JPH04255459A - Switching regulator - Google Patents

Abstract

PURPOSE:To limit a current when an input voltage is high and to control a duty when the voltage decreases in a step-down type switching regulator. CONSTITUTION:An output voltage V0 of a switching regulator is monitored by an error amplifier OP, its output and an output of a triangular wave oscillator OS is input to a PWM comparator COMP1, and an ON duty of an output transistor T1 is controlled by the output of the comparator COMP1. When an input voltage V1 is high, a maximum ON duty of the transistor T1 is limited by a fixed voltage applied to a DTC terminal of a DTC comparator COMP 2. When the voltage V1 decreases, an output side of a comparator COMP3 becomes a high level, a transistor T3 is turned ON, and a limit of the ON duty is released.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is a step-down type pulse width control method that maintains a constant output voltage by changing the duty ratio in response to fluctuations in input voltage and output current. Related to switching regulators (stepped down from input voltage).

0002

[Prior Art] Conventionally, this type of pulse width control type switching regulator changes the output voltage by changing the on-duty (the period during which the output transistor is on in one cycle) in response to fluctuations in the input voltage and output current. is kept constant.

FIG. 4 shows general characteristics of this type of switching regulator. FIG. 4(A) shows its duty VI (input voltage) characteristic, and FIG. 4(B) shows its duty I0 (output current) characteristic. It shows. As shown in the figure, when the input voltage VI decreases, the duty ratio is increased, and conversely, when the input voltage VI increases, the duty ratio is decreased. On the other hand, when the output current increases due to a decrease in load resistance, the duty ratio is increased accordingly, and conversely, when the output current decreases due to an increase in load resistance, the duty ratio is decreased, thus maintaining the output voltage constant. .

FIG. 2 shows an example of the circuit configuration of a switching regulator as a conventional technique, in which T1 is an output transistor, and D, L, and C are each inserted in a circuit between the output transistor T1 and a load RL. Diode, inductance and smoothing capacitor are shown.

The output voltage VO applied to the load RL is resistance-divided by resistors RA and RB and input to one side input terminal (for example, the + side input terminal) of the error amplifier (for example, an operational amplifier) OP. A predetermined reference voltage Vr is applied to the other side input terminal (for example, - side input terminal).
ef is input. In this way, the error amplifier OP
The output voltage V0 is monitored by the reference voltage Vre.
compared with f.

Here, the output of the error amplifier OP is such that the output voltage V0 on the load side is V0 = Vref (RA +R
B) When /RB, the reference voltage Vref
Then, the output voltage VO is the above Vref (RA
+RB) /RB, the output voltage of the error amplifier OP is also equal to the reference voltage Vref.
increase or decrease based on

Next, the output voltage of the error amplifier OP is PW
It is input to one side input terminal (in this case, - side input terminal) of M comparator COMP1, and the other side input terminal (+
Oscillator (triangular wave oscillator) OS input to side input terminal)
is compared with the output of As a result, as the output voltage V0 increases or decreases as described above, the P
On the contrary, the period during which the output side of the WM comparator COMP1 is at a high level decreases or increases.

Since the output side of the PWM comparator COMP1 is input to one side input terminal of the AND gate G, the output side of the DTC comparator COMP2 (to be described later) inputted to the other side input terminal of the AND gate is at a high level. As long as it is, the output side of the AND gate G becomes high level according to the high level period of the output side of the PWM comparator COMP1, and the switching transistor T2 becomes high level according to the high level period of the AND gate G.
turns on. During the period when the transistor T2 is on, a predetermined voltage is applied between the emitter and the base of the output transistor T1 through the resistor R1, and the output transistor T1 is turned on.

Therefore, the period during which the output transistor T1 is on (on duty) is equal to the output voltage V
0 increases or decreases from the Vref (RA +RB)/RB as a reference, the output voltage V0 conversely decreases or increases, thereby maintaining the output voltage V0 at the Vref (RA +RB)/RB.

Generally, in a switching regulator, a current limit is applied to prevent the output transistor (corresponding to the above-mentioned transistor T1) from being destroyed due to overcurrent. As a result, a method for limiting the on-duty (dead time control, commonly known as DTC) is adopted.

To this end, the circuit shown in FIG. 2 is provided with a DTC comparator COMP2, and one input terminal (the negative input terminal in this case) of the DTC comparator COMP2 is set as the DTC terminal, and the voltage applied to the DTC terminal is set as the DTC terminal. The voltage is compared with the output of the oscillator (triangular wave oscillator) OS input to the other side input terminal (in this case, the + side input terminal) of the DTC comparator COMP2. Regardless of the output level of the PWM comparator COMP1, the maximum on-duty (maximum value of on-duty) Dmax of the output transistor T1 is
It is determined by the output state of the TC comparator COMP2 (in other words, the DTC terminal voltage applied to the DTC terminal).

That is, in FIG. 3, the DTC terminal voltage applied to the DTC terminal is VDTC, and the oscillator O
The maximum output voltage value and the minimum output voltage value of S are respectively V
H and VL, the DTC comparator CO
The periods during which the output voltage of MP2 is at high level and low level are TON and TOFF, respectively. During the period of TON, the output transistor T1 is turned on via the transistor T2, so the maximum value Dmax of the on-duty is as follows: Dmax = TON/(TON+TOFF) = (VH
-VDTC)/(VH -VL).

[0013] Conventionally, as shown in FIG.
Generally, the DTC terminal voltage is fixed at a constant voltage (for example, a voltage obtained by dividing the reference voltage Vref by resistors R3 and R4), thereby setting the above-mentioned Dmax at a fixed value.

However, as explained above with reference to FIG. 4(A), when the input voltage VI changes, the duty ratio changes, so the maximum value D of the on-duty
If we limit max to a predetermined fixed value as described above, then
As shown in FIG. 5, there is a problem in that the duty ratio cannot be controlled when the input voltage VI decreases (in FIG. 5, when the input voltage VI decreases below V2). Note that V1 in FIG. 5 above indicates that the duty ratio is 100 when the duty ratio is not limited as described above.
%, and the V1 is (V0 +VC
E·sat ) (wherein VCE·sat indicates the saturation voltage of the output transistor T1).

[0015]

[Problems to be Solved by the Invention] The present invention has been made to solve this problem, and when the input voltage is high (normal operating state), the maximum value of the on-duty is limited (that is, the current When the input voltage drops (however, the required minimum value of input voltage is as mentioned above (V
0 +VCE·sat)), the purpose is to obtain a step-down switching regulator that can control the duty ratio.

[0016]

[Means for Solving the Problems] In order to solve the problems, according to the present invention, the output voltage is maintained constant by changing the duty ratio in response to fluctuations in the input voltage and output current. Provided is a step-down switching regulator characterized in that it includes means for limiting the maximum value Dmax of the duty ratio, and means for canceling the limitation when the input voltage drops.

That is, the present invention provides the step-down switching regulator with a duty ratio of 1.
We focused on the fact that there is no particular problem in the operation of keeping the output voltage constant even when the input voltage drops, and when the input voltage drops, we remove the limit on the maximum value of the on-duty and the duty ratio becomes 100%. In this way, the duty ratio can be controlled even when the input voltage drops.

[0018]

[Function] According to the above configuration, in a step-down switching regulator, current is limited when the input voltage is high (normal operating state), and the duty ratio is limited even when the input voltage is low (VI > V0 + VCE・sat ). can be controlled.

[0019]

[Embodiment] FIG. 1 shows the circuit configuration of a switching regulator as an embodiment of the present invention. As described above, a fixed voltage obtained by dividing the reference voltage Vref by the resistors R3 and R4 is applied, and the transistor T3 is connected via the resistor R5. and the transistor T
The output side of the comparator COMP3 is connected to the base of 3, and one side input terminal of the comparator COMP3 (
- side input terminal), input voltage VI is connected to resistors R6 and R7.
The voltage divided by resistance is input to the comparator CO
It is compared with a reference voltage Vref applied to the other side input terminal (+ side input terminal) of MP3, and an on-duty limit switching circuit is configured by these additional circuits.

In the above-described circuit, while the input voltage VI is high, the output side of the comparator COMP3 is at a low level, the transistor T3 is turned off, and the resistors R3 . The maximum value of the on-duty is limited by a fixed voltage resistance-divided by R4. On the other hand, as the input voltage VI decreases, the resistor R6 . When the voltage divided by R7 becomes lower than the reference voltage Vref,
The output side of the comparator COMP3 is inverted to high level, thereby turning on the transistor T3 and turning on the D
The TC terminal voltage is set below the minimum output voltage value VL of the oscillator. As a result, the maximum value of the on-duty is no longer limited by the fixed voltage, and the duty ratio can be set to 100%.

In this way, in the switching regulator according to the present invention, the on-duty is not limited when the input voltage drops, so the switching regulator can be used in the power supply circuit of in-vehicle electronic equipment (particularly microcomputer-controlled electronic equipment). When used, input voltage (battery voltage) such as when the ignition switch is turned on.
It is possible to control the electronic device even when the voltage has not risen sufficiently (for example, in the case of a microcomputer, the VCC voltage only needs to be 5V, so the battery voltage that is the input voltage of the switching regulator must be approximately 5V or more). (This enables the operation of the above microcontroller.) Therefore, the switching regulator is particularly used in engine control equipment (such as fuel injection control).
If used in the power supply circuit of a device controlled by a microcomputer, the microcomputer can operate not only when the ignition switch is turned on as described above, but also when the battery voltage drops, as long as the battery voltage is approximately 5V or higher. , thereby making it possible to start the engine.

[0022]

According to the present invention, it is possible to obtain a step-down switching regulator that is capable of current limitation when the input voltage is high and duty control when the input voltage is low.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a circuit configuration of a switching regulator as an embodiment of the present invention.

FIG. 2 is a diagram illustrating a circuit configuration of a switching regulator as a conventional technique.

FIG. 3 is a diagram explaining the function of dead time control (DTC).

FIG. 4 is a diagram showing general characteristics of a switching regulator.

FIG. 5 is a diagram illustrating problems in the prior art.

[Explanation of symbols]

OP...Error amplifier OS...Oscillator COMP1...PWM comparator COMP2...DTC comparator COMP3...Comparator that configures the on-duty limit switching circuit

Claims (1)

[Claims] 1. A step-down switching regulator that maintains a constant output voltage by changing the duty ratio in response to fluctuations in input voltage and output current, the step-down switching regulator having a maximum value of the duty ratio. 1. A switching regulator comprising: means for limiting; and means for canceling the limit when the input voltage drops.