Load control device

A load control device includes a triangular wave generation portion which generates a triangular wave signal by charging/discharging a capacitor based on a constant current supplied from a constant current source, a load control portion which controls a load based on the triangular wave signal, and a temperature compensation element whose characteristic changes with a rise in temperature, which is provided to the constant current source.

BACKGROUND OF THE INVENTION

The present invention relates to a load control device for controlling a load such as a lamp of a vehicle.

Some of the related art load control devices include a triangular wave generation portion, a set voltage generation portion, a comparison portion and a driving control portion. In case a driving instruction signal to instruct driving of a load at a certain level corresponding to a fixed input is supplied, the triangular wave generation portion generates a triangular wave. The set voltage generation portion holds and generates a second set voltage set between the maximum voltage and the minimum voltage of the triangular wave. The comparison portion compares the triangular wave with the second set voltage. The driving control portion thus generates a driving control signal that changes its level with a constant frequency and duty based on the comparison result of the comparison portion.

In case a driving instruction signal to instruct stoppage of driving of a load at a certain level corresponding to a fixed input is supplied, the triangular wave generation portion generates a triangular wave. The set voltage generation portion holds and generates a third set voltage lower than the second set voltage. The comparison portion compares the triangular wave with the third set voltage. The driving control portion thus generates a driving control signal that changes its level with a constant frequency and duty based on the comparison result of the comparison portion.

In case a driving instruction signal that changes its level with a predetermined frequency and duty corresponding to a pulse input, the triangular wave generation portion generates a first set voltage set between the second set voltage and the third set voltage. The set voltage generation portion selectively generates the second set voltage or third set voltage in correspondence to the frequency and duty of the driving instruction signal. The comparison portion compares the first set voltage with the second set voltage or the third set voltage. The driving control portion thus generates a driving control signal that changes its level with the same frequency and duty as those of the driving control signal (for example, refer to JP-A-2001-148294 (claim 1, [A0019] to [A0053], FIGS. 1 to 3)).

In the above related load control device, the driving control portion generates and outputs a driving control signal that changes its level with certain frequency and duty even when the temperature changes. The ON resistance of a power MOSFET as a load driving element is substantially proportional to temperature and heat increases with temperature. Thus, it is necessary to perform heat dissipation design so that heat dissipation will be permitted at the expected maximum operating temperature. As a result, the device scale increases.

Also, the above load control device according to the related art uses a headlamp mounted on a vehicle such as a two-wheeled vehicle or a four-wheeled vehicle as a load. The headlamp mounted on a vehicle may be one including a low-beam lamp and a high-beam lamp attached to a single reflector or a single headlamp including a filament for low beams and a filament for high beams. Low beams are preferably turned ON so as not to cause glare on the eyes of the driver of a vehicle in front or an oncoming vehicle, if any, in night driving. High beams are preferably turned ON in the absence of a vehicle in front or an oncoming vehicle in night driving.

Some of the above vehicles have a feature called DRL (Daytime Running Light) that forcibly turns ON a headlamp in the daytime also in order to let pedestrians or oncoming cars recognize the presence of the vehicle and prevent possible traffic accidents. Some vehicles equipped with the DRL feature use low beams for DRL while others use high beams for DRL.

The related art load control device is composed of ICs and has a capacitor interposed therein as an external component between a connection terminal and a ground so as to set the frequency of a triangular wave generated by the triangular wave generation portion. In case the capacitor has shorted by some cause, the FET as a load driving element is maintained ON. As a result, in case the load is the headlamp, the headlamp is maintained ON with a 100% duty ratio.

With a vehicle using low beams for DRL, there are no particular problems even when the headlamp keeps lighting. The headlight lighting state ensures safety of the people on the vehicle, pedestrians and oncoming vehicles so that the lighting state is rather favorable from the viewpoint of a fail-safe design. With a vehicle using high beams for DRL, the headlight lighting state is maintained with a 100% duty ratio. This could cause glare with respect to the driver of a vehicle in front or an oncoming vehicle which leads to a traffic accident.

This advantage could be common to any device in general that controls a load based on a generated triangular wave signal.

SUMMARY OF THE INVENTION

The invention has been accomplished in view of the foregoing circumstances. An object of the invention is to provide a load control device that solves the above problems.

In order to solve the above problems, the invention provides a load control device, comprising:

a triangular wave generation portion which generates a triangular wave signal by charging/discharging a capacitor based on a constant current supplied from a constant current source;

a load control portion which controls a load based on the triangular wave signal; and

a temperature compensation element whose characteristic changes with a rise in temperature, which is provided to the constant current source.

Preferably, the load control portion includes a pulse width modulation wave generation portion which generates a pulse width modulation wave signal based on the triangular wave signal, and a load driving portion which supplies a load current to the load based on the pulse width modulation wave signal.

Preferably, the temperature compensation element is a diode having a characteristic that the reverse-direction leakage current increases with the rise in temperature.

Preferably, the temperature compensation element is a thermistor having a characteristic that the resistance value drops with the rise in temperature.

In the above configurations, the load control device operates normally at normal temperatures. When the temperature has approached an operating limit, the frequency of a pulse width modulation signal is corrected to decrease the heat value. It is thus unnecessary to make a heat dissipation design to allow heating at an expected maximum operating temperature unlike in the related art practices. As a result, a heat dissipation portion is simplified thus downsizing the load control device.

According to the present invention, there is also provided a load control device for controlling a load based on a generated triangular wave signal, comprising:

a triangular wave generation portion which generates the triangular wave signal having the same frequency in a first interposing state where a capacitor for setting the frequency of the triangular wave signal is interposed between a power source and an input end of a comparison portion and a second interposing state where the capacitor is interposed between a ground and the input end of the comparison portion,

wherein the capacitor is configured to be interposed in either the first interposing state or the second interposing state.

Preferably, the load control device further comprises a pulse width modulation wave generation portion which generates a pulse width modulation wave signal based on the triangular wave signal, and a load control portion which controls the load based on the pulse width modulation wave signal.

According to the above configurations, it is possible to enhance the safety of a load control device subjected to a short of a capacitor. In case the load control device is mounted on a vehicle and the headlamp of a vehicle is used as a load and low beams are used for DRL, it is possible to assure a fail-safe design. In case high beams are used for DRL, it is possible to enhance the safety. It is unnecessary to manufacture two types of printed circuit boards depending on the type of vehicle using the load control device. This contributes to reduced costs.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

FIG. 1is a block diagram showing the configuration of a load control device according to a first embodiment of the invention. The load control device according to the first embodiment includes a triangular wave generation circuit1, a pulse width modulation (PWM) wave generation portion2, an OR gate3, a driving circuit4, and a load driving element5. The triangular wave generation circuit1generates a triangular wave signal of a predetermined frequency and shape by switching between charging and discharging of an external capacitor C1for frequency setting.

The PWM generation circuit2generates a PWM signal (at “H” (High) level or “L” (Low) level) based on a triangular wave signal supplied from a triangular wave generation circuit1. The OR gate3supplies to the driving circuit4a logical value (at “H” (High) level or “L” (Low) level) obtained through logical OR operation of a control signal (at “H” (High) level or “L” (Low) level) supplied externally and a PWM signal (at “H” (High) level or “L” (Low) level) supplied from the PWM generation circuit2. The driving circuit4amplifies and inverts the logical value supplied from the OR gate3and applies a driving voltage to the load driving element5. When the driving voltage is applied from the driving circuit4, the load driving element5supplies a load current to a load6.

FIG. 2is a circuit diagram as a particular implementation of the block diagram of a load control device shown inFIG. 1. InFIG. 2, a portion enclosed by alternate long and short dashed lines constitutes a load control device. Components of the load control device including transistors Q1to Q10, resistors R1to R12, comparators CP1, CP2, an OR gate3, a driving circuit4and a constant voltage power source21are composed of ICs. That is, a capacitor C1and an N-channel MOSFET22as a load driving element5are external components of the ICs.

The load control device of this embodiment is a device (low-side switching device) that includes an N-channel MOSFET22as a load driving element5downstream a lamp11as a load6. The load control device is mounted for example on a vehicle.FIG. 2shows a lamp11used as a headlamp which serves as the load6shown inFIG. 1. The lamp11is connected between the power terminal Tb and the output terminal To of the load control device. InFIG. 2, a battery12mounted on a vehicle is used as a power source. A battery voltage Vbatis connected between the power terminal Tb and the ground terminal Tg of the load control device.

InFIG. 2, a control signal (at “H” (High) level or “L” (Low) level) (fixed input) outputted from an ECU (Electronic Control Unit)13mounted on a vehicle is supplied to the load control device. The ECU controls the fuel injection amount or ignition timing of the engine of a vehicle to control the engine or controls an automatic transmission or traction control.

InFIG. 2, PNP transistors Q1to Q4, PNP transistors Q5to Q10, resistors R1to R9, a comparator CP1and a capacitor C1constitute a triangular wave generation circuit1shown inFIG. 1. The transistors Q2to Q4constitute a current mirror circuit (constant current source). The emitter area of each of the transistors Q2to Q4is the same. Thus, collector currents I2to I4flowing through the collectors of the transistors Q2to Q4are the same. That is, Expression (1) is satisfied.
I2=I3=I4  (1)

where a current I0flowing through a resistor R2is represented by Expression (2) using a constant voltage Vc, the base-emitter voltage VBE2of the transistor Q2and the resistor R2.
I0=(Vc−VBE2)/R2  (2)

The transistor Q1is used for amplification. A diode D1has a p-n junction and a characteristic that a reverse-direction leakage current increases with a rise in temperature. A current I1flowing through the resistor R1is a current that bypasses part of the current I0from the transistor Q2. Given the reverse-direction leakage current of the diode D1as Ird1and the dc current amplification ratio of the transistor Q1as hfeq1, the current I1is represented by the expression (3) in the state that the transistor Q1is not saturated.
I1=Ird1×hfeq1  (3)

From Expression (2) and Expression (3), the current I2is represented by Expression (4).
I2=I0−I1={(Vc−VBE2)/R2}−Ird1×hfeq1  (4)

The collector currents I2to I4are constant currents as a reference for charging or discharging the capacitor C1. The collector current I4is a current used to charge the capacitor C1with electric charges.

The transistors Q5to Q7constitute a current mirror circuit (constant current source). The resistor R3is provided to compensate for the base current of the transistor Q5. The ratio of the emitter area of the transistor Q5to the total emitter area of the transistors Q6and Q7is 1:2. The collector current flowing in the collector of the transistor Q5is equal to the collector current I3of the transistor Q3. Further, from Expression (1), the collector current I3of the transistor Q3is equal to the collector current I2of the transistor Q2.

Thus, the collector current I6flowing in the transistor Q6is twice each of the collector currents I2to I4of the transistors Q2to Q4. That is, Expression (5) is satisfied.
I6=2×I2=2×I3=2×I4  (5)

The collector current I6is a current used to discharge the electric charges accumulated on the capacitor C1.

The transistor Q8is provided to shut down the supply of the collector current I6when turned ON. The transistor Q8and the resistors R4to R6generate a reference voltage Vt1for generating the triangular wave signal. The resistor R7is a base resistor connected between the base of the transistor Q9and the output terminal of the comparator CP1.

The transistor Q10and resistors R8and R9constitute a circuit for turning ON/OFF the transistor Q8by way of the output signal of the comparator CP1. In the triangular wave generation circuit1, the comparator CP1compares the voltage VC1of the capacitor C1with a reference voltage Vt1based on a constant current obtained by a current mirror circuit (constant current source) composed of transistors Q2to Q4, a current mirror circuit (constant current source) composed of transistors Q5to Q7and a resistor R2respectively connected to a constant voltage Vc. The triangular wave generation circuit1thus switches between charging and discharging of the capacitor C1to generate a triangular wave signal.

The comparator CP2and resistors R10and R11constitute a PWM generation circuit2shown inFIG. 1. The resistors R10and R11generate a reference voltage Vk for generating the PWM signal. The reference voltage Vk is represented by Expression (6).
Vk=Vc×R11/(R10+R11)  (6)

In the PWM generation circuit2, the comparator CP2compares a triangular wave signal supplied from the triangular wave generation circuit1with the reference voltage Vk. The PWM generation circuit2thus generates a PWM signal.

The resistor R12is interposed between a power source Vc and an input terminal Ti and functions as a pull-up resistor to stably hold the potential of a control signal supplied from the ECU13. The constant voltage power source21generates a constant voltage Vc from a battery voltage Vbatsupplied from a battery12and supplies the constant voltage Vc to each part of the load control device. The MOSFET22has its gate connected to the output terminal of the driving circuit4and its drain connected to the output terminal To of the load control device and its source grounded.

Operation of the load control device of this configuration will be described referring to the timing chart shown inFIG. 3. As shown inFIG. 3, in case the control signal supplied from the ECU13is High, the output signal of the OR gate3is always High. The driving circuit4amplifies and inverts the logical value of High level supplied from the OR gate3and applies a Low driving voltage to the MOSFET22. While the Low driving voltage is applied from the driving circuit4, the MOSFET22has its gate voltage driven Low so that it is turned OFF. In this case, the source voltage of the MOSFET22is almost equal to the battery voltage Vbatso that a load current does not flow into a load6, or a lamp11in this example as shown inFIG. 3.

As shown inFIG. 3, in case the control signal supplied from the ECU13is Low, the output signal of the comparator CP2of the PWM generation circuit2serves as an output signal of the OR gate3.

In case the voltage VC1of the capacitor C1is lower than the reference voltage Vt1at a certain time, the output signal of the comparator CP1is driven Low and the transistors Q9and Q10are turned OFF. While the transistor Q9is turned OFF, the reference voltage Vt1is the upper limit voltage Vb of the triangular wave signal as shown inFIG. 3. The upper limit voltage Vb is represented by Expression (7).
Vb=Vc×R5/(R4+R5)  (7)

When the transistor Q10is turned OFF, a current flows into the base of the transistor Q8from the resistor R9so that the transistor Q8is turned ON. When the transistor Q8is turned ON, supply of a collector current I6is stopped. As a result, a collector current I4flows, which charges the capacitor C1with electric charges and the voltage across the terminals of the capacitor C1increases. The voltage VC1of the capacitor C1rises.

When the voltage VC1of the capacitor C1exceeds the upper limit voltage Vb even by a small amount, the output signal of the comparator CP1is driven High, which turns ON the transistors Q9and Q10. While the transistor Q9is turned ON, without considering the saturation voltage of the transistor Q9, the reference voltage Vt1becomes a resistance dividing voltage of the composite resistance value of the resistors R5and R6and the resistance value of the resistor R4, and as shown inFIG. 3, becomes the lower limit voltage Va of the triangular wave signal. The lower limit voltage Va is represented by Expression (8).
Va=Vc×(R5×R6)/(R4×R5+R4×R6+R5×R6)  (8)

When the transistor Q10is turned ON, a current does not flow from the resistor R9to the base of the transistor Q8. This turns OFF the transistor Q8. When the transistor Q8is turned OFF, supply of the collector current I6starts. As mentioned above, the collector current I6is double the collector current I4. Thus, the electric charges accumulated on the capacitor C1are discharged with a current value obtained by subtracting the collector current I4from the collector current I6. When the electric charges accumulated on the capacitor C1are discharged, the voltage across the terminals of the capacitor C1drops. Thus, the voltage VC1of the capacitor C1drops. When the voltage VC1of the capacitor C1drops below the lower limit voltage Va even by a small amount, the output signal of the comparator CP1is inverted to Low. These operations are repeated to generate the triangular wave signal shown inFIG. 3.

When the triangular wave signal supplied from the triangular wave generation circuit1, that is, the voltage VC1of the capacitor C1exceeds the reference voltage Vk, the output signal of the comparator CP2of the PWM generation circuit2is driven Low. When the voltage VC1of the capacitor C1drops below the reference voltage Vk, the output signal of the comparator CP2is driven High. These operations are repeated to generate the PWM signal shown inFIG. 3.

The output signal of the comparator CP2, that is, the PWM signal is supplied to the driving circuit4via the OR gate3. The driving circuit4amplifies and inverts the logical value of the PWM signal supplied from the OR gate3and applies a driving voltage to the MOSFET22. While the driving voltage applied from the driving circuit4is High, the MOSFET22has its gate voltage driven High so that it is turned ON. The source voltage of the MOSFET22is almost equal to the ground voltage. As shown inFIG. 3, a load current flows into a load6, or a lamp11in this example.

In case the driving voltage from the driving circuit4is Low, the MOSFET22is turned OFF. Thus, the source voltage of the MOSFET22rises until it is almost equal to the battery voltage Vbat. As shown inFIG. 3, a load current does not flow into a load6, or a lamp11in this example.

These operations are repeated to drive the lamp11to blink based on the supplied driving voltage.

The period T of the triangular wave signal will be described. While the transistor Q1is not saturated, the current I1is represented by Expression (3) above. The period T of the triangular wave signal is represented by Expression (9).

The voltage VBE2across the base and emitter of the transistor Q2has a temperature characteristic of around 2 mV/° C. so that it may be regarded as almost constant. Thus, the period T of the triangular wave signal is influenced by the leakage current Ird1of the diode D1and the dc current amplification ratio hfeq1of the transistor Q1. The reverse-direction leakage current Ird1of the diode D1, similar to the dc current amplification ratio hfeq1of the transistor Q1, has a characteristic that it increases with a rise in temperature. As the temperature rises, the period T of the triangular wave signal becomes longer than Expression (9). In other words, the frequency of the triangular wave signal drops.

When the current I1increases and the transistor Q1is saturated, the saturation voltage is almost 0 volts so that the current I1is represented by

In case the reverse-direction current Ird1of the diode D1increases and the transistor Q1is saturated, it is necessary to apply Expression (10) in place of Expression (3) for the current I1. The period T of the triangular wave signal thus becomes constant as represented by Expression (11). As a result, even when the reverse-direction Ird1of the diode D1has increased, the period T of the triangular wave signal does not become longer than necessary.

FIG. 4shows an exemplary result of comparison between a case where a lamp of a certain rating is actually driven by a load control device of the above configuration and a case where the same lamp is actually driven in accordance with the related art. In this comparison experiment, setting is made so that the frequency of a PWM signal is 100 Hz at a temperature of 25° C. both in First embodiment and related art example. The duty ratio of the PWM signal is 50%, the ON resistance of the MOSFET as a load driving element5at 25° C. is set to 30 mΩ, and the ON resistance temperature coefficient of the MOSFET is 0.8%/° C.

InFIG. 4, the switching heat refers to a total of heat generated when the MOSFET is turned ON and turned OFF. The ON heat refers to the heat in a period where the MOSFET is on past the turn-on period until it shifts to the turn-off period. The total heat is a total of switching heat and ON heat, that is, heat of the actual MOSFET. FromFIG. 4, it is understood that the total heat increases with a rise in temperature in the related art example while the total heat drops at 125° C., an operation limit temperature in First embodiment.

In this way, according to First embodiment of the invention, A diode D1having a characteristic that the reverse-direction leakage current increases with a rise in temperature, a fixed resistor R1and a transistor Q1for amplification are added to a transistor Q2of the current mirror circuit (constant current source). Thus, the load control device operates normally at normal temperatures. When the temperature has exceeded 75° C. and approached an operation limit, the frequency of the PWM signal is corrected to decrease heat. It is thus unnecessary to make a heat dissipation design to allow heating at an expected maximum operating temperature unlike in the related art practices. As a result, a heat dissipation portion is simplified thus downsizing the load control device.

Second Embodiment

While the diode D1having a p-n junction is used in the first embodiment, the invention is not limited thereto. For example, a Schottky barrier diode may be used instead of the diode D1. In case a high temperature leakage current that flows with a rise in the temperature of a Schottky barrier diode is large enough, a PNP transistor Q1may be omitted as shown inFIG. 5.

FIG. 5is a circuit diagram showing the configuration of a load control device according to Second embodiment of the invention. InFIG. 5, a same component as that inFIG. 2is given a same sign and the corresponding description is omitted. The load control device shown inFIG. 5differs from that shown inFIG. 2in that a Schottky barrier diode D2is provided anew instead of a diode D1having a p-n junction and a PNP transistor Q1is removed. Operation of the load control device in this example is almost the same as that in First embodiment so that its description is omitted.

In this way, according to Second embodiment of the invention, a Schottky barrier diode D2having a characteristic that a high temperature leakage current increases substantially with a rise in temperature and a fixed resistor R1are added to a transistor Q2of the current mirror circuit (constant current source). This provides almost the same effect as that of First embodiment.

Third Embodiment

FIG. 6is a circuit diagram showing the configuration of a load control device according to the third embodiment of the invention.

InFIG. 6, a same component as that inFIG. 2is given a same sign and the corresponding description is omitted. The load control device shown inFIG. 6differs from that shown inFIG. 2in that a thermistor TH1is provided anew instead of PNP transistor Q1and a diode D1is removed.

The thermistor TH1has a characteristic that the resistance value drops with a rise in temperature. The thermistor TH1is called an NTC (negative temperature coefficient) thermistor where a change in temperature is proportional to a change in resistance value. The thermistor TH1is produced for example by mixing oxides such as nickel (Ni), manganese (Mn), cobalt (Co) and iron (Fe) and sintering the resulting mixture.

Referring toFIG. 6, a thermistor TH1, transistors Q2to Q10, resistors R1to R9, a comparator CP1and a capacitor C1constitute a triangular wave generation circuit1shown inFIG. 1. The transistors Q2to Q4constitute a current mirror circuit (constant current source). The emitter area of each of the transistors Q2to Q4is the same. Thus, a collector current I2to I4flowing through each of the collectors of the transistors Q2to Q4is the same. That is, Expression (1) is satisfied.
I2=I3=I4  (1)
where a current I0flowing through a resistor R2is represented by Expression (2) using a constant voltage Vc, the base-emitter voltage VBE2of the transistor Q2and the resistor R2.
I0=(Vc−VBE2)/R2  (2)

A current I1flowing through the resistor R1is a current that bypasses part of the current I0from the transistor Q2. Given the resistance value of the thermistor TH1as Rth1, the current I1is represented by the expression (12).
I1=VBE2/(Rth1+R1)  (12)

From Expression (2) and Expression (12), the current I2is represented by Expression (13).
I2=I0−I1={(Vc−VBE2)/R2}−{VBE2/(Rth1+R1)}  (13)

The collector currents I2to I4are constant currents as a reference for charging or discharging the capacitor C1. The collector current I4is a current used to charge the capacitor C1with electric charges.

The configuration of a load control device according to Third embodiment of the invention after a transistor Q5is the same as that of the load control device according to First embodiment (refer toFIG. 2) described earlier so that the corresponding description is omitted. The operation of the load control device of the above configuration is substantially the same as the operation of the load control device explained above with reference to the timing chart shown inFIG. 3.

The period T of the triangular wave signal generated by the triangular wave generation circuit1is represented by Expression (14).

The voltage VBE2across the base and emitter of the transistor Q2has a temperature characteristic of around 2 mV/° C. so that it may be regarded as almost constant. Thus, the period T of the triangular wave signal is influenced only by the resistance value Rh1of the thermistor TH1. The thermistor TH1has a characteristic that the resistance value drops with a rise in temperature. As the temperature rises, the period T of the triangular wave signal becomes longer than Expression (14). In other words, the frequency of the triangular wave signal drops.

FIG. 7shows an exemplary result of comparison between a case where a lamp of a certain rating is actually driven by a load control device of the above configuration and a case where the same lamp is actually driven in accordance with the related art. In this comparison experiment, an NTC thermistor is used as a thermistor TH1with the resistance value Rth1at a temperature of 25° C. being 100 kΩ and the B constant being 4500. The other conditions and meanings of words shown inFIG. 7are the same as those of First embodiment described referring toFIG. 4. FromFIG. 7, it is understood that, while the total heat increases with a rise in temperature in the related art example, increase in the total heat is suppressed despite a rise in temperature in the third embodiment.

In this way, according to The third embodiment of the invention, a thermistor TH1having a characteristic that the resistance value drops with a rise in temperature and a fixed resistor R1are added to a transistor Q2of the current mirror circuit (constant current source). This provides almost the same effect as that of the first embodiment.

Fourth Embodiment

FIG. 8is a circuit diagram as an another particular implementation of the block diagram of a load control device shown inFIG. 1. InFIG. 8, a portion enclosed by alternate long and short dashed lines constitutes a load control device. Components of the load control device including transistors Q101to Q109, resistors R101to R11, comparators CP101, CP102, an OR gate3, a driving circuit4and a constant voltage power source121are composed of ICs. That is, a capacitor C101and an N-channel MOSFET122as a load driving element5are external components of the ICs.

The load control device in this embodiment is a device (low-side switching device) that includes an N-channel MOSFET122as a load driving element5downstream a lamp111as a load6. The load control device is mounted for example on a vehicle. As the load6shown inFIG. 1, a lamp111used as a headlamp is used inFIG. 8. The lamp111is connected between the power terminal Tb and the output terminal To of the load control device. InFIG. 8, a battery112mounted on a vehicle is used as a power source. A battery voltage Vbatis connected between the power terminal Tb and the ground terminal Tg of the load control device.

InFIG. 8, a control signal (at “H” (High) level or “L” (Low) level) (fixed input) outputted from an ECU (Electronic Control Unit)113mounted on a vehicle is supplied to the load control device. The ECU is designed to control the fuel injection amount or ignition timing of the engine of a vehicle to control the engine or control an automatic transmission or traction control.

InFIG. 8, PNP transistors Q101to Q103, PNP transistors Q104to Q109, resistors R101to R108, a comparator CP101and a capacitor C101constitute a triangular wave generation circuit1shown inFIG. 1. The transistors Q101to Q103constitute a current mirror circuit. The emitter area of each of the transistors Q101to Q103is the same. Thus, collector currents I1to I3flowing through the collectors of the transistors Q101to Q103are the same. That is, Expression (1) is satisfied.
I1=I2=I3  (1)
where a collector current I1is represented by Expression (2) using a constant voltage Vc, the base-emitter voltage VBE1of the transistor Q1and the resistor R1.
I1=(Vc−VBE1)/R1  (2)

The collector currents I1to I3are constant currents as a reference for charging or discharging the capacitor C101. The collector current I3is a current used to discharge the electric charges accumulated on the capacitor C101or charge the capacitor C101with electric charges.

The transistors Q104to Q106constitute a current mirror circuit (constant current source). The resistor R102is provided to compensate for the base current of the transistor Q104. The ratio of the emitter area of the transistor Q104to the total emitter area of the transistors Q105and Q106is 1:2. The collector current flowing in the collector of the transistor Q104is equal to the collector current I2of the transistor Q102. Further, from Expression (1), the collector current I2of the transistor Q102is equal to the collector current I1of the transistor Q101.

Thus, the collector current I5flowing in the transistor Q105is twice each of the collector currents I1to13of the transistors Q101to Q103. That is, Expression (3) is satisfied.
I5=2×I1=2×I2=2×I3  (3)

The collector current I5is a current used to charge the capacitor C101with electric charges or discharge the electric charges accumulated on the capacitor C101.

The transistor Q107is provided to shut down the supply of the collector current I5when turned ON. The transistor Q108and the resistors R103to R105generate a reference voltage Vt1for generating the triangular wave signal. The resistor R106is a base resistor connected between the base of the transistor Q108and the output terminal of the comparator CP101.

The transistor Q109and resistors R107and R108constitute a circuit for turning ON/OFF the transistor Q107by way of the output signal of the comparator CP1. In the triangular wave generation circuit1, the comparator CP101compares the voltage VC1of the capacitor C101with a reference voltage Vt1based on a constant current obtained by a current mirror circuit composed of transistors Q101to Q103, a current mirror circuit composed of transistors Q104to Q106and a resistor R101respectively connected to a constant voltage Vc. The triangular wave generation circuit1thus switches between charging and discharging of the capacitor C101to generate a triangular wave signal.

The comparator CP102and resistors R109and R110constitute a PWM generation circuit2shown inFIG. 1. The resistors R109and R110generate a reference voltage Vk for generating the PWM signal. The reference voltage Vk is represented by Expression (15).
Vk=Vc×R110/(R109+R110)  (15)

In the PWM generation circuit2, the comparator CP102includes a triangular wave signal supplied from the triangular wave generation circuit1with the reference voltage Vk. The PWM generation circuit2thus generates a PWM signal.

The resistor R111is interposed between a power source Vc and an input terminal Ti and functions as a pull-up resistor to stably hold the potential of a control signal supplied from the ECU113. The constant voltage power source121generates a constant voltage Vc from a battery voltage Vbatsupplied from a battery112and supplies the constant voltage Vc to each part of the load control device. The MOSFET122has its gate connected to the output terminal of the driving circuit4and its drain connected to the output terminal To of the load control device and its source grounded.

Operation of the load control device of this configuration will be described. It is assumed that the load control device of the above configuration includes an IC where transistors Q101to Q109, resistors R101to R111, comparator CP101, CP102, an OR gate3, a driving circuit4and a constant voltage power source121are arranged on its internal chip, a capacitor C101and a MOSFET122mounted on a single printed circuit board.

For example as shown inFIG. 9, on this printed circuit board is formed patterns P1to P3for mounting a capacitor C101as an external component of the IC. At the ends of the patterns P1to P3are respectively formed lands L1to L3. The pattern P1is connected to a power line that connects to the output terminal of the constant voltage power source121shown inFIG. 8. The pattern P2is connected to the non-inverted input terminal of the comparator CP101shown inFIG. 8. The pattern P3is connected to the ground line shown inFIG. 8.

(1) In case a vehicle where this load control device is mounted uses low beams for DRL:

In this case, as shown inFIG. 9, one terminal of the capacitor C101is inserted into a through hole made almost in the center of the land L1formed at an end of the pattern P1and the other terminal of the capacitor C101is inserted into a through hole made almost in the center of the land L2formed at an end of the pattern P2. Next, for example by melting cream solder previously applied on the lands L1and L2, one terminal of the capacitor C101and the land L1are electrically connected to each other and the other terminal of the capacitor C101and the land L2are electrically connected to each other.

Next, operation of the load control device of the above configuration will be described referring to the timing chart shown inFIG. 10. As shown inFIG. 10, in case the control signal supplied from the ECU113is High, the output signal of the OR gate3is always High. The driving circuit4amplifies and inverts the logical value of High level supplied from the OR gate3and applies a Low driving voltage to the MOSFET122. While the Low driving voltage is applied from the driving circuit4, the MOSFET122has its gate voltage driven Low so that it is turned OFF. In this case, the source voltage of the MOSFET122is almost equal to the battery voltage Vbatso that a load current does not flow into a load6, or a lamp111in this example as shown inFIG. 10.

As shown inFIG. 10, in case the control signal supplied from the ECU113is Low, the output signal of the comparator CP102of the PWM generation circuit2serves as an output signal of the OR gate3.

In case the voltage VC1of the capacitor C101is lower than the reference voltage Vt1at a certain time, the output signal of the comparator CP101is driven Low and the transistors Q108and Q109are turned OFF. While the transistor Q108is turned OFF, the reference voltage Vt1is the upper limit voltage Vb of the triangular wave signal as shown inFIG. 10. The upper limit voltage Vb is represented by Expression (16).
Vb=Vc×R104/(R103+R104)  (16)

When the transistor Q109is turned OFF, a current flows into the base of the transistor Q107from the resistor R108so that the transistor Q107is turned ON. When the transistor Q7is turned ON, supply of a collector current I5is stopped. As a result, a collector current I3flows, which discharges the electric charges accumulated on the capacitor C101and the voltage across the terminals of the capacitor C101decreases. The voltage VC1of the capacitor C101rises.

When the voltage VC1of the capacitor C101exceeds the upper limit voltage Vb even by a small amount, the output signal of the comparator CP101is driven High, which turns ON the transistors Q108and Q109. While the transistor Q108is turned ON, without considering the saturation voltage of the transistor Q108, the reference voltage Vt1becomes a resistance dividing voltage of the composite resistance value of the resistors R104and R105and the resistance value of the resistor R103, and as shown inFIG. 10, becomes the lower limit voltage Va of the triangular wave signal. The lower limit voltage Va is represented by Expression (17).
Va=Vc×(R104×R105)/(R103×R104+R103×R105+R104×R105)  (17)

When the transistor Q109is turned ON, a current does not flow from the resistor R8to the base of the transistor Q107. This turns OFF the transistor Q107. When the transistor Q107is turned OFF, supply of the collector current I5starts. The collector current I5is double the collector current I3. Thus, subtracting the collector current I3from the collector current I5, the collector current I3flows so that the capacitor C101is charged with electric charges. When the capacitor C101is charged, the voltage across the terminals of the capacitor C101increases. Thus, the voltage VC1of the capacitor C101drops. When the voltage VC1of the capacitor C101drops below the lower limit voltage Va even by a small amount, the output signal of the comparator CP101is inverted to Low. These operations are repeated to generate the triangular wave signal shown inFIG. 10.

The period T of the triangular wave signal is represented by Expression (18).

When the triangular wave signal supplied from the triangular wave generation circuit1, that is, the voltage VC1of the capacitor C101exceeds the reference voltage Vk, the output signal of the comparator CP102of the PWM generation circuit2is driven Low. When the voltage VC1of the capacitor C101drops below the reference voltage Vk, the output signal of the comparator CP102is driven High. These operations are repeated to generate the PWM signal shown inFIG. 10.

The output signal of the comparator CP2, that is, the PWM signal is supplied to the driving circuit4via the OR gate3. The driving circuit4amplifies and inverts the logical value of the PWM signal supplied from the OR gate3and applies a driving voltage to the MOSFET122. While the driving voltage applied from the driving circuit4is High, the MOSFET122has its gate voltage driven High so that it is turned ON. The source voltage of the MOSFET122is almost equal to the ground voltage. As shown inFIG. 10, a load current flows into a load6, or a lamp11in this example.

In case the driving voltage from the driving circuit4is Low, the MOSFET122is turned OFF. Thus, the source voltage of the MOSFET122rises until it is almost equal to the battery voltage Vbat. As shown inFIG. 10, a load current does not flow into a load6, or a lamp111in this example.

These operations are repeated to drive the lamp111to blink based on the supplied driving voltage.

For example, in case the capacitor C101has shorted by some cause in this normal operation, the voltage VC1of the capacitor C101becomes a constant voltage Vc. The constant voltage Vc is higher than the reference voltage Vk as understood from Expression (15). Thus, the voltage VC1of the capacitor C101is higher than the reference voltage Vk. Thus, the output signal of the comparator CP102, that is, the PWM signal is fixed to Low level.

The PWM signal fixed to Low level is supplied to the driving circuit4via the OR gate3. The driving circuit4amplifies and inverts the logical value of the PWM signal supplied from the OR gate3and keeps applying a High driving voltage to the MOSFET122. While the driving voltage that is fixed to High level is applied from the driving circuit4, the MOSFET122is maintained ON and keeps feeding a load current to the lamp111. In other words, the lamp111keeps lighting with a 100% duty ratio.

In this case, low beams are used for DRL so that the lamp111in constant lighting does not present no particular problems. The lighting state of the lamp111ensures safety of the people on the vehicle, pedestrians and oncoming vehicles so that the lighting state is rather favorable from the viewpoint of a fail-safe design.

(2) In case a vehicle where this load control device is mounted high beams for DRL:

In this case, one terminal of the capacitor C101is inserted into a through hole made almost in the center of the land L2formed at an end of the pattern P2and the other terminal of the capacitor C101is inserted into a through hole made almost in the center of the land L3formed at an end of the pattern P3. Next, for example by melting cream solder previously applied on the lands L2and L3, one terminal of the capacitor C101and the land L2are electrically connected to each other and the other terminal of the capacitor C101and the land L3are electrically connected to each other.

Next, operation of the load control device of the above configuration will be described. In case the control signal supplied from the ECU113is High, the output signal of the OR gate3is always High. The driving circuit4amplifies and inverts the logical value of High level supplied from the OR gate3and applies a Low driving voltage to the MOSFET122. While the Low driving voltage is applied from the driving circuit4, the MOSFET122is turned OFF. In this case, the source voltage of the MOSFET122is almost equal to the battery voltage Vbatso that a load current does not flow into a load6, or a lamp11in this example.

In case the control signal supplied from the ECU113is Low, the output signal of the comparator CP102of the PWM generation circuit2serves as an output signal of the OR gate3.

In case the voltage VC1of the capacitor C101is lower than the reference voltage Vt1at a certain time, the output signal of the comparator CP1is driven Low and the transistors Q108and Q109are turned OFF. While the transistor Q108is turned OFF, the reference voltage Vt1is the upper limit voltage Vb of the triangular wave signal.

When the transistor Q109is turned OFF, a current flows into the base of the transistor Q107from the resistor R108so that the transistor Q107is turned ON. When the transistor Q107is turned ON, supply of a collector current I5is stopped. As a result, a collector current I3flows, which charges the capacitor C101with electric charges and the voltage across the terminals of the capacitor C101increases. The voltage VC1of the capacitor C101rises.

When the voltage VC1of the capacitor C101exceeds the upper limit voltage Vb even by a small amount, the output signal of the comparator CP101becomes “H” level and the transistors Q108and Q109are turned ON. While the transistor Q108is turned ON, the reference voltage Vt1becomes the lower limit voltage Va of the triangular wave signal.

When the transistor Q109is turned ON, a current does not flow from the resistor R108to the base of the transistor Q107. This turns OFF the transistor Q107. When the transistor Q107is turned OFF, supply of the collector current I5starts. The collector current I5is double the collector current I3as mentioned above. Thus, subtracting the collector current I3from the collector current I5, the collector current I3flows so that the electric charges accumulated on the capacitor C101are discharged.

When the electric charges accumulated on the capacitor C101are discharged, the voltage across the terminals of the capacitor C101decreases. Thus, the voltage VC1of the capacitor C101drops. When the voltage VC1of the capacitor C101drops below the lower limit voltage Va even by a small amount, the output signal of the comparator CP101is inverted to Low. These operations are repeated to generate the triangular wave signal. The period T of the triangular wave signal is represented by Expression (18) mentioned above.

When the triangular wave signal supplied from the triangular wave generation circuit1, that is, the voltage VC1of the capacitor C101exceeds the reference voltage Vk, the output signal of the comparator CP102of the PWM generation circuit2is driven Low. When the voltage VC1of the capacitor C101drops below the reference voltage Vk, the output signal of the comparator CP102is driven High. These operations are repeated to generate the PWM signal.

The output signal of the comparator CP102, that is, the PWM signal is supplied to the driving circuit4via the OR gate3. The driving circuit4amplifies and inverts the logical value of the PWM signal supplied from the OR gate3and applies a driving voltage to the MOSFET122. While the driving voltage applied from the driving circuit4is High, the MOSFET122is turned ON. The source voltage of the MOSFET122is almost equal to the ground voltage, and thus a load current flows into a lamp111in this example.

In case the driving voltage from the driving circuit4is Low, the MOSFET122is turned OFF. Thus, the source voltage of the MOSFET122rises until it is almost equal to the battery voltage Vbat. As a result, a load current does not flow into a lamp111.

These operations are repeated to drive the lamp111to blink based on the supplied driving voltage.

For example, in case the capacitor C101has shorted by some cause in this normal operation, the voltage VC1of the capacitor C101becomes 0V. The voltage VC1of the capacitor C101is 0V and thus is lower than the reference voltage Vk in Expression (15). The output signal of the comparator CP102, i.e., the PWM signal, is fixed to High level.

The PWM signal fixed to High level is supplied to the driving circuit4via the OR gate3. The driving circuit4amplifies and inverts the logical value of the PWM signal supplied from the OR gate3and keeps applying a Low driving voltage to the MOSFET122. While the driving voltage that is fixed to Low level is applied from the driving circuit4, the MOSFET122is maintained OFF and maintains a state where a load current does not flow into the lamp111. In other words, the lamp111stays OFF.

In this case, the high beams are used for DRL so that the lamp111stays OFF. There is no possibility of glare occurring on the eyes of the driver of a vehicle in front or an oncoming vehicle, thus previously preventing a traffic accident.

In this way, according to Fourth embodiment of the invention, the triangular wave generating circuit1is configured such that a triangular wave signal of the same frequency and same shape is generated in a first interposing state where a capacitor C101for setting the frequency is interposed between the constant voltage Vc and the non-inverted input terminal of the comparator CP101and a second interposing state where the capacitor C101is interposed between a ground and the non-inverted input terminal of the comparator CP101.

According to the fourth embodiment of the invention, for example, as shown inFIG. 9, patterns P1to P3for mounting the capacitor C101for frequency setting are formed on a printed circuit board where a load control device is mounted, in accordance with the capacitor C101and the first or second interposing form. In case a vehicle where the load control device is mounted uses the low beams for DRL, both terminals of the capacitor C101are electrically connected to the land L1of the pattern P1and the land L2of the pattern P2. In case a vehicle where the load control device is mounted uses the high beams for DRL, both terminals of the capacitor C1are electrically connected to the land L2of the pattern P2and the land L3of the pattern P3.

It is thus possible to enhance the safety of the load control device assumed when the capacitor C101has shorted. With a vehicle using low beams for DRL, a fail-safe design is ensured. With a vehicle using high beams for DRL, safety is enhanced. It is unnecessary to manufacture two types of printed circuit boards depending on the type of vehicle using the load control device. This contributes to reduced costs.

Fifth Embodiment

While the comparator CP2(CP102) does not exhibit hysteresis in each of the foregoing embodiments, the invention is not limited thereto but the comparator CP2(CP102) may exhibit hysteresis.FIG. 11is a circuit diagram showing an exemplary configuration of a comparator CP2(CP102) and its peripheral circuitry where a hysteresis circuit31is added to the comparator CP2(CP102).

The hysteresis circuit31is composed of an inverter INV, a PNP transistor Q21, and resistors R21and R22. The inverter INV inverts the output signal of the comparator CP2(CP102), that is, the PWM signal. The resistor R22is a base resistor connected between the base of the transistor Q21and the output end of the inverter INV. The PNP transistor Q21changes the reference voltage Vk when it is turned ON by a High output signal of the inverter INV supplied via the resistor R22. Configuration of the other parts of the load control device than the comparator CP2(CP102) and its peripheral circuitry may be the same as that inFIG. 2in the first embodiment, the same as that inFIG. 5in the second embodiment, the same as that inFIG. 6in the third embodiment, and the same as that inFIG. 8in the fourth embodiment.

Next, operation of the comparator CP2(CP102) and its peripheral circuitry of the load control device will be described referring to the timing chart shown inFIG. 12.

In case a triangular wave signal supplied from the triangular wave generation circuit1is above a reference voltage Vk, the output signal of the comparator CP2(CP102) of the PWM generation circuit2is driven Low. The output signal of the inverter INV is driven High and the transistor Q21is turned ON.

While the transistor Q21is turned ON, without considering the saturation voltage of the transistor Q21, the reference voltage Vk becomes a resistance dividing voltage of the composite resistance value of the resistors R11and R21and the resistance value of the resistor R10, and as shown inFIG. 12, becomes a second reference voltage Vk2. The second reference voltage Vk2is represented by Expression (19).
Vk2=Vc×{(R11×R21)/(R11+R21)}/[R10+(R11×R21)/(R11+R21)]  (19)

Next, when the triangular wave signal drops below the second reference voltage Vk2, the output signal of the comparator CP2(CP102) is driven High. Thus, the output signal of the inverter INV is driven Low and the transistor Q21is turned OFF. While the transistor Q21is turned OFF, the reference voltage Vk2changes to a value represented by Expression (6) as shown inFIG. 12. In other words, the comparator CP2has hysteresis. These operations are repeated to generate a PWM signal with a larger pulse width than in the foregoing embodiments as shown inFIG. 12. In this way, according to Fifth embodiment, the comparator CP2(CP102) has hysteresis so that a noise resistance can be enhanced further than the above embodiments.

Sixth Embodiment

While the invention is applied to a device (low-side switching device) that includes an N-channel MOSFET22as a load driving element5arranged downstream a lamp11as a load6in the foregoing embodiments, the invention is not limited thereto. For example, the invention may be applied to a device (high-side switching device) that includes an N-channel MOSFET22as a load driving element5arranged upstream a lamp11as a load6. In this case, a P-channel MOSFET may be used in place of an N-channel MOSFET22as a load driving element5.

Seventh Embodiment

While an N-channel MOSFET22or a P-channel MOSFET is used as a load driving element5in the foregoing embodiments, the invention is not limited thereto. The load driving element5may be a bipolar transistor, a thyristor, an IGBT (Insulated Gate Bipolar Transistor), an SIT (Static Induction Transistor) or any other type of switching element.

While embodiments of the invention have been detailed referring to drawings, specific configurations of the invention are not limited thereto but modifications to the design within the scope of the invention are also included in the invention.

For example, while the constant voltage power source21is provided in the above embodiments, the invention is not limited thereto but the constant voltage power source21may be done without. In this case, in the first embodiment, the emitter of each of the transistors Q1to Q4and one end of each of the resistors R4, R9, R10and R12are directly connected to the power terminal Tb. In the second embodiment, the emitter of each of the transistors Q2to Q4, the cathode of the Schottky barrier diode D2, and one end of each of the resistors R4, R9, R10and R12are directly connected to the power terminal Tb. In the third embodiment, the emitter of each of the transistors Q2to Q4, the thermistor TH1, and one end of each of the resistors R4, R9, R10and R12are directly connected to the power terminal Tb. Note that, in Third embodiment, the connecting position of the thermistor TH1may be changed with that of the resistor R1.

Also, while the constant voltage power source21is provided in the above embodiments, the invention is not limited thereto. The constant voltage power source21may be not provided to the load control device. In this case, the emitter of each of the transistors Q1to Q3(Q101to Q103) and one end of each of the resistors R3, R8, R9and R11(R103, R108, R109and R111) are directly connected to the power terminal Tb. The pattern P1to which one terminal of the capacitor C1(C101) is to be connected is also directly connected to the power terminal Tb.

While the patterns P1to P3shown inFIG. 9are formed on a printed circuit board in the foregoing embodiments, the invention is not limited thereto. For example, the following configuration may be used. One terminal of the capacitor C101is electrically connected to the land of a pattern that is connected to the non-inverted input terminal of the comparator CP101. The pattern P2shown inFIG. 9is formed in a very short length and the other terminal of the capacitor C101is electrically connected to one land (not shown inFIG. 9). In case an automobile where this load controller is mounted uses low beams for DRL, a jumper pin is electrically connected across the lands L1and L2shown inFIG. 9. In case an automobile where this load controller is mounted uses high beams for DRL, a jumper pin is electrically connected across the lands L2and L3shown inFIG. 9.

While the load control device according to the invention is mounted on a vehicle, and the load6is a lamp11used as a headlamp in the above embodiments, the invention is not limited thereto. The invention may be generally applied to a device for controlling a load based on the generated PWM signal or the like.

The foregoing embodiments may use techniques of each other unless its purpose and configuration are not contradictory or problematic.

Although the invention has been illustrated and described for the particular preferred embodiments, it is apparent to a person skilled in the art that various changes and modifications can be made on the basis of the teachings of the invention. It is apparent that such changes and modifications are within the spirit, scope, and intention of the invention as defined by the appended claims.

The present application is based on Japan Patent Application No. 2006-161862 filed on Jun. 12, 2006 and Japan Patent Application No. 2006-161873 filed on Jun. 12, 2006, the contents of which are incorporated herein for reference.