JPH04255460A - Switching regulator - Google Patents

Abstract

PURPOSE:To maintain an output current limiting value constant even if an input voltage is varied. 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. A maximum ON duty of the transistor T1 is so controlled that an input voltage limiting value is maintained constant even if an input voltage V1 is varied by connecting a maximum ON duty controller CNT to a DTC terminal of a DTC comparator COMP2 for controlling the maximum ON duty of the transistor T1 and so varying that the DTC terminal voltage to be applied to the DTC terminal becomes inversely proportional to the voltage V1.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pulse width control type switching regulator that maintains a constant output voltage by changing the duty ratio in response to fluctuations in input voltage and output current.

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. 5 shows general characteristics of this type of switching regulator. FIG. 5(A) shows its duty VI (input voltage) characteristic, and FIG. 5(B) shows its duty IO (output current) characteristic. It shows the characteristics. 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, where T1 is an output transistor, DL and C are a diode and an inductance inserted in the circuit between the output transistor T1 and the load RL, respectively. and a smoothing capacitor.

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 negative 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 VO is monitored by the reference voltage Vre.
compared with f.

Here, the output of the error amplifier OP is such that the output voltage VO on the load side is VO = Vref (RA +R
B)/RB, the reference voltage Vref
Then, the output voltage VO is equal to the above Vref (RA +
RB)/RB, the output voltage of the error amplifier OP conversely decreases or increases with respect to the reference voltage Vref.

Next, the output voltage of the error amplifier OP is PW
It is input to one side input terminal (+ side input terminal in this case) 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 VO 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
As O increases or decreases from the Vref (RA + RB )/RB as a reference, the output voltage VO conversely decreases or increases, thereby maintaining the output voltage VO 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, one input terminal (+ side 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 negative 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 determined by the output state of the DTC comparator COMP2 (in other words, at the DTC terminal). (applied DTC terminal voltage).

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) = (VD
TC −VL )/(VH −VL ).

[0013] Conventionally, as shown in FIG.
The DTC terminal voltage is set to a constant voltage (for example, reference voltage Vref
is fixed at a voltage divided by resistors R3 and R4,
Therefore, it is common to set the above-mentioned Dmax to a fixed value.

However, as explained with reference to FIG. 5A, when the input voltage VI changes, the duty ratio changes, so the maximum value Dmax of the on-duty
If it is limited to a predetermined fixed value as described above, the output current value to which the current is limited changes as I1, I2, I3, etc. depending on the input voltage VI, as shown in Figure 6. The problem with this system was that it was not practical.

[0015]

Problems to be Solved by the Invention The present invention has been made to solve such problems, and it is an object of the present invention to
By changing the TC terminal voltage and controlling the maximum value Dmax of the on-duty, the output current limit value remains constant even if the input voltage changes.

[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. The switching regulator is provided with means for controlling the maximum value Dmax of the duty ratio to vary in inverse proportion to the input voltage at that time, so that the output current limit value is maintained regardless of fluctuations in the input voltage. Provided is a switching regulator characterized in that the switching regulator is constant.

[0017]

[Operation] According to the above configuration, the maximum value D of the duty ratio
By controlling max so that it changes in inverse proportion to the input voltage VI at that time, the change characteristics of the Dmax with respect to the input voltage VI can be made to correspond to a predetermined output current limit value, and the As a result, the output current limit value can be kept constant regardless of fluctuations in the input voltage.

[0018]

[Embodiment] FIG. 1 shows the circuit configuration of a switching regulator as an embodiment of the present invention. , maximum on-duty (Dmax) control circuit CNT is connected. The maximum on-duty control circuit CNT receives the input voltage VI of the switching regulator and outputs a DTC terminal voltage such that the output current limit value remains constant even if the input voltage VI changes.

Here, as shown in FIG. 5(A) above, the duty ratio (Duty) of the switching regulator is
and input voltage VI, Duty∝1/VI
There is a relationship between Further, as explained in relation to FIG. 3, there is a relationship between the maximum on-duty (Dmax) and the DTC terminal voltage VDTC as Dmax∝VDTC.

Therefore, by using a division circuit as the maximum on-duty control circuit CNT, the DTC terminal voltage VD
If TC is changed inversely proportional to the input voltage VI (that is, VDTC ∝1 /VI), the maximum on-duty (Dmax) also changes inversely proportional to the input voltage VI. (that is, Dmax∝1/VI), thereby making it possible to keep the output current limit value constant even if the input voltage VI changes.

FIG. 4 shows D obtained by the apparatus shown in FIG.
The solid line curve in FIG. 4 shows the duty-VI characteristic as a general characteristic of a switching regulator, for example, the duty-VI characteristic for an output current of 100 mA. On the other hand, the dotted line curve shows the Dmax-VI characteristic obtained by the device of the present invention shown in FIG. 1, and is the Dmax-VI characteristic for a predetermined output current limit value (for example, 1 A). This shows that even if the input voltage VI changes, the Dmax changes so that the output current limit value remains constant.

[0022]

According to the present invention, it is possible to obtain a switching regulator in which the output current limit value remains constant even if the input voltage changes.

[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: Dmax-VI obtained by the apparatus of FIG. 1
FIG. 3 is a diagram showing characteristics.

FIG. 5 is a diagram showing general characteristics of a switching regulator.

FIG. 6 is a diagram illustrating problems in the prior art.

[Explanation of symbols]

OP…Error amplification device OS…oscillator COMP1...PWM comparator COMP2...DTC comparator

Claims (1)

[Claims] 1. A switching regulator that maintains an output voltage constant by changing a duty ratio in response to fluctuations in input voltage and output current, wherein the maximum value of the duty ratio is set to the current input voltage. 1. A switching regulator comprising means for controlling the output current to vary in inverse proportion to the voltage, so that the output current limit value remains constant regardless of fluctuations in the input voltage.