JP5332590B2 - CURRENT DETECTION CIRCUIT, SWITCHING POWER SUPPLY USING THE CURRENT DETECTION CIRCUIT, AND ELECTRONIC DEVICE - Google Patents

Description

  The present invention relates to a current detection circuit that detects an overcurrent in a DC / DC converter, that is, a switching power supply that converts a certain input voltage into a desired output voltage, and more particularly, a switching element (output transistor) that supplies energy to an inductor and a load. It is an object of the present invention to provide a current detection circuit capable of detecting an overcurrent in a state where a P-type MOS transistor functioning as a state does not reach a saturation region, a switching power supply using the current detection circuit, and an electronic device.

  The switching power supply is provided with a switching element between a terminal connected to a load and outputting a desired DC voltage and an input power supply voltage, or between a terminal connected to a load and outputting a desired DC voltage, and a ground. A desired DC voltage is maintained by controlling the switching element. Then, in order to prevent the overcurrent from flowing and destroying the switching element at the time of abnormality such as when the load is short-circuited, the overcurrent detection circuit detects that the overcurrent has flowed, and the detection signal ( The current flowing through the switching element is controlled by an overcurrent detection signal) to protect against destruction.

FIG. 5 is a diagram illustrating an example of a step-down DC / DC converter including an overcurrent detection circuit.
The DC / DC converter 10 shown in the figure controls an output transistor (switching element) M1 composed of a P-type MOS transistor, a rectifying switch M2 using an N-type MOS transistor, and ON / OFF timing of the output transistor M1. A control circuit 11 for detecting the overcurrent flowing through the output transistor M1 in order to protect the output transistor M1 from destruction caused by the overcurrent, an inductor L1, and an output capacitance Cout of the converter. Is done.

  Here, the control circuit 11 inputs a control signal to the gates of the output transistor M1 and the rectifying switch M2 so that the output voltage Vout of the DC / DC converter becomes constant. The output transistor M1 (P-type MOS transistor) whose source is connected to the power supply voltage repeats ON / OFF according to the control signal from the control circuit 11 input to the gate, and outputs an oscillation output. The inductor L1 and the output capacitance Cout constitute a smoothing circuit for smoothing the oscillation output of the output transistor (P-type MOS transistor) M1 and outputting it as a DC voltage Vout.

  The overcurrent detection circuit 20 includes an output current detection resistor 24 connected to a power supply voltage, a constant current source 22 having one end connected to the output current detection resistor 24 and the other end grounded, a power supply voltage, and an output. P-type MOS transistors M3 and M4 connected in series between a connection point (hereinafter referred to as “node B”) between the transistor (P-type MOS transistor) M1 and the inductor L1, and an inverting input terminal serving as an output current detection resistor 24 and the constant current source 22 are connected to a connection point (hereinafter referred to as “node A”), and a non-inverting input terminal is connected to a connection point between the source of the P-type MOS transistor M3 and the drain of the P-type MOS transistor M4. An overcurrent detection output comparator 21 whose terminal is connected to the input terminal of the control circuit 11 and a signal input to the gate of the output transistor (P-type MOS transistor) M1 are inverted and output. And an inverter 23 that.

  The same signal as that inputted to the gate of the output transistor (P-type MOS transistor) M1 is inputted to the gate of the P-type MOS transistor M3, and the output of the inverter 23 which inverts and outputs the signal is the output of the P-type MOS transistor M4. It is input to the gate.

  When the output transistor (P-type MOS transistor) M1 is on, the P-type MOS transistor M3 is on and the P-type MOS transistor M4 is off. Conversely, when the output transistor (P-type MOS transistor) M1 is off, the P-type MOS transistor M3 is also off and the P-type MOS transistor M4 is on.

With this configuration, when the output transistor (P-type MOS transistor) M1 is on, the P-type MOS transistor M3 is on and the P-type MOS transistor M4 is off. It is entered into the non-inverting input terminal of the output comparator 21. On the other hand, when the output transistor (P-type MOS transistor) M1 is off, the P-type MOS transistor M3 is off and the P-type MOS transistor M4 is on, so that the power supply voltage is an overcurrent detection output comparator. 21 is input to the non-inverting input terminal.
As a result, the voltage at the node A and the voltage at the node B are compared by the overcurrent detection output comparator 21 only when the output transistor (P-type MOS transistor) M1 is on.

  If the current value flowing through the constant current source 22 is Iref, the current Iref flows through the output current detection resistor 24, and a voltage corresponding to the current and the resistance value of the output current detection resistor 24 is used for output current detection. It occurs at both ends of the resistor 24. This voltage becomes the limit voltage Vlimref that determines the overcurrent detection level.

  When the output transistor M1 is turned on, a voltage corresponding to the output current Iout and the on-resistance of the output transistor M1 drops with respect to the power supply voltage and is generated at the node B. When the current flowing through the output transistor M1 becomes an overcurrent, the voltage generated at the node B is lower than the voltage at the node A. Therefore, the overcurrent detection output comparator 21 outputs a low level (overcurrent detection signal). An overcurrent detection signal is input to the control circuit 11. As a result, the control circuit 11 turns off the output transistor M1, assuming that an overcurrent has flowed through the output transistor M1. The current value at this time is defined as a limit current.

  The on-resistances of the output current detection resistor 24 and the output transistor M1 have different temperature / voltage dependencies. For this reason, the limit current varies in various ways. A structure for avoiding this problem is disclosed in Patent Document 1.

  FIG. 6 is a known circuit diagram disclosed in FIG. 3 of Patent Document 1, and a current sense circuit that introduces a structure that substantially cancels on-resistance variation by using a replica element 32 in addition to the switch 12. FIG. In the figure, if the on-resistance of the replica element 32 is 1 / K times that of the switch 12, a current value obtained by multiplying Iswitch by K will flow to Isense. In other words, a current value obtained by multiplying Isense by 1 / K flows to Isswitch.

FIG. 7 is a diagram showing a circuit of a step-down DC / DC converter (switching power supply) including an overcurrent detection circuit in which the structure of FIG. 6 is introduced.
The difference from FIG. 5 is that a monitoring transistor M5 composed of the same P-type MOS transistor as the output transistor M1 is provided instead of the output current detection resistor 24. The source of the monitor transistor M5 is the power supply voltage, the gate is the signal input to the gate of the output transistor M1, the drain is the constant current source 22 and the inverting input terminal of the overcurrent detection output comparator 21, Each is connected. Here, the size of the monitoring transistor M5 is set to 1 / N of the output transistor M1.

  In this configuration, when the output transistor M1 is turned on, a current Iref flows through the monitor transistor M5, and the limit voltage Vlimref generated at both ends of the monitor transistor M5 varies according to this current and the on-resistance of the monitor transistor M5. Therefore, the limit current is always N times the current value of Iref, and the on-resistance variation can be substantially canceled.

JP 2006-106989 A

  However, in the case of the step-down DC / DC converter (switching power supply) 10 having the overcurrent detection circuit of FIG. 7, when the gate-source voltage of the output transistor M1 decreases, the drain current of the output transistor M1 is illustrated in FIG. To decrease.

  Here, FIG. 8 is a diagram showing the relationship between the drain current (vertical axis) and the source-drain voltage (horizontal axis) in the output transistor M1, and is indicated by an arrow in the figure when the gate-source voltage decreases. As shown, the drain current of the output transistor M1 decreases.

Therefore, as shown in FIG. 8, a gate - if the source voltage is decreased, the output transistor M1 will reach the saturation region, the drain current is also increased output current Iout that a not reach the limit current .

To solve this problem, if the drain current falls below the limit current due to the drain current drop due to the gate-source voltage drop, the output transistor does not always reach the saturation region by lowering the limit current. It is only necessary to detect that a current exceeding the limit current has flowed .
In other words, considering that the on-resistance of the monitoring transistor M5 increases as the power supply voltage decreases with respect to the step-down DC / DC converter 10 including the overcurrent detection circuit of FIG. 7, the limit voltage has a certain value. It can be seen that it is sufficient to employ a circuit configuration for preventing the deterioration.

Therefore, the present invention provides a DC / DC converter, that is, a switching power supply technology for converting a certain input voltage into a desired output voltage, and a switching element (output) that supplies energy to an inductor or a load even when the power supply voltage drops. Provided is a current detection circuit capable of detecting that a current exceeding a limit current has passed without a P-type MOS transistor functioning as a transistor reaching a saturation region, a switching power supply using the current detection circuit, and an electronic device The purpose is to do.

The invention of claim 1, in order to achieve the above object, a current detection circuit for detecting a current flowing to the load via the MOS transistor which functions as an output transistor, connected between the power supply voltage and a ground voltage Further, it is defined by a monitoring transistor composed of MOS transistors, a first constant current source for generating a reference current, a reference current generated by the first constant current source, and a value of an on-resistance of the monitoring transistor. The first limit voltage to be detected is compared with the drain voltage when the MOS transistor functioning as the output transistor is turned on to detect and detect that a current exceeding a predetermined limit current flows. comprising a comparator for outputting a signal, and means for setting an upper limit or the lower limit value to said first limit voltage, the first Limi A means for setting an upper limit or a lower limit for the second voltage, generating a voltage independent of the power supply voltage from the power supply voltage, generating a second limit voltage based on the voltage independent of the power supply voltage, Is set to the upper limit or lower limit value of the first limit voltage, and the comparator outputs the detection signal in a state where the MOS transistor functioning as the output transistor is in the linear region without reaching the saturation region. It is characterized by output.

Further, in the invention of claim 2, means for setting the upper or lower limit value to the first limit voltage, the resistance provided between the power supply voltage and a ground voltage, a diode coupled MOS transistors, the second A series circuit composed of a constant current source, a gate connected to the gate of the diode-coupled MOS transistor, a drain connected to the first constant current source, and a source connected to the monitor transistor It is characterized by.

Further, the invention of claim 3, a current detecting circuit for detecting a current flowing to the load via the MOS transistor which functions as an output transistor, connected between a power supply voltage and a ground voltage, a MOS transistor Monitoring transistor and a first constant current source for generating a reference current, a first limit defined by a reference current generated by the first constant current source and a value of an on-resistance of the monitoring transistor Functions as a voltage, a second limit voltage generated from the power supply voltage and not dependent on the power supply voltage, a higher or lower one of the first limit voltage and the second limit voltage, and the output transistor by comparing the drain voltage when the MOS transistor is turned on to, electrostatic exceeds a predetermined limit current Comprising a comparator having at least three inputs and outputs a detection signal by detecting that flows, the comparator the state in the linear region without MOS transistor which functions as the output transistor has reached the saturation region A detection signal is output . Further, in the invention of claim 4, the first limit voltage is obtained from a connection point between the monitoring transistor and the first constant current source, and the second limit voltage is the power supply voltage and the ground voltage. It is characterized in that it is obtained from the connection point of the resistor connected between the second constant current source and the second constant current source.

A switching power supply according to the present invention is a switching power supply that uses the current detection circuit to detect a current flowing through a load via a MOS transistor that functions as an output transistor. The electronic device according to the present invention includes the switching power supply described above. Is an electronic device using

According to the present invention, it is possible to provide a current detection circuit capable of detecting a limit current in a state where an output transistor is in a linear region even when a power supply voltage is lowered, a switching power supply using the current detection circuit, and an electronic device.

(Example)
Hereinafter, embodiments of a step-down DC / DC converter (switching power supply) including a current detection circuit according to the present invention will be described in detail with reference to the drawings.

(First embodiment)
FIG. 1 is a block diagram of a step-down DC / DC converter (switching power supply) 10 including a current detection circuit according to a first embodiment of the present invention.
The step-down DC / DC converter (switching power supply) 10 according to the first embodiment of the present invention shown in FIG. 1 is different from the step-down DC / DC converter (switching power supply) 10 provided with the detection circuit of FIG. A circuit configuration for preventing the voltage between the power supply voltage and the node A from dropping below a certain voltage value is provided.

  That is, the step-down DC / DC converter (switching power supply) 10 shown in FIG. 1 is interposed between the monitoring transistor M5 and the constant current source 22 with respect to FIG. 7, and the source is an overcurrent detection output comparator. A P-type MOS transistor M6 connected to the inverting input terminal 21; a P-type MOS transistor M7 having a gate connected to the gate of the P-type MOS transistor M6 and forming a diode connection; and one end grounded and the other end A constant current source 25 for generating a current Iref2 equal to the current Iref generated by the constant current source 22 connected to the drain of the P-type MOS transistor M7, one end connected to the power supply voltage, and the other end for the P-type MOS transistor M7 The resistor 26 connected to the source of the power source is added, and the circuit between the power source voltage and the node A can be obtained by these added circuit configurations. The voltage does not drop below a constant voltage value.

  Here, it is assumed that the sizes of the P-type MOS transistor M6 and the P-type MOS transistor M7 are equal.

  When the P-type MOS transistor M7 operates in the saturation region, a potential of Vcc−R26 × Iref2−VthM7 is generated at the gate of the transistor M7. Here, Vcc is a power supply voltage, R26 is a resistance value of the resistor 26, Iref2 is a current value supplied by the constant current source 25, and VthM7 is a threshold voltage of the P-type MOS transistor M7.

  As the power supply voltage Vcc decreases, the on-resistance of the monitor transistor M5 increases, and the voltage between the power supply voltage Vcc and the node A increases (see (X) in FIG. 2A). It drops more rapidly than the voltage drops. Then, the gate-source voltage in the P-type MOS transistor M6 decreases, and the transistor M6 operates in the saturation region at a certain time.

  When the transistor M6 is in the saturation region, the voltage at the node A is the drain voltage of the P-type MOS transistor M6. At this time, the voltage at the node A is Vcc−R26 × Iref2−VthM7 + VthM6. Since (−R26 × Iref2−VthM7 + VthM6) is constant, the voltage at the node A thereafter decreases by the same value as the decrease in Vcc, that is, the power supply voltage Vcc decreases and the transistor M6 enters the saturation region. Thereafter, the voltage between the power supply voltage Vcc and the node A becomes constant (the voltage at the node A is constant in terms of the power supply voltage based on the power supply voltage). Here, VthM6 is a threshold voltage of the P-type MOS transistor M6.

  Therefore, before the transistor M6 enters the saturation region, the voltage at the node A decreases more rapidly than the decrease in the power supply voltage Vcc due to the increase in the on-resistance in the monitoring transistor M5, but the power supply voltage Vcc decreases and the transistor M6 decreases. After the voltage reaches the saturation region, the decrease in the voltage at the node A is caused by the decrease in the power supply voltage Vcc and decreases by the same value as the decrease in the power supply voltage Vcc (constant in terms of the power supply voltage based on the power supply voltage). (Refer to “Vlimref fluctuation in FIG. 1 circuit” in FIG. 2B).

  Considering the power supply voltage reference, the voltage at the contact A decreases as the power supply voltage decreases before the transistor M6 enters the saturation region, but decreases due to the increase in the on-resistance of the transistor M5. It can be said. As a result, the limit voltage can be prevented from decreasing below a certain value, and as a result, the limit current can be decreased.

(Second embodiment)
Next, a second embodiment for solving the problem will be described.
FIG. 3 is a block diagram of a step-down DC / DC converter (switching power supply) 10 including a current detection circuit according to the second embodiment of the present invention. Here, the same code | symbol is used with respect to the structure similar to the structure in FIG.

  The step-down DC / DC converter (switching power supply) 10 according to the second embodiment shown in FIG. 3 is different from the step-down DC / DC converter 10 having the current detection circuit shown in FIG. 7 described above to generate a current Iref2. A constant current source 25, a resistor 26 having one end connected to the power supply voltage and the other end connected to the constant current source 25, and an inverting input terminal added to the overcurrent detection output comparator 21. is there.

  An inverting input terminal added to the overcurrent detection output comparator 21 is connected to a connection point (hereinafter referred to as “node C”) between the resistor 26 and the constant current source 25.

  Here, it is assumed that the current value of the constant current source 25 is equal to the current value of the constant current source 22. Further, the overcurrent detection output comparator 21 has a function of comparing the larger of the voltages generated at the node A and the node C with the switching output voltage.

A voltage corresponding to Iref and the on-resistance of the monitoring transistor M5 is generated at the node A. Further, a voltage corresponding to the resistor 26 and the current Iref2 is constantly generated at the node C. Here, when the power supply voltage decreases, the on-resistance of the monitoring transistor M5 increases, so the voltage at the node A starts to decrease.
On the other hand, for the same reason as described in the first embodiment, the voltage appearing at both ends of the resistor 26 (that is, the voltage between the power supply voltage and the node C) is constant (see FIG. 4A). As is apparent from FIG. 4B, the rate of decrease in the voltage at the node C due to the decrease in the power supply voltage decreases by a smaller amount than the rate of decrease in the power supply voltage at the node A due to the decrease in the power supply voltage. I understand that.

  As a result, as shown in FIG. 4B, a point where the voltage at the node A and the voltage at the node C cross each other appears. The voltage at the node A decreases more rapidly than the decrease in the power supply voltage due to the ON resistance increasing action of the transistor M5 as the power supply voltage Vcc decreases, but at a certain point, the voltage at the node A and the voltage at the node C cross each other. When the voltage to be compared is switched to the voltage at node C (a decrease similar to the decrease in the power supply voltage (a constant voltage in terms of the power supply voltage based on the power supply voltage)), the voltage to be compared is Switch.

Considering the power supply voltage reference, the voltage at the node A decreases as the power supply voltage decreases due to an increase in the on-resistance of the transistor M5. When the power supply voltage falls below a certain value, the voltage to be compared is apparent from FIG. In this way, the voltage is switched to the voltage at the node C between the resistor 26 that maintains a constant voltage between both ends and the second point current power source 25 .

  Considering that the limit current is expressed as Vlimref / (ON resistance of MOS transistor M1), that Vlimref is constant, and that the ON resistance of M1 increases when the power supply voltage decreases, this method allows It turns out that it becomes possible to reduce the limit current which was the subject of the invention.

  It should be noted that the P-type MOS transistor and the N-type MOS transistor used in the above embodiments are merely examples, and different types of MOS transistors may be used. For example, in the above embodiment, a P-type MOS transistor is used as the output transistor (switching element) M1, but it may be an N-type MOS transistor. In this case, it goes without saying that the power supply voltage and other circuit configurations need to be changed accordingly.

  The above-described current detection circuit for detecting overcurrent and the switching power supply incorporating this current detection circuit are widely applied to various electronic devices such as PCs (personal computers) that require DC / DC converters, various acoustic devices, and portable devices. it can.

1 is a circuit diagram showing a step-down DC / DC converter (switching power supply) according to a first embodiment of the present invention. It is a figure which shows transition of the voltage of the node A in FIG. It is a circuit diagram which shows the step-down DC / DC converter (switching power supply) which concerns on the 2nd Example of this invention. It is a figure which shows transition of the voltage of the node A and the node C in FIG. It is a figure of the overcurrent detection circuit which concerns on background art, and a DC / DC converter (switching power supply) provided with the same. FIG. 6 is a circuit diagram of a known technique for minimizing a measurement difference due to a change in on-resistance of a high-side switch described in Japanese Patent Laid-Open No. 2006-109689. It is a figure which shows the other overcurrent detection circuit of background art, and a DC / DC converter (switching power supply) provided with the same. It is a figure which shows the current level (limit current) which detects the characteristic curve of the drain-source voltage (Vds: horizontal axis) and drain current (Ids: vertical axis) of a transistor, and an overcurrent.

Explanation of symbols

M1: Output transistor (P-type MOS transistor, switching element)
M2: Rectification switch M3, M4: P-type MOS transistor M5: Monitor transistor 10: Step-down DC / DC converter with overcurrent detection circuit 11: Control circuit 20: Overcurrent detection circuit 21: Comparison for overcurrent detection output 22 and 25: Constant current source 24: Resistance for detecting output current 26: Resistance

Claims (6)

A current detection circuit for detecting a current flowing to the load via the MOS transistor which functions as an output transistor, connected between the power supply voltage and the ground voltage, the monitor transistor and the reference current composed of MOS transistors The first constant current source to be generated, the reference current generated by the first constant current source, the first limit voltage defined by the on-resistance value of the monitoring transistor, and the output transistor By comparing the drain voltage when the MOS transistor is turned on, a comparator that detects that a current exceeding a predetermined limit current has flowed and outputs a detection signal; and the first limit voltage and means for setting an upper limit or the lower limit value, means for setting the upper or lower limit value to the first limit voltage, A voltage independent of the power supply voltage is generated from the power supply voltage, a second limit voltage is generated based on the voltage independent of the power supply voltage, and the second limit voltage is set to an upper limit of the first limit voltage or A current detection circuit characterized in that the comparator outputs the detection signal in a state where the MOS transistor functioning as the output transistor is in a linear region without reaching a saturation region. . 2. The current detection circuit according to claim 1, wherein means for setting an upper limit or a lower limit value for the first limit voltage includes a resistor provided between the power supply voltage and the ground voltage, a diode-coupled MOS transistor, A series circuit composed of two constant current sources, a gate connected to the gate of the diode-coupled MOS transistor, a drain connected to the first constant current source, a source connected to the monitor transistor, and a MOS transistor connected to each other. A current detection circuit comprising: A current detection circuit for detecting a current flowing through a load via a MOS transistor functioning as an output transistor, comprising a MOS transistor connected between a power supply voltage and a ground voltage and a reference transistor Generated from the first constant current source to be generated, the reference voltage generated by the first constant current source, the first limit voltage defined by the on-resistance value of the monitoring transistor, and the power supply voltage When the MOS transistor functioning as the output transistor is turned on, the second limit voltage that is not dependent on the power supply voltage, the higher or lower of the first limit voltage and the second limit voltage, and the output transistor By comparing with the drain voltage of the A comparator having at least three inputs for outputting an output signal, wherein the comparator outputs the detection signal in a state where the MOS transistor functioning as the output transistor is in a linear region without reaching a saturation region; current detecting circuit, characterized in that. 4. The current detection circuit according to claim 3, wherein the first limit voltage is obtained from a connection point between the monitoring transistor and the first constant current source, and the second limit voltage is the power supply voltage. A current detection circuit obtained from a connection point between a resistor connected between a ground voltage and a second constant current source .   5. A switching power supply, wherein the current detection circuit according to claim 1 is used to detect an overcurrent flowing through a load via a MOS transistor functioning as an output transistor.   An electronic device using the switching power supply according to claim 5.

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