JP2013062721A - Overcurrent detection device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To maximally prevent wrong detection even if a load having an electrical characteristic deviating from a recommended value suitable for drive is driven.SOLUTION: A threshold generation circuit 11 applies threshold generation currents ITH, ITL to resistances 13, 14 to generate threshold voltages VTH, VTL, and a comparator 12 compares therewith a detection voltage Va across a shunt resistance 7 to produce an overcurrent detection signal Sc. A threshold correction circuit 17 increases the threshold voltages VTH, VTL with increasing average load current, load current AC change and supply voltage VB, and increases the threshold voltages VTH, VTL with decreasing temperature of a load 4.

Description

  The present invention relates to an overcurrent detection device that outputs an overcurrent detection signal by comparing a voltage generated according to a current flowing through a load with a threshold voltage.

  The user of the vehicle can increase the number of lamps installed in the vehicle within the range that meets the safety standards of road transport vehicles after acquiring the vehicle, replace it with a lamp that consumes a large amount of current to increase the amount of light, Various automotive electrical parts may be purchased and installed. Among these electric parts for automobiles, in addition to genuine products provided by automobile manufacturers for each vehicle, there are parts manufactured and sold independently by parts manufacturers.

  Electrical parts such as lamps are driven by a load driving device configured as an IC. This load driving device includes a high-accuracy overcurrent protection circuit, and cuts off the current when the driving current exceeds a predetermined threshold current (see Patent Documents 1 and 2).

  Genuine products have electrical characteristics that conform to the electrical specifications of the vehicle unless they are selected and installed incorrectly. In contrast, electrical parts sold by general parts manufacturers are often designed and manufactured as general-purpose products that can be commonly applied to multiple types of vehicles. Therefore, the electrical characteristics are likely to be shifted.

  For example, if the power consumption of an electrical part is larger than the recommended value suitable for the electrical specifications of the vehicle, the drive current increases and the ripple of the drive current also increases, so the temperature fluctuation of the electrical part itself and the fluctuation of the battery voltage The overcurrent protection function may operate due to slight fluctuation factors. In addition, since the power supply voltage range necessary for normal operation is narrow, some electrical parts themselves include a booster circuit or a step-down circuit, and there are those that give noise to the load driving device when the booster circuit or the step-down circuit operates. This noise increases due to fluctuations in temperature and power supply voltage, and appears superimposed on the drive current.

JP 2009-21071 A JP 2005-204375 A

  When a load (electrical part) is connected to the power line of the vehicle in this way, if the electrical characteristics of the load deviate from the recommended value suitable for the electrical specifications of the vehicle, the overcurrent detection device is inherently problematic. There is a risk of erroneous detection as an overcurrent condition even in normal usage. Since this defect appears only after incompatibility with the electrical parts connected to the vehicle load drive device, it is difficult for the user to take a precaution. In addition, it is difficult for vehicle manufacturers to provide load drive devices that are compatible with various electric parts other than genuine products.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide an overcurrent detection device capable of preventing erroneous detection as much as possible even when a load having electrical characteristics deviated from a recommended value is driven.

  The overcurrent detection device according to claim 1 is a current detection circuit that converts a current flowing through a load into a voltage and outputs the voltage, a threshold value generation circuit that generates a threshold voltage, and a voltage output by the current detection circuit And a comparison circuit that outputs an overcurrent detection signal. The threshold generation circuit detects an average current flowing through the load, detects a first state detection circuit that increases the threshold voltage as the detected average current increases, and detects an AC change in the current flowing through the load. At least one of the second state detection circuits that increase the threshold voltage as the detected AC change is larger is provided.

  According to the first state detection circuit, an average current that steadily flows through the load is detected according to the electrical characteristics of the load, and the threshold voltage for determining an overcurrent increases as the average current increases. . In addition, according to the second state detection circuit, an AC change such as a ripple or noise that constantly appears in the load current is detected according to the electrical characteristics of the load. The threshold voltage at the time of determination is increased and corrected. As a result, unless the load drive device malfunctions due to overheating or noise, it will cause an overcurrent and error when driving a load with electrical characteristics that deviate from the recommended values suitable for the vehicle electrical specifications. Detection can be prevented as much as possible.

  According to the means described in claim 2, the threshold value generation circuit includes at least one of the third state detection circuit and the fourth state detection circuit in addition to the first state detection circuit and / or the second state detection circuit. I have. The third state detection circuit detects the temperature of the load or the ambient temperature of the load, and increases or decreases the threshold voltage depending on the type of load as the detected temperature is lower. For example, in a load such as a halogen light bulb or an incandescent light bulb whose resistance decreases as the temperature decreases, the threshold voltage is increased as the detection temperature is lower. On the other hand, in a load such as a light-emitting diode, where the forward voltage increases as the temperature decreases, the threshold voltage decreases as the detection temperature decreases. Thereby, the erroneous detection of the overcurrent depending on the temperature of the load or the environmental temperature of the load can be prevented as much as possible. The fourth state detection circuit detects a power supply voltage applied to the load, and increases the threshold voltage as the detected power supply voltage is higher. Thereby, erroneous detection of overcurrent depending on the power supply voltage can be prevented as much as possible.

  According to the means described in claim 3, the comparison circuit compares the voltage output from the current detection means with a high level threshold voltage and a low level threshold voltage separated by a predetermined hysteresis width, and detects an overcurrent. Output a signal. The state detection circuit increases or decreases the high level threshold voltage and the low level threshold voltage while maintaining a predetermined hysteresis width. Thereby, the hysteresis width can be changed in accordance with the correction amount of the threshold voltage, and the overcurrent detection operation and the recovery operation from the overcurrent can be stably performed.

  According to the means described in claim 4, each state detection circuit outputs a threshold generation current having a magnitude corresponding to an increase or decrease of the threshold voltage. The threshold generation circuit generates a threshold voltage by causing a threshold generation current to be superimposed and flowing through a resistive circuit. Thereby, a comprehensive threshold voltage reflecting a plurality of types of states can be generated easily and accurately.

The block diagram of the load drive circuit provided with the overcurrent detection circuit which shows the 1st Embodiment of this invention Configuration diagram of threshold correction circuit Configuration diagram of first state detection circuit The figure which shows the relationship between the average value of detection voltage Va, and correction | amendment electric current ITH1, ITL1 Configuration diagram of second state detection circuit The figure which shows the relationship between AC change amount V2 of voltage VS, and correction electric current ITH2, ITL2. Waveform diagram of voltage VS Configuration diagram of third state detection circuit A diagram showing the relationship between the forward voltage VD and the correction currents ITH3 and ITL3 in the case of a lamp load. A diagram showing the relationship between the detected temperature and the correction currents ITH3 and ITL3 in the case of a lamp load The figure which shows the relationship between the forward voltage VD and correction | amendment electric current ITH3 and ITL3 in case a load is a light emitting diode. The figure which shows the relationship between detection temperature and correction current ITH3 and ITL3 when the load is a light emitting diode Configuration diagram of fourth state detection circuit A diagram showing the relationship between the power supply voltage VB and the correction currents ITH4 and ITL4 Waveform diagram of voltage and current when power supply voltage VB increases Waveform diagram of voltage and current when overcurrent is detected FIG. 1 equivalent diagram showing a second embodiment of the present invention The block diagram of the threshold value generator circuit which shows the 3rd Embodiment of this invention

(First embodiment)
Hereinafter, a first embodiment will be described with reference to FIGS. FIG. 1 shows a configuration relating to a load driving circuit incorporating an overcurrent detection circuit. The load driving circuit 1 configured as an in-vehicle IC operates by receiving a power supply voltage VB from a battery (not shown), and when the light switch 2 is turned on, the relay 3 is turned on to energize the load 4 composed of a lamp. Let When the high potential side power supply line and the low potential side power supply line inside and outside the load drive circuit 1 are the power supply lines 5 and 6, respectively, the shunt resistor 7, the contact 3b of the relay 3 and the load 4 are interposed between the power supply lines 5 and 6. Connected in series. The shunt resistor 7 having the resistance value Ra corresponds to a current detection circuit that converts the current IL flowing through the load 4 into a voltage Va (= IL · Ra) and outputs the voltage Va, and constitutes a part of an overcurrent detection circuit 8 described later. doing.

  The load drive circuit 1 includes an overcurrent detection circuit 8, a control circuit 9, and a P-channel type MOS transistor 10 for high side drive. The overcurrent detection circuit 8 corresponding to the overcurrent detection device includes, in addition to the shunt resistor 7 described above, a threshold generation circuit 11 that generates a high level threshold voltage VTH and a low level threshold voltage VTL, and a shunt. It comprises a hysteresis comparator 12 (corresponding to a comparison circuit) that compares the detection voltage Va of the resistor 7 with threshold voltages VTH and VTL and outputs an overcurrent detection signal Sc.

  The high level threshold voltage VTH and the low level threshold voltage VTL are separated by a predetermined hysteresis width. The hysteresis comparator 12 sets the overcurrent detection signal Sc to the H level (overcurrent detection state) when the detection voltage Va becomes larger than the threshold voltage VTH. Thereafter, when the detection voltage Va becomes equal to or lower than the threshold voltage VTL, the overcurrent detection signal Sc is returned to the L level on condition that the state continues for a predetermined waiting time.

  The threshold generation circuit 11 includes resistors 13 and 14 (resistive circuit) having one end connected to the power supply line 5, current output circuits 15 and 16, and a threshold correction circuit 17. The threshold generation circuit 11 generates threshold voltages VTH and VTL by flowing threshold generation currents ITH and ITL through the resistors 13 and 14, respectively. The current output circuit 15 superimposes (adds) the standard current ITH0 having a constant value and the correction currents ITH1 to ITH4 flown by the threshold correction circuit 17 to flow through the resistor 13. The current output circuit 16 superimposes (adds) the standard current ITL0 having a constant value and the correction currents ITL1 to ITL4 flown by the threshold correction circuit 17 to flow through the resistor 14.

  As shown in FIG. 2, the threshold correction circuit 17 includes a first state detection circuit 18, a second state detection circuit 19, a third state detection circuit 20, a fourth state detection circuit 21, and an addition composed of diodes 22H to 25H. The circuit 26H and an adder circuit 26L including diodes 22L to 25L are included.

  The control circuit 9 is connected to the power supply line 5 via the light switch 2. When the light switch 2 is on and the overcurrent detection signal Sc is at L level (overcurrent non-detection state), for example, a gate drive signal having 0 V is output to drive the MOS transistor 10 on. In contrast, when the light switch 2 is on and the overcurrent detection signal Sc is at the H level (overcurrent detection state) and when the light switch 2 is off, for example, a gate drive signal having the power supply voltage VB. Is output to turn off the MOS transistor 10. Between the drain of the MOS transistor 10 (the output terminal of the load driving circuit 1) and the power supply line 6, the coil 3a of the relay 3 is connected outside the IC.

  Next, the configuration and operation of the first state detection circuit 18 to the fourth state detection circuit 21 will be described with reference to FIGS. In these state detection circuits 18 to 21, the same components (including components that differ only in resistance value and reference voltage value) are assigned the same reference numerals.

The first state detection circuit 18 shown in FIG. 3 detects the average current flowing through the load 4, and increases the correction currents ITH1 and ITL1 to increase the threshold voltages VTH and VTL as the detected average current increases. The operational amplifier 31 is a buffer circuit that receives the voltage VS of the low potential side terminal of the shunt resistor 7. The differential amplifier circuit composed of the operational amplifier 32 and the resistors 33 to 36 outputs the detection voltage Va corresponding to the load current IL as shown in the equation (1) because the resistance values of the resistors 33 to 36 are equal.
Va = VB−VS (1)

A differential amplifier circuit comprising an operational amplifier 37, resistors 38 to 41, a reference voltage generation circuit 42 and a capacitor 43 has a reference voltage of the reference voltage generation circuit 42 as VT1, resistors 38 and 40 as resistance values R1, and resistors 39 and 41. Assuming that the resistance value is R2, the averaged voltage V1 obtained by offsetting the detection voltage Va by the reference voltage VT1 and amplifying the gain R2 / R1 times as shown in the equation (2) is output.
V1 = R2 / R1 (Va-VT1) (2)

The voltage V1 is converted into correction currents ITH1 and ITL1 by the voltage-current conversion circuit 44. That is, the transistor 46 connected in series with the constant current circuit 45 has the form of an emitter follower and performs impedance conversion. The transistor 47 and the resistor 48 having the resistance value R3 convert the voltage V1 into the current Im1, as shown in the equation (3). The conversion rate of voltage-current conversion can be adjusted by the resistance value R3.
Im1 = V1 / R3 (3)

This current Im1 flows into a current mirror circuit composed of transistors 49, 50 and 51. Then, it is converted into a correction current ITH1 that is a sink current by a current mirror circuit having a mirror ratio of 1 consisting of transistors 52 and 53, and is converted into a correction current ITL1 that is a sink current by a current mirror circuit having a mirror ratio of 1 consisting of transistors 54 and 55. Is done. The slopes and magnitudes of the correction currents ITH1 and ITL1 can be adjusted by resistors 56 and 57 connected between the power supply line 5 and the transistors 50 and 51, respectively. The relational expressions at this time are expressed by the following expressions (4) and (5) if the resistance values of the resistors 56 and 57 are R4 and R5, respectively.
VBE of transistor 49 = VBE of transistor 50 + R4 · ITH1 (4)
VBE of transistor 49 = VBE of transistor 51 + R5 · ITL1 (5)

  FIG. 4 shows the input / output characteristics of the first state detection circuit 18. The horizontal axis indicates the average value of the detection voltage Va by the shunt resistor 7, and the vertical axis indicates the correction currents ITH1 and ITL1. The reference voltage VT1 corresponds to a correction start voltage. When the average value of the detection voltage Va is lower than the reference voltage VT1, that is, when the average value of the load current IL is lower than VT1 / Ra, the correction currents ITH1 and ITL1 do not flow. When the average value of the load current IL exceeds VT1 / Ra, the correction currents ITH1 and ITL1 increase as the average current increases, so that it is necessary for the overcurrent detection signal Sc to shift to the H level (overcurrent detection state). Threshold voltages VTH and VTL increase.

The second state detection circuit 19 shown in FIG. 5 detects the AC change of the current IL flowing through the load 4 and increases the correction currents ITH2 and ITL2 as the detected AC change is larger to increase the threshold voltages VTH and VTL. Increase. The peak hold circuit 58 detects the peak value VSmax of the voltage VS, and the bottom hold circuit 59 detects the bottom value VSmin of the voltage VS. The sampling time and hold time here are set so that the peak-to-peak value corresponding to AC fluctuations such as ripples and noise that constantly appear in the load current IL can be captured. The differential amplifier circuit composed of the operational amplifier 32 and the resistors 33 to 36 outputs the AC change V2 as shown in the equation (6) because the resistance values of the resistors 33 to 36 are equal.
V2 = VSmax−VSmin (6)

The voltage V2 is converted into correction currents ITH2 and ITL2 by the voltage-current conversion circuit 44. The transistor 47 and the resistor 48 having the resistance value R6 convert the voltage V2 into the current Im2, as shown in the equation (7). The conversion rate of voltage-current conversion can be adjusted by the resistance value R6.
Im2 = V2 / R6 (7)

If the resistance values of the resistors 56 and 57 are R7 and R8, respectively, the following equations (8) and (9) are established.
VBE of transistor 49 = VBE of transistor 50 + R7 · ITH2 (8)
VBE of the transistor 49 = VBE + R8 · ITL2 of the transistor 51 (9)

  FIG. 6 shows the input / output characteristics of the second state detection circuit 19. The horizontal axis indicates the AC change V2 of the voltage VS, and the vertical axis indicates the correction currents ITH2 and ITL2. The AC change V2 here is a peak-to-peak value including both ripple and noise. FIG. 7 shows the waveform (a) of the voltage VS when the load current IL is large and the waveform (b) of the voltage VS when the load current IL is small on the same scale. The two-dot chain line is the detected peak value VSmax and bottom value VSmin.

  As the load current IL increases, the AC change amount V2 of the voltage VS increases, so that the correction currents ITH2 and ITL2 are increased. As a result, the threshold voltages VTH and VTL necessary for the overcurrent detection signal Sc to shift to the H level (overcurrent detection state) increase.

  The third state detection circuit 20 shown in FIG. 8 detects the temperature of the load 4 or the ambient temperature of the load 4, and increases the correction currents ITH3 and ITL3 as the detected temperature is lower. The temperature is detected by the forward voltage VD of the diode 61 connected in series with the constant current circuit 60. As is well known, the forward voltage VD has a negative temperature characteristic. The diode 61 is arranged in the vicinity of the load 4 so that the temperature of the load 4 or the ambient temperature of the load 4 can be detected, but may be provided in the relay 3 shown in FIG.

The differential amplifier circuit composed of an operational amplifier 37, resistors 38 to 41 and a reference voltage generation circuit 42 has a reference voltage VT2 as a reference voltage of the reference voltage generation circuit 42, a resistance value R9 of the resistors 38 and 40, and a resistance value of the resistors 39 and 41. Assuming R10, the voltage V3 obtained by offsetting the forward voltage VD by the reference voltage VT2 and amplifying the gain R10 / R9 times as shown in the equation (10) is output.
V3 = R10 / R9 (VD-VT2) (10)

This voltage V3 is converted into correction currents ITH3 and ITL3 by the voltage-current conversion circuit 44. The transistor 47 and the resistor 48 having the resistance value R11 convert the voltage V3 into the current Im3 as shown in the equation (11). The conversion rate of voltage-current conversion can be adjusted by the resistance value R11.
Im3 = V3 / R11 (11)

If the resistance values of the resistors 56 and 57 are R12 and R13, respectively, the following equations (12) and (13) are established.
VBE of transistor 49 = VBE of transistor 50 + R12 · ITH3 (12)
VBE of transistor 49 = VBE of transistor 51 + R13 · ITL3 (13)

  FIG. 9 shows the input / output characteristics of the third state detection circuit 20. The horizontal axis indicates the forward voltage VD of the diode 61, and the vertical axis indicates the correction currents ITH3 and ITL3. FIG. 10 shows FIG. 9 as a relationship between the detected temperature and the correction currents ITH3 and ITL3. That is, the correction currents ITH3 and ITL3 do not flow when the forward voltage VD is equal to or lower than the reference voltage VT2 (for example, 0.45 V), that is, when the detected temperature is equal to or higher than 150 ° C. When the detection temperature is lower than 150 ° C., the correction currents ITH3 and ITL3 increase as the detection temperature decreases, so that the threshold voltage VTH necessary for the overcurrent detection signal Sc to shift to the H level (overcurrent detection state). VTL increases.

  The characteristics of the third state detection circuit 20 shown here are suitable for loads such as halogen bulbs and incandescent bulbs whose resistance decreases as the temperature decreases. This is because with such a load 4, the rush current increases as the temperature when the light switch 2 is turned on is lower. On the other hand, in the load 4 in which the forward voltage increases as the temperature decreases, such as a light emitting diode, the correction currents ITH3 and ITL3 increase as the forward voltage VD increases, that is, as the detection temperature decreases, as shown in FIGS. Is a characteristic that reduces

The fourth state detection circuit 21 shown in FIG. 13 detects the power supply voltage VB applied to the load 4, and increases the correction currents ITH4 and ITL4 as the detected power supply voltage VB is higher. Between the power supply lines 5 and 6, voltage dividing resistors 62 and 63 having resistance values R14 and R15 are connected in series. The divided voltage VR of the power supply voltage VB is as shown in equation (14).
VR = R15 / (R14 + R15) · VB (14)

The differential amplifier circuit composed of an operational amplifier 37, resistors 38 to 41 and a reference voltage generation circuit 42 has a reference voltage VT3 as a reference voltage of the reference voltage generation circuit 42, a resistance value R16 of resistors 38 and 40, and a resistance value of resistors 39 and 41. Assuming R17, the voltage V4 obtained by offsetting the divided voltage VR of the power supply voltage VB by the reference voltage VT3 and amplifying the gain R17 / R16 times as shown in the equation (15).
V4 = R17 / R16 (VR-VT3) (15)

This voltage V4 is converted into correction currents ITH4 and ITL4 by the voltage-current conversion circuit 44. The transistor 47 and the resistor 48 having the resistance value R18 convert the voltage V4 into the current Im4 as shown in the equation (16). The conversion rate of voltage-current conversion can be adjusted by the resistance value R18.
Im4 = V4 / R18 (16)

If the resistance values of the resistors 56 and 57 are R19 and R20, respectively, the following equations (17) and (18) are established.
VBE of transistor 49 = VBE of transistor 50 + R19 · ITH4 (17)
VBE of transistor 49 = VBE of transistor 51 + R20 · ITL4 (18)

  FIG. 14 shows the input / output characteristics of the fourth state detection circuit 21. The horizontal axis shows the power supply voltage VB, and the vertical axis shows the correction currents ITH4 and ITL4. When the power supply voltage VB is less than (R14 + R15) / R15 · VT3, the correction currents ITH4 and ITL4 do not flow. When the power supply voltage VB exceeds (R14 + R15) / R15 · VT3, the correction currents ITH4 and ITL4 increase as the voltage value increases, so that the overcurrent detection signal Sc shifts to the H level (overcurrent detection state). Necessary threshold voltages VTH and VTL increase. This is because the load current IL increases as the power supply voltage VB applied to the load 4 increases. The reference voltage VT3 corresponds to the correction start voltage.

  FIG. 15 shows voltage and current waveforms when the power supply voltage VB increases. The light switch 2 is turned on in the period from time t1 to t2 and in the period from time t5 to t6. Since the temperature of the load 4 (lamp filament temperature) is low when the light switch 2 is turned on, a rush current flows, and the load current IL gradually decreases as the lighting state continues. In this figure, since the detection voltage Va of the shunt resistor 7 does not exceed the threshold voltage VTH, energization to the load 4 is not interrupted. As the power supply voltage VB gradually increases from time t3 to time t4, the fourth state detection circuit 21 gradually increases the correction currents ITH4 and ITL4. As a result, threshold generation currents ITH and ITL and threshold voltages VTH and VTL also increase.

  FIG. 16 shows waveforms of voltage and current when an overcurrent is detected. The light switch 2 is turned on in the period from time t11 to t12 and in the period from time t13 to t15. When the terminals of the load 4 are short-circuited at time t14, the detection voltage Va of the shunt resistor 7 exceeds the threshold voltage VTH, so that the overcurrent detection signal Sc becomes H level and the energization to the load 4 is cut off. Here, the light switch 2 is turned off at time t15. However, if the light switch 2 continues to be turned on, the overcurrent detection signal Sc returns to the L level when a predetermined waiting time elapses after the light switch 2 is turned off, and energization is resumed. Is done.

  According to the present embodiment described above, the overcurrent detection circuit 8 outputs the overcurrent detection signal Sc based on the comparison between the detection voltage Va by the shunt resistor 7 and the threshold voltages VTH and VTL. In this case, the threshold value generation circuit 11 independently performs the correction currents ITH1 to ITH4 and the correction according to four types of states (average load current, AC change amount of the load current IL, temperature, power supply voltage VB). The threshold voltages VTH and VTL are generated by changing the currents ITL1 to ITL4 and superimposing these correction currents to flow through the shunt resistor 7. Thereby, comprehensive threshold voltages VTH and VTL reflecting each state can be generated easily and accurately, and an overcurrent can be detected in a well-balanced manner based on each state.

  The threshold generation circuit 11 includes a first state detection circuit 18 to a fourth state detection circuit 21. Of these, the first state detection circuit 18 detects the average current flowing through the load 4 and increases the threshold voltages VTH and VTL as the detected average current increases. The second state detection circuit 19 detects the AC change V2 of the current IL flowing through the load 4, and increases the threshold voltages VTH and VTL as the detected AC change V2 increases.

  If the first state detection circuit 18 and the second state detection circuit 19 are provided, even if the power consumption of the load 4 is shifted to a larger value than the recommended value suitable for the electrical specifications of the load driving circuit 1, a short circuit or As long as there is no abnormality such as deterioration and the operation is normal and the load drive circuit 1 does not cause a malfunction such as malfunction due to overheating or noise, erroneous detection of overcurrent can be prevented as much as possible. As a result, a load such as a lamp designed and manufactured by a parts manufacturer as a general-purpose product that can be commonly applied to a plurality of types of vehicles can be used without any problem as much as a genuine product designed and manufactured by a vehicle manufacturer.

  The third state detection circuit 20 detects the temperature of the load 4 or the ambient temperature of the load 4, and increases the threshold voltages VTH and VTL as the detected temperature is lower. As a result, erroneous detection of overcurrent due to the rush current of the load 4 can be prevented as much as possible. The fourth state detection circuit 21 detects the power supply voltage VB applied to the load 4 and increases the threshold voltages VTH and VTL as the detected power supply voltage VB is higher. As a result, it is possible to prevent erroneous detection of overcurrent caused by fluctuations in battery voltage.

  The overcurrent detection circuit 8 includes a hysteresis comparator 12. Each state detection circuit 18 to 21 can increase or decrease the threshold voltages VTH and VTL while maintaining a desired hysteresis width by trimming the resistors 56 and 57. it can. As a result, the overcurrent detection operation and the return operation from the overcurrent detection state can be stably performed.

(Second Embodiment)
FIG. 17 shows a second embodiment of the load drive circuit incorporating the overcurrent detection circuit, and the same reference numerals are given to the components corresponding to the components shown in FIG. In the load drive circuit 71, the connection mode of the constituent elements to the power supply lines 5 and 6 is opposite to that of the load drive circuit 1 shown in FIG. For example, one end of the load 4 is connected to the power supply line 5, and one end of the shunt resistor 7 is connected to the power supply line 6. In the load drive circuit 71, the N-channel MOS transistor 10 is low-side drive, and one ends of the resistors 13 and 14 are connected to the power supply line 6. The load drive circuit 71 and the load drive circuit 1 have substantially the same configuration. Therefore, according to this embodiment, the same operation and effect as the first embodiment can be obtained.

(Third embodiment)
FIG. 18 shows a third embodiment representing the threshold value generation circuit of the overcurrent detection circuit. Also in this embodiment, as shown in FIG. 1 or FIG. 17, the shunt resistor 7, the contact 3 b of the relay 3, and the load 4 are connected in series between the power lines 5 and 6. When the light switch 2 is turned on, the power supply voltage VB is applied to the load 4 and a load current IL flows. The overcurrent detection circuit compares the detection voltage Va by the shunt resistor 7 with the threshold voltages VTH and VTL and outputs an overcurrent detection signal Sc.

  The threshold value generation circuit 81 performs part of the processing for generating the threshold voltages VTH and VTL by a microcomputer 82 (hereinafter referred to as a microcomputer 82). A voltage detector 83 that detects a voltage VS corresponding to the load current IL, a temperature detector 84 that detects a voltage VD corresponding to the temperature of the load 4 or the ambient temperature of the load 4, and a power supply voltage VB are provided outside the microcomputer 82. A voltage detection unit 85 to detect, A / D converters 86 to 88, and D / A converters 89 and 90 are provided. The A / D converters 86 to 88, the D / A converters 89 and 90, and the like may be built in the microcomputer 82. The voltage detection unit 83 includes a shunt resistor 7, and the temperature detection unit 84 includes a diode, for example.

  The first correction unit 91 to the fourth correction unit 94 and the correction value calculation unit 95 show the processing executed by the CPU of the microcomputer 82 according to the program stored in the ROM as a block. The first state detection unit 96 to the fourth state detection unit 99 surrounded by a two-dot chain line correspond to the first state detection circuit to the fourth state detection circuit in the present invention. The correction value calculation unit 95 adds the correction values of the threshold voltages VTH and VTL calculated by the first correction unit 91 to the fourth correction unit 94, respectively, and calculates the total threshold voltage values VTH and VTL. To do. The digital values of the threshold voltages VTH and VTL are converted into threshold voltages VTH and VTL, which are analog values, through D / A converters 89 and 90, respectively.

  The first correction unit 91 calculates the average value of the load current IL based on the voltage VS input through the A / D converter 86, and the correction values of the threshold voltages VTH and VTL are calculated as the average value increases. increase. The second correction unit 92 calculates the AC change of the input voltage VS, and increases the correction values of the threshold voltages VTH and VTL as the AC change is larger. The AC change is a peak-to-peak value of ripples and noise that constantly appear in the load current IL. The third correction unit 93 calculates the temperature of the load 4 or the ambient temperature of the load 4 based on the voltage VD input via the A / D converter 87, and the threshold voltages VTH and VTL are reduced as the temperature decreases. Increase the correction value. In this case, the correction values of the threshold voltages VTH and VTL may be reduced as the temperature is lower according to the characteristics of the load 4. The fourth correction unit 94 increases the correction values of the threshold voltages VTH and VTL as the power supply voltage VB input via the A / D converter 88 is higher.

  According to the present embodiment, as in the first and second embodiments, when a general-purpose load supplied by a parts manufacturer is driven, it operates normally without abnormality such as overheating, noise failure, short circuit, and deterioration. As long as it is detected, erroneous detection of overcurrent can be prevented as much as possible. Further, since the threshold voltage VTH and VTL correction processing is executed using the microcomputer 82, it corresponds to four types of driving conditions (average load current, AC change of load current IL, temperature, power supply voltage VB). Complex correction processing is also possible.

  If a microcomputer inherent in an ECU (Electronic Control Unit) equipped with a load drive circuit can be used, it is not necessary to add a new microcomputer for overcurrent detection. The threshold generation circuit 11 shown is also unnecessary. Therefore, the circuit size of the overcurrent detection device can be reduced.

(Other embodiments)
As mentioned above, although preferred embodiment of this invention was described, this invention is not limited to embodiment mentioned above, A various deformation | transformation and expansion | extension can be performed within the range which does not deviate from the summary of invention.

  In the first and second embodiments, the first state detection circuit 18 to the fourth state detection circuit 21 are provided. However, it is sufficient that at least one of the first state detection circuit 18 and the second state detection circuit 19 is provided. . In addition to this, at least one of the third state detection circuit 20 and the fourth state detection circuit 21 may be provided. The same applies to the third embodiment.

  Since the hysteresis comparator 12 is used for the comparison circuit, the threshold generation circuits 11 and 81 generate two threshold voltages VTH and VTL. However, when the hysteresis characteristic is omitted, only one threshold voltage VT is generated. do it.

  The threshold generation circuit 11 may generate the threshold voltages VTH and VTL by causing the threshold generation currents ITH and ITL to flow through a resistive circuit instead of the resistors 13 and 14. The resistive circuit is a circuit element that outputs a voltage having a certain relationship with an input current.

The load 4 may be a load other than a lamp such as a motor, a solenoid, or an LED.
The output transistor driven by the control circuit 9 is not limited to a MOS transistor but may be a bipolar transistor, an IGBT, or the like. The output transistor may be arranged on either the high side or the low side. Either a P channel type or an N channel type may be used, and either a PNP type or an NPN type may be used.
A semiconductor switching element may be used instead of the relay 3.

  In the drawing, 4 is a load, 7 is a shunt resistor (current detection circuit), 8 is an overcurrent detection circuit (overcurrent detection device), 11 and 81 are threshold generation circuits, 12 is a hysteresis comparator (comparison circuit), 13 , 14 are resistors (resistive circuits), 18 and 96 are first state detection circuits, 19 and 97 are second state detection circuits, 20 and 98 are third state detection circuits, and 21 and 99 are fourth state detection circuits, VTH is a high level threshold voltage, VTL is a low level threshold voltage, ITH1 to ITH4, ITL1 to ITL4 are threshold generation currents, and Sc is an overcurrent detection signal.

Claims (4)

A current detection circuit that converts the current flowing through the load into a voltage and outputs the voltage, a threshold value generation circuit that generates a threshold voltage, and compares the voltage output from the current detection circuit with the threshold voltage In an overcurrent detection device including a comparison circuit that outputs a current detection signal,
The threshold generation circuit detects an average current flowing through the load, and a first state detection circuit that increases the threshold voltage as the detected average current increases, and an AC change amount of the current flowing through the load. And an at least one of second state detection circuits for increasing the threshold voltage as the detected AC change is larger.   The threshold value generation circuit detects the temperature of the load or the ambient temperature of the load in addition to the first state detection circuit and / or the second state detection circuit, and the lower the detected temperature, A third state detection circuit for increasing or decreasing the threshold voltage according to the type, a power supply voltage applied to the load is detected, and the threshold voltage is increased as the detected power supply voltage is higher. The overcurrent detection device according to claim 1, further comprising at least one of four state detection circuits. The comparison circuit compares the voltage output from the current detection means with a high level threshold voltage and a low level threshold voltage separated by a predetermined hysteresis width, and outputs the overcurrent detection signal;
3. The overcurrent detection device according to claim 1, wherein the state detection circuit increases or decreases the high level threshold voltage and the low level threshold voltage while maintaining the predetermined hysteresis width. . Each of the state detection circuits outputs a threshold generation current having a magnitude corresponding to an increase or decrease of the threshold voltage,
4. The threshold value generation circuit according to claim 1, wherein the threshold value generation circuit generates the threshold voltage by superimposing the threshold value generation current and flowing it through a resistive circuit. Current detection device.

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