JP2003244936A - Power unit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a power unit which prevents nonconformity arising in starting the supply of a power voltage to a load, and also can prolong a battery lifetime, in a power circuit using a battery. <P>SOLUTION: This power unit applies a rising step voltage, which rises in a stair form, to a drive voltage for controlling it in such a conduction state as to let a drain terminal output power voltage to the gate terminal of an FETQ4, when starting the supply of the power voltage to the load 20. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply device, and more particularly to a power supply device that supplies power to a digital camera or the like.

[0002]

2. Description of the Related Art Conventionally, a power supply device of this type is known as shown in FIG. As shown in the figure, the power supply device 10 includes a battery 11 that outputs a battery voltage,
A DC / DC converter 12 that boosts the battery voltage output from the battery 11 and outputs it as a power supply voltage;
P-ch MOSFET Q1 (hereinafter, FET Q1) for turning on / off the supply of the battery voltage to the C converter 12
It has and.

On / off of the FET Q1 is controlled by a controller 12a via a transistor Q2. That is, when an H level control signal is output from the control terminal T1 of the controller 12a, the transistor Q
2 turns on. By turning on this transistor Q2,
Since the gate voltage is applied to the gate terminal of the FET Q1, conduction is established between the source terminal and the drain terminal of the FET Q1, and supply of the battery voltage to the DC / DC converter 12 is started.

On the other hand, when the output of the H level control signal from the control terminal T1 is stopped, the transistor Q2 is turned off. By turning off the transistor Q2, the FET Q1
Since the application of the gate voltage to the gate terminal of the FET is stopped, the source terminal and the drain terminal of the FET Q1 become non-conductive, and the supply of the battery voltage to the DC / DC converter 12 is stopped.

A load 20 is connected in series to the power supply device 10, and the power supply device 10 supplies the power supply voltage to the load 20. Further, a capacitor C2 for removing the influence of noise and ripple on the power supply voltage is connected in parallel to the load 20.

Next, the detailed structure of the DC / DC converter 12 will be described. DC / DC converter 12
Is an inductor L1 that stores electric energy from the battery 11, a diode D1 for preventing backflow, a smoothing capacitor C1, and resistors R3 and R4 for detecting a power supply voltage.
And a contact point between the inductor L1 and the diode D1 and a switch element Q3 provided between the ground.

Therefore, when the battery voltage is supplied from the battery 11 to the DC / DC converter 12, the switching element Q3
When is turned on, a current flows through the inductor L1 and electric energy is stored. On the other hand, when the switch element Q3 is turned off, the electric energy stored in the inductor L1 is sent to the load 20 side through the diode D1.

The DC / DCIC 12b has a resistor R
3 is an IC that controls on / off of the switch element Q3 based on the power supply voltage detected by using R3 and R4 to obtain a desired power supply voltage. This DC / DCIC 12b
Starts ON / OFF control of the switch element Q3 in response to the output of the control signal from the control terminal T2 of the controller 12a, and stops ON / OFF control of the switch element Q3 in response to the stop of the output of the control signal.

[0009]

However, in the power supply device 10 described above, when the supply of the power supply voltage to the load 20 is started, at the same time when the FET Q1 is turned on, the DC / D
When the control by the CIC 12b is started and the battery voltage is boosted, the DC / DC converter 12 charges the capacitor C2 and operates at full power until a desired power supply voltage is obtained, so a rush current flows to the load 20 side. Will end up.

In the above power supply device 10, since the internal resistance of the battery 11 is not zero, a voltage drop occurs when a rush current flows to the load 20 side, and a supply necessary for the operation of the load 20 and the DC / DC converter 12 is generated. If the voltage falls below the lower limit of the voltage, the power supply to the load 20 is interrupted. When the load is a control element such as a CPU, this causes a serious problem such as a stop of processing or a runaway. Further, there is a problem that the rush current consumes a large capacity of the battery 11 and shortens the life of the battery 11.

Particularly, in the digital camera, the consumption of the battery 11 due to the rush current is fatal. In other words, the capacity of the built-in battery is limited in the digital camera due to the size of the camera. For example, a laptop PC or a video camcorder may be equipped with a battery such that the battery terminal voltage does not drop significantly even when a large rush current flows. Have difficulty. Further, if an AC adapter is used, it has sufficient supply capability and can prevent a large drop in the voltage of the power input terminal, but this is impractical because of portability.

Therefore, in order to reduce the rush current, a method is considered in which the output voltage from the power supply device 10 has two levels. This method will be described below with reference to the time chart of FIG. In the figure, (a) is shown in FIG.
The voltage at point a, (b) is the output current of the battery 11,
(C) is the output of the control terminal T1, and (d) is the control terminal T1.
2 output is shown.

First, the controller 12a is a DC / DC
Before activating the IC 12b, an H-level control signal is output from the control terminal T1 (see FIG. 6C), and DC / DC
The battery voltage of the battery 11 is supplied to the converter 12. In response to the supply of the battery voltage, the voltage at the point a, which is the output of the DC / DC converter 12, becomes the battery voltage (see FIG. 6A). Also, depending on the supply of the battery voltage,
Since the capacitor C2 is charged, a rush current is generated in the battery 11 (see FIG. 6 (b)).

However, in the conventional method, the rush current is from 0 V to a desired power supply voltage, but in this method, the battery voltage of the battery 11 is 0 V (<power supply voltage).
Since the rush current is up to, it can be made smaller than the conventional one. Next, the controller 12a outputs an H level control signal from the control terminal T2 (FIG. 6 (d)).
), And activates the DC / DCIC 12b.

The DC / DCIC 12b enters the normal operation period T5 after the dead period T3 from the output of the control signal to the startup and the soft start period T4 for slowly charging the capacitor C2.

By the way, as the DC / DC converter 12, not only the one using the above-mentioned inductor L1 but also the one using a transformer capable of coping with a load such as a CCD requiring two positive and negative power supplies. is there. However, if the output voltage from the power supply device 10 is 2
In the step-by-step method, the DC / D using the inductor L1 is used.
It can be applied only to the C converter 12. Even if this method is used, a rush current from 0 V to the battery voltage (<power supply voltage) of the battery 11 is generated.

Further, even when the power supply voltage to the load 20 is cut off, if the power supply voltage is reduced to 0 V at once, a rush current is generated, which causes a great drain of the battery and malfunction of the controller 12a. We also know that there will be defects.

In view of this, the present invention focuses on the above-mentioned problems and prevents problems that occur when the supply of the power supply voltage to the load is started or cut off.
In the case of a power supply circuit using a battery, it is an object to provide a power supply device that can extend the battery life.

[0019]

The invention according to claim 1 made in order to solve the above problems is a power supply device having a power supply circuit for outputting a DC power supply voltage, wherein the power supply circuit and the power supply are provided. A power supply side terminal that is provided between a load receiving a voltage and connected to the power supply circuit side, a load side terminal connected to the load side, and a conduction state between the power supply side terminal and the load side terminal. A semiconductor switch element having a control terminal for controlling the load, and the semiconductor switch element of the semiconductor switch element in a state where the power supply voltage is being output from the power supply circuit when starting to supply the power supply voltage to the load. A rising step voltage that applies to the control terminal a rising step voltage that rises stepwise toward the drive voltage that controls the conduction state such that the power supply voltage is output from the load side terminal. Resides in the power supply apparatus characterized by comprising a pressurizing means.

According to the first aspect of the present invention, the semiconductor switch element is provided between the power supply circuit that outputs a DC power supply voltage and the load that receives the power supply voltage, and is connected to the power supply circuit side. A power source side terminal, a load side terminal connected to the load side, and a control terminal for controlling a conduction state between the power source side terminal and the load side terminal. When the rising step voltage applying means starts supplying the power supply voltage to the load, the power supply circuit is outputting the power supply voltage,
To the control terminal of the semiconductor switch element, a step voltage that rises stepwise is applied toward the drive voltage that controls the conductive state such that the power supply voltage is output from the load side terminal.

Therefore, by applying a rising step voltage rising stepwise toward the drive voltage to the control terminal of the semiconductor switching element provided between the power supply circuit and the load, the load side of the semiconductor switching element is applied. The terminal voltage also rises stepwise, that is, gradually rises to reach the power supply voltage. Therefore, the voltage of the load side terminal does not reach the power supply voltage instantaneously, and the power supply voltage and the rush current at the start of supply can be reduced. Moreover, the rush current can be reduced regardless of the type of the power supply circuit by controlling the voltage supplied to the load side by using the semiconductor switch element provided on the downstream side of the power supply circuit.

According to a second aspect of the present invention, there is provided a power supply device having a power supply circuit for outputting a DC power supply voltage, the power supply device being provided between the power supply circuit and a load supplied with the power supply voltage. A semiconductor switch element having a power supply side terminal connected to a circuit side, a load side terminal connected to the load side, and a control terminal for controlling a conduction state between the power supply side terminal and the load side terminal, and the load On the other hand, when the supply of the power supply voltage is stopped, with the power supply voltage being output from the power supply circuit, with respect to the control terminal of the semiconductor switch element, the power supply side terminal-the load side terminal And a step-down voltage applying means for applying a step-down voltage that drops stepwise toward a non-conducting voltage for controlling the gap between them to be in a non-conducting state.

According to the second aspect of the present invention, the semiconductor switch element is provided between the power supply circuit that outputs a DC power supply voltage and the load that receives the power supply voltage, and is connected to the power supply circuit side. A power source side terminal, a load side terminal connected to the load side, and a control terminal for controlling a conduction state between the power source side terminal and the load side terminal. When the step-down voltage applying means stops the supply of the power supply voltage to the load, the power supply circuit is outputting the power supply voltage,
Power supply side terminal to the control terminal of the semiconductor switch element
A step-down voltage that drops stepwise toward a non-conducting voltage that makes the load-side terminals non-conducting is applied.

Therefore, by applying a descending step voltage stepwise descending toward the non-conducting voltage to the control terminal of the semiconductor switching element provided between the power supply circuit and the load, the load side of the semiconductor switching element is applied. The voltage of the terminal also becomes stepwise, that is, gradually decreases and reaches the non-conduction voltage.
Therefore, it is possible to reduce the power supply voltage and the rush current when the supply is stopped, and to prevent problems caused by the voltage at the load side terminal instantaneously dropping to the non-conducting voltage. Moreover, the rush current can be reduced regardless of the type of the power supply circuit by controlling the voltage supplied to the load side by using the semiconductor switch element provided on the downstream side of the power supply circuit.

According to a third aspect of the present invention, there is provided a power supply device having a power supply circuit for outputting a DC power supply voltage, the power supply device being provided between the power supply circuit and a load supplied with the power supply voltage. A semiconductor switch element having a power supply side terminal connected to a circuit side, a load side terminal connected to the load side, and a control terminal for controlling a conduction state between the power supply side terminal and the load side terminal, and the load. Then, when the supply of the power supply voltage is started, the power supply voltage is output from the load side terminal to the control terminal of the semiconductor switch element while the power supply voltage is being output from the power supply circuit. And an ascending step voltage applying means for applying an ascending step voltage rising stepwise toward the drive voltage for controlling the conductive state, and stopping the supply of the power supply voltage to the load. When the power supply voltage is output from the power supply circuit, a non-conduction voltage for controlling the power supply side terminal and the load side terminal in a non-conduction state with respect to the control terminal of the semiconductor switch element. Descending step voltage applying means for applying a descending step voltage that descends in a stepwise manner toward the driving voltage, and the rising step voltage is the driving voltage when the falling step voltage reaches the non-conduction voltage from the driving voltage. A power supply device is characterized in that it is shorter than the time from when the voltage reaches the non-conducting voltage.

According to the third aspect of the present invention, in addition to the actions of the first and second aspects, the rush current generated according to the voltage change of the load side terminal when the power source voltage is cut off is the load side terminal when the power source voltage is supplied. Paying attention to the fact that it is smaller than the rush current that occurs according to the voltage change of, the time between the rising step voltage and the non-conduction voltage from the driving voltage to the non-conduction voltage increases from the driving voltage to the non-conduction voltage. Shorter than time. Therefore, the power supply voltage cutoff operation can be performed quickly.

The invention according to claim 4 is the power supply device according to claim 3, wherein the number of steps of the falling step voltage is made smaller than the number of steps of the rising step voltage, or each of the falling step voltages. By setting the time for maintaining the voltage shorter than the time for maintaining each voltage of the rising step voltage, the time until the falling step voltage reaches the non-conduction voltage from the driving voltage, the rising step voltage is the driving voltage. In the power supply device, the time is shorter than the time from when the voltage reaches the non-conducting voltage.

According to the fourth aspect of the invention, the number of steps of the falling step voltage is made smaller than the number of steps of the rising step voltage, and the time for maintaining each voltage of the falling step voltage is set to By making the time shorter than the holding time, it is possible to easily make the time required for the falling step voltage to reach the non-conduction voltage from the driving voltage shorter than the time for the rising step voltage to reach the non-conduction voltage in the driving voltage. .

The invention according to claim 5 is the power supply device according to any one of claims 1 to 4, wherein the step voltage applying means generates a voltage having a height corresponding to an inputted digital value. A power supply device is characterized by having a digital / analog conversion means.

According to the invention of claim 5, the step voltage applying means has a digital (D) / analog (A) converting means. Therefore, by using the D / A conversion means, the step voltage can be easily controlled by a microcomputer or the like, and the size can be reduced.

[0031]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing an embodiment of a power supply device of the present invention. In the figure, the same parts as those of the conventional device described above with reference to FIG. 5 are designated by the same reference numerals, and detailed description thereof will be omitted.

As shown in the figure, the power supply device 30 boosts the battery voltage output from the battery 31a and the battery 31a which outputs the battery power, and outputs DC / DC voltage which is output as the battery voltage.
A power supply circuit 31 including a DC converter 31b is provided. As the DC / DC converter 31b,
In addition to the one using the inductor described above in the related art, one using a transformer is also conceivable. Also, the DC
The operation of the / DC converter 31b is controlled by a control signal output from the control terminal T6 in the controller 32.

A FET Q4 is provided between the power supply circuit 31 and the load 20 described above. This FETQ
The source terminal (terminal on the power supply circuit side) of No. 4 is connected to the power supply circuit 31, and the drain terminal (terminal on the load side) is connected to the load 20. The gate terminal (control terminal) of the FET Q4 is connected to the D / A converter 34 (D / A conversion means) via the buffer amplifier 33. This D / A
The converter 34 outputs a voltage according to the digital value output from the control terminal T7 in the controller 32.

The operation of the power supply device having the above configuration will be described below with reference to the time chart of FIG. In the figure, (a)
Is the output of the power supply circuit 31, (b) is the output of the control terminal T6, (c) is the output of the drain terminal of the FET Q4, (d).
Indicates the output current of the battery 31a, and (e) indicates the gate terminal input of the FET Q4.

First, the operation at the start of supplying the power supply voltage to the load 20 will be described. The controller 32 outputs an H level control signal from the control terminal T6 (see FIG. 2).
(See (b)), and activates the DC / DC converter 31b. At this time, a predetermined power supply voltage is output from the DC / DC converter 31b (FIG. 2 (a)). At this time, the starting current of the DC / DC converter 31b flows from the battery 31a (see FIG. 2D), which is a relatively small value.

Next, in order to supply the power supply voltage to the load 20, the operation of the D / A converter 34 is started under the control of the controller 32. With this operation start, the buffer amplifier 33 that amplifies the output of the D / A converter 34 outputs F
As the power supply voltage is output from the drain terminal of ETQ4, an increasing step voltage that increases stepwise is output toward the drive voltage Vd that controls the conduction state between the source terminal and the drain terminal of FETQ4. As is clear from this, the controller 32 and the D / A converter 34 are
It constitutes a rising step voltage applying means.

When this rising step voltage is input to the gate terminal of the FET Q4, the drain terminal of the FET Q4 also gradually rises from 0V and reaches the power supply voltage. For this reason, the voltage of the drain terminal does not instantaneously reach the power supply voltage, and the capacitor C2 can be charged slowly.

Therefore, it is possible to reduce the power supply voltage and the rush current at the start of supply, prevent problems occurring at the start of supply of the power supply voltage to the load 20, and extend the life of the battery 31a. Moreover, by using the FET Q4 provided on the downstream side by the power supply circuit 31, the load 2
By controlling the voltage supplied to the 0 side, the rush current can be reduced regardless of the type of the power supply circuit 31, and versatility can be improved.

Next, the operation when the supply of the power supply voltage to the load 20 is stopped will be described. The controller 32 stops the supply of the power supply voltage to the load 20 by the D /
The operation of the A converter 34 is ended. At the end of this operation, D
From the buffer amplifier 33 that amplifies the output of the A / A converter 34, a non-conduction voltage that causes a non-conduction state between the source terminal and the drain terminal of the FET Q4 (hereinafter, non-conduction voltage = 0 V for simplification of description). ), A step-down falling voltage is output. As is clear from this, the controller 32 and the D / A converter 34 constitute a step-down voltage applying means.

When the falling step voltage is input to the gate terminal of the FET Q4, the drain terminal of the FET Q4 also gradually drops from the power supply voltage and reaches 0V. Therefore, the voltage of the drain terminal does not instantly drop from the power supply voltage to 0V.

Therefore, it is possible to reduce the power supply voltage and the rush current at the time of stopping the supply, and to prevent problems caused by the voltage at the drain terminal instantaneously dropping from the power supply voltage to 0V. Moreover, by controlling the voltage supplied to the load 20 side by using the FET Q4 provided on the downstream side of the power supply circuit 31, it is possible to reduce the rush current regardless of the type of the power supply circuit 31, and thus the versatility is improved. It can also be increased.

Note that the controller 32 pays attention that the rush current generated in response to the voltage change of the drain terminal when the power supply voltage is cut off is smaller than the rush current generated in response to the voltage change of the drain terminal when the power supply voltage is supplied, The time required for the falling step voltage to reach 0V from the drive voltage is shorter than the time required for the rising step voltage to reach the drive voltage from 0V. As a result, the power supply voltage cutoff operation can be performed quickly.

As a method of making the time required for the falling step voltage to reach 0V from the driving voltage shorter than the time required for the rising step voltage to reach the driving voltage from 0V, for example, the number of steps of the falling step voltage is increased. It is conceivable that the number of steps is smaller than the number of steps of the voltage, or that the time for holding each voltage of the step voltage is shorter than the time for holding each voltage of the step voltage.

Although the buffer amplifier 33 is provided between the output of the D / A converter 34 and the gate terminal of the FET Q4 in the above-described embodiment, for example, as shown in FIG. A transistor Q5 may be provided between the output of the A converter 34 and the gate terminal of the FET Q4 so that a voltage according to the output voltage from the D / A converter 34 can be applied to the gate terminal of the FET Q4.

Further, in the above-mentioned embodiment, FIG.
Although the rising or falling step voltage as shown in FIG. 3 (e) is input to the gate terminal, it is also conceivable to use a capacitor to dull the rising or falling step voltage.

Further, the D / A converter 34 may be either an integrated one or a discrete one.

[0047]

As described above, according to the first aspect of the invention, the voltage at the load side terminal does not instantaneously reach the power supply voltage, and the power supply voltage and the rush current at the start of supply are reduced. Therefore, it is possible to prevent a problem that occurs at the time of starting the supply of the power supply voltage to the load and to extend the life of the battery in the case of the power supply circuit using the battery. Moreover, since the semiconductor switch element provided on the downstream side of the power supply circuit is used to control the voltage supplied to the load side, the rush current can be reduced regardless of the type of the power supply circuit. A high power supply device can be obtained.

According to the second aspect of the invention, the power supply circuit-
To the control terminal of the semiconductor switching element provided between the loads, by applying a step-down voltage that decreases stepwise toward the non-conducting voltage, the voltage of the load-side terminal of the semiconductor switching element also stepwise, That is, it gradually drops and reaches the non-conduction voltage. Therefore, it is possible to reduce the power supply voltage and the rush current when the supply is stopped, and to prevent problems caused by the voltage at the load side terminal instantaneously dropping to the non-conducting voltage. Moreover, since the semiconductor switch element provided on the downstream side of the power supply circuit is used to control the voltage supplied to the load side, the rush current can be reduced regardless of the type of the power supply circuit. A high power supply device can be obtained.

According to the third aspect of the present invention, it is possible to obtain a power supply device capable of performing a power supply voltage cutoff operation quickly.

According to the fourth aspect of the present invention, the time taken for the falling step voltage to reach the non-conducting voltage from the driving voltage is simply defined as the time taken for the rising step voltage to reach the driving voltage from the non-conducting voltage. A power supply device that can be shortened can be obtained.

According to the fifth aspect of the present invention, by using the D / A conversion means, the step voltage can be easily controlled by a microcomputer or the like, and a power supply device that can be downsized can be obtained. be able to.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of a power supply device of the present invention.

FIG. 2 is a time chart for explaining the operation of the power supply device of FIG. 1 at the start of power supply voltage supply.

FIG. 3 is a time chart for explaining the operation of the power supply device of FIG. 1 when the power supply voltage is cut off.

FIG. 4 is a diagram showing another embodiment of the power supply device of the present invention.

FIG. 5 is a diagram showing an example of a conventional power supply device.

FIG. 6 is a time chart for explaining the operation of the conventional power supply device.

[Explanation of symbols]

30 power supply device 31 power supply circuit 20 load Q4 FET (semiconductor switch element) 32 controller (rising step voltage applying means, falling step voltage applying means) 34 D / A converter (D / A converting means, rising step voltage applying means, falling) Step voltage applying means)

Claims (5)

[Claims] 1. A power supply device having a power supply circuit that outputs a DC power supply voltage, the power supply device being provided between the power supply circuit and a load receiving the supply of the power supply voltage, and connected to the power supply circuit side. A semiconductor switch element having a power supply side terminal, a load side terminal connected to the load side, and a control terminal for controlling a conduction state between the power supply side terminal and the load side terminal; and the power supply for the load. When the supply of voltage is started, a conduction state in which the power supply voltage is output from the power supply circuit, and the power supply voltage is output from the load side terminal to the control terminal of the semiconductor switch element. A step-up voltage applying means for applying a step-up voltage that rises stepwise toward a controlled drive voltage. 2. A power supply device having a power supply circuit that outputs a DC power supply voltage, the power supply device being provided between the power supply circuit and a load receiving the supply of the power supply voltage, and being connected to the power supply circuit side. A semiconductor switch element having a power supply side terminal, a load side terminal connected to the load side, and a control terminal for controlling a conduction state between the power supply side terminal and the load side terminal; and the power supply for the load. When the supply of voltage is stopped, in a state where the power supply voltage is being output from the power supply circuit, with respect to the control terminal of the semiconductor switch element, the power supply side terminal-the load side terminal is brought into a non-conductive state. And a step-down voltage applying means for applying a step-down voltage that drops stepwise toward a controlled non-conduction voltage. 3. A power supply device having a power supply circuit that outputs a DC power supply voltage, the power supply device being provided between the power supply circuit and a load receiving the supply of the power supply voltage, and being connected to the power supply circuit side. A power supply side terminal, a load side terminal connected to the load side, and a semiconductor switch element having a control terminal for controlling a conduction state between the power supply side terminal and the load side terminal; When the supply is started, the power supply voltage is output from the power supply circuit, and the control terminal of the semiconductor switch element is controlled to be in a conductive state in which the power supply voltage is output from the load side terminal. Increasing step voltage applying means for applying an increasing step voltage that increases stepwise toward the driving voltage, and when stopping the supply of the power supply voltage to the load, In the state in which the power supply voltage is output from the path, with respect to the control terminal of the semiconductor switch element, a step shape is formed toward a non-conduction voltage for controlling the non-conduction state between the power supply side terminal and the load side terminal. Falling step voltage applying means for applying a falling step voltage that drops to the non-conducting voltage from the driving voltage until the falling step voltage reaches the non-conducting voltage from the driving voltage. A power supply characterized by being shorter than the time it takes to reach a voltage. 4. The power supply device according to claim 3, wherein the number of steps of the falling step voltage is made smaller than the number of steps of the rising step voltage, or a time period during which each voltage of the falling step voltage is maintained, By setting the rising step voltage to be shorter than the time for holding each voltage, the rising step voltage changes from the driving voltage to the non-conducting voltage until the falling step voltage reaches the non-conducting voltage. A power supply characterized by being shorter than the time it takes to reach it. 5. The power supply device according to claim 1, wherein the step voltage application unit includes a digital / analog conversion unit that generates a voltage having a height according to an input digital value. A power supply device having.