upper limit voltage, the count value is increased or decreased.

According to a third aspect of the present invention, in the power supply circuit according to any one of the first to second aspects, the conduction period setting circuit sets the conduction period of the switching element to:
The apparatus is configured to set a time corresponding to a value obtained by multiplying a unit time determined by a clock signal by the count value.

According to a fourth aspect of the present invention, there is provided the power supply circuit according to any one of the first to third aspects, wherein the conduction period setting circuit is configured to output one of a start time and an end time of the conduction period. One time is fixed, and when the count value is increased or decreased, the other time is changed.

According to the present invention, the switching element is turned off during the video signal period in which the video signal is transmitted from the video signal, and the switching element is turned on every horizontal blanking period, so that the video signal is transmitted. The DC output voltage is supplied to the electronic circuits handled.

The conduction period of the switching element is configured to be a time corresponding to the count value stored in the counter, and the comparator outputs a sample voltage corresponding to the magnitude of the DC output voltage to a predetermined reference voltage. The count value is manipulated according to the result, and the conduction period is corrected so that the sample voltage approaches the reference voltage. As a result, even when the DC output voltage fluctuates, the DC output voltage is stabilized at a predetermined voltage.

Therefore, an amplifier for processing an analog signal is not required, and since the sample voltage is digitally processed by the comparator and the counter, there is no danger of malfunction such as oscillation.

The reference voltage is composed of a lower limit voltage and an upper limit voltage, and a comparator compares the sample voltage with the lower limit voltage and the upper limit voltage, respectively. If it is outside, the count value is increased or decreased, and if it is within the range of the voltage, the count value is not changed, thereby improving the stability of the DC output voltage.

Since a high-speed clock signal is generated in an electronic device that handles a video signal, using the clock signal reduces the conduction period of the switching element.
The time can be set according to a value obtained by multiplying the unit time determined by the clock signal by the count value.

In the above power supply circuit, since the switching element needs to conduct during the horizontal blanking period and to change from conduction to cutoff before the end of the horizontal blanking period, the start time or the end time of the conduction period is required. If one of the times is fixed within the horizontal blanking period, and the count value is changed, the other time is changed within the horizontal blanking period, so that the conduction period is changed according to the count value. It can be set.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference numeral 1 in FIG. 1 denotes a power supply circuit according to an example of the present invention, which includes a switching element 11, an inductance element 12, a smoothing capacitor 13, and a diode 1.
4, a drive circuit 21, a control circuit 22, and a clock signal generator 23.

The switching element 11 is a p-channel MO
The source terminal is connected to the input-side power supply 18 to which the input voltage _Vb is applied. Further, the gate terminal of the switching element 11 is connected to the drive circuit 21, and the drain terminal is connected to the cathode terminal of the diode 14 and one end of the inductance element 12.

The other end of the inductance element 12 is connected to an electronic circuit 29 which is a load of the power supply circuit 1 and a smoothing capacitor 1 having one terminal connected to a ground voltage line.
3 is connected to the other terminal.

On the other hand, the anode terminal of the diode 14 is connected to the ground voltage line,
When a ground voltage is applied to the gate terminal of the switching element 11 to make the switching element 11 conductive, one end of the inductance element 12 is connected to the input-side power supply 18 while the diode 14 is reverse-biased. A current is supplied from 18 to the electronic circuit 29 and the smoothing capacitor 13 to generate a DC output voltage V _OUT .

In this state, when the drive circuit 21 applies a high voltage to the gate terminal and switches the switching element 11 from conduction to interruption, the electromotive force generated in the inductance element 12 causes the diode 14 to be forward-biased, Is converted into a current and supplied to the smoothing capacitor 13 and the electronic circuit 29.

When the energy stored in the inductance element 12 disappears, the current supplied to the smoothing capacitor 13 and the electronic circuit 29 is stopped, and the current is supplied to the electronic circuit 29 by discharging the smoothing capacitor 13. .

FIG. 2 shows an example of the electronic circuit 29. The electronic circuit 29 includes a CCD circuit 61 and an AD converter 6.
2 and a driver 67. The CCD circuit 61 is driven by the driver 67, and is configured to capture incident light as image data.

The image data captured by the CCD circuit 61 is output as an analog signal.
After removing noise from the analog signal, the signal is converted into a digital signal, and is output to the DSP circuit 65 under the control of the logic circuit (ASIC) 63.

The digital signal input to the DSP circuit 65 is subjected to image processing according to a program stored in the memory 64, and is output to the DA converter 68 and the protocol circuit 69 via the logic circuit 63. The digital signal subjected to the image processing is converted into an analog signal by the DA converter 68, transmitted under the control of the protocol circuit 69, and displayed on the display device on the receiving side.

The maximum output of the analog signal output from the CCD circuit 61 is as small as 100 mV.
The AD converter 62 converts the input signal into a digital signal after voltage amplification. When noise enters a signal processing system that handles such a weak analog signal, the signal is amplified together with the analog signal and converted into a digital signal, so that a finally obtained image is deteriorated.

In the power supply circuit 1 of the present invention, the driving circuit 21 for operating the switching element 11 is controlled by the control circuit 22. The driving circuit 21 controls the switching element 11 only during a period that does not affect image data. It is configured to conduct.

The power supply circuit 1 has two resistors 27 and 28 and the electronic circuit 2
9, the DC output voltage V _OUT is connected to its resistor 27,
28 is divided by the sample voltage V _S that is proportional to the DC output voltage V _OUT are generated, the sample voltage V _S is supplied to the control circuit 22.

The control circuit 22 includes two comparators 4
1 _1, and 41 _2, the reference voltage generator 42 _1, 42 _2, a counter operating circuit (decoder) 44, a counter 45, the conduction period setting circuit 46 and is provided, the input sample voltage V _S Is supplied to the non-inverting input terminals of the comparators 41 ₁ and 41 ₂ .

[0034] to the inverting input terminal of the comparators 41 _1, 41 _2, the reference voltage is inputted to the reference voltage generator 42 _1, 42 ₂ outputs, so as determine the magnitude relationship between the sample voltage V _S ing.

The reference voltage generator 42 _1, 42 reference _{voltage 2} is output, the upper limit voltage V _ref1, is constituted by a low voltage lower limit voltage _{V ref2 (V ref2 <V ref1} ) than the upper limit voltage, Assuming that one comparator 41 ₁ is on the upper bit side and the other comparator 41 ₂ is on the lower bit side, the upper limit voltage V _ref1 is input to the upper bit side comparator 41 ₁ and the lower limit voltage V _ref
2 is inputted to the comparator 41 _{and second} low-order bit side.

Assuming that the output of the comparators 41 ₁ and 41 ₂ is high by “1” and low by “0”, the sample voltage V _S is higher than the upper limit voltage V _ref1 (V _ref1).
<V _S ), “11” is output. If the sample voltage V _S is within the range of the voltage constituted by the upper limit voltage V _ref1 and the lower limit voltage V _ref2 (V _ref2 ≦ V _S ≦ V _ref1 ), “01” is output. Is output, and when the sample voltage V _S is smaller than the lower limit voltage V _ref1 (V _s <V _ref1 ), “00” is output.

The outputs of the comparators 41 ₁ and 41 ₂ are input to a counter operation circuit 44. The counter operation circuit 44 is configured to operate the count value in the counter 45 according to the input signal. In the circuit 44, "1"
When "1" is input, the count value is increased by "1",
When “00” is input, it is reduced by “1” and “01”
Is configured not to be increased or decreased when is input.

The count value in the counter 45 is read by a conduction period setting circuit 46. The conduction period setting circuit 46 sets one cycle period of the clock signal input from the clock signal generator 23 as a unit time. A point in time corresponding to a value obtained by multiplying the unit time by the count value is set at the start of conduction of the switching element 11, and the switching element 11 is made conductive through the drive circuit 21.

The timing of the conduction period will be described with reference to FIG. Referring to FIG. 3, reference numeral 50 denotes a video signal, and a video signal included in the video signal 50 is output within a video signal period T _A and a horizontal blanking period T
It is assumed that no signal is output within _B (10 μsec).

Further, reference numeral 51 in the horizontal blanking period T _B, a signal indicating a period when the switching element 11 is capable conduction, from the time h _max initiating the horizontal blanking period T _B, a video signal The signal is output until time _hstop at which the horizontal synchronization signal included in 51 ends. Therefore,
The maximum value of the conduction time of the switching element 11 is the time h
It is represented by _Tmax between _max and time _hstop .

In the control circuit 22, the time at which the conduction of the switching element 11 ends is fixed to the time h _stop , and the time h _{ON at} which the switching element 11 starts conduction is set.
Can be changed.

Then, time h_stopThan the clock signal
Time h earlier by one period_minIs set and the switch
Conduction time h of the switching element 11_ONIs the time h_minFrom time
Time_{s top}It is not possible to set it during. Follow
And the conduction start time h_ONIs h_minWhen matches
Conduction period T of the switching element 11_ONIs the minimum conduction period T
_minbecome. Conduction period T_ONIs the maximum value T_maxWhen it comes to
Time_ONIs time h_maxWhen it matches.

Reference numeral 56 denotes a waveform of the gate voltage of the switching element 11, which becomes low during the conduction period T _ON to make the switching element 11 conductive. Symbol 57 is
Shows the waveform of the drain voltage of the switching element 11, the switching element 11 is conductive, the drain voltage rises to the input voltage V _b, Turning to cut off from the conductive,
The diode 1 is generated by the electromotive force of the inductance element 12.
4 is forward biased and falls to a negative voltage.

The conduction period T _ON of the switching element 11 is
h _max point unit time (1 cycle period of the clock signal)
The have time to time h _ON corresponding to the value obtained by multiplying the count values S ₁ is set as a period obtained by subtracting the period T _max, in the horizontal blanking period T _B, the switching element 11 is conductive only that period Shall be. And
Then before the switching element 11 is conductive, the sample voltage V _s is compared with a reference voltage, as a result, when the "11" from the counter operation circuit 44 is output, the new count value S ₂ is

S ₂ = S ₁ +1. When “01” is output, S ₂ = S _{1. When} “00” is output, S ₂ = S ₁ −1. , the conduction period setting circuit 46, according to the count value S _2, move the conduction start time h _ON. Thus, the conduction period of the switching element 11 becomes a period obtained by subtracting the period obtained by multiplying the unit time count value S ₂ from the time T _max.

[0046] Thus, the count value indicating the conduction start time h _ON of the switching element 11, so being reviewed every horizontal blanking period T _B, and when the power circuit 1 starts operating, the load change When the sample voltage V _S becomes a voltage outside the range of the reference voltage, the voltage gradually approaches the reference voltage by the unit of conduction for each conduction period T _ON . Then, the sample voltage V _S is stabilized when it reaches a voltage between the lower limit voltage V _ref1 and the upper limit voltage V _ref2 , so that the output voltage V _OUT eventually stabilizes at a predetermined voltage.

Further, since the conduction period T _ON is a horizontal blanking period T in _B, and when the switching element 11 becomes conductive, even when the noise is generated when the turn to cut off from the conductive, included in the video signal 50 It does not affect the video signal.

Although the power supply circuit 1 is of a step-down type, it may be of a step-up type or a polarity inversion type as shown by reference numerals 2 and 3 in FIGS. 5 (a) and 5 (b).

In FIGS. 5A and 5B, the same circuit elements as those in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted.
In the step-up type power supply circuit 2 of FIG.
One end of 2a is connected to the input side power supply 18, the other end is connected to the anode terminal of the diode 14a, and the cathode terminal of the diode 14a is connected to an electronic circuit 29 as a load.

The switching element 11a formed of an n-channel MOS transistor has a drain terminal connected to the anode terminal of the diode 14a, a source terminal connected to the ground voltage line, and a gate terminal connected to the switching terminal 11a.
It is connected to a control circuit 22 via a drive circuit 21 and is configured such that the conduction period is controlled as in the power supply circuit 1 described above.

The voltage waveform at the gate terminal of the switching element 11a is indicated by reference numeral 56a in FIG. 4A, and the voltage waveform at the drain terminal is indicated by reference numeral 57a in FIG. When the gate voltage 56a becomes high and the switching element 11a conducts, the diode 14a is reverse-biased and a current flows through the inductance element 12a.

When the conduction changes from the conduction state to the interruption state, a voltage in which the electromotive force of the inductance element 12a is superimposed on the output voltage _Vb of the input side power supply 18 is applied to the anode terminal of the diode 14a, and the diode 14a is forward-biased. The magnetic energy stored in the element 12a is converted into a current and supplied to the electronic circuit 29 and the smoothing capacitor 13.
At this time, since the electromotive force generated in the inductance element 12a is superimposed on the input voltage _Vb , the DC output voltage V _OUT of the power supply circuit 2 is boosted. This power supply circuit 2
But, the switching element 11a is turned on every horizontal blanking period T _B of the video signal, the output voltage V _OUT by conduction period is operation is stabilized.

The power supply circuit 3 of the polarity inversion type shown in FIG.
The source terminal of the switching element 11b composed of a channel MOS transistor is connected to the input side power supply 18,
The drain terminal is connected to the cathode terminal of the diode 14b, and the anode terminal of the diode 14b is connected to the electronic circuit 29.
And the capacitor 13. One end of the inductance element 12b is connected to the drain terminal of the switching element 11b, and the other end is connected to the ground voltage line.

The waveform of the gate voltage of the switching element 11b is indicated by reference numeral 56b in FIG. 4B, and the waveform of the drain voltage is indicated by reference numeral 57b in FIG. When the gate voltage 56b becomes low and the switching element 11b conducts, the diode 14a is reverse-biased, and a current flows through the inductance element 12b. Then, when the gate voltage becomes high and the switching element 11b changes from conduction to interruption, the diode 14b is forward-biased by the electromotive force of the inductance element 12b, and the electronic circuit 29 is set to a negative potential.

[0055] In this switching power supply 3, similarly to the switching power supply 1, the switching element 11b are adapted to conduct at the horizontal blanking period T _B, that the conduction period is controlled, the output voltage V _OUT is stabilized.

In the above power supply circuits 1 to 3, the switching elements 11, 11a, and 11b increase or decrease the conduction period using one cycle of the clock signal as a unit time. However, the present invention is not limited to this. A unit time may be a period of a half cycle of the clock signal or two or more cycles. Further, the end time of the conduction time may not be fixed, but the start time may be fixed, and the end time may be changed when the count value is increased or decreased.

Further, the upper limit voltage V _ref1 and the lower limit voltage V
_In addition to _ref2 , a voltage for comparing the sample voltage V _S is provided,
The conduction period may be finely controlled.

[0058]

Since no analog processing is performed, there is no need to set the time constant of the circuit. Since the switching element conducts during the horizontal blanking period, a high-quality image can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an example of a power supply circuit of the present invention.

FIG. 2 is a block diagram showing an example of an electronic circuit serving as a load of the power supply circuit;

FIG. 3 is a timing chart for explaining the operation of the power supply circuit of the present invention.

4A is a timing chart showing a drain waveform of a power supply circuit according to a second embodiment of the present invention; FIG. 4B is a timing chart showing a drain waveform of a power supply circuit according to a third embodiment of the present invention;

5A is a block diagram of a power supply circuit according to a second embodiment of the present invention. FIG. 5B is a block diagram of a power supply circuit according to a third embodiment of the present invention.

FIG. 6 is a block diagram showing an example of a conventional power supply circuit.

[Explanation of symbols]

1, 2, 3 ... power supply circuit 11, 11a, 11b ... switching elements 41 ₁ , 41 ₂ ... comparator 44 ...
... Counter operation circuit 45 ... Counter 46 ...
Conduction period setting circuit 50: Video signals S ₁ , S ₂
...... Count value T _A ...... Video signal period T _B ...... Horizontal blanking period h _ON ...... Start time of conduction period h _stop ...... End time of conduction period V _S ...... Sample voltage V _{ref 1} ......
Upper limit voltage V _ref2 …… Lower limit voltage V _OUT …… DC output voltage

Claims (4)

[Claims] 1. A control circuit for shutting off a switching element during a video signal period in a video signal including a video signal period and a horizontal blanking period, and turning on the switching element during at least the horizontal blanking period. A power supply circuit configured to convert a current flowing through the switching element into a DC output voltage and supply the DC output voltage to an electronic circuit that handles the video signal, wherein the control circuit performs the switching during the horizontal blanking period. A conduction period setting circuit for setting a conduction period of the element to a time corresponding to a count value stored in the counter; a comparator for comparing a sample voltage corresponding to the DC output voltage with a predetermined reference voltage; and the comparator Counter operation for operating the count value such that the DC output voltage becomes a predetermined value based on the comparison result of And a power supply circuit having the circuit. 2. The reference voltage includes a lower limit voltage and an upper limit voltage, the comparator compares the sample voltage with the lower limit voltage and the upper limit voltage, and the counter operation circuit determines that the sample voltage is lower than the lower limit voltage. 2. The power supply circuit according to claim 1, wherein the count value is increased or decreased when the count value is out of a range of a voltage constituted by a voltage and the upper limit voltage. 3. The circuit according to claim 1, wherein the conduction period setting circuit is configured to set the conduction period to a time corresponding to a value obtained by multiplying a unit time determined by a clock signal by the count value. Item 3. The power supply circuit according to any one of items 2. 4. The conduction period setting circuit is configured to fix one of a start time and an end time of the conduction period, and to change the other time when the count value is increased or decreased. The power supply circuit according to claim 1.