JPH0816764B2 - Camera battery circuit - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to a battery check circuit for a camera.

(Background of the Invention) A light receiving element such as a silicon photodiode or cadmium sulfide is generally used to measure the brightness of an object. Since the outputs of these light receiving elements are affected by the ambient temperature, temperature compensation of the light receiving element outputs has been conventionally performed in various forms.

As one of the temperature compensations, there is one provided with a voltage generation circuit that generates a voltage proportional to absolute temperature. In the case of using an analog system and a digital system for exposure control etc., the voltage proportional to the absolute temperature is used as a voltage source of the analog system, and also as an analog-digital converter (A / D converter) or digital-analog converter (D / It is used as a reference voltage for the A converter.

By the way, when the voltage of the battery of the camera falls below a predetermined value, proper exposure control and the like cannot be performed. Therefore, a camera having an automatic exposure control function or the like has conventionally been provided with a battery check mechanism. The battery check is performed by comparing the battery voltage with a reference value. Normally, the battery voltage is not affected by the ambient temperature. Therefore, in the camera having the analog system and the digital system for performing the temperature compensation, the battery voltage is checked. A / by a reference voltage proportional to temperature
When performing D-conversion and comparing the digital value with the reference value to check the battery, even if the battery voltage is at the same level, the digital value changes depending on the temperature, and the reference value itself does not change in proportion to the temperature. If not, the circuit becomes complicated and expensive.

(Object of the invention) In order to solve such a conventional problem, an object of the present invention is to convert a battery voltage into a digital value with an A / D converter using a voltage proportional to absolute temperature as a reference voltage, and convert the digital value to It is an object of the present invention to provide a battery check circuit for a camera that does not need to change the reference value according to the ambient temperature when the battery is compared with the reference value.

(Summary of the Invention) The present invention has a voltage generating means for generating a voltage proportional to an absolute temperature according to a first temperature coefficient, and its output voltage is used as a reference voltage of an analog-digital converting means.
The present invention further includes temperature coefficient applying means for applying a second temperature coefficient smaller than the first temperature coefficient to the battery voltage,
The battery check voltage provided with the second temperature coefficient is
Based on the reference voltage, it is converted into a digital value by the analog-digital conversion means, the value is compared with a predetermined reference value by the comparison and determination means, and the quality of the battery voltage is judged from the comparison result.

(Embodiment) FIG. 1 shows an example of an exposure control circuit of a camera including an embodiment of a battery check circuit of the present invention, and 1 is a battery E
And a voltage dividing resistors R1 and R2 for extracting the battery voltage at a predetermined value.
Is connected to the voltage generation circuit 3 described later, and is connected to the resistor R1.
The connection point between the resistor R2 and the resistor R2 is connected to the base of the transistor Q1. The emitter of the transistor Q1 is connected to the anode of the diode D1 and its cathode is grounded via the resistor R3. The cathode of the diode D1 is connected to the channel CH2 of the A / D converter 4. Here, the transistor Q1 and the diode D1 constitute a temperature coefficient giving circuit 2, and the giving circuit 2 causes a resistor R1 and
A predetermined temperature coefficient is given to the voltage Vx at the connection point of R2.

The voltage generation circuit 3 is configured to generate a voltage proportional to absolute temperature. That is, resistor R4, diode D
2 and D3 are connected in series with the battery E, and the constant current source C1
The connection point between the resistor R4 and the resistor R4 is connected to the inverting input terminal of the operational amplifier A1 via the resistor R5, and the connection point between the resistor R4 and the diode D2 is connected to the non-inverting input terminal of the operational amplifier A1. Operational amplifier
The output terminal of A1 is the reference voltage input terminal VR of the A / D converter 4.
It is also connected to a photometric circuit 5 described later and is grounded via resistors R6 and R7. Diodes D4 and D5 are connected between the connection point of the resistors R6 and R7 and the inverting input terminal of the operational amplifier A1.

Reference numeral 5 indicates a photometric circuit, in which a photodiode PD is interposed between two input terminals of the operational amplifier A2,
Two diodes D6 and D7 are connected in series between the output terminal and the inverting input terminal of the operational amplifier A2. Further, the output terminal of the operational amplifier A2 is connected to the diodes D8 and D9 connected in series, and the cathode of the diode D9 is grounded via the constant current source C2 and also connected to the input channel CH1 of the A / D converter 4. ing.

The A / D converter 4 is followed by a microprocessor 6, which is well known and is not shown.
It consists of a CPU, ROM, RAM, etc., and various programs such as the main routine and the battery check subroutine described later are in ROM.
Is stored in advance and various controls such as exposure control of the camera are executed based on the result calculated according to the program.

The battery voltage check in this example will be described with reference to the battery check subroutine of FIG.

This subroutine is started by jumping from the main routine that is always activated while the power switch (not shown) of the camera is turned on, and when jumping to this routine from the main routine at a predetermined timing, , A / D converter 4 in step S1
The input voltage of the input channel CH2, that is, the digital data Vin obtained by quantizing the battery check voltage Vch2 is read. Then, in step S2, the digital data
Vin is compared with the reference value RN for battery check stored in the ROM in advance and its magnitude relationship. Digital data Vin
If is larger, the operation returns to the main routine without doing anything, and if the digital data Vin is smaller than the reference value RN, a negative determination is made and the operation proceeds to step S3 to prohibit the release and display the battery replacement or confirm that the battery voltage is low. Display it, then return to the main routine.

Here, in determining the battery check reference value RN, it is necessary to consider that a voltage proportional to the absolute temperature is used as the reference voltage of the A / D converter 4.

That is, the output voltage VA1 of the voltage generation circuit 3 is The photometric output VA2 of the photometric circuit 5 to which the voltage VA1 is supplied is However, it can be expressed as q: electronic charge K: Boltzmann constant T: absolute temperature.

Therefore, if the temperature coefficients of the currents I2 and photocurrent IL of the resistors R4 to R7 and the constant current source C2 are the same, the voltage generating circuit 3
It can be seen that the output voltages VA1 and VA2 of the photometric circuit 5 are proportional to the absolute temperature T.

Here, when the amount of light incident on the photodiode PD doubles, the photocurrent IL also doubles, and the photometric output associated with the change in the amount of light, that is, the change ΔVA2 in the output voltage of the operational amplifier A2.
Is Can be expressed as That is, when the amount of incident light is doubled, the output voltage of the operational amplifier A2 becomes Will only increase. Double the amount of light is 1 EV
Since this is nothing but an increase in the equivalent light amount, in this embodiment, at an ambient temperature of 25 ° C., the amount of voltage change due to a change in a factor related to exposure equivalent to 1 EV was calculated. And In addition, if the resolution of 1/6 of 1EV is enough for exposure control of the camera, the voltage corresponding to the LSB of the A / D converter 4 should be Have chosen. If the A / D converter 4 has 8 bits, the reference voltage Vref (= VA1) of the A / D converter 4 at an ambient temperature of 25 ° C. Therefore, the resistance values of the resistors R4 to R7 of the voltage generating circuit 3 are determined, and thereby each exposure factor is controlled with a desired resolution.

Now, in order to obtain the temperature coefficient of the reference voltage Vref supplied to the reference voltage input terminal VR of the A / D converter 4, when both sides of the fourth equation are differentiated by the absolute temperature T, Therefore, the temperature coefficient of the reference voltage Vref is about 5.1 [mV /
deg].

On the other hand, in Fig. 1, the voltage at the connection point of resistors R1 and R2 is
Vx, the base-emitter junction voltage of the transistor Q1 is Ve, and the forward voltage of the diode D1 is Vd, the battery check voltage V supplied to the channel CH2 of the A / D converter 4
ch2 can be expressed as Vch2 = Vx-Ve-Vd (6). Differentiating this equation 6 by the absolute temperature T, Becomes The voltage Vx is However, since it can be expressed as E: battery voltage, if resistors R1 and R2 have the same temperature coefficient, Becomes Also, since the base-emitter junction voltage of the transistor and the forward voltage of the diode have a temperature coefficient of about -2 [mV / deg], In other words, formula 7 is Becomes That is, in this example, to check the battery
It can be seen that the temperature coefficient of the battery check voltage Vch2 supplied to the input channel CH2 of the A / D converter 4 is 4 [mV / deg].

As described above, in the camera of this embodiment, the ambient temperature is 25
Considering ℃, the reference voltage Vref of A / D converter 4
Must be 1.52 [V] or higher, and the battery voltage at that time must also be higher than that value.

Therefore, the battery check reference value RN is determined as follows in consideration of the above-mentioned temperature coefficient of the reference voltage Vref and the battery check voltage Vch2. Ie ambient temperature
If the reference voltage Vref becomes 1.52 [V] at 25 ℃, the battery check voltage Vch2
Is It means that it has become. Here, since the full scale (256: decimal number) of the A / D converter 4 corresponds to 1.52 [V], when 1.18 [V] is input to the channel CH2, the A / D output is Becomes Therefore, in the case of this embodiment, the reference value RN is 201
(Decimal number) should be set. Of course, regarding the values of the voltage dividing resistors R1 and R2 of the power source 1, the battery check voltage Vch2 is 1.18 [V] in consideration of the forward voltage of the diode D1.
It is necessary to set it so that the ambient temperature is 25 ° C.

As described above, in the present embodiment, the transistor Q1 and the diode D1 are used to give a positive temperature coefficient of 4 [mV / deg] to the battery voltage, and the temperature coefficient of the reference voltage of the A / D converter 4 is set to 5.1 [mV]. / deg] and A / D
The resolution of the converter 4 and 2 ^8, a battery check reference value RN I decided. Therefore, since both the reference voltage Vref and the battery check voltage Vch2 have a temperature coefficient, it is possible to accurately check the battery voltage without changing the reference value RN every time the temperature changes.

 The general expression of the tenth formula is as follows.

However, N is the number of bits of the A / D converter 4.

By the way, the temperature coefficient of the battery check voltage of the eleventh equation is a temperature coefficient due to the junction of the semiconductors of the transistor and the diode in the temperature coefficient giving circuit 2, and is set smaller than the temperature coefficient of the reference voltage Vref in this embodiment. ing. Here, in the temperature coefficient giving circuit 2, the battery voltage Vc
The temperature coefficient given to h2 is the reference voltage V of the A / D converter 4.
If it is larger than the temperature coefficient of ref, the battery check voltage Vc will rise at a certain temperature depending on the voltage division ratio of the battery voltage.
h2 exceeds the reference voltage Vref, the A / D output from the A / D converter 4 is not reflected in the battery check voltage Vch2, and accurate battery check cannot be performed. Therefore, the temperature coefficient applied to the battery check voltage Vch2 must be smaller than the temperature coefficient of the reference voltage Vref. This is generally expressed as follows: m × αj ≦ β (12) where m is the number of semiconductors used in the temperature coefficient giving circuit 2,
αj can be expressed as a junction temperature coefficient of the semiconductor, and β can be expressed as a temperature coefficient of the reference voltage Vref. When the temperature coefficients of the semiconductors used differ, (α ₁ + α ₂ + ... α _m ) ≦ β (13) On the other hand, normally, the junction temperature coefficient of a transistor or a diode is about 2 [mV / deg], and the temperature coefficient applying circuit 2
Since the number of semiconductors used in (3) is an integer, the temperature coefficient of the temperature coefficient giving circuit 2 shown in the 11th and 12th equations is a discontinuous value that cannot be arbitrarily selected.

3 and 4 show other two examples of the temperature coefficient giving circuit 2. In FIG. 3, two transistors Q1 and Q
1'is used and FIG. 4 shows a case where two diodes D1 and D1 'are used.

According to the present invention, the battery check voltage having the second temperature coefficient smaller than the first temperature coefficient is A / D converted according to the reference voltage having the first temperature coefficient, and the first and Since the battery voltage is checked by comparing the battery check reference value determined according to the second temperature coefficient with the A / D converted value, the battery check reference value is changed according to the temperature. There is no need and the configuration is simplified.

Also, the reference voltage supplied to the photometric circuit is used as the reference voltage for A / D conversion of the battery check voltage, so there is no need to generate a new reference voltage for A / D conversion, and the circuit scale Can be downsized.

Further, since the second temperature coefficient is determined according to the junction temperature coefficient of the semiconductor, the second temperature coefficient value can be easily changed / adjusted by adjusting the number of semiconductors, and the second temperature coefficient can be easily optimized. Become.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a flowchart showing an example of a battery check processing procedure, and FIG.
FIG. 4 and FIG. 4 are circuit diagrams showing a second embodiment of the temperature coefficient giving circuit. 1: Power supply E: Battery, R1, R2: Resistor 2: Temperature coefficient giving circuit Q1: Transistor, D1: Diode R3: Resistor 3: Reference voltage generation circuit D2 to D5: Diode R4 to R7: Resistor C1: Constant current source 4 : A / D converter 5: Photometric circuit D6 to D9: Diode PD: Photodiode C2: Constant current source 6: Microprocessor

Claims (1)

[Claims] 1. A voltage generator that is connected to a power source, generates a voltage proportional to an absolute temperature according to a first temperature coefficient, and supplies a voltage to a photometric circuit, and a voltage of the power source that is higher than the first temperature coefficient. Temperature coefficient applying means for applying a small second temperature coefficient, and analog-digital for converting the voltage from the temperature coefficient applying means into a digital value using the voltage supplied from the voltage generating means to the photometric circuit as a reference voltage. When the number of bits of the digital value of the conversion means and the analog-digital conversion means is N, a comparison determination means for determining the voltage of the power supply by comparing with a reference value for battery check obtained by the equation (a). And a battery check circuit for the camera. Reference value for battery check = 2 ^N × (the second temperature coefficient) / (the first temperature coefficient)
… (A)