JPH02181664A - Current sensing circuit - Google Patents

Abstract

PURPOSE:To accurately detect a current by providing a switching element between the drain electrode of a MOS transistor and a capacitor and charging by the current flowing in the transistor. CONSTITUTION:By impressing gate voltage on a gate terminal 12, the MOS transistors Tr 10a and Tr 10b are turned on and the currents are applied to the Tr 10a and Tr 10b respectively. Even if large capacity current is applied to the MOSTr 10a which is a power MOS transistor, the current can be easily detected since the current flowing in the MOSTr 10b is small. In such a case, clocks phi1-phi3 are given to the switching elements 14-18 and the capacitor 15 is charged and discharged by the current flowing in the MOSTr 10b and a reference current source. The electrode potential of the capacitor 15 is decided by a comparator 17, thereby accurately detecting the current. The above- mentioned elements 14 and 16 are provided between the drain electrode of the MOSTr 10a and Tr 10b and the capacitor 15.

Description

DETAILED DESCRIPTION OF THE INVENTION A. Field of Industrial Application The present invention relates to a current sense circuit used for overcurrent detection 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, the drain electrodes of MOS transistors (N channel) la and lb constituting the current mirror are connected to the power supply terminal 2, and their shared upper electrodes are connected to the gate terminal 3.
It is connected to the. Further, 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. 6 is a comparator for overcurrent judgment;
A resistor 5 and a MOS transistor 1 are connected to one input terminal.
A load Lo is connected to the connection point P with the source electrode of b, and a reference voltage V raf is applied to the other input terminal.
is connected between source terminal 4 and ground.

Therefore, when the MoS transistor 1a is turned on by applying a 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 to 1R (R is the resistor 5). A voltage drop corresponding to the resistance value of The voltage at the connection point P due to this voltage drop is determined by the comparator 6 as the reference voltage V re
f, and based on the comparison result, it is determined whether or not there is an overcurrent, and the detection signal is outputted from the output terminal 7 of the comparator 6.

In such a current sense circuit, MOS transistor 1
By making the channel width of b extremely smaller than that of MOS transistor 1a to reduce the current flowing through resistor 5, current detection can be performed with a simple configuration using resistor 5 with a low current capacity even in a power IC. Note that in the power IC, since a large current flows through the MOS transistor 1a, it is difficult to directly handle the current.

C0 Problems to be Solved by the Invention However, in the conventional current sensing circuit as described above, current is detected by detecting a voltage drop across a resistive element connected to the source electrode of the MOS transistor 1b constituting a current mirror. Therefore, if the voltage drop across the resistor element is ΔV=i−R (where R is the resistance value of the resistor 5 and i is the current value of the MOS transistor 1b), the detected current value is determined by the semiconductor manufacturing process. Due to variations in resistance, in the case of diffused resistors and polysilicon resistors, the resistance value R may vary by ±10%.
There were problems in that the degree of current detection varied and that accurate current detection was difficult.

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

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, in which a source electrode or a drain electrode of a first MOS transistor is connected to a load.

The above-mentioned technical problem is solved by the following configuration.

and a capacitor connected to be charged by a current flowing through the first and second MOS transistors.

The capacitor is connected between the source electrode or the drain electrode of the first MOS transistor and the capacitor, and the capacitor is charged in advance with the current flowing through the first MOS transistor at the time of current detection, and the electrode potential of the capacitor is set to the capacitor.
A first switch element that sets the potential of the load-side electrode of the S transistor, and a source or drain electrode of the second MoS transistor and a capacitor;
a second switch element that further charges the capacitor charged to the load-side electrode potential of the OS transistor by a current flowing through the second MoS transistor; a third switch element that discharges the capacitor with a constant current; and a comparator that determines the magnitude of the electrode potential of the capacitor when the capacitor is charged and discharged by the third switch element.

86 action 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 this charging and discharging is compared with a reference value in a comparator to determine whether there is an overcurrent.

Therefore, according to the present invention, accurate current detection is possible without being affected by semiconductor process variations or chip temperature.

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

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

In the figure, the first and second
The drain electrodes of the MoS transistors (N-channel MOS transistors) 10a and 10b are connected to the terminal 11 of the power supply Vo.
, whose shared gate electrode is connected to gate terminal 12
It is connected to the.

Here, the first. The second MoS transistor is connected to the first M
The current flowing through the second MOS transistor is set to be smaller than the current flowing through the OS transistor.

As a setting method, the channel width of the second MOS transistor may be made smaller than that of the first MOS transistor, or the first and second MOS transistors may be configured from a plurality of cells, and the channel width of the second MOS transistor may be smaller than that of the first MOS transistor. Second compared to transistor
The number of MOS transistor cells may be reduced. In addition, the source electrode of the MOS transistor 10a is connected to the load L
o is connected to the source terminal 13. Further, its source electrode is connected to one electrode of a charging/discharging capacitor 15 via a first switching element 14 made of a MOS transistor whose gate input is a clock φ□ (see FIG. 2), and the other electrode of the capacitor 15 is The electrode is grounded.

The source electrode of the MOS transistor lOb is connected to the clock φ
2 (see FIG. 2) is connected to one electrode of a capacitor 15 via a second switch element 16 made of a MOS transistor having a gate input. Furthermore, one electrode of the capacitor 15 is connected to the (+) terminal of the overcurrent detection comparator 17.
A reference voltage V ref is applied to the (-) input terminal of the comparator 17 . Also, capacitor 1
A series circuit of a reference current source 19 and a third switch element 18 made of a MOS transistor whose gate input is a clock φ3 (see FIG. 2) is connected between one electrode of the switch 5 and the ground.

Clock φ0. φ2. φ is generated from a clock generation circuit (not shown).

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

First, when a gate voltage is applied to the gate terminal 12, M
OS transistors 10a and 10b are turned on, and current flows through each MOS transistor 10a (for example, a power MOS transistor) and MOS transistor 10b. At this time, if the current flowing through the MOS transistor 10a is ia, then the current ib flowing through the MOS transistor 10b is as follows: 1b=a-ia (1). However, a is the MOS transistor 10a.

10b. If we set a to 1 here,
ib<<ia, and even if the MOS transistor 10a is a power MOS transistor and passes a large amount of current, ib is small, so the current can be easily detected.

When detecting the current, the clock φ19. shown in the time chart of FIG. 2 is used. φ2. φ, for each switch element 1
4, 16.18, the capacitor 15 is charged and discharged by the current flowing through the MOS transistor 10b and the reference current source 19, and the electrode potential of the capacitor 15 at this time is determined by the comparator 17.
Overcurrent is detected by making a judgment.

First, the switch element 14 is turned on by the high level of the clock φ□ applied to the gate of the switch element 14, thereby setting the potential of the connection point A, that is, the electrode potential of the capacitor 15, to the voltage Vs of the source terminal 13. Then clock φ
1 is set to low level, the clock φ2 shown in FIG. 2(b) is activated (H) and the second switch element 16 is activated.
When the current ib is applied to the gate of the clock φ2, the switch element 16 is turned on for the active time of the clock φ2, and a current ib flows, thereby charging the capacitor 15.

At this time, if the active time of the clock φ2 is 0 and the capacitance of the capacitor 15 is C, then the rising potential ΔV□ of the electrode potential of the capacitor 15 becomes ΔV,=ib-to/C-(2).

Next, after setting the clock φ2 to low level, as shown in FIG.
When the clock φ1 is made active (H) for the same time as the active time to of the clock φ2 at the timing shown in Q), the switch element 18 is turned on for that time, and the electrode (connection point A) of the capacitor 15 is connected to the reference current source 19. connected to the battery and discharged with a constant reference current. At this time, if the current of the reference current source 19 is ir and the active time of the clock φ is to, then the voltage drop Δv at the electrode of the capacitor 15
2 becomes ΔV,=ir-to/C-(3).

Therefore, the change ΔV in the electrode potential of the capacitor 15 in one cycle shown in FIG. 2 is ΔV=ΔV, -ΔV,
=(i b - i r)to/C-(4).

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 rises, and the voltage applied to the (+) input terminal of the comparator 17 becomes the reference voltage. When the voltage exceeds V ref, a detection signal is output from the comparator 17 to the output terminal 17a.

In this embodiment, the capacitance of the capacitor 15, which is charged and discharged by the source current ib and the reference current ir, can be maintained even if the capacitor 15 is charged and discharged by the source current ib and the reference current ir. Since C and the active time to for charging and discharging are common, the current i
Variation components of the capacitors and the like when comparing b and ir are canceled out, and currents ib and ir can be accurately compared, and accurate current detection becomes possible.

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

This FIG. 3 differs from FIG. 2 described above in that the clock φ2.
The clock φ3 applied to the gate of the third switch element 18 has a waveform that continues for a predetermined period of time in response to the low level of the clock φ, as shown in FIGS. 3(b) and 3(c).

Lock φ2 like this. When the switch elements 16 and 18 are controlled using φ, the same effect as in the above embodiment can be obtained. Note that the method shown in FIG. 2 can save more current consumption.

Furthermore, FIG. 4 is a circuit diagram when an N-channel MOS transistor is used as a low-side switch.
The operation is similar to the case of FIG. 1 in which 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 part numbers as in FIG. 1 are given the same reference numerals, and the explanation thereof will be omitted, and the parts different from FIG. 1 will be mainly described.

That is, FIG. 5 differs from FIG. 1 in the following two points.

■Resistor 20 between the first switch element 14 and connection point A
are connected in series, and this resistor 20 allows the electrode of the capacitor 15 to receive the influence of potential fluctuations at the source terminal 13 due to noise or the like.

■Third switch element 18 for discharging between connection point A and ground
When the clock φ' applied to the gate of the third switching element 18 is active, the switching element 18 is in the saturation region, so that the switching element 18 functions as a constant current source. did.

In this second embodiment, in addition to obtaining the same effects as in the first embodiment, the electrode potential of the capacitor 15 is less susceptible to potential fluctuations at the source terminal 13, and the reference current source can be omitted. effective.

G1 Effect of the invention According to the invention, the first and second
A capacitor is connected to the MoS transistor through a switch element, and after this capacitor is charged to the load side electrode potential of the first MOS transistor, the second MOS transistor is charged with a current flowing through it and discharged with a constant current. Since the current is detected by determining the magnitude of the electrode potential of the capacitor, the current can be detected accurately and with high precision without being affected by variations in the semiconductor process.

[Brief explanation of the drawing]

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 the switch control clock in this 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 shown in FIG. 1. FIG. 5 is a block diagram showing a second embodiment of the current sensing circuit according to the present invention. FIG. 6 is a block diagram of a conventional current sense circuit. 10a, 10b: MOS transistor 11: Power supply terminal 13: Source terminal: First switch element: Second switch element: Third switch element Two reference current sources 15: Capacitor 17: Comparator Patent applicant Nissan Motor Co., Ltd.

Claims (1)

[Scope of Claims] A current sensing circuit having first and second MOS transistors sharing a gate electrode, wherein a source electrode or a drain electrode of the first MOS transistor is connected to a load, comprising: a capacitor connected to be charged by the current flowing through the second MOS transistor; and a capacitor connected between the source electrode or the drain electrode of the first MOS transistor and the capacitor, the capacitor being connected to the first MOS transistor in advance at the time of current detection. a first switch element that charges the first MOS transistor with a current flowing through it and sets the electrode potential of the capacitor to the load-side electrode potential of the first MOS transistor; a source electrode or drain electrode of the second MOS transistor; connected between the capacitor and the first
a second switch element that further charges a 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 that discharges the capacitor with a constant current; A current sensing circuit comprising: a comparator that determines the magnitude of the electrode potential of the capacitor when the capacitor is charged and discharged by second and third switching elements.