Load driving device

The present invention relates to a load drive device including a synchronous rectifier circuit.

It may be necessary to detect a ceiling fault, a ground fault, or a disconnection as an abnormality detection in a load drive device. Especially for automobiles, due to fuel efficiency and safety issues, a wide range of drive conditions are required. It is required to be able to detect anomalies. By the way, as a disconnection detection in the synchronous rectifier circuit which is one of the circuits for driving the load, there is a method of detecting the current flowing in the synchronous rectifier circuit and determining the disconnection when the current does not flow. However, when driving at a low duty ratio or a low power supply voltage, the current flowing through the synchronous rectifier circuit may not meet the accuracy of current detection, and in this case, there is a risk of erroneous detection as a disconnection.

As a conventional technique that enables disconnection detection under such low current conditions, the drive side switching element is turned ON / OFF while the return side switching element is in the OFF state, and the presence or absence of a pulse of the output voltage is detected to determine the disconnection. There is a method. For example, in Patent Document 1, the synchronous rectification operation is stopped for a certain period of time at regular intervals, and during the stop period, a pulse drive is performed with a fixed duty by a drive switching element, and the output terminal of the drive switching element is used. It is described that the disconnection is determined depending on whether or not a pulsed voltage signal is output.

<p><patcit num="1"><text>Japanese Unexamined Patent Publication No. 2012-143048</text></patcit></p>

<p> In the conventional method, a diagnosis time of at least one cycle is required for pulse detection, and there is a problem that the diagnosis time becomes long.</p>

<p> The load drive device according to the present invention includes a synchronous rectifier circuit having a drive-side switching element and a return-side switching element, a driver control circuit that controls the synchronous rectifier circuit, and a voltage monitor that monitors the voltage of the output terminal of the synchronous rectifier circuit. The driver control circuit comprises a circuit, and when a diagnostic command is received, the driver control circuit controls the return side switching element to be turned off when the drive side switching element is switched from ON to OFF, and the voltage monitor circuit is provided. Detects a normal state when the voltage to be monitored is within a predetermined voltage range during a period in which both the drive-side switching element and the return-side switching element are turned off.</p>

<p> According to the present invention, it is possible to detect in a short time whether the output side of the synchronous rectifier circuit is in a disconnected state or a normal state while driving with a low load current.</p>

<figref num="1">It is a circuit block diagram in 1st Example.</figref><figref num="2">It is a timing chart which shows an example of the operation when the duty ratio is high in 1st Example.</figref><figref num="3">It is a timing chart which shows an example of the operation when the duty ratio is low in 1st Example.</figref><figref num="4">It is a circuit block diagram in the 2nd Example.</figref><figref num="5">It is a timing chart which shows an example of the operation of the 2nd Example.</figref><figref num="6">It is a circuit block diagram in the 3rd Example.</figref><figref num="7">It is a timing chart which shows an example of the operation of the 3rd Example.</figref><figref num="8">It is a circuit block diagram in 4th Example.</figref><figref num="9">It is a circuit block diagram of the diagnostic current generation circuit.</figref><figref num="10">It is a timing chart which shows an example of the operation of 4th Example.</figref><figref num="11">It is a circuit block diagram in the 5th Example.</figref>

[First Example]

FIG. 1 shows a circuit configuration diagram in this embodiment. The load drive device 101 includes a synchronous rectifier circuit 104 composed of a high-side FET 102 and a low-side FET 103. An inductive load 106 and a terminal capacitor 107 are connected to the synchronous rectifier circuit output terminal 105 of the synchronous rectifier circuit 104. In this embodiment, since the synchronous rectifier circuit 104 has a low-side configuration in which the load 106 is connected to the power supply voltage VB, the high-side FET 102 serves as a return-side switching element and the low-side FET 103 serves as a drive-side switching element. Operate. The terminal capacitor 107 has a role of protecting the synchronous rectifier circuit 104 from an external surge.

The driver control circuit 108 controls the gate voltage of each of the high-side FET 102 and the low-side FET 103 to turn ON / OFF according to the input PWM command. The current detection circuit 109 detects the current flowing through the synchronous rectifier circuit 104, and inputs the detection result to the diagnosis necessity determination circuit 110.

The diagnosis necessity determination circuit 110 compares the duty ratio of the PWM control input from the microcomputer or the like with the current detection result of the current detection circuit 109 with the threshold value, and ORs the disconnection diagnosis result A based on the comparison result. A diagnostic command is output to the driver control circuit 108 at the gate 112. When the driver control circuit 108 detects that the diagnostic command becomes H (High), it shifts to the diagnostic mode described later, and outputs the diagnostic signal as H to the voltage monitor circuit 111 during the diagnostic mode.

The voltage monitor circuit 111 monitors the voltage of the synchronous rectifier circuit output terminal 105 in the diagnostic mode, and ORs the disconnection diagnosis result B based on the comparison result with the voltage threshold set between the power supply voltage VB and the GND voltage. Output to the gate 112 and the driver control circuit 108. When either the disconnection diagnosis result A or the disconnection diagnosis result B is an H signal indicating a disconnection, the OR gate 112 sets the state of the synchronous rectifier circuit output terminal 105 as the disconnection state and outputs the disconnection diagnosis result C to H. do.

FIG. 2 is a timing chart showing an example of operation when the duty ratio is high in this embodiment. Specifically, it is an example when the duty ratio is higher than the duty ratio threshold value. In normal control, the driver control circuit 108 alternately turns on / off the high-side FET 102 and the low-side FET 103 in accordance with the PWM command to perform PWM control. When the PWM command shown in FIG. 2 (a) is H, the high-side FET 102 is controlled as OFF as shown in FIG. 2 (b), and the low-side FET 103 is controlled as ON as shown in FIG. 2 (c). When the PWM command shown in FIG. 2A is L, the high-side FET 102 is controlled as ON and the low-side FET 103 is controlled as OFF.

As shown in FIG. 2 (f), when the duty ratio is higher than the duty ratio threshold, the state of the synchronous rectifier circuit output terminal 105 is based only on the current detection result of the current detection circuit 109 shown in FIG. 2 (d). It is determined by the diagnosis necessity determination circuit 110. Therefore, the diagnosis necessity determination circuit 110 always outputs the diagnosis command shown in FIG. 2 (g) as L output.

When the current detection result of the current detection circuit 109 is larger than the current threshold value, the diagnosis necessity determination circuit 110 outputs the disconnection diagnosis result A as a normal state as shown in FIG. 2 (i). On the other hand, at time t1, when the current detection result of the current detection circuit 109 is smaller than the current threshold value, the disconnection diagnosis result A shown in FIG. 2 (i) is output as H as the disconnection state. As a result, as shown in FIG. 2 (k), the OR gate 112 outputs the disconnection diagnosis result C as a disconnection state.

At this time, when the Duty ratio threshold is set when the synchronous rectifier circuit output terminal 105 is in a normal connection state and is driven with a Duty ratio equal to or higher than the Duty ratio threshold, the current flowing through the synchronous rectifier circuit 104 is always higher than the current threshold. By setting the value so that it becomes high, the state of the load 106 can be reliably determined only by the current threshold value.

In this way, when the Duty ratio is high, the disconnection diagnosis is performed using only the current detection result of the current detection circuit 109, so that the control of the synchronous rectifier circuit 104 does not shift to the disconnection diagnosis mode, and the disconnection is performed only by the normal control. Can be diagnosed.

FIG. 3 is a timing chart showing an example of operation when the duty ratio is low in this embodiment. Specifically, it is an example when the duty ratio is equal to or less than the duty ratio threshold value. The diagnosis necessity determination circuit 110 is shown when the current detection result of the current detection circuit 109 shown in FIG. 3 (d) is equal to or less than the current threshold value and the Duty ratio shown in FIG. 3 (f) is equal to or less than the Duty ratio threshold value. As shown in 3 (g), set the diagnostic command to H. In other cases, the diagnosis necessity determination circuit 110 outputs the diagnosis command as L to the driver control circuit 108 as shown in FIG. 3 (g).

The driver control circuit 108 receives the diagnostic command H shown in FIG. 3 (g), shifts to the diagnostic mode at the Fall edge (falling edge) t2 of the PWM command shown in FIG. 3 (a), and shifts to the diagnostic mode, and the high side FET 102 and the low side. Both FET 103 are turned off. Then, the driver control circuit 108 outputs the under-diagnosis signal H shown in FIG. 3H to the voltage monitor circuit 111.

When the connection state of the synchronous rectifier circuit output terminal 105 is normal, the voltage of the synchronous rectifier circuit output terminal 105 in the diagnostic mode rises to the power supply voltage VB to which the load 106 is connected. The voltage monitor circuit 111 compares the voltage of the synchronous rectifier circuit output terminal 105 in the diagnostic mode with the voltage threshold value. Then, as shown in FIG. 3 (e), the voltage of the synchronous rectifier circuit output terminal 105 continues to be higher than the voltage threshold during the predetermined filter time T, that is, the voltage monitor circuit 111 is the synchronous rectifier circuit output terminal. When it is detected that the voltage of 105 is within a predetermined voltage range, the state of the synchronous rectifier circuit output terminal 105 is determined to be normal, and the disconnection diagnosis result B is output to L as shown in FIG. 3 (i). The driver control circuit 108 terminates the diagnostic mode and returns to normal control at the same time that the voltage monitor circuit 111 determines that the state of the synchronous rectifier circuit output terminal 105 is normal. That is, when the voltage monitor circuit 111 detects a normal state, the voltage monitor circuit 111 transmits a signal indicating the normal state (L of the disconnection diagnosis result B) to the driver control circuit 108, and the driver control circuit 108 sends a signal indicating the normal operation (L). When L) of the disconnection diagnosis result B is received, the ON / OFF operation of the high-side FET 102 which is the return-side switching element and the low-side FET 103 which is the drive-side switching element is restarted.

The low-side FET 103, which is the drive-side switching element, shifts to the diagnostic mode at the timing when it switches from ON to OFF. During the diagnostic mode, both the high-side FET 102 and the low-side FET 103 are in the OFF state. Reflux current flows through the body diode of the high side FET 102. At this time, the resistance is larger than that of the normal drive in which the high-side FET 102 is turned on during reflux. Therefore, the longer the reflux current flows through this diode, the more heat is generated, which affects the behavior of the load 106. There is a risk of doing so. To solve this problem, by setting the filter time T to be sufficiently shorter than the L period of PWM, the normal state is detected in a short time within one cycle as shown in Fig. 3, and normal control is started. Since the high-side FET 102 can be turned on by returning it, it is possible to reduce the increase in heat generation and the influence on the behavior of the load 106.

The filter time T is set to prevent misdiagnosis due to noise or the like and to perform the diagnosis more reliably, but the voltage monitor circuit 111 is normal when the output voltage exceeds the voltage threshold without providing the filter time T. It is also possible to detect the state. In this case, the detection of the normal state is completed in a very short period of time, and the increase in heat generation and the influence on the behavior of the load 106 can be minimized.

A case where the synchronous rectifier circuit output terminal 105 is disconnected at time t3 will be described. The driver control circuit 108 receives the diagnostic command H shown in FIG. 3 (g), shifts to the diagnostic mode at the Fall edge t4 of the PWM command shown in FIG. 3 (a), and turns off both the high-side FET 102 and the low-side FET 103. do. At this time t4, the voltage of the synchronous rectifier circuit output terminal 105 in the diagnostic mode is GND as shown in FIG. 3 (e) in order to maintain the voltage value when the low-side FET 103 immediately before shifting to the diagnostic mode was ON. It becomes an electric potential. The voltage monitor circuit 111 detects that the voltage of the synchronous rectifier circuit output terminal 105 continues to be below the voltage threshold for the filter time during the diagnostic mode, and determines that the state of the synchronous rectifier circuit output terminal 105 is disconnected. Then, the disconnection diagnosis result B shown in FIG. 3 (j) is output as H. As a result, as shown in FIG. 3 (k), the OR gate 112 outputs the disconnection diagnosis result C as a disconnection state.

As described above, in this embodiment, the diagnosis mode is entered at the timing when the low-side FET 103, which is the drive-side switching element, switches from ON to OFF. This is because if the high-side FET 102 is switched from ON to OFF and an attempt is made to turn off the FETs on both sides for diagnosis, the voltage of the synchronous rectifier circuit output terminal 105 is in the normal state and the wire is disconnected. This is because both times are close to the power supply voltage VB, and it becomes difficult to distinguish between normal and disconnection. In order to avoid such a situation, in this embodiment, the diagnosis is performed when the low-side FET 103 is switched from ON to OFF so that the power supply voltage is VB in the normal state and GND in the case of disconnection. Diagnosis is performed at the timing when the voltage can be reached, making it easy to distinguish between disconnection and normal.

According to this embodiment, the disconnection diagnosis in the diagnostic mode performed with both the high-side FET 102 and the low-side FET 103 turned off is based on the voltage value of the synchronous rectifier circuit output terminal 105 and the disconnection state of the synchronous rectifier circuit output terminal 105. Since the normal state is determined, it does not depend on the magnitude of the current flowing through the synchronous rectifier circuit 104. Therefore, the current flowing through the synchronous rectifier circuit 104 does not meet the accuracy of the current detection circuit 109, and there is a risk of erroneous detection of disconnection based on the current detection result alone. However, it is possible to reliably detect disconnection.

In FIG. 3, the diagnostic mode is continued even after the voltage monitor circuit 111 detects the disconnection, but immediately after the disconnection is detected, the Rise edge (rising edge) t5 of the PWM command, etc., is arbitrary after the disconnection is detected. You may return to the normal control at the timing of.

In addition, when a normal state or disconnection is detected and the diagnostic mode returns to normal control, the diagnostic command is set to L, and then the current detection result is below the current threshold and the duty ratio is below the duty ratio threshold for a certain period. If it continues, the diagnostic command may be set to H again to shift to the diagnostic mode. By doing so, the frequency of diagnosis can be adjusted as necessary, and the increase in heat generation and the influence on the behavior of the load 106 can be further reduced.

In FIG. 1, the input of the diagnosis necessity determination circuit 110 is the duty ratio and the current detection result, but this is because the diagnosis mode is entered only under the condition that the current flowing through the synchronous rectifier circuit 104 becomes small. As another example, the diagnosis necessity determination circuit 110 inputs the power supply voltage VB of the synchronous rectifier circuit 104 as an input instead of the Duty ratio as a reference for determining the magnitude of the current to flow in the synchronous rectifier circuit 104. When the power supply voltage VB of the synchronous rectifier circuit 104 is equal to or less than a predetermined voltage threshold value, a diagnostic command may be output. Further, as another example, the diagnosis necessity determination circuit 110 uses the power supply voltage VB of the synchronous rectifier circuit 104 as an input instead of the Duty ratio as a reference for determining the magnitude of the current to flow in the synchronous rectifier circuit 104. , When the current detection result from the current detection circuit 109 is not more than a predetermined current and the input power supply voltage VB of the synchronous rectifier circuit 104 is not more than a predetermined voltage, a diagnostic command may be output. As yet another example, the diagnosis necessity determination circuit 110 uses the Duty ratio and the power supply voltage VB of the synchronous rectifier circuit 104 as a reference for determining the magnitude of the current to flow in the synchronous rectifier circuit 104, instead of the Duty ratio. The product of the Duty ratio of the input synchronous rectifier circuit 104 and the power supply voltage VB of the input synchronous rectifier circuit 104 when the current detection result from the current detection circuit 109 is less than or equal to the predetermined current. May output a diagnostic command when is less than or equal to a predetermined product. Further, the diagnosis necessity determination circuit 110 may set the determination criterion of the diagnosis command output to only one of the current detection result, the duty ratio, the power supply voltage VB, or the product of the duty ratio and the power supply voltage VB. Alternatively, the diagnostic command can be output as H at regular intervals of the PWM command or at regular time intervals without providing a criterion for the current value flowing through the synchronous rectifier circuit 104 as described above.

[Second Example]

FIG. 4 is a circuit configuration diagram in the second embodiment. In this embodiment, the configuration of the load 401 is different from that of the first embodiment. The same reference numerals are given to the configurations common to those of the first embodiment, and the description thereof will be omitted. In the present embodiment, the synchronous rectifier circuit 104 has a high-side configuration in which the load 401 is connected to GND, the high-side FET 102 operates as a drive-side switching element, and the low-side FET 103 operates as a return-side switching element. Other configurations are the same as those of the first embodiment, and the following description mainly describes the parts different from the first embodiment.

FIG. 5 is a timing chart showing an example of the operation of this embodiment. Since the synchronous rectifier circuit 104 has a high-side configuration, in normal control, when the PWM command shown in FIG. 5 (a) is H, the high-side FET 102 shown in FIG. 5 (b) is ON, and FIG. 5 (c). The low-side FET 103 shown in is turned off. When the PWM command shown in FIG. 5 (a) is L, the high-side FET 102 shown in FIG. 5 (b) is turned off and the low-side FET 103 shown in FIG. 5 (c) is turned on. In this embodiment, the conditions regarding the voltage of the synchronous rectifier circuit output terminal 105 for the voltage monitor circuit 111 to determine the normal state and the disconnection state of the synchronous rectifier circuit output terminal 105 are different from those of the first embodiment.

When the state of the synchronous rectifier circuit output terminal 105 is normal, the voltage of the synchronous rectifier circuit output terminal 105 in the diagnostic mode at the filter time T starting from time t2 is connected to the load 401 as shown in FIG. 5 (e). It drops to the previous GND potential. Therefore, the voltage monitor circuit 111 determines that the state of the synchronous rectifier circuit output terminal 105 is normal when the voltage of the synchronous rectifier circuit output terminal 105 in the diagnostic mode is lower than the voltage threshold value, and the disconnection shown in FIG. 5 (j). Output the diagnosis result B to L.

A case where the synchronous rectifier circuit output terminal 105 is disconnected at time t3 will be described. The driver control circuit 108 receives the diagnostic command H shown in FIG. 5 (g), shifts to the diagnostic mode at the Fall edge t4 of the PWM command shown in FIG. 5 (a), and turns off both the high-side FET 102 and the low-side FET 103. do. As shown in FIG. 5 (e), the voltage of the synchronous rectifier circuit output terminal 105 in the diagnostic mode at this time t4 maintains the voltage value when the high-side FET 102 immediately before shifting to the diagnostic mode was ON. The power supply voltage is VB. Therefore, the voltage monitor circuit 111 determines that the state of the synchronous rectifier circuit output terminal 105 is a disconnection when the voltage of the synchronous rectifier circuit output terminal 105 is equal to or higher than the voltage threshold value, and the disconnection diagnosis result shown in FIG. 5 (j). Output B as H. As a result, as shown in FIG. 5 (k), the OR gate 112 outputs the disconnection diagnosis result C as a disconnection state.

As described in the first and second embodiments, it is judged to be normal when the voltage of the synchronous rectifier circuit output terminal 105 in the diagnostic mode is higher than the threshold voltage in the low-side configuration and lower than the threshold voltage in the high-side configuration. By changing the judgment condition of the monitor circuit 21, whether the synchronous rectifier circuit 104 has a low-side configuration or a high-side configuration, the normality or disconnection state of the synchronous rectifier circuit output terminal 105 is determined. Can be done.

[Third Example]

FIG. 6 is a circuit configuration diagram in the third embodiment. In this embodiment, the configurations of the input / output signals of the PWM command generation circuit 601 and the current detection circuit 109 are different from those of the first embodiment. The same reference numerals are given to the configurations common to those of the first embodiment, and the description thereof will be omitted. The current detection circuit 109 inputs the under-diagnosis signal from the driver control circuit 108, and outputs the current detection result and the collect signal indicating that the current detection result is inaccurate to the PWM command generation circuit 601.

The PWM command generation circuit 601 inputs the current detection result, the collect signal, and the current command value, and generates a PWM command so that the current flowing through the synchronous rectifier circuit 104 becomes equal to the current command value. That is, when the collect signal is L (indicating that the current detection result is accurate), the PWM command generation circuit 601 is equal to the current command value based on the current detection result from the current detection circuit 109. Generate a PWM command so that Further, when the in-collect signal is H (indicating that the current detection result is inaccurate), the PWM command generation circuit 601 ignores the current detection result from the current detection circuit 109 and the in-collect signal. Based on the current value before H, a PWM command is generated so that it becomes equal to the current command value.

FIG. 7 is a timing chart showing an example of the operation of this embodiment. The current detection circuit 109 calculates the average value of the currents (synchronous rectifier circuit currents) shown in FIG. 7 (d) flowing through the synchronous rectifier circuit 104 for one cycle from the Rise edge of the PWM command to the next Rise edge. The calculation result is output as the current detection result shown in Fig. 7 (e) in the next cycle. Therefore, the average current value of the first cycle in FIG. 7 is output in the period of the second cycle, the average current value of the second cycle is output in the period of the third cycle, and so on. The value is output with a delay of one cycle.

By the way, as a means for detecting the current of the synchronous rectifier circuit 104, there is a method of obtaining the current from the source-drain voltage of the high-side FET 102 and the low-side FET 103 and their respective ON resistances. In this case, the high-side FET 102 and the low-side FET 103 If any of the above is not ON, the current cannot be detected correctly. Therefore, the current detection result detected during the diagnostic mode period in which both the high-side FET 102 and the low-side FET 103 are turned off becomes inaccurate.

Therefore, the current detection circuit 109 detects the cycle including the diagnostic mode period from the signal under diagnosis shown in FIG. 7 (j), and while outputting the average current during that period, the collect signal shown in FIG. 7 (f) is H. By outputting as, the PWM command generation circuit 601 is notified that the current detection result is inaccurate. Specifically, as shown in FIG. 7 (d), the diagnostic mode period is included in the second cycle, the third cycle, the fifth cycle, and the sixth cycle, and the average current value of each is output. The collect signal shown in Fig. 7 (f) is H during the period of one cycle delay. Although the period during which the 6th period average is output is not shown in FIG. 7, the collect signal is H for this period as well.

The diagnosis necessity determination circuit 110 compares the current detection result shown in FIG. 7 (e) with the current threshold value, and outputs the diagnosis command shown in FIG. 7 (i) to the driver control circuit 108. In response to this, the driver control circuit 108 outputs the diagnostic signal shown in FIG. 7 (j) to the voltage monitor circuit 111 and the current detection circuit 109. At the filter time T starting at time t6, the collect signal shown in FIG. 7 (f) is L. On the other hand, since the detected value of the output voltage by the voltage monitor circuit 111 shown in FIG. 7 (g) is H, the disconnection diagnosis result B shown in FIG. 7 (l) is L.

Next, since the collect signal shown in FIG. 7 (f) is H at the filter time T starting from the time t7, the PWM command generation circuit 601 is notified that the current detection result is inaccurate. Since the detected value of the output voltage by the voltage monitor circuit 111 shown in FIG. 7 (g) is H, the disconnection diagnosis result B shown in FIG. 7 (l) is L.

At the filter time T starting at time t8, the collect signal shown in FIG. 7 (f) is L. On the other hand, since the detected value of the output voltage by the voltage monitor circuit 111 is L, the disconnection diagnosis result B shown in FIG. 7 (l) becomes H, and the disconnection is detected.

In this way, by notifying the PWM command generation circuit 601 that the current detection result is inaccurate due to the disconnection detection during the diagnostic mode, the PWM command is generated based on the erroneous current detection result. Can be avoided. In this embodiment, the current detection result is the average value of the previous PWM cycle, but when the current of the synchronous rectifier circuit 104 is output as the current detection result in real time as in the first and second embodiments, diagnosis is in progress. The signal can also be treated as an collect signal.

[Fourth Example]

FIG. 8 is a circuit configuration diagram in the fourth embodiment. In this embodiment, the configuration of the diagnostic current generation circuit 801 is different from that in the first embodiment. The same reference numerals are given to the configurations common to those of the first embodiment, and the description thereof will be omitted.

As shown in FIG. 8, a signal during diagnosis is input from the driver control circuit 108 to the diagnostic current generation circuit 801. The diagnostic current generation circuit 801 is connected to the synchronous rectifier circuit output terminal 105.

FIG. 9 is a circuit configuration diagram of the diagnostic current generation circuit 801. The diagnostic current generation circuit 801 is activated by being connected to the synchronous rectifier circuit output terminal 105 by the switch C901 when the diagnostic current signal is H, that is, during the diagnostic mode. The synchronous rectifier circuit output terminal 105 is connected to the current source A904 and the current source B905 via the switch A902 and the switch B903, respectively. The comparator 906 compares the voltage of the synchronous rectifier circuit output terminal 105 with the voltage Vbias, and outputs H when the synchronous rectifier circuit output terminal 105 is higher than Vbias and L when the synchronous rectifier circuit output terminal 105 is lower than Vbias. The output of the comparator 906 is directly input to the switch A902 via the inverter 907 and to the switch B903. When the output of the comparator 906 is H, that is, the synchronous rectifier circuit output terminal 105 is higher than Vbias, the switch A902 is turned off, the switch B903 is turned on, and the output of the comparator 906 is L, that is, the synchronous rectifier circuit output terminal 105. If is lower than Vbias, turn on switch A902 and turn off switch B903.

The current source A904 causes the diagnostic current to flow toward the synchronous rectifier circuit output terminal 105 while the switch A902 is ON and the switch C901 is ON, and charges the terminal capacitor 107. Further, the current source B905 draws a diagnostic current from the synchronous rectifier circuit output terminal 105 while the switch B903 is ON and the switch C901 is ON, and draws out the electric charge of the terminal capacitor 107. As a result, the diagnostic current generation circuit 801 charges and discharges the terminal capacitor 107 by the diagnostic current during the diagnostic mode, and controls the voltage of the synchronous rectifier circuit output terminal 105 to Vbias.

At this time, in order to prevent the diagnostic current from affecting the operation of the load 106, it is set sufficiently smaller than the drive current of the load 106. Further, by this, even when the load 106 is normally connected, the voltage drop in the load 106 due to the diagnostic current can be sufficiently reduced, and the voltage of the synchronous rectifier circuit output terminal 105 in the diagnostic mode can be reduced. When the state of the synchronous rectifier circuit output terminal 105 is normal, the voltage of the synchronous rectifier circuit output terminal 105 rises to near the power supply voltage VB without becoming Vbias. On the other hand, when the synchronous rectifier circuit output terminal 105 is disconnected, the voltage of the synchronous rectifier circuit output terminal 105 is raised to Vbias by the diagnostic current. At this time, Vbias needs to be set between the voltage threshold and the GND potential.

FIG. 10 is a timing chart showing an example of the operation of this embodiment. When the connection state of the synchronous rectifier circuit output terminal 105 is normal, at times t10 and t11, if the diagnostic current is set sufficiently small as described above, the voltage of the synchronous rectifier circuit output terminal 105 in the diagnostic mode will be. , As shown in FIG. 10 (f), it is higher than Vbias, and the diagnostic current shown in FIG. 10 (e) is drawn from the synchronous rectifier circuit output terminal 105, but the synchronous rectifier circuit output terminal shown in FIG. 10 (f). The voltage of 105 rises to near the power supply voltage VB.

If the synchronous rectifier circuit output terminal 105 is in a disconnected state, the voltage of the synchronous rectifier circuit output terminal 105 is GND as shown in FIG. 10 (f) at time t12 immediately after the transition to the diagnostic mode. The diagnostic current shown in FIG. 10 (e) flows into the synchronous rectifier circuit output terminal 105, and the voltage of the synchronous rectifier circuit output terminal 105 is raised to Vbias as shown in FIG. 10 (f). When the voltage of the synchronous rectifier circuit output terminal 105 reaches Vbias, the diagnostic current shown in FIG. 10 (e) is in equilibrium between the outflow side and the drawn side, so that the total is 0A.

The voltage monitor circuit 111 detects that the voltage of the synchronous rectifier circuit output terminal 105 continues to be below the voltage threshold for the filter time during the diagnostic mode, and determines that the state of the synchronous rectifier circuit output terminal 105 is disconnected. Then, the disconnection diagnosis result B shown in FIG. 10 (k) is output as H. As a result, as shown in FIG. 10 (l), the OR gate 112 outputs the disconnection diagnosis result C as a disconnection state.

As described above, by controlling the voltage of the synchronous rectifier circuit output terminal 105 in the diagnostic mode by the diagnostic current generation circuit 801, more stable diagnosis becomes possible. For example, if the voltage of the synchronous rectifier circuit output terminal 105 exceeds the voltage threshold due to disturbance noise despite the disconnection state, the diagnostic current can be drawn in and the voltage of the synchronous rectifier circuit output terminal 105 can be lowered within the filter time. , It is possible to prevent erroneous normal state judgment.

Further, if this embodiment is used, the case where the synchronous rectifier circuit output terminal 105 is about to be disconnected and the resistance between the synchronous rectifier circuit output terminal 105 and the power supply voltage VB of the load 106 becomes high is regarded as the disconnection state. It can be detected. Specifically, assuming that the resistance value between the power supply of the synchronous rectifier circuit output terminal 105 and the load 106 to be detected as a disconnection state is R and the diagnostic current drawn in is I, the voltage of the synchronous rectifier circuit output terminal 105 is VB-I. It becomes × R. By setting the value of the diagnostic current I so that this voltage value is lower than the voltage threshold value, the resistance value R can be detected as a disconnection state.

In the fourth embodiment shown in FIGS. 8 to 10, a low-side configuration in which the load 106 is connected to the power supply voltage VB has been described as an example, but a high-side configuration may be used. Similarly, even in the case of the high-side configuration, stable diagnosis and a state of disconnection can be detected. However, the value of Vbias at this time needs to be set between the voltage threshold value and the power supply voltage VB.

[Fifth Example]

FIG. 11 is a circuit configuration diagram in the fifth embodiment. In this embodiment, the configurations of the bulb timing control system (VTC: Valve Timing Control System) 1101 of the engine, the failure determination circuit 1102, and the warning light 1103 are different from those of the first embodiment. The same reference numerals are given to the configurations common to those of the first embodiment, and the description thereof will be omitted.

In this embodiment, the load drive device 101 controls the VTC1101 by driving a solenoid valve mounted on the automobile and incorporated in the VTC1101. The VTC1101 plays a role of continuously changing the valve timing of the engine according to the operating conditions such as the engine rotation speed, temperature, load, etc., which obtains the optimum valve timing in the entire operating range, so that the engine is discharged from the engine. It reduces air pollutants in the exhaust gas and improves fuel consumption and engine output / performance.

When the disconnection diagnosis result C is H, the failure determination circuit 1102 turns on the warning light 1103 to notify the user of the disconnection of VTC1101. At this time, the failure determination circuit 1102 determines that the VTC1101 is definitely disconnected and turns on the warning light 1103 only when H of the disconnection diagnosis result C is continuously detected for a certain period of time, for example. It is also possible to carry out a more reliable warning by such means.

By applying the disconnection diagnosis performed by turning off both the high-side FET 102 and the low-side FET 103 to the VTC1101 during the diagnostic mode as in this embodiment, in the entire operating range from high Duty ratio to low Duty ratio. Therefore, the disconnection diagnosis can be performed, and even in the operating region where the drive current of the VTC1101 becomes small, the disconnection diagnosis can be surely performed while suppressing the influence on the operation of the VTC1101. It is possible to reduce the increase in air pollutants in the exhaust gas and the deterioration of fuel consumption and engine output / performance.

According to the embodiment described above, the following effects can be obtained. (1) The load drive device 101 is a synchronous rectifier circuit 104 having a drive-side switching element (high-side FET 102 or low-side FET 103) and a return-side switching element (low-side FET 103 or high-side FET 102), and a driver that controls the synchronous rectifier circuit 104. A control circuit 108 and a voltage monitor circuit 111 that monitors the voltage of the output terminal of the synchronous rectifier circuit 104 are provided, and the driver control circuit 108 receives a diagnostic command and when the drive-side switching element switches from ON to OFF. The recirculation side switching element is also controlled to be OFF, and the voltage monitor circuit 111 is used when the voltage to be monitored is within a predetermined voltage range during the period when both the drive side switching element and the recirculation side switching element are OFF. , Detects normal condition. This makes it possible to detect in a short time whether the output side of the synchronous rectifier circuit is in a disconnected state or a normal state while driving with a low load current.

The present invention is not limited to the above examples, and other embodiments considered within the scope of the technical idea of the present invention are also included within the scope of the present invention as long as the features of the present invention are not impaired. .. Further, the configuration may be a combination of the above-mentioned examples.

101 Load drive 102 High-side FET 103 Low-side FET 104 Synchronous rectifier circuit 105 Synchronous rectifier circuit Output terminal 106 Load 107 Terminal Condenser 108 Driver control circuit 109 Current detection circuit 110 Diagnosis necessity judgment circuit 111 Voltage monitor circuit 112 OR Gate 401 Load 601 PWM command generation circuit 801 Diagnostic current generation circuit 901 Switch C 902 Switch A 903 Switch B 904 Current source A 905 Current source B 906 Comparator 907 Inverter 1101 VTC (Valve Timing Control System) 1102 Failure judgment circuit 1103 Warning light