JPH06180332A - Current detection circuit - Google Patents

Abstract

PURPOSE:To detect the electric current flowing to an output transistor with high accuracy. CONSTITUTION:The title circuit is constituted in such a way that a current detecting transistor Q2 which is resemble to an output transistor Q1 and has a current driving ability which is 1/N of that of the transistor Q1 is provided and an active load A which generates an electric current following the voltage drop of the transistor Q2 so that the voltage drop of the transistor Q1 can become equal to that of the transistor Q2 is connected to the transistor Q2 as the load of the transistor Q2. In addition, the electric current which is 1/N of the electric current flowing to the transistor Q1 can be detected with high accuracy and a current detecting voltage proportional to the current can be obtained by means of a current detecting resistor.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detecting a load current of a power transistor that allows a large current to flow.

[0002]

2. Description of the Related Art A conventional detection circuit of this type is shown in FIG.
A transistor Q1 (or Q1b) that outputs a main current (most of the output current Io) as shown in (a) and (b).
And a drain (or collector) and a control electrode (gate or base) connected in common to the transistor Q1 (or Q1b) and a small current detecting transistor Q2 (or Q2b) having similar characteristics, and a drain (or The collector) is connected to the output terminal, the control electrode (gate or base) is connected to the control input terminal, and the source of transistor Q2 (or transistor Q2).
The current detection resistor Rs was connected between the emitter of 2b) and the drain of the transistor Q1 (or the emitter of the transistor Q1b), and the source of the transistor Q1 (or the emitter of the transistor Q1b) was connected to GND.

In this operation, when the current detecting resistor Rs is small, the transistors Q1 and Q2 (or the transistors Q1b and Q2b) become a current mirror operation, and the current Io flowing through the output terminal is the transistor Q1 (or Q1b) and the transistor Q2. (Or Q2
When the relative ratio of b) is N: 1, a current of Io / (N + 1) flows in the transistor Q2 (or Q2b). Here, when N is much larger than 1, it becomes Io / N.

A detection voltage (Vs) Vs = Io.1 / N.Rs is obtained across the current detection resistor Rs.

[0005]

According to the conventional example of FIG. 5, the current flowing through the current detecting transistor Q2 (or Q2b) is proportional to the output current Io when Rs = 0, and it is desired to increase the detection voltage Vs. In this case, Rs had a practical value, and the proportional relationship was broken and the detection accuracy was poor.

This is because the gate-source potential of the transistor Q2 or the base-emitter potential of the transistor Q2b is lowered by the current detection voltage Vs.

[0007]

According to the present invention, the operating conditions of the transistors Q1 (or Q1b) and Q2 (or Q2b), that is, the gate-source (or base-emitter) voltage and the drain-source (or collector- An active load that acts so as to make the voltages between the emitters equal is used as the load of the transistor Q2 (or Q2b), and the output current of the active load is proportional to the output current. It was configured.

[0008]

According to the above structure, the transistor Q1 (or Q
1b) and the transistor Q2 (or Q2b) relative ratio 1
A large detection voltage which is almost completely proportional to / N can be generated across the current detection resistor.

[0009]

EXAMPLE FIG. 1A shows a first example. Q1 is an N-channel MOS transistor for main current output, Q2 is an N-channel MOS transistor for current detection, and the relative ratio is set to N: 1. The drain is commonly connected to the power supply terminal, the gate is commonly connected to the control input terminal, and the source of the transistor Q1 is connected to the output terminal.
There is a load between and. The source of the transistor Q2 is an N-channel MOS that outputs the current of the active load A.
The drain of the transistor Q3 is connected, and the source of the transistor Q3 is connected to GND through the current detection resistor Rs.

The gate of the transistor Q3 is an operational amplifier O.
The output of P is connected, the + input terminal of the operational amplifier OP is connected to the source of the transistor Q2, and the − input terminal of the operational amplifier OP is connected to the source of the transistor Q1.

According to this structure, even if the load current Il due to the external load changes and the drain-source voltage of the transistor Q1 changes, the source potentials of the transistors Q1 and Q2 are always equal to each other. Since the gate of Q3 is controlled, transistors Q1 and Q2
Performs a current mirror operation close to ideal, and transistor Q
A current of I _L / N can be applied to 2. Therefore, Vs = I _L / N × RS is obtained as a detection voltage (Vs) across the current detection resistor Rs.

In FIG. 1B, NPN transistors Q1b and Q1 are used instead of N channel MOS transistors Q1 and Q2.
This is an example using 2b.

Since the operation is similar to that of the example shown in FIG. 1A, the description thereof will be omitted.

[0014]

[Embodiment 2] FIGS. 2A and 2B show a second embodiment, in which FIG. 2A shows P channel MOS transistors Q1 and Q.
The two sources are connected in common, and the drain is connected to the power source at the output terminal and the current output transistor Q3 of the active load A, respectively.
Of the operational amplifier OP and the transistor Q3 so that the drain voltages of the transistors Q1 and Q2 are equal to each other.

FIG. 2B shows an example in which the P-channel MOS transistors Q1 and Q2 are PNP transistors Q1b and q2b. The operation is similar to the example of FIG.

[0016]

[Third Embodiment] FIGS. 3A and 3B show a third embodiment. The difference from the first and second embodiments is that the transistors Q1 (or Q1b) and Q2 (or Q2b) are arranged on the GND side, and accordingly, the current output transistor Q3 forming the active load A is a P-channel MOS transistor. The point where the counter Rs for current detection is connected to the power supply side. Since the operation is similar, it will be omitted.

[0017]

Fourth Embodiment FIG. 4 shows a fourth embodiment. This circuit is the first
The source of the current output transistor Q3 of the active load A of the above embodiment is directly connected to GND, and a transistor Q4 having a gate and a source commonly connected to the transistor Q3 is provided. In the example in which is inserted, a current proportional to the current flowing in the transistor Q3 is obtained from the transistor Q4.

This construction can be similarly constructed in the second to fourth embodiments.

[0019]

As described above, by adopting the configuration according to the present invention, the current detection voltage proportional to the load current can be taken out with high efficiency and degree of configuration.

[Brief description of drawings]

FIG. 1 is a first embodiment.

[FIG. 2] Second embodiment

FIG. 3 Third embodiment

FIG. 4 is a fourth embodiment.

FIG. 5 Conventional example

[Explanation of symbols]

 Q1, Q1b output transistor Q2, Q2b current detection transistor Q3 active load current output transistor Q4 active load current shunt transistor OP operational amplifier circuit A active load circuit Rs current detection resistor

Claims (3)

[Claims] 1. A first transistor that outputs a main current, and a second transistor for current detection that is small but similar in characteristics to the first transistor. The second transistor has a control electrode (gate or base) commonly connected and a drain. (Or collector) or source (or emitter) is commonly connected to one of the power supplies, the other of the first transistors is connected to the other of the power supplies via a load, and The other is connected to the other of the power supply through an active load, and the active load makes the other potential of the second transistor equal to the other potential of the first transistor. Current detection circuit. 2. The current detection circuit according to claim 1, wherein the active load is a series circuit of a third transistor for current output and a current detection resistor arranged in the power source. 3. A third transistor for current output of an active load, a similar fourth transistor mirror-connected to the third transistor is provided, and an active load output current detection resistor is inserted in the output terminal of the fourth transistor. The described current detection circuit.