JP5959548B2 - Light emitting diode display device and control method thereof - Google Patents

Description

  The present invention relates to a display device using a light emitting diode and a control method thereof, and more particularly to a light emitting diode display device using a power semiconductor for power supply to the light emitting diode and a control method thereof.

  In recent years, light-emitting diodes have been widely used as light emitters used in lighting fixtures and display devices, and light-emitting diodes have been adopted in lighting fixtures and the like mounted on vehicles (for example, Patent Document 1). ). Further, for example, Patent Document 2 discloses a technique for preventing damage to the light emitting diode due to heat generation. In Patent Document 2, the temperature in the lamp chamber in which the light emitting diode is housed is monitored, and a switching regulator that supplies current to the light emitting diode is controlled by a PWM controller in order to prevent the temperature from rising excessively. The switching regulator has a configuration including a transformer and an NMOS transistor as a switch.

  Light-emitting diodes are also being used in warning display devices for in-vehicle radars, but for in-vehicle radars, the radar body is reduced in size, weight, and cost from the standpoints of installation and cost. There is a strong demand. The warning display device for in-vehicle radars notifies the danger information detected by the radar by lighting the light emitting diodes, but is installed on the dashboard or door mirror in the vehicle so that the user can easily detect this. ing. On the other hand, the radar main body is attached to the outside of the vehicle such as a bumper of the vehicle, but a drive circuit for driving the light emitting diode of the warning display device may be provided on the radar main body side outside the vehicle. In that case, a reduction in size and weight of the drive circuit is strongly demanded.

  Since the switching regulator described in Patent Document 2 includes a transformer and an NMOS transistor, it is difficult to reduce the size and weight and reduce the cost. In order to reduce the size and weight of the drive circuit and reduce the cost, it is desirable to use a drive circuit configured to supply current to a light emitting diode from a battery in a single-stage power supply circuit configuration. The drive circuit used (hereinafter simply referred to as a linear regulator circuit) has a simple configuration and is suitable for miniaturization.

JP 2010-522113 A JP 2004-276738 A

  However, in the linear regulator circuit, power loss corresponding to the voltage difference between the input voltage and the output voltage occurs, and this power loss is converted into heat. In particular, when the light emitting diode display device is mounted on a vehicle and used, the voltage of the battery as a power source greatly varies depending on the running state and the like, and therefore the input voltage of the linear regulator circuit also varies greatly.

  The battery voltage is normally about 12.6 V, but the voltage varies depending on factors such as alternator operation and battery deterioration. Therefore, in-vehicle devices are often required to operate stably in a voltage range from about 8V to about 16V as a product specification. Therefore, it is necessary to design the total forward voltage of the light-emitting diodes so that the light-emitting diode display device is sufficiently lit even when the battery voltage becomes the minimum voltage in the specification, for example. In that case, when the battery voltage increases, the input / output voltage difference of the linear regulator circuit increases, and as a result, heat generation increases.

  Heat generation due to power loss in the linear regulator circuit increases the junction temperature of the heating element in the linear regulator circuit. If the junction temperature exceeds the upper limit (absolute rating), the heating element may be damaged. is there.

  Designed for stable operation even under the most severe specifications (for example, when the battery voltage is at the upper limit of the specification and the environmental temperature is also at the upper limit of the specification), even if the junction temperature has sufficient margin in the normal operating environment When it is necessary to do so, it is necessary to install a large heat sink or use a large power element in order to improve heat dissipation. As a result, the size of the linear regulator circuit becomes large and the cost becomes high, so that there is a problem that the merit of using the linear regulator circuit is reduced.

  Therefore, when it is assumed that the input / output voltage difference becomes large, a DCDC converter with high conversion efficiency, for example, is used for the drive circuit instead of the linear regulator circuit. However, in addition to the problem that the DCDC converter has a larger number of components and a higher component cost than the linear regulator circuit, there is also a problem that a large mounting space is required. Further, since the DCDC converter is a switching power supply, there is a problem that noise caused by switching occurs.

  The present invention has been made in view of the above problems, and controls power supply to a light-emitting diode using a linear regulator circuit including a power transistor and prevents excessive heat generation of the power transistor. An object of the present invention is to provide a light emitting diode display device and a control method thereof.

In order to solve the above problems, a first aspect of a light-emitting diode display device according to the present invention includes a display unit having one or more light-emitting diodes, and a power transistor that controls a current supplied from a predetermined power source to the light-emitting diodes. A linear regulator circuit that controls the output current of the power transistor to match a predetermined target current value, a temperature measuring circuit that measures the ambient temperature of the linear regulator circuit, and at least an input of the linear regulator circuit A voltage is input and the ambient atmosphere temperature is input from the temperature measurement circuit to calculate a junction temperature of the power transistor, and it is determined whether or not the junction temperature reaches a predetermined upper limit value. The linear regulator when it is determined that a predetermined upper limit is reached Wherein the determining output current determination unit and the output current of the road is excessively large, a PWM signal having a reduced duty ratio when the junction temperature in the output current determining unit determines that reaches the upper limit linear regulator circuit A control unit having a PWM pulse controller that outputs to the power supply, and the linear regulator circuit further controls on / off of the output current of the power transistor in accordance with the PWM signal input from the PWM pulse controller. It is characterized by.

  In another aspect of the light emitting diode display device of the present invention, the control unit further includes a voltage difference calculation circuit that inputs the input voltage and calculates a voltage difference from an output voltage of the linear regulator circuit, and the output current The determination unit inputs the voltage difference from the voltage difference calculation circuit and calculates the junction temperature.

  Another aspect of the light emitting diode display device of the present invention is characterized in that the voltage difference calculation circuit further calculates the voltage difference by inputting an output voltage of the linear regulator circuit.

  In another aspect of the light-emitting diode display device of the present invention, the voltage difference calculation circuit calculates the voltage difference with an output voltage of the linear regulator circuit as a predetermined constant value.

  In another aspect of the light emitting diode display device of the present invention, the control unit further includes a memory that stores in advance a temperature characteristic table of a forward voltage of the light emitting diode, and the output current determination unit is configured to store the output current determination unit from the memory. A temperature characteristic table is read to estimate a forward voltage of the light emitting diode, and an output voltage of the linear regulator circuit is estimated from the forward voltage to calculate the junction temperature.

  In another aspect of the light emitting diode display device of the present invention, the linear regulator circuit inputs a reference voltage source that outputs a target voltage with respect to a reference voltage determined from a current of the display unit, and the reference voltage and the target voltage. A differential amplifier that outputs an error signal of the both, and a control transistor that controls an output current of the power transistor in accordance with the error signal, wherein the PWM pulse controller transmits the PWM signal to the reference voltage source. And the reference voltage source turns on / off the output of the target voltage in accordance with the PWM signal.

According to a first aspect of the control method of a light emitting diode display device of the present invention, the display unit includes one or more light emitting diodes, and a power transistor that controls a current supplied from a predetermined power source to the light emitting diodes. A linear regulator circuit that controls the output current of the power transistor to match a predetermined target current value, a temperature measuring circuit that measures the ambient temperature of the linear regulator circuit, and a junction temperature of the power transistor is calculated to be predetermined. A control unit that outputs a PWM signal of the light emitting diode display device, wherein the control unit inputs at least the input voltage of the linear regulator circuit and the ambient temperature from the temperature measurement circuit To calculate the junction temperature of the power transistor, It is determined whether or not the junction temperature reaches a predetermined upper limit value, and when it is determined that the junction temperature reaches the upper limit value, a PWM signal with a reduced duty ratio is output to the linear regulator circuit, and the linear regulator In the circuit, the output current of the power transistor is further on / off controlled in accordance with the PWM signal input from the control unit .

  According to the present invention, a light-emitting diode display device capable of controlling power supply to a light-emitting diode using a linear regulator circuit having a power transistor and preventing excessive heat generation of the power transistor and control thereof It becomes possible to provide a method.

It is a block diagram which shows the structure of the light emitting diode display apparatus which concerns on 1st Embodiment of this invention. It is a block diagram which shows the structure of the light emitting diode display apparatus which concerns on 2nd Embodiment of this invention. It is a block diagram which shows the structure of the light emitting diode display apparatus which concerns on 3rd Embodiment of this invention.

  A light emitting diode display device and a control method thereof according to a preferred embodiment of the present invention will be described in detail with reference to the drawings. Each component having the same function is denoted by the same reference numeral for simplification of illustration and description.

  In the light emitting diode display device and the control method thereof according to the present invention, a low-cost linear regulator circuit with a simple configuration is used as a drive circuit for the light emitting diode. The linear regulator circuit is configured to reduce the input voltage to a desired output voltage and supply a current to the light emitting diode, but power corresponding to the voltage difference between the input voltage and the output voltage is lost. . This power loss is converted into heat to increase the junction temperature of the heating element inside the linear regulator circuit. In the light emitting diode display device and the control method thereof according to the present invention, power loss in the linear regulator circuit can be reduced, thereby suppressing an increase in junction temperature and facilitating downsizing of the light emitting diode display device.

When the power loss amount in the linear regulator circuit is P, the power loss amount P is given by the following equation.
P = (Vin−Vout) × I + Vin × Iq (1)
here,
Vin: input voltage of the linear regulator circuit Vout: output voltage of the linear regulator circuit I: output current of the linear regulator circuit Iq: quiescent current of the linear regulator circuit

Further, the junction temperature Tj of the heating element in the linear regulator circuit is given by the following equation.
Tj = Ta + θJa × P (2)
here,
Ta: ambient temperature around linear regulator circuit θJa: thermal resistance from heating element to ambient atmosphere

  As shown in Expression (1), power loss occurs corresponding to the voltage difference between the input voltage Vin and the output voltage Vout. This power loss increases the junction temperature Tj of the heating element inside the linear regulator circuit, as shown in Equation (2).

(First embodiment)
A light-emitting diode display device and a control method thereof according to a first embodiment of the present invention will be described below with reference to FIG. FIG. 1 is a block diagram illustrating a configuration of a light emitting diode display device 100 according to the present embodiment. The light emitting diode display device 100 includes a display unit 110 having a light emitting diode 111, a linear regulator circuit 120 that supplies power to the light emitting diode 111, a control unit 130 that controls the linear regulator circuit 120, a reference resistor 140, and a temperature measurement. Circuit 150.

  The display unit 110 can include one or a plurality of light emitting diodes 111. Here, an example is shown in which two light emitting diodes 111 are arranged in series. The indicator 110 using the light emitting diode 111 can be used, for example, as a warning indicator of a radar device mounted on a vehicle.

  The linear regulator circuit 120 includes a power transistor 121, a control transistor 122, a differential amplifier 123, and a reference voltage source 124. Here, as an example, a pnp type bipolar transistor is used for the power transistor 121 and an npn type bipolar transistor is used for the control transistor 122, but this is not restrictive. As the reference voltage source 124, for example, a band gap reference can be used.

The differential amplifier 123 has a positive input terminal connected to the reference voltage source 124 and a negative input terminal connected to the input side (the display unit 110 side) of the reference resistor 140. Since the output side of the reference resistor 140 is grounded, the input side voltage of the reference resistor 140 (hereinafter referred to as the reference voltage Vref) is given by the following equation.
Vref = Rref × ILED
Here, the resistance value of the reference resistor 140 is Rref, and the current of the light emitting diode 111 equal to the current flowing through the reference resistor 140 is ILED.

  The differential amplifier 123 controls the output current of the power transistor 121 so that the reference voltage Vref matches the voltage (target voltage) of the reference voltage source 124. Thereby, the current ILED of the light emitting diode 111 is controlled to a predetermined target current value corresponding to the target voltage of the reference voltage source 124. Here, when the target voltage of the reference voltage source 124 is V0, the target current value is given by V0 / Rref.

  By controlling the output current of the power transistor 121 by the differential amplifier 123 as described above, the input voltage Vin of the linear regulator circuit 120 is dropped in the power transistor 121 to the output voltage Vout. The power loss amount P in the linear regulator circuit 120 is caused by a voltage drop from Vin to Vout in the power transistor 121. The light emitting diode display device 100 according to the present embodiment is configured to prevent the junction temperature from becoming excessive due to the power loss amount P in the power transistor 121, thereby avoiding damage to the power transistor 121. It is possible to do.

  The control unit 130 includes a voltage difference calculation circuit 131, an output current determination unit 132, and a PWM pulse controller 133. The voltage difference calculation circuit 131 inputs the input voltage Vin and the output voltage Vout of the power transistor 121, calculates the voltage difference (Vin−Vout) between them, and outputs this to the output current determination unit 132. The output current determination unit 132 receives the voltage difference (Vin−Vout) from the voltage difference calculation circuit 131 and the ambient atmosphere temperature Ta of the linear regulator circuit 120 from the temperature measurement circuit 150. The temperature measurement circuit 150 is configured to measure the ambient atmosphere temperature Ta of the linear regulator circuit 120 using, for example, a thermistor.

  The output current determination unit 132 calculates the power loss amount P in the power transistor 121 using the voltage difference (Vin−Vout), and calculates the junction temperature Tj from the power loss amount P and the ambient atmosphere temperature Ta. When the obtained junction temperature Tj is excessive, it is determined that the output current of the linear regulator circuit 120 is excessive, and the PWM pulse controller 133 is requested to decrease the output current of the linear regulator circuit 120. Is output.

  The PWM pulse controller 133 outputs a control signal (PWM signal) for turning on / off the reference voltage source 124, and adjusts a duty ratio (0 to 1) that is a ratio for turning on the reference voltage source 124. To output a PWM signal. When a request signal for reducing the output current of the linear regulator circuit 120 is input from the output current determination unit 132, a PWM signal with a reduced duty ratio is output to the reference voltage source 124. As a result, the period during which the reference voltage source 124 is turned off increases and the period during which the output current of the linear regulator circuit 120 is reduced to zero increases. As a result, the average output current of the linear regulator circuit 120 is lowered, and the junction temperature of the power transistor 121 can be lowered.

In the above formula (1), the output current I of the power transistor 121 that is the output current of the linear regulator circuit 120 is equal to the current ILED of the light emitting diode 111 and the current of the reference resistor 140. Further, the output voltage Vout of the linear regulator circuit 120 is equal to the sum of the total forward voltage (referred to as VLED) of the light emitting diode 111 that is the amount of voltage drop in the display unit 110 and the reference voltage Vref,
Vout = VLED + Vref
It can be expressed as.

Thus, the junction temperature Tj of the power transistor 121 can be expressed as the following equation using equations (1) and (2).
Tj = Ta + θja × {(Vin−VLED−Vref) × ILED × D
+ Vin × Iq} (3)

In Expression (3), the current ILED of the light emitting diode 111 can be calculated using the following expression.
ILED = Vref / Rref
Here, the reference resistance Rref can be acquired in advance. D represents the duty ratio of the PWM signal output from the PWM pulse controller 133 to the reference voltage source 124, and the output current of the power transistor 121 corresponds to ILED or 0 corresponding to the on / off of the reference voltage source 124. Therefore, ILED × D represents the average output current of the power transistor 121.

  The quiescent current Iq of the linear regulator circuit 120 can be obtained by measuring in advance the current flowing in the linear regulator circuit 120 while the output current of the linear regulator circuit 120 is stopped. When the amount of power loss due to the quiescent current Iq is sufficiently small, the term Vin × Iq may be ignored. Furthermore, the thermal resistance θJa from the heating element (here, the power transistor 121) to the ambient atmosphere can be acquired in advance based on the package characteristics of the linear regulator circuit 120.

  In the light emitting diode display device 100 of this embodiment, the output current determination unit 132 uses the voltage difference (Vin−VLED−Vref) input from the voltage difference calculation circuit 131 and the ambient atmosphere temperature Ta input from the temperature measurement circuit 150. Thus, the junction temperature Tj is calculated from the equation (3). The output current determination unit 132 determines whether or not a value obtained by adding a predetermined margin to the calculated junction temperature Tj reaches the maximum absolute rating value (allowable upper limit value) Tjmax of the junction temperature, and determines that Tjmax is reached. At this time, a request signal is output to the PWM pulse controller 133 so as to reduce the duty ratio D. When the PWM pulse controller 133 decreases the duty ratio D according to the request signal, the average output current of the power transistor 121 is decreased, the junction temperature Tj is decreased, and the power transistor 121 is prevented from being damaged.

  In the above description, the output current determination unit 132 calculates the junction temperature Tj using Expression (3). However, the present invention is not limited to this. For example, in the voltage difference calculation circuit 131, the input voltage Vin and the output voltage Vout are calculated. (= VLED + Vref) may be used to calculate the power loss amount P from Equation (1), and the output current determination unit 132 may calculate the junction temperature Tj using Equation (2).

  According to the light emitting diode display device 100 and the control method thereof according to the present embodiment, a small, light and low cost linear regulator circuit can be used as a drive circuit for supplying power to the light emitting diode. The power transistor 121 can be prevented from being damaged by calculating and monitoring the junction temperature of the power transistor 121 in the linear regulator circuit.

(Second Embodiment)
A light emitting diode display device and a control method thereof according to the second embodiment of the present invention will be described below with reference to FIG. FIG. 2 is a block diagram showing a configuration of the light emitting diode display device 200 of the present embodiment.

  In the light emitting diode display device 100 according to the first embodiment, the voltage difference calculation circuit 131 inputs both the input voltage Vin and the output voltage Vout (= VLED + Vref) of the power transistor 121 and calculates the voltage difference (Vin−Vout). Was configured. Here, although the total forward voltage VLED of the light emitting diode 111 varies due to variations in physical characteristics and temperature characteristics of the light emitting diode 111, the variation range is considered to be smaller than the voltage variation range of the battery power supply. . Therefore, the output voltage Vout of the power transistor 121 can be set in advance as a constant value.

  In the light emitting diode display device 200 of the present embodiment, the output voltage Vout is set to a constant value set in advance as described above, and the voltage difference calculation circuit 231 inputs only the measured value of the input voltage Vin, and the voltage difference (Vin− Vout) is calculated. Thereby, the structure of the control part 230 of this embodiment and the voltage difference calculating circuit 231 can be simplified.

(Third embodiment)
A light emitting diode display device and a control method thereof according to the third embodiment of the present invention will be described below with reference to FIG. FIG. 3 is a block diagram showing a configuration of the light-emitting diode display device 300 of the present embodiment.

  In the light emitting diode display device 200 of the second embodiment, the output voltage Vout of the power transistor 121 is set to a constant value. However, in the light emitting diode display device 300 of the present embodiment, Vout is considered in consideration of only the temperature characteristics of the light emitting diode 111. It is configured to calculate. Since the forward voltage of the light emitting diode 111 varies depending on the temperature, the temperature characteristics of the forward voltage of the light emitting diode 111 can be tabulated in advance.

  The light emitting diode display device 300 of the present embodiment is configured to include a memory 334 in the control unit 330, and the temperature characteristics of the total forward voltage VLED of the light emitting diode 111 are tabulated in advance and stored in the memory 334. The output current determination unit 332 of the present embodiment reads a table of temperature characteristics of the total forward voltage VLED from the memory 334, and calculates the total forward voltage VLED of the light emitting diode 111 using the table. Then, the output voltage Vout of the power transistor 121 is obtained from the total forward voltage VLED, and the voltage difference (Vin−Vout) is calculated.

  In the light emitting diode display device 300 of the present embodiment, as described above, the control unit 330 can be prevented from inputting the output voltage Vout, and the configuration of the control unit 330 can be simplified by eliminating the voltage difference calculation circuit. it can. Note that as the temperature used for referring to the table of the temperature characteristics of the forward voltage, the temperature of the light emitting diode 111 measured separately may be used.

(Other embodiments)
A light emitting diode display device and a control method thereof according to another embodiment of the present invention will be described below. In the light emitting diode display device 100 according to the first embodiment, when the junction temperature Tj calculated by the output current determination unit 132 is excessive, a request signal for reducing the output current of the power transistor 121 is output to the PWM pulse controller 133. Was configured to do.

  On the other hand, as another embodiment, the output current determination unit 132 provides power to the PWM pulse controller 133 so that the junction temperature Tj always has a certain margin with respect to the maximum absolute rating value (allowable upper limit value) Tjmax. A signal requesting increase or decrease in the output current of the transistor 121 can be output. Thus, the light emitting diode display device can be controlled so that the light emitting diode 111 always emits light with the maximum luminance allowed.

  As a light emitting diode display device and a control method thereof according to still another embodiment of the present invention, the resistance value of the reference resistor 140 can be variably configured. As the reference resistor 140 having a variable resistance value, for example, a digital potentiometer can be used, or a plurality of resistors having different resistance values can be switched. When the junction temperature Tj of the power transistor 121 becomes excessive by making the resistance value of the reference resistor 140 variable, the voltage difference (Vin−Vout) is reduced by increasing the resistance value of the reference resistor 140. The output current of the power transistor 121 can be reduced. Thereby, damage to the power transistor 121 can be prevented.

  As described with reference to the above embodiment, according to the light emitting diode display device and the control method thereof according to the present invention, a small, light and low cost linear regulator circuit can be used as a drive circuit for supplying power to the light emitting diode. It becomes. And it is possible to prevent the power transistor from being damaged by calculating and monitoring the junction temperature of the power transistor in the linear regulator circuit. The light-emitting diode display device of the present invention does not require a large heat sink to prevent the junction temperature from becoming excessive, and can operate even when the battery voltage and the junction temperature change over a wide range. As a result, the power transistor can be prevented from being damaged, and a light-emitting diode display device having a small size and low cost and high reliability and a control method thereof can be provided.

  Note that the description in the present embodiment shows an example of the light emitting diode display device and the control method thereof according to the present invention, and the present invention is not limited to this. The detailed configuration and detailed operation of the light-emitting diode display device and the control method thereof in the present embodiment can be changed as appropriate without departing from the spirit of the present invention.

100, 200, 300 Light emitting diode display device 110 Display unit 111 Light emitting diode 120 Linear regulator circuit 121 Power transistor 122 Control transistor 123 Differential amplifier 124 Reference voltage source 130, 230, 330 Control unit 131, 231 Voltage difference calculation circuit 132, 332 Output current determination unit 133 PWM pulse controller 140 Reference resistance 150 Temperature measurement circuit 334 Memory

Claims (7)

A display unit having one or more light emitting diodes;
A linear regulator circuit having a power transistor for controlling a current supplied from a predetermined power source to the light emitting diode, and controlling the output current of the power transistor to match a predetermined target current value;
A temperature measurement circuit for measuring the ambient temperature of the linear regulator circuit;
At least the input voltage of the linear regulator circuit is input and the ambient atmosphere temperature is input from the temperature measurement circuit to calculate the junction temperature of the power transistor and determine whether the junction temperature reaches a predetermined upper limit value An output current determination unit that determines that the output current of the linear regulator circuit is excessive when it is determined that the junction temperature reaches the predetermined upper limit value, and the junction temperature is determined to be the upper limit by the output current determination unit. A PWM pulse controller that outputs a PWM signal with a reduced duty ratio to the linear regulator circuit when it is determined that the value reaches a value, and a control unit,
The linear regulator circuit further performs on / off control of the output current of the power transistor in accordance with the PWM signal input from the PWM pulse controller. The control unit further includes a voltage difference calculation circuit that inputs the input voltage and calculates a voltage difference from the output voltage of the linear regulator circuit,
The light emitting diode display device according to claim 1, wherein the output current determination unit calculates the junction temperature by inputting the voltage difference from the voltage difference calculation circuit. The light emitting diode display device according to claim 2, wherein the voltage difference calculation circuit further inputs an output voltage of the linear regulator circuit to calculate the voltage difference. The light-emitting diode display device according to claim 2, wherein the voltage difference calculation circuit calculates the voltage difference with an output voltage of the linear regulator circuit as a predetermined constant value. The control unit further includes a memory for storing in advance a temperature characteristic table of the forward voltage of the light emitting diode,
The output current determination unit reads the temperature characteristic table from the memory, estimates a forward voltage of the light emitting diode, calculates an output voltage of the linear regulator circuit from the forward voltage, and calculates the junction temperature. The light-emitting diode display device according to claim 1. The linear regulator circuit includes a reference voltage source that outputs a target voltage with respect to a reference voltage determined from the current of the display unit, a differential amplifier that inputs the reference voltage and the target voltage and outputs an error signal of both, A control transistor for controlling an output current of the power transistor according to the error signal,
The PWM pulse controller outputs the PWM signal to the reference voltage source;
The light emitting diode display device according to claim 1, wherein the reference voltage source turns on / off the output of the target voltage according to the PWM signal. A display unit having one or more light emitting diodes and a power transistor for controlling a current supplied to the light emitting diode from a predetermined power source, and controlled so that an output current of the power transistor matches a predetermined target current value LED regulator comprising: a linear regulator circuit that performs a temperature measurement circuit that measures an ambient temperature of the linear regulator circuit; and a controller that calculates a junction temperature of the power transistor and outputs a predetermined PWM signal Control method,
The control unit inputs at least an input voltage of the linear regulator circuit and inputs the ambient atmosphere temperature from the temperature measurement circuit to calculate a junction temperature of the power transistor, and the junction temperature reaches a predetermined upper limit value. Output a PWM signal with a reduced duty ratio to the linear regulator circuit when it is determined that the junction temperature reaches the upper limit value,
In the linear regulator circuit, the output current of the power transistor is further on / off controlled in accordance with the PWM signal input from the control unit .

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