JP2001085627A - Switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent erroneous function of a circuit for protecting an output power transistor in switching the output power transistor when the output power transistor and the protective circuit are not formed on one chip. SOLUTION: In an IC 15 for switching power supply, a high potential insular region 40 is formed between a region where an overcurrent detection circuit 1 is formed and a region where an output power transistor Tr1 is formed along with a cross under 16 having a minus voltage when the output power transistor Tr1 is turned off. The insular region 40 is isolated from other circuits and provided with a high potential by applying a voltage VIN inputted to the IC 15 for switching power supply.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply device in which a transistor for controlling an output voltage by switching and a protection circuit for protecting the transistor from overcurrent are formed on one chip.

[0002]

2. Description of the Related Art FIG. 5 is a block diagram of a switching power supply. In this switching power supply, reference numeral 11 denotes an input terminal to which an input voltage VIN is input. C1 is a capacitor for removing noise and the like.

An overcurrent 1 intervenes between the input terminal 11 and the collector of the output power transistor Tr1 and detects whether the current flowing from the input terminal 11 to the output power transistor Tr1 exceeds a predetermined overcurrent detection level. It is a detection circuit. The output power transistor Tr1 is an NPN transistor.

A driving circuit 7 is connected to the base of the output power transistor Tr1 and drives the output power transistor Tr1 ON / OFF. L1 is a coil that stores energy when the output power transistor Tr1 is on and releases energy when the output power transistor Tr1 is off. One end is connected to the emitter of the output power transistor Tr1. . C2 is a capacitor having one end grounded and the other end connected to the coil L1. The voltage VOUT appearing at the emitter of the output power transistor Tr1 is smoothed by the coil L1 and the capacitor C2 to become the output voltage VO.

F1 is a ferrite bead provided to reduce noise radiated by switching of the output power transistor Tr1. D1 is a freewheeling diode whose anode is grounded and whose cathode is connected to the emitter of the output power transistor Tr1 via the ferrite bead F1. Reference numeral 12 denotes an output terminal for outputting the output voltage VO.

[0006] R3 represents a load connected to the output terminal 12 by a resistor. R1 and R2 are output terminals 12
And a resistor connected in series between the ground. 2
Is the voltage Va at the midpoint of connection of the resistors R1 and R2 to the inverting input terminal.
Is input, and a reference voltage Vref is input to a non-inverting input terminal to amplify a difference between the voltages Va and Vref. Reference numeral 3 denotes a reference voltage source that outputs a reference voltage Vref.

Reference numeral 4 compares the output voltage Vb of the differential amplifier 2 with the voltage of the triangular wave supplied from the oscillator 5, and outputs a high level when the voltage Vb is higher than the voltage of the triangular wave. This is a comparator that outputs a low level when it is lower. Reference numeral 6 denotes an RS flip-flop circuit, which is set when the overcurrent detection circuit 1 detects an overcurrent, and is reset by a reset signal from the oscillator 5. At the point Pa, the output terminal of the comparator 4 and the set output terminal of the RS flip-flop circuit 6 are connected. Thus, the signal SP output from the comparator 4 when the overcurrent detection circuit 1 does not detect an overcurrent.
Is input to the drive circuit 7, while the overcurrent detection circuit 1 detects an overcurrent, the RS flip-flop circuit 6 shuts off the signal SP until reset by the reset signal from the oscillator 5, and the drive circuit 7 Input level signal.

Reference numeral 9 denotes a constant voltage source for supplying a constant voltage Vc. 8 compares the voltage Va with the constant voltage Vc and outputs a high level when the voltage Va is higher than the constant voltage Vc;
This is a comparator that outputs a low level when a is lower than the constant voltage Vc. An oscillation frequency changing circuit 10 changes the oscillation frequency of the triangular wave output from the oscillator 5 and the reset signal by the signal output from the comparator 8. The frequency of the triangular wave output from the oscillator 5 and the frequency of the reset signal are equal.

Reference numeral 15 denotes a switching power supply IC (integrated circuit) which drives the output power transistor Tr1, the overcurrent detection circuit 1, the differential amplifier 2, the reference voltage source 3, the comparator 4, the oscillator 5, the RS flip-flop circuit 6, and the like. The circuit 7, the comparator 8, the constant voltage source 9, and the oscillation frequency changing circuit 10 are formed on one chip.

FIG. 6 is a circuit diagram showing an example of an internal circuit of the overcurrent detection circuit 1. In the overcurrent detection circuit 1, RM1 is a resistor inserted between the input terminal 11 and the output power transistor Tr1. Numeral 63 denotes a PNP transistor whose emitter is connected to a point P1 between the resistor RM1 and the input terminal 11. Reference numeral 64 denotes a PNP transistor whose emitter is connected to a point P2 between the resistor RM1 and the collector of the output power transistor Tr1, whose base is connected to the base of the PNP transistor 63, and whose collector is connected to the base.

Numeral 67 denotes an NPN transistor, whose emitter is grounded and whose collector is a PNP transistor 63.
Connected to the collector. Reference numeral 68 denotes an NPN transistor whose emitter is grounded and whose collector is connected to the collector of the PNP transistor 64.

Reference numeral 69 denotes an NPN transistor, whose emitter is grounded and whose base is an NPN transistor 67;
68 and the collector is connected to the base. Reference numeral 66 denotes an NPN transistor whose emitter is grounded, whose base is connected to the collector of the NPN transistor 67, and whose collector is connected to the set input terminal of the RS flip-flop circuit 6.

Reference numeral 65 denotes a constant current source circuit, which supplies a constant current to NP.
The signal is output toward the collector of the N-type transistor 69 and the base of the NPN-type transistors 67, 68, 69. 13
Is an output terminal for outputting a voltage VOUT appearing at the emitter of the output power transistor Tr1.

Numeral 30 denotes a parasitic NPN transistor formed in the switching power supply IC 15,
The emitter is connected to the base of the output power transistor Tr1, the P-type silicon substrate serves as the base, and the collector is connected to the bases of the PNP transistors 63 and 64.

In a normal state where the collector current I1 of the output power transistor Tr1 does not exceed the overcurrent detection level, P
The NP transistors 63 and 64 and the NPN transistors 67, 68 and 69 are ON. The NPN transistor 67 is saturated, and its collector voltage V1 is sufficiently low, so that the NPN transistor 66 is off. Thereby, the RS flip-flop circuit 6 sets the set output terminal to a high level.

When the current I1 increases, the base potential of the PNP transistors 63 and 64 decreases due to the voltage drop of the resistor RM1, so that the base potential of the PNP transistors 63 and 64 decreases.
The emitter-to-emitter voltage VBE increases, and the collector current I2 of the PNP transistor 63 increases. When the collector current I1 of the output power transistor Tr1 further increases and exceeds the overcurrent detection level set by the resistance value of the resistor RM1, the current I2 can flow from the collector to the emitter of the saturated NPN transistor 67. Since the current becomes larger than the current, the collector voltage V1 of the NPN transistor 67 increases, and the NPN transistor 66 is turned on.
I do. As a result, the RS flip-flop circuit 6 sets the set output terminal to a low level.

FIG. 7 is a circuit diagram showing an example of the internal circuit of the drive circuit 7. Reference numeral 51 denotes a PNP transistor whose emitter has an overcurrent detection circuit 1 and an output power transistor Tr1.
And the base is connected to the line f via the resistor 58, and the collector is connected to the base. 52 is a PNP transistor;
The emitter is connected to the line f, the base is connected to the base of the PNP transistor 51, and the collector is connected to the resistor 61.
Is connected to the emitter of the output power transistor Tr1.

Reference numeral 53 denotes a PNP transistor, the emitter of which is connected to the line f, the base of which is connected to the line f via the resistor 59, and the collector of which is connected to the base. 54 turns ON / OF the output power transistor Tr1
A PNP type transistor driven by F, an emitter connected to the line f, and a base connected to the PNP type transistor 53
And the collector is connected to the base of the output power transistor Tr1.

An NPN transistor 55 has an emitter connected to the emitter of the output power transistor Tr1, a base connected to the collector of the PNP transistor 52, and a collector connected to the base of the output power transistor Tr1. An NPN transistor 56 has an emitter grounded and a base connected to an inverter 62.
, And the collector is connected to the collector of the PNP transistor 51. An NPN transistor 57 has an emitter grounded and a base connected to a point P.
a, and the collector is connected to the collector of the PNP transistor 53.

Reference numeral 60 denotes a resistor interposed between the base and the emitter of the output power transistor Tr1. The inverter 62 sets the low level to NP when the point Pa is at the high level.
The signal is output to the base of the N-type transistor 56, and when the point Pa is at the low level, the high level is
6 to the base.

When the point Pa is at a high level by the RS flip-flop circuit 6 and the comparator 4,
Since the base voltage of the NPN transistor 57 becomes high level, the NPN transistor 57 is turned on. NPN
Since the base voltage of the type transistor 56 becomes low level by the inverter 62, the NPN type transistor 56 is turned off. Since the NPN transistor 56 is OFF, the PNP transistor 51,
52, the voltage between the emitter and the base becomes zero, and PNP
The type transistors 51 and 52 are turned off. This allows
The voltage between the emitter and the base of the NPN transistor 55 becomes zero by the resistor 61, and the NPN transistor 55 is turned off.

Since the NPN transistor 57 is ON, the voltage between the emitter and the base of the PNP transistors 53 and 54 increases, and the PNP transistors 53 and 54 are turned ON. The base of the output power transistor Tr1 is maintained at a high voltage by the PNP transistor 54, and the output power transistor Tr1 is turned on.

On the other hand, when the point Pa is at a low level, the NPN transistor 57 is turned off,
The NPN transistor 56 turns on. Since the NPN transistor 57 is off, the PNP transistors 53 and 54 are off. Since the voltage between the emitter and the base of the output power transistor Tr1 becomes zero by the resistor 60, the output power transistor Tr1 is turned off.
I do.

Since the NPN transistor 56 is turned on, the PNP transistors 51 and 52 are turned on. As a result, the base voltage of the NPN transistor 55 becomes a high voltage.
N. When the NPN transistor 55 is ON,
The NPN transistor 55 quickly turns off the output power transistor Tr1 when the point Pa changes from the high level to the low level, because the current has the effect of extracting current from the base of the output power transistor Tr1.

Returning to FIG. 5, in this switching power supply, the input voltage VIN is switched by the output power transistor Tr1. While the output power transistor Tr1 is ON, the voltage VOUT appearing at the emitter of the output power transistor Tr1 causes the capacitor C2 and the load R3 to pass through the coil L1.
Is supplied with power. Output power transistor T
During the period when r1 is OFF, a current is supplied through the diode D1 by the energy stored in the coil L1.

The output voltage VO is divided by a ratio according to the resistance values of the resistors R1 and R2 to become a voltage Va. The voltage Vb obtained by amplifying the difference between the voltage Va and the reference voltage Vref by the differential amplifier 2 is obtained. Voltage Vb and 1 output from oscillator 5
The comparator 4 compares the triangular wave of 00 KHz.

The comparator 4 outputs a PWM (pulse width modulation) signal SP having a pulse width corresponding to the voltage Vb output from the differential amplifier 2. In a normal state in which the overcurrent detection circuit 1 does not detect an overcurrent, the PWM signal SP
Is given to the drive circuit 7 through the point Pa, so that PWM
The drive circuit 7 controls the ON period and the OFF period of the output power transistor Tr1 according to the duty cycle of the signal SP. Thus, the output voltage VO is controlled such that the voltage Va matches the reference voltage Vref.

On the other hand, in an overcurrent state when the overcurrent detection circuit 1 detects an overcurrent, a low-level signal is driven from the RS flip-flop circuit 6 via the point Pa regardless of the PWM signal SP. Since the driving power is input to the circuit 7, the driving circuit 7 turns off the output power transistor Tr1.
Thus, the RS flip-flop circuit 6 controls the drive circuit 7 so as to suppress the current flowing through the output power transistor Tr1.

FIG. 8 shows a time change of the voltage VOUT, the current IL flowing through the coil L1, the set output terminal voltage and the reset terminal voltage of the RS flip-flop circuit 6 in the normal state where the overcurrent detection circuit 1 does not detect the overcurrent. FIG. FIG. 9 shows a voltage VOU in an overcurrent state in which the overcurrent detection circuit 1 detects an overcurrent and the overcurrent protection function operates.
FIG. 9 is a waveform diagram illustrating a time change of a set output terminal voltage and a reset terminal voltage of T, a current IL, and an RS flip-flop circuit 6;

In the normal state, the set output terminal voltage of the RS flip-flop circuit 6 remains at a high level as shown in FIG. The reset terminal voltage of the RS flip-flop circuit 6 is a pulse having a period T from the oscillator 5. RS
Since the output of the flip-flop circuit 6 remains at the high level, the PWM signal SP is input to the drive circuit 7.
The drive circuit 7 turns ON / OFF the output power transistor Tr1 according to the PWM signal SP.

At the moment when the output power transistor Tr1 is turned off, the voltage VOUT greatly swings to a negative potential as described later, but is recovered immediately and stabilized at a voltage slightly lower than the ground level. During the period when the output power transistor Tr1 is OFF, the current IL gradually decreases. During the period in which the output power transistor Tr1 is ON, the voltage VOUT is kept on the high voltage side, and the current IL gradually increases. The transistor Tr1 repeats switching at a period T by the PWM signal SP, and the output voltage VO of the switching power supply is controlled by the duty cycle of the PWM signal SP. In the normal state, the current IL
Does not exceed the overcurrent level Iref.

On the other hand, in the overcurrent state, as shown in FIG. 9, the overcurrent detection circuit 1 detects the overcurrent at time t1 when the current IL exceeds the overcurrent level Iref. Immediately after that, at time t2, the set output terminal voltage of the RS flip-flop circuit 6 is set to low level. Thus, when the output power transistor Tr1 is turned off at the time point t3, the voltage VOUT swings significantly to a negative potential, and then stabilizes at a voltage slightly lower than the ground level. Current I
L starts to decrease from time t3. The RS flip-flop circuit 6 holds the set output terminal voltage at a low level until time t4 when the next reset signal is input.

When the RS flip-flop circuit 6 is reset by the reset signal at time t4, the current IL falls below the overcurrent level Iref due to the decrease in the current IL, so that the overcurrent detection circuit 1 does not detect the overcurrent, and R
The S flip-flop circuit 6 sets the set output terminal voltage to a high level. Thereafter, the switching of the output power transistor Tr1 is controlled by the PWM signal SP, so that the output power transistor t5 is turned on at time t5.
I do. Then, the voltage VOUT becomes a high voltage and the current IL
Turns to increase.

Thereafter, the current IL is again changed to the overcurrent level Ir.
When ef is exceeded, the overcurrent detection circuit 1 detects an overcurrent, so that the set output voltage of the RS flip-flop circuit 6 is set to a low level. When the set output voltage of the RS flip-flop circuit 6 is at a low level, a low-level signal is input to the drive circuit 7,
The drive circuit 7 turns off the output power transistor Tr1 and sets the voltage VOUT to a low level.

When the overcurrent protection function operates as described above, the ON duty of the output power transistor Tr1 becomes smaller, and eventually the output power transistor Tr1 becomes smaller.
Is the width from the time when is turned ON until the overcurrent protection is activated. O
When the N duty becomes small, the switching power supply does not supply the current required by the load R3, so that the output voltage VO decreases.

As a result, the output voltage VO decreases and the voltage V
When “a” becomes lower than the constant voltage Vc, the comparator 8 switches the output to the oscillation frequency changing circuit 10 from the high level to the low level. Then, a signal for lowering the oscillation frequency is output from the oscillation frequency changing circuit 10 to the oscillator 5.
When the oscillator 5 receives this signal, it changes the constant current value for determining the oscillation frequency to lower the oscillation frequency.

When arranging elements such as the output power transistor Tr1 on the chip of the switching power supply IC 15, the driving is performed so as not to complicate the signal line in consideration of the current flow from the drive circuit 7 to the transistor Tr1. The distance between the circuit 7 and the transistor Tr1 is reduced. Further, since the drive circuit 7 and the output power transistor Tr1 which handle a relatively large current are also sources of noise, the overcurrent detection circuit 1 and its detection location are taken into consideration so as not to affect other circuits. The collector of the output power transistor Tr1 is kept from becoming far.

In general, a circuit for handling a relatively weak current or voltage in an analog IC is liable to malfunction as the signal is attenuated or the surrounding noise is picked up as the distance from the signal source increases. Therefore, in general, in an analog IC, it is necessary to place a circuit that handles such a relatively weak current or voltage near a signal generation source. The overcurrent detection circuit 1 shown in FIG.
1 is a circuit for handling a relatively weak voltage generated at both ends of the circuit 1. Therefore, care is taken to arrange an element such as a transistor constituting the overcurrent detection circuit 1 near the resistor RM1. Also,
Consider that the chip area is as small as possible within the range where performance can be exhibited.

As a result of these considerations, the arrangement of the main elements of the switching power supply IC 15 is conventionally as shown in FIG. The input terminal 11 has an elongated rectangular shape, and is arranged so as to be in contact with the left side of the switching power supply IC 15 and the longer side of the rectangle is parallel to the left side. The output power transistor Tr1 is also elongated and rectangular and has a larger area than the input terminal 11;
And the longer side of the rectangle is arranged so as to be parallel to the longer side of the input terminal 11.

A plurality of cross-unders 16 are provided on the side opposite to the side of the output power transistor Tr1 facing the input terminal 11 so as to extend in a direction perpendicular to the side. The cross-under 16 is used instead of metal when a metal wiring does not pass through a single-layer metal IC or the like. In many cases, the cross-under 16 is an N ⁺ diffusion layer. An output terminal 13 is provided at a position parallel to the cross-under 16 and in contact with the lower side of the switching power supply IC 15. The drive circuit 7 is provided at the upper left corner of the switching power supply IC 15.

The PNP transistor 54 that drives the ON / OFF of the output power transistor Tr1 in the drive circuit 7 is provided at a position in contact with the output power transistor Tr1. The overcurrent detection circuit 1 has an output power transistor Tr
It is provided at a position facing 1. The input voltage VIN input from the input terminal 11 is 1
And input to the overcurrent detection circuit 1 via the

FIG. 11 shows a cross section of the switching power supply IC 15 from the point (A) to the point (B) in FIG.
First, from the point (A) to the point (B), an N ⁺ buried layer 21a is provided in a portion corresponding to the output power transistor Tr1 on the P-type silicon substrate 20. Next, an N ⁺ buried layer 21 is formed in a portion corresponding to the cross under 16 on the substrate 20.
b is provided. Next, an N ⁺ buried layer 21c is provided in a portion corresponding to the overcurrent detection circuit 1 on the substrate 20.

An N-type semiconductor epitaxial growth portion 22a is provided above the N ⁺ buried layer 21a. An epitaxial growth portion 2 of an N-type semiconductor is formed on the N ⁺ buried layer 21b.
2b is provided. The epitaxial growth portions 22a and 22b are separated by an element isolation layer 28 of a P-type semiconductor. An N-type semiconductor epitaxial growth portion 22c is provided above the N ⁺ buried layer 21c. The epitaxial growth portions 22b and 22c are separated by a P-type semiconductor device isolation layer 29.

An N ⁺ diffusion layer 23 is provided in the epitaxial growth portion 22a. In addition, the epitaxial growth part 2
2a, a P diffusion layer 24 is provided at a position distant from the N ⁺ diffusion layer 23. An N ⁺ diffusion layer 25 is provided inside the P diffusion layer 24.
Is provided. Thus, an NPN-type output power transistor Tr1 is formed in which the N ⁺ diffusion layer 23 serves as a collector, the P diffusion layer 24 serves as a base, and the N ⁺ diffusion layer 25 serves as an emitter.

In the epitaxial growth portion 22b, N ⁺
A diffusion layer 26 is provided. Thus, the cross-under 16 is provided in the epitaxial growth portion 22b. P diffusion layer 27 is provided in epitaxial growth portion 22c. Thus, the epitaxial growth portion 2
An overcurrent detection circuit 1 having a configuration as shown in FIG. 6 is formed in 2c. The P diffusion layer 24 serving as the base of the output power transistor Tr1 and the N ⁺ buried layer of the cross under 16 are connected by a wiring 50.

In the parasitic NPN transistor 30, the epitaxial growth portion 22b serves as an emitter, the P-type silicon substrate 20 serves as a base, and the epitaxial growth portion 22c
Is formed to be a collector.

[0047]

When the above-mentioned conventional switching power supply is used in a circuit which dislikes noise such as an audio reproducing apparatus, a ferrite bead F1 is inserted into the switching power supply as shown in FIG. May be done. The ferrite bead F1 is a tube made of a magnetic material having high magnetic permeability and inserted into leads of a wiring or a component to easily form a coil.

Generally, a large short-circuit current flows through the diode D1 at the moment when the applied voltage is reversed from the forward direction to the reverse direction. This is called a reverse recovery characteristic. In the switching power supply, when the ferrite bead F1 is not inserted, a large short-circuit current flows through the diode D1 due to the reverse recovery characteristic, and the diode D1 becomes a noise source. Therefore, the coil is equivalently inserted by inserting the ferrite bead F1 into the lead of the diode D1. Then
The short-circuit current is suppressed by the impedance component of the ferrite bead F1, and the amount of generated noise is reduced.

However, when the ferrite bead F1 is inserted, the voltage VOUT greatly swings to a negative potential due to the impedance component of the ferrite bead F1 when the output power transistor Tr1 is switched off, but the impedance component transiently decreases. ,
The voltage VOUT is stabilized at a voltage lower than the ground level by the forward voltage of the diode D1.

The degree to which the voltage VOUT swings to a negative potential depends on the load R3. When the load R3 is light, as shown in FIG. 12, the degree to which the voltage VOUT swings to a negative potential is small, and no particular problem occurs. However, when the load R3 becomes heavy, the voltage VO appearing at the emitter of the output power transistor Tr1 as shown in FIG.
Because the degree to which the UT swings to a negative potential increases,
The output power transistor Tr1 is turned on, and the base of the output power transistor Tr1 also has the negative potential at the same time. FIG. 12 is a waveform diagram showing a time change of the set output terminal voltage and the voltage VOUT when the load R3 is light,
FIG. 13 is a waveform diagram showing a change over time of the set output terminal voltage and the voltage VOUT when the load R3 is heavy.

Since the base of the output power transistor Tr1 is connected to the drive circuit 7 via the cross under 16, the cross under 16 also has a negative potential. Then, as shown in FIG. 11, the N ⁺ diffusion layer 26, the N-type semiconductor epitaxial growth portion 22b, and the N ⁺ buried layer 21b constituting the cross-under 16 have a negative potential, so that the parasitic NPN transistor 30 is turned on. Then, as shown by an arrow 31, a current flows from the overcurrent detection circuit 1 to the cross-under 16.

In the overcurrent detection circuit 1, the bases of the PNP transistors 63 and 64 (see FIG. 6) correspond to the N-type semiconductor epitaxial growth portion 22c (see FIG. 11) of FIG.
The base voltages of the P-type transistors 63 and 64 decrease.

That is, when the emitter voltage of the output power transistor Tr1 greatly swings to the minus side,
The base voltages of the NP-type transistors 63 and 64 are greatly reduced. Then, since the base-emitter voltage VBE of the PNP transistor 63 greatly increases, the collector current I2 of the PNP transistor 63 increases, and the NPN transistor 66 turns on.
This means that the overcurrent detection circuit 1 has erroneously detected the overcurrent state.

When the overcurrent detection circuit 1 erroneously detects an overcurrent state, the RS flip-flop circuit 6 sets the set output terminal to a low level. Output power transistor Tr
If the time during which the voltage VOUT appearing at the emitter 1 is a negative potential is long, the RS flip-flop circuit 6 is reset by the reset signal from the oscillator 5 while the RS flip-flop circuit 6 is set as in the operation in the overcurrent state as shown in FIG. Therefore, the pulse width of the PWM signal SP is narrowed to narrow the switching pulse width. This also lowers the output voltage VO.

When the voltage Va falls below the constant voltage Vc, a high-level signal is transmitted from the comparator 8 to the oscillation frequency changing circuit 10, and the oscillation frequency of the oscillator 5 is reduced by the oscillation frequency changing circuit 10. As a result, FIG.
As shown in (2), the cycle in which the voltage VOUT repeats the high voltage side and the low voltage side becomes longer. As described above, in the conventional switching power supply device (see FIG. 5), when the load R3 is heavy, the overcurrent detection circuit 1 erroneously detects when the output power transistor Tr1 is turned off, and the switching power supply IC 15 does not operate normally. there were.

An object of the present invention is to solve the above-mentioned problems, and an object of the present invention is to provide a switching power supply device which does not malfunction at the time of switching of an output power transistor.

[0057]

In order to achieve the above object, according to a first aspect of the present invention, an NPN-type switching transistor, a drive circuit for switchingly driving the switching transistor, and a current flowing through the switching transistor are overcurrent. An overcurrent detection circuit for detecting whether or not the switching transistor is in a state; a circuit for controlling the drive circuit so as to suppress a current flowing through the switching transistor when the overcurrent is in the state; And a diode whose other end is connected to an output terminal for connecting to a load, and a diode whose cathode is connected to the emitter side of the switching transistor and a connection node of the coil and whose anode is connected to a predetermined voltage. Consisting of at least the switching transistor and the In a switching power supply device in which a current detection circuit is formed on one chip, a constant voltage is applied between a region where the switching transistor is formed and a region where the overcurrent detection circuit is formed. A low-impedance region is formed.

According to such a configuration, the switching power supply device performs switching driving of the switching transistor by the drive circuit. Switching transistor is O
During the N period, energy is supplied to the coil and the load by the output of the switching transistor. At this time, energy is stored in the coil. While the switching transistor is off, a current flows in the forward direction of the diode due to the energy stored in the coil. Therefore, electric power is supplied to the load through the coil. The overcurrent detection circuit detects whether the current flowing through the switching transistor is in an overcurrent state. When in an overcurrent state, a circuit for controlling the drive circuit controls the drive circuit so as to suppress the current flowing through the switching transistor. At least a switching transistor and an overcurrent detection circuit are formed on one chip. On the chip, a low-impedance region is formed between the region where the switching transistor is formed and the region where the overcurrent detection circuit is formed, and a constant voltage is applied. O
Even if the parasitic transistor formed between the switching transistor and the low-impedance region is turned on when FF is performed, the parasitic transistor formed between the low-impedance region and the overcurrent detection circuit is turned on. And the overcurrent detection circuit does not perform erroneous detection.

Further, in the second invention, in the first invention, the constant voltage is obtained from an input voltage input to the switching power supply. According to such a configuration, the low impedance region becomes high potential by the input voltage.

Further, in the third invention, an NPN-type switching transistor, and a PNP having a collector connected to a base of the switching transistor and an emitter connected to a collector of the switching transistor for switching-driving the switching transistor.
A drive circuit having a transistor of a type, an overcurrent detection circuit for detecting whether or not the current flowing through the switching transistor is in an overcurrent state, and suppressing a current flowing through the switching transistor when the overcurrent state is in the overcurrent state. A coil that has one end connected to the emitter side of the switching transistor and the other end connected to an output terminal for connecting to a load,
An emitter side of the switching transistor and a diode having a cathode connected to a connection node of the coil and an anode connected to a predetermined voltage, wherein at least the switching transistor, the drive circuit, and the overcurrent detection circuit are one chip. In the switching power supply device formed above, a region where the PNP transistor is formed is provided between a region where the switching transistor is formed and a region where the overcurrent detection circuit is formed. I am trying to be.

According to such a configuration, the switching power supply device performs switching driving of the switching transistor using the PNP type transistor constituting the drive circuit. During the period when the switching transistor is ON, energy is supplied to the coil and the load by the output of the switching transistor. At this time, energy is stored in the coil. Switching transistor is OF
During the F period, a current flows in the forward direction of the diode due to the energy stored in the coil. Therefore, electric power is supplied to the load through the coil. The overcurrent detection circuit detects whether the current flowing through the switching transistor is in an overcurrent state. When in an overcurrent state, a circuit for controlling the drive circuit controls the drive circuit so as to suppress the current flowing through the switching transistor. At least a switching transistor, a drive circuit, and an overcurrent detection circuit are formed on one chip. On the chip, a region where a PNP transistor is formed is provided between a region where the switching transistor is formed and a region where the overcurrent detection circuit is formed.
Even if a parasitic transistor formed between the switching transistor and the PNP transistor is turned on when the switching transistor is turned on / off, the PNP transistor requires a relatively large current to drive the switching transistor. Because it operates, it is not affected by the operation. Therefore, since the parasitic transistor formed between the PNP transistor and the overcurrent detection circuit does not turn on, the overcurrent detection circuit does not erroneously detect.

Further, in the fourth invention, an NPN-type switching transistor, a drive circuit for switchingly driving the switching transistor, and an overcurrent detection for detecting whether or not a current flowing through the switching transistor is in an overcurrent state A circuit for controlling the drive circuit so as to suppress a current flowing through the switching transistor in the overcurrent state; and a circuit for connecting one end to an emitter of the switching transistor and the other end to a load. A coil connected to an output terminal; and a diode having a cathode connected to the emitter of the switching transistor and a connection node of the coil and an anode connected to a predetermined voltage, and at least the switching transistor and the overcurrent detection circuit. And 1
In a switching power supply device formed on one chip, a region where a high voltage is applied is formed between a region where the switching transistor is formed and a region where the overcurrent detection circuit is formed. .

According to such a configuration, the switching power supply device performs switching driving of the switching transistor by the drive circuit. Switching transistor is O
During the N period, energy is supplied to the coil and the load by the output of the switching transistor. At this time, energy is stored in the coil. While the switching transistor is off, a current flows in the forward direction of the diode due to the energy stored in the coil. Therefore, electric power is supplied to the load through the coil. The overcurrent detection circuit detects whether the current flowing through the switching transistor is in an overcurrent state. When in an overcurrent state, a circuit for controlling the drive circuit controls the drive circuit so as to suppress the current flowing through the switching transistor. At least a switching transistor and an overcurrent detection circuit are formed on one chip. On the chip, a high-voltage application area is formed between the area where the switching transistor is formed and the area where the overcurrent detection circuit is formed. Even if a parasitic transistor formed between this region and the region to which a high voltage is applied is turned on, the parasitic transistor formed between this region and the overcurrent detection circuit does not turn on, and The detection circuit does not erroneously detect.

According to a fifth aspect, in any one of the first to fourth aspects, a ferrite bead is connected between the emitter of the switching transistor and the diode. .

According to such a configuration, ferrite beads are inserted to suppress a large short-circuit current flowing through the diode. In this case, the parasitic transistor formed by the switching transistor is easily turned on at the time of switching, but the overcurrent detection circuit does not erroneously detect.

[0066]

Embodiments of the present invention will be described below. Also in the first and second embodiments of the present invention, description will be made on the assumption that an overcurrent detection circuit 1 is provided to protect the switching power supply device from overcurrent as shown in FIG. Therefore, the first embodiment and the second embodiment
The block diagram of the switching power supply device of this embodiment is the same as that shown in FIG. 5, and the same block as the conventional one is formed on one chip for the switching power supply IC 15 as well. Therefore, these points are the same as those described in the related art, and the description will be omitted.

<First Embodiment> FIG. 1 is a diagram showing an arrangement of main elements of a switching power supply IC 15 according to a first embodiment of the present invention. In the arrangement of this embodiment, the overcurrent detection circuit 1 is provided at a position further away from the output power transistor Tr1 than the arrangement of the conventional switching power supply IC 15 (FIG. 10), and becomes a negative potential when the output power transistor Tr1 is switched off. A low-impedance, high-potential island region 40 is formed between the region where the cross-under 16 is provided and the region where the overcurrent detection circuit 1 is provided. The island-shaped region 40 is separated from the cross-under 16 and the overcurrent detection circuit 1, and has a high potential when an input voltage VIN is applied from a wiring 71 connecting the cross-under 16 and the overcurrent detection circuit 1. .

FIG. 2 shows a cross section of the switching power supply IC 15 from the point (A) to the point (B) in FIG. First, an output power transistor Tr1 is provided from point (A) to point (B). Next, a cross-under 16 is provided adjacent to the output power transistor Tr1. Next, a high-potential island region 40 is provided adjacent to the cross-under 16. Next, the overcurrent detection circuit 1 is provided adjacent to the high-potential island region 40.

The configuration of the output power transistor Tr1, the cross-under 16, and the overcurrent detection circuit 1 are the same as those shown in FIG. 11, which shows a cross section of the conventional switching power supply IC 15, and therefore, in FIG. Are denoted by the same reference numerals and description thereof is omitted.

In the high potential island region 40, an N ⁺ buried layer 21 d is provided in a portion corresponding to the high potential island region 40 on the P-type silicon substrate 20. N ⁺ buried layer 21d
Is provided with an epitaxial growth portion 22d of an N-type semiconductor. The epitaxial growth portions 22b and 22d are separated by a P-type semiconductor device isolation layer 29. The epitaxial growth portions 22c and 22d are separated by an element isolation layer 41 of a P-type semiconductor. Epitaxial growth part 22d
Are at a high potential due to the application of the input voltage VIN.

In the parasitic NPN transistor 30, the epitaxial growth portion 22b serves as an emitter, the P-type silicon substrate 20 serves as a base, and the epitaxial growth portion 22d
Is formed to be a collector. Reference numeral 70 designates the epitaxial growth portion 22c as an emitter, the P-type silicon substrate 20 as a base, and the epitaxial growth portion 22c.
Is a parasitic NPN transistor formed to be a collector.

When the output power transistor Tr1 is switched off, the ferrite beads F1 (see FIG. 5)
Is provided, the output power transistor Tr
The voltage VOUT appearing at one of the emitters may fluctuate to a negative potential.

When the degree of the swing is large, the output power transistor Tr1 is turned ON, the base of the output power transistor Tr1 also has a negative potential, and the cross-under 16 connected to the base by the wiring 50 also has the negative potential. Since the N-type semiconductor epitaxial growth portion 22d constituting the high-potential island region 40 adjacent to the cross-under 16 has a high potential, the parasitic NPN transistor 30 is turned on.

However, since the high-potential island region 40 is separated from the overcurrent detection circuit 1 and has a high potential by the input voltage VIN, the parasitic transistor 70 is turned on.
Accordingly, the parasitic transistor 30 does not affect the operation of the overcurrent detection circuit 1. The provision of the high potential island region 40 prevents the overcurrent detection circuit 1 from malfunctioning when the output power transistor Tr1 is switched off. Parasitic transistor 30
Therefore, the island region 40 needs to have a low impedance. This is because if the impedance of the island region 40 is high, the high impedance greatly amplifies the high-frequency noise and affects the peripheral circuits.

Note that there is no guarantee that malfunction will be prevented if the overcurrent detection circuit 1 is simply distant from the cross-under 16 at which the potential becomes negative when the output power transistor Tr1 is switched off. By providing the high-potential island region 40 as described above, malfunction is reliably prevented.

When the terminal 13 (see FIGS. 6 and 7) of the switching power supply IC 15 to which the voltage VOUT is output is forcibly reduced to a negative potential by another power supply, the voltage VOUT becomes- When the voltage drops to 0.8 V, a malfunction occurs due to the same symptoms as when overcurrent is detected. However, in the present embodiment, the malfunction does not occur unless the voltage VOUT becomes -1.8 V or less.

This means that the output power transistor Tr1
When a large current flows through the switch, the voltage V
Since OUT largely swings to the minus side, the voltage VOUT
When a large current flows to −1.8 V, the overcurrent detection circuit 1 detects the overcurrent and performs the overcurrent protection normally, so that the load R3 is used under the use condition in which the ferrite bead F1 is inserted. This means that the switching power supply IC 15 can normally perform overcurrent protection even when it is heavy.

Therefore, when the load R3 is light, the switching power supply of this embodiment operates as shown in FIG. 12 similarly to the conventional switching power supply. On the other hand, when the load R3 is heavy, the switching power supply device of the present embodiment does not malfunction, so that the voltage VOUT largely swings to a negative potential, but operates almost the same as in FIG. At this time, since the set output terminal voltage is at the high level, the ON duty of the output power transistor Tr1 does not decrease, and the oscillation frequency of the oscillator 5 does not decrease.

In FIG. 1, the high potential island region 4
0 is provided not only between the region where the cross-under 16 is formed and the region where the overcurrent detection circuit 1 is formed, but also in a wider range than the region where the output power transistor Tr1 is switched off. This is for preventing the island-shaped region serving as a potential from affecting other circuit portions (not shown).

<Second Embodiment> FIG. 3 is a diagram showing an arrangement of main elements of a switching power supply IC 15 according to a second embodiment of the present invention. In the arrangement of this embodiment, the overcurrent detection circuit 1 is provided at a position further away from the output power transistor Tr1 than the arrangement of the conventional switching power supply IC 15 (FIG. 10), and becomes a negative potential when the output power transistor Tr1 is switched off. A PNP transistor 54 constituting the drive circuit 7 is formed between a region where the cross-under 16 is formed and a region where the overcurrent detection circuit 1 is formed. The PNP transistor 54 turns on the output power transistor Tr1
· A transistor that is driven OFF. The input voltage VIN input from the input terminal 11 is input to the overcurrent detection circuit 11 without passing through the cross-under 16.

FIG. 4 shows a cross section of the switching power supply IC 15 from the point (A) to the point (B) in FIG. First, an output power transistor Tr1 is provided from point (A) to point (B). Next, a cross-under 16 is provided adjacent to the output power transistor Tr1. Next, a PNP transistor 54 is provided adjacent to the cross-under 16. Next, the overcurrent detection circuit 1 is provided adjacent to the PNP transistor 54.

The configuration of the output power transistor Tr1, the cross-under 16, and the overcurrent detection circuit 1 is the same as that of FIG. 11 showing a cross section of the above-mentioned conventional switching power supply IC 15, and therefore, in FIG. Are denoted by the same reference numerals and description thereof is omitted.

In the PNP transistor 54, an N ⁺ buried layer 21e is provided on a portion corresponding to the PNP transistor 54 on the P-type silicon substrate 20. An N-type semiconductor epitaxial growth portion 2 is formed on the N ⁺ buried layer 21e.
2e is provided. The epitaxial growth portions 22b and 22e are separated by a P-type semiconductor device isolation layer 29. The epitaxial growth portions 22c and 22e are separated by an element isolation layer 41 of a P-type semiconductor. A P diffusion layer 43 is provided in the epitaxial growth portion 22e, and a PNP transistor 54 is formed.

In the parasitic NPN transistor 30, the epitaxial growth portion 22b serves as an emitter, the P-type silicon substrate 20 serves as a base, and the epitaxial growth portion 22e
Is formed to be a collector. Parasitic NPN
In the type transistor 70, the epitaxial growth portion 22e serves as an emitter, the P-type silicon substrate 20 serves as a base,
The epitaxial growth portion 22c is formed to be a collector.

When the PNP transistor 54 turns off, the output power transistor Tr1 turns off. At the time of this OFF, the voltage VOUT appearing at the emitter of the output power transistor Tr1 may swing to a minus potential.

When the degree of the swing is large, the output power transistor Tr1 is turned ON, the base of the output power transistor Tr1 also has a negative potential, and the cross-under 16 connected to the base by the wiring 50 also has the negative potential. Therefore, the parasitic NPN transistor 30 is turned on. Parasitic NPN transistor 30
As a result, a current of about 10 μA flows from the PNP transistor 54 to the cross under 16 as shown by the arrow 31.

On the other hand, the PNP transistor 54
When the emitter current IE of the output power transistor Tr1 (see FIG. 5) is several A, the base current of the output power transistor Tr1 is IE / h when the current amplification factor of the output power transistor Tr1 is hF1.
F1 and this current IE / hF1 is substantially equal to the collector current of the PNP transistor 54, and if the current amplification factor of the PNP transistor 54 is hF2, IE / (h
F1 · hF2).

Since the current amplification factors hF1 and hF2 are about 100, the base current of the PNP transistor 54 is several hundred μA. Therefore, even if a current of about several tens of μA flows through the PNP transistor due to the parasitic NPN transistor Tr1, the operation of the PNP transistor 54 is not affected. Therefore, the PNP transistor 54
The overcurrent detection circuit 1 is not affected by the overcurrent detection circuit 1 even if the cross-under 16 has a negative potential, and does not make an erroneous detection even if the crossover 16 becomes a negative potential.

In this embodiment, when the terminal of the switching power supply IC 15 from which the voltage VOUT is output is forcibly reduced to a negative potential by another power supply, a malfunction may occur unless the voltage VOUT becomes -1.8 V or less. lost.
This means that even when the load R3 is heavy under the use condition in which the ferrite beads F1 are inserted, the switching power supply I
This means that C15 can normally perform overcurrent protection.

Therefore, when the load R3 is light, the switching power supply of this embodiment operates as shown in FIG. 12 similarly to the conventional switching power supply. On the other hand, when the load R3 is heavy, the switching power supply device of the present embodiment does not malfunction, so that the voltage VOUT largely swings to a negative potential, but operates almost the same as in FIG. At this time, the ON duty of the output power transistor Tr1 does not decrease and the oscillation frequency of the oscillator 5 does not decrease due to the high level of the set output terminal voltage.

In this embodiment, the high potential island region 40 (see FIG. 1) is not newly provided as in the first embodiment, but can be dealt with by rearranging the PNP transistor 54. There is an advantage that the area of the switching power supply IC 15 does not increase.

[0092]

As described above, according to the first aspect, in the switching power supply device in which at least the switching transistor and the overcurrent detection circuit are formed on one chip, the area where the switching transistor is formed is provided. A low-impedance region is formed between the switching transistor and the region where the overcurrent detection circuit is formed. Since a constant voltage is applied, the region is formed between the switching transistor and the low-impedance region. Even if the parasitic transistor is turned on, the parasitic transistor formed between the low impedance region and the overcurrent detection circuit does not turn on, and the overcurrent detection circuit does not erroneously detect.

According to the second aspect of the present invention, the low-impedance region easily becomes high potential by the input voltage.

According to the third aspect, in a switching power supply device in which at least a switching transistor, a drive circuit, and an overcurrent detection circuit are formed on one chip, the area where the switching transistor is formed is A PNP transistor forming a drive circuit is formed between the current detection circuit and the region where the current detection circuit is formed. Since the PNP transistor operates with a relatively large current to drive the switching transistor, the operation is affected even if a parasitic transistor formed between the switching transistor and the PNP transistor is turned ON. Nothing. Therefore, since the parasitic transistor formed between the PNP transistor and the overcurrent detection circuit does not turn on, the overcurrent detection circuit does not erroneously detect. Further, since no new element is added on the chip, the chip area does not increase.

According to the fourth invention, in a switching power supply device in which at least a switching transistor and an overcurrent detection circuit are formed on one chip, a region where the switching transistor is formed and an overcurrent detection circuit Is formed between the switching transistor and the region to which the high voltage is applied, even if the parasitic transistor formed between the switching transistor and the region to which the high voltage is applied is turned on. The parasitic transistor formed between this region and the overcurrent detection circuit does not turn on, and the overcurrent detection circuit does not erroneously detect.

Further, according to the fifth aspect, since the ferrite beads are connected between the emitter of the switching transistor and the diode, a large short-circuit current is suppressed by the reverse recovery characteristic of the diode. This reduces the amount of noise generated. Further, even though the ferrite beads are connected, the erroneous detection by the overcurrent detection circuit does not occur.

[Brief description of the drawings]

FIG. 1 is a diagram showing an arrangement of main elements of a switching power supply IC of a switching power supply according to a first embodiment of the present invention.

FIG. 2 is a sectional view of a main part of the switching power supply IC.

FIG. 3 is a diagram illustrating an arrangement of main elements of a switching power supply IC of a switching power supply according to a second embodiment of the present invention.

FIG. 4 is a sectional view of a main part of the switching power supply IC.

FIG. 5 is a block diagram of a switching power supply device.

FIG. 6 is an internal circuit diagram of the overcurrent detection circuit of the switching power supply IC.

FIG. 7 is an internal circuit diagram of a drive circuit of the switching power supply IC.

FIG. 8 is a waveform chart showing an operation of the switching power supply device in a normal state.

FIG. 9 is a waveform diagram showing an operation of the switching power supply device in an overcurrent detection state.

FIG. 10 is a diagram showing an arrangement of main elements of a switching power supply IC of a conventional switching power supply device.

FIG. 11 is a sectional view of a main part of the switching power supply IC.

FIG. 12 is a waveform diagram showing a change over time of the set output voltage and the voltage VOUT when the load is light in the switching power supply device.

FIG. 13 is a waveform diagram showing a change over time of a set output voltage and a voltage VOUT when a load is heavy in a conventional switching power supply device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Overcurrent detection circuit 2 Differential amplifier 3 Reference voltage source 4 Comparator 5 Oscillator 6 RS flip-flop circuit 7 Drive circuit 8 Comparator 9 Constant voltage source 10 Oscillation frequency change circuit 11 Input terminal 12 Output terminal 15 Switching power supply IC 16 Cross under Reference Signs List 30 parasitic NPN transistor 40 high-potential island region 54 PNP transistor 70 parasitic NPN transistor C1, C2 capacitor D1 diode F1 ferrite bead L1 coil R1, R2 resistor R3 load Tr1 output power transistor (switching transistor) VIN input Voltage

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) EY12 EY17 EY29 EZ00 EZ03 EZ04 EZ07 EZ08 EZ10 EZ28 EZ32 EZ51 FX02 FX04 FX07 FX32 FX36 GX01 GX02 GX04 GX07 GX08

Claims (5)

[Claims] 1. An NPN-type switching transistor, a drive circuit for switchingly driving the switching transistor, an overcurrent detection circuit for detecting whether a current flowing through the switching transistor is in an overcurrent state, and an overcurrent detection circuit. A circuit for controlling the drive circuit so as to suppress a current flowing through the switching transistor when the switching transistor is in a state; one end connected to an emitter side of the switching transistor and the other end connected to an output terminal for connecting to a load; A diode having a cathode connected to the emitter side of the switching transistor and a connection node of the coil and an anode connected to a predetermined voltage, and at least the switching transistor and the overcurrent detection circuit are mounted on a single chip. Switchon formed on In the power supply device, a low impedance region to which a constant voltage is applied is formed between a region where the switching transistor is formed and a region where the overcurrent detection circuit is formed. Switching power supply. 2. The switching power supply according to claim 1, wherein the constant voltage is obtained from an input voltage input to the switching power supply. 3. A drive circuit comprising: an NPN-type switching transistor; and a PNP-type transistor having a collector connected to a base of the switching transistor and an emitter connected to a collector of the switching transistor for switching-driving the switching transistor. And an overcurrent detection circuit for detecting whether or not the current flowing through the switching transistor is in an overcurrent state, and controlling the drive circuit so as to suppress the current flowing through the switching transistor when the overcurrent state is established. A circuit, one end of which is connected to the emitter side of the switching transistor and the other end of which is connected to an output terminal for connecting to a load; A switching power supply device comprising a diode connected to a predetermined voltage, wherein at least the switching transistor, the drive circuit, and the overcurrent detection circuit are formed on a single chip. Between the region where the overcurrent detection circuit is formed and the region where the overcurrent detection circuit is formed.
A switching power supply device, wherein an NP-type transistor is formed. 4. An NPN-type switching transistor, a driving circuit for switchingly driving the switching transistor, an overcurrent detection circuit for detecting whether a current flowing through the switching transistor is in an overcurrent state, and an overcurrent detection circuit. A circuit for controlling the drive circuit so as to suppress a current flowing through the switching transistor when the switching transistor is in a state; one end connected to an emitter side of the switching transistor and the other end connected to an output terminal for connecting to a load; A diode having a cathode connected to the emitter side of the switching transistor and a connection node of the coil and an anode connected to a predetermined voltage, and at least the switching transistor and the overcurrent detection circuit are mounted on a single chip. Switchon formed on A switching power supply device, wherein a region to which a high voltage is applied is formed between a region where the switching transistor is formed and a region where the overcurrent detection circuit is formed. . 5. The switching power supply according to claim 1, wherein a ferrite bead is connected between an emitter of the switching transistor and the diode.