JPH0527129B2 - - Google Patents

Description

[Detailed Description of the Invention] <Technical Field> The present invention provides a transistor output circuit that includes:
The present invention relates to a current limiting circuit proposed as a countermeasure for driving a load such as a light bulb through which an inrush current flows when the power is turned on, or as a countermeasure against short-circuiting of the output terminal due to an accident, or as a circuit for easily obtaining a constant current output.

<Prior art> Conventionally, a circuit as shown in FIG. 1 was used. For example, when the emitter current of the output transistor TR ₅ increases, the voltage generated by the emitter current across the resistor R ₂ increases. As a result, negative feedback is applied by turning on the transistor TR ₄ and lowering the base current of TR ₅ , so that an output current exceeding a certain current value is not generated. I ₃ is
A constant current source connected to the connection point between the base of TR ₅ and the collector of TR ₄ , and TR ₆ is the switching transistor of this circuit.

This circuit is a constant current circuit with negative feedback, and the specific design example shown in FIG. 1 and the specific numerical values of the current flowing through each part in that case are shown below. When you want the output current _I6 of TR5 to be 1mA, the transistor
If _hFE is 100, 10μA is required as base current _I5 . Therefore, I ₃ supplies enough of it,
In addition, TR4 also needs a value that supplies a stable current. Therefore, assume that I ₃ is now set to 110 μA. The current _I4 flowing through TR4 becomes 100μA.
Since TR4's V _BE ≒0.72V, TR5's output current
If you want I ₆ to be 1mA, the value of R ₂ will be 720Ω.
The above stable state where I ₄ = 100 μA, I ₅ = 10 μA, and I ₆ = 1 mA is called a steady state.

As is clear from the operating principle described above, this conventional circuit has the disadvantage that when the current gets close to the limit value (the base current of TR ₅ decreases), the output saturation voltage as seen from the ground level becomes extremely high. .

Also, especially when integrated into an IC, the base-emitter voltage of TR ₄ has negative temperature characteristics;
Since R ₂ created by the diffused resistance inside the IC has positive temperature characteristics, it is given as the limiting current of the emitter.
V _BE /R ₂ had the disadvantage of having a large positive temperature coefficient.

<Objective of the Invention> The present invention has a structure that has little influence on the output saturation voltage and has a small temperature coefficient even when integrated into an IC.
The present invention provides a current limiting circuit that eliminates the conventional drawbacks.

<Example> An example of the present invention will be described below with reference to FIG.

FIG. 2 is a circuit diagram showing this embodiment.

TR ₂ is an output transistor whose collector is used as an output terminal and whose emitter is grounded via resistor R ₁ . I ₁ is a constant current source (current value is also explained using I ₁ )
and is connected to the base of output transistor _TR2 . _D1 is a diode-connected transistor, and its cathode is connected to the emitter of the output transistor _TR2 . _I2 is a constant current source (current value _I2 ) connected to the anode of the diode _D1 .
TR ₁ has the emitter grounded and the base diode
This transistor is connected to the anode of _D1 , and its collector is connected to the base of the output transistor _TR2 . _TR3 is a switching transistor in the circuit described above, and its emitter is grounded and its collector is connected to the constant current power supply _I1 .

Now, assume that D ₁ is a diode-connected transistor with the same characteristics as TR ₁ . Also constant current source
The ratio of the current values of I ₁ and I ₂ is I ₁ /I ₂ =n. Assuming that h _FE of each transistor is large enough,
I ₁ is approximately equal to the collector current of TR ₁ in steady state, and I ₂ is approximately equal to the forward current of D ₁ (collector current if D ₁ is considered as a transistor). Therefore, from the base-emitter voltage vs. collector current characteristics of the transistor, the base-emitter voltage V _BE (TR ₁ ) of TR ₁ and the forward voltage V _F (D ₁ ) of D ₁ are as follows: V _BE (TR ₁ )= KT/q・_o I ₁ /I ₀ V _E (D ₁ )=KT/q・_o I ₂ /I ₀ (However, I ₀ is the dark current, also called the collector cut-off current in transistors.) Therefore, V _BE (TR ₁ ) −V _F (D ₁ ) = KT/q _o I ₁ / I ₂ = KT/q _o n, so in this case, the output will generate a voltage of KT/q _o n across R ₁ . A steady state is achieved with current. For example, when n=10, this voltage is approximately
60mVat25℃, which has little effect on the saturation voltage of TR ₂ .

A specific design example of FIG. 2 and, in that case, specific numerical values of the current flowing through each part are shown below.
When the output current I ₁₀ of TR2 is desired to be 1 mA and the h _FE of the transistor is 100, 10 μA is required as the base current I ₈ . Suppose now that I ₁ is set to 110 μA. At this time, the current _I7 flowing through TR1 is 100 μA. Further, if I ₂ is set to 10 μA, ΔV _BE of both transistors TR1 and D1, that is, the voltage generated across R1 is KT/qn10, which is about 60 mV at T=25°C. Therefore, R1
If the value of is set to 60Ω, 1mA can be passed through TR2, and each current (I ₇ =
100 μA, I ₈ = I ₉ = 10 μA, I ₁₀ = 1 mA) flows to each part and becomes stable. This stable state is the steady state.

Furthermore, since the voltage KT/q _o n generated across R ₁ has a positive temperature coefficient, it cancels out the positive temperature coefficient of the resistance when R ₁ is made of a diffused resistor within the IC.
Since this voltage is 1/298/°C≒3300ppM/°C, the limiting current has a temperature characteristic of +300 to 1300ppM/°C, whereas the diffused resistance has a temperature characteristic of 2000 to 3000ppM/°C.

Generally, the temperature coefficient of the difference in V _BE voltage between two transistors with different current densities is +3300ppm/℃
The only condition is that the transistors be manufactured in the same process on the same chip and have the same physical properties.

The V _BE of the transistor is expressed by the following equation.

V _BE = KT/q・n (Ic/Io) V _BE of a transistor with a current density that is n times higher than this
is expressed by the following formula.

V _BE (n)=KT/q・n (n・Ic/Io) The difference in V _BE between these two transistors, ΔV _BE , is as follows.

ΔV _BE =V _BE (n)−V _BE =KT/q· _o n The temperature coefficient of ΔV _BE is obtained by differentiating ΔV _BE with respect to T. That is, dΔV _BE /dT=K/q・_o n=1/TΔV _BE Therefore, ΔV _BE is +1/at 25°C (=298°K).
It has a temperature coefficient of 298≒+3300ppm/℃.

By the way, the current limiting operation is basically the same as the conventional circuit; when the emitter current of TR ₂ becomes large, the voltage generated across R ₁ turns on TR ₁ via diode D ₁ . Negative feedback that lowers the base current of TR ₂ prevents the output current from exceeding a certain current value. At this time, the voltage across _R1 is usually about 60mVat25℃
It does not increase the output saturation voltage as seen from the ground level.

In the above embodiment, the current ratio of constant current sources I ₁ and I ₂ is I ₁ /
Although I ₂ =n, the same effect as changing n can be obtained by changing the sizes of TR ₁ and D ₁ (emitter area). In other words, the size of D ₁ is set to n of TR ₁
If it is doubled, the dark current I ₀ of D ₁ will also be multiplied by n, and even if the current values of constant current sources I ₁ and I ₂ are the same (I ₁ = I ₂ ), V _BE (TR ₁ ) −V _F (D ₁ )=KT/ _qon .

That is, it is sufficient to make the emitter current density of TR ₁ larger (n times) than the current density of D ₁ , and there is no problem in using either one or both together.

Switching transistor TR ₃ (in Fig. 1
TR ₆ ) is inserted to turn on and off light bulbs, etc., and is not necessary when used only for constant current operation.

<Effects of the Invention> As described above, the present invention can provide a useful current limiting circuit that has little influence on the output saturation voltage and has a small temperature coefficient even when implemented as an IC.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a conventional example, and FIG. 2 is a circuit diagram showing an embodiment of the present invention. TR ₁ ...transistor, _D1 ...diode,
TR ₂ ... Output transistor, I ₁ , I ₂ ... Constant current source.

Claims (1)

[Claims] 1. The emitter is grounded through a resistor _R1 , the constant current source _I1 is connected to the base, the output transistor _TR2 has its collector as an output terminal, the cathode is connected to the emitter of the output transistor TR2, and the anode is connected to the output transistor _TR2 . A diode _D1 is connected to a constant current source _I2 , the base is connected to the anode of the diode _D1 , the emitter is grounded, and the collector is connected to the output transistor.
a transistor TR ₁ connected to the base of TR ₂ , said transistor TR ₁ and said diode;
1. A current limiting circuit characterized in that the emitter current density of the transistor TR ₁ is made larger than the current density of the diode D ₁ so that a current flows through D ₁ .