JPH0644205B2 - Constant voltage power supply circuit - Google Patents

Description

The present invention relates to a constant voltage power supply circuit, and more particularly to a low loss type constant voltage power supply circuit using a PNP type transistor as an output transistor.

(B) Conventional Technique When an NPN type transistor is used as an output transistor of a constant voltage power supply circuit, there is a problem that power loss becomes large depending on the base-emitter voltage of the transistor. For that reason,
When creating a low-loss constant voltage power supply circuit, a PNP transistor is usually used as an output transistor.

As a low loss type constant voltage power supply circuit, for example, there is one shown in FIG. In FIG. 2, (1) is a starting circuit, (2) is a reference voltage generating circuit, (3) is a constant current source,
(4) is an error amplifier circuit. The error amplifier circuit (4) is PN
P-type transistors (5), (6) and NPN transistor (7),
(8) and resistor (9). Further, (10) is an NPN type output transistor driving transistor controlled by the output of the error amplification circuit (4), (13) is a PNP type output transistor, and (11) is a bias setting resistor of the output transistor (13). , (12) are base current limiting resistors, and (14)
And (15) are resistors for setting the output voltage.

Note that (16) is an input terminal to which a voltage of, for example, about 6 V is input. An output terminal (17) outputs a voltage of, for example, about 5.2V. (18) is a ground power supply.

Next, the operation of the conventional circuit shown in FIG. 2 will be described. When a power supply voltage (for example, 6 V) is applied to the input terminal (16), the starting circuit (1) first operates. Then the reference voltage generator (2)
Operates and a predetermined reference voltage (for example, 1.2 V) is applied to the error amplification circuit (4). As a result, the NPN transistor (7) is turned on, so that a current flows through the PNP transistor (5). Therefore, the base current also flows through the PNP transistor (6) connected in a current mirror relationship with the PNP transistor (5) to turn on the transistor, and the current is supplied to the ground power supply (18) via the driving transistor (10). Flowing. This causes a voltage drop across the resistor (11) to turn on the output transistor (13), and a current is also generated in the output voltage setting resistors (14) and (15). When the voltage divided by the resistors (14) and (15) is applied to the base of the NPN transistor (8) and the base voltage is lower than the base voltage of the NPN transistor (7), the error amplifier circuit (4) Since the output becomes higher than the predetermined voltage value, the current flowing through the driving transistor (10) and the output transistor (13) increases. Therefore, the resistance (1
The divided voltage by 4) and (15), that is, the base voltage of the transistor (8) rises and approaches the base voltage of the transistor (7).

On the other hand, when the divided voltage of the resistors (14) and (15), that is, the base voltage of the transistor (8) becomes higher than the base voltage of the transistor (7), the base voltage of the driving transistor (10) relatively decreases. , The current flowing through the transistor (10) and the output transistor (13) decreases. As a result, the base voltage of the transistor (8) drops and again approaches the base voltage of the transistor (7). In this way, the base voltage of the transistor (8) becomes equal to the base voltage (reference voltage) of the transistor (7) after a lapse of a certain time after power-on, and as a result, a predetermined voltage is output from the output terminal (17). A voltage (for example, 5.2V) is generated.

In the circuit of FIG. 2, since the potential difference between the input and output terminals can be reduced to the collector-emitter saturation voltage of the output transistor (13), it becomes a constant voltage power supply circuit with low loss.

(C) Problems to be Solved by the Invention By the way, the power supply voltage is applied to the input terminal (16), the starting circuit (1) operates, and a predetermined reference voltage is supplied from the reference voltage generating circuit (2) to the transistor (7). The base voltage of the transistor (8) does not reach the reference voltage immediately even when applied to the base of (4). Therefore, immediately after startup, the transistor (8) is off, and almost all of the current flowing through the transistor (6) is used as the base current of the driving transistor (10). Therefore, the collector current of the driving transistor (10) increases and overdrives the output transistor (13). As a result, the current flowing through the output transistor (13) sharply increases and power consumption increases.

As described above, according to the conventional circuit, there is a problem that the power consumption increases from the time of start-up until the steady state is reached.

Further, when the input voltage V _IN applied to the input terminal (16) is lower than a predetermined voltage, the base voltage of the transistor (8) remains the same even if a reference voltage is applied to the base of the transistor (7). The value of the reference voltage cannot be reached, so a large current continues to flow in the output transistor (13), and in the worst case,
The element may be destroyed.

In particular, when a dry battery is used as a power source for the input voltage V _IN applied to the input terminal (16), the output voltage of the dry battery gradually decreases due to use, but the power consumption is accelerated as the output voltage decreases. Therefore, the life of the dry battery is rapidly shortened, and a large current flows, which causes deterioration of the semiconductor circuit element.

Therefore, a base current limiting resistor (12) is provided to prevent a large base current from flowing and suppress an increase in power consumption. However, if the resistance value of the resistor (12) is large, the base current can be reduced, but the voltage drop of the resistor (12) becomes large, and in order to obtain the desired output characteristics of the output transistor (13), A higher input voltage V _IN is required, and the features of the low loss type constant voltage power supply circuit are lost. Therefore, generally, the resistance value of the resistor (12) is set to several tens to several hundreds Ω.
However, there is a problem in that the base current is not sufficiently limited and power consumption is large.

The present invention was created in view of the above conventional problems, and provides a low-loss constant-voltage power supply circuit capable of reducing power consumption at startup while maintaining the features of a low-loss type. To aim.

(D) Means for Solving the Problems A constant voltage power supply circuit of the present invention includes a differential amplifier circuit for comparing a reference voltage and an output voltage, and a current mirror circuit arranged as a collector load of the differential amplifier circuit. A drive transistor to which the output of the differential amplifier circuit is applied to the base, and an output transistor driven by the drive transistor,
And a means for preventing saturation of the drive transistor at the time of startup.

(E) Action When the power is turned on, the output of the output transistor has not yet risen sufficiently at startup. That is,
Since the difference between the reference voltage and the output of the output transistor is large at this point, the error amplifier circuit tries to turn on the driving transistor at the next stage deeply to flow a large collector current. Since the collector current of the driving transistor is also the base current of the output transistor, the output transistor also tries to turn on deeply.

However, according to the present invention, since the means for preventing the driving transistor from being saturated is provided, the driving transistor is not deeply turned on, and the collector current thereof does not become excessively large. Therefore, it is possible to prevent an increase in power consumption at startup.

(F) Embodiment An embodiment of the present invention will be described with reference to FIG. FIG. 1 is a configuration diagram of a low-loss constant voltage power supply circuit according to an embodiment of the present invention, in which (19) is a starting circuit, (20) is a reference voltage generating circuit, and (21) is a constant current source. Reference numeral (22) is an error amplification circuit composed of a differential amplification circuit for comparing the output of the reference voltage generation circuit (20) and the divided voltage output A of resistors (31) and (32) described later. The error amplifier circuit (22) includes PNP transistors (23) and (24) that form a current mirror circuit as a load, an NPN transistor (25) whose base input is the output of the reference voltage generation circuit (20), and a resistor divider. It is composed of an NPN transistor (26) having a voltage output A as a base input and a common emitter resistor (27). The base and collector of the PNP transistor (23) are short-circuited to form a diode connection. (28) is for driving an output transistor whose base is connected to the output of the error amplification circuit (22), whose emitter is connected to the ground power supply (36), and whose collector is connected to the bias resistor (29) of the output transistor (30). NPN transistor. (30) is an output transistor, the base is a bias resistor
It is connected to the input terminal (34) (V _IN ) via (29), the emitter is connected to the input terminal (34), and the collector is the output terminal (35).
_Is connected to (V _OUT ). (31) and (32) are resistors for setting the output voltage, and the resistance-divided voltage output A is a transistor
Applied to the base of (26). (33) is a PNP transistor for reducing the starting current, and the base is an output transistor
It is connected to the base of (30), the emitter is connected to the output terminal (35), and the collector is connected to the bases of PNP transistors (23) and (24) that form the current mirror circuit of the error amplification circuit (22). There is.

Next, the operation of the embodiment circuit of the present invention will be described. When a positive power supply voltage (for example, V _IN = 6V) is applied to the input terminal (34), the starting circuit (19) activates the reference voltage generating circuit (20) and a predetermined reference voltage (for example, 1.2V). Is output.
As a result, the reference voltage is applied to the base of the transistor (25) to turn on the transistor (25),
Current flows in 3). This causes the same amount of current to flow in the transistor (24). By the way, since the voltage level of the resistance-divided voltage output A remains at the ground level at this time, the transistor (26) is off, and therefore almost all the current flowing through the transistor (24) is at the base of the transistor (28). Used as an electric current. For this reason, the collector voltage of the transistor (28) drops to almost the ground potential level, and a large amount of base current flows into the output transistor (30).

However, at the same time that the output transistor (30) turns on, the voltage level of the output terminal (35) (which is also the collector voltage of the output transistor) rises to a value almost equal to the voltage of the input terminal, so the transistor (33) turns on. Collector current flows. This current flows into the base of the NPN transistor (25) of the error amplification circuit (22) and flows into the ground power supply via the resistor (27). By the way, since the current flowing through the transistor (25) is set to a constant value by the reference voltage, only the current flowing from the transistor (33) side is
The current flowing into the transistor (25) via (23) is reduced. That is, since the base current of the transistor (23) decreases, the base current of the transistor (24) also decreases and the collector current thereof also decreases. Therefore, the current flowing into the base of the transistor (28) is also reduced,
It is possible to prevent the collector potential of the transistor (28), that is, the base voltage of the output transistor (30), from dropping sharply. And the resistance division voltage output A by resistance (31), (32)
When reaches the reference voltage, the output voltage of the output (35) also stabilizes (for example, 5.2 V) and reaches a steady state.

By the way, in the steady state, a reverse bias is applied between the base and emitter of the output transistor (33), and the output transistor (33) becomes inoperative. Now, the base-emitter voltage of the output transistor (30) is V _BE (30) and the collector-emitter voltage is V _CE (3
0) and the base-emitter voltage of the transistor (33) is V _BE (33), V _BE (30) = V _BE (30) −V _CE (30)
It is represented by. Therefore, when the output transistor (30) is in a non-saturated state, that is, when the output voltage is stable, the transistor
Since the base-emitter of (33) is reverse-biased and becomes inoperative, the constant voltage output function is not impaired.

Next, a description will be given by comparing the power consumptions of the conventional example circuit and the example circuit of the present invention when the input power supply voltage becomes lower than a predetermined voltage. FIG. 3 is an experimental characteristic diagram showing the relationship between the input power supply voltage V _IN and the power supply current I _CC of the conventional circuit, and FIG. 4 is the input power supply voltage V of the embodiment circuit of the present invention.
It is a characteristic view by experiment showing the relation between _IN and power supply current I _CC . Parameter I ₀ is the output current, I ₀ = 0 and I ₀
= 250 mA is shown.

As described above, according to the circuit of the embodiment of the present invention, the power supply current I _CC is 60 mA even when the input power supply voltage V _IN is as small as 3 to 5V.
It can be suppressed to the following. On the other hand, according to the conventional circuit, when I ₀ = 0, it reaches about 330 mA. Therefore, in the case of the conventional circuit, when the input power supply voltage is low,
Not only does a large current flow and wasteful power consumption increase,
The element may be deteriorated and may be destroyed if a large current continues to flow, but this is not the case in the embodiment circuit of the present invention. Further, when a dry battery is used as the V _IN power source, the output voltage of the dry battery also gradually decreases. However, according to the conventional circuit, the lower the output voltage, the more rapidly the power consumption increases and the dry battery Although the life is shortened, the circuit of the embodiment of the present invention can extend the life of the dry battery.

(G) Effect of the Invention As described above, according to the present invention, it is possible to suppress a sharp increase in the base current of the output transistor at the time of startup to reduce power consumption and to output a stable constant voltage in a steady state. it can.

Further, when the input power supply voltage is small, the base current of the output transistor increases as in the case of startup, but according to the present invention, this can also be solved. Especially when a dry battery is used as the input power source, the life of the dry battery can be extended.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a circuit diagram showing a conventional constant voltage power supply circuit, and FIG. 3 is I of FIG.
A characteristic diagram showing the _CC- V _IN characteristic, and FIG. 4 shows I _{CC of} FIG.
It is a characteristic view which shows -V _IN characteristic. (20) …… Reference voltage generation circuit, (22) …… Error amplification circuit, (2
8) …… Drive transistor, (30) …… Output transistor,
(33) …… Starting current reduction transistor, (34) …… Input terminal, (35) …… Output terminal.

Claims (1)

[Claims] 1. A differential amplifier circuit including a pair of transistors whose emitters are commonly connected, for comparing a reference voltage applied to the base of one transistor with an output voltage applied to the other base, and the difference. A current mirror circuit arranged as a collector load of the dynamic amplifier circuit, a drive transistor to which the output of the differential amplifier circuit is applied to the base, an emitter driven by the drive transistor, an emitter at an unregulated power supply terminal, and a collector at PNPs whose bases are connected to the output terminals and the collectors of the drive transistors, respectively.
Type output transistor, a PNP type saturation prevention transistor having an emitter connected to the output terminal, a base connected to the base of the output transistor, and a collector connected to the collector of the one transistor. A constant voltage power supply circuit characterized in that a transistor is turned on to prevent saturation of the drive transistor.