JP5552813B2 - Current limiting circuit and electronic device - Google Patents

Description

  The embodiments discussed herein relate to current limiting techniques that limit the current through a load to a predetermined amount or less.

Various circuits are known as current limiting circuits that limit a current flowing through a load to a predetermined amount or less.
FIG. 10 will be described. FIG. 10 is a configuration example of a conventional current limiting circuit.

This circuit limits the output current i flowing from the output _Vout to a predetermined amount or less. The operation of this circuit will be briefly described on the assumption that the resistance value r of the current detection resistor R11 in this circuit is sufficiently smaller than the bias current supply resistor R12 that limits the bias current of the transistor TR11.

In the circuit of FIG. 10, the output current i flowing from the input V _in to an output V _out (i.e. the collector current of the TR11) is increased, the current detection resistor R11 potential difference across (r × i) is increased. Eventually, when this potential difference reaches the base-emitter conduction threshold voltage V _th2 of the transistor TR12, the collector-emitter of TR12 transitions from the OFF state to the ON state, and the base-emitter voltage V _BE1 of TR11 decreases. Let Then, TR11 reduces its collector current, that is, output current i. In the current limiting circuit of FIG. 10, the output current i is limited to a predetermined amount or less in this way.

As described above, in the circuit of FIG. 10, the output current i flows _out from the output _Vout within the range where the potential difference (r × i) across R11 reaches the threshold voltage _Vth2 . Therefore,
The output current i is limited to V _th2 / r or less.

Some constant current circuits are known in which the temperature dependence of the output current value is reduced.
FIG. 11 will be described. FIG. 11 is a configuration example of a conventional constant current circuit as a second example of the conventional current limiting circuit.

  The operation of this circuit will be briefly described on the assumption that the resistance value r of the current detection resistor R21 in this circuit is sufficiently smaller than the bias current supply resistor R22 that supplies a bias current to the transistor TR21.

In the circuit of FIG. 11, the operational amplifier (operational amplifier) OP21 controls the base voltage of TR21 via R22, and always maintains the potential difference across R21 at Vi. Therefore, the collector current (ie, output current) _ic of TR21 flowing through R21 from the power source having the voltage Vcc is
It becomes. As is apparent from the equation [2], the value of _ic does not depend on the base-emitter threshold voltage of the transistor, and the temperature dependency is extremely low.

  Next, FIG. 12 will be described. FIG. 12 is a configuration example of a conventional overheat protection circuit as a third example of the conventional current limiting circuit. This circuit uses a thermistor to detect the overheating condition of the load, temporarily limits the load current during overheating, and when the overheating condition disappears and returns to normal, the restriction is removed. is there.

  In FIG. 12, when the switch SW31 is closed, the output current i that the power source E31 passes through the load RL31 is limited by the transistor TR31 and the resistor R31. Here, the collector current of TR31 (ie, output current i) is controlled by the base current of TR31, and this base current is determined by the base potential of TR31. This base potential is a value obtained by dividing the voltage value of the power source E31 by the resistor module 31 and the resistor R32. Here, the resistor module 31 is configured by a series connection of a resistor R33 and a resistor unit 32, and the resistor unit 32 is configured by a parallel connection of a resistor R34 and a thermistor R35.

  The thermistor R35 is thermally coupled to the load RL31 (or TR31). In this circuit, the resistance value of the resistance module 31 is substantially equal to the value of R33 in the normal state, and is substantially equal to the combined resistance value of R33 and R34 in the overheated state of the load RL31 (or TR31). , R33, R34, and R35 are set in advance. In this way, the load RL31 is protected from the overheated state by controlling the output current i by changing the base potential of the TR31 between the normal state and the overheated state.

  As another background art, there is also known a technique for suppressing a change due to an ambient temperature of a protection characteristic against an overcurrent to a load.

JP 2005-51873 A JP 2002-305840 A

  In general, the base-emitter conduction threshold voltage of a transistor has a high temperature dependency. Specifically, the threshold voltage fluctuates by about ± 20% in a temperature range of about −20 ° C. to + 70 ° C. There are many such things. For this reason, in the above-described current protection circuit of FIG. 10, it is extremely difficult to strictly set the upper limit value (current limit value) of the output current i. In addition, since the temperature characteristic of the threshold voltage is strongly nonlinear, it is difficult to compensate the temperature characteristic with a simple circuit.

In this regard, the constant current circuit of FIG. 11 has extremely low temperature dependence because the value of the output current _ic does not depend on the base-emitter threshold voltage of the transistor, as described above. However, since this circuit uses an operational amplifier, a power supply that supplies a voltage high enough to drive the circuit is required, and the power consumption of the circuit also increases. Also, the use of an operational amplifier leads to an increase in cost, and circuit transient characteristics may become a problem.

  On the other hand, since the overheat protection circuit of FIG. 12 does not use an operational amplifier like the circuit of FIG. 11, a power source for driving the operational amplifier is unnecessary, and the circuit is composed of relatively inexpensive parts. Yes. However, when the accuracy of the upper limit value of the output current i in this circuit is numerically calculated, it has been found that the error in the temperature range of −20 ° C. to + 70 ° C. is limited to about ± 10% or less. For this reason, a further reduction in the temperature dependence of the current limiting circuit is desired.

  The present invention has been made in view of the above-described problems, and a problem to be solved is to provide a current limiting circuit that realizes appropriate compensation for temperature dependence with a simple circuit configuration.

One of the current limiting circuits described later in this specification includes a first transistor and a second transistor, both of which are bipolar transistors, a bias current supply resistor, and a current detection resistor unit. Here, the first transistor has an emitter connected to the base of the second transistor, a collector connected to the circuit output terminal, and a base connected to the collector of the second transistor. The second transistor has an emitter connected to the circuit input terminal, a collector connected to the base of the first transistor, and a base connected to the emitter of the first transistor. The bias current supply resistor is inserted between a node between the base of the first transistor and the collector of the second transistor and the circuit ground terminal. The current detection resistor section is inserted between a node between the circuit input terminal and the emitter of the second transistor, and a node between the emitter of the first transistor and the base of the second transistor. the series connection of the resistor and the second resistor is configured by parallel connection, it detect the current output from the circuit output terminal.

  The current limiting circuit described later in this specification realizes appropriate compensation for temperature dependence with a simple circuit configuration.

It is a block diagram of the current limiting circuit which concerns on this embodiment. 3 is a graph (part 1) illustrating an example of a voltage characteristic between a base and an emitter of a transistor. It is the graph (the 2) which illustrated an example of the base-emitter voltage characteristic of a transistor. It is the graph which illustrated an example of the temperature-resistance value characteristic of a thermistor. It is the graph which illustrated an example of the relationship between selection of a thermistor and the change of the error of an electric current upper limit. It is an example of the setting result of the characteristic value with respect to the component of the current detection resistance part Ri. It is an example of the setting result of the characteristic value with respect to the component of the current detection resistance part Ri of a structure different from FIG. It is a block diagram of the current detection resistance part Ri comprised by connecting two thermistors in series. It is an example of the setting result of the characteristic value with respect to the component of the current detection resistance part Ri comprised as shown in FIG. It is a structural example of the electronic device using the current limiting circuit of FIG. It is a structural example of the conventional current limiting circuit. It is a structural example of the conventional constant current circuit. It is a structural example of the conventional overheat protection circuit.

First, FIG. 1 will be described. FIG. 1 illustrates the configuration of a current limiting circuit according to the present embodiment.
The current limiting circuit 10 in FIG. 1 includes transistors TR1 and TR2, both of which are PNP bipolar transistors, a current detection resistor Ri, and a bias current supply resistor Rb. This circuit has an input terminal 1, an output terminal 2, and a ground terminal 3. When the input voltage Vin is applied to the input terminal 1, the circuit outputs the output voltage Vout from the output terminal 2. The ground terminal 3 is maintained at a reference potential (ground potential).

The circuit of FIG. 1 has a configuration in which the current detection resistor R11 in the circuit of FIG. 10 described above is replaced with a current detection resistor Ri.
In FIG. 1, the transistor TR1 has an emitter connected to the base of the transistor TR2, a collector connected to the output terminal 2, and a base connected to the collector of the transistor TR2. The transistor TR2 has an emitter connected to the input terminal 1, a collector connected to the base of the transistor TR1, and a base connected to the emitter of the transistor TR1.

  The bias current supply resistor Rb is inserted between the node between the base of the transistor TR1 and the collector of the transistor TR2 and the ground terminal 3, and limits the bias current of the transistor TR1.

The current detection resistor Ri is for detecting a current flowing from the input terminal 1 to the output terminal 2, that is, an output current i flowing from the output terminal 2. The current detection resistor Ri is inserted between a node between the input terminal 1 and the emitter of the transistor TR2 and a node between the emitter of the transistor TR1 and the base of the transistor TR2, and a series connection between the thermistor Rt and the resistor Rs. A resistor Rp is connected in parallel for connection. In the following description, the combined resistance value of the current detection resistor unit Ri and r _i, the value of this r _i shall be sufficiently small as compared with the resistance value r _b of the bias current supply resistor Rb.

In the circuit of FIG. 1, when the output current i flowing from the input terminal 1 to the output terminal 2 (that is, the collector current of TR1) increases, the potential difference (r _i × i) across the current detection resistor Ri increases. Eventually, when this potential difference reaches the base-emitter conduction threshold voltage V _th2 of TR2, the collector-emitter of TR2 transitions from the off state to the on state, and the base-emitter voltage V _BE1 of TR1 is lowered. . Then, TR1 decreases its collector current, that is, the output current i. In the current limiting circuit 10 of FIG. 1, the output current i is limited to a predetermined amount or less in this way.

As described above, in the circuit of FIG. 1, the output current i flows out from the output Vout within a range where the potential difference (r _i × i) across Ri reaches the threshold voltage V _th2 . Therefore,
There satisfied, the output current i is limited to less than V _th2 / r _i.

[Resistance value of bias current supply resistor Rb]
By the way, the base current in the ON state of TR1 is approximately (Vin / r _b ). Therefore, when the current amplification factor of the TR1 and h _FE, the collector current (that is, the output current i) of TR1 is _{approximately, {h FE × (Vin /} r b)}. Therefore, if the minimum value of the current amplification factor of TR1 used in the circuit of FIG. 1 is _{hFE_min} , the condition to be satisfied by the resistance value _rb of the bias current supply resistor Rb is as _follows from the equation [3].
It becomes.

[Temperature dependency of transistor base-emitter conduction threshold voltage]
The magnitude of the change of the base-emitter voltage Vj per 1 ° C., that is, the temperature coefficient α _t , where the absolute temperature is T and the base-emitter band gap at that temperature T is Eg, It is given by the equation (5).

In this equation, depending on whether the diffusion current is the main component or the recombination current is the main component, an Eg compensation term of about several tens of mV is required. The temperature coefficient α _t varies depending on the current density or the like, and is usually about −1.5 to −3.0 [mV / ° C.].

The base-emitter conduction threshold voltage of the transistor is the base-emitter voltage Vj when the collector current is almost zero. In general small signal transistors, the difference in temperature dependence of the base-emitter conduction threshold voltage between the types is small. 2A and 2B are graphs illustrating an example of the base-emitter voltage characteristics of the transistor (2SA1312), in which the temperature dependency is expressed. The base-emitter conduction threshold voltage of this transistor can be read from these figures as follows.
-25 [℃] …… 0.650 [V]
-20 [℃] …… 0.640 [V]
+25 [℃] ………… 0.550 [V]
+70 [℃] …… 0.442 [V]
+100 [° C] ............ 0.370 [V]

[Thermistor temperature-resistance characteristics and B constant]
Next, the temperature characteristics of the thermistor will be described.

In the thermistor, the relationship between the resistance value and the temperature in the temperature range is approximately given by the following [Equation 6].

In the above [Expression 6], T ₁ and T ₂ are absolute temperatures [K], and R ₁ and R ₂ are zero load resistance values (changes due to self-heating) at temperatures T ₁ and T ₂ , respectively. Resistance value with sufficiently low power consumption to be negligible) [Ω]. B is a B constant [K] indicating the magnitude of the change in resistance value of each thermistor.

  FIG. 3 is a graph illustrating an example of the temperature-resistance value characteristic of the thermistor, in which the nominal resistance value (resistance value at + 25 ° C.) is 150 [Ω]. For example, a thermistor having a nominal resistance value of about 30 [Ω] to 150 [kΩ] and a B constant of about 2150 [K] to 4100 [K] depending on the resistance value is easily available. Yes (for example, TN10 series manufactured by Mitsubishi Materials Corporation).

[Setting of characteristic values of components of current detection resistor Ri in FIG. 1]
Next, the setting of the characteristic values of the resistors Rs and Rp and the thermistor Rt constituting the current detection resistor unit Ri will be described.

In the following description, the following characters are used.
Current upper limit of output current i: i _max
Lower limit temperature at which the circuit of FIG. 1 is allowed to use: T _low
Maximum allowable temperature for use of the circuit of FIG. 1: T _high
Nominal resistance value of thermistor Rt: r _t
Resistance value of resistor Rs: r _s
Resistance value of resistor Rp: r _p
The resistance of the thermistor at the lower limit temperature _{T low:} α × r _t
Resistance value of the thermistor at the upper limit temperature _{T high:} β × r _t
TR2 base-emitter conduction threshold voltage at the lower limit temperature T _low : V _{th2_low}
TR2 base-emitter conduction threshold voltage at the upper limit temperature T _high : V _{th2_high}

In the circuit of FIG. 1, it is clear that the following [Equation 7] and [Equation 8] are established from the above [Equation 3].

Here, when the formula [7] is arranged with respect to r _p , the following [formula 9] is obtained.

It clears r _p by substituting this Equation 9] expression [Expression 8] expression and rearranging the r _s, [Expression 10] the following equation is obtained.

The [Expression 10] where is a quadratic equation in r _s, the discriminant D is
It is.

In general, the base-emitter conduction threshold voltage of a transistor decreases as the temperature increases. In general, the resistance value of the thermistor decreases as the temperature increases. Therefore, the following formula generally holds.

Therefore,
Therefore, D> 0 is established. Therefore, it can be seen that r _s has a real solution in the equation [10].

Next, a condition for obtaining a positive value for r _s is obtained. Solving [Equation 10] for r _s ,

Considering the [Equation 12], in order for the value of the [Equation 14] to be positive, the value of the numerator on the right side of the [Equation 14] needs to be negative. If the value of this numerator is set as N and the formula of N is arranged,

However,
It is.

Here, considering [Equation 12], since γ> 0 and α−β> 0, N is negative.
This is only the case. Solving this condition for r _t,
It becomes. Therefore, if r _t satisfying this condition is selected, the value of r _s can be calculated from [Equation 14] considering [Equation 17].
Can be set. Furthermore, if the values of r _t and r _s are set in this way, r _p can be set from the equation [9].

Therefore, the characteristic value of each component of the current detection resistor Ri is set by the following procedure.
[Step 1] First, the current upper limit value i _max of the output current i is determined, and the base-emitter conduction threshold voltages V _{th2_low} and V _{th2_high} at the temperatures T _low and T _high for the transistor TR2 to be used, respectively. get information.

[Step 2] Next, the value obtained in the above (1) is substituted into the right side of the equation [18], and the thermistors Rt satisfying this equation are selected one by one.
Many of the readily available thermistors have their nominal resistance values aligned with the E6 series in a standard number sequence, and this step selects from these nominal resistance values.
The values of α and β on the right side of [Formula 18] are calculated using Formula [6]. However, in the general thermistor, the nominal B constant differs depending on the nominal resistance value. Therefore, first, α and β calculated using provisional values (for example, B = 3000) are applied to the formula [18]. Then, after selecting the thermistor Rt to be used, α and β are calculated again using the nominal B constant of the selected thermistor Rt, and it is confirmed that [Formula 18] is satisfied.

[Step 3] Next, the value obtained in the above [Step 1] and each value r _t , α, and β of the thermistor Rt selected in the above [Step 2] are substituted into the [Equation 19] and selected. setting the resistance value r _s of the corresponding resistor Rs to the thermistor Rt. However, the value of D in [Equation 19] is obtained using [Equation 11].

[Step 4] Next, the value obtained in [Step 1], the characteristic values r _t , α, and β of the thermistor Rt selected in [Step 2], and the resistance Rs set in [Step 3]. Substituting the resistance value r _{s in the} equation (9) for calculation. Thus, the resistance value r _p of the resistor Rp corresponding to the selected thermistor Rt is set.

[Step 5] Next, the temperature T _low to T _high when the circuit of FIG. 1 is constituted by the thermistor Rt selected as described above and the values of the resistors Rs and Rp set in accordance with the selection. The amount of fluctuation of the current upper limit value i _{max in} the range is calculated using the formulas [6] and [7]. Then, an error between this calculation result and the initially determined current upper limit value i _max is calculated, and the combination of the thermistor Rt and the resistors Rs and Rp when this error is within the allowable range is determined as the current detection resistor Ri. This is finally set as the characteristic value of the component.

FIG. 4 is a graph illustrating an example of the relationship between the selection of the thermistor and the change in the error of the current upper limit value in the current limiting circuit 10 of FIG. In this graph, the horizontal axis represents the ratio of the nominal resistance value r _t of the thermistor Rt for [Equation 18] where the right side of r _tmax of (r _t / r _tmax), the vertical axis represents the error for the current upper limit value i _max Represents. From this graph, in the current limiting circuit 10, for example, when it is desired that the error with respect to the current upper limit value i _max be within 2%, the value of the nominal resistance value r _t is 0.41 × _{rt max} ≦ r. _It can be seen that the thermistor Rt in the range of _t ≦ 0.78 × _rtmax may be selected.

  When the setting is performed according to the above procedure, it is possible to set a characteristic value that can appropriately compensate for the temperature dependence of the current limiting circuit 10 of FIG. 1 for the components of the current detection resistor Ri.

  Next, specific setting results of characteristic values for the components of the current detection resistor Ri in the current limiting circuit 10 of FIG. 1 will be presented.

The table of FIG. 5 is an example of setting results of characteristic values for the components of the current detection resistor Ri.
This table shows that in the current limiting circuit 10 of FIG. 1, when the current upper limit value i _max is set to the value shown in the column “theoretical value”, the thermistor Rt is selected by the above-described procedure, and the resistors Rs and Rp are selected. A list of errors calculated by setting the resistance value is provided.

In the calculation of the numerical values presented in this table, the lower limit temperature T _low was set to −20 [° C.], and the upper limit temperature: T _high was set to +70 [° C.]. At this time, the base-emitter conduction threshold voltages V _{th2_low} and V _{th2_high} of TR2 at the lower limit temperature T _low and the upper limit temperature T _high are values read from FIG. 2A and FIG. It was set to 640 [V] and 0.442 [V]. Moreover, the thermistor used the nominal resistance value and nominal B constant of the TN10 series made by Mitsubishi Materials Corporation mentioned above.

In this table, the current upper limit value i _max is 1.0, 1.5, 2.2, 3.3, 4.7, 6.8, 10.0, 15.0, and 22.0 [mA]. The error when each value is used is presented. In each current upper limit value, a combination of the thermistor Rt and the resistances Rs and Rp when the error is minimized on each of the positive side and the negative side is presented.

  In the table of FIG. 5, the column of “Thermistor R at 25 ° C. == RT” shows the nominal resistance value of the selected thermistor Rt. In addition, in each of the columns “α” and “β”, the values of α and β calculated by the equation [6] using the nominal resistance value and the nominal B constant of the selected thermistor Rt are shown, respectively. Has been.

Further, in each column of “Rs” and “Rp”, resistance values r _s and r _{p of} the resistors Rs and Rp calculated by the procedure of [Step 3] and [Step 4] described above are shown, respectively. Yes. Then, in the column "Error (%)", to constitute a current detecting resistor portion R by using the thermistor Rt that is selected, and the calculated r _s and r resistor resistance value _p is set Rs and Rp In this case, the error of the current upper limit value with respect to the initially determined current upper limit value i _max is shown.

  In this table, each numerical value obtained in the middle of the above calculation is shown in the column of each row not specifically described.

In the table of FIG. 5, as an example, a column of “theoretical value (1 mA)” is referred to.
The left side of the column, if you select the thermistor Rt nominal resistance value "2200.0" [Omega], the resistance value r _s and r _p of the resistor Rs and Rp are "853.1" [Omega] And “663.2” [Ω]. Then, a the selected thermistor Rt, the resistance value of the calculated r _s and r _p are forming the current detection resistor portion R by using the set resistor Rs and Rp, the error of the current upper limit value, the initial It is shown that the current upper limit value i _{max is} “−0.94” [%].

On the other hand, when the thermistor Rt having the nominal resistance value “3300.0” [Ω] is selected, the resistance values r _s and r _p of the resistors Rs and Rp are “697.7” [ Ω] and “654.2” [Ω]. Then, a the selected thermistor Rt, the resistance value of the calculated r _s and r _p are forming the current detection resistor portion R by using the set resistor Rs and Rp, the error of the current upper limit value, the initial It is shown that the current upper limit value i _{max is} “2.22” [%]. The value of this error is larger in absolute value than “−0.94” [%] described above. That is, in the current detection circuit of FIG. 1, when the current upper limit value i _{max is set} to “1 mA”, the error is minimized by selecting the thermistor Rt having the nominal resistance value “2200.0” [Ω]. It can be seen from the table of FIG.

Referring to the table of FIG. 5, the absolute value of the error is 0.94 [%] at the maximum within the range of the presented current upper limit value i _max . In other words, in the current limiting circuit 10 of FIG. 1, the temperature dependence of the current upper limit value can be set to about ± 1% in the range of −20 to +70 [° C.] when the current upper limit value i _max is in the range of 1 to 22 mA. From the table in FIG.

Here, the table of FIG. 6 is presented for reference. This table is an example of setting results of characteristic values for the components of the current detection resistor Ri having a configuration different from that in FIG.
In the setting result example of FIG. 6, the current detection resistor Ri is configured similarly to the resistor unit 32 in the overheat protection circuit shown in FIG. 12 (configuration in which a resistor Rs is connected in series to a parallel connection of a thermistor Rt and a resistor Rp). It is a case. In this case, the selection of the thermistor Rt and the setting of the resistors Rp and Rs are derived in the same manner as in the circuit of FIG. 1 using the selection condition equation for the nominal resistance value of the thermistor Rt and the derivation equation for the resistance values of the resistors Rp and Rs. It was done.

The way of viewing the table of FIG. 6 is the same as the way of viewing the table of FIG. Referring to FIG. 6, in the current limiting circuit 10 in which the current detection resistor Ri is configured as described above, when the current upper limit value i _max is 10 mA, the temperature dependence of the current upper limit value is −20 to +70. In the range of [° C.], only 6.30% can be achieved. From this, it can be seen that in the current limiting circuit 10 in which the configuration of the current detection resistor Ri is configured as shown in FIG. 1, the temperature dependence of the current upper limit value is significantly reduced.

  Note that the thermistor Rt of the current detection resistor Ri in the current limiting circuit 10 of FIG. 1 is formed by connecting a plurality of (two in FIG. 7) thermistors Rt1 and Rt2 having different nominal resistance values as shown in FIG. You may make it comprise.

As described above, in many of the thermistors that are readily available, the nominal resistance values are aligned in the E6 series in the standard number sequence. For this reason, the selection of only one thermistor Rt may not be able to reduce the error of the current upper limit value i _max . On the other hand, when the current detection resistor Ri is configured as shown in FIG. 7, it is possible to select a more appropriate resistance value of the thermistor, so that the error of the current upper limit value i _max can be further reduced. It becomes like this.

Here, the table of FIG. 8 is presented. This table is an example of the result of setting the characteristic values for the components of the current detection resistor Ri configured as shown in FIG.
The way of viewing the table of FIG. 8 is the same as the way of viewing the table of FIG.

In the table of FIG. 8, the column of “theoretical value (1 mA)” is referred to as an example.
On the left side of this column, a thermistor Rt1 having a nominal resistance value of 2200.0 [Ω] and a thermistor Rt2 having a nominal resistance value of 220.0 [Ω] are selected and connected in series (the combined resistance value is “2420.0”). “[Ω]”). In this case, the resistance value r _s and r _p of the resistor Rs and Rp are shown to be set to the respective "797.5" [Omega] and "661.8" [Omega]. Then, a the selected thermistor Rt, the resistance value of the calculated r _s and r _p are forming the current detection resistor portion R by using the set resistor Rs and Rp, the error of the current upper limit value, the initial It is shown that the current upper limit value i _{max is} “−0.20” [%].

On the other hand, the thermistor Rt1 having a nominal resistance value of 2200.0 [Ω] and the thermistor Rt2 having a nominal resistance value of 330.0 [Ω] are selected and connected in series on the right side of this column (the combined resistance value is “2530”). .0 ”[Ω]). In this case, the resistance values r _s and r _{p of} the resistors Rs and Rp are set to “774.2” [Ω] and “661.0” [Ω], respectively. Then, a the selected thermistor Rt, the resistance value of the calculated r _s and r _p are forming the current detection resistor portion R by using the set resistor Rs and Rp, the error of the current upper limit value, the initial It is shown that the current upper limit value i _{max is} “0.15” [%]. Therefore, in the current detection circuit of FIG. 1, when the current upper limit value i _{max is set} to “1 mA”, a thermistor having a nominal resistance value of 2200.0 [Ω] and 330.0 [Ω] is connected in series. Then, it can be seen from the table of FIG. 8 that the error can be minimized.

As can be seen by comparing the table of FIG. 8 with the table of FIG. 5, by configuring the current detection resistor Ri as shown in FIG. 7, the current upper limit value of all current upper limit values i _max presented in the table is The error is improved from the case where the current detection resistor Ri having the configuration of FIG. 1 is used. When the current detection resistor Ri is configured as shown in FIG. 7, the temperature dependence of the current upper limit value is ± 0 in the range of −20 to +70 [° C.] when the current upper limit value i _max is in the range of 1 to 22 mA. It can be seen from the table in FIG.

Next, FIG. 9 will be described. FIG. 9 is a configuration example of an electronic device using the current limiting circuit 10 of FIG.
The electronic device 100 includes a current limiting circuit 10 (the current detection resistor Ri may have the configuration shown in FIG. 7), a battery 11, an MPU 12, a buzzer 13, a light emitting diode 14, a resistor 15, and switches 16a and 16b. Has been.

The battery 11 is connected to the input terminal 1 of the current limiting circuit 10.
The buzzer 13 is one of the loads. When the switch 16a is closed, the buzzer 13 operates by receiving power from the output terminal 2 of the current limiting circuit 10 and emits a sound.

The light-emitting diode 14 is also one of the loads. When the switch 16b is closed, the light-emitting diode 14 operates by receiving power from the output terminal 2 of the current limiting circuit 10, and emits light.
The resistor 15 determines the magnitude of the current flowing through the light emitting diode 14.

  The MPU (microprocessor unit) 12 is a control unit that controls the operation of the entire electronic device 100. The MPU 12 operates by receiving power supply directly from the battery 11 (that is, without going through the current limiting circuit 10) and controls the opening and closing of the switches 16a and 16b, thereby driving the buzzer 13 and the light emitting diode 14. Take control.

  In the configuration of FIG. 9, when there is no current limiting circuit 10 (when the buzzer 13 and the light-emitting diode 14 operate by receiving power directly from the battery 11), the MPU 12 closes the switches 16a and 16b and the buzzer 13 and the light-emission. Consider the case where the diode 14 is driven. In this case, the battery 11 causes a voltage drop due to the current flowing through the buzzer 13 and the light emitting diode 14 and the internal resistance of the battery 11. At this time, if the internal resistance of the battery 11 is high and the output voltage falls below a value necessary for the operation of the MPU 12, the MPU 12 may not be able to perform a normal control operation.

On the other hand, in the electronic device 100 configured as shown in FIG. 9, the current limiting circuit 10 can limit the current flowing through the buzzer 13 and the light emitting diode 14 to a predetermined value or less. Therefore, by setting the current upper limit value i _max in the current limiting circuit 10 to a value that does not fall below the value necessary for the operation of the MPU 12, the MPU 12 can be controlled normally even if the buzzer 13 and the light emitting diode 14 are driven. To be able to.

DESCRIPTION OF SYMBOLS 1 Input terminal 2 Output terminal 3 Ground terminal 10 Current limiting circuit 11 Battery 12 MPU
13 Buzzer 14 Light emitting diode 15 Resistor 16a, 16b, SW31 Switch 31 Resistor module 32 Resistor unit E31 Power supply OP21 Operational amplifier R11, R21 Current detection resistor R12, R22, Rb Bias current supply resistor RL31 Load Ri Current detection resistor R31, R32, R33 , R34, Rp, Rs Resistance R35, Rt, Rt1, Rt2 Thermistors TR1, TR2, TR11, TR12, TR21, TR31 Transistors

Claims (5)

Both have bipolar transistors, a first transistor and a second transistor,
The first transistor has an emitter connected to the base of the second transistor, a collector connected to a circuit output terminal, and a base connected to the collector of the second transistor;
The second transistor has an emitter connected to the circuit input terminal, a collector connected to the base of the first transistor, a base connected to the emitter of the first transistor,
A bias current supply resistor inserted between a node between the base of the first transistor and the collector of the second transistor and a circuit ground terminal; and a node between the circuit input terminal and the emitter of the second transistor; , Inserted between the node of the emitter of the first transistor and the base of the second transistor, and is configured by connecting a second resistor in parallel to a series connection of the thermistor and the first resistor , current detecting resistor section that detect the current output from the circuit output terminal,
And a current limiting circuit. The current limiting circuit according to claim 1,
The upper limit value of the current output from the circuit output terminal is i _max ,
Let the nominal resistance of the thermistor be r _t ,
The resistance value of the thermistor at temperature lower limit use of the current limiting circuit is permitted expressed as α × r _t,
The resistance value of the thermistor at a temperature below which use of the current limiting circuit is allowed represents a β × r _t,
The base-emitter conduction threshold voltage at the lower limit temperature of the second transistor is V _{th2_low} ,
When the base-emitter conduction threshold voltage at the upper limit temperature of the second transistor is V _{th2_high} ,
The nominal resistance value r _t is
A current limiting circuit characterized by satisfying The resistance value r _{s of the} first resistor and the resistance value r _{p of the} second resistor are
However,
The current limiting circuit according to claim 2, wherein:   4. The current limiting circuit according to claim 1, wherein the thermistor is constituted by a series connection of a plurality of thermistors having different nominal resistance values. 5. A current limiting circuit according to any one of claims 1 to 4;
A battery connected to the circuit input terminal of the current limiting circuit;
A load receiving power supply from a circuit output terminal of the current limiting circuit;
A microprocessor that receives power from the battery and operates to control driving of the load;
An electronic device comprising: