JP2009195022A - Dc-dc converter and semiconductor integrated circuit for controlling power supply - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a control technology capable of avoiding degradation of power efficiency in a current feedback type DC-DC converter. <P>SOLUTION: The DC-DC converter includes current detecting circuits Q1, Q2, AMP1 that apply a DC input voltage supplied from DC power supply to an inductor to flow a current and detect a current flowing through a switching device SW1 for driving and storing energy in a coil to output as a ground contact potential reference voltage, a low-pass filter LPF connected to a rear stage of the current detecting circuits, level shift means Q4 for raising a potential of a signal that passes through the low-pass filter, and waveform composite means CS1, C3, Q3 for generating a RAMP signal (triangular wave) only during current detection by the current detection circuits and adds the signal to the voltage raised by the level shift means. An output voltage of the waveform composite means is supplied to a PWM comparator 22 as a reference potential. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a switching regulator type DC-DC converter for converting a DC voltage and a semiconductor integrated circuit for controlling the power supply thereof, and more particularly, current feedback for detecting and controlling a current when a switching element on a voltage input terminal side is turned on. The present invention relates to a technique effective when applied to a DC-DC converter of a type.

  There is a switching regulator type DC-DC converter as a circuit that converts an input DC voltage and outputs DC voltages of different potentials. Such a DC-DC converter includes a driving switching element that applies a DC power source voltage supplied from a DC power source such as a battery to an inductor (coil) to flow current and accumulate energy in the coil, and the driving switching element. A diode rectification type DC-DC converter is provided with a rectifying switching element that rectifies the coil current during the off-energy release period, and is turned on and off in a complementary manner with the driving switching element and the rectifying switching element. There is a synchronous rectification type DC-DC converter with higher power efficiency.

In a synchronous rectification type DC-DC converter, an output voltage is detected by an error amplifier and fed back to a PWM (pulse width modulation) comparator or a PFM (pulse frequency modulation) comparator. In general, voltage feedback control is performed to shorten the ON time of the switching element when the output voltage is increased and the output voltage is increased. Furthermore, in order to limit the peak current at light load by detecting the current flowing through the switching element (high side switch) on the input terminal side, a control called current mode is also used in which the current is detected and controlled. It is. As inventions related to DC / DC converters to which current mode control is applied, for example, there are those described in Patent Document 1 and Patent Document 2.
JP 2006-0149056 A Japanese Patent Laid-Open No. 08-298771

  The present applicant has developed and studied a current feedback type DC-DC converter having a configuration as shown in FIG. 4 in order to prevent sub-harmonic oscillation of the inductor current occurring in a region where the duty is 50% or more. The DC-DC converter of FIG. 4 performs PWM control by feeding back the output voltage, receives an electric potential of a current detection resistor Rs connected in series with the switching element, detects an electric current, and an RAMP signal ( An amplifier AMP2 that converts voltage to current of the triangular wave), a resistor R0 that synthesizes the currents of these amplifiers and converts them into a composite voltage as shown in FIG. 5B, and a power source Vb for level shift that raises the voltage. And the raised voltage is supplied to the non-inverting input terminal of the PWM comparator 22 as a reference potential.

  The regulator of FIG. 4 can prevent subharmonic oscillation by performing phase compensation by synthesizing the RAMP waveform and the current detection waveform. However, the current detection of the signal after the synthesis of the RAMP signal and the current detection signal is possible. Spike-like noise is generated at points such as t1 to t5 in FIG. 5B where the signal disappears, and the combined waveform (triangular wave) is destroyed, which may reduce the linearity of the duty. When driving with a duty of 100% is required, the pre-start signal at the beginning of the next cycle is masked due to the response delay of the PWM comparator, and the flip-flop 25 at the subsequent stage is not set and the switching at the next cycle is performed. May fall out.

  Furthermore, a current detection resistor Rs is used, and the value of this resistor is preferably as small as possible to reduce the loss. However, considering manufacturing variation and area, it is difficult to make the value 100 mΩ or less. For this reason, when the on-resistance of the switching element is reduced by improving the process, there is a problem that the loss of the detection resistor Rs is relatively increased and the power efficiency is deteriorated.

  The present invention has been made paying attention to the above-described problems. The object of the present invention is to improve the linearity of the duty in the current feedback type DC-DC converter and to reduce the following due to the response delay of the PWM comparator. It is an object of the present invention to provide a control technique capable of avoiding an undesired situation in which a mask is applied to a pre-start signal at the beginning of a cycle.

  Another object of the present invention is to provide a control technique capable of improving power efficiency in a current feedback type DC-DC converter.

  In order to achieve the above object, the present invention provides an inductor (coil) for voltage conversion, and a driving switching element for applying a DC input voltage supplied from a DC power source to the inductor so as to pass a current and accumulate energy in the inductor. And a rectifying element that rectifies the current of the inductor during an energy release period in which the driving switching element is turned off, and a PWM comparator to which an output voltage is fed back to detect a current flowing through the driving switching element and In a DC-DC converter including a switching control circuit that performs on-time control of the driving switching element, the switching control circuit detects a current flowing through the driving switching element and outputs the detected current as a ground potential reference voltage. The current detection circuit and the current detection circuit detects the current. Only While in a waveform synthesis means for generating a predetermined waveform signal (RAMP signal) is the output voltage of the waveform synthesizing means those to be supplied to the PWM comparator.

  According to the DC-DC converter having the above-described configuration, phase compensation can be performed by combining a waveform in which the current flowing through the switching element is detected and the RAMP waveform, thereby preventing harmonic oscillation and improving the linearity of the duty. Will be able to.

  Preferably, the current detection circuit includes an on period detection circuit that extracts and transmits a current flowing through the driving switching element connected to an output node, a dummy element having the same structure as the driving switching element, A transistor and a resistor connected in series with the dummy element; and a differential amplifier for receiving a signal from the on-period detection circuit and outputting a signal for biasing the transistor, the output of the differential amplifier being the current detection circuit Is configured to be output of

  Thereby, it is not necessary to provide a current detection resistor in series with the driving switching element, and the power efficiency can be improved. Instead of using a dummy element, a current detection resistor is connected in series with the driving switching element, and a differential amplifier that receives the voltage dropped by the current detection resistor is input to the current detection circuit. It is also possible to configure so that the output of the differential amplifier is the output of the current detection circuit.

  The switching control circuit includes level shift means for raising the potential of the output signal of the current detection circuit, and the waveform synthesis means is configured to add the voltage raised by the level shift means to the waveform signal. . Thereby, a waveform signal of a desired level can be supplied to the PWM comparator.

  The switching control circuit includes a low-pass filter connected to a subsequent stage of the current detection circuit, and the level shift means raises the potential of the signal that has passed through the low-pass filter. By providing a low-pass filter, noise can be removed from the signal supplied to the PWM comparator.

  Further, an error amplifier that outputs a voltage corresponding to the potential difference between the feedback voltage from the output side and the reference voltage is provided in the previous stage of the PWM comparator, and the output of the PWM comparator is reset in the subsequent stage of the PWM comparator. A flip-flop that is input to the terminal and to which a pre-start signal having a predetermined period is input is provided to the set terminal. As a result, the ON timing of the switching element can be accurately given by the pre-start signal, and the output of the PWM comparator becomes a whisker-like pulse, so that the pre-start signal at the head of the next cycle is masked by the response delay of the PWM comparator It is possible to avoid the occurrence of such an undesirable situation.

  Further, the level shift means uses a PNP bipolar transistor or a P-channel FET that has a collector terminal connected to a ground point, a signal that has passed through the low-pass filter is input to a base terminal, and a level-shifted voltage is output from the emitter terminal. You may do it. As a result, the level shift means can be realized by a relatively simple circuit.

  According to the present invention, in the current feedback type DC-DC converter, the linearity of the duty is improved, and an undesired situation in which the pre-start signal at the head of the next period is masked due to the response delay of the PWM comparator occurs. Can be avoided. Further, since it is not necessary to use a current detection resistor, there is an effect that power efficiency can be improved in the current feedback type DC-DC converter.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the drawings.

  FIG. 1 shows a first embodiment of a switching regulator type DC-DC converter to which the present invention is applied.

  The DC-DC converter of this embodiment is connected between a coil L1 as an inductor, a voltage input terminal IN to which a DC input voltage Vin is applied, and one terminal of the coil L1, and a drive current is directed toward the coil L1. A driving switch transistor SW1 composed of a P-channel MOSFET (insulated gate field effect transistor), a rectifying switch transistor SW2 composed of an N-channel MOSFET, a switching control circuit 20 for controlling on and off of these switch transistors SW1, SW2, A smoothing capacitor C1 connected between the other terminal of the coil L1 and the ground point is provided.

  Although not particularly limited, among the elements constituting the DC-DC converter, elements other than the coil L1 and the smoothing capacitor C1 are formed on the semiconductor chip, and the control circuit 20 and the switching elements SW1 and SW2 are semiconductor integrated. The circuit is configured as a circuit (IC), and the coil L1 and the capacitor C1 are connected as external elements to an external terminal provided in the IC.

  In the DC-DC converter of this embodiment, the switching control circuit 20 generates a driving pulse that turns on and off the transistors SW1 and SW2 in a complementary manner. In a steady state, the driving transistor SW1 Is turned on, the DC input voltage Vin is applied to the coil L1 and a current directed to the output terminal flows to charge the smoothing capacitor C1. When the driving transistor SW1 is turned off, the rectifying transistor SW2 is turned on instead. Then, a current is supplied to the coil L1 through the turned-on transistor SW2. Then, the pulse width of the drive pulse input to the control terminal (gate terminal) of SW1 is controlled according to the output voltage and the current of the switch transistor SW1, thereby generating a DC output voltage Vout obtained by stepping down the DC input voltage Vin. Is done.

  The switching control circuit 20 is connected in series between the voltage feedback terminal FB and the ground point, and bleeder resistors R1 and R2 that divide the output voltage Vout by a resistance ratio, and the voltage divided by the bleeder resistor and the reference voltage Vref1. And an error amplifier 21 that outputs a voltage corresponding to the potential difference, and a PWM comparator 22 to which the output of the error amplifier 21 is input to a non-inverting input terminal. The output of the PWM comparator 22 is an OR gate. The signal is input to the reset terminal of the reset priority RS flip-flop 25 via the circuit 24.

  A waveform signal from a waveform generation circuit 23 (described later) that generates a waveform signal corresponding to the detected current is input to the inverting input terminal of the PWM comparator 22. The reset signal RES is input to the other input terminal of the OR gate circuit 24. This is a function for limiting the duty, and is used when a maximum on-duty is specified to provide a protection function. In a system that requires 100% driving of SW1, this is done by not inputting the reset signal RES. The function can be disabled.

  A prestart signal PST is input to the set terminal of the RS flip-flop 25. The pre-start signal PST is a pulse having a predetermined frequency supplied from an oscillator or the like. When the flip-flop 25 is set by the pre-start signal PST, the driver 27a is connected to the high-side switch transistor SW1 via the logic circuit 26. A trigger for turning on the drive signal S1 is given. The logic circuit 26 generates a drive signal S2 that turns on the switch transistor SW2 in a complementary manner with SW1, and at the same time, prevents the through current from flowing through the switch transistors SW1 and SW2 being turned on at the same time. It has a function of generating S1 and S2 so that the low level period of S1 and the high level period of the drive signal S2 of SW2 do not overlap.

  The control circuit 20 of the present embodiment detects the ON period of the high side switch SW1 connected to the output node N1, extracts and transmits the potential V1 of the output node N1 only during that period, and outputs a signal S3 indicating the ON period. The ON period detection circuit 28 is provided, and the potential V1 of the output node N1 extracted by the detection circuit 28 is fed back to the waveform generation circuit 23.

  The waveform generation circuit 23 has the same structure as that of the high-side switch SW1, and is connected to the dummy transistor Q1, which is a P-channel MOSFET having a source connected to the power supply voltage terminal Vdd and a gate connected to the ground point, and the power supply voltage terminal Vdd. A differential amplifier AMP1 that applies a voltage corresponding to the potential passed through the P-channel MOSFET Q2 and the current-voltage conversion resistor R3, the on-period detection circuit 28 connected in series with Q1 to the gate of Q2. A low-pass filter LPF comprising a resistor R4 and a capacitor C4 connected to a connection node N2 between the transistor Q2 and the resistor R3, and a level shift PNP bipolar transistor Q4 having a base connected to the resistor R4 and the capacitor C4 connection node N4. The transistor Q4 is connected in series between the emitter of the transistor Q4 and the power supply voltage terminal Vdd. Capacitance C3 and the constant current source CS1, and a like switch transistor Q3, which is provided in parallel with the capacitor C3.

  The dummy transistor Q1 is for flowing a current proportional to the size ratio with the high-side switch SW1. For example, when a current having a magnitude 1 / 100,000 of the current of the high-side switch SW1 is to be supplied to the transistor Q2. , Q1 is designed so that its W / L ratio (ratio of gate width and gate length) is 1 / 100,000 of the W / L ratio of SW1. The voltage at the node N2 converted by the resistor R3 through which the current from Q2 flows is a ground potential reference voltage, and the transistor Q4 raises the voltage from which noise has been removed by passing through the filter by the emitter-base voltage Vf. To the emitter. By raising the voltage of the ground potential reference, it is possible to avoid the occurrence of an error due to collapse of the lower apex of the waveform of the voltage node N4 that changes in a triangular waveform.

  The switch transistor Q3 is ON / OFF controlled by the control signal S3 formed by the signal generation circuit 29 based on the pre-start signal PST and the signal indicating the ON period supplied from the ON period detection circuit 28, and the ON of Q3 The emitter voltage of the transistor Q4 is transmitted to the connection node N3 between the capacitor C3 and the constant current source CS1 during the period, and the potential of the capacitor C3 charged by the current from the constant current source CS1 is added during the off period. As a result, the potential of the waveform obtained by adding the RAMP waveform (sawtooth wave) generated by the constant current source CS1 and the capacitor C3 to the current detection waveform appearing at the emitter of the level shift transistor Q4 is supplied to the PWM comparator 22. The

  In the converter according to the present embodiment, the low-pass filter LPF is provided in the waveform generation circuit 23 where the current detection waveform before synthesis is generated. Therefore, in the converter of FIG. 4, t1 to t5 in FIG. Spike-like noise that could occur at any point is no longer generated. Further, since no current detection resistor is used, there is an advantage that the loss in the detection resistor is eliminated and the power efficiency is improved.

  Further, since the RAMP waveform is generated in accordance with the control signal S3 formed based on the pre-start signal PST and the signal indicating the on period, there is an advantage that the error is small. That is, the signal indicating the ON period generated by detecting the current of the drive switch transistor SW1 is earlier than the feedback voltage from the output terminal but is not a feedback signal, and is not a parasitic capacitance of the circuit. It may be delayed by the influence of etc. However, as in the embodiment, the generation of the RAMP waveform is started by the feed-forward method using the pre-start signal PST as a trigger, thereby eliminating the delay and increasing the phase margin of the switching control signal generated based on the synthesized waveform. Can do.

  In addition, since the converter of FIG. 4 synthesizes the waveform using the RAMP signal generated externally, it is provided with a plurality of converters and has an oscillator to generate a reset signal, a prestart signal, and a RAMP signal. In the case of the multi-output DC / DC converter provided with the waveform generation circuit and distributed to the control circuit of each converter, when the converter of FIG. 4 is used, noise from the peripheral circuit is added to the RAMP signal to form a waveform. Although there is a risk of distortion, since the RAMP signal is generated internally in the converter of the present embodiment, such noise is difficult to be applied and distortion generated in the waveform can be reduced.

  FIG. 3 shows the operation timing of the DC-DC converter of this embodiment.

  As is clear from the operation timing of the converter of FIG. 4 shown in FIG. 5, the timings of the pre-start signal PST and the reset signal RES are the same. The waveform signal from the waveform generation circuit 23 input to the inverting input terminal of the PWM comparator 22 becomes a waveform obtained by adding the RAMP waveform to the detected triangular current waveform as shown in FIG. The peak point of the waveform changes along the output Verror of the error amplifier 21. Further, in the converter of this embodiment, as shown in FIG. 3D, the output of the PWM comparator 22 is always a beard-like pulse, the flip-flop 25 is reset by this pulse, and the falling timing of the drive pulse of SW1 give.

  FIG. 5D shows the design operation timing of the PWM comparator 22 in the converter of FIG. 4, but in an actual product, as shown in FIG. 5D ′, the output of the PWM comparator 22 is affected by the parasitic capacitance. The waveform collapses, the falling edge is later than the pulse of the pre-start signal PST, the flip-flop 24 is not set, and the output pulse is lost as shown by the alternate long and short dash line in FIG. There was a fear. On the other hand, in the converter of this embodiment, the output of the PWM comparator 22 is always a narrow pulse having a narrow width as shown in FIG. 3D, so that the flip-flop 25 is reliably set by the pre-start signal PST. There is an advantage that there is no possibility that the output pulse is lost.

  FIG. 2 shows a second embodiment of a DC-DC converter to which the present invention is applied.

  In the DC-DC converter according to this embodiment, a current detection resistor Rs is provided between the voltage input terminal IN and the driving switch transistor SW1, and a difference for amplifying a potential of a connection node between the detection resistors Rs and SW1 is provided. A dynamic amplifier AMP2 is provided. Further, a low-pass filter LPF including a resistor R4 and a capacitor C4, a level shift PNP bipolar transistor Q4, a capacitor C3, a constant current source CS1, and a switch transistor Q3 are provided at the subsequent stage of the differential amplifier AMP2. The RAMP signal synthesizing means is provided.

  The converter of this embodiment also has an advantage that spike-like noise does not occur in the synthesized waveform because the low-pass filter is provided in the waveform generating circuit 23 where the current detection waveform before synthesis is generated. . Further, since the output of the PWM comparator 22 is always a beard-like pulse, there is an advantage that the flip-flop 25 is reliably set by the pre-start signal PST, and there is no possibility that the output pulse is lost.

  Although the invention made by the present inventor has been specifically described based on the embodiment, the present invention is not limited to the above embodiment. For example, in the above embodiment, on-chip elements formed on the same semiconductor chip as the control circuit are used as the switching elements SW1 and SW2, but external elements may be used.

  In the first embodiment, the dummy transistor Q1 is used in the circuit for generating the current detection waveform. However, a resistor may be used instead of this transistor. However, by using a dummy transistor having the same structure as that of the drive switch transistor SW1, when the characteristics of the SW1 are shifted due to process variations, the dummy transistor also varies in the same manner, so that the detection error can be reduced. Furthermore, although the PNP bipolar transistor is used as the level shift element Q4 in the above embodiment, a P-channel FET may be used.

  In the above-described embodiment, the rectifying switch transistor SW2 is connected in series with the driving switch transistor SW1 and the synchronous rectification type DC-DC converter is turned on and off complementarily with the SW1. It is also possible to apply to a diode rectification type DC-DC converter using a diode instead of SW2. Further, the present invention can be applied not only to a step-down DC-DC converter as in the embodiment but also to a step-up DC-DC converter and an inverting DC-DC converter that generates a negative voltage.

  In the above description, the example in which the present invention is applied to a non-insulated DC-DC converter has been described. However, the present invention is not limited thereto, and a switching element in an insulated DC-DC converter using a transformer. It can be applied to a circuit that controls

FIG. 1 is a circuit configuration diagram showing a first embodiment of a DC-DC converter to which the present invention is applied. FIG. 2 is a circuit configuration diagram showing a second embodiment of a DC-DC converter to which the present invention is applied. FIG. 3 is a timing chart showing changes in signals and potentials at various parts of the DC-DC converter of the first embodiment. FIG. 4 is a block diagram showing an example of a DC-DC converter examined prior to the present invention. FIG. 5 is a timing chart showing changes in signals and potentials at various parts of the DC-DC converter of FIG.

Explanation of symbols

20 switching control circuit 21 error amplifier 22 PWM comparator 23 waveform generation circuit 24 OR gate circuit 25 flip-flop 26 logic circuit 27a, 27b driver 28 on-period detection circuit L1 coil (inductor)
C1 Smoothing capacitor Q1 Dummy transistor SW1 Coil driving switch transistor (driving switching element)
SW2 Switch transistor for synchronous rectification (switching element for rectification)
LPF Low pass filter

Claims (9)

An inductor for voltage conversion; a driving switching element that applies a DC input voltage supplied from a DC power source to the inductor to cause a current to flow and store energy in the inductor; and an energy release in which the driving switching element is turned off A rectifying element that rectifies the current of the inductor in a period, and a switching control circuit that includes a PWM comparator to which an output voltage is fed back and detects a current flowing through the driving switching element to control an on-time of the driving switching element In a DC-DC converter comprising:
The switching control circuit detects a current flowing through the driving switching element and outputs it as a ground potential reference voltage, and generates a predetermined waveform signal only while the current is detected by the current detection circuit. A DC-DC converter comprising: a waveform synthesizing unit configured to supply an output voltage of the waveform synthesizing unit to the PWM comparator.   The current detection circuit includes an on-period detection circuit that extracts and transmits a current flowing through the driving switching element connected to an output node, and a dummy having the same structure as the driving switching element and a predetermined W / L ratio. An element, a transistor and a resistor connected in series with the dummy element, and a differential amplifier that receives a signal from the on-period detection circuit and outputs a signal for biasing the transistor, and the output of the differential amplifier is 2. The DC-DC converter according to claim 1, wherein the DC-DC converter is an output of a current detection circuit.   A current detection resistor is connected in series with the driving switching element, and the current detection circuit includes a differential amplifier that receives a voltage dropped by the current detection resistor, and an output of the differential amplifier is the current The DC-DC converter according to claim 1, wherein the DC-DC converter is an output of a detection circuit.   The switching control circuit includes level shift means for raising a potential of an output signal of the current detection circuit, and the waveform synthesis means adds the voltage raised by the level shift means to the waveform signal. Item 4. The DC-DC converter according to Item 2 or 3.   5. The DC− according to claim 4, wherein the switching control circuit includes a low-pass filter connected to a subsequent stage of the current detection circuit, and the level shift unit raises the potential of the signal that has passed through the low-pass filter. DC converter.   An error amplifier that outputs a voltage corresponding to the potential difference between the feedback voltage from the output side and a reference voltage is provided in the previous stage of the PWM comparator, and the output of the PWM comparator is a reset terminal in the subsequent stage of the PWM comparator. 6. A DC-DC converter according to claim 5, wherein a flip-flop is provided, wherein a flip-flop is input to the set terminal and a pre-start signal having a predetermined period is input to the set terminal.   The level shift means is a PNP bipolar transistor or a P channel FET that has a collector terminal connected to a ground point, a signal that has passed through the low-pass filter is input to a base terminal, and outputs a level shifted voltage from the emitter terminal. The DC-DC converter according to any one of claims 4 to 6, characterized in that   A detection circuit configured to detect an ON period of the driving switching element; and the waveform synthesizing unit includes: a capacitive element having one terminal connected to an output side of the level shift unit; and the other terminal of the capacitive element; A constant current source connected between the power supply voltage terminal and a switching element connected in parallel with the capacitive element, the switching element being on / off controlled by a signal indicating an on period from the detection circuit; The DC-DC converter according to any one of claims 4 to 7, wherein A power supply control semiconductor integrated circuit having a switching control circuit that generates a control signal of a driving switching element that controls a current flowing through an inductor for voltage conversion,
The switching control circuit includes an error amplifier that outputs a voltage corresponding to a potential difference between a feedback voltage from an output side and a reference voltage, a PWM comparator that receives an output of the error amplifier at one input terminal, and an output of the PWM comparator Is input to the reset terminal, a pre-start signal having a predetermined period is input to the set terminal, a current detection circuit that detects a current flowing through the driving switching element and outputs the current as a ground potential reference voltage, A low-pass filter connected to the subsequent stage of the current detection circuit, level shift means for raising the potential of the signal that has passed through the low-pass filter, and a predetermined waveform signal are generated only while the current is detected by the current detection circuit. Waveform synthesis means for adding to the voltage raised by the level shift means, Configured power control semiconductor integrated circuit so that the output voltage is supplied to the other input terminal of the PWM comparator forms combining means.

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