JP2005088748A - Airbag ignition circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To decrease the capacity of a backup capacitor in an airbag ignition circuit. <P>SOLUTION: Both of the drain voltage of a mirror side transistor Tr1 of a sense MOS and the drain voltage of a source side transistor Tr2 are equalized. For this, both of the voltage drop VF of an adverse current protecting capacitor D1 and the voltage drop V1 of a current detecting resistor R1 are set equally, connecting the resistor R1 to the power source 3 through a PchMOS (Tr4). Thereby, the sense ratio n of the sense MOS can be made constant even in the case of the power voltage VSU drop, preventing a sudden increase of the ignition current. Thus, an airbag ignition guarantee voltage VL can be decreased, enabling the capacity of the backup capacitor CBU to be decreased. Alternatively, the reverse current avoiding capacitor D1 and a discharge diode D2 can be replaced with a lower saturation adverse current protecting circuit to decrease the capacity of the capacitor CBU while by increasing the power voltage. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an airbag ignition circuit in an airbag device that protects an occupant when a collision accident occurs in an automobile or the like.

The airbag ignition circuit protects an occupant by causing an electric current to flow through the squib when a collision accident occurs to ignite and explode and deploy the airbag.
In the event of a collision, the battery is expected to be disconnected from the power supply circuit. Therefore, a backup capacitor is connected to the power supply so that the squib can be reliably fed even when the battery is disconnected.

  When current flows through the squib, discharge from the backup capacitor proceeds and the power supply voltage decreases. Therefore, a capacitor for maintaining a predetermined voltage value is required as a backup capacitor until all the squibs operate. On the other hand, various proposals have heretofore been made in order to minimize the capacitance of the capacitor. (For example, see Patent Document 1 and Patent Document 2.)

A general airbag ignition circuit will be described with reference to FIG. In the following description, the circuit of FIG. 1 is referred to as a “basic circuit”.
A power supply voltage VSU is supplied from the battery 1 to the ignition power supply 3 through the ignition switch IG and the step-up converter 2. A backup capacitor CBU is connected to the power source 3 through the charger 4 and the discharge diode D2. The capacitor CBU is always charged by the charger 4.

When the battery 1 is disconnected from the power source 3 due to an impact such as a collision, the capacitor CBU discharges the electric charge accumulated in the power source 3 through the discharge diode D2. Thus, the power supply voltage VSU is continuously supplied to the airbag ignition circuit.
A sense MOS (Tr1, Tr2), a squib 5, and a MOS transistor Tr3 are connected between the power supply 3 and the ground.

The sense MOSs (Tr1, Tr2) have a structure in which a current n times as large as the current flowing in the mirror side circuit flows in the source side circuit. A backflow prevention diode D1 is inserted between the source side transistor Tr2 of the sense MOS and the power supply 3. A current detection resistor R1 is inserted between the mirror-side transistor Tr1 of the sense MOS and the power supply 3.
In the circuit configuration shown in FIG. 5, a portion surrounded by a square is configured by one airbag control IC 6.

  Since there is a limit to the electrical energy that can be stored in the capacitor CBU, the ignition current that flows to the squib is controlled so that a constant current flows for a predetermined time so that the electrical energy of the capacitor CBU is not wasted. The duration of the ignition current is the same as the ON time of the ignition signal and is controlled by the ECU (not shown). The constant current control is performed by a constant current control unit configured by the current detection resistor R1 and the differential amplifier AMP2.

  A resistor R1 through which the mirror-side current of the sense MOS (Tr1, Tr2) flows and a resistor R2 through which a constant current flows are input to the inverting input terminal (−) and the non-inverting input terminal (+) of the differential amplifier AMP2. The differential amplifier AMP2 controls the sense MOS transistors (Tr1, Tr2) so that the voltage drop V1 generated by the mirror current coincides with the constant voltage drop generated in the resistor R2. As a result, the ignition current is subjected to constant current control.

The inverting input terminal of the differential amplifier AMP2 is grounded through the switch S3. The switch S3 is always off, whereby the output of the differential amplifier AMP2 is turned off, and the sense MOSs (Tr1, Tr2) continue to be in the off state.
The switch S3 is turned on by an ignition signal output from the ECU when a vehicle collision is detected. As a result, the differential amplifier AMP2 starts operating, and the sense MOSs (Tr1, Tr2) are turned on. The sense MOSs (Tr1, Tr2) are turned on, and an ignition current flows through the squib 5 to ignite and explode, and the air bag opens with the momentum.

A backflow prevention diode D1 is inserted between the power supply 3 and the source side transistor Tr2 of the sense MOS.
The backflow prevention diode D1 will be described with reference to FIG.
When the ignition IG is off and the downstream side of the squib 5 is short-circuited with the battery wiring, a current flows toward the backup capacitor CBU of the power supply 3 through the path shown in FIG. When this reverse current flows, the squib 5 operates and the airbag is opened. In order to prevent such a situation, a diode D1 is inserted to prevent a reverse current from flowing.

Japanese Patent Laid-Open No. 11-78771 JP 2001-341612 A

However, in the circuit of FIG. 1, when the power supply voltage VSU decreases, there arises a problem that the current flowing through the squib 5 increases despite constant current control as described below.
FIG. 3 shows the VDS-ID characteristics of the sense MOSs (Tr1, Tr2). In the figure, the horizontal axis represents the drain-source voltage VDS (hereinafter, abbreviated as “drain voltage VDS”), and the vertical axis represents the drain current ID. The drain voltage VDS increases and decreases in the same manner according to the increase and decrease of the power supply voltage VSU.

The drain current ID of the sense MOS shows a substantially constant value while the drain voltage VDS is high, and rapidly decreases when the drain voltage VDS becomes low.
The drain voltage VDS of the mirror side transistor Tr1 of the sense MOS of FIG. 1 is lower than the drain voltage VDS of the source side transistor Tr2 by the voltage drop V1 of the current detection resistor R1.

  As the power supply voltage VSU decreases, the drain voltage VDS also decreases. While the drain voltage VDS is high, the drain currents ID of the transistors Tr1 and Tr2 are substantially equal even if there is a voltage difference corresponding to the voltage drop V1. As the voltage VDS decreases, a current difference occurs between the transistors Tr1 and Tr2 due to the voltage difference V1. This means that the sense ratio n of the sense MOS 1 changes and increases.

FIG. 4 shows the relationship between the power supply voltage VSU and the ignition current in the circuit of FIG. The horizontal axis in FIG. 4 is the power supply voltage VSU, and the vertical axis is the ignition current.
In FIG. 4, a solid line shows an ideal operating current characteristic. The ideal state is when the drain voltages VDS of the transistors Tr1 and Tr2 are equal even if the power supply voltage VSU is lowered. In this case, the current difference of the drain current ID does not occur.

  Even if the drain voltage VDS is lowered, the mirror-side current is kept constant by constant current control until the drain voltage VDS is lowered to a certain voltage VL1. When the drain voltage VDS decreases below the voltage VL1, the mirror side current also decreases. In an ideal state, since the sense ratio of the sense MOSs (Tr1, Tr2) is constant, the ignition current also maintains a constant value until the drain voltage VDS decreases to the voltage VL1. The ignition of the airbag may be completed by the time it drops from the power supply voltage VSU to the voltage VL1 (airbag ignition guarantee voltage).

  The dotted line in FIG. 4 shows the actual operating current characteristics in the circuit of FIG. When the power supply voltage VSU decreases and the drain voltage VDS reaches the voltage VL2 that is higher than VL1, the sense ratio n of the sense MOSs (Tr1, Tr2) increases as described with reference to FIG. As a result, as shown in FIG. 4, even if the mirror-side current is controlled at a constant current, the source-side current (ignition current) rapidly increases below the voltage VL2. Therefore, the ignition of the airbag must be completed by VL2 higher than the voltage VL1.

  The ideal characteristics shown in FIG. 4 are compared with actual characteristics. When the drain voltage VDS decreases, in the ideal characteristic, the voltage VL1 becomes the airbag ignition guarantee voltage. In actual characteristics, the larger voltage VL2 becomes the airbag ignition guarantee voltage, and the squib ignition operation must be terminated before the power supply voltage VSU decreases to the voltage VL2.

For this purpose, in an actual circuit, it is necessary to flow a necessary ignition current before the voltage decreases to the voltage VL2 by increasing the capacity of the backup capacitor CBU. Increasing the capacitance of the capacitor CBU causes an increase in cost.
An object of the present invention is to reduce the capacity of a backup capacitor in an airbag ignition circuit.

  The present invention includes a power source to which a battery and a backup capacitor are connected, a squib, a sense MOS that is connected between the power source and the squib and is turned on in response to an ignition signal, and a source-side circuit of the sense MOS A backflow prevention diode connected between the power supply and a current detection resistor connected between the mirror-side circuit of the sense MOS and the power supply, and the current flowing through the squib by the sense MOS is a constant current An airbag ignition circuit including a constant current control unit to be controlled is an object.

In the first aspect of the present invention, even if the power supply voltage is lowered, the drain voltage of the mirror transistor of the sense MOS and the drain voltage of the source transistor are made equal. For this purpose, the voltage drop of the backflow prevention diode is set equal to the voltage drop of the current detection resistor, and the current detection resistor is connected to the power supply through the low saturation backflow prevention circuit.
With this configuration, even if the power supply voltage is lowered, the sense ratio of the sense MOS can be made constant, and the airbag ignition guarantee voltage VL can be lowered. As a result, the capacity of the backup capacitor can be reduced.

In the second aspect of the present invention, the discharge diode inserted between the backup capacitor and the power supply or the backflow prevention diode inserted between the power supply and the sense MOS is replaced with a low saturation backflow prevention circuit.
The discharge diode and the backflow prevention diode are between the backup capacitor and the sense MOS, and reduce the power supply voltage due to the forward voltage drop. On the other hand, the low saturation backflow prevention circuit has a function of preventing backflow and has a small on-resistance. Therefore, the voltage supplied to the sense MOS can be increased, and the capacitor capacity can be reduced accordingly.

  According to the present invention, the capacity of the backup capacitor in the airbag ignition circuit can be reduced, and thereby the cost of the airbag ignition circuit can be reduced.

  Embodiments of the present invention will be described below with reference to the drawings.

A first embodiment of the present invention will be described with reference to FIG. In the following description, only portions different from the basic circuit in FIG. 1 will be described, and redundant description will be omitted.
In the mirror side circuit of the sense MOS (Tr1, Tr2), a P-channel MOS (Tr4) as a low saturation backflow prevention circuit is inserted between the power supply 3 and the current detection resistor R1 (hereinafter referred to as “P-channel MOS”). Is abbreviated as “PchMOS”). The PchMOS (Tr4) is externally attached to the IC 6 in consideration of heat generation.

The PchMOS (Tr4) has a drain connected to the power supply side and a source connected to the load side. Further, a parasitic diode exists in the Pch MOS (Tr4).
The PchMOS (Tr4) is turned on / off by the operation of the switch S1. Although details of the on / off operation of the switch S1 will be described later, the switch S1 is turned on when the ignition signal is input to the gate, or is turned on before that.

  The PchMOS (Tr4) inserts a resistor between the source and the gate so that the gate and the source have the same potential when the switch S1 is in the OFF state. This prevents backflow without turning on even when a voltage is applied to the source. When the switch S1 is turned on, a voltage drop is generated in the source-gate resistance by a constant current, and the PchMOS (Tr4) is turned on.

When the PchMOS (Tr4) is OFF, the reverse flow due to the battery voltage short circuit described with reference to FIG. 2 is prevented by a parasitic diode. When turned on, the voltage drop is very small and energization is possible.
When the ignition signal is input, the sense MOS (Tr1, Tr2) is turned on and the ignition current flows, as in the basic circuit (FIG. 1).

  On the power supply side of the mirror side transistor Tr1 of the sense MOS, a voltage drop V2 when the PchMOS (Tr4) is turned on and a voltage drop V1 of the current detection resistor R1 are generated. On the power supply side of the transistor Tr2 on the source side, a forward voltage drop VF occurs in the backflow prevention diode D1. Therefore, the drain voltage VDS of the mirror side transistor Tr1 is VSU− (V1 + V2), and the drain voltage VDS of the source side transistor Tr2 is VSU−VF.

  Since the on-resistance of the PchMOS Tr4 is sufficiently small, V2≈0. Therefore, if V1 = VF, the drain voltages VDS of the transistors Tr1 and Tr2 can be made equal. That is, if the voltage drop V1 of the current detection resistor R1 is set to be equal to the voltage drop VF of the backflow prevention diode D1, the drain voltages VDS of the transistors Tr1 and Tr2 become equal.

As a result, in the VDS-ID characteristics of FIG. 3, even if the drain voltage VDS of the transistors Tr1 and Tr2 is equal, and the drain voltage VDS is lowered due to the drop of the power supply voltage VSU, the difference between the drain currents ID of the transistors Tr1 and Tr2 is Does not occur. As a result, the power supply voltage-ignition current characteristic also becomes the ideal operating current characteristic of FIG.
Therefore, according to this example, since the airbag ignition guarantee voltage VL can be lowered, it is not necessary to increase the capacity of the capacitor CBU unnecessarily.

A second embodiment of the present invention will be described with reference to FIG.
The airbag ignition circuit of FIG. 6 is different from the circuit of FIG. 5 in that PchMOS (Tr4) is built in the airbag control IC 6. Therefore, the description which overlaps with the description so far is abbreviate | omitted.

  In the circuit of the first embodiment (FIG. 5), the PchMOS (Tr4) is provided outside the IC 6 in consideration of the heat generation amount of the PchMOS (Tr4). For this reason, the PchMOS (Tr4) must be manufactured separately from the IC6. Further, a terminal for connecting the PchMOS (Tr4) and the resistor R1 must be provided in the IC6.

On the other hand, by increasing the mirror ratio n of the sense MOSs (Tr1, Tr2) (for example, n = 1000), the current of the mirror side transistor Tr1 can be reduced. Thereby, the calorific value of PchMOS (Tr4) can be reduced. Therefore, the PchMOS (Tr4) can be built in the IC6.
According to this example, the same effects as those of the first embodiment can be obtained, and the PchMOS (Tr4) can be manufactured at the time of manufacturing the airbag control IC 6, thereby reducing the cost. Also, the number of terminals of the IC 6 can be reduced.

A third embodiment of the present invention will be described with reference to FIG.
This example embodies the on / off control of the PchMOS (Tr4) described in the first and second embodiments. Therefore, the description which overlaps with the description so far is abbreviate | omitted.
The circuit in FIG. 7 turns on / off the switch S1 that controls on / off of the PchMOS (Tr4) in the circuit of the second embodiment (FIG. 6) by an ignition signal.

  In this example, the ignition signal is input from the ECU, and the PchMOS (Tr4) is turned on while the sense MOSs (Tr1, Tr2) are turned on. Further, the PchMOS (Tr4) is turned off during the unignited operation or after the ignition operation is completed. At this time, the parasitic diode is in the reverse direction with respect to the reverse current described in FIG. 2, and prevents the reverse current from flowing through the transistor Tr4. Therefore, also in this example, the operation as described in the first embodiment is executed.

In this example, the switch S1 can be omitted, and the transistor Tr4 can be directly turned on / off by the ECU.
In the circuit of FIG. 7, the PchMOS (Tr4) is built in the IC 6, but the PchMOS (Tr4) and the switch S1 can be externally attached as in the first embodiment (FIG. 5).

Embodiment 4 of the present invention will be described with reference to FIG.
The airbag ignition circuit of FIG. 8 provides a different method of on / off control of the PchMOS (Tr4) in the third embodiment (FIG. 7). Therefore, the description which overlaps with the description so far is abbreviate | omitted.

  A differential amplifier AMP3 is provided, a power supply 3 is connected to its inverting input terminal (−), a reference voltage source 7 for generating an arbitrary reference voltage is connected to its non-inverting input terminal (+), and a switch D1 is output at its output. Is turned on and off. Therefore, when the ignition signal is input when the power supply voltage VSU is equal to or higher than the reference voltage, the switch S1 is turned on. On the other hand, while the power supply voltage VSU is higher than the reference voltage, even if the ignition signal is input, the differential amplifier AMP3 does not output, the switch S1 is kept off, and the PchMOS (Tr4) remains off.

According to this example, even if the ignition signal is input from the ECU, the PchMOS (Tr4) remains off while the power supply voltage VSU is high. During this time, the mirror side current of the sense MOS (Tr1, Tr2) flows through the parasitic diode of the PchMOS (Tr4).
Therefore, in this example, the drain voltage VDS of the mirror side transistor Tr1 of the sense MOS is lower than the drain voltage VDS of the source side transistor Tr2 due to the voltage drop V2 of the parasitic diode. However, since the power supply voltage VSU is high, there is no problem that the ignition current increases as described with reference to FIGS.

When the power supply voltage VSU decreases due to the discharge of the capacitor CBU, the switch S1 is turned on, the PchMOS (Tr4) is turned on, and the operation described in the first embodiment is executed.
In the circuit of FIG. 8, the PchMOS (Tr4) is built in the IC 6, but the PchMOS (Tr4) and the switch S1 can be externally attached as in the first embodiment (FIG. 5).

Embodiment 5 of the present invention will be described with reference to FIG.
In the airbag ignition circuit of FIG. 9, the connection relationship of the differential amplifier AMP3 is only different from the circuit of the fourth embodiment (FIG. 8). Therefore, the description which overlaps with the description so far is abbreviate | omitted.
In the differential amplifier AMP3 of FIG. 9, the power supply 3 is connected to the inverting input terminal (−), and the reference voltage source 7 is connected to the non-inverting input terminal (+). Further, it operates regardless of the input of the ignition signal.

The differential amplifier AMP3 compares the power supply voltage VSU with the battery voltage, and turns on the switch S1 when the power supply voltage VSU becomes higher. When the power supply voltage VSU is lower, the switch S1 is turned off.
When the ignition switch IG is turned on and the power supply voltage VSU rises by the step-up converter 2 and becomes higher than the battery voltage, the PchMOS (Tr4) is turned on. Therefore, when an ignition signal is input from the ECU, the PchMOS (Tr4) is on and the operation described in the first embodiment is executed.

When the switch S1 is on, the power supply voltage VSU is higher than the battery voltage even if the battery wiring short circuit shown in FIG. 2 occurs. Therefore, the PchMOS (Tr4) is turned off. Even if it is not, no reverse current flows to the power source 3 side.
On the other hand, when the power supply voltage VSU is lower than the battery voltage, the PchMOS (Tr4) is turned off. Therefore, even if the short circuit shown in FIG. 2 occurs, the current flows from the squib 5 side to the power supply 3 through the PchMOS (Tr4). Will not flow backwards.

Further, when the battery 1 is disconnected during a car collision, the power supply voltage VSU becomes higher than the battery voltage, so that the PchMOS (Tr4) is turned on. Therefore, when the power supply voltage VSU decreases due to the discharge of the capacitor CBU, the operation described in the embodiment 1-4 is executed.
Although the circuit of FIG. 9 includes the PchMOS (Tr4) in the IC 6, the PchMOS (Tr4) and the switch S1 can be externally attached as in the first embodiment (FIG. 5).

A sixth embodiment of the present invention will be described with reference to FIG.
The airbag control IC usually includes a plurality of airbag ignition circuits. In this example, the PchMOS (Tr4) is shared by a plurality of airbag ignition circuits.
The circuit of FIG. 10 is a multiple of the airbag ignition circuit of the second embodiment (FIG. 6). Therefore, the description which overlaps with the description so far is abbreviate | omitted.

A plurality of airbag ignition circuits are formed in the IC 6. In this example, the backflow prevention diode D1 and the PchMOS (Tr4) are commonly used for a plurality of circuits. Each airbag ignition circuit receives a different ignition signal and operates separately.
In this example, the PchMOS (Tr4) is turned on regardless of whether the ignition signal is input to only one circuit or simultaneously input to a plurality of circuits. Therefore, the operation as described in the first embodiment is performed in each circuit.

According to this example, by sharing the backflow prevention diode D1 and the PchMOS (Tr4), the chip area of the airbag control IC can be reduced, and the cost of the IC can be reduced.
In addition, the example shown in FIG. 9 is a case where there are two airbag ignition circuits, but the same can be implemented when there are three or more airbag ignition circuits.
Further, in the circuit of FIG. 10, the PchMOS (Tr4) is built in the IC 6, but the PchMOS (Tr4) and the switch S1 can be externally attached as in the first embodiment (FIG. 5).

A seventh embodiment of the present invention will be described with reference to FIG.
The on / off control of the PchMOS (Tr4) of the multiple airbag ignition circuit (FIG. 10) of the sixth embodiment can be performed by any method of the third to fifth embodiments.
When the PchMOS (Tr4) is to be turned on by the ignition signal as in the third embodiment (FIG. 7), the operation may be performed with the OR of the ignition signal of each circuit as described below. .

The circuit of FIG. 11 is the circuit of the sixth embodiment (FIG. 10), and turns on the switch S1 by an ignition signal. Therefore, the description which overlaps with the description so far is abbreviate | omitted.
The ignition signal input to each airbag ignition circuit is input to the OR circuit OR simultaneously with turning on the switch S3 of each addition circuit. The switch S1 is on / off controlled by the output of the OR circuit OR. Accordingly, if even one airbag ignition circuit operates, the PchMOS (Tr4) is turned on.

When the method of the fourth embodiment (FIG. 8) is applied to the sixth embodiment, the ignition signal inputted to the differential amplifier AMP3 may be passed through the OR circuit OR as shown in FIG.
When the method of the fifth embodiment (FIG. 9) is applied to the sixth embodiment, since the ignition signal is not used for the on / off control of the PchMOS (Tr4), the circuit relating to the differential amplifier AMP3 of FIG. What is necessary is just to apply to a circuit.

  In the circuit of FIG. 11, the PchMOS (Tr4) and the switch S1 are built in the IC 6. However, as in the first embodiment (FIG. 5), the PchMOS (Tr4) and the switch S1 can be externally attached. .

Embodiment 8 of the present invention will be described with reference to FIG.
The circuit of FIG. 12 is obtained by applying a PchMOS (Tr5) which is a low saturation backflow prevention circuit in place of the backflow prevention diode D1 of the basic circuit (FIG. 1). Therefore, the description which overlaps with the description so far is abbreviate | omitted.
The PchMOS (Tr5) has a drain connected to the power supply side and a source connected to the PchMOS (Tr1, Tr2) side. On / off of the PchMOS (Tr5) is controlled by a switch S1 built in the IC6.

  The on / off control of the switch S1 can be performed by any of the methods of Example 3-5. First, it is turned on in response to the ignition signal as in the third embodiment. Second, as in the fourth embodiment, a differential amplifier AMP3 for comparing the power supply voltage VSU and the reference voltage is provided, and only when the power supply voltage VSU is lower than the reference voltage, the PchMOS (Tr5 ) Is turned on. Thirdly, as in the fifth embodiment, a differential amplifier AMP3 for comparing the battery voltage and the power supply voltage VSU is provided, and when the power supply voltage VSU is higher than the battery voltage, the PchMOS (Tr5) is turned on.

  If a short circuit with the battery circuit as shown in FIG. 2 occurs while the PchMOS (Tr5) is off, a reverse current will flow into the power supply 3 through the parasitic diodes of the squib 5 and sense MOS (Tr1, Tr2). To do. However, this reverse current is prevented because the PchMOS (Tr5) is off and the parasitic diode is in the reverse direction.

When an ignition signal is input from the ECU, the PchMOS (Tr5) is turned on at the same time or turned on before that.
In this example, since the connection relationship between the sense MOS (Tr1, Tr2) and the current detection resistor R1 is the same as that of the basic circuit (FIG. 1), the power supply voltage is the same as described with reference to FIGS. Due to the decrease in VSU, a difference occurs in the drain currents of the transistors Tr1 and Tr2, and when the voltage decreases to a certain voltage VL2, the ignition current increases. However, in this example, the adverse effects due to this decrease can be reduced as described below.

  In the basic circuit (FIG. 1), since there is a forward voltage drop VF due to the backflow prevention diode D1, the drain voltage VDS is lower than the power supply voltage VSU by the voltage drop VF. On the other hand, in this example, the on-resistance of the PchMOS (Tr5) is almost 0, so that the drain voltage VDS of the transistors Tr1 and Tr2 is equal to the power supply voltage VSU, and the drain voltage VDS is equal to the voltage drop VF compared to the conventional example. Becomes higher.

  As a result, the voltage difference between the drain voltage VDS at the start of discharging of the capacitor CBU and the airbag ignition guarantee voltage VL2 increases. Therefore, it is possible to obtain an ignition current necessary for operating the airbag without increasing the capacitor capacity.

A ninth embodiment of the present invention will be described with reference to FIG.
The circuit of FIG. 13 is an example in which a PchMOS (Tr5) is used for backflow prevention of the eighth embodiment (FIG. 12), and the mirror side of the sense MOS (Tr1, Tr2) of the first to seventh embodiments (FIG. 5 to FIG. 11). This is an example in which a PchMOS (Tr4) is inserted in the electric circuit. Therefore, the description which overlaps with the description so far is abbreviate | omitted.

  In FIG. 13, a PchMOS (Tr5) for preventing backflow is inserted between the source transistor Tr2 of the sense MOS and the power supply. A current detection resistor R1 and a PchMOS (Tr4) are connected to the mirror side transistor Tr1. PchMOS (Tr4) is connected not to the power supply 3 but to the backup capacitor CBU.

  In Example 1-7, the voltage drop VF of the backflow prevention diode D1 and the voltage drop V1 upstream on the mirror side are made equal, so that the drain voltages VDS of the transistors Tr1 and Tr2 of the sense MOS are made equal. If a PchMOS (Tr5) is used upstream of the source side as in this example, the voltage drop VF of the backflow prevention diode D1 becomes zero. For this reason, a voltage drop VF must be created.

  In this example, the forward voltage drop VF of the discharge diode D2 of the backup capacitor CBU is used as the voltage drop corresponding to the voltage drop V1 of the resistor R1. Therefore, the PchMOS (Tr4) is connected to the backup capacitor CBU. Thereby, the drain voltages VDS of the transistors Tr1 and Tr2 of the sense MOS can be made equal.

  In Example 1-7, the drain voltage VDS of the sense MOS (Tr1, Tr2) is lower than the power supply voltage VSU by the voltage drop VF of the backflow prevention diode D1. In this example, since the on-resistance of the PchMOS (Tr5) is 0, VDS = VSU, and the drain voltage VDS of the transistors Tr1 and Tr2 increases by the voltage drop VF of the diode D1.

According to this example, the capacitance of the backup capacitor CBU is reduced by equalizing the drain voltages VDS of the sense MOS transistors Tr1 and Tr2 as in the case of the first to seventh embodiments. Further, similarly to the eighth embodiment, the capacitance of the capacitor CBU can be further reduced by increasing the drain voltage VDS of the transistors Tr1 and Tr2 at the start of discharge.
Note that the switch S1 for turning on and off the PchMOS (Tr4) and the PchMOS (Tr5) can be implemented by any of the methods in the third to fifth embodiments.

Embodiment 10 of the present invention will be described with reference to FIG.
The circuit of FIG. 14 is obtained by applying a low saturation backflow prevention circuit in place of the discharge diode D2 of the backup capacitor CBU in the basic circuit (FIG. 1). Therefore, the description which overlaps with the description so far is abbreviate | omitted.

  A PchMOS (Tr6) of a low saturation backflow prevention circuit is inserted between the backup capacitor CBU and the power supply 3 instead of the discharge diode D2. At this time, the drain side of the PchMOS (Tr6) is connected to the capacitor CBU, and the parasitic diode is connected in the same direction as the discharge diode D2. Therefore, even if the PchMOS 3 (Tr6) is off, the supply of power from the capacitor CBU to the power source 3 is not interrupted.

A switch S2 for controlling on / off of the PchMOS (Tr6) is built in the IC6. The on / off control of the switch S2 can be performed by any method of the embodiment 3-5.
When the ignition signal is input or before that, the PchMOS (Tr6) is turned on, and the power supply voltage VSU is higher than the basic circuit (FIG. 1) because the voltage drop VF of the discharge diode D2 disappears. It has become.

  In this example, like the basic circuit (FIG. 1), the current detection resistor R1 is connected to the load side of the backflow prevention diode D1. Therefore, as with the basic circuit, when the power supply voltage VSU decreases, the ignition current increases rapidly below the voltage VL2 (FIG. 4). That is, the airbag ignition guarantee voltage VL2 remains high. However, when the PchMOS (Tr6) is on, the power supply voltage VSU is higher than the basic circuit by the voltage drop VF. This increases the voltage difference between the power supply voltage VSU and the airbag ignition guarantee voltage VL2. Therefore, it is possible to obtain an ignition current necessary for operating the airbag without increasing the capacitor capacity.

Embodiment 11 of the present invention will be described with reference to FIG.
The circuit of FIG. 15 is obtained by applying the PchMOS (Tr6) of Example 10 in place of the discharge diode D2 of the backup capacitor CBU in the circuit of Example 2 (FIG. 6). Therefore, the description which overlaps with the description so far is abbreviate | omitted.

In this example, the current detection resistor R1 is connected to the power source 3 through the PchMOS (Tr4) on the mirror side of the sense MOS (Tr1, Tr2), as in the second embodiment (FIG. 6). Therefore, by making the voltage drop VF of the backflow prevention diode D1 equal to the voltage drop V1 of the resistor R1, the airbag ignition guarantee voltage VL described with reference to FIGS. 3 and 4 can be lowered.
In this example, similarly to Example 10 (FIG. 14), the PchMOS (Tr6) is applied instead of the discharge diode D2, so that the power supply voltage VSU is increased.

According to this example, the capacity of the backup capacitor CBU can be further reduced by the decrease in the airbag ignition guarantee voltage VL due to the use of the PchMOS (Tr4) and the increase in the power supply voltage VSU due to the use of the PchMOS (Tr6). .
In this example, the PchMOS (Tr6) of Example 10 is applied to the circuit of Example 2. Similarly, the PchMOS (Tr6) of Example 10 is applied to the circuits of Example 1 and Example 8. Even if applied, the same effect can be obtained.

Embodiment 12 of the present invention will be described with reference to FIG.
As described above, ON / OFF of the switch S2 that controls the PchMOS (Tr6) can be controlled by any one of the methods of the embodiment 3-5.
This example shows still another example of the on / off control of the switch S2.
The circuit of FIG. 16 is obtained by adding a differential amplifier that performs on / off control of the switch S2 to the circuit of the eleventh embodiment (FIG. 15). Therefore, the description which overlaps with the description so far is abbreviate | omitted.

  Normally, the backup capacitor CBU is in a charged state while the battery voltage is being applied. For this reason, when the discharge PchMOS (Tr6) is turned on, a large current flows through the backup capacitor CBU. Therefore, the PchMOS (Tr6) must be turned off while the battery voltage is detected. When the battery voltage is not detected, the capacitor CBU is discharged through the PchMOS (Tr6). Therefore, in order to reduce the heat generation of the PchMOS (Tr6) itself, it is necessary to turn on the PchMOS (Tr6).

A differential amplifier AMP4 is provided for controlling on / off of the switch S2. In the differential amplifier AMP4, the battery voltage is input to the non-inverting input terminal, and the reference voltage source 8 is connected to the inverting input terminal.
The differential amplifier AMP4 monitors the battery voltage, turns on the PchMOS (Tr6) when the voltage is lower than the reference voltage, and reduces heat generation of the PchMOS (Tr6) due to the discharge current from the capacitor CBU. If the battery voltage is equal to or higher than the reference voltage, the PchMOS (Tr6) is turned off to prevent a large current from flowing from the power supply 3 to the capacitor CBU.

A thirteenth embodiment of the present invention will be described with reference to FIG.
In the circuit of FIG. 17, the drain voltage of the transistors Tr1 and Tr2 of the sense MOS is made equal by linearly controlling the PchMOS (Tr4) of the second embodiment (FIG. 6) with an amplifier. Therefore, the description which overlaps with the description so far is abbreviate | omitted.
The drain voltage VDS of the mirror MOS transistor Tr1 of the sense MOS is connected to the non-inverting input terminal of the differential amplifier AMP1, and the drain voltage VDS of the source transistor Tr2 is connected to the inverting input terminal of the differential amplifier AMP1.

  When the ignition signal is input, the differential amplifier AMP1 starts operation, and a voltage drop V2 generated in the PchMOS (Tr4), a voltage drop V1 generated in the current detection resistor R1, a voltage drop VF generated in the backflow prevention diode, During this period, the PchMOS (Tr4) is controlled so that the relationship of VF = V1 + V2 is established. As a result, the on-resistance of the PchMOS (Tr4) is controlled linearly.

As a result, the drain voltages of the sense MOS transistors Tr1 and Tr2 can be matched with high accuracy.
Further, according to this example, compensation is also performed when the voltage drop VF of the backflow prevention diode D1, which is an external diode, changes due to temperature fluctuation.
In the circuit of FIG. 17, the PchMOS (Tr4) and the switch S1 are built in the IC 6, but the PchMOS (Tr4) and the switch S1 can be externally attached as in the first embodiment (FIG. 5).

Embodiment 14 of the present invention will be described with reference to FIG.
The circuit of FIG. 18 is an airbag ignition circuit having a plurality of ignition circuits of the sixth embodiment (FIG. 10) so that the on-resistance of the PchMOS (Tr4) can be linearly controlled as in the thirteenth embodiment (FIG. 17). It is a thing. Therefore, the description which overlaps with the description so far is abbreviate | omitted.

  When the on-resistance of the PchMOS (Tr4) is controlled linearly as shown in FIG. 17, the drain voltage VDS of the mirror-side transistor Tr1 of the sense MOS must be connected to the differential amplifier AMP1. However, when the differential amplifier AMP1 is shared by a plurality of ignition circuits, the drain voltage VDS of the transistor Tr1 of different ignition circuits cannot be connected together. Further, it is not possible to connect only the transistor Tr1 of one ignition circuit and omit the other ignition circuit.

  On the other hand, it is conceivable to connect the sense MOSs (Tr1, Tr2) of the circuit that performs the ignition operation to the differential amplifier AMP1 using a changeover switch or the like. However, in this case, when a plurality of ignition circuits perform an ignition operation at the same time, both switches are closed, and the mirror side transistors Tr1 of the sense MOSs of the plurality of ignition circuits are simultaneously connected to the differential amplifier AMP1, Operation becomes unstable.

Therefore, in the circuit of FIG. 18, the ground-side potential of the resistor R2 that generates the reference of the ignition current is used as the drain voltage VDS of the mirror-side transistor Tr2 of the sense MOS.
The differential amplifier AMP2 that performs constant current control controls the sense MOS transistors (Tr1, Tr2) so that the voltage drop V1 of the resistor R1 is equal to the voltage drop V1 of the resistor R2. Therefore, by using the ground side potential of the resistor R2, a voltage equivalent to the drain voltage VDS of the mirror side transistor Tr1 of the sense MOS is input to the non-inverting input terminal of the differential amplifier AMP1.

The differential amplifier AMP1 starts operation by the output of the OR circuit OR. An ignition signal input from the ECU to each ignition circuit is input to the OR circuit OR. Therefore, when an ignition signal is input to one or a plurality of ignition circuits, the differential amplifier AMP1 starts an operation for linearly controlling the PchMOS (Tr4).
At this time, even if a plurality of ignition circuits operate simultaneously, each constant current control circuit can operate separately and does not affect each other.

Embodiment 15 of the present invention will be described with reference to FIG.
In the thirteenth embodiment (FIG. 17), the input section of the differential amplifier AMP1 that controls the PchMOS (Tr4) is changed. Therefore, the description which overlaps with the description so far is abbreviate | omitted.

  A diode D3 is provided in the IC6. A constant current is supplied to the diode D3 by a constant current source to generate a voltage drop equivalent to the voltage drop VF of the backflow prevention diode D1, and is input to the inverting input terminal of the differential amplifier AMP1. Since the power supply voltage VSU−voltage drop VF = the drain voltage VDS of the transistor Tr2 of the sense MOS, a voltage equivalent to the drain voltage VDS of the transistor Tr2 is input to the inverting input terminal of the operational amplifier AMP1.

In this example, the control accuracy of the drain voltage VDS of the sense MOS (Tr1, Tr2) is slightly lower than that in the thirteenth embodiment. However, as a method for correcting the temperature variation of the external backflow prevention diode D1, the present embodiment can be sufficiently implemented.
In addition, this example is applicable also to the circuit which shares PchMOS (Tr4) with the some ignition circuit shown in Example 14 (FIG. 18).

It is a figure which shows a general airbag ignition circuit. It is a figure which shows the backflow in the circuit of FIG. It is a figure which shows the characteristic of the sense MOS of FIG. It is a figure which shows the relationship between the power supply voltage VSU and the ignition current in the circuit of FIG. It is a figure which shows the 1st Example of the airbag ignition circuit of this invention. It is a figure which shows the 2nd Example of the airbag ignition circuit of this invention. It is a figure which shows the 3rd Example of the airbag ignition circuit of this invention. It is a figure which shows the 4th Example of the airbag ignition circuit of this invention. It is a figure which shows the 5th Example of the airbag ignition circuit of this invention. It is a figure which shows the 6th Example of the airbag ignition circuit of this invention. It is a figure which shows the 7th Example of the airbag ignition circuit of this invention. It is a figure which shows the 8th Example of the airbag ignition circuit of this invention. It is a figure which shows the 9th Example of the airbag ignition circuit of this invention. It is a figure which shows the 11th Example of the airbag ignition circuit of this invention. It is a figure which shows the 12th Example of the airbag ignition circuit of this invention. It is a figure which shows the 13th Example of the airbag ignition circuit of this invention. It is a figure which shows the 14th Example of the airbag ignition circuit of this invention. It is a figure which shows the 15th Example of the airbag ignition circuit of this invention. It is a figure which shows the 16th Example of the airbag ignition circuit of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Battery 2 ... Step-up converter 3 ... Ignition power supply 4 ... Charger 5 ... Squib 6 ... Airbag control IC
Reference voltage sources AMP1-AMP4 ... Amplifier CBU ... Back-up capacitor D1 ... Back-flow prevention diode D2 ... Discharge diode D3 ... IC built-in diode ID ... Drain current IG ... Ignition switch OR ... OR circuit R1 ... Current detection resistor R2 ... Ignition current generation reference resistance S1-S3 ... Switch Tr1, Tr2 ... Sense MOS
Tr3 ... MOS
Tr4-Tr6 ... PchMOS
V1 ... Voltage drop V2 of resistor R1 ... Voltage drop VDS of diode D2 ... Drain voltage VF of sense MOS ... Voltage drop VL of diode D1 ... Airbag ignition guarantee voltage VSU ... Power supply voltage

Claims (20)

A power supply with a battery and a backup capacitor connected;
With a squib,
A sense MOS connected between the power source and the squib and turned on in response to an ignition signal;
A backflow prevention diode connected between the source-side circuit of the sense MOS and the power source;
A constant current control unit that includes a current detection resistor connected between the mirror-side circuit of the sense MOS and the power supply, and that controls the current flowing through the squib by the sense MOS;
A low saturation backflow prevention circuit connected between the current detection resistor and the power source and controlled to be turned on and off;
An air bag ignition circuit comprising:   2. The low saturation backflow prevention circuit is built in the same IC as the sense MOS by increasing the mirror ratio of the sense MOS and reducing the heat generation amount of the low saturation backflow prevention circuit. The described airbag ignition circuit.   The airbag ignition circuit according to claim 1 or 2, wherein the low saturation backflow prevention circuit is turned on in response to the ignition signal.   A comparator that compares the power supply voltage with a predetermined reference voltage and turns on the low saturation backflow prevention circuit in response to the ignition signal only when the power supply voltage falls below the reference voltage; 4. The airbag ignition circuit according to claim 3, wherein   The comparison between the battery voltage and the power supply voltage is provided, and when the power supply voltage is higher than the battery voltage, a comparison unit that turns on the low saturation backflow prevention circuit is provided. Airbag ignition circuit.   An airbag ignition circuit comprising a plurality of airbag ignition circuits according to claim 1 or 2, wherein only one low-saturation backflow prevention circuit is provided and also used as the plurality of airbag ignition circuits.   The airbag ignition circuit according to claim 6, wherein the low saturation backflow prevention circuit is turned on in response to the ignition signal.   The power supply voltage is compared with a reference voltage, and a comparison unit is provided that turns on the low saturation backflow prevention circuit in response to the ignition signal only when the power supply voltage drops below the reference voltage. The airbag ignition circuit according to claim 7.   The air according to claim 6, further comprising: a comparison unit that compares the battery voltage with the power supply voltage and turns on the low saturation backflow prevention circuit when the power supply voltage is higher than the battery voltage. Bag ignition circuit. A power supply with a battery and a backup capacitor connected;
With a squib,
A sense MOS connected between the power source and the squib and turned on in response to an ignition signal;
A low-saturation backflow prevention circuit connected between the source-side circuit of the sense MOS and the power supply and controlled on and off;
A constant current control unit that includes a current detection resistor connected between the mirror side circuit of the sense MOS and the squib side of the low saturation backflow prevention circuit, and controls the current flowing through the squib by the sense MOS; ,
An air bag ignition circuit comprising:   The airbag ignition circuit according to claim 10, wherein the low saturation backflow prevention circuit is turned on in response to the ignition signal.   Comparing the power supply voltage with a predetermined reference voltage, and providing a comparison unit that turns on the low saturation backflow prevention circuit in response to the ignition signal only when the power supply voltage drops below the reference voltage. The airbag ignition circuit according to claim 11.   11. The comparison unit according to claim 10, further comprising: a comparison unit that compares the battery voltage with the power supply voltage and turns on the low saturation backflow prevention circuit when the power supply voltage is higher than the battery voltage. Airbag ignition circuit. A power supply with a battery connected and a backup capacitor connected through a discharge diode;
With a squib,
A sense MOS connected between the power source and the squib and turned on in response to an ignition signal;
A first low saturation backflow prevention circuit which is connected between the source side circuit of the sense MOS and the power supply and is controlled to be turned on and off;
A constant current control unit that includes a current detection resistor connected to the power supply side of the mirror-side circuit of the sense MOS, and that controls the current flowing through the squib by the sense MOS;
A second low saturation backflow prevention circuit connected between the current detection resistor and the backup capacitor and controlled to be turned on and off;
An air bag ignition circuit comprising: A power supply with a battery and a backup capacitor connected;
With a squib,
A sense MOS connected between the power source and the squib and turned on in response to an ignition signal;
A backflow prevention diode connected between the source-side circuit of the sense MOS and the power source;
A constant current control unit that includes a current detection resistor connected between the mirror-side circuit of the sense MOS and the backflow prevention diode, and that controls the current flowing through the squib by the sense MOS;
A low saturation backflow prevention circuit connected between the backup capacitor and the power source and controlled to be turned on and off;
An air bag ignition circuit comprising:   The airbag ignition circuit according to claim 15, further comprising a second low saturation backflow prevention circuit connected between the current detection resistor and the power source and controlled to be turned on and off.   17. The airbag according to claim 15, further comprising a comparison unit that compares the battery voltage with a reference voltage and turns on the low saturation backflow prevention circuit when the battery voltage is lower than the reference voltage. Ignition circuit.   3. The control unit for linearly controlling the low saturation backflow prevention circuit so that a drain voltage of the mirror side transistor of the sense MOS and a drain voltage of the source side transistor are equal to each other. The described airbag ignition circuit.   19. The airbag ignition circuit according to claim 18, wherein a plurality of airbag ignition circuits are provided, and the low saturation backflow prevention circuit and the control unit are provided in common to the plurality of airbag ignition circuits.   A diode connected to the power supply and generating a voltage drop similar to the voltage drop of the backflow prevention diode is built in the same IC as the sense MOS, and the voltage drop is used as the drain voltage of the source side transistor. The airbag ignition circuit according to claim 18.

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