JPH02266407A - Constant current circuit - Google Patents

Abstract

PURPOSE:To correct the variance of a constant current value due to the variance of a process and to attain a highly accurate constant current circuit by adding a resistor to an ordinary constant current circuit and trimming the resistor by laser beams. CONSTITUTION:A resistor 7 is inserted between the source of a constant current transistor(TR) and a ground and the substrate of the TR 2 is connected to the ground potential. When a link member is cut out by laser beams from the lower part of the member 13, a resistance values of resistors 11 to 8 are added to a resistor 12 in order, so that the value of the resistor 7 is changed. When the resistor 7 is trimmed by laser beams while monitoring the constant current value I by said method, the value I can be allowed to coincide with a required constant current value. Thus, the highly accurate constant current circuit can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a constant current circuit formed into a monolithic IC, and in particular, a constant current value is adjusted by a laser trimming method.

(Summary of the Invention) The present invention corrects variations in constant current values due to process variations by adding a resistor to a conventional constant current circuit and trimming the resistor with a laser beam, thereby realizing a highly accurate constant current circuit. It is something to do.

[Conventional technology]

In order to explain a conventional constant current circuit, a constant current charging circuit, which is an application of the conventional constant current circuit, will be explained as an example. Figure 2 shows a constant current charging circuit using a conventional constant current circuit.
A reference voltage l is input to the constant current transistor 2, and the transistor 3 connected to saturation is used as a load of the constant current transistor 2. Therefore, the current {circle around (1)} flowing through transistors 2 and 3 is expressed by equation (1).

1-K CV, -t VtN)" -...
-(1) Here, K is the conductivity coefficient, V r *f is the voltage of the reference voltage 1, and vyH is the dark value voltage of the transistor 2.

K, V, , and VTN are power supply voltages ■. Since the current flowing through transistors 2 and 3 is constant regardless of
It becomes i. However, the values of transistors 2 and 3 are set so that constant current transistor 2 operates in the saturation region. Furthermore, the gate of transistor 3 is
Therefore, under conditions where the values of transistors 3 and 4 are equal and transistor 4 operates in the saturation region, the current flowing through transistor 4 is 2 (
1). This is called a current mirror circuit. The drain or output terminal 5 of the transistor 4 is connected to a capacitor 6. The transistor 4 and capacitor 6 constitute a constant current charging circuit. At this time, the time change of the output terminal 5 is expressed by the following formula (2
).

t ■=□ --... (2) With a smile, ■ is the voltage of output 5 terminal, ■ is the constant current value of equation (1), t is time, and C is the capacitance value of capacitor 6. . Therefore, since I and C are constant, the voltage at output terminal 5 is
It becomes a linear function of time t.

However, when this circuit is made into a monolithic IC, the value of the transistor, the dark value voltage, and the reference voltage value V rat vary due to process variations. ) also varies depending on the process because the constant current value I, which is its coefficient, varies depending on the process.
It cannot be used for C.

[Means for Solving the Problems] The present invention aims to solve the problems of the conventional technology, and achieves this by obtaining a highly accurate constant current circuit and realizing a highly accurate constant current charging circuit. could be used for IC.

Specifically, by inserting a resistor between the source of the constant current transistor 2 and the ground, and trimming this with a laser beam, the voltage drop generated across the resistor lowers the threshold voltage of the constant current transistor 2 to the 4ft'tl voltage. The constant current value is adjusted to a desired value by changing the constant current value.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the drawings.

Figure 1 shows the constant current light according to the present invention! It is a constant current circuit diagram with a function. A resistor 7 is inserted between the source of the constant current transistor 2 and the ground, and the substrate of the transistor 2 is connected to the ground potential.

Next, the circuit operation will be explained. When the current flowing through the constant current transistor 2 flows through the resistor 7, a voltage drop occurs. Let this voltage drop be Δ■. When this Δ■ occurs, since the source and substrate of the constant current transistor 2 are connected through the resistor 7, the bank gate effect works, and as a result,
The dark value voltage Va of the constant current transistor 2 increases, and becomes approximately as shown in the following equation.

Here, ■1. ' is the dark value voltage of transistor 2 after rising. That is, by adjusting the value of the resistor 7, ΔV changes and the threshold voltage can also be changed. therefore,
If the process-related variations in the value and reference voltage of Equation +11 are corrected by the threshold voltage, a constant constant 'r12
The it value I can be obtained.

As a result, the time coefficient in equation (2) can be made uniform.

Next, a method of adjusting the value of the resistor 7 using a laser beam will be explained. FIG. 3 shows a shape diagram of a resistor 7 formed by polysilicon or diffusion.
It is divided into 12 parts and connected in series. A link member 13 that can be cut by a laser beam from the connection point of each resistor
are connected, and the other ends of the link members 13 are all connected to the sources of the transistors 2. In this state, the link member 13 has a resistance value considerably lower than the resistance value of the resistors 8 to 12, so that it is equivalent to having the resistors 8 to 11 short-circuited. Therefore, the resistor 7 has the value of the resistor 12. Next, if the link member 13 is cut by a laser beam from below, the resistance 1110, 9.
Since the resistance value is added to resistor 12 in the order of resistor 8, resistor 7
The value of will change. By this method, by trimming the resistor 7 with a laser beam while monitoring the constant current value (2), it is possible to adjust the constant current value to a desired value.

〔Effect of the invention〕

As described above, according to the present invention, by inserting a resistor between the source of the constant current transistor and the ground, the dark value voltage of the cough transistor can be changed by the back gate effect, and the amount of change in the threshold voltage can be changed. can be adjusted by trimming the resistance with a laser beam. As a result, a high-precision constant current circuit can be realized, and this circuit can be used to create a monolithic I/O circuit with high added value.
This has the effect of realizing C.

[Brief explanation of drawings]

FIG. 1 is a constant current circuit diagram of the present invention, FIG. 2 is a conventional constant current circuit diagram, and FIG. 3 is a shape diagram of a resistor section for laser trimming. 2, 3゜6 ・ ・ ・ 7 ・ ・ ・ 13 ・ ・ ・ 4... Transistor, capacitor, resistor, link member and above Applicant: Seiko Electronic Industries Co., Ltd. Representative Patent attorney Takayuki Hayashi Run-up radio circuit diagram Figure 1 Shape diagram of resistor part Figure 3 Conventional wave circuit diagram Figure 2

Claims (1)

[Claims] A constant current circuit configured as a monolithic IC configured by MOS transistors biased by a reference voltage, characterized in that a resistor is inserted between the source and the substrate of the MOS transistor, and the resistor is trimmed by a laser beam. current circuit.