Power supply device having boosting circuit and boosting control applicable to electronic devices

A power supply device includes a first load circuit that operates by a first voltage; a second load circuit that operates by a second voltage higher than the first voltage; a boosting circuit that generates the first voltage if the power supply device is in a first driving mode, and generates the second voltage if the power supply device is in a second driving mode; a first feedback circuit that connects the first load circuit and the boosting circuit if the power supply device is in the first driving mode; and a second feedback circuit that connects the first load circuit and the boosting circuit if the power supply device is in the second driving mode. The first load circuit is operated in the first driving mode. The first load circuit and the second load circuit are operated in the second driving mode.

BACKGROUND

Field of the Invention

The present invention relates to a power supply device including a boosting circuit.

Description of the Related Art

A portable device such as a digital camera uses a liquid crystal display device as a display device for displaying an image or the like. A liquid crystal display device generally uses as a backlight circuit a lighting circuit including white light emitting diodes (LEDs). A liquid crystal display device having an approximately 3-inch display unit often uses a lighting circuit in which four to six white LEDs are connected in series. Since a forward voltage Vf of each white LED is about 3.0 V, a power supply of about 12 to 18 V is required to drive the lighting circuit. Recently, mainstream digital cameras employ a system using a one cell lithium-ion battery or two cell lithium-ion battery. In such a system, a voltage input from the battery is 3 to 9 V. Thus, a boosting circuit is required to drive the above lighting circuit.

Further, to remove dust attached to an image sensor when a lens is replaced in an interchangeable lens digital camera, there has been proposed a technique for vibrating a low pass filter (a quartz plate) placed in the image sensor using a piezoelectric element, thereby removing the dust. In this case, it is necessary to drive the piezoelectric element at a relatively high voltage, namely about 20 to 25 V. Thus, an interchangeable lens digital camera using a one cell lithium-ion battery or two cell lithium-ion battery requires at least two types of boosting circuits. A first boosting circuit is a boosting circuit used for a backlight circuit of a liquid crystal display device, and a second boosting circuit is a boosting circuit used to drive a piezoelectric element. These boosting circuits need to employ a high voltage semiconductor device having excellent switching responsiveness and therefore are relatively expensive.

Japanese Patent Application Laid-Open No. 2006-101637 describes a technique for generating, in a single boosting circuit, driving voltages for driving a first load circuit and a second load circuit, which is driven at a higher voltage than the first load circuit.

A system described in Japanese Patent Application Laid-Open No. 2006-101637 includes two types of feedback circuits (30, 40) for two load circuits (80, 82). A first feedback circuit (30) is a circuit that is highly efficient as a circuit for passing a constant current through LEDs (80), which are a first load circuit. A second feedback circuit (40) is a circuit used to drive a second load circuit (82). If the second feedback circuit (40) is used, excessive power is supplied to the LEDs (80). Thus, a constant-current circuit (42) is connected to a cathode side of the LEDs (80). When the second feedback circuit (40) is used, however, a feedback voltage is varied depending on individual variations and temperature variations in a forward voltage Vf of the LEDs (80). This results in varying an output voltage from a boosting circuit (10).

Further, in the system described in Japanese Patent Application Laid-Open No. 2006-101637, the first feedback circuit (30) may not be able to perform a feedback control for generating a constant current under a condition that an input voltage of the boosting circuit (10) is higher than an output voltage of the boosting circuit (10). For example, the first feedback circuit (30) may not be able to perform the feedback control under a condition that an input voltage from a power source is 5 V to 10 V and a driving voltage of the LEDs (80) operated as a backlight circuit is about 9 V. In such a case, the first feedback circuit (30) cannot perform a constant-current control, and therefore, it is necessary to perform the constant-current control using only the second feedback circuit (40). The system described in Japanese Patent Application Laid-Open No. 2006-101637, however, does not include a circuit for determining such condition. Japanese Patent Application Laid-Open No. 2006-101637 does not describe a condition for determining such condition, either. Further, in the system described in Japanese Patent Application Laid-Open No. 2006-101637, when the first feedback circuit (30) is in a connected state and if the lighting up of the LEDs (80), which operate as the backlight circuit of a liquid crystal display device, has been started, an illuminance of the liquid crystal display device gradually increases. Thus, the liquid crystal display device may display an uncomfortable image.

SUMMARY

According to an aspect of the present invention, it is directed to a novel power supply device or power supply circuit applicable to an electronic device such as an imaging device.

According to another aspect of the present invention, it is directed to a novel boosting circuit and boosting control applicable to an electronic device such as an imaging device.

According to another aspect of the present invention, a power supply device includes a first load circuit that operates by a first voltage; a second load circuit that operates by a second voltage higher than the first voltage; a boosting circuit that generates the first voltage if the power supply device is in a first driving mode, and generates the second voltage if the power supply device is in a second driving mode; a first feedback circuit that connects the first load circuit and the boosting circuit if the power supply device is in the first driving mode; a second feedback circuit that connects the first load circuit and the boosting circuit if the power supply device is in the second driving mode; and a control unit that causes the first load circuit to operate if the power supply device is in the first driving mode, and causes the first load circuit and the second load circuit to operate if the power supply device is in the second driving mode.

DESCRIPTION OF THE EMBODIMENTS

Exemplary embodiments, features, and aspects of the present invention will be described below with reference to the drawings. Exemplary embodiments of the present invention, however, is not limited to the following exemplary embodiments.

FIG. 1is a diagram illustrating the components of a power supply circuit100according to a first exemplary embodiment. The power supply circuit100according to the first exemplary embodiment operates as a power supply device applicable to an electronic device such as an imaging device.

A power source1supplies power to the power supply circuit100. The power source1is, for example, a direct-current power source in which an input voltage Vin input from the power source1to the power supply circuit100is assumed to be about 5 V to 10 V. The power source1is assumed to be a lithium-ion battery having two cells or six AA batteries connected in series. The power source1, however, is not limited to these batteries.

A boosting circuit2boosts the input voltage Vin input from the power source1to the boosting circuit2and outputs an output voltage VOUT. The boosting circuit2includes an input capacitor20, an output capacitor21, a switching transistor22, an inductor23, a rectifying diode24, a comparator25, and an oscillation circuit26. The input capacitor20is connected between an input node of the boosting circuit2and the ground potential. The output capacitor21is connected between an output node of the boosting circuit2and ground potential. One end of the inductor23is connected to the input node of the boosting circuit2, and the other end of the inductor23is connected to the anode of the rectifying diode24. The cathode of the rectifying diode24is connected to the output node of the boosting circuit2.

The switching transistor22is a transistor for performing switching driving in the boosting circuit2. The switching transistor22is a high voltage transistor of which a switching operation has excellent responsiveness and which can be driven up to a boosted voltage. The drain of the switching transistor22is connected to a connection point between the inductor23and the rectifying diode24, and the source of the switching transistor22is connected to the ground potential. Further, a control signal output from the comparator25is input to the gate of the switching transistor22. The oscillation circuit26generates a triangle wave at a fixed frequency and outputs the generated triangle wave. The comparator25compares a signal level of output of a first feedback circuit7or a second feedback circuit8with a triangle wave output from the oscillation circuit26. As a result of the comparison by the comparator25, a control signal subjected to duty control according to a signal level of output of the first feedback circuit7or the second feedback circuit8is generated and output to the switching transistor22.

The switching transistor22is controlled by the control signal from the comparator25. If the switching transistor22is on, energy is stored in the inductor23. Then, if the switching transistor22has been turned off, the energy stored in the inductor23is charged to the output capacitor21via the rectifying diode24. This enables the boosting circuit2to boost the input voltage Vin input from the power source1to the boosting circuit2and supply necessary power to a first load circuit3and a second load circuit4. The second load circuit4corresponds to a driver4.

The first load circuit3is a backlight circuit of a liquid crystal display device. The backlight circuit according to the first exemplary embodiment is assumed to be, for example, a circuit in which four white LEDs, each having a forward voltage Vf of about 3.0 V, are connected in series. Thus, a voltage required to drive the first load circuit3is assumed to be about 12 V or more. Further, the first load circuit3is a circuit that operates as a constant-current driven circuit. The first load circuit3can operate in either of a first driving mode and a second driving mode, which will be described below. A liquid crystal display unit12and the first load circuit3are included as an integrated unit in the liquid crystal display device. The liquid crystal display unit12is driven in synchronization with the operation of the first load circuit3.

If the power supply circuit100is in the first driving mode, the power supply circuit100performs control to feed back a voltage dropped by a detection resistor circuit5, which is connected in series to the first load circuit3, to the boosting circuit2via the first feedback circuit7. This enables the power supply circuit100to perform control so that a desired constant current flows through the first load circuit3. If, however, the power supply circuit100is in the first driving mode, a constant-current circuit6and the second feedback circuit8are not selected. If the power supply circuit100is in the first driving mode, the boosting circuit2can boost a voltage only as required to pass a constant current through the first load circuit3. This enables an efficient driving of the boosting circuit2. If the power supply circuit100is in the first driving mode, a change in a resistance value of the detection resistor circuit5can vary a current flowing through the first load circuit3.

Further, if the power supply circuit100is in the second driving mode, the power supply circuit100selects the constant-current circuit6, which is connected in series to the first load circuit3. In this case, the boosting circuit2performs a boosting operation based on a voltage detected by an output voltage detection circuit9. Further, in this case, the boosting circuit2performs the boosting operation so that the output voltage VOUT would be equal to or greater than a voltage obtained by adding a voltage required to drive the first load circuit3and a voltage required for a normal operation of the constant-current circuit6. This enables a constant current to flow through the first load circuit3.

The second load circuit4, which corresponds to the driver4, is a load circuit that requires a driving voltage higher than that of the first load circuit3. The second load circuit4operates only if the power supply circuit100is in the second driving mode. If the power supply circuit100is applied to an imaging device such as an interchangeable lens digital camera, for example, a driving circuit for driving a piezoelectric element corresponds to the second load circuit4. In this case, to remove dust near an image sensor, the piezoelectric element included in the second load circuit4operates as a component for vibrating a low pass filter (a quartz plate) placed in an image sensor. For example, if a voltage required to drive the second load circuit4is about 20 V, the boosting circuit2performs, based on a voltage detected by the output voltage detection circuit9, the boosting operation so that the output voltage VOUT of the boosting circuit2would be 20 V. This boosting operation enables the boosting circuit2to drive the second load circuit4. A change in a voltage division ratio of resistors in the output voltage detection circuit9can change the output voltage VOUT of the boosting circuit2. Thus, the voltage division ratio of the resistors in the output voltage detection circuit9can be appropriately changed according to the voltage required to drive the second load circuit4.

The detection resistor circuit5includes a detection resistor31and a switch32, which corresponds to a first switch. One end of the detection resistor31is connected to an output end (a node on lower potential side) of the first load circuit3via the switch32, and the other end of the detection resistor31is connected to the ground potential.

The constant-current circuit6includes a constant-current supply35and a switch36, which corresponds to a second switch. A series circuit including the constant-current supply35and the switch36is connected between the output end (the node on the lower potential side) of the first load circuit3and the ground potential.

The first feedback circuit7includes a first error amplifier33and a switch34, which corresponds to a third switch. To the first error amplifier33, a potential of the connection point between one end of the detection resistor31and the switch32(i.e., a voltage dropped by the detection resistor31) in the detection resistor circuit5and a first reference voltage REF1, which is generated by a first reference voltage supply, are input. The output of the first error amplifier33is supplied to the comparator25in the boosting circuit2via the switch34.

The output voltage detection circuit9includes voltage dividing resistors39and40. The voltage dividing resistors39and40are connected in series between the output node of the boosting circuit2, that is, an input end (a node on higher potential side) of the first load circuit3, and the ground potential. A potential of the connection point between the voltage dividing resistors39and40is output as a voltage detected by the output voltage detection circuit9.

The second feedback circuit8includes a second error amplifier37and a switch38, which corresponds to a fourth switch. To the second error amplifier37, a voltage detected by the output voltage detection circuit9and a second reference voltage REF2, which is generated by a second reference voltage supply, are input. The output of the second error amplifier37is supplied to the comparator25in the boosting circuit2via the switch38.

A control unit10is a microprocessor for controlling an operation of the power supply circuit100. The control unit10selects either of the first feedback circuit7and the second feedback circuit8and selects either of the circuits5and6. Further, the control unit10controls the second load circuit4which is connected to the power supply circuit100, and also controls the liquid crystal display unit12which operates in conjunction with the first load circuit3. A switch control signal output from the control unit10is input to the switches32and34and an inverter11. The switch control signal inverted by the inverter11is input to the switches36and38. In other words, the switches32and34operate as a first switch group, and the switches36and38operate as a second switch group. The control unit10can exclusively control on/off states of these switch groups.

In the first exemplary embodiment, the first driving mode is a mode for driving only the first load circuit3. In the first driving mode, the detection resistor circuit5is selected as a driving circuit of the first load circuit3, and also the first feedback circuit7is selected. The boosting circuit2performs a boosting operation based on an output of the first feedback circuit7, thereby outputting the output voltage VOUT according to the sum of the forward voltages Vf of the plurality of LEDs included in the first load circuit3. This drives the first load circuit3at a constant current.

Further, the second driving mode is a mode for driving both the first load circuit3and the second load circuit4. In the second driving mode, the constant-current circuit6is selected as a driving circuit of the first load circuit3, and also the output voltage detection circuit9and the second feedback circuit8are selected. This determines the output voltage VOUT of the boosting circuit2. Based on a voltage detected by the output voltage detection circuit9, the boosting circuit2generates the output voltage VOUT required to drive the second load circuit4. At this time, if the output voltage VOUT of the boosting circuit2is equal to or greater than a voltage obtained by adding a voltage required to drive the first load circuit3and a voltage that allows a normal operation of the constant-current circuit6, the first load circuit3can be driven to operate at a constant current.

Next, with reference toFIGS. 2 and 3, an operation of the power supply circuit100according to the first exemplary embodiment will be described.FIG. 2is a diagram illustrating an exemplary relationship between the output voltage VOUT from the boosting circuit2and a current IL (an LED current), which flows through the first load circuit3, according to the first exemplary embodiment.FIG. 3is a flow chart illustrating an exemplary operation of the power supply circuit100according to the first exemplary embodiment.

In step S101, power source1starts supplying power to the power supply circuit100. In step S102, the control unit10turns off the switches32and34and turns on the switches36and38to select the constant-current circuit6and the second feedback circuit8. While the constant-current circuit6and the second feedback circuit8are selected, the power supply circuit100operates in the second driving mode. If the constant-current circuit6and the second feedback circuit8are selected, then in step S103, the boosting circuit2performs a boosting operation based on a voltage detected by the output voltage detection circuit9. If the boosting operation for boosting the input voltage Vin to a target output voltage (e.g., a second voltage V2) has been completed by the boosting circuit2(YES in step S104), the control unit10causes the constant-current supply35in the constant-current circuit6to operate. Then, in step S105, the control unit10lights up the plurality of LEDs included in the first load circuit3(the backlight circuit) and also simultaneously drives the liquid crystal display unit12. The processes in steps S101to S105are performed in a period300illustrated inFIG. 2. In the period300, the output voltage VOUT of the boosting circuit2rises to the target output voltage (e.g., the second voltage V2) with a predetermined slope. The plurality of LEDs in the first load circuit3are lit up after the boosting operation for boosting the input voltage Vin to this target output voltage (e.g., the second voltage V2) has been completed. Such operation is performed, thereby, when displaying an image or the like on the liquid crystal display device, enabling instantaneous display of the image or the like with an excellent responsiveness.

Next, in this state, power consumption of the first load circuit3is: (a voltage required to drive the second load circuit4)×(an amount of constant current). This is inefficient to drive the backlight circuit which is the first load circuit3. Thus, in step S106, the control unit10subsequently turns off the switches36and38and turns on the switches32and34to select the first feedback circuit7and the detection resistor circuit5. While the first feedback circuit7and the detection resistor circuit5are selected, the power supply circuit100operates in the first driving mode. If the first feedback circuit7and the detection resistor circuit5are selected, the boosting circuit2is controlled by the first feedback circuit7and the detection resistor circuit5. The boosting circuit2performs a boosting operation based on a voltage dropped by the detection resistor circuit5. In step S106, the boosting circuit2operates so that the output voltage VOUT of the boosting circuit2would be a first voltage V1. The first voltage V1 is a voltage lower than the second voltage V2. At this time, in step S107, since the desired LED current IL continues to be supplied to the first load circuit3, the plurality of LEDs in the first load circuit3continue to be lit up. In this state, power consumption of the first load circuit3is: (a power required to drive the first load circuit3+a voltage that allows a normal operation of the constant-current circuit6)×(an amount of constant current). Thus, it is possible to drive the first load circuit3more efficiently than driving the first load circuit3using the second feedback circuit8. The process in step S107is performed in a period301illustrated inFIG. 2. As also illustrated inFIG. 2, when the second feedback circuit8is switched to the first feedback circuit7, the current IL flowing through the first load circuit3slightly changes due to a responsiveness of the error amplifier33of the first feedback circuit7. The plurality of LEDs in the first load circuit3, however, continue to be lit up while the processes in steps S105to S107are performed.

Next, if the control unit10has issued a request to drive the second load circuit4(YES in step S108), then in step S109, the control unit10turns off the switches32and34and turns on the switches36and38to select the constant-current circuit6and the second feedback circuit8. While the constant-current circuit6and the second feedback circuit8are selected, the power supply circuit100operates in the second driving mode. If the constant-current circuit6and the second feedback circuit8are selected, the boosting circuit2performs a boosting operation based on a voltage detected by the output voltage detection circuit9. If the boosting operation for boosting the input voltage Vin to a predetermined output voltage (e.g., the second voltage V2) has been completed by the boosting circuit2(YES in step S110), then in step S111, the control unit10outputs a driving command and drives the second load circuit4using the output voltage VOUT from the boosting circuit2. At this time, in step S112, the plurality of LEDs in the first load circuit3can continue to be lit up by being driven by the constant-current supply35in the constant-current circuit6. Thus, it is possible to continue to display an image or the like on the liquid crystal display device while the second load circuit4is driven. The processes in steps S108to S112correspond to the operation performed in a period302illustrated inFIG. 2. Consequently, the plurality of LEDs in the first load circuit3continue to be lit up while the processes in steps S105to S112are performed.

Next, if the driving of the second load circuit4has ended (YES in step S113), then in step S114, the control unit10turns off the switches36and38and turns on the switches32and34to select the first feedback circuit7and the detection resistor circuit5again. While the first feedback circuit7and the detection resistor circuit5are selected, the power supply circuit100operates in the first driving mode. If the first feedback circuit7and the detection resistor circuit5are selected, the boosting circuit2performs a boosting operation based on a voltage dropped by the detection resistor circuit5. In step S114, the boosting circuit2operates so that the output voltage VOUT of the boosting circuit2would be the first voltage V1. Consequently, in step S115, it is possible to reduce power consumption and continue to light up the plurality of LEDs in the first load circuit3. This operation is the operation performed in a period303illustrated inFIG. 2. As described above, when the second feedback circuit8is switched to the first feedback circuit7, the current IL flowing through the first load circuit3slightly changes. The plurality of LEDs in the first load circuit3, however, continue to be lit up while the processes in steps S105to S115are performed.

In a second exemplary embodiment, description will be given of a case where a condition of the input voltage Vin from the power source1to the boosting circuit2and a condition for lighting up the plurality of LEDs included in the first load circuit3are changed.FIG. 4is a diagram illustrating the components of a power supply circuit100according to the second exemplary embodiment. InFIG. 4, the components having functions similar to those of the components illustrated inFIG. 1are designated by the same reference numerals. In the second exemplary embodiment, among the components illustrated inFIG. 4, the components having functions similar to those of the components illustrated inFIG. 1will not be described. Similarly to the power supply circuit100according to the first exemplary embodiment, the power supply circuit100according to the second exemplary embodiment also operates as a power supply device applicable to an electronic device such as an imaging device.

One of the differences between the power supply circuit100according to the second exemplary embodiment and the power supply circuit100according to the first exemplary embodiment lies in the power source1. In the second exemplary embodiment, the power source1is assumed to be a lithium-ion battery having one or two cells, or four to six AA batteries connected in series. The input voltage Vin of the boosting circuit2is assumed to be about 4 V to 10 V. Another one of the differences lies in the first load circuit3. The first load circuit3is a circuit that operates as the backlight circuit of a liquid crystal display device and includes a plurality of LEDs. In the second exemplary embodiment, a case is assumed where the first load circuit3is a circuit in which three to six white LEDs are connected in series. Thus, in the second exemplary embodiment, a voltage required to drive the first load circuit3is assumed to be about 8 V to 18 V.

Further, yet another one of the differences lies in that the power supply circuit100includes a current detection circuit50, which is connected in series between the output node of the boosting circuit2and the input end of the first load circuit3. The current detection circuit50includes a current detection resistor51and an error amplifier52. The current detection resistor51is connected in series between the output node of the boosting circuit2and the input end of the first load circuit3. To the error amplifier52, potentials of both ends of the current detection resistor51are input, and the error amplifier52amplifies and outputs the difference voltage between the input potentials. The control unit10can detect the current flowing through the first load circuit3, using an output of the current detection circuit50. The control unit10controls switching between the first feedback circuit7and the second feedback circuit8based on the detected current.

Next, with reference toFIG. 5, an operation of the power supply circuit100according to the second exemplary embodiment will be described.FIG. 5is a flow chart illustrating an exemplary operation of the power supply circuit100according to the second exemplary embodiment.

In step S200, the power source1starts supplying power to the power supply circuit100. In step S201, the control unit10turns off the switches32and34and turns on the switches36and38to select the constant-current circuit6and the second feedback circuit8. While the constant-current circuit6and the second feedback circuit8are selected, the power supply circuit100operates in the second driving mode. If the constant-current circuit6and the second feedback circuit8are selected, then in step S202, the boosting circuit2performs a boosting operation based on a voltage detected by the output voltage detection circuit9. If the boosting operation for boosting the input voltage Vin to a target output voltage (e.g., a second voltage V2) has been completed by the boosting circuit2(YES in step S203), the control unit10causes the constant-current supply35in the constant-current circuit6to operate. Then, in step S204, the control unit10lights up the plurality of LEDs included in the first load circuit3(the backlight circuit) and also simultaneously drives the liquid crystal display unit12. Such operation is performed, thereby, when displaying an image or the like on the liquid crystal display device, enabling instantaneous display of the image or the like with an excellent responsiveness.

Next, in this state, power consumption of the first load circuit3is: (a voltage required to drive the second load circuit4)×(an amount of constant current). This is inefficient to drive the backlight circuit, which is the first load circuit3. Thus, in step S205, the control unit10subsequently turns off the switches36and38and turns on the switches32and34to select the first feedback circuit7and the detection resistor circuit5. While the first feedback circuit7and the detection resistor circuit5are selected, the power supply circuit100operates in the first driving mode. If the first feedback circuit7and the detection resistor circuit5are selected, the boosting circuit2is controlled by the first feedback circuit7and the detection resistor circuit5. The boosting circuit2performs a boosting operation based on a voltage dropped by the detection resistor circuit5. The first load circuit3is driven at a constant current. In step S205, the boosting circuit2operates so that the output voltage VOUT of the boosting circuit2would be a first voltage V1. The first voltage V1 is a voltage lower than the second voltage V2. Consequently, the plurality of LEDs in the first load circuit3continue to be lit up while the processes in steps S204and S205are performed. Next, in step S206, the control unit10detects the current IL flowing through the first load circuit3, using the current detection circuit50. In step S207, the control unit10determines whether a desired constant-current value has been obtained. At this time, if a condition (an input voltage of the current detection circuit50)<(a voltage required to drive the first load circuit3) is satisfied, the desired constant-current value is obtained, whereas if this condition is not satisfied, the desired constant-current value cannot be obtained.

Next, if it has been determined in step S207that the desired constant-current value has been obtained (YES in step S207), then in step S208, the control unit10controls the first feedback circuit7so that the desired LED current IL continues to be supplied to the first load circuit3. Consequently, the desired LED current IL continues to be supplied to the first load circuit3, and therefore, the plurality of LEDs in the first load circuit3continue to be lit up.

Next, if the control unit10has issued a request to drive the second load circuit4(YES in step S209), then in step S210, the control unit10turns off the switches32and34and turns on the switches36and38to select the constant-current circuit6and the second feedback circuit8. While the constant-current circuit6and the second feedback circuit8are selected, the power supply circuit100operates in the second driving mode. If the constant-current circuit6and the second feedback circuit8are selected, the boosting circuit2performs a boosting operation based on a voltage detected by the output voltage detection circuit9. If the boosting operation for boosting the input voltage Vin to a predetermined output voltage (e.g., the second voltage V2) has been completed by the boosting circuit2(YES in step S211), then in step S212, the control unit10outputs a driving command and drives the second load circuit4using the output voltage VOUT from the boosting circuit2. At this time, in step S213, the plurality of LEDs in the first load circuit3can continue to be lit up by being driven by the constant-current supply35in the constant-current circuit6. Thus, it is possible to continue to display an image or the like on the liquid crystal display device while the second load circuit4is driven.

Next, if the driving of the second load circuit4has ended (YES in step S214), then in step S215, the control unit10turns off the switches36and38and turns on the switches32and34to select the first feedback circuit7and the detection resistor circuit5again. While the first feedback circuit7and the detection resistor circuit5are selected, the power supply circuit100operates in the first driving mode. If the first feedback circuit7and the detection resistor circuit5are selected, the boosting circuit2performs a boosting operation based on a voltage dropped by the detection resistor circuit5. In step S215, the boosting circuit2operates so that the output voltage VOUT of the boosting circuit2would be the first voltage V1. Consequently, in step S216, it is possible to reduce power consumption and continue to light up the plurality of LEDs in the first load circuit3. Thus, the plurality of LEDs in the first load circuit3continue to be lit up while the processes in steps S204to S216are performed.

If, on the other hand, it has been determined in step S207that the desired constant-current value has not been obtained (NO in step S207), then in step S218, the control unit10turns off the switches32and34and turns on the switches36and38to select the constant-current circuit6and the second feedback circuit8. While the constant-current circuit6and the second feedback circuit8are selected, the power supply circuit100operates in the second driving mode. If the constant-current circuit6and the second feedback circuit8are selected, then in steps S219, S220, and S221, the control unit10controls the voltage division ratio of the resistors of the output voltage detection circuit9to boost the output voltage VOUT of the boosting circuit2until the current detection circuit50detects the desired constant-current value. If the desired constant-current value has been obtained (YES in step S221), then in step S222, the plurality of LEDs in the first load circuit3are lit up by the output voltage VOUT of the boosting circuit2obtained by the control of the second feedback circuit8.

Next, if the control unit10has issued a request to drive the second load circuit4(YES in step S223), the control unit10maintains a state of selecting the second feedback circuit8, and the boosting circuit2performs a boosting operation based on a voltage detected by the output voltage detection circuit9. At this time, a boosting rate of the boosting circuit2is changed by the control unit10controlling the voltage division ratio of the resistors of the output voltage detection circuit9. Further, the control unit10changes the boosting rate of the boosting circuit2so that a voltage required to drive the second load circuit4is obtained as the output voltage VOUT of the boosting circuit2. If, in step S224, the boosting operation has been completed by the boosting circuit2(YES in step S224), then in step S225, the control unit10outputs a driving command and drives the second load circuit4using the output voltage VOUT from the boosting circuit2.

Next, if the driving of the second load circuit4has ended (YES in step S226), then in step S227, the control unit10controls the voltage division ratio of the resistors of the output voltage detection circuit9, thereby changing the boosting rate of the boosting circuit2to a value determined in step S220. In step S227, the boosting circuit2performs a boosting operation based on a voltage detected by the output voltage detection circuit9. Consequently, the desired LED current IL continues to be supplied to the first load circuit3, and therefore, in step S228, the plurality of LEDs in the first load circuit3continue to be lit up. Thus, the plurality of LEDs in the first load circuit3continue to be lit up while the processes in steps S204to S207and S218to S228are performed.

Next, a third exemplary embodiment will be described.

In the third exemplary embodiment, description will be given of a case where a condition of the input voltage Vin from the power source1to the boosting circuit2and a condition for lighting up the plurality of LEDs included in the first load circuit3are changed.FIG. 6is a diagram illustrating the components of a power supply circuit100according to the third exemplary embodiment. InFIG. 6, the components having functions similar to those of the components illustrated inFIG. 1are designated by the same reference numerals. In the third exemplary embodiment, among the components illustrated inFIG. 6, the components having functions similar to those of the components illustrated inFIG. 1will not be described. Similarly to the power supply circuit100according to the first exemplary embodiment, the power supply circuit100according to the third exemplary embodiment also operates as a power supply device applicable to an electronic device such as an imaging device.

One of the differences between the power supply circuit100according to the third exemplary embodiment and the power supply circuit100according to the first exemplary embodiment lies in the power source1. In the third exemplary embodiment, the power source1is assumed to be a lithium-ion battery having one or two cells or four to six AA batteries connected in series. As a result, the input voltage Vin of the boosting circuit2is assumed to be about 4 V to 10 V. Another one of the differences lies in the first load circuit3. The first load circuit3is a circuit that operates as the backlight circuit of a liquid crystal display device and includes a plurality of LEDs. In the third exemplary embodiment, a case is assumed where the first load circuit3is a circuit in which three to six white LEDs are connected in series. Thus, in the third exemplary embodiment, a voltage required to drive the first load circuit3is assumed to be about 8 V to 18 V.

Further, yet another one of the differences lies in that the power supply circuit100includes a voltage detection circuit60, which detects the voltage at the input node of the boosting circuit2. The voltage detection circuit60includes resistors61and62. The resistors61and62are connected in series between the input node of the boosting circuit2and the ground potential. A potential of the connection point between the resistors61and62is output as a voltage detected by the voltage detection circuit60. The control unit10can detect an input voltage of the boosting circuit2based on an output of the voltage detection circuit60. The control unit10controls switching between the first feedback circuit7and the second feedback circuit8based on a voltage detected by the voltage detection circuit60.

Next, with reference toFIG. 7, an operation of the power supply circuit100according to the third exemplary embodiment will be described.FIG. 7is a flow chart illustrating an exemplary operation of the power supply circuit100according to the third exemplary embodiment.

In step S301, the power source1starts supplying power to the power supply circuit100. In step S302, the control unit10turns off the switches32and34and turns on the switches36and38to select the constant-current circuit6and the second feedback circuit8. While the constant-current circuit6and the second feedback circuit8are selected, the power supply circuit100operates in the second driving mode. If the constant-current circuit6and the second feedback circuit8are selected, then in step S303, the boosting circuit2performs a boosting operation based on a voltage detected by the output voltage detection circuit9. If the boosting operation for boosting the input voltage Vin to a target output voltage (e.g., a second voltage V2) has been completed by the boosting circuit2(YES in step S304), the control unit10causes the constant-current supply35in the constant-current circuit6to operate. Then, in step S305, the control unit10lights up the plurality of LEDs included in the first load circuit3(the backlight circuit) and also simultaneously drives the liquid crystal display unit12. Such operation is performed, thereby, when displaying an image or the like on the liquid crystal display device, enabling instantaneous display of the image or the like with an excellent responsiveness.

Next, in this state, power consumption of the first load circuit3is: (a voltage required to drive the second load circuit4)×(an amount of constant current). This is inefficient to drive the backlight circuit, which is the first load circuit3. Thus, it is desirable that the boosting circuit2is controlled by the first feedback circuit7and the detection resistor circuit5. In step S306, however, the control unit10determines whether an input voltage Vin of the boosting circuit2detected by the voltage detection circuit60exceeds a predetermined value corresponding to a voltage required to drive the first load circuit3. In this case, the first voltage V1 is a voltage lower than the second voltage V2.

Next, if it has been determined in step S306that the input voltage Vin of the boosting circuit2does not exceed the predetermined value corresponding to the voltage required to drive the first load circuit3(YES in step S306), then in step S307, the control unit10turns off the switches36and38and turns on the switches32and34to select the first feedback circuit7and the detection resistor circuit5. While the first feedback circuit7and the detection resistor circuit5are selected, the power supply circuit100operates in the first driving mode. If the first feedback circuit7and the detection resistor circuit5are selected, the boosting circuit2is controlled by the first feedback circuit7and the detection resistor circuit5. The boosting circuit2performs a boosting operation based on a voltage dropped by the detection resistor circuit5. In step S307, the boosting circuit2operates so that the output voltage VOUT of the boosting circuit2would be the first voltage V1. In this case, the first voltage V1 is a voltage lower than the second voltage V2. At this time, in step S308, since the desired LED current IL continues to be supplied to the first load circuit3, the plurality of LEDs in the first load circuit3continue to be lit up.

Next, if the control unit10has issued a request to drive the second load circuit4(YES in step S309), then in step S310, the control unit10turns off the switches32and34and turns on the switches36and38to select the constant-current circuit6and the second feedback circuit8. While the constant-current circuit6and the second feedback circuit8are selected, the power supply circuit100operates in the second driving mode. If the constant-current circuit6and the second feedback circuit8are selected, the boosting circuit2performs a boosting operation based on a voltage detected by the output voltage detection circuit9. If the boosting operation for boosting the input voltage Vin to a predetermined output voltage (e.g., the second voltage V2) has been completed by the boosting circuit2(YES in step S311), then in step S312, the control unit10outputs a driving command and drives the second load circuit4using the output voltage VOUT from the boosting circuit2. At this time, in step S313, the plurality of LEDs in the first load circuit3can continue to be lit up by being driven by the constant-current supply35in the constant-current circuit6. Thus, it is possible to continue to display an image or the like on the liquid crystal display device while the second load circuit4is driven.

Next, if the driving of the second load circuit4has ended (YES in step S314), then in step S315, the control unit10turns off the switches36and38and turns on the switches32and34to select the first feedback circuit7and the detection resistor circuit5again. While the first feedback circuit7and the detection resistor circuit5are selected, the power supply circuit100operates in the first driving mode. If the first feedback circuit7and the detection resistor circuit5are selected, the boosting circuit2performs a boosting operation based on a voltage dropped by the detection resistor circuit5. In step S315, the boosting circuit2operates so that the output voltage VOUT of the boosting circuit2would be the first voltage V1. Consequently, in step S316, it is possible to reduce power consumption and continue to light up the plurality of LEDs in the first load circuit3. Thus, the plurality of LEDs in the first load circuit3continue to be lit up while the processes in steps S305to S316are performed.

If, on the other hand, it has been determined in step S306that the input voltage Vin of the boosting circuit2exceeds the predetermined value corresponding to the voltage required to drive the first load circuit3(NO in step S306), then in steps S318and S319, the control unit10maintains a state of selecting the second feedback circuit8, and the boosting circuit2performs a boosting operation based on a voltage detected by the output voltage detection circuit9. The boosting circuit2appropriately changes a boosting rate and performs the boosting operation for boosting the input voltage Vin to the voltage required to drive the first load circuit3. In step S320, the first load circuit3is driven at a constant current by the output voltage VOUT of the boosting circuit2. Consequently, the plurality of LEDs in the first load circuit3continue to be lit up while the processes in steps S305, S306, and S318to S320are performed.

At least one of the various functions, processes, and methods described in the first to third exemplary embodiments can be achieved using a program. Hereinafter, in a fourth exemplary embodiment, a program for realizing at least one of the various functions, processes, and methods described in the first to third exemplary embodiments will be referred to as a “program X”. Further, in the fourth exemplary embodiment, a computer for executing the program X will be referred to as a “computer Y”. Examples of the computer Y include a personal computer, a microcomputer, and a central processing unit (CPU).

At least one of the various functions, processes, and methods described in the first to third exemplary embodiments can be realized by the computer Y executing the program X. In this case, the program X is supplied to the computer Y via a computer readable storage medium. A computer readable storage medium according to the fourth exemplary embodiment includes at least one of a hard disk device, an optical disc, a Compact Disc Read Only Memory (CD-ROM), a Compact Disc Recordable (CD-R), a memory card, a read only memory (ROM), and a random access memory (RAM). Further, the computer readable storage medium according to the fourth exemplary embodiment is a non-transitory storage medium.

While the present invention is described with reference to exemplary embodiments, it is to be understood that the present invention is not limited to the exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications and equivalent structures.

This application claims the benefit of Japanese Patent Application No. 2013-210484, filed Oct. 7, 2013, which is hereby incorporated by reference herein in its entirety.