JPH04134271A - Output circuit - Google Patents

Abstract

PURPOSE:To quicken the response in over-current sensing, by generating a reference voltage between a transistor for comparison and a constant current circuit. CONSTITUTION:The over-current sensing value of the current 10 flowing in an output terminal 100 can be represented by I/O=IS.n when VS=VO, where (n) derives from the ratio n:1 of the transistor 3 size to the transistor 2 size, and IS represents the current value flowing in a constant current circuit 4. Therein the output transistor 3 and the transistor 2 for comparison have the same characteristic in the transient where the output transistor 3 transfers from off to on, or vice versa, and also are driven by the same driver circuit 1, so that the voltage at the transient varies in the same manner, wherein there is no fear of mis-judging likely according to the conventional system. Therefore, it is no more necessary to prohibit a current sensing signal 101 in the transient from off to on, or vice versa. Accordingly a comparator can judge the overcurrent at the output terminal irrespective of voltage variation in the transient of the output transistor.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an output circuit included in a semiconductor device or the like, and more particularly to an overcurrent detection circuit that detects a current flowing through an output terminal.

[Prior Art] Conventionally, this type of output circuit includes a MOS transistor 3 controlled by a driver circuit 1, as shown in FIG. In addition, the potential VO of the output terminal 300 is compared with the reference voltage Vr generated by the constant voltage circuit 7 by the comparator 5. For example, if the on-resistance of the MOS transistor 3 is RON, and the current flowing to the output terminal 300 is 10, then ■0=RON・■0, and the current detection signal of the comparator 5 is Vr=VO=RON-LO. Then, it will be reversed. Therefore, the current detection value of this circuit is l0=Vr/RON
When IO>Vr/RON, it is determined that there is an overcurrent.

[Problems to be Solved by the Invention] The conventional output circuit described above compares the output terminal voltage VO with the reference voltage V', and when Vr<VO, it is determined that the current is a steady current value, and when Vr>V2O, the current value is determined to be a steady current value. It is determined that there is an overcurrent.

This functions normally as long as the output transistor is sufficiently turned on, but when the output is turned off, Vr > VO and an overcurrent is determined, so it is necessary to disable the current detection signal when the output is turned off. There is. MOS transistor 3
Since the comparator 5 makes an erroneous judgment during the transition period when the i\! transitions from off to on or from on to off, it is necessary to prohibit the judgment of the comparator 5 during this transition period as well. There was a problem in that the detection of true overcurrent was delayed during the transition period.

[Means for Solving the Problems] The gist of the present invention is to provide a driver circuit, an output transistor connected to a first voltage source and controlled by the driver circuit,
A load connected between the output transistor and the second voltage source, and a comparator that compares the voltage of the output terminal with a reference voltage in order to detect an overcurrent flowing through the output terminal provided between the output transistor and the load. In the output circuit, a comparison transistor and a constant current circuit are connected in series between the first voltage source and the second voltage source, and the reference voltage is generated between the comparison transistor and the constant current circuit. In addition, the characteristics of the comparison transistor are made equal to those of the output transistor, the transistor size of the comparison transistor is made different from the transistor size of the output transistor, and the comparison transistor is controlled in synchronization with the output transistor. be.

[Operation of the Invention] The output transistor and the comparison transistor are synchronously switched between the on state and the off state.

Although the output transistor and the comparison transistor are different, since the characteristics of the transistors are the same, the voltage change occurring at the output terminal during the switching transition period is similar to the voltage change of the reference voltage. Therefore, the comparator can determine the overcurrent at the output terminal regardless of voltage fluctuations during the transition period of the output transistor.

[Example] Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a circuit diagram of a first embodiment of the present invention. 1 is a driver circuit, 3 is an output transistor, 2 is a comparison transistor which has the same characteristics as the output transistor 3 but has a different transistor size, 4 is a constant current circuit, 5 is a comparator, 6 is a load, and 100 is an output terminal. This is an example of a high side switch. When the transistor size ratio of transistor 3 and transistor 2 is n:1, and the current value flowing through constant current circuit 4 is IS, output terminal 100
The overcurrent detection value of the current IO flowing in is when vs=vo, and therefore can be expressed as l10=IS-n. In the case of this embodiment, during the transition period when the output transistor 3 transitions from off to on and from on to off, the output transistor 3 and comparison transistor 2 have the same characteristics and are driven by the same driver circuit. Therefore, the voltage during the transition period also changes in the same way, and there is no such determination as in the conventional method. Therefore, there is no need to inhibit the current detection signal 101 during the transition period from off to on and from on to off.

FIG. 2 is a circuit diagram showing a second embodiment of the present invention.

In this embodiment, the output format is a low-side switch, but the configuration and operation are similar to those in the first embodiment, so corresponding components will only be designated with the same reference numerals and a description thereof will be omitted.

[Effects of the Invention] As explained above, the present invention includes an output transistor and a second transistor having the same characteristics as the output transistor and a different transistor size, and the output electrode of the output transistor is used as an output terminal to connect a load. A constant current circuit is connected to the output electrode of the second transistor, the gate is driven by the same driver circuit, and the output is switched from OFF to ON by comparing the potentials of each output electrode and detecting the current. Prohibiting the on→off transition period is no longer necessary,
This has the effect of speeding up the overcurrent detection response.

If it is a conventional circuit, the overcurrent detection response speed is several tens to several tens.
Although a delay of 0 μs occurs, according to the present invention, the delay time can be reduced to several μs or less due to the response speed of the comparator.

[Brief explanation of drawings]

FIG. 1 is an equivalent circuit diagram showing a first embodiment of the present invention, FIG. 2 is an equivalent circuit diagram showing a second embodiment of the invention, and FIG. 3 is an equivalent circuit diagram of a conventional example. 1 ・ ・ ・ ・ tsu001. . 3 ・ ・ ・ ・ ・ 4− ・ ・ ・ 5 φ ・ ・ ・ 6 φ ・ ・ ・ ・ 7 ・ ・ ・ ・ ・ 100 ・ ・ ・

Claims (2)

[Claims] (1) A driver circuit, an output transistor connected to the first voltage source and controlled by the driver circuit, a load connected between the output transistor and the second voltage source, and an output transistor provided between the output transistor and the load. In an output circuit having a comparator that compares the voltage of the output terminal with a reference voltage in order to detect an overcurrent flowing through the output terminal, a comparison transistor and a constant current are connected between the first voltage source and the second voltage source. The circuit is connected in series to generate the reference voltage between the comparison transistor and the constant current circuit, and
The characteristics of the comparison transistor are made equal to the characteristics of the output transistor, the transistor size of the comparison transistor is made different from the transistor size of the output transistor, and the comparison transistor is controlled in synchronization with the output transistor. Output circuit. (2) The output circuit according to claim 1, wherein the comparison transistor is controlled by the driver circuit.