JP2011091938A - Abnormality detecting circuit - Google Patents

Abstract

An abnormality of an output voltage is detected even at a soft start.
Soft start reference voltage circuits 16 and 18 output a soft start reference voltage that is maintained at a reference voltage after the voltage gradually rises at the time of power-on and reaches a reference voltage Vref. The output voltage control circuit 12 controls the output voltage according to the soft start reference voltage Vs. The threshold voltage generation circuit 30 generates a threshold voltage that is a predetermined amount lower than the soft start reference voltage Vs. The abnormality detection circuit 32 compares the threshold voltage with the output voltage, and detects a decrease in the output voltage that is greater than or equal to a predetermined value.
[Selection] Figure 1

Description

  The present invention relates to an abnormality detection circuit that detects an abnormality in an output of a power supply circuit.

  Conventionally, a power supply circuit IC for supplying a constant voltage to a semiconductor integrated circuit (IC) has been widely used. In the power supply circuit IC, when the output is grounded, there is a problem that a large current is output and the output transistor is destroyed.

  Therefore, a function of protecting a circuit such as an output transistor by detecting that an output voltage has become a predetermined value or less, detecting an abnormal state such as a ground fault, and limiting an output current is known.

JP 2001-238440 A

  Here, when the circuit is started, the output voltage of the power supply IC starts from 0V. In particular, a soft start function for gradually increasing the output voltage is usually provided in order to prevent an adverse effect caused by a sudden rise in voltage in the circuit. In this case, there is a problem in that a comparison with a predetermined value is performed before the output voltage rises, and it is determined that there is an abnormality for a predetermined period. On the other hand, if detection is not performed during this period, an abnormality such as a ground fault at the output end during the soft start period cannot be detected.

  The present invention is an abnormality detection circuit that detects an abnormality in the output of a power supply circuit, and at the time of power supply startup, the voltage gradually increases, and after reaching the reference voltage, a soft start reference voltage that is maintained at the reference voltage A soft-start reference voltage circuit that outputs, an output voltage control circuit that controls the output voltage according to the soft-start reference voltage, and a threshold voltage generator that generates a threshold voltage that is a predetermined amount lower than the soft-start reference voltage And an abnormality detection circuit that compares the threshold voltage with the output voltage and detects a decrease in the output voltage more than a predetermined value.

  The soft start reference voltage circuit includes a reference voltage circuit that outputs a constant reference voltage and a soft start circuit that outputs a soft start reference voltage that gradually increases at the time of rising. Preferably, the lower voltage of the reference voltages is selected.

  The threshold voltage generation circuit preferably includes an amplifier that amplifies the soft start reference voltage at a predetermined magnification of less than 1.

  Further, it is preferable that the abnormality detection circuit detects an abnormality when a decrease in the output voltage more than a predetermined value continues for a predetermined time.

  According to the present invention, it is possible to detect an abnormality in the output voltage by comparison with an appropriate threshold value even during soft star.

It is a circuit which shows the structure of embodiment. It is a figure which shows the various voltages for abnormality detection.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a circuit diagram illustrating an overall configuration of a power supply circuit including an abnormality detection circuit according to an embodiment. The voltage dividing circuit 10 is composed of resistors R1 and R2 connected in series, and outputs a voltage obtained by dividing the output voltage Vout of the power supply circuit by the resistors R1 and R2 from an intermediate point between the resistors R1 and R2. The power supply circuit as a whole is a circuit that negatively feeds back the output voltage Vout to an error amplifier to which the reference voltage is input, so that the output voltage Vout matches the reference voltage. By configuring the voltage dividing circuit 10 with a resistor ladder and changing the output voltage from the voltage dividing circuit 10, the output voltage Vout can be changed without changing the reference voltage.

  The output voltage of the voltage dividing circuit 10 is input to the negative input terminal of the error amplifier 12. The error amplifier 12 has two positive input terminals, one receiving a reference voltage Vref and the other receiving a soft start reference voltage. The error amplifier 12 compares the smaller input voltage of the two positive input terminals with the input voltage of the positive input terminal, and outputs a signal regarding the comparison result.

  Here, the reference voltage Vref is an output of the reference power supply 14, and the reference power supply 14 is connected to one positive input terminal of the error amplifier 12. The soft start reference voltage is a gradually increasing voltage obtained by charging a constant current from the constant current source 16 to an external capacitor 18 whose other end is connected to the ground when the power supply is turned on. Therefore, the soft start reference voltage exceeds the reference voltage Vref as the predetermined time elapses. After that, the soft start reference voltage is unnecessary, and it is preferable to maintain the voltage at the reference voltage Vref or higher.

  The soft start reference voltage is a voltage that is used only at the start, and the reference voltage Vref is normally used. Therefore, in the normal time, the error amplifier 12 outputs an error signal about the difference voltage between the output voltage of the voltage dividing circuit 10 and the reference voltage Vref from the error amplifier 12. This difference signal becomes a larger positive signal as the output voltage Vout is lower, and a larger negative signal as the output voltage Vout is higher.

  This error signal is input to the positive input terminal of the comparator 20 and is compared with a triangular wave input to the negative input terminal. For example, the output of the comparator 20 has a duty ratio of 50% when the error signal is 0, and the duty ratio decreases as the error signal becomes a large positive signal, and the duty ratio increases as the negative signal increases.

  The output of the comparator 20 is supplied to the gate of the transistor 22. In the transistor 22, the input voltage Vin is supplied to the drain, and the source is connected to Vout of the output voltage Vout through the coil 24. Further, a cathode of a diode 28 whose anode is connected to the ground is connected to an intermediate portion between the transistor 22 and the coil 24, and one end of a capacitor 26 whose other end is connected to the ground is connected to the output end 24. . Therefore, the output voltage Vout obtained by stepping down the input voltage Vin is obtained at the output terminal Vout according to the switching duty ratio of the transistor 22. The output terminal Vout is connected to the voltage dividing circuit 10.

  Therefore, the output of the error amplifier 12 is determined so that Vout × R1 / (R1 + R2) = Vref is satisfied, and the transistor 22 is PWM-controlled with the duty ratio corresponding to this, so that the output voltage Vout is controlled. For example, a stepped down output voltage of about Vin = 5V and Vout = 3V can be obtained. In the case of boosting, a transistor may be provided in parallel with the diode 28 to switch the transistor.

  In the present embodiment, a 0.8 amplifier 30 is provided. The negative input terminal of the 0.8 amplifier 30 is connected to the output terminal Vout. The 0.8 amplifier 30 multiplies the signal input to the positive input terminal by 0.8, and has two positive input terminals. The 0.8 amplifier 30 gives priority to the lower voltage and outputs the voltage.

  A reference voltage Vref is input to one of the two positive input terminals, and a soft start reference voltage is input to the other. Therefore, the output of the 0.8 amplifier 30 is 0.8 times the reference voltage Vref or 0.8 times the soft start reference voltage. Note that the magnification of the 0.8 amplifier 30 may be not less than 0.8 as long as it is less than 1 and greater than 0, but a value suitable for abnormality detection, for example, about 0.6 to 0.9 is adopted. Is done.

  The output of the 0.8 amplifier 30 is input to the positive input terminal of the comparator 32 as a threshold voltage. The output of the voltage dividing circuit 10 (the negative input of the error amplifier 12) is supplied to the negative input terminal of the comparator 32. Therefore, the comparator 32 outputs a high level when the output of the voltage dividing circuit 10 falls below the threshold voltage that is the output of the 0.8 amplifier 30. The output of the comparator 32 is supplied to the timer circuit 34. The timer circuit 34 has an external capacitor 34a, charges the output from the comparator 32 to the capacitor 34a, and outputs an abnormality detection signal when the charging voltage exceeds a predetermined threshold voltage. . The waiting time of the timer circuit 34 is determined by the capacitance of the capacitor 34a, the resistance arranged in the path, and the like. The time of the timer circuit 34 is set depending on how long the period during which the output of the comparator 32 is at the H level is continued. Further, the charge of the capacitor 34a may be reset by natural discharge, but it is also preferable that the charge is discharged through a predetermined resistance. Note that the timer circuit 34 may be configured using a counter that starts counting in accordance with the H level of the comparator 32 instead of such an analog circuit.

  In this manner, an abnormality detection signal is obtained at the output of the timer circuit 34, and a large voltage drop of the output voltage Vout due to a ground fault of the output terminal Vout can be detected and the abnormality detection signal can be output.

  FIG. 2 shows the voltage change of each part of this embodiment. Thus, at the time of start-up, the soft start reference voltage Vs is obtained by charging the capacitor 18 with the constant current from the constant current source 16. The error amplifier 12 controls the output voltage Vout with the lower of the soft start reference voltage and the reference voltage Vref as a target. Therefore, Vout × R1 / (R1 + R2), which is the output of the error amplifier 12, first increases sequentially with the same voltage as the soft start reference voltage Vs. When the soft start reference voltage Vs exceeds the reference voltage Vref, the reference voltage Vref is reached. Be controlled.

  On the other hand, the threshold voltage, which is the output of the 0.8 amplifier 30, is a voltage of the error amplifier target voltage × 0.8. Therefore, when a voltage drop as shown by a two-dot chain line in FIG. 2 occurs due to a ground fault at the output terminal Vout, the comparator 32 outputs an abnormality detection signal. Therefore, although not shown in FIG. 2, the power supply circuit can be protected by limiting the output current with another circuit.

  Here, in the error amplifier 12 or the 0.8 amplifier 30, in order to select a lower one of the input signals at the positive input terminal, for example, a differential amplifier having a pair of p-channel transistor diodes as a differential transistor, Two differential transistors on the positive input side may be arranged in parallel.

  In addition, when the soft start reference voltage Vs exceeds the reference voltage Vref, it is preferable to limit charging so that the soft start reference voltage Vs does not become higher than that. For example, the constant current source 16 may be stopped and clamped at a predetermined potential.

  As described above, according to the present embodiment, since the voltage considering the soft start reference voltage is used as the threshold voltage of the comparator 32, a voltage abnormality due to an output ground fault or the like is detected from the time of startup. It becomes possible.

  In the figure, the terminals of the semiconductor integrated circuit IC are indicated by circles.

  10 voltage divider circuit, 12 error amplifier, 14 reference power supply, 16 constant current source, 18 capacitor, 20 comparator, 22 transistor, 24 coil, 26 capacitor, 28 diode, 30 0.8 amplifier, 32 comparator, 34 timer circuit. 34a Capacitor.

Claims (4)

An abnormality detection circuit for detecting an abnormality in the output of the power supply circuit,
A soft start reference voltage circuit that outputs a soft start reference voltage that is maintained at the reference voltage after the voltage gradually rises and reaches the reference voltage at the time of power-on,
An output voltage control circuit for controlling the output voltage according to the soft start reference voltage;
A threshold voltage generation circuit that generates a threshold voltage that is lower by a predetermined amount than the soft start reference voltage;
An abnormality detection circuit that compares the threshold voltage with the output voltage and detects a decrease in the output voltage over a predetermined value;
An abnormality detection circuit comprising: The abnormality detection circuit according to claim 1,
Soft start reference voltage circuit
A reference voltage circuit that outputs a constant reference voltage;
A soft start circuit that outputs a soft start reference voltage that gradually rises at the time of rising;
Including
An abnormality detection circuit, wherein a lower one of a reference voltage and a soft start reference voltage is selected. In the abnormality detection circuit according to claim 1 or 2,
The abnormality detection circuit, wherein the threshold voltage generation circuit includes an amplifier that amplifies the soft start reference voltage at a predetermined magnification of less than 1. In the abnormality detection circuit according to any one of claims 1 to 3,
The abnormality detection circuit detects an abnormality when a decrease in output voltage more than a predetermined value continues for a predetermined time.

Images