JP2008129757A - Constant current circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a constant current circuit for adjusting a current value, and for reducing a circuit area. <P>SOLUTION: A non-volatile memory 17 stores data D1 for setting a voltage VA in a non-volatile manner. A voltage setting part 13 sets the voltage VA based on the data D1 stored in the non-volatile memory 17. Then, the voltage setting part 13 outputs the voltage VA to the gate of a MOS transistor 11. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a constant current circuit, and more particularly to a constant current circuit capable of reducing a circuit area.

  Light emitting elements such as semiconductor lasers or light emitting diodes are widely used as light sources. These light emitting elements emit light having power corresponding to the applied current.

  When a device including a light emitting element is manufactured, adjustment is often performed at the time of product inspection so that the power of light output from the light emitting element is within a predetermined range. In this case, the current flowing through the light emitting element is generally adjusted. As a method for adjusting the current value, a plurality of trimming resistors are formed in advance in the circuit, and one or a plurality of resistors are cut from these resistors.

For example, Japanese Patent Laid-Open No. 5-129734 (Patent Document 1) discloses that when light of a predetermined power is output from a semiconductor laser, a current value of a monitor circuit that monitors the intensity of the semiconductor laser becomes a predetermined value. A method for adjusting the current value is disclosed. According to this document, the monitor circuit has a plurality of trimming resistors. By trimming part or all of the plurality of trimming resistors, the current value of the monitor circuit can be adjusted to a predetermined value.
JP-A-5-129734

  In the case of an adjustment circuit including a trimming resistor, it is necessary to increase the number of resistors as the current value and the voltage value are finely adjusted, so that the circuit area increases. In order to cut the trimming resistor, a laser device that normally outputs high-power light is used. For this reason, when a certain trimming resistor is cut, the influence of laser light irradiation also occurs around the resistor.

  For this reason, in an adjustment circuit including a trimming resistor, for example, the distance between the resistors needs to be as large as possible in order to prevent the laser operation from affecting the circuit operation. For this reason, the circuit area increases.

  An object of the present invention is to provide a constant current circuit capable of adjusting a current value and reducing a circuit area.

  In summary, the present invention is a constant current circuit for supplying a constant drive current to a load, wherein the first electrode is connected to the load, the second electrode is electrically connected to the fixed potential node, and the control electrode A transistor that receives a control voltage and causes a drive current to flow, and a drive circuit that supplies the control voltage to the transistor to drive the transistor are provided. The drive circuit includes a storage unit that stores first setting data for setting the control voltage in a nonvolatile manner, and a voltage that sets the control voltage based on the first setting data and outputs the control voltage to the control electrode Including a setting unit.

  Preferably, the voltage setting unit includes a reference voltage circuit that outputs a reference voltage, a voltage dividing circuit that divides the control voltage and can change a voltage dividing ratio, and a voltage dividing circuit that divides the voltage according to the first setting data. A voltage dividing ratio setting circuit for setting the pressure ratio; and a differential unit for outputting a control voltage corresponding to a difference between the reference voltage and the output voltage from the voltage dividing circuit.

  More preferably, the transistor is a MOS transistor. The constant current circuit further includes a resistor connected between the second electrode and the fixed potential node. The sign of the temperature characteristic of the resistance value of the resistor is opposite to the sign of the temperature characteristic of the threshold voltage of the MOS transistor.

  More preferably, the storage unit further stores second setting data for setting a voltage division ratio different from the voltage division ratio set by the first setting data.

  More preferably, the voltage division ratio setting circuit selects one of the first and second setting data in accordance with an instruction from the outside, and reads the selected data from the storage unit.

  According to the present invention, a constant current circuit with a reduced circuit area can be realized.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and description thereof will not be repeated.

[Embodiment 1]
FIG. 1 is a schematic diagram of an electronic device including the constant current circuit according to the first embodiment.

  Referring to FIG. 1, electronic device 100 includes a light emitting diode 1 and a constant current circuit 2. The anode of the light emitting diode 1 is connected to the power supply node. The voltage of the power supply node is Vcc.

  The cathode of the light emitting diode 1 is connected to the constant current circuit 2. The constant current circuit 2 makes the current Iop flowing through the light emitting diode 1 constant. As a result, the light emitting diode 1 emits light of a predetermined power.

  In addition, the wavelength of the light which the light emitting diode 1 shown in FIG. 1 emits is not specifically limited. For example, the light emitting diode 1 may emit visible light or infrared light. Further, the electronic device 100 may include a semiconductor laser instead of the light emitting diode 1. Further, if the device operates by applying a constant current, the load driven by the constant current circuit 2 is not limited to a light emitting element such as a light emitting diode or a semiconductor laser, and other loads (for example, a DC motor or the like). )

FIG. 2 is a configuration diagram of the constant current circuit 2 shown in FIG.
Referring to FIG. 2, constant current circuit 2 includes a MOS transistor 11, a resistor 12, a voltage setting unit 13, and a nonvolatile memory 17.

  MOS transistor 11 is, for example, an N-type MOS transistor. The drain (first electrode) of the MOS transistor 11 is connected to the cathode of the light emitting diode 1. The source (second electrode) of MOS transistor 11 is electrically connected to ground node GND. The gate (control electrode) of MOS transistor 11 receives voltage VA. As a result, the MOS transistor 11 is turned on, and a current Iop flows between the drain and the source.

  Resistor 12 is connected between the source of MOS transistor 11 and ground node GND. The resistor 12 has a function of canceling the temperature characteristic of the current Iop. As a result, the current Iop can be made substantially constant regardless of the temperature. The resistor 12 is a resistor using a diffusion layer in a semiconductor substrate, for example.

  The nonvolatile memory 17 stores data D1 for setting the voltage VA in a nonvolatile manner. The voltage setting unit 13 sets the voltage VA based on the data D1 stored in the nonvolatile memory 17. The voltage setting unit 13 outputs a voltage VA to the gate of the MOS transistor 11.

  The voltage setting unit 13 includes a differential circuit 14, a voltage dividing circuit 15, a reference voltage circuit 16, and a voltage dividing ratio setting circuit 18. Differential circuit 14 receives voltage VREF and voltage VB. The differential circuit 14 outputs a voltage VA corresponding to the difference between the voltage VREF and the voltage VB.

  The voltage dividing circuit 15 divides the voltage VA and outputs a voltage VB. The reference voltage circuit 16 outputs a voltage VREF.

  The voltage VREF is a constant voltage (for example, 1.1 V). By appropriately setting voltage VB, voltage VA can be set to a desired voltage (for example, 1.3 V).

  The nonvolatile memory 17 includes a logic circuit 17A and a data storage unit 17B. Data D1 is stored in the data storage unit 17B. The logic circuit 17A reads out the data D1 from the data storage unit 17B and outputs it to the voltage division ratio setting circuit 18 when the electronic device 100 shown in FIG. For example, the non-volatile memory 17 is a serial EEPROM (Electronically Erasable and Programmable Read Only Memory).

The voltage dividing ratio setting circuit 18 sets the voltage dividing ratio in the voltage dividing circuit 15 according to the data D1.
According to the conventional technique, as the voltage VA is finely adjusted, more trimming resistors are required. Further, in order to adjust the voltage VA, it is necessary to cut off some or all of the plurality of trimming resistors. However, for example, when the trimming resistor is cut by a laser device that outputs high-power light, the periphery of the resistor is also affected by laser light irradiation. For this reason, in a circuit including a trimming resistor, it is necessary to make the distance between the resistors as large as possible. For these reasons, the circuit area is increased to some extent in a circuit including a trimming resistor.

  On the other hand, in this embodiment, the voltage VA can be adjusted without using a trimming resistor. Therefore, the area of the constant current circuit can be reduced. Further, an apparatus (for example, a laser apparatus) for cutting the trimming resistor is not necessary. Therefore, an increase in cost in the inspection process of the electronic device 100 shown in FIG. 1 can be suppressed.

FIG. 3 is a diagram showing an example of the configuration of the voltage dividing circuit 15 of FIG.
Referring to FIG. 3, voltage dividing circuit 15 includes resistors 21A to 21E and transmission gates 22A to 22D.

  The resistors 21A to 21E are connected in series between the node N1 to which the output terminal of the differential circuit 14 is connected and the ground node. Transmission gate 22A is connected between nodes NA and N2. The node N2 is a node to which one of the two input terminals of the differential circuit 14 is connected.

  Transmission gate 22B is connected between nodes NB and N2. Transmission gate 22C is connected between node NC and node N2. Transmission gate 22D is connected between nodes ND and N2.

  The voltage division ratio setting circuit 18 includes a register 25 and a decoder 26. The register 25 receives serial data (data D1 in FIG. 2) from the nonvolatile memory 17 and converts the serial data into parallel data. The decoder 26 decodes the parallel data from the register 25 and outputs the decoded result to the transmission gates 22A to 22D.

  Each of transmission gates 22A to 22D is rendered conductive when receiving an L (logic low) level signal and is rendered non-conductive when receiving an H (logic high) level signal. Each of the transmission gates 22A to 22D is set to a conductive state or a non-conductive state by a signal from the decoder 26, whereby the voltage dividing ratio in the voltage dividing circuit 15 is determined.

  Next, a function for canceling the temperature characteristic of the current Iop in the constant current circuit 2 will be described.

  FIG. 4 is a circuit diagram for explaining a current flowing through MOS transistor 11 and resistor 12 in FIG.

  Referring to FIG. 4, the drain of MOS transistor 11 is connected to the power supply circuit, and the drain voltage is set to voltage VD. The source of the MOS transistor 11 is grounded via the resistor 12. A voltage VA is applied to the gate of the MOS transistor 11. The current flowing through the MOS transistor 11 is defined as a current ID. When the voltage VD is changed, the current ID changes. Further, the current ID changes according to the temperature of the MOS transistor 11.

FIG. 5 is a diagram for explaining the relationship between the current ID and the voltage VD shown in FIG.
Referring to FIGS. 5 and 4, curve k1 is a curve showing the relationship between current ID and voltage VD when ambient temperature T of MOS transistor 11 is equal to temperature T0. A curve k2 is a curve showing the relationship between the current ID and the voltage VD when the ambient temperature T is higher than the temperature T0. Curves k1, k2 both show the relationship between current ID and voltage VD when the source of MOS transistor 11 is directly connected to the ground node.

The threshold voltage of MOS transistor 11 is set to voltage Vth. When the source of the MOS transistor 11 is directly connected to the ground node, the current ID is proportional to (VA−Vth) ² . Here, the voltage Vth has a negative temperature characteristic. That is, the voltage Vth decreases as the temperature increases. Therefore, if the voltage VA is constant regardless of the ambient temperature T, the current ID increases as the ambient temperature T increases.

In the present embodiment, resistor 12 is connected between the source of MOS transistor 11 and the ground node. When the resistance value of the resistor 12 is R, the current ID is proportional to {(VA−ID × R) −Vth} ² . Here, since the resistance value R has a positive temperature characteristic (a characteristic in which the resistance value increases as the temperature increases), both (VA−ID × R) and the voltage Vth decrease as the temperature increases. Therefore, as can be seen by comparing the curve k2 and the curve k3, in the present embodiment, it is possible to suppress an increase in the current ID even when the ambient temperature T becomes higher than the temperature T0. Furthermore, the curve k3 and the curve k1 can be overlapped by appropriately setting the resistance value R. That is, the temperature dependence of the current ID can be reduced.

  Even when the ambient temperature T decreases, the same action as described above occurs. As a result, the temperature dependency of the current ID can be reduced.

  As described above, according to the first embodiment, the circuit area in the constant current circuit can be reduced. Further, according to the first embodiment, a constant current circuit with little temperature variation can be realized.

[Embodiment 2]
FIG. 6 is a configuration diagram of the constant current circuit according to the second embodiment.

  Referring to FIG. 6, the second embodiment is different from the first embodiment in that data D1 and D2 are stored in data storage unit 17B of nonvolatile memory 17. The data D2 is data for setting the voltage division ratio in the voltage dividing circuit 15 as with the data D1. However, the voltage division ratio determined by the data D1 is different from the voltage division ratio determined by the data D2.

  The voltage division ratio setting circuit 18 reads either data D1 or data D2 from the nonvolatile memory 17. In the second embodiment, the data read by the voltage division ratio setting circuit 18 is fixed to only one of the data D1 and D2.

  When the voltage dividing ratio setting circuit 18 sets the voltage dividing ratio of the voltage dividing circuit 15 based on the data D2, the voltage VB1 is output from the voltage dividing circuit 15, and the voltage VA1 is output from the differential circuit 14. The voltage VB1 is a voltage different from the voltage VB, and the voltage VA1 is a voltage different from the voltage VA.

  The current Iop changes according to the voltage input to the gate of the MOS transistor 11. In the second embodiment, the magnitude of the constant current flowing through the load can be variously set by storing a plurality of data in the nonvolatile memory 17. Therefore, according to the second embodiment, it is possible to realize a constant current circuit that can easily cope with various loads having different drive currents.

  Note that the types of data stored in the nonvolatile memory are not limited to two, and more types of data may be stored.

[Embodiment 3]
FIG. 7 is a schematic diagram of an electronic device including the constant current circuit according to the third embodiment.

  7 and 1, in the third embodiment, electronic device 100 includes a constant current circuit 2 </ b> A instead of constant current circuit 2. In Embodiment 3, electronic device 100 further includes a key input unit 4 and a key input determination unit 5.

  The key input unit 4 is a part that receives input from the user. The key input determination unit 5 sends a signal SIG to the constant current circuit 2A according to the result of the user operating the key input unit 4 (for example, the number of times the key is pressed or whether a specific key is pressed). The constant current circuit 2A changes the current Iop according to the signal SIG. Thereby, the power of the light output from the light emitting diode 1 can be changed.

  For example, when the light emitting diode 1 is a backlight of the display, the brightness of the light output from the light emitting diode 1 can be changed according to the user's key operation, so that the brightness of the display can be changed. Further, when the electronic device 100 is a communication device used for infrared communication and the light emitting diode 1 is a light emitting diode that emits infrared light, the intensity of the infrared light output from the light emitting diode 1 is set according to the key operation content by the user. It becomes possible to strengthen.

FIG. 8 is a configuration diagram of the constant current circuit 2A of FIG.
Referring to FIGS. 8 and 6, constant current circuit 2 </ b> A includes a voltage setting unit 13 </ b> A instead of voltage setting unit 13. The voltage setting unit 13A includes a voltage division ratio setting circuit 18A instead of the voltage division ratio setting circuit 18. The configuration of the other parts of the constant current circuit 2A is the same as the configuration of the corresponding part of the constant current circuit 2.

  The voltage division ratio setting circuit 18A selects one of the data D1 and D2 according to the signal SIG, and reads the selected data from the nonvolatile memory 17. For example, if the level of signal SIG is L level, voltage division ratio setting circuit 18A reads data D1, and if the level of signal SIG is H level, voltage division ratio setting circuit 18A reads data D2.

  As described above, according to the third embodiment, the value of the constant current can be changed according to an instruction (signal SIG) from the outside, so that the high functionality of the constant current circuit and the electronic device including the constant current circuit is realized. It becomes possible to do.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

3 is a schematic diagram of an electronic device including the constant current circuit of Embodiment 1. FIG. It is a block diagram of the constant current circuit 2 shown in FIG. FIG. 3 is a diagram illustrating an example of a configuration of a voltage dividing circuit 15 in FIG. 2. FIG. 3 is a circuit diagram for explaining a current flowing through a MOS transistor 11 and a resistor 12 in FIG. 2. FIG. 5 is a diagram for explaining a relationship between a current ID and a voltage VD shown in FIG. FIG. 6 is a configuration diagram of a constant current circuit according to a second embodiment. 10 is a schematic diagram of an electronic device including the constant current circuit according to Embodiment 3. FIG. It is a block diagram of the constant current circuit 2A of FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Light emitting diode, 2, 2A constant current circuit, 4 key input part, 5 key input determination part, 11 transistor, 12 resistance, 13, 13A voltage setting part, 14 differential circuit, 15 voltage divider circuit, 16 reference voltage circuit, 17 non-volatile memory, 17A logic circuit, 17B data storage unit, 18, 18A voltage division ratio setting circuit, 21A-21E resistance, 22A-22D transmission gate, 25 registers, 26 decoder, 100 electronic equipment, GND ground node, N1, N2 , NA to ND nodes.

Claims (5)

A constant current circuit for supplying a constant drive current to a load,
A transistor in which a first electrode is connected to the load, a second electrode is electrically connected to a fixed potential node, and a control voltage is received by a control electrode to flow the drive current;
A drive circuit for applying a control voltage to the transistor to drive the transistor;
The drive circuit is
A storage unit that nonvolatilely stores first setting data for setting the control voltage;
A constant current circuit, comprising: a voltage setting unit that sets the control voltage based on the first setting data and outputs the control voltage to the control electrode. The voltage setting unit includes:
A reference voltage circuit for outputting a reference voltage;
A voltage dividing circuit capable of dividing the control voltage and changing a voltage dividing ratio;
A voltage dividing ratio setting circuit for setting a voltage dividing ratio of the voltage dividing circuit according to the first setting data;
The constant current circuit according to claim 1, further comprising: a differential unit that outputs the control voltage according to a difference between the reference voltage and an output voltage from the voltage dividing circuit. The transistor is a MOS transistor,
The constant current circuit is:
A resistor connected between the second electrode and the fixed potential node;
The constant current circuit according to claim 2, wherein the sign of the temperature characteristic of the resistance value of the resistor is opposite to the sign of the temperature characteristic of the threshold voltage of the MOS transistor.   The constant current circuit according to claim 2, wherein the storage unit further stores second setting data for setting a voltage division ratio different from the voltage division ratio set by the first setting data.   5. The constant current according to claim 4, wherein the voltage division ratio setting circuit selects one of the first and second setting data in accordance with an instruction from the outside and reads the selected data from the storage unit. circuit.

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