JP2009300199A - Rc discharge circuit, power source management circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an RC discharge circuit having improved measuring resolution, or the like. <P>SOLUTION: This RC discharge circuit (33) for discharging the charge accumulated in a capacitor (C) through a resistance part including electric resistances (R1, R2) has a constitution equipped with a time constant changing part (33a) including a detection part for detecting a charge voltage of the capacitor, and operated when the charge voltage of the capacitor becomes below a prescribed threshold (Vth), for changing a charge time constant specified by a capacitance of the capacitor and a resistance value of the resistance part by changing the whole resistance value of the resistance part. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an RC discharge circuit and a power management circuit.

2. Description of the Related Art Electronic devices such as printers are known that include a CR circuit (RC circuit) that measures the time from when power supply is stopped (see, for example, Patent Document 1).
JP-A-8-104013

This type of timing circuit is desired to be able to measure the elapsed time after power supply is stopped more accurately.
An object of the present invention is to provide an RC discharge circuit and a power management circuit with improved measurement resolution.

The present invention solves the above problems by the following means. In addition, in order to make an understanding easy, although the code | symbol corresponding to embodiment of this invention is attached | subjected and demonstrated, it is not limited to this.
The invention according to claim 1 is an RC discharge circuit for discharging the electric charge accumulated in the capacitor (C) through a resistance unit including electric resistances (R1, R2), and a detection unit for detecting a charging voltage of the capacitor And when the charging voltage of the capacitor becomes less than a predetermined threshold value (Vth), the capacitance of the capacitor and the entire resistance portion are changed by changing the overall resistance value of the resistance portion. An RC discharge circuit (33) comprising a time constant changing unit (33a) for changing a discharge time constant defined by a specific resistance value.

According to a second aspect of the present invention, in the RC discharge circuit according to the first aspect, the time constant changing unit (33a) is configured such that the charging voltage of the capacitor (C) is less than the predetermined threshold value (Vth). The RC discharge circuit (33) is characterized in that the discharge time constant is increased.
According to a third aspect of the present invention, in the RC discharge circuit according to the first or second aspect, the resistance unit includes a plurality of resistors (R1, R2), and the time constant changing unit (33a) includes the plurality of resistors. An RC discharge circuit (33) comprising a switch mechanism for changing a connection mode of the resistors, and changing the discharge time constant by changing a connection mode of the plurality of resistors.
According to a fourth aspect of the present invention, in the RC discharge circuit according to the third aspect, a first resistor (R1) and a second resistor (R2) are connected in parallel, and the charging voltage of the capacitor (C) is provided. Is equal to or greater than the predetermined threshold (Vth), the capacitor is discharged through the first resistor and the second resistor, and the charge voltage of the capacitor is less than the predetermined threshold. Is an RC discharge circuit (33), wherein the capacitor is discharged through the second resistor.
According to a fifth aspect of the present invention, in the RC discharge circuit according to the third aspect, the first resistor (R10) and the second resistor (R20) having different resistance values are respectively connected via the switching elements (S10, S20). When the charging voltage of the capacitor (C) is equal to or higher than a predetermined threshold, the capacitor is discharged through the first resistor, and the charging voltage of the capacitor is set to the predetermined voltage. The capacitor is discharged through the second resistor, and the resistance value of the first resistor is smaller than the resistance value of the second resistor. It is a discharge circuit (330).
According to a sixth aspect of the present invention, in the RC discharge circuit according to the first or second aspect, the resistance portion includes a plurality of resistors (R1 to R4), and the capacitor includes the first capacitor (C1). A second capacitor (C2) is provided in parallel connection, and the time constant changing unit (33a) changes a connection mode of the plurality of resistors, the first capacitor, and the second capacitor. A switching mechanism, and when the charging voltage of the capacitor is equal to or higher than the predetermined threshold value (Vth), the electric charge accumulated in the first capacitor and the second capacitor is discharged through the plurality of resistors. When the charging voltage of the capacitor becomes less than the predetermined threshold, the charge accumulated in the first capacitor is discharged through a part (R3) of the plurality of resistors, and the second Capacitor A RC discharge circuit (40), characterized in that discharging the accumulated charge via the other portion of the plurality of resistors (R2, R4).
According to a seventh aspect of the present invention, in the RC discharge circuit according to the sixth aspect, a circuit formed by the first capacitor (C1) and a part of the plurality of resistors (R3), and the second capacitor ( The RC discharge circuit (40) is characterized in that a discharge time constant is different from that of a circuit formed by C2) and other portions (R2, R4) of the plurality of resistors.

According to an eighth aspect of the present invention, there is provided an RC discharge circuit (33) according to any one of the first to seventh aspects, and a power supply state monitoring unit for monitoring a power supply state with respect to a power supply target (23). 31) and an elapsed time information acquisition unit (36) for obtaining a time from when power supply to the power supply target is stopped until power supply is resumed, the RC discharge circuit The capacitor (C) is charged in advance, and when the power supply to the power supply object is stopped based on the output of the power supply state monitoring unit, the capacitor starts discharging, and the elapsed time information acquisition unit Is the elapsed time from when power supply is stopped to when it is restarted based on the amount of change in the charging voltage of the capacitor when power supply to the power supply object is restarted. A power management circuit (30) and finding.
According to a ninth aspect of the present invention, in the power management circuit according to the eighth aspect, the elapsed time information acquisition unit (36) records the amount of change in the charging voltage of the capacitor (C) and the elapsed time in association with each other. The power management circuit (30) is characterized in that the elapsed time is obtained by referring to the elapsed time information.
The power management circuit according to claim 10 includes a charge control circuit (34) for charging a capacitor (C) provided in the RC discharge circuit (33) in the power management circuit according to claim 8 or 9, wherein The charge control circuit is a power management circuit (30) including a first suppression unit (Tr1, Tr2) that suppresses leakage of charges charged in the capacitor.
An eleventh aspect of the invention is the power management circuit according to any one of the eighth to tenth aspects, wherein the charging voltage information of the capacitor (C1) provided in the RC discharge circuit (33) is read out. A readout control unit (35) that inputs to the elapsed time information acquisition unit (36) is provided, and the readout control unit includes a second suppression unit (Tr3, Tr4) that suppresses leakage of charges charged in the capacitor. This is a power management circuit (30).
Note that the configuration described with reference numerals may be modified as appropriate, and at least a part of the configuration may be replaced with another component.

  According to the present invention, it is possible to provide an RC discharge circuit and a power management circuit with improved measurement resolution.

[First Embodiment]
A camera including an embodiment of a power management circuit to which the present invention is applied will be described below with reference to the drawings and the like.
FIG. 1 is a block diagram showing the configuration of the camera of the first embodiment.
The camera 10 includes an image sensor 11 that converts subject light into an electrical signal and outputs the image, a photographing lens 12 that guides the subject light to the image sensor 11, an image processing circuit 13 that generates image data based on the output of the image sensor 11, and an image A display device 14 that displays an image generated based on data, a control unit 15 that comprehensively controls electrical elements such as the image sensor 11, and a battery 16 that functions as a power source for the electrical elements such as the image sensor 11. The digital camera includes a battery chamber 17 that is detachably accommodated, an illumination device 20 that emits illumination light (flash) to a subject at the time of photographing, and the like.

The illumination device 20 includes a xenon lamp 21 that is a light emitting element, and a diffusion plate 22 that diffuses the illumination light forward of the illumination light with respect to the xenon lamp 21. The diffusion plate 22 is made of, for example, a synthetic resin material.
The lighting device 20 includes a light emission control circuit 23 including a capacitor, a microcomputer, and the like (both not shown) that function as a power source for the xenon lamp 21. The light emission control circuit 23 performs light emission start control, light emission stop control, and the like of the illumination light by the xenon lamp 21 according to an instruction from the control unit 15.

Here, since the illuminating device 20 irradiates the subject with illumination light such as flash light emitted from the xenon lamp 21 via the diffusion plate 22, the diffusion plate 22 is heated by the illumination light each time light is emitted. Then, in order to prevent the diffusion plate 22 from being deformed by heating, the lighting device 20 causes the light emission control circuit 23 to perform, for example, light emission inhibition control when the temperature of the diffusion plate 22 exceeds a certain value. I do.
When performing such light emission inhibition control, it is necessary to constantly monitor the temperature of the diffusion plate 22 in order to determine whether or not the illumination light can be emitted. However, since the diffusion plate 22 allows the illumination light to pass, It is practically impossible to provide a temperature sensor or the like.

  For this reason, the light emission control circuit 23 provided in the illumination device 20 of the present embodiment measures the elapsed time after light emission by a timer circuit (not shown) provided in the light emission control circuit 23. The light emission control circuit 23 depends on the outside air temperature measured by a temperature sensor (not shown) provided in the lighting device 20, the temperature rise (predicted value) of the diffusion plate 22 due to one light emission, and the time elapsed after light emission. The temperature change of the diffusion plate 22 is calculated in real time by comprehensively considering the temperature drop (predicted value) and the like.

  Here, the light emission control circuit 23 including the timer circuit is operated by power supply from the battery 16. For this reason, when the battery 16 is removed from the battery chamber 17 while the elapsed time after the light emission is being measured by the timer circuit, the accurate elapsed time after the completion of the light emission cannot be measured. In addition, when it becomes impossible to measure the elapsed time after the light emission is completed, in addition to the case where the battery 16 is removed, when the battery 16 is exhausted and the voltage is reduced, or when the camera 10 performs power consumption reduction control (when not photographing, etc. The control for stopping the power supply to the image pickup device 11, the display device 14, etc.) is executed, and the power supply to the timer circuit of the light emission control circuit 23 is also stopped.

On the other hand, the camera 10 of this embodiment includes a power management circuit 30 including an RC discharge circuit (also referred to as a CR discharge circuit) that can operate even when the battery 16 is removed from the battery chamber 17. ing.
Since the power management circuit 30 can measure the elapsed time from when the battery 16 is removed until the battery 16 is mounted again, the light emission control circuit 23 can accurately measure the elapsed time after the completion of light emission together with the output of the timer circuit. Time can be calculated.

Hereinafter, the power management circuit 30 will be described.
FIG. 2 is a circuit diagram showing a power management circuit provided in the camera shown in FIG.
The power management circuit 30 includes a DC-DC converter 31, a low voltage detection IC 32, an RC discharge circuit 33, a charge control circuit 34, a read control circuit 35, a CPU 36, and the like.

The DC-DC converter 31 is electrically connected to the battery 16 accommodated in the battery chamber 17, and the DC power source voltage output from the battery 16 is about 3.7 V, for example, about 3 V DC, for example. Transform to power supply voltage (Vcc) and output.
The low voltage detection IC 32 is electrically connected to the DC-DC converter 31 and the CPU 36 and monitors the output voltage of the DC-DC converter 31. The low voltage detection IC 32 detects this when the output voltage from the DC-DC converter 31 has dropped significantly (detection of power supply stoppage), for example, when the battery 16 is removed from the battery chamber 17 of the camera 10. This is transmitted to the CPU 36.

  The RC discharge circuit 33 includes a capacitor C and a plurality of resistors R1 and R2. The capacitor C is charged in advance by a charge control circuit 34 described later. The charge control circuit 34 stops the charging of the capacitor C when the low voltage detection IC 32 detects the power supply stop. Capacitor C automatically starts discharging when charging is stopped.

The charge control circuit 34 is a circuit that charges the capacitor C provided in the RC discharge circuit 33. The charge control circuit 34 includes a PNP transistor Tr1 and an NPN transistor Tr2, and the voltage drop of the power supply voltage Vcc supplied from the DC-DC converter 31 can be substantially ignored. Thereby, the charging control circuit 34 can perform charging until the charging voltage (Vc) of the capacitor C becomes equal to the power supply voltage Vcc.
The PNP transistor Tr <b> 1 and the NPN transistor Tr <b> 2 suppress the leakage of the charge accumulated in the capacitor C to the charge control circuit 34 when the power supply from the DC-DC converter 31 is stopped. Therefore, the RC discharge circuit 33 can substantially ignore the voltage drop of the capacitor C even when the charge control circuit 34 is in the OFF state.

The read control circuit 35 is a circuit that reads the charging voltage of the capacitor C provided in the RC discharge circuit 33 and outputs it to an A / D converter (not shown) provided in the CPU 36. In a state where power is supplied from the DC-DC converter 31 to the capacitor C, the read control circuit 35 is not operating (the read control circuit is off).
Similarly to the charge control circuit 34, the read control circuit 35 includes a PNP transistor Tr3 and an NPN transistor Tr4, and suppresses leakage of charge from the capacitor C to the read control circuit 35 in the off state. As a result, the capacitor C can substantially ignore the voltage drop from the fully charged state (voltage = Vcc).

  The CPU 36 outputs the output from the read control circuit 35 (capacitor C charging voltage information), the charging voltage of the capacitor C and the elapsed time after the power supply is stopped, which is stored in the nonvolatile memory 37 in advance. Based on the data (table) associated with, the time elapsed since the power supply was stopped is obtained. Note that when the power supply is stopped, the CPU 36 itself stops operating, and thus the elapsed time is obtained after the power supply is resumed.

Next, the configuration of the RC discharge circuit 33 will be described.
As described above, the power management circuit 30 according to the present embodiment is based on the charging voltage (remaining voltage) of the capacitor C provided in the RC discharge circuit 33 until the power supply is resumed after the power supply is stopped. Elapsed time (hereinafter simply referred to as elapsed time).
Here, in the camera 10, as an example in which the power supply is stopped, the battery 16 can be replaced. The replacement of the battery 16 is normally considered to be completed in several seconds to several tens of seconds although it depends on the types of the camera 10 and the battery 16.
As described above, the power management circuit 30 is assumed to often have a measurement target time of several tens of seconds or less. Therefore, the power supply circuit 30 has a particularly high accuracy with an elapsed time of several tens of seconds after the power supply is stopped. It is preferable that measurement is possible. On the other hand, as an example in which the power supply is stopped, it can be considered other than battery replacement. Therefore, it is desirable that the elapsed time can be measured at most several tens of minutes.

  Therefore, the RC discharge circuit 33 provided in the power management circuit 30 of the present embodiment has a discharge time constant (hereinafter simply referred to as a time constant) when the charging voltage of the capacitor C becomes less than a predetermined threshold value (Vth). By increasing the value, the elapsed time can be measured with high accuracy for several tens of seconds after the power supply is stopped, and the elapsed time can be measured for several tens of minutes. The time constant is a value defined by the product of the capacitance of the capacitor C provided in the RC discharge circuit 33 and the resistance value of the resistor R (R1, R2) used for discharging the capacitor C.

As shown in FIG. 2, the RC discharge circuit 33 includes a capacitor C, a resistor R1, and an overvoltage detection IC 33a. The capacitor C is charged by the above-described charging control circuit 34 until the charging voltage becomes the voltage Vcc.
The overvoltage detection IC 33a is a part that monitors the charging voltage of the capacitor C, and includes a resistor R2 connected in parallel to the resistor R1. In the present embodiment, the resistance value of the resistor R1 is set smaller than the resistance value of the resistor R2 (R1 <R2). However, since the condition of “R1 // R2 <R2” (R1 // R2 indicates a combined resistance value when the resistor R1 and the resistor R2 are connected in parallel) is always satisfied, the resistance value May be R1> R2. In some cases, the relationship between the resistance values may be R1 = R2.

The overvoltage detection IC 33a includes a switch mechanism that is activated by a current flowing from the capacitor C to the resistor R2. The RC discharge circuit 33 discharges the capacitor C using both the resistor R1 and the resistor R2 when the charging voltage of the capacitor C is equal to or higher than the threshold Vth, and when the charging voltage becomes lower than the threshold Vth, The detection IC 33a automatically cuts off the connection between the resistor R1 and the resistor R2, and switches the circuit so that the capacitor C is discharged only by the resistor R2.
Hereinafter, the operation of the power management circuit 30 will be specifically described with reference to FIG.
FIG. 3 is a timing chart showing the operation of the power management circuit shown in FIG.

FIG. 3 starts from a state where the charging of the capacitor C is completed (charging voltage Vcc).
At time Ta, for example, the battery 16 is removed from the battery chamber 17 and the power supply to the power management circuit 30 is stopped.
When the power supply is stopped, the low voltage detection IC 32 detects this and stops the operation of the CPU 36 (Tb). When the CPU 36 stops operating, the output of the output port OUT2 of the CPU 36 switches from High to Low, whereby the charge control circuit 34 stops charging the capacitor C, and at the same time, the RC discharge circuit 33 starts discharging the capacitor C. To do.
At this time, the RC discharge circuit 33 discharges the capacitor C using the resistors R1 and R2 connected in parallel. The time constant t1 at this time is defined by t1 = (R1 // R2) * C. Here, (R1 // R2) is a combined resistance value of the resistor R1 and the resistor R2, and C is a capacitance of the capacitor C.

When the charging voltage of the capacitor C decreases with the passage of time and becomes less than the predetermined threshold Vth (Tc), the overvoltage detection IC 33a detects this and the switch mechanism provided in the overvoltage detection IC 33a Resistor R1 is disconnected from the circuit. As a result, the capacitor C is discharged only through the resistor R2. The time constant t2 of the RC discharge circuit 33 at this time is defined by t2 = R2 * C. Since the combined resistance value R1 // R2 of the resistors R1 and R2 is smaller than the resistance value of the resistor R2 (R1 // R2 <R2), the time constant t1 <time constant t2.
Here, when the time constant is large and the case where the time constant is small, the RC discharge circuit 33 can discharge the capacitor C faster when the time constant is small. Then, the discharge speed of the capacitor C becomes slower at this timing (Tc) (see the discharge curve in FIG. 3).

For example, when the battery 16 is mounted again and the power supply is resumed before the discharge of the electric charge accumulated in the capacitor C is completed, the low voltage detection IC 32 detects this (Td), Accordingly, the CPU 36 resumes operation.
When the operation is resumed, the CPU 36 activates the output port OUT1 (Te) while keeping the output port OUT2 inactive (Low). As a result, the read control circuit 35 is turned on, and the charging voltage of the capacitor C is measured and output to the CPU 36. The CPU 36 performs A / D conversion on the charging voltage information of the capacitor C, refers to a table recorded in the nonvolatile memory 37, and determines the elapsed time (Tb to Td) from when the battery 16 is removed to when it is mounted again. It calculates and outputs to the light emission control circuit 23 (Tf). Thereafter, the CPU 36 switches the output of the output port OUT2 to High, and the RC discharge circuit 33 stops discharging the capacitor C (Tg).
The power management circuit 30 according to the present embodiment obtains the elapsed time with reference to a table prepared in advance, but is not limited thereto, and may be calculated based on the following Equation 1.
T = f (t) (Formula 1)
In the above (Formula 1), T represents temperature and t represents elapsed time.

Next, power management control performed by the power management circuit 30 provided in the camera 10 of the present embodiment will be described step by step using a flowchart.
FIG. 4 is a flowchart showing power management control performed by the power management circuit shown in FIG.

(Step S01: Charge)
In the power management circuit 30, the charge control circuit 34 charges the capacitor C with the battery 16 mounted on the camera 10 (proceeds to step S02).
(Step S02: Power supply determination)
For example, when the battery 16 is removed from the battery chamber 17, the low voltage detection IC 32 detects this based on the output of the DC-DC converter 31, and proceeds to step S03. If stoppage of power supply is not detected, the standby state is entered.

(Step S03: Stop charging)
When the power supply is stopped, the operation of the CPU 36 is stopped, and accordingly, the charge control circuit 34 stops the charging of the capacitor C and proceeds to step S04.
(Step S04: Start of discharge)
The RC discharge circuit 33 discharges the electric charge accumulated in the capacitor C through the resistor R1 and the resistor R2, and proceeds to step S05. The time constant of the RC discharge circuit 33 at this time is t1.

(Step S05: Power supply determination)
If the power supply is resumed while discharging the capacitor C with the time constant t1, the process proceeds to step S09. On the other hand, if the power supply is not resumed, the process proceeds to step S06.
(Step S06: Charge voltage determination)
When the overvoltage detection IC 33a detects that the charging voltage of the capacitor C has become less than the threshold value Vth, the RC discharge circuit 33 proceeds to step S07. If the charging voltage of the capacitor is equal to or higher than the threshold value Vth, the process returns to step S05.

(Step S07: Change time constant)
The overvoltage detection IC 33a changes the time constant of the RC discharge circuit 33 from t1 to t2, and proceeds to step S08. Thereby, the voltage drop amount per time of the capacitor C becomes small.
(Step S08: Power supply determination)
If the power supply is resumed while discharging the capacitor C with the time constant t2, the process proceeds to step S09. On the other hand, when the power supply is not resumed, the discharge of the capacitor C is continued until the charge voltage cannot be detected. In the case of the power management circuit 30 according to the present embodiment, when the charging voltage is about 0.5 V, for example, the change in the charging voltage per elapsed time becomes minute, so it is difficult to obtain the elapsed time based on the charging voltage. become.
However, in the power management circuit 30 of the present embodiment, the charging voltage of the capacitor C becomes about 0.5 V, for example, after 30 minutes or more have passed since the power supply was stopped. Although the power management circuit 30 is for protecting the diffusion plate 22 of the lighting device 20, for example, 30 minutes or more have passed since the power supply was stopped even if the light was emitted just before the power supply was stopped. After that, there is no practical problem because the temperature of the diffusion plate 22 is considered to have decreased to about the outside air temperature.

(Step 09: Find the elapsed time)
When the power supply is resumed, the read control circuit 35 reads the charge voltage of the capacitor C and outputs it to the CPU 36. The CPU 36 refers to the table recorded in the nonvolatile memory 37 and based on the charge voltage of the capacitor C. The time elapsed since the power supply was stopped is obtained.
The camera 10 finally illuminates based on the time from the last light emission until the power supply is stopped (timed by a timer circuit provided in the light emission control circuit 23) and the elapsed time obtained by the power management circuit 30. The total elapsed time after emitting light is obtained. Then, the temperature of the diffusion plate 22 is obtained based on the total elapsed time after the light emission, and the light emission inhibition control or the like is performed as necessary.

Next, the effect of the power management circuit 30 of this embodiment is demonstrated using the graph shown in FIG.
FIG. 5 is a graph showing an example of a change in the charging voltage of the capacitor provided in the RC discharge circuit shown in FIG.
In FIG. 5, (a) shows a change in charging voltage until about 2200 seconds (about 37 minutes) have elapsed from the start of discharge, and (b) is a partially enlarged view of the graph shown in (a). There is a change in the charging voltage until about 40 seconds elapse from the start of discharging.

  In FIG. 5, graph A shows the change in the charging voltage of the capacitor C by the RC discharge circuit 33 of the present embodiment, and graph B shows the charging voltage of the capacitor C by the RC discharge circuit (not shown) of the comparative form. Shows changes. The RC discharge circuit of the comparative form is a known RC discharge circuit including one capacitor C and one resistor R.

  In the graph shown in FIG. 5, as an example, the charging voltage Vcc of the capacitor C is, for example, 3.0 V, and the threshold Vth that is a reference when the overvoltage detection IC 33 a switches the time constant is, for example, 2. Although the case where it is set to 4 V is shown, the charging voltage Vcc and the threshold value Vth are not limited to these values, and can be appropriately changed.

  As described above, the RC discharge circuit 33 discharges the capacitor C with the time constant t1 until the charging voltage of the capacitor C becomes less than the threshold value Vth (for example, 2.4 V in the example shown in FIG. 5). Do. In the case of the graph shown in FIG. 5, the charging voltage of the capacitor C becomes less than 2.4, for example, when about 30 seconds elapses from the start of discharge.

  Then, the overvoltage detection IC 33a changes the time constant of the RC discharge circuit 33 from t1 to t2 larger than t1 when the charging voltage of the capacitor C becomes less than 2.4V, for example. Since the RC discharge circuit 33 has a slower discharge rate as the time constant increases, the rate of decrease of the charging voltage per time is slower (gradient of the graph) as shown in FIG.

  As described above, the power management circuit 30 according to the present embodiment needs to measure the elapsed time in detail, for example, compared with the power management circuit according to the comparative example for about 30 seconds after the power supply is stopped. The discharge speed of the capacitor C is fast. Therefore, the amount of change in voltage is large even at the same elapsed time, and the resolution when obtaining the elapsed time based on the change in voltage is high.

More specifically, as shown in FIG. 5B, the RC discharge circuit 33 of the present embodiment has a charging voltage of the capacitor C, for example, at the time when 20 seconds have elapsed since the power supply stop, for example, In contrast to the voltage drop of about 0.42 V, the RC circuit of the comparative form has a voltage drop of only about 0.05 V, for example.
For example, in the case of the RC discharge circuit of the comparative form, when the 21 seconds have passed since the power supply stop, although a slight voltage drop can be confirmed, the change in the charging voltage of the capacitor compared to when 20 seconds passed is It is only small (0.01 V or less), and it is difficult to accurately obtain the elapsed time based on the change in the charging voltage.
In contrast, in the same example, the RC discharge circuit according to the present embodiment has a charging voltage of about 0.02 V when the power supply is stopped, for example, when 20 seconds and 21 seconds have elapsed. It is descending. Therefore, the power management circuit 30 including the RC discharge circuit 33 of the present embodiment can reliably and accurately obtain the elapsed time from the power supply stop based on the charging voltage of the capacitor C.

The power management circuit 30 of the first embodiment can obtain the following effects in addition to the effects described above.
(1) Since the switch mechanism provided in the RC discharge circuit 33 of the embodiment operates with the discharge voltage from the capacitor C, the resistor R1 and the resistor R2 are connected even when the power supply from the DC-DC converter 31 is stopped. The connection mode of can be changed. Therefore, even when the power supply is completely stopped, such as when the battery 16 is removed, the power management circuit 30 can reliably measure the time after the power supply is stopped.
(2) Since the time constant is changed according to the connection mode of the resistors, the time constant can be easily tuned by selecting the resistors (selecting the resistance values of the resistors R1 and R2). For example, it is easier to further reduce the time constant t1 and further increase the time constant t2 than in the above example.
(3) Since the timing for changing the time constant is determined by the detection voltage of the overvoltage detection IC 33a, the timing for switching the time constant can be easily tuned by changing the threshold value Vth. For example, in the above-described example, the time constant is changed in about 30 seconds, for example, after the power supply is stopped. However, by reducing the threshold value Vth, for example, highly accurate elapsed time measurement is performed over 30 seconds or more. Can do. .
(4) Since the transistors Tr for suppressing leakage of the charge charged in the capacitor C are provided in the charge control circuit 34 (Tr1, Tr2) and the readout control circuit 35 (Tr3, Tr4), respectively, the voltage of the capacitor C is surely The elapsed time can be obtained based on the change.

[Second Embodiment]
Next, a camera including a second embodiment of the power management circuit to which the present invention is applied will be described with reference to FIG. In this second embodiment and other embodiments described below, parts that perform the same functions as those of the first embodiment described above are given the same reference numerals or unified reference numerals at the end, and overlapping explanations or The drawings are omitted as appropriate.
The camera of the second embodiment differs from the camera of the first embodiment in the configuration of the RC discharge circuit provided in the power management circuit. Hereinafter, the configuration of the RC discharge circuit will be described.
FIG. 6 is a block diagram illustrating a power management circuit provided in the camera of the second embodiment.

  The RC discharge circuit 33 of the first embodiment includes one capacitor C, and switches the connection mode of the two resistors R1 and R2 when switching the time constant. In contrast, the RC discharge circuit 43 of the second embodiment is provided with two capacitors (C1, C2). When switching the time constant, these two capacitors C1, C2 are electrically connected in the circuit. Are separated to form two RC discharge circuits.

  The RC discharge circuit 43 provided in the power management circuit 40 of the second embodiment includes a first capacitor C1 and a second capacitor C2 connected in parallel as capacitors charged by the charge control circuit 44. ing. The capacitance of the first capacitor C1 is set larger than the capacitance of the second capacitor C2. Hereinafter, these combined capacitances are defined as (C1 // C2).

The RC discharge circuit 43 of the second embodiment is provided with four resistors R1 to R4. The relationship between the resistance values of the four resistors is R3>R2>R4> R1.
The first capacitor C1, the second capacitor C2, and the first to fourth resistors R1 to R4 form a first RC discharge circuit. The output of the first RC discharge circuit is input to the A / D1 port of the CPU.

  The resistor R2 is provided in the overvoltage detection IC 43a. The overvoltage detection IC includes a switch mechanism that is activated by a current flowing from the capacitors C1 and C2 to the resistor R2. When the charging voltage of the first capacitor C1 and the second capacitor C2 is equal to or higher than the threshold value Vth (Vc ≧ Vth), the power management circuit 40 sets the capacitors (C1, C2) by the first RC discharge circuit described above. When discharging is performed and the charging voltage of the first capacitor C1 and the second capacitor C2 reaches the threshold value Vth (Vc <Vth), the switch mechanism includes the second capacitor C2, the resistor R2, and the resistor R4. A second RC discharge circuit, and a third RC discharge circuit including a first capacitor C1 and a resistor R3 are formed. The output of the second RC discharge circuit is input to the A / D1 port of the CPU 46. The output of the third RC discharge circuit is input to the A / D2 port of the CPU 46.

  The time constant t1 of the RC discharge circuit 43 in the case of Vc ≧ Vth is defined by t1 = (R1 // R2 // R3 // R4) * (C1 // C2). On the other hand, when Vc <Vth, the time constant t2 of the second RC discharge circuit is defined by t2 = (R2 // R4) * C2. The time constant t3 of the third RC discharge circuit is defined by t3 = R3 * C1. A relationship of t1 <t2 <t3 is established between the time constants t1 to t3.

In the RC discharge circuit 43 of the second embodiment, immediately after the low voltage detection IC 42 detects the stop of power supply (Vc ≧ Vth), the output of the first RC discharge circuit is input to the A / D1 port of the CPU.
When the overvoltage detection IC 43a detects that the charging voltage of the capacitor C1 and the capacitor C2 is less than Vth (Vc <Vth), the output of the second RC discharge circuit is connected to the A / D1 port of the CPU. The output of the RC discharge circuit is input to the A / D2 port of the CPU. Since the second RC discharge circuit and the third RC discharge circuit have different time constants, the amount of voltage drop per time is different. As a result, the CPU 46 changes the charging voltage of the capacitors (C1, C2) in two ways. Can be obtained in a pattern.

FIG. 7 is a graph showing a change in the charging voltage of the capacitor provided in the RC discharge circuit shown in FIG.
Since the first capacitor C1 and the second capacitor C2 are connected in parallel, when the charging voltage Vc is equal and Vc ≧ Vth (time constant = t1), a graph showing the change of these charging voltages (hereinafter, referred to as “the charging voltage Vc”). As shown in FIG. 7, there is only one type of discharge curve.
On the other hand, when Vc <Vth (time constant = t2 and t3), two RC discharge circuits having different time constants are formed, so two types of discharge curves are formed.
The discharge curve based on the output of the third RC discharge circuit (C1 discharge curve) has a smaller amount of voltage drop per time than the discharge curve based on the output of the second RC discharge circuit (C2 discharge curve). So it is suitable for long time measurement. In addition, the discharge curve based on the output of the second RC discharge circuit (C2 discharge curve) has a voltage drop amount per time as compared to the discharge curve based on the output of the third RC discharge circuit (C1 discharge curve). Therefore, it is possible to measure the elapsed time since the power supply is stopped in more detail.
As described above, the power management circuit 40 according to the second embodiment can measure the elapsed time with higher accuracy for a longer time than the power management circuit 30 according to the first embodiment. It is also possible to measure the elapsed time.

[Third Embodiment]
Next, a camera including a third embodiment of a power management circuit to which the present invention is applied will be described.
FIG. 8 is a block diagram showing a configuration of an RC power management circuit provided in the camera of the third embodiment.
The RC discharge circuit 53 provided in the power management circuit 50 of the third embodiment includes two capacitors C1 and C2 connected in parallel as in the second embodiment. Three resistors (R1 to R3) are provided, and the resistor R2 is provided in the overvoltage detection IC 53a.

The RC discharge circuit 53 of the third embodiment discharges the charges accumulated in the capacitors C1 and C2 via the resistors R1 to R3 when Vc ≧ Vth.
When Vc <Vth, two RC discharge circuits are formed by the switch mechanism provided in the overvoltage detection IC 53a, similarly to the RC discharge circuit 43 of the second embodiment.
In the RC discharge circuit 53 of the third embodiment, a second RC discharge circuit (time constant t2 = C2 * R2) including a capacitor C2 and a resistor R3, and a third RC discharge circuit including a capacitor C1 and a resistor R3. (Time constant t3 = C1 * R3) is formed. The capacitance of the capacitor C1 is set larger than the capacitance of the capacitor C2 (C1> C2). Further, the resistance values of the resistors R1 to R3 (respectively R1 to R3) have a relationship of R2>R1> R3, and the relationship of t1 <t2 <t3 is satisfied between the time constants t1 to t3. ing.

In addition, since the graph (discharge curve) which shows the change of the charging voltage of capacitor | condenser C1, C2 with which RC discharge circuit 53 of 3rd Embodiment was provided is substantially the same as 2nd Embodiment, the illustration is abbreviate | omitted. It shall be.
The power management circuit 50 of the third embodiment described above can also obtain the same effects as those of the second embodiment.

[Deformation]
The present invention is not limited to the embodiment described above, and various modifications and changes as shown below are possible, and these are also included in the technical scope of the present invention.
(1) In the RC discharge circuit of the first embodiment, the discharge time constant is changed by changing the connection of two resistors. However, the present invention is not limited to this, and a variable resistor is provided in the RC discharge circuit to charge the capacitor. The resistance value of the variable resistor may be changed according to the above.
(2) The RC discharge circuit provided in the power management circuit of the embodiment detects the charging voltage of the capacitor by the overvoltage detection IC. However, the present invention is not limited to this as long as the charging voltage threshold can be detected. And an inverter may be used in combination.
(3) In the RC discharge circuit of the embodiment, only one threshold is set. However, the present invention is not limited to this, and the circuit may be configured such that a plurality of thresholds are provided and the time constant is switched in a plurality of stages.
(4) The RC discharge circuit of the embodiment performs control to increase the time constant when the charging voltage of the capacitor becomes less than the threshold, but the type of electrical equipment using the RC discharge circuit and the purpose of power management Depending on the case, for example, the time constant may be controlled to be small at the timing when the charging voltage becomes less than the threshold.
(5) The power management circuit of the embodiment is provided in the camera, but is not limited thereto, and may be provided in another electrical device such as a printer.
(6) A modification in which the circuit (FIG. 2) in the first embodiment is modified as follows is also conceivable.
FIG. 9 is a circuit diagram of an RC discharge circuit 330 in which the RC discharge circuit 33 shown in FIG. Hereinafter, the RC discharge circuit 330 will be described. In the present modification, only the RC discharge circuit 33 in FIG. 2 is replaced with FIG. 9, and the other circuit configuration is the same as that in FIG. The same components as those in FIG. 2 are denoted by the same reference numerals, and description thereof is omitted here.
In the RC discharge circuit 330 of FIG. 9, two resistors R10 and R20 are connected in parallel to the capacitor C via switching elements S1 and S2, respectively. The switching element is composed of, for example, a MOS-FET. The resistance values of the resistors R10 and R20 have a magnitude relationship of R10 <R20. The switching element S1 is an element that is turned on when the charging voltage of the capacitor C is equal to or higher than the first voltage value V1 shown in FIG. 10A, and is turned off if it is less than the first voltage value V1. On the other hand, the switching element S2 is a switch that is turned on if the charging voltage of the capacitor C is less than the first voltage value V1, and is turned off if it is greater than or equal to the first voltage value. The time constant t10 of the RC discharge circuit 330 when the switch S1 is ON is smaller than the time constant t20 when the switch S2 is ON.
The operation of this modification will be described with reference to a graph showing a change in the charging voltage (residual voltage) of the capacitor C by the RC discharge circuit 330 in FIG. Although the scale is omitted in FIG. 10A, the same scale as in FIG. 5A is used. Immediately after the battery 16 is removed from the battery chamber 17, sufficient charge is accumulated in the capacitor C, so that the detected remaining voltage is larger than the first voltage value V <b> 1. Therefore, at T10, the switch S1 is turned on and the switch S2 is turned off. For a while, the capacitor C is discharged with a time constant t10. After the discharge, when the charging voltage of the capacitor C becomes less than the first voltage value V1 (at time T20), the switch S1 is turned off while the switch S2 is turned on. As a result, thereafter, discharge is performed with a time constant t20.
(7) The modification of the above (6) shown in FIG. 9 can also be configured as follows by changing the operating voltage of the switching element S2.
The operating voltage of the switching element S1 is the same as described in (6) above. On the other hand, the switching element S2 is turned off when the charging voltage of the capacitor C is less than the second voltage value V2 (V1 <V2) and is equal to or higher than the first voltage value V1 (in other words, when the voltage value is equal to or higher than the voltage value V2). And OFF when the voltage value is less than V1).
The time constant t10 of the RC discharge circuit 330 when only the switch S1 is ON is smaller than the time constant t20 when only the switch S2 is ON, and when both the switches S1 and S2 are both ON Is smaller than t10 (t30 <t10 <t20).
The operation of this modification will be described with reference to a graph showing a change in the charging voltage (residual voltage) of the capacitor C by the RC discharge circuit 330 in FIG. Although the scale is omitted in FIG. 10B, it is the same as the scale in FIG. Immediately after the battery 16 is removed from the battery chamber 17, sufficient charge is accumulated in the capacitor C, so that the detected remaining voltage is larger than the first voltage value V <b> 1. Therefore, at T10, both the switch S1 and the switch S2 are turned on, and the capacitor C is discharged for a while with a time constant t30. After the discharge, when the charging voltage of the capacitor C becomes less than the voltage value V2 (at time T30), the switch S2 is turned OFF. At this time, the switch S1 is maintained in the ON state. As a result, thereafter, discharge is performed with a time constant t10. Further, after the discharge, when the charging voltage of the capacitor C becomes less than the voltage value V1 (at time T20), the switch S1 is turned off while the switch S2 is turned on again. As a result, thereafter, discharge is performed with a time constant t20.
(8) The RC discharge circuit 330 used in the modified examples of the above (6) and (7) has various combinations of resistors (types of time constants to be switched) other than those described above by changing the operating voltage of the switching element. Can be made. For example, the combination of resistors to be switched may be R10 and R1 // R20, or may be R20 and R10 // R20.

It is a block diagram which shows the structure of the camera of 1st Embodiment. It is a circuit diagram which shows the power management circuit with which the camera shown in FIG. 1 was equipped. 3 is a timing chart showing an operation of the power management circuit shown in FIG. 2. 3 is a flowchart showing power management control performed by the power management circuit shown in FIG. 2. It is a graph which shows the change of the charge voltage of the capacitor | condenser with which the RC discharge circuit shown in FIG. 2 was equipped. It is a block diagram which shows the power management circuit with which the camera of 2nd Embodiment was equipped. It is a graph which shows the change of the charging voltage of the capacitor | condenser with which the RC discharge circuit shown in FIG. 6 was equipped. It is a block diagram which shows the power management circuit with which the camera of 3rd Embodiment was equipped. It is a circuit diagram of the RC discharge circuit in the modification of 1st Embodiment. It is a graph which shows the change of the charge voltage (remaining voltage) of the capacitor | condenser C with which the RC discharge circuit in the modification of 1st Embodiment was equipped.

Explanation of symbols

  30 Power management circuit: 33 RC discharge circuit: 33a Overvoltage detection IC: C capacitor: R1, R2 resistance

Claims (11)

An RC discharge circuit that discharges electric charges accumulated in a capacitor through a resistance portion including an electric resistance,
A detection unit that detects a charging voltage of the capacitor, and operates when the charging voltage of the capacitor becomes less than a predetermined threshold, and changes the overall resistance value of the resistance unit to change the capacitance of the capacitor. An RC discharge circuit comprising: a time constant changing unit that changes a discharge time constant defined by a capacity and an overall resistance value of the resistor unit. The RC discharge circuit according to claim 1,
The RC discharge circuit, wherein the time constant changing unit increases the discharge time constant when a charging voltage of the capacitor becomes less than the predetermined threshold value. In the RC discharge circuit according to claim 1 or 2,
The resistance portion includes a plurality of resistors,
The RC discharge circuit, wherein the time constant changing unit includes a switch mechanism that changes a connection mode of the plurality of resistors, and changes the discharge time constant by changing a connection mode of the plurality of resistors. The RC discharge circuit according to claim 3,
A first resistor and a second resistor are provided in parallel;
When the charging voltage of the capacitor is equal to or higher than the predetermined threshold, the capacitor is discharged through the first resistor and the second resistor,
The RC discharge circuit, wherein when the charging voltage of the capacitor becomes less than the predetermined threshold, the capacitor is discharged through the second resistor. The RC discharge circuit according to claim 3,
A first resistor and a second resistor having different resistance values are provided in parallel through switching elements, respectively.
When the charging voltage of the capacitor is equal to or higher than a predetermined threshold, the capacitor is discharged through the first resistor,
When the charging voltage of the capacitor is less than the predetermined threshold, the capacitor is discharged via the second resistor,
The RC discharge circuit, wherein a resistance value of the first resistor is smaller than a resistance value of the second resistor. In the RC discharge circuit according to claim 1 or 2,
The resistance portion includes a plurality of resistors,
The capacitor is provided by connecting a first capacitor and a second capacitor in parallel,
The time constant changing unit includes a switch mechanism that changes a connection mode of the plurality of resistors, the first capacitor, and the second capacitor,
When the charging voltage of the capacitor is equal to or higher than the predetermined threshold, the charge accumulated in the first capacitor and the second capacitor is discharged through the plurality of resistors.
When the charging voltage of the capacitor becomes less than the predetermined threshold value, the charge accumulated in the first capacitor is discharged through a part of the plurality of resistors and accumulated in the second capacitor. An RC discharge circuit, wherein the discharged electric charge is discharged through other portions of the plurality of resistors. The RC discharge circuit according to claim 6,
A discharge time constant is different between a circuit formed by a part of the first capacitor and the plurality of resistors and a circuit formed by another part of the second capacitor and the plurality of resistors. RC discharge circuit. An RC discharge circuit according to any one of claims 1 to 7,
A power supply state monitoring unit for monitoring a power supply state for a power supply object;
An elapsed time information acquisition unit for obtaining a time from when power supply to the power supply object is stopped until power supply is resumed,
The RC discharge circuit pre-charges the capacitor and starts discharging the capacitor when power supply to the power supply object is stopped based on the output of the power state monitoring unit,
The elapsed time information acquisition unit obtains an elapsed time from when power supply is stopped to when it is restarted based on a change amount of a charging voltage of the capacitor when power supply to the power supply target is restarted. A power management circuit characterized by that. The power management circuit according to claim 8,
The elapsed time information acquisition unit obtains the elapsed time by referring to elapsed time information recorded in association with an amount of change in the charging voltage of the capacitor and an elapsed time. In the power management circuit according to claim 8 or 9,
A charge control circuit for charging a capacitor provided in the RC discharge circuit;
The power supply management circuit according to claim 1, wherein the charge control circuit includes a first suppression unit that suppresses leakage of charges charged in the capacitor. In the power management circuit according to any one of claims 8 to 10,
A readout control unit that reads out charging voltage information of a capacitor provided in the RC discharge circuit and inputs the information to the elapsed time information acquisition unit,
The power supply management circuit according to claim 1, wherein the read control unit includes a second suppression unit that suppresses leakage of electric charge charged in the capacitor.

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