JP2764984B2 - Current sense circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a current sensing circuit used for detecting an overcurrent in a power IC.

B. Prior Art FIG. 6 shows a conventional current sensing circuit known, for example, from US Pat. No. 4,553,084.

In the figure, drain electrodes of MOS transistors (N-channel) 1a and 1b constituting a current mirror are connected to a power supply terminal 2
, And the common gate electrode is connected to the gate terminal 3. The source electrode of the MOS transistor 1a is connected to the source terminal 4, and the source electrode of the MOS transistor 1b is connected to the source terminal 4 via the current detection resistor 5. Reference numeral 6 denotes a comparator for judging an overcurrent. One input terminal of the comparator 6 is connected to a connection point P between the resistor 5 and the source electrode of the MOS transistor 1b, and the other input terminal has a reference voltage Vref.
Has been added. In addition, it is turned on by MOS transistor 1a.
The load Lo to be turned off is connected between the source terminal 4 and the ground.

Therefore, when the MOS transistor 1a is turned on by the application of the gate voltage, the voltage at the connection point P becomes the voltage at the power supply terminal 2 and the current i flowing through the MOS transistor 1b flows through the resistor 5, so that iR (R is ) Occurs. The voltage at the connection point P due to this voltage drop is compared with the reference voltage Vref by the comparator 6, it is determined whether or not an overcurrent is present based on the comparison result, and the detection signal is output from the output terminal 7 of the comparator 6. It has become.

In such a current sensing circuit, the MOS transistor 1b
If the current flowing through the resistor 5 is reduced by making the channel width of the resistor 5 extremely smaller than that of the MOS transistor 1a, current detection can be performed with a simple configuration using the resistor 5 having a low current capacity even in a power IC. In the power IC, the MOS transistor 1a
Therefore, it is difficult to directly measure the current.

C. Problems to be Solved by the Invention However, in the conventional current sensing circuit as described above, current detection is performed by detecting a voltage drop of a resistance element connected to the source electrode of the MOS transistor 1b constituting a current mirror. Because of the system, the voltage drop of the resistance element is represented by ΔV = i · R (where R is the resistance value of the resistor 5 and i is the MOS
(The current value of the transistor 1b), the detected current value varies by about ± 10% in the case of a diffused resistor or a polysilicon resistor due to a variation in a semiconductor manufacturing process, and it is difficult to accurately detect a current. There was a problem.

A technical object of the present invention is to accurately perform current detection without being affected by variations in a semiconductor manufacturing process.

D. Means for Solving the Problems The present invention provides first and second MOs sharing a gate electrode.
The present invention is applied to a current sensing circuit having an S transistor and having a source electrode or a drain electrode of a first MOS transistor connected to a load.

The technical problem described above is solved by the following configuration.

A capacitor connected between a capacitor connected to be charged with a current flowing through the first and second MOS transistors and a source electrode or a drain electrode of the first MOS transistor, and a capacitor connected in advance when a current is detected; Is charged with a current flowing through the first MOS transistor to set the electrode potential of the capacitor to the load-side electrode potential of the first MOS transistor.
A second switch element connected between the source electrode or the drain electrode of the transistor and the capacitor and further charging the capacitor charged to the load-side electrode potential of the first MOS transistor by the current flowing through the second MOS transistor; And a third switch element for discharging the capacitor with a constant current, and a comparator for determining the magnitude of the electrode potential of the capacitor when the capacitor is charged and discharged by the second and third switch elements.

E. Operation When the first switch element is turned on, the electrode potential of the capacitor becomes the load-side electrode potential of the first MOS transistor. When the first switch element is turned off and the second switch element is turned on, the capacitor is charged by the current flowing through the second MOS transistor, and when the third switch element is turned on, the capacitor is charged with a constant current. Discharge. The electrode potential of the capacitor due to the charging and discharging is compared with a reference value in a comparator to determine whether or not an overcurrent occurs.

Therefore, in the present invention, accurate current detection can be performed without being affected by variations in semiconductor processes and chip temperatures.

F. Examples Hereinafter, examples of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a configuration diagram showing a first embodiment of a current sensing circuit according to the present invention.

In the figure, first and second MOS transistors (N-channel MOS transistors) constituting a current mirror 10
a, a drain electrode of 10b are connected to terminal 11 of the power source V _D,
The common gate electrode is connected to the gate terminal 12. Here, the first and second MOS transistors are the first MOS transistor.
The current flowing through the second MOS transistor is set to be smaller than the current flowing through the transistor. As a setting method, the channel width of the second MOS transistor may be smaller than that of the first MOS transistor,
The first and second MOS transistors are composed of a plurality of cells,
The number of cells of the second MOS transistor may be smaller than that of the first MOS transistor. Also, MOS transistor 1
The source electrode 0a is connected to the source terminal 13 to which the load Lo is connected. Further, the source electrode is connected to the clock φ ₁
2 (see FIG. 2) is connected to one electrode of a charge / discharge capacitor 15 via a first switch element 14 composed of a MOS transistor having a gate input, and the other electrode of the capacitor 15 is grounded.

The source electrode of the MOS transistor 10b has a clock φ
₂ (see FIG. 2) is connected to one electrode of a capacitor 15 via a second switch element 16 composed of a MOS transistor having a gate input. Further, one electrode of the capacitor 15 is connected to the (+) input terminal of the comparator 17 for overcurrent detection, and the reference voltage Vref is applied to the (−) input terminal of the comparator 17. A series circuit of a third switch element 18 composed of a MOS transistor having a gate input of clock φ ₃ (see FIG. 2) and a reference current source 19 is connected between one electrode of the capacitor 15 and the ground. ing.

The clocks φ ₁ , φ ₂ , φ ₃ are generated from a clock generation circuit (not shown).

Next, the operation of the present embodiment configured as described above will be described.

First, when a gate voltage is applied to the gate terminal 12, the MO
The S transistors 10a and 10b are turned on, and a current flows through each of the MOS transistors 10a (for example, power MOS transistors) and the MOS transistor 10b. At this time, MOS
Assuming that the current flowing through the transistor 10a is ia, the current ib flowing through the MOS transistor 10b is ib = a · ia (1). Here, a is the size ratio of the MOS transistors 10a and 10b. Here, if a << 1, ib << ia, and even if the MOS transistor 10a is a power MOS transistor and flows a large amount of current, the current ib can be easily detected because ib is small.

At the time of current detection, the clocks φ ₁ , φ ₂ , φ ₃ shown in the time chart of FIG.
The capacitor 15 is charged and discharged by the current flowing through the MOS transistor 10b and the reference current source 19, and the comparator 17 determines the electrode potential of the capacitor 15 at this time to detect overcurrent.

First, turns on the switch element 14 in the clock phi ₁ of the high level applied to the gate of the switching element 14, thereby the potential of the node A, that is, the electrode potential of the capacitor 15 to the voltage Vs of source terminal 13. Then the addition of the clock phi ₁ to the gate of the clock phi ₂ shown in FIG. 2 in a state of a low level (b) to activate (H) a second switch element 16, switch only Ann active time of the clock phi ₂ The element 16 is turned on, and the current ib flows, whereby the capacitor 15 is charged. At this time, the active time of the clock phi ₂ and-to, and the capacitance of the capacitor 15 is C, the potential rise [Delta] V ₁ of the electrode potential of the capacitor 15 becomes _{ΔV 1 = ib · to / C} ... (2).

Next, clock phi ₂ after the low level, the only clock phi ₃ same time as the active time to the clock phi ₂ at the timing shown in FIG. 2 (c) to activate (H), the switch element only that time 18 turns on, and the electrode (connection point A) of the capacitor 15 is connected to the reference current source 19 to discharge at a constant reference current. At this time, the current of the reference current source 19 and ir, the active time of the clock phi ₃ and-to, the voltage drop [Delta] V ₂ electrodes of the capacitor 15 becomes _{ΔV 2 = ir · to / C} ... (3).

Therefore, in one cycle shown in FIG. 2, the change ΔV in the electrode potential of the capacitor 15 is as follows: ΔV = ΔV ₁ −ΔV ₂ = (ib−ir) to / C (4)

Here, when ib> ir, when the capacitor 15 is charged and discharged at the timing shown in FIG. 2, the electrode potential of the capacitor 15 gradually increases, and the voltage applied to the (+) input terminal of the comparator 17 becomes the reference voltage. When Vref is exceeded, a detection signal is output from the comparator 17 to the output terminal 17a.

In the present embodiment, even if the capacitor 15 charged / discharged by the source current ib and the reference current ir varies from one device to another in the manufacturing process, or the temperature changes, the capacitance of the capacitor 15 Since C and the active time to for charging / discharging are common, the variation component of the capacitor when comparing the currents ib and ir is cancelled, and the currents ib and ir can be accurately compared with each other. , Accurate current detection becomes possible.

FIG. 3 shows another embodiment of a switch control timing chart according to the present invention.

3 is different from FIG. 2 in that FIG.
The clock φ ₂ applied to the gate of the switch element 16 and the clock φ ₃ applied to the gate of the third switch element 18 are changed to the low level of the clock φ ₁ as shown in FIGS. 3 (b) and (c). , A waveform that is continuous for a predetermined time.

A switch element using such clocks φ ₂ and φ ₃
The same effects as in the above-described embodiment can be obtained also when the control of 16, 18 is performed. Note that the method shown in FIG. 2 can save current consumption.

FIG. 4 is a circuit diagram when an N-channel MOS transistor is used as a low-side switch.
The operation is the same as in the case of FIG. 1 where the channel MOS transistor is used as a high side switch.

Second Embodiment FIG. 5 is a block diagram showing a second embodiment of the current sensing circuit according to the present invention.

In the figure, the same parts as those in FIG. 1 are denoted by the same reference numerals, and the description thereof will be omitted, and parts different from FIG. 1 will be mainly described.

That is, FIG. 5 differs from FIG.
Is a point.

A resistor 20 is connected in series between the first switch element 14 and the connection point A, and the resistance 20 makes it difficult for the electrode of the capacitor 15 to be affected by a potential change of the source terminal 13 due to noise or the like.

Connect only the third switching element 18 for discharge between the ground and the connection point A, the switching element clock phi _'3 applied to the gate of the third switch element 18 when active
The switch element 18 is made to function as a constant current source by setting the switch 18 to a saturation region.

In the second embodiment as well, the same effects as those of the first embodiment can be obtained, the electrode potential of the capacitor 15 is hardly affected by the fluctuation of the potential of the source terminal 13, and the reference current source can be omitted. effective.

G. Effects of the Invention According to the present invention, the first and second
A capacitor is connected to the MOS transistor through a switch element, and the capacitor is charged to the load-side electrode potential of the first MOS transistor, and then charged by the current flowing through the second MOS transistor and discharged by a constant current. Since the current detection is performed by determining the magnitude of the electrode potential of the capacitor, the current can be detected accurately and accurately without being affected by variations in the semiconductor process.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first embodiment of a current sensing circuit according to the present invention. FIG. 2 is a timing chart of a switch control clock in the present embodiment. FIG. 3 is a timing chart of a switch control clock showing another embodiment of the present invention. FIG. 4 is a circuit diagram showing a modification of the circuit of FIG. FIG. 5 is a block diagram showing a second embodiment of the current sensing circuit according to the present invention. FIG. 6 is a configuration diagram of a conventional current sensing circuit. 10a, 10b: MOS transistor 11: power supply terminal, 13: source terminal 14: first switch element, 15: capacitor 16: second switch element, 17: comparator 18: third switch element 19: reference current source

Claims (1)

(57) [Claims] A first and a second MO sharing a gate electrode
In a current sense circuit having an S transistor and having a source electrode or a drain electrode of a first MOS transistor connected to a load, a capacitor connected so as to be charged by a current flowing through the first and second MOS transistors Is connected between the source electrode or the drain electrode of the first MOS transistor and the capacitor, and when the current is detected, the capacitor is charged in advance with the current flowing through the first MOS transistor, and the electrode potential of the capacitor is set to this value. First
A first switch element for setting a potential of a load-side electrode of a MOS transistor; a first switch element connected between a source electrode or a drain electrode of the second MOS transistor and the capacitor;
A second switch element for further charging the capacitor charged to the load-side electrode potential of the MOS transistor with a current flowing through the second MOS transistor; a third switch element for discharging the capacitor with a constant current; And a comparator for determining the magnitude of the electrode potential of the capacitor when the capacitor is charged and discharged by the second and third switch elements.