JPS6079810A - Constant current source circuit - Google Patents

Abstract

PURPOSE:To attain operation of the circuit at a low power supply voltage and also ease of circuit integration while eliminating the need for a high resistance by connecting a transistor (TR) poled oppositely to cancel mutual base-emitter voltage to a feedback path. CONSTITUTION:A TRQ7 for feedback is inserted to the feedback path to cancel the base-emitter voltage of a feedback TRQ5. Since the base-emitter voltage VBE of the Q5 is cancelled by the base-emitter voltage of a Q7, the operation limit of the circuit is decided by the base-emitter voltage VBE of the Q2, a saturation voltage VCE of a Q4, and a voltage drop of a resistor R4, or the base-emitter voltage VBE of a Q3, a saturated voltage VCE of a Q1 and a voltage drop of a resistor R3. Since the saturated voltage VCE of the Q4 is larger in a TR in an IC generally, the lower limit of the operating voltage is decided by the voltage value of the former, which is larger than the latter. Moreover, no lower limit of the resistance value is required for a resistor R2 for start and a far smaller resistance value is enough, because the power is supplied from the Q7.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to a constant current source circuit that utilizes a so-called bandgap voltage and can operate at a lower power supply voltage.

<Prior art> Figure 1 shows a conventional circuit. This circuit is well known, and its basic operation will be briefly described. Ql is a transistor having an emitter area N times larger than Q2. Q2 is connected to the collector base.

At this time, the entire circuit operates so that the collector currents of Ql and Q2 are equal, so the emitter of Qo and GND
(earth) between the so-called bandgap voltage Δ■B. (
j6 fnNrnv ) occurs at room temperature, and the following equation is obtained. R, is a resistor that determines the current value inserted between the emitter of Q□ and GND.

In such conventional circuits, the limit of low power supply voltage is Q
3 and Q, the base-emitter voltage VB.

and the saturation voltage V of Q. It is determined by E(sat). Therefore, when converting to IC, VB]! ,=0.65
V.

voE(sat) - (approximately 1.4 V d as klV
; Operation limit has been reached.

In addition, in the conventional circuit, if the value of the starting resistor R2 is small,
Q... Q4's base current flowed too much, and in a steady state, a current of more than ``BE'' was forced to become a high resistance in R1.

This is not suitable for IC implementation.

<Purpose of the Invention> The present invention provides a constant current source circuit which can operate at a lower power supply voltage than the conventional one and is advantageous when implementing an IC with a KL that does not require a very high resistance. According to FIGS. 2 and 3, the embodiments of the invention are provided with the same reference numerals.

Q7 is the base emitter 1iL of Q5 in the return path.
Q
, or PNP) transistor, a Q7KNPN) transistor is used. QahQg.

Qla is a transistor constituting a constant current source for supplying the base current of Q7. Rs, R4 is Q
This resistor is used to ensure the operating voltage of Q7 and Q8 (to ensure that Q7 operates as an emitter follower). By inserting R3 and R6, a voltage is generated across them, and the base-emitter voltage of Q7 is Vゎ and Q
The sum voltage of the saturation voltage VC0 (sat) of 8 is expressed as Q3 and Q
is generated between the base voltage of , and the power supply voltage Vcc.

Q1 to Q4 and Q6 are transistors constituting a conventionally known circuit section similar to that shown in FIG. 1, and R1 is a resistor that determines a constant current value. IR2 is a resistor that forms a collective current between Q3 and Q4.

When the power is turned on without switching, the emitter-base junction of R3-Q3 - the emitter-base junction of R2, and the emitter-base junction of R4-Q -
Two path 1c currents of R3 flow, and collector currents of Q3 and Q flow. Here, it is assumed that the resistance values of R3 and R4 are equal, and the dimensions and shapes of Q3 and Q4 are the same. This causes Q3 and Q4 to flow the same collector current.

VBE The collector current of Q3 and Q4 is calculated by □ R.

If the current is less than the collector current of Ql or Q2
(in other words, the collector current of Q4, and (in other words, the collector current of Q3) becomes larger, pulling the base of Q5 to a lower level.Then, of course, the emitter of Q5 also becomes a low level, and furthermore, the emitter of Q7 In other words, the base potentials of Q3 and Q are also at a low level, and when the current is greater than the current calculated by the equations Q3 and Q4, the opposite operation to that described above is performed to reduce the collector currents of Q3 and Q,1.

In this way, a constant current operation similar to the conventional one is performed.

However, the operating limit in this circuit is that the base-emitter voltage of Q5 vB]D is the base-emitter voltage of Q7■3.
, the base-emitter voltage Vl(+) of Q2 and the saturation voltage V CE (S a j
) and R2 voltage drop, or ■ Q3's base-emitter voltage VBE and Ql's saturation voltage VCE (sat) and R3
determined by the voltage drop. Generally, in the case of transistors in an IC, the saturation voltage V is higher for the PNP transistor Q4. p
Since (sat) is large, the lower limit of the operating voltage is determined by the voltage value on the ■ side, which is a large one shift.

The voltage drop across R4 is the base-emitter voltage of Qs and Q7.
Since the base and emitter voltages of Q8 are approximately equal, the saturation voltage V of Q8 is required for constant current source operation. 0 (sat) minutes is sufficient;
This produces a voltage drop of approximately 0.2V. Ultimately, the operating limit in this circuit is the base-emitter voltage VB0 of Q2.
-065v, Q, saturation voltage 'VcBB(Saj)==
o, 2v, 1. , 05 V, and conventional circuit 1
This means that the operating limit has been lowered by 0.35 V.

FIG. 3 shows the power supply voltage versus power current characteristics of the circuit example of the present invention and the conventional circuit. (a) is the characteristic of the conventional circuit;
(b) shows the characteristics of the circuit of the present invention.

This improvement is extremely valuable for ICs that operate on batteries; for example, what used to require 21 manganese batteries now only requires one.

Q: ;i' Regarding the starting resistance R2VC, the circuit of the present invention Q? Since current is supplied from the resistor, there is no particular lower limit on the resistance value as mentioned above, and it is possible to get by with a much smaller resistor.

<Effects of the Invention> As described above, the present invention provides a constant current source circuit that can operate with a lower power supply voltage than conventional circuits, does not require a high resistance for starting, and is advantageous when integrated into an IC. can be provided.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing a conventional example, Figure 2 is a circuit diagram showing an example of the present invention, and Figure 3 is a diagram showing voltage versus current characteristics of the conventional circuit and the circuit example of the present invention. be. Q5 ・Q7...Feedback transistor, R2...
・Resistance for starting, R3, R4...Resistance for ensuring emisoctor follower operation.

Claims (1)

[Claims] 1. In a constant current source circuit using a bandgap voltage, a transistor Q51Q7 of opposite polarity is connected to the feedback path to cancel mutual base-emitter voltage, and the output side transistor Q7 of the feedback path is used for starting. resistance R
2, and a resistor R3 that ensures that the transistor QsjQ4, into which the feedback signal is introduced, operates as an emitter follower of the output side transistor Q7.
, R4 is connected to the constant current source circuit.