JP3755770B2 - Load drive device and portable device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a load driving device that drives a load with an output voltage converted from a power supply voltage using a DC-DC conversion type power supply circuit, and a portable device using the load driving device.
[0002]
[Prior art]
Conventionally, a load driving device that drives a load such as an LED by using a DC-DC conversion type power supply circuit that generates a voltage different from the power supply voltage has been widely used. In order to generate a predetermined output voltage and output current from a power supply circuit for driving the load driving device, the load driving device detects the input voltage or the load voltage applied to the load or the load current flowing through the load, It returns to the control circuit of a circuit (refer patent document 1).
[0003]
In this conventional driving device, the control voltage is detected by dividing the input voltage or the load voltage with a high resistance. The load current is detected by a voltage drop across a resistor connected in series with the load. The control voltage and load current are compared with a reference value to control the output voltage and output current from the power supply circuit to predetermined values.
[0004]
[Patent Document 1]
Japanese Patent Laid-Open No. 2001-313423
[Problems to be solved by the invention]
In a portable electronic device (mobile device) such as a mobile phone, the load current for driving the load is increased or decreased within a certain range depending on the usage on the electronic device side where the load is provided. Sometimes. For example, when the load is a light emitting element (such as a light emitting diode), the light emission amount may be adjusted to an arbitrary value.
[0006]
In such a case, if a load current detection resistor is inserted in series with the load, the loss due to the current detection resistor increases as the load current increases. Therefore, there has been a problem that the efficiency of the electronic apparatus as a whole including the power supply circuit and the load is reduced when the current is large (that is, when the load is heavy).
[0007]
Therefore, the present invention uses an output circuit such as a DC-DC conversion type power supply circuit that converts the power supply voltage and supplies it to the load, and changes the load current for driving the load to a predetermined range and increases the load current. It is an object of the present invention to provide a load driving device capable of efficiently driving a load while avoiding an increase in loss accompanying the above, and a portable device using the load driving device.
[0008]
[Means for Solving the Problems]
The load driving device according to claim 1, wherein the output circuit converts the power supply voltage and supplies the plurality of loads as a predetermined output voltage;
A plurality of constant current sources that are connected in series to the plurality of loads, respectively , and that allow each adjustable constant current to flow;
The output circuit controls the predetermined output voltage so that a lowest voltage among voltages at connection points between the plurality of loads and the plurality of constant current sources becomes a constant voltage.
[0009]
Current load driving apparatus according to claim 2, in the load driving device according to claim 1, wherein each of said plurality of constant current sources, which are connected to a constant current circuit of the current variable, to the constant current circuit in series A current mirror circuit is configured to include a mirror input-side transistor and a current mirror output-side transistor to which the same control input as the current mirror input-side transistor is applied, and the current mirror output-side transistor includes the adjustment It is characterized in that a possible constant current is passed.
[0010]
Load driving apparatus according to claim 3, in the load driving device according to claim 2, wherein each of the plurality of loads, characterized in that it is a light emitting diode group including at least one light emitting diode.
[0011]
According to a fourth aspect of the present invention, there is provided the load driving device according to the second aspect, wherein the constant voltage is higher than a saturation voltage of the output transistor for the current mirror.
[0012]
The load driving apparatus according to claim 5, in the load driving device according to any one of claims 1 to 4, wherein the output circuit includes a coil, a switch for switching the current supply to the coil, and the said coil switch A switching power supply circuit having a rectifying element provided between the connection point of the output and the output terminal.
[0013]
Load driving apparatus according to claim 6, in the load driving device according to claim 5, wherein the switching power supply circuit further comprises a connection to a smoothing circuit to the output voltage point side of the rectifier element, and the plurality of loads characterized by and a control circuit for performing on-off switching of the switch to the lowest voltage is equal to the reference voltage in the voltage of the connection point between the plurality of constant current sources.
[0015]
Load driving apparatus according to claim 7, the portable device being characterized in that by using the load driving device according to any one of claims 1 to 6.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of a load driving device of the present invention will be described with reference to the drawings. FIG. 1 is a diagram illustrating a configuration of a load driving device according to a first embodiment of the present invention.
[0017]
In FIG. 1, a switching power supply circuit 100 as an output circuit is a boost type power supply circuit that boosts an input power supply voltage Vcc and outputs a boosted output voltage Vo1.
[0018]
A coil L1 and a switch Q1, which is an N-type MOS transistor, are connected in series between the power supply voltage Vcc and the ground. From the series connection point A, the voltage at the series connection point A is rectified and smoothed by the rectifying diode D1 and the smoothing capacitor C1, and output as the output voltage Vo1. Note that the voltage is a potential with respect to the ground unless otherwise specified.
[0019]
The external load 10 and the constant current source I1 are connected in series between the output voltage Vo1 point (P1) and the ground. The external load 10 is a load determined by a current flowing through a point to be operated, and a predetermined drive current Io set by the constant current source I1 flows through the external load 10. The voltage generated at one end P2 of the constant current source I1 becomes the detection voltage Vdet.
[0020]
The control circuit Cont receives the detection voltage Vdet and the reference voltage Vref from the reference voltage source B1, and generates a switching signal for switching the switch Q1 so that the detection voltage Vdet becomes equal to the reference voltage Vref. Here, an error amplifier Eamp that amplifies and outputs the difference between the reference voltage Vref and the detection voltage Vdet, and a pulse width modulation control circuit Pwm that forms a PWM signal based on the output of the error amplifier Eamp and outputs it as a switching signal. It is comprised including.
[0021]
The external load 10 is connected between the terminal P1 and the terminal P2. The present invention can be configured as an electronic device such as a portable device including an external load of a load driving device. In this case, the terminals P1 and P2 may be omitted.
[0022]
As the external load 10, light emitting diodes LED1 to LED3 are used. The light-emitting diode LED is, for example, a white light-emitting diode, and is used for LCD (liquid crystal display panel) and key backlight. Therefore, FIG. 1 shows an example of three series, but different combinations such as series connection, parallel connection, or series-parallel connection, depending on the required light emitting area and light emission amount, etc. There are various forms.
[0023]
The current-voltage characteristics of the light emitting diode LED are shown in FIG. This characteristic is an example of the current If-voltage Vf characteristic of the white light emitting diode LED. In FIG. 2, the horizontal axis represents the logarithmic display current If, and the vertical axis represents the voltage Vf. The light emitting diode LED emits light in a current If range of 20 mA (point A in the figure) to 1.5 mA (point B in the figure). By changing the current If, the amount of light emitted from the light emitting diode LED changes according to the magnitude of the current If.
[0024]
When the current If is used at 20 mA, the forward voltage Vf operates at a voltage of 3.4 V, as indicated by point A in FIG. However, the characteristics of each light emitting diode LED are not uniform, and the forward voltage Vf varies within a range of, for example, about 3.4 V to 4.0 V even with the same current of 20 mA. Thus, in general, the white light emitting diode LED has a higher forward voltage Vf than the light emitting diode LEDs of other colors, and the output voltage Vo1 is required to be 12.0 V or more in order to use three of them in series. .
[0025]
FIG. 3 is a diagram illustrating an example of a circuit configuration of the constant current source I1. In FIG. 3, a constant current circuit I11 and an N-type MOS transistor (hereinafter referred to as an N-type transistor) Q2 are connected in series between a power supply voltage Vcc and the ground. The drain and gate of this N-type transistor Q2 are directly connected. In addition, an N-type transistor Q3 having a driving capability higher than that of the N-type transistor Q2 is provided to allow the drive current Io to flow. The gate of the N-type transistor Q2, which is an input side transistor, is connected to the gate of the N-type transistor Q3, which is an output side transistor, to form a current mirror circuit.
[0026]
In FIG. 3, the magnitude of the drive current Io flowing through the N-type transistor Q3 is set to a desired value by adjusting the current of the constant current circuit I11.
[0027]
Again, returning to FIG. The constant current source I1 can operate at a constant current if there is a voltage equal to or higher than the saturation voltage (about 0.3 V) of the transistor (N-type transistor Q3 in FIG. 3) used therein. The voltage in the portion exceeding the saturation voltage (about 0.3 V) becomes an unnecessary loss in the constant current source I1 (that is, the loss is voltage × current). The output voltage Vo1 of the power supply circuit 100 is controlled so that the voltage drop Vdet of the constant current source I1 becomes the reference voltage Vref. Accordingly, the reference voltage Vref is set to a value that allows a slight margin of voltage to the saturation voltage (about 0.3 V) of the transistor used for the constant current source I1.
[0028]
The operation of the thus configured load driving device will be described with reference to FIG. 4 showing the characteristics of the driving current Io and the output voltage Vo1. First, the drive current Io that flows through the light emitting diode that is the load 10 is set in the constant current circuit I11. In the switching power supply circuit 100, the on / off switching control of the switch Q1 is started so that the detection voltage Vdet becomes equal to the reference voltage Vref. As a result, the output voltage Vo1 gradually increases.
[0029]
As a result, the detection voltage Vdet and the reference voltage Vref become equal, and the drive current Io flows through the light emitting diodes LED1 to LED3 that are the load 10. Thereby, the light emitting diode emits light with a predetermined light emission amount.
[0030]
In this case, even if the characteristic of the forward Direction voltage Vf of the light emitting diode LED1~LED3 was varied, the output voltage Vo1 is only varies from a predetermined value in accordance with the variation of light emission from the light-emitting diode LED1~LED3 There is no particular effect on control. The voltage drop of the constant current source I11, that is, the detection voltage Vdet is constant. Therefore, the output voltage Vo1 has a magnitude obtained by adding the drop voltage Vled (= 3 × Vf) of the light emitting diodes LED1 to LED3 corresponding to the driving current Io at that time to the constant detection voltage Vdet.
[0031]
Next, when the light emission amount from the light emitting diodes LED1 to LED3 is changed, the magnitude of the drive current Io is changed. For example, when the drive current Io is increased, the amount of light emitted from the light emitting diodes LED1 to LED3 increases accordingly. Along with this, the drop voltage Vled of the light emitting diodes LED1 to LED3 also increases depending on the characteristics of FIG. The slope of the output voltage Vo1 in FIG. 4 follows the If-Vf characteristic in FIG.
[0032]
Accordingly, the drop voltage Vled of the light emitting diodes LED1 to LED3 increases as the drive current Io increases, and the output voltage Vo1 increases with the characteristics shown in FIG. However, since the detection voltage Vdet maintains a constant value, the loss in the constant current source I1 does not increase even if the drive current is increased to increase the amount of light emission. Therefore, the efficiency of the load driving device is maintained high.
[0033]
FIG. 5 is a diagram showing a configuration of a load driving device according to the second embodiment of the present invention. In FIG. 5, another load 20 is added in addition to the load 10, and a constant current source I20 is provided corresponding to the load 20 as the load increases. The other load to be added may be two or more.
[0034]
In FIG. 5, a constant current source I10 is provided in series with a load 10, and a drive current Io1 flows through the series circuit. The voltage drop of the constant current source I10 becomes the first detection voltage Vdet1. Similarly, a constant current source I20 is provided in series with the load 20, and a drive current Io2 flows through the series circuit. The voltage drop of the constant current source I20 becomes the second detection voltage Vdet2. P11, P12, P21, and P22 are terminals for load connection.
[0035]
The error amplifier Eamp of the control circuit Cont has two non-inverting input terminals (+) and one inverting input terminal (−). The first detection voltage Vdet1 and the second detection voltage Vdet2 are input to the two non-inverting input terminals (+), respectively, and the reference voltage Vref is input to the inverting input terminal (−). In the error amplifier Eamp, the lower one of the first detection voltage Vdet1 and the second detection voltage Vdet2 input to the two non-inverting input terminals (+) is selected and compared with the reference voltage Vref. Other configurations are the same as those in the first embodiment of FIG.
[0036]
In the load drive circuit of FIG. 5, the drive currents Io1 and Io2 of the plurality of loads 10 and 20 can be individually adjusted. Then, the lower one of the drop voltages Vdet1 and Vdet2 of the constant current sources I10 and I20 is automatically selected, and the switching control of the power supply circuit 100 is performed.
[0037]
The constant current operation of the constant current sources I10 and I20 for flowing the drive currents Io1 and Io2 of the plurality of loads 10 and 20 is ensured. Therefore, the same effect as in the first embodiment can be obtained for a plurality of loads.
[0038]
【The invention's effect】
According to the present invention, an output voltage is obtained by converting an output voltage from a power supply voltage using an output circuit (DC-DC conversion type power supply circuit), and an operating point is determined by a flowing current. Since a constant current source whose value can be adjusted is connected, the current required on the load side can be stably supplied to the load.
[0039]
Further, the output voltage of the DC-DC conversion type power supply circuit is controlled so that the voltage drop of the constant current source becomes equal to a reference voltage set to a voltage value at which the constant current source can stably operate. Accordingly, the output voltage of the DC-DC conversion type power supply circuit is automatically adjusted so that a current necessary for a predetermined light emission amount flows even if there is a variation in characteristics among the light emitting diode groups as the load.
[0040]
In addition, since the voltage drop of the constant current source is automatically controlled to the reference voltage so that the set current value can be maintained, even if the current flowing through the light emitting diode group that is the load becomes large, the voltage across the resistor There is no increase in loss as in detection. Therefore, the loss accompanying the increase in load current does not substantially increase, so that the load can be driven with high efficiency in a wide load current range.
[0041]
In addition, since the DC-DC conversion type power supply circuit is a switching power supply circuit that switches the energization to the coil and converts the power supply voltage to the output voltage, the conversion efficiency is good and the necessary high voltage can be output. it can.
[0042]
Also, a variable current type constant current source is provided for each light emitting diode group that is a load, and the DC-DC conversion type power supply circuit is controlled according to the lower voltage drop of those constant current sources. Therefore, a predetermined constant current can be stably supplied to each of the light emitting diode groups that are loads.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of a load driving device according to a first embodiment of the present invention. FIG. 2 is a diagram showing current-voltage characteristics of a light emitting diode LED. FIG. 3 is a circuit configuration of a constant current source I1. FIG. 4 is a diagram showing characteristics of drive current Io and output voltage Vo1. FIG. 5 is a diagram showing a configuration of a load driving device according to a second embodiment of the present invention.
100 Power supply circuit 10, 20 Load LED1 to LED3 Light emitting diode L1 Coil C1 Smoothing capacitor D1 Diode Q1 Switch Cont Control circuit B1 Reference voltage source I1, I10, I20 Constant current source I11 Constant current circuit Q2, Q3 N-type MOS transistor Vcc Input Power supply voltage Vo1 Output voltage Io, Io1, Io2 Drive current Vled, Vled1, Vled2 Light emitting diode drop voltage Vdet, Vdet1, Vdet2 Detection voltage Vref Reference voltage

Claims (7)

An output circuit that converts a power supply voltage and supplies it to a plurality of loads as a predetermined output voltage;
A plurality of constant current sources that are connected in series to the plurality of loads, respectively , and that allow each adjustable constant current to flow;
The output circuit controls the predetermined output voltage so that the lowest voltage among the voltages at the connection points of the plurality of loads and the plurality of constant current sources becomes a constant voltage. Load drive device. Each of the plurality of constant current sources is provided with a current variable type constant current circuit, a current mirror input side transistor connected in series with the constant current circuit, and the same control input as the current mirror input side transistor. 2. The load according to claim 1, wherein a current mirror circuit is configured to include a current mirror output-side transistor, and the adjustable constant current is supplied to the current mirror output-side transistor. Drive device. Each of the plurality of loads, characterized in that it is a light emitting diode group including at least one light emitting diode, the load driving device according to claim 2. The load driving device according to claim 2 , wherein the constant voltage is higher than a saturation voltage of the output transistor for the current mirror. The output circuit is a switching power supply circuit having a coil, a switch for switching energization to the coil, and a rectifying element provided between a connection point of the coil and the switch and an output terminal. The load driving device according to any one of claims 1 to 4 , wherein the load driving device is characterized. The switching power supply circuit further includes a smoothing circuit connected to the output voltage point side of the rectifier element, the lowest voltage is the reference in the voltage of the connection point of the plurality of constant current source and the plurality of loads 6. The load driving device according to claim 5, further comprising a control circuit that performs on / off switching of the switch so as to be equal to a voltage. Portable device characterized in that it uses a load drive device according to any one of claims 1 to 6.

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