JP4984565B2 - DC-DC converter - Google Patents

Description

  The present invention relates to a step-down or step-up DC-DC converter used for a portable terminal or the like.

  2. Description of the Related Art A step-down or step-up DC-DC converter that performs DC-DC (direct current-direct current) conversion by turning a switching element on (ON) or off (OFF) is widely used in portable terminals (for example, patents). Reference 1). FIG. 4 is a diagram showing a circuit configuration example of such a general DC-DC converter. Here, an example of a step-down DC-DC converter is shown.

  The DC-DC converter includes an error amplifier Q1 that amplifies an error between a feedback signal from the output voltage Vo and a reference voltage Vref from the reference voltage generator 1, a triangular wave from the triangular wave generator 2, and an output signal of the error amplifier Q1. A comparator Q2 that compares (error signal) and outputs a PWM (Pulse Width Modulation) pulse with a duty ratio corresponding to the output signal (error signal), and outputs the PWM pulse output from the comparator Q2 A P-channel MOS transistor Tr1 which is a switching element is turned on and off via the MOS driver 3, and a DC voltage generated thereby is smoothed and supplied to a load.

  The error amplifier Q1 and comparator Q2, the reference voltage generator 1, the triangular wave generator 2 and the output MOS driver 3 are configured in the control IC 10 together with an internal power supply 4 for generating 3V. A commutation diode D1 and a coil L1 as an inductor are connected to the MOS transistor Tr1, and a smoothing capacitor C1 is connected to the opposite side of the coil L1. A load resistor R1 is connected via a switch SW1 as a load to which the output voltage Vo is supplied.

  The output voltage Vo is divided by the resistors Ra and Rb, and the divided voltage is input to the error amplifier Q1 as a feedback signal and compared with the reference voltage Vref. Further, a series circuit of a resistor R2 and a capacitor C2 is connected as a phase compensation circuit between the input and output of the error amplifier Q1, and a series circuit of a resistor R3 and a capacitor C3 is connected as a phase compensation circuit in parallel with the voltage dividing resistor Ra. Yes. These phase compensation circuits are provided to stabilize the operation of the DC-DC converter.

  The DC-DC converter configured as described above compares the feedback signal from the output voltage Vo with the reference voltage Vref in the error amplifier Q1, amplifies the error voltage, and outputs the amplified error signal to the comparator Q2. At this time, the output (error signal) of the error amplifier Q1 is controlled to increase when the feedback signal is lower than the reference voltage Vref, and to decrease when the feedback signal is higher. The comparator Q 2 compares the error signal output from the error amplifier Q 1 with a triangular wave and outputs a PWM pulse to the output MOS driver 3. Thereby, the ON / OFF duty ratio of the MOS transistor Tr1 is controlled, and the output voltage Vo is controlled. Although the triangular wave is output from the triangular wave generator 2, a sawtooth wave may be output instead of the triangular wave.

  Actually, in a DC-DC converter, the input voltage Vcc and the load current Io often fluctuate within a range that cannot be ignored. In addition, there may be a problem in stability depending on the frequency characteristics of the parts forming the feedback loop. On the other hand, in order to keep the output voltage Vo stable, a phase compensation circuit may be added around the error amplifier and the sensing resistor (see, for example, Non-Patent Document 1). In the circuit of FIG. 4, a phase compensation circuit of a resistor R2 and a capacitor C2 is connected between the input and output of the error amplifier Q1, and a phase compensation circuit of a resistor R3 and a capacitor C3 is connected in parallel with the resistor Ra that is a sensing resistor. .

  That is, since the error amplifier Q1 is a negative feedback circuit, the phase is delayed by 180 degrees, and the phase delay of the LC filter by the coil L1 on the output side and the smoothing capacitor C1 and the phase delay of the error amplifier Q1 overlap with each other. When phase delay occurs, the output voltage oscillates. Therefore, output voltage oscillation is prevented by performing phase compensation in the feedback circuit of the error amplifier Q1.

  The reason why the phase compensation circuit is added around the sensing resistor is that in the voltage mode, the phase compensation (delay compensation) of the error amplifier Q1 is often insufficient. The phase compensation of the sensing resistor is partial phase advance compensation, and the oscillation phenomenon is improved by advancing the phase of a part of the frequency domain as shown in FIG. FIG. 5 is a diagram illustrating the frequency characteristics of the phase compensation circuit around the sensing resistor, where A indicates the phase (Phase) [deg] and B indicates the gain (Gain) [dB].

  Here, when there is a phase compensation circuit around the sensing resistor, the voltage across the capacitor C3 is held by the charge accumulated in the capacitor C3 of the phase compensation circuit when a load short circuit occurs, and therefore, the input to the error amplifier Q1. The terminal IN becomes a negative voltage. As shown in FIG. 6, the input terminal IN of the control IC 10 is usually provided with a protection circuit composed of diodes D2 and D3 to protect against input positive (+) and negative (-) overvoltages. However, since the ground (GND) side of the diode D2 is actually connected to the emitter of the parasitic transistor Tr2, when the input terminal IN becomes a negative voltage, a bias current flows through the parasitic transistor Tr2 and the parasitic transistor Tr2 is turned on. . For this reason, an excessive current flows through the parasitic transistor Tr2, and the control IC 10 may be destroyed.

  Therefore, a countermeasure example as shown in FIG. 7 is taken. FIG. 7 is a diagram showing a circuit example for preventing an excessive current from flowing through the control IC 10. In this circuit, a Schottky barrier diode D4 having a low forward voltage (about 0.3 V) is connected between the input terminal IN and the ground outside the control IC 10.

In the above circuit, when a load short circuit occurs, the electric charge accumulated in the capacitor C3 is released. However, the Schottky barrier against the forward voltage (about 0.6 V) of the diode D2 of the protection circuit inside the control IC 10 is released. Since the forward voltage of the diode D4 is lower, the charge stored in the capacitor C3 is discharged by the Schottky barrier diode D4. Since the negative voltage does not drop to 0.6V, the diode D2 of the protection circuit is not turned on, the parasitic transistor Tr2 is also kept off, and no excessive current flows.
JP 2002-84741 A 4. Sanyo Electric Co., “Application of switching power supply to smoothing capacitor” Design example for OS-CON, [on line], [Search December 7, 2005], Internet <URL: http://www.secc.co.jp/pdf/2005/oscon/j/j82. pdf>

  However, the conventional DC-DC converter as described above has a problem that the cost is increased due to an increase in the number of components and the component layout, since measures against excessive current are taken with external components.

  The present invention has been made in view of these points, and an object of the present invention is to provide a highly reliable DC-DC converter that can prevent destruction of a control IC due to an excessive current with an inexpensive configuration.

In the present invention, in order to solve the above-described problem, a feedback in which the output voltage is divided by a first resistor having one end connected to the output voltage and a second resistor connected to the other end of the first resistor. An error amplifier that compares the signal with a reference voltage to generate an error signal, and a control integrated circuit that includes a comparator that compares the error signal with a triangular wave or sawtooth wave to output a PWM pulse, and is output from the comparator A DC-DC converter for generating a DC voltage by turning on and off a switching element by a PWM pulse, and a capacitor connected in parallel to the first resistor in a feedback circuit of the feedback signal input to the error amplifier together and a phase compensation circuit with a the abnormality of the load which the output voltage is supplied, against the other end of the first resistor DC-DC converter is provided, characterized in that it comprises a switch for switching on the ground side end of the connection of the phase compensating circuit from the input terminal of the error amplifier, which is in the control integrated circuit.

  According to such a DC-DC converter, the connection of one end of the phase compensation circuit is switched from the input terminal of the error amplifier to the ground side by a switch when the load is abnormal, so that the control IC is destroyed by an excessive current with an inexpensive configuration. Can be prevented, and it becomes highly reliable.

  In the DC-DC converter of the present invention, the connection of one end of the phase compensation circuit is switched to the ground side when the load is abnormal by the switch. Therefore, the control IC can be prevented from being destroyed by an excessive current, and has high reliability. There is an advantage.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a circuit diagram showing a main part of a DC-DC converter according to a first embodiment of the present invention. The overall configuration of the DC-DC converter of the present embodiment is the same as that of FIG. 4, and the comparator Q2 compares the triangular wave with the error signal that is the output signal of the error amplifier Q1, and the duty according to the error signal. A ratio PWM pulse is generated. Then, the PWM pulse output from the comparator Q2 turns on and off the MOS transistor Tr1 which is a switching element to generate a DC voltage, which is smoothed and supplied to the load. In addition, a phase compensation circuit of a resistor R3 and a capacitor C3 is provided on the input side of the error amplifier Q1, which is a feedback circuit for a feedback signal of the output voltage Vo, and one end of the phase compensation circuit is provided when a load to which the output voltage Vo is supplied is abnormal. Are connected to the ground side from the input terminal of the error amplifier Q1. The switches SW2 and SW3 connect the input terminal IN to which one end of the phase compensation circuit is connected to the inverting input terminal of the error amplifier Q1 during normal operation of the DC-DC converter, and when a load short circuit (output short circuit) is detected. Connects both to ground.

  Next, the control operation of the DC-DC converter configured as described above will be described. The DC-DC converter of the present embodiment has a configuration in which two switches SW2 and SW3 are added between the error amplifier Q1 and the input terminal IN of the control IC 10 with respect to the circuit shown in FIG. Here, a case will be described in which the voltage at the input terminal IN is detected as an overcurrent when the voltage is 0.4 V or less, for example.

  Since the output voltage Vo is stably supplied during normal operation (steady time), the voltage of the input terminal IN is 1 V (output from the reference voltage generator 1 and input to the non-inverting input terminal of the error amplifier Q1. The reference voltage is 1V and is virtually short-circuited to this reference voltage). When a load short-circuit occurs, the output voltage and the voltage at the input terminal IN rapidly decrease, and when the overcurrent is detected (the voltage at the input terminal IN = 0.4 V), the switches SW2 and SW3 are switched to the ground side. As a result, the electric charge accumulated in the capacitor C3 is discharged to the ground side, and the electric charge can be discharged without the level of the input terminal IN becoming a negative voltage. Therefore, the minimum value of the level of the input terminal IN becomes 0 V, and the protection circuit The diodes D2 and D3 and the parasitic transistors Tr2 and Tr3 do not operate.

  As described above, the DC-DC converter of the present embodiment is a circuit having a phase compensation circuit around the sensing resistor. Even when the output is short-circuited, the signal from the phase compensation circuit is switched to the ground side by the switches SW2 and SW3. In other words, a negative voltage is not applied to the input terminal IN of the control IC 10, and the destruction of the control IC 10 due to an excessive current can be prevented with an inexpensive configuration, so that the reliability is high.

Since the switch SW3 is switched for the purpose of fixing the input level of the MOS transistor Tr1, it may be switched to the input voltage Vcc side.
FIG. 2 is a circuit diagram showing a specific configuration example of the switches SW2 and SW3 of the present embodiment. In this circuit, the voltage of the input terminal IN is simply determined by the comparator Q3 with hysteresis, and the switch composed of the N-channel MOS transistors Tr5 and Tr8 is operated via the driver 5. Absent. The reference voltage (V−) of the comparator Q3 is generated from the reference voltage Vref of 1V from the reference voltage generator 1 in the control IC 10 of FIG. 4 and a resistor. For example, the threshold value at the time of falling is 0.3V, The threshold is set to 0.5V. This is an example, and the absolute level and the hysteresis level are adjusted so as to operate stably without hunting or the like. The switch connecting the input terminal IN of the control IC 10 and the error amplifier Q1 is a transmission gate circuit composed of an N-channel MOS transistor Tr6, a P-channel MOS transistor Tr7, and an inverter Q4 so as to cope with a voltage change. In FIG. 2, the diodes D2 and D3 are not shown.

  FIG. 3 is a circuit diagram showing a main part of the DC-DC converter according to the second embodiment of the present invention. In the present embodiment, the control IC 10 includes a sensing resistor. That is, there are resistors Ra and Rb for dividing the output voltage in the control IC 10, a feedback signal is input from the connection point to the error amplifier Q1, and a series circuit of a resistor R3 and a capacitor C3 which is a phase compensation circuit in parallel with the resistor Ra. Are connected. 1 is input to the inverting input terminal of the error amplifier Q1 during the normal operation of the DC-DC converter by the switch SW4 corresponding to the switch SW2 in FIG. 1, and when a load short circuit (output short circuit) is detected. Is input to the ground side.

  Also in this embodiment, like the circuit shown in FIG. 1, even if a load short-circuit occurs, the input terminal IN does not become a negative voltage, and it is possible to prevent destruction of the control IC 10 due to an excessive current with an inexpensive configuration. It will be highly reliable. In the case of the present embodiment, when the switch SW4 is switched to the ground side, the input of the error amplifier Q1 is fixed at 0 V (short circuit voltage of the output), so that the one corresponding to the switch SW3 in FIG. 1 is not necessary.

  In each of the above embodiments, the step-down DC-DC converter is shown. However, the same effect can be obtained with the step-up DC-DC converter. However, in the case of a step-up DC-DC converter, when the output is short-circuited, a short-circuit current continues to flow through the path of input voltage Vcc → coil (inductor) → output terminal (short-circuit). This is a difference from the step-down DC-DC converter. A step-up DC-DC converter in which a load switch is provided between the coil and the output terminal and a circuit for turning off the load switch when an overcurrent is detected has substantially the same operation as the step-down DC-DC converter.

It is a circuit diagram which shows the principal part of the DC-DC converter of the 1st Embodiment of this invention. It is a circuit diagram which shows the specific structural example of the switch of 1st Embodiment. It is a circuit diagram which shows the principal part of the DC-DC converter of the 2nd Embodiment of this invention. It is a figure which shows the circuit structural example of a general DC-DC converter. It is a figure which shows the frequency characteristic of the phase compensation circuit around sensing resistance. It is a figure which shows the protection circuit of the input terminal of control IC. It is a figure which shows the example of a circuit which prevents that an overcurrent flows into control IC.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Reference voltage generator 2 Triangular wave generator 3 Output MOS driver 4 Internal power supply 5 Driver 10 Control IC
C1 smoothing capacitor C2, C3, capacitor D1 commutation diode D2, D3 diode L1 coil Q1 error amplifier Q2, Q3 comparator R1 load resistance R2, R3, Ra, Rb resistance SW1, SW2, SW3, SW4 switch Tr1, Tr5, Tr6 Tr7, Tr8 MOS transistor

Claims (2)

A feedback signal obtained by dividing the output voltage by a first resistor having one end connected to the output voltage and a second resistor connected to the other end of the first resistor is compared with a reference voltage to generate an error signal. An error amplifier, and a control integrated circuit including a comparator that compares the error signal with a triangular wave or a sawtooth wave and outputs a PWM pulse, and the switching element is turned on and off by the PWM pulse output from the comparator. A DC-DC converter for generating a DC voltage,
The feedback circuit of the feedback signal input to said error amplifier, and having a phase compensation circuit comprising a first capacitor connected in parallel to resistor,
The control integrated switch for switching the connection of one end of the phase compensation circuit connected to the other end of the first resistor from the input terminal of the error amplifier to the ground side when the load supplied with the output voltage is abnormal A DC-DC converter characterized by having in a circuit . 2. The DC-DC converter according to claim 1, wherein the load abnormality is a load short circuit.

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