JP4224967B2 - Current detection circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a current detection circuit, and more particularly to a current detection circuit that detects a current flowing through a load.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, there is a current detection circuit that is used for an AC adapter, a secondary battery charging circuit, and the like and detects a current flowing through a load.
[0003]
FIG. 1 is a circuit diagram showing an example of a conventional current detection circuit. In the figure, a power input terminal (VCC) 10 is connected to a power output terminal (VOUT) 12 through a detection resistor RLS. The base of the npn transistor QA1 is connected to the power input terminal 10 side of the detection resistor RLS, and the base of the npn transistor QA2 is connected to the power output terminal 12 side of the detection resistor RLS.
[0004]
The collector of the transistor QA1 is connected to the power supply input terminal 10, and the emitter is connected to the ground terminal (GND) 11 via the constant current source 13, and is also connected to the voltage buffer 14 formed of an operational amplifier. The collector of the transistor QA2 is connected to the power input terminal 10, and the emitter is connected to the ground terminal 11 via the constant current source 15, and is connected to a voltage buffer 16 formed of an operational amplifier.
[0005]
The output terminal of the voltage buffer 14 is connected to a non-inverting input terminal of a current amplifier 17 constituted by an operational amplifier via a resistor RA1, and the output terminal of the voltage buffer 16 is connected to an inverting input terminal of the current amplifier 17 via a resistor RA3. It is connected. The inverting input terminal of the current amplifier 17 is grounded via a resistor RA4 (= RA3), and the non-inverting input terminal and the output terminal of the current amplifier 17 are connected via a resistor RA2 (= RA1).
[0006]
The output terminal of the current amplifier 17 is connected to the non-inverting input terminal of the comparator 18. A reference voltage obtained by dividing the constant voltage generated by the constant current source 19 and the Zener diode Dz provided between the power input terminal 10 and the ground terminal 11 by the resistors RA5 and RA6 is supplied to the inverting input terminal of the comparator 18. ing. The output terminal of the comparator 18 is connected to an open collector npn transistor QA3.
[0007]
Here, the voltage on the power supply input terminal 10 side of the detection resistor RLS is voltage-converted by the transistor QA1, and then supplied to the current amplifier 17 through the voltage buffer 14, and the voltage on the power supply output terminal 12 side of the detection resistor RLS is supplied by the transistor QA2. After the conversion, the voltage is supplied to the current amplifier 17 through the voltage buffer 16. As a result, the current amplifier 17 amplifies the potential difference between both ends of the detection resistor RLS, that is, the current flowing through the detection resistor RLS. The output of the current amplifier 17 is compared with a reference voltage by a comparator 18, and the comparator 18 generates a signal that is at a high level when the current flowing through the detection resistor RLS becomes large and becomes a low level when the current is small. Output from the collector of the transistor QA3.
[0008]
[Problems to be solved by the invention]
The above-described conventional current detection circuit is composed of many circuit blocks called a reference voltage circuit including voltage buffers 14 and 16, a current amplifier 17, a comparator 18, a constant current source 19, a zener diode Dz, and the like. There were many problems.
[0009]
The present invention has been made in view of the above points, and an object thereof is to provide a current detection circuit that has a simple circuit configuration and can significantly reduce the number of circuit elements.
[0010]
[Means for Solving the Problems]
According to the first aspect of the present invention, the current detection output changes the current detection output depending on whether the current flowing through the detection resistor (RLS) connected between the input terminal (20) and the output terminal (22) is equal to or less than a predetermined value. A circuit,
A first transistor (Q7) for supplying a collector current based on the input terminal side potential of the detection resistor (RLS);
The collector of the first transistor (Q7) is connected to the power source through the third resistor (R1), the emitter is grounded through the fourth resistor (R4), and the base is connected to the second constant current. The collector of the eighth transistor (Q15) connected to the source (Q5) and the base of the eighth transistor (Q15) is connected, the base is connected to the emitter of the eighth transistor (Q15), and the emitter is grounded. A temperature compensation circuit (Q14, Q15, R1, R4) having a ninth transistor (Q14) ;
A second transistor (Q8) for flowing a collector current based on the output terminal side potential of the detection resistor (RLS);
The collector current flowing through the first transistor (Q7) and the second transistor (Q8) are connected to the first and second transistors (Q7, Q8) via the first and second resistors (R2, R3). ), The third and fourth transistors (Q9, Q10) having a current mirror configuration that operates so as to have a predetermined ratio to the collector current flowing through
A fifth transistor (Q11) that supplies a collector current of the fourth transistor (Q10) and a sixth transistor (Q12) that has a current mirror configuration and that flows a collector current based on the collector current of the fifth transistor (Q11); Current circuit (24, Q1 to Q4, Q6) including a constant current source (Q6) and having a predetermined ratio between a collector current flowing through the third transistor (Q9) and a collector current flowing through the fourth transistor (Q10). , Q11, Q12)
The base of the sixth transistor (Q12) is connected to the base of the first constant current source (Q6) together with the collector of the sixth transistor (Q12), and the current is switched when the collector current of the sixth transistor (Q12) exceeds a predetermined value. By having the seventh transistor (Q13) that changes the detection output,
With detectable detecting a current flowing through resistor precisely, the circuit configuration can greatly reduce the number of circuit elements is simple, Ru can perform temperature compensation.
[0012]
The invention according to claim 2 is the current detection circuit according to claim 1 ,
The current circuit is
A tenth transistor (Q1) having a collector and a base connected to the third constant current source (24);
An eleventh transistor (Q2) having a base connected in common with the tenth transistor (Q1) to form a current mirror circuit;
A twelfth transistor (Q3) having a base connected in common with the tenth transistor (Q1) to form a current mirror circuit and a collector connected to the collector of the third transistor;
A thirteenth transistor (Q4) having a collector and a base connected to the collector of the eleventh transistor (Q2);
The first constant current source (Q6) comprising a transistor having a base connected in common with the thirteenth transistor (Q4) to form a current mirror circuit and an emitter connected to the collector of the sixth transistor (Q12). Including.
[0013]
Note that the reference numerals in the parentheses are given for ease of understanding, are merely examples, and are not limited to the illustrated modes.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 2 shows a circuit configuration diagram of an embodiment of the current detection circuit of the present invention. In the figure, a power input terminal (VCC) 20 is connected to a power output terminal (VOUT) 22 through a detection resistor RLS. Further, one end of a constant current source 24 is connected to the power input terminal 20, and the other end of the constant current source 24 is connected to the collector of the npn transistor Q1. The transistor Q1 has a collector and base connected to the bases of the npn transistors Q2 and Q3 to form a current mirror circuit. The emitters of the transistors Q1, Q2 and Q3 are connected to the ground terminal (GND) 21. The emitter areas of the transistors Q1, Q2, and Q3 are 1: 1: 1.
[0015]
The collector of the transistor Q2 is connected to the collector of the pnp transistor Q4. Transistor Q4 has a collector and base connected to the bases of pnp transistors Q5 and Q6 to form a current mirror circuit, and the emitters of transistors Q4, Q5 and Q6 are connected to power input terminal 20. The emitter areas of the transistors Q4, Q5, and Q6 are 2: 1: 1, and the transistors Q5 and Q6 function as a constant current source.
[0016]
One end of the resistor R1 is connected to the power input terminal 20 side of the detection resistor RLS, and the other end of the resistor R1 is connected to the base of the npn transistor Q7 and the collector of the npn transistor Q15. The collector of the transistor Q7 is connected to the power input terminal 20, and the emitter is connected to one end of the resistor R2. The other end of the resistor R2 is connected to the emitter of the pnp transistor Q9.
[0017]
On the other hand, the base of the npn transistor Q8 is connected to the power output terminal 22 side of the detection resistor RLS. The collector of the transistor Q8 is connected to the power input terminal 20, and the emitter is connected to one end of the resistor R3. The other end of the resistor R3 is connected to the emitter of the pnp transistor Q10. The base and collector of transistor Q9 are connected to the base of transistor Q10 to form a current mirror circuit, and the collectors of transistors Q9 and Q10 are connected to the collectors of npn transistors Q3 and Q11. The emitter of the transistor Q3 is grounded.
[0018]
The collector of the transistor Q5 is connected to the collector of the npn transistor Q14 and the base of the npn transistor Q15. The base of the transistor Q14 is connected to the emitter of the transistor Q15 and one end of the resistor R4, and the emitter of the transistor Q14 and the other end of the resistor R4 are grounded. The transistors Q14 and Q15 and the resistor R4 limit the collector current of the transistor Q15 and operate as a temperature compensation circuit.
[0019]
The collector of the transistor Q6 is connected to the collector of an npn transistor Q12 and the base of an open collector output transistor (npn transistor Q13). The base of the transistor Q12 is connected to the base and collector of the transistor Q11 to form a current mirror circuit, and the emitters of the transistors Q11 and Q12 are grounded.
[0020]
Here, when the constant current fed by the constant current source 24 and _{I 1,} the transistors Q1, Q2, the collector current _{I CQ9} transistor Q9 by the current mirror circuit of Q3 is _{I 1} becomes the transistors Q4, Q5, Q6 current mirror circuit Since the emitter area of the transistor Q4 is twice that of the other transistors Q5 and Q6 and the transistors Q11 and Q12 are current mirrors, the transistors Q6, Q12, and Q11 are at the current detection switching point at which the transistor Q13 switches from off to on. , Q10 respectively, collector currents I _CQ6 , I _CQ12 , I _CQ11 , I _CQ10 are I _1/2 .
[0021]
Current flows through the sense resistor RLS, the relative power input terminal 20, the potential of the power supply output terminal 22 decreases to decrease the collector current _{I CQ10} of transistor Q10, to the collector current _{I Cq6} of the transistor Q6 is _{constant, I Cq6} The remaining portion becomes the base of the transistor Q13, and the transistor Q13 is turned on. The value at which the current flowing in the detection resistor RLS is turned on is determined by the selection of each element constant, and between both ends of the detection resistor RLS (that is, between the power input terminal 20 and the power output terminal 22). FIG. 3 shows the relationship between the currents I _CQ6 and I _CQ10 when the potential difference between them is 0.2V.
[0022]
By the way, the potential difference V _LS1 between the transistors Q9 and Q10 is expressed as follows.
[0023]
V _LS1 = (V _BEQ9 + I _CQ9 × R2) − (V _BEQ10 + I _CQ10 × R3)
= KT / q × ln (I _CQ9 / I _CQ10 ) + I _CQ9 × R2
-I _CQ10 × R3
= KT / q × ln2 + I ₁ (R2−R3 / 2)
(∵I _CQ9 = 2 · I _CQ10 = I ₁ )
The voltage drop V _LS2 of the resistor R1 is expressed as follows when the base-emitter voltage V _BEQ14 of the transistor Q14 is used.
[0024]
V _LS2 = V _BEQ14 · R1 / R4
The potential difference V _LS3 between the transistors Q7 and Q8 is expressed as follows when the base-emitter voltages V _BEQ7 and V _BEQ8 of the transistors Q7 and Q8 are used. Here, q is the charge amount of electrons, k is the Boltzmann constant, and T is the absolute temperature.
[0025]
_{V LS3 = (V BEQ7 -V BEQ8} )
= KT / q × ln (I _CQ7 / I _CQ8 )
= KT / q × ln2
(∵I _CQ7 = I _CQ9 , I _CQ8 = I _CQ10 , I _CQ9 = 2 · I _CQ10 = I ₁ )
Thus, the potential difference V _LS between the power supply input terminal 20 and the power supply output terminal 30 is expressed by equation (1).
[0026]
V _LS = V _LS1 + V _LS2 + V _LS3
= 2 · kT / q × ln2 + I ₁ (R2−R3 / 2)
+ V _BEQ14 · R1 / R4 (1)
The temperature coefficient ∂V _LS / ∂T in equation (1) is expressed by equation (2). In a bipolar transistor, generally, ∂V _BE /∂T=−0.002.
[0027]
∂V _LS / ∂T = 2 · k / q × ln2 + ∂I ₁ / ∂T (R2-R3 / 2)
−0.002 · R1 / R4 (2)
Therefore, by setting the current I ₁ and the resistors R1, R2, R3, and R4 to be 0.002 · R1 / R4 = 2 · k / q × ln2 + ∂I ₁ / ∂T (R2−R3 / 2) The temperature fluctuation of the detection voltage (potential difference between both ends of the detection resistor RLS where the transistor Q13 is turned on) can be eliminated.
[0028]
【The invention's effect】
As described above, according to the invention described in claim 1, with a current flowing through the detection resistor can be accurately detected, the circuit configuration can greatly reduce the number of circuit elements is simple, Ru can perform temperature compensation.
[Brief description of the drawings]
FIG. 1 is a circuit configuration diagram of an example of a conventional current detection circuit.
FIG. 2 is a circuit configuration diagram of an embodiment of a current detection circuit of the present invention.
FIG. 3 is a diagram illustrating a relationship between a potential difference between both ends of a detection resistor RLS and currents I _CQ6 and I _CQ10 ;
[Explanation of symbols]
20 Power input terminal 21 Ground terminal 22 Power output terminal 24 Constant current sources Q1 to Q3, Q7, Q8, Q11 to Q15 npn transistors Q4 to Q6, Q9, Q10 pnp transistors R1 to R4 Resistor RLS Detection resistor

Claims (2)

A current detection circuit that changes a current detection output depending on whether a current flowing through a detection resistor connected between an input terminal and an output terminal is less than or equal to a predetermined value,
A first transistor for flowing a collector current based on the input terminal side potential of the detection resistor;
An eighth transistor having a collector connected to the base of the first transistor and a power source via a third resistor, an emitter grounded via a fourth resistor, and a base connected to a second constant current source; A temperature compensation circuit having a ninth transistor having a collector connected to the base of the eighth transistor, a base connected to the emitter of the eighth transistor, and a grounded emitter ;
A second transistor for flowing a collector current based on the output terminal side potential of the detection resistor;
A current mirror that is connected to the first and second transistors via first and second resistors and operates so that a collector current flowing through the first transistor and a collector current flowing through the second transistor have a predetermined ratio. A third and a fourth transistor configured;
A third transistor including a fifth transistor for supplying a collector current of the fourth transistor, a sixth transistor configured to flow a collector current based on a collector current of the fifth transistor, and a first constant current source; A current circuit having a predetermined ratio between a collector current flowing through the first transistor and a collector current flowing through the fourth transistor;
A seventh transistor that has a base connected to the first constant current source together with the collector of the sixth transistor, and is turned on / off to change the current detection output when the collector current of the sixth transistor exceeds a predetermined value; A current detection circuit comprising: The current detection circuit according to claim 1 ,
The current circuit is
A tenth transistor having a collector and a base connected to a third constant current source;
An eleventh transistor having a base connected in common to the tenth transistor to form a current mirror circuit;
A twelfth transistor having a base connected in common to the tenth transistor to form a current mirror circuit and a collector connected to the collector of the third transistor;
A thirteenth transistor having a collector and a base connected to the collector of the eleventh transistor;
A current mirror circuit having a base connected in common to form a current mirror circuit and an emitter connected to the collector of the sixth transistor; Detection circuit.

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