JP5839863B2 - STEP-DOWN SWITCHING REGULATOR, ITS CONTROL CIRCUIT, AND ELECTRONIC DEVICE USING THE SAME - Google Patents

Description

  The present invention relates to a step-down switching regulator, and more particularly to a control technology for a synchronous rectification type switching regulator.

  Various electronic devices such as mobile phones, PDAs (Personal Digital Assistants), and notebook personal computers in recent years are equipped with microcomputers that perform digital signal processing. The power supply voltage required for driving such a microcomputer has been reduced with the miniaturization of the semiconductor manufacturing process, and there is one that operates at a low voltage of 1.5 V or less. On the other hand, a battery such as a lithium ion battery is mounted on such an electronic device as a power source. The voltage output from the lithium ion battery is about 3V to 4V. If this voltage is supplied to the microcomputer as it is, wasteful power consumption occurs. Therefore, the battery voltage is reduced using a step-down switching regulator. Generally, the voltage is made constant and supplied to the microcomputer.

  There are two types of step-down switching regulators: a method using a rectifying diode (hereinafter referred to as a diode rectifying method) and a method using a rectifying transistor instead of a diode (hereinafter referred to as a synchronous rectifying type). In the former case, there is an advantage that high efficiency can be obtained when the load current flowing through the load is low. However, since a diode in addition to the inductor and the output capacitor is required outside the control circuit, the circuit area becomes large. In the latter case, the efficiency when the current supplied to the load is small is inferior to the former, but since a transistor is used instead of a diode, it can be integrated inside the LSI, and the circuit area including peripheral components As a result, downsizing is possible. When an electronic device such as a cellular phone is required to be downsized, a switching regulator using a rectifying transistor (hereinafter referred to as a synchronous rectification switching regulator) is often used.

FIGS. 1A and 1B are diagrams showing time waveforms of current at the time of heavy load and light load of the synchronous rectification switching regulator, respectively. In the figure, _{I L} is the current flowing through the _{inductor, I OUT} represents the load current, the time average value of the current _{I L} flowing through the inductor becomes a load current _{I OUT.} As shown in FIG. 1 (a), at the time of heavy load, since the load current I _OUT increases, the current I _L flowing through the inductor continues to take a positive value. However, as shown in FIG. 1 (b), the load current I _OUT decreases at light load, the current I _L flowing through the inductor is negative as the hatched portion, the direction of the current I _L flowing through the inductor reverses . As a result, in the synchronous rectification type, a current flows from the inductor to the ground through the synchronous rectification transistor at light load. Since this current is not supplied to the load, power is wasted.

In order to solve this problem, a circuit that switches the control method of the switching regulator between a heavy load and a light load has been proposed. Specifically, when the inductor current I _L is positive, it is determined that the heavy load, determines when it is detected that the inductor current I _L becomes negative and light loads switch the operation mode.

  FIG. 2 is a circuit diagram showing a configuration of a switching regulator according to a comparative technique examined by the present inventors. The step-down switching regulator 2r includes a control circuit 100r and an output circuit 102. The output circuit 102 includes a switching transistor M1, a synchronous rectification transistor M2, an inductor L1, and an output capacitor C1.

The control circuit 100r stabilizes the output voltage _VOUT supplied to the load 6 near the target value by controlling the on / off states of the switching transistor M1 and the synchronous rectification transistor M2.

The control circuit 100 includes a first controller 10, a second controller 20, a driver 30, and a light load detection comparator 40r. The output voltage _VOUT of the step-down switching regulator 2r is divided by the resistors R1 and R2, and the feedback voltage _VFB is generated. The control circuit 100 operates in the first mode (normal mode) in a heavy load state where the load current I _OUT flowing through the load 6 is somewhat large, and in the light load state where the load current I _OUT is close to zero, Operates in mode (light load mode).

In the normal mode, the first controller 10 becomes active, and generates a pulse width modulation (PWM) signal S _PWM whose duty ratio is adjusted so that the feedback voltage V _FB matches a predetermined reference voltage V _REF . The driver 30 complementarily switches the switching transistor M1 and the synchronous rectification transistor M2 based on the PWM signal _SPWM .

In the light load mode, the second controller 20 is active. The second controller 20 turns on the switching transistor M1 for a certain period, subsequently turns on the synchronous rectification transistor M2 for a certain period, and then turns off both the switching transistor M1 and the synchronous rectification transistor M2 to set the LX terminal to high impedance. As a result, the output voltage V _OUT slightly increases. In the light load mode, switching of the switching transistor M1 and the synchronous rectification transistor M2 is stopped, so that power consumption of the circuit is reduced.

The light load detection comparator 40r is provided for detecting a transition from the heavy load state to the light load state. When the inductor current _IL is positive in the heavy load state, the current I _M2 flows through the synchronous rectification transistor M2 from the source to the drain. In the light load state, the inductor current _IL becomes negative, and the current I _M2 flows through the synchronous rectification transistor M2 from the drain to the source.

The light load detection comparator 40r monitors the current I _M2 flowing through the synchronous rectification transistor M2 during the on period of the synchronous rectification transistor M2 in the heavy load state, and compares the detection signal corresponding to the current I _M2 with a predetermined threshold value. By doing so, a light load state is detected.

JP 2004-32875 A JP 2002-252971 A

The duty ratio of the switching transistor M1 and the synchronous rectification transistor M2 in the normal mode of the step-down switching regulator 2r is controlled according to the input voltage _VIN and the output voltage _VOUT . Specifically, the duty ratio _T ON / _{T P is} given by _V OUT _/ V _IN. T _ON is the ON time of the switching transistor M1, and _TP is the switching period.

Therefore, as the input voltage _VIN decreases and approaches the output voltage _VOUT , the duty ratio approaches 100%, and the on-time of the synchronous rectification transistor M2 is shortened. As described above, the light load detection comparator 40r compares the current I _M2 during the on-time of the synchronous rectification transistor _M2 with a threshold value. Here, the response speed of the light load detection comparator 40r is finite, and if the ON time of the synchronous rectification transistor M2 becomes shorter than the response time of the light load detection comparator 40r, the light load cannot be detected. This means that although the output current I _OUT is very small, the operation continues in the normal mode and the power consumption increases.

  It should be noted that the above consideration should not be taken as a range of common general knowledge in the field of the present invention. Furthermore, the above-mentioned consideration itself is the first time the applicant has conceived.

  SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and one of exemplary objects of an embodiment thereof is to provide a control circuit capable of reliably detecting a light load state in a synchronous rectification step-down switching regulator. .

  One embodiment of the present invention relates to a control circuit for a synchronous rectification step-down switching regulator. The control circuit is active in the normal mode, and generates a first pulse signal whose duty ratio is adjusted so that the feedback voltage according to the output voltage of the switching regulator approaches the target value, and in the light load mode. A second controller that generates a second pulse signal that becomes active and intermittently switches the switching transistor and the synchronous rectification transistor of the switching regulator, and converts the second pulse signal in the light load mode based on the first pulse signal in the normal mode. First, a driver for driving the switching transistor and the synchronous rectification transistor, and a detection current flowing through the switching transistor during the ON period of the switching transistor are compared with a predetermined first threshold current. Comprises a light-load detection comparator for generating a comparison signal detection current is asserted exceeds the first threshold current, the. The control circuit is set to the normal mode when the comparison signal is asserted and to the light load mode when the comparison signal is not asserted.

  When the input voltage decreases and approaches the output voltage, the on-time of the switching transistor becomes longer. According to this aspect, since the light load state is detected according to the current flowing through the switching transistor during the ON period of the switching transistor, a sufficient detection period can be ensured even if the input voltage is low, so that the light load state can be ensured. It can be detected.

  The light load detection comparator may compare the voltage drop of the switching transistor during the ON period of the switching transistor with a threshold voltage corresponding to the first threshold current.

  The control circuit may further include a threshold voltage generation circuit that generates a threshold voltage having a potential difference lower than the input voltage of the step-down switching regulator according to the first threshold current. The light load detection comparator may compare the potential of the connection point between the switching transistor and the synchronous rectification transistor during the ON period of the switching transistor with a threshold voltage.

  The threshold voltage generation circuit is a first transistor of the same type as the switching transistor, the first terminal of which is connected to the input terminal of the step-down switching regulator, and the control terminal controls the switching transistor in the ON state of the switching transistor. A first transistor to which a potential of the terminal is applied, and a current source connected to the second terminal of the first transistor and generating a reference current corresponding to the first threshold current, the second terminal of the switching transistor May be output as a threshold voltage.

Depending on the normal mode and the light load mode, hysteresis may be set for the threshold current.
In this case, when the current flowing through the switching transistor fluctuates in the vicinity of the threshold current, oscillation that alternates between the normal mode and the light load mode can be suppressed.

  In the light load mode, the second controller (1) turns on the switching transistor, and then turns off the switching transistor when the current flowing through the switching transistor reaches a predetermined second threshold current, and (2) continues synchronous rectification. When the transistor is turned on and the current flowing through the synchronous rectification transistor becomes zero, the synchronous rectification transistor is turned off. (3) Thereafter, the second pulse signal may be generated so that both the switching transistor and the synchronous rectification transistor are turned off.

The control circuit may be integrated on a single semiconductor substrate.
“Integrated integration” includes the case where all of the circuit components are formed on a semiconductor substrate and the case where the main components of the circuit are integrated. A resistor, a capacitor, or the like may be provided outside the semiconductor substrate.

  Another aspect of the present invention relates to a step-down switching regulator. The step-down switching regulator includes a switching transistor and a synchronous rectification transistor provided in series between an input terminal and a ground terminal, an inductor provided between a connection point of the switching transistor and the synchronous rectification transistor, and an output terminal, and an output terminal And an output capacitor provided between the control terminal and the ground terminal, and any one of the above-described control circuits for driving the switching transistor and the synchronous rectification transistor.

  Another embodiment of the present invention relates to an electronic device. The electronic device includes a battery that outputs a battery voltage, and the above-described step-down switching regulator that steps down the battery voltage and supplies the voltage to a load.

  Note that any combination of the above-described constituent elements and the constituent elements and expressions of the present invention replaced with each other among methods, apparatuses, systems, etc. are also effective as an aspect of the present invention.

  The step-down switching regulator according to the present invention can reliably detect a light load state.

FIGS. 1A and 1B are diagrams showing time waveforms of current at the time of heavy load and light load of the synchronous rectification switching regulator, respectively. It is a circuit diagram which shows the structure of the switching regulator which concerns on the comparison technique which the present inventors examined. It is a block diagram which shows the structure of the electronic device carrying the synchronous rectification type | mold step-down switching regulator which concerns on embodiment. 4A to 4H are waveform diagrams showing the operation of the step-down switching regulator of FIG. It is a figure which shows the efficiency of normal mode (I) and light load mode (II). FIG. 4 is a circuit diagram illustrating a specific configuration example of a part of the control circuit of FIG. 3. It is a circuit diagram which shows the specific structural example of a light load detection comparator.

  FIG. 3 is a block diagram illustrating a configuration of the electronic device 1 in which the synchronous rectification step-down switching regulator 2 according to the embodiment is mounted. The electronic device 1 is, for example, a mobile phone terminal, a digital camera, or a portable audio player, and includes a step-down switching regulator 2, a battery 4, and a load 6.

Battery 4 is, for example, a lithium ion battery, and outputs a battery voltage _{V BAT} of about 3 to 4V. A battery voltage V _BAT (also referred to as an input voltage _VIN ) is input to the input terminal P1 of the step-down switching regulator 2. The step-down switching regulator 2 steps down the input voltage _VIN to a predetermined voltage level and supplies it to a load 6 connected to the output terminal P2. The load 6 includes, for example, a microprocessor that operates with a power supply voltage of about 1.5V.

  The step-down switching regulator 2 includes a control circuit 100 and an output circuit 102. The output circuit 102 includes a switching transistor M1, a synchronous rectification transistor M2, an inductor L1, and an output capacitor C1. The switching transistor M1 and the synchronous rectification transistor M2 are sequentially connected between the input terminal IN and the ground terminal GND. A connection point between the switching transistor M1 and the synchronous rectification transistor M2 is connected to the switching terminal LX. In the present embodiment, the switching transistor M1 is a P-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor), and the synchronous rectification transistor M2 is an N-channel MOSFET, but may be configured by another transistor.

  The control circuit 100 is a functional IC (Integrated Circuit) integrated on one semiconductor substrate, and the switching transistor M1 and the synchronous rectification transistor M2 are built in the control circuit 100.

The output capacitor C1 is provided between the output terminal P2 and the ground. The inductor L1 is provided between one end of the output capacitor C1 and the switching terminal LX of the control circuit 100. In the control circuit 100, the switching voltage V _LX generated at the switching terminal _LX is applied to one end of the inductor L1.

The control circuit 100 includes a first controller 10, a second controller 20, a driver 30, and a light load detection comparator 40. The control circuit 100 is configured to be able to switch between a normal mode and a light load mode. The control circuit 100 is set to the normal mode when the load current I _OUT is large to some extent, and is set to the light load mode when the load current I _OUT is small.

The first controller 10 is active in the normal mode, and generates the first pulse signal S _PWM whose duty ratio is adjusted so that the feedback voltage V _FB corresponding to the output voltage _VOUT of the switching regulator 2 approaches the target value V _REF. To do. The configuration of the first controller 10 is not particularly limited, and the first controller 10 can be configured using known modulators such as hysteresis control, fixed on-time method, fixed off-time method, voltage mode, peak current mode, and average current mode.

The second controller 20 becomes active in the light load mode, and the second pulse for intermittently switching the switching transistor M1 and the synchronous rectification transistor M2 of the switching regulator 2 so that the feedback voltage _VFB approaches the target value _VREF. A signal _SPFM is generated. The configuration of the second controller 20 is not particularly limited, and can be configured using a modulator capable of lowering the switching frequency at a light load in a synchronous rectification step-down switching regulator.

The driver 30 drives the switching transistor M1 and the synchronous rectification transistor M2 based on (1) the first pulse signal _SPWM in the normal mode and (2) based on the second pulse signal _SPFM in the light load mode.

Light-load detection comparator 40, a detection current _{I M1} flowing through the switching transistor M1 is compared with a first threshold current _{I TH1} predetermined in ON period _{T ON} of the switching transistor M1, the detected current _{I M1} is the first threshold value When the current _ITH1 is exceeded, a comparison signal S1 that is asserted (for example, high level) is generated.

The control circuit 100 is set to the normal mode when the comparison signal S1 is asserted (high level), in other words, when the detection current I _M1 exceeds the threshold current I _TH1 . Conversely, in the negated state where the comparison signal S1 is not asserted, in other words, when the detection current I _M1 does not exceed the threshold current I _TH1 , the light load mode is set.

The above is the configuration of the control circuit 100. Next, the operation will be described. 4A to 4H are waveform diagrams showing the operation of the step-down switching regulator 2 of FIG.
FIG. 4A shows the first pulse signal S _PWM in the normal mode. FIG. 4B shows inductor currents I _{L1 to} I _L3 corresponding to three different load currents. The average value of the inductor current _ILi is the load current _IOUT . 4C and 4D show the detection current I _M1 and the comparison signal S1 ₁ corresponding to the inductor current I _L1 , and FIGS. 4E and 4F show the detection current I corresponding to the inductor current I _L2. the _M2 and the comparison signal S1 _2, FIG. 4 (g), (h) shows the detected current _{I M3} and the comparison signal S1 ₃ corresponds to the inductor current _{I L3.}

As shown in FIGS. 4C and 4E, when the load current I _OUT is large to some extent, the detection current I _M1 exceeds the threshold current I _TH1 in each cycle of the first pulse signal S _PWM . Therefore, as shown in FIGS. 4D and 4F, the comparison signal S1 is asserted every cycle, and the control circuit 100 is set to the normal mode.

As shown in FIG. 4G, when the load current I _OUT becomes small, that is, when the light load state is entered, the detection current I _M1 becomes lower than the threshold current I _TH1 . As a result, the comparison signal S1 is not asserted in each cycle, and the control circuit 100 shifts to the light load mode.

When shifting to the light load mode, the switching transistor M1 and the synchronous rectification transistor M2 are driven by the second pulse signal _SPFM . As a result, in the light load mode, the switching frequency of the switching transistor M1 and the synchronous rectification transistor M2 is reduced as compared with the normal mode, and the efficiency is improved.

Even in the light load mode, the monitoring of the detection current I _M1 by the light load detection comparator 40 is continued. When the detection current I _M1 exceeds the threshold current I _TH1 , the comparison signal S1 is asserted and the normal mode is entered.

The above is the basic operation of the control circuit 100.
According to the control circuit 100, it is possible to determine a light load according to the current flowing through the switching transistor M1, not the current flowing through the synchronous rectification transistor M2. This has the following advantages.

(First advantage)
As described in relation to the comparative technique, the duty ratio of the switching transistor M1 and the synchronous rectification transistor M2 in the normal mode of the step-down switching regulator 2 is controlled according to the input voltage _VIN and the output voltage _VOUT . Specifically, the duty ratio _T ON / _{T P is} given by _V OUT _/ V _IN.

In the comparative technique for determining a light load based on the current I _M2 flowing through the synchronous rectification transistor M2, the duty ratio approaches 100% as the input voltage _VIN decreases and approaches the output voltage _VOUT , and synchronous rectification is performed. The on-time of the transistor M2 is shortened, and there is a possibility that a light load cannot be detected.

In contrast, according to the control circuit 100 of FIG. 3, the input voltage V _IN is the on-time T _ON of the switching transistor M1 becomes longer in accordance with decreases, a longer detection period in a switching cycle T _P, the light load It can be detected reliably.

In the control circuit 100 of FIG. 3, the detection period becomes shorter as the input voltage _VIN becomes higher, and the detection period may be shorter than the response time of the comparator. Therefore, when the input voltage _VIN is high, it cannot be detected that the detection current I _M1 exceeds the threshold current I _TH1 and the comparison signal S1 is not asserted. In this case, the control circuit 100 enters the light load mode. Therefore, it is possible to prevent the operation in the normal mode despite the light load.

(Second advantage)
Further, the control circuit 100 of FIG. 3 has an advantage that the threshold current _ITH1 can be arbitrarily set. FIG. 5 is a diagram showing the efficiency in the normal mode (I) and the light load mode (II). The horizontal axis represents the load current I _OUT and the vertical axis represents the efficiency. In the region where the load current I _OUT is larger than the threshold value Ia, the normal mode is more efficient, and in the region where the load current I _OUT is smaller than the threshold value Ia, the light load mode is more efficient. Here, the efficiency in the normal mode and the light load mode varies depending on the target value of the output voltage _VOUT , the input voltage _VIN , the inductance of the inductor L1, the capacitance value of the output capacitor C1, the switching frequency in the normal mode, and the like. Therefore, the threshold value Ia at the point where the two curves intersect can also change.

In the comparative technique, when the detection current I _M2 flowing through the synchronous rectification transistor M2 becomes zero, that is, when the inductor current _IL is inverted, the light load mode is set. Therefore, in the comparative technique, the light load mode and the normal mode may be switched by the load current _IOUT at a point far from the threshold value Ia shown in FIG.

On the other hand, in the control circuit 100 of FIG. 3, by optimizing the threshold current I _TH1 , the light load mode and the normal mode are switched at the load current I _OUT close to the threshold value Ia shown in FIG. And the efficiency can be increased as compared with the conventional case.

Note that the threshold current I _TH1 in the normal mode may be set a threshold current I _TH1 in the light load mode to a different value. Specifically, the threshold current I _TH1 in the normal mode, and a value lower than the threshold current I _TH1 in the light load mode, it is preferable to have a hysteresis threshold current I _TH1. By providing hysteresis to the threshold current _ITH1 , the oscillation that alternates between the normal mode and the light load mode is suppressed when the detection current I _M1 flowing through the switching transistor M1 fluctuates in the vicinity of the threshold current _ITH1. it can.

  Next, a specific configuration example of the control circuit 100 will be described.

FIG. 6 is a circuit diagram illustrating a specific configuration example of a part of the control circuit 100 of FIG. The first controller 10 in FIG. 6 is a hysteresis-controlled modulator and includes a hysteresis comparator 12. The hysteresis comparator 12 compares the feedback voltage V _FB corresponding to the output voltage _VOUT with a threshold voltage V _REF having hysteresis, and generates a first pulse signal S _PWM .

The second controller 20 includes a first comparator 22, a second comparator 24, a third comparator 26, and a logic unit 28. The first comparator 22 compares the detection current I _M1 flowing through the switching transistor M1 with the second threshold current I _TH2 while the switching transistor M1 is on, and asserts (high level) when I _M1 > I _TH2. A comparison signal S2 is generated.

The second comparator 24 compares the detection current I _M2 flowing through the synchronous rectification transistor _M2 with a predetermined third threshold current I _TH3 during the ON period of the synchronous rectification transistor M2, and the detection current I _M2 is the third threshold value. When the current decreases to the current _ITH3, a comparison signal S3 that is asserted (high level) is generated.

The third threshold current I _TH3 is preferably zero. Thereby, the energy stored in the inductor L1 during the ON period of the switching transistor M1 can be efficiently supplied to the output capacitor C1.

The third comparator 26 compares the feedback voltage V _FB corresponding to the output voltage _VOUT with the reference voltage V _REF, and generates a comparison signal S4 that is asserted when V _FB becomes lower than V _REF .

When transitioning to the light load mode, the logic unit 28 sets the second pulse signal _SPFM to the first level (for example, high level), turns on the switching transistor M1, and turns off the synchronous rectification transistor M2. When the comparison signal S2 is asserted, the second pulse signal _{SPFM is set} to the second level (for example, low level), the switching transistor M1 is turned off, and the synchronous rectification transistor M2 is turned on. Subsequently, when the comparison signal S3 is asserted, the high impedance signal Hi-Z is asserted (for example, high level). When the high impedance signal Hi-Z is asserted, the driver 30 turns off both the switching transistor M1 and the synchronous rectification transistor M2, and puts the switching terminal LX into a high impedance state. Thereafter, when the comparison signal S4 is asserted, the logic unit 28 turns on the switching transistor M1. The configuration and operation of the second controller 20 are not limited to those shown in FIG.

FIG. 7 is a circuit diagram illustrating a specific configuration example of the light load detection comparator 40.
In the switching transistor M1, a voltage drop ΔV _M1 (= I _M1 × R _ON1 ) proportional to the detection current I _M1 occurs. R _ON1 is the on-resistance of the switching transistor M1. The light load detection comparator 40 compares the voltage drop ΔV _M1 of the switching transistor M1 during the ON period of the switching transistor M1 with the threshold voltage ΔV _TH1 corresponding to the first threshold current I _TH1 .

The threshold voltage generation circuit 42 generates a threshold voltage _VTH1 . The threshold voltage V _TH1 is lower than the input voltage _VIN by a potential difference ΔV _TH1 .
V _TH1 = V _IN −ΔV _TH1 (1)

The threshold voltage generation circuit 42 includes a first transistor M3 of the same type as the switching transistor M1, that is, a P-channel MOSFET, and a current source 46. Current source 46 generates a constant current _{I C} which is proportional to the threshold current _{I TH1.} The first terminal (source) of the first transistor M3 is connected to the input terminal IN, and the current source 46 is connected to the second terminal (drain). The voltage level applied to the gate of the switching transistor M1, that is, the ground voltage (0 V) when the switching transistor M1 is on is applied to the control terminal (gate) of the first transistor M1. The threshold voltage generation circuit 42 outputs the potential at the connection point between the first transistor M3 and the current source 46 as the threshold voltage _VTH1 .

The threshold voltage V _TH1 generated by the threshold voltage generation circuit 42 is given by Expression (2).
_{V TH1 = V IN -R ON3 ·} I C ... (2)
ΔV _TH1 = R _ON3 × I _C (3)

The potential V _LX of the switching terminal LX is given by equation (4).
V _LX = V _IN −ΔV _M1 = V _IN −R _ON1 × I _M1 (4)

The voltage comparator 44 generates a comparison signal S1 by comparing the threshold voltage V _TH1 with the potential V _LX of the switching terminal LX. Therefore, the detection current I _M1 can be compared with the threshold current I _TH1 by setting the constant current IC to satisfy the equation (5). I _C = I _TH1 × R _ON1 / R _ON3 ( 5)
When the transistor size of the switching transistor M1 is N times that of the first transistor M3, since R _ON1 / R _ON3 = 1 / N, I _C = I _TH1 / N may be set.

Note that the method of current detection and comparison is not limited to that shown in FIG. 7, and another method may be used. For example, a detection resistor may be provided in series with the switching transistor M1, and the voltage drop may be compared with a threshold value. Alternatively, the detection transistor may be connected to the switching transistor M1 so that the gate and the source are common, and the current flowing through the detection transistor may be compared with the threshold current _ITH1 .

  Note that the first comparator 22 of the second controller 20 may be configured by a circuit similar to the light load detection comparator 40 of FIG.

  Those skilled in the art will understand that the above-described embodiment is an exemplification, and that various modifications can be made to combinations of the respective constituent elements and processing processes, and such modifications are also within the scope of the present invention. is there.

  In the embodiment, the microcomputer is described as an example of the load circuit driven by the step-down switching regulator 2 including the control circuit 100. However, the present invention is not limited to this, and the load circuit is reduced and various operations that operate in a light load state are performed. A driving voltage can be supplied to the load circuit.

  In the embodiment, the case where the control circuit 100 is integrated in one LSI has been described. However, the present invention is not limited to this, and some components are provided as discrete elements or chip components outside the LSI. Or you may comprise by several LSI.

  Further, in this embodiment, the setting of the high level and low level logical values of each signal is an example, and can be freely changed by appropriately inverting it with an inverter or the like.

DESCRIPTION OF SYMBOLS 1 ... Electronic device, 2 ... Step-down switching regulator, 4 ... Battery, 6 ... Load, 100 ... Control circuit, 102 ... Output circuit, P1 ... Input terminal, P2 ... Output terminal, LX ... Switching terminal, C1 ... Output capacitor, L1 ... Inductor, M1 ... Switching transistor, M2 ... Synchronous rectification transistor, 10 ... First controller, 12 ... Hysteresis comparator, 20 ... Second controller, 22 ... First comparator, 24 ... Second comparator, 26 ... Logic unit, 30 ... driver, 40 ... light load detection comparator, 42 ... threshold voltage generation circuit, 44 ... voltage comparator.

Claims (9)

A control circuit for a synchronous rectification step-down switching regulator,
A first controller that is active in a normal mode and generates a first pulse signal in which a duty ratio is adjusted such that a feedback voltage corresponding to an output voltage of the switching regulator approaches a target value;
A second controller that is active in a light load mode and generates a second pulse signal that intermittently switches the switching transistor and the synchronous rectification transistor of the switching regulator;
A driver for driving the switching transistor and the synchronous rectification transistor based on the first pulse signal in the normal mode and based on the second pulse signal in the light load mode;
Compares the current flowing through the switching transistor with a predetermined first threshold current during the ON period of the switching transistor, and generates a comparison signal that is asserted when the current flowing through the switching transistor exceeds the first threshold current A light load detection comparator
With
When the comparison signal is asserted, the normal mode is set; when the comparison signal is not asserted, the light load mode is set;
In the light load mode, the second controller (1) turns on the switching transistor, and then turns off the switching transistor when a current flowing through the switching transistor reaches a predetermined second threshold current, (2 Next, the synchronous rectification transistor is turned on, and when the current flowing through the synchronous rectification transistor becomes zero, the synchronous rectification transistor is turned off. (3) Thereafter, both the switching transistor and the synchronous rectification transistor are turned off. that control circuit to and generates a second pulse signal. The light load detection comparator is
2. The control circuit according to claim 1, wherein a voltage drop of the switching transistor in an ON period of the switching transistor is compared with a threshold voltage corresponding to the first threshold current. A threshold voltage generating circuit for generating a threshold voltage having a potential difference lower than the input voltage of the step-down switching regulator according to the first threshold current;
2. The control circuit according to claim 1, wherein the light load detection comparator compares the threshold voltage with a potential at a connection point between the switching transistor and the synchronous rectification transistor in an ON period of the switching transistor. . The threshold voltage generation circuit includes:
A first transistor of the same type as the switching transistor, the first terminal of which is connected to the input terminal of the step-down switching regulator, and the potential of the control terminal of the switching transistor in the ON state of the switching transistor is An applied first transistor;
A current source connected to the second terminal of the first transistor and generating a reference current corresponding to the first threshold current;
4. The control circuit according to claim 3, wherein a potential of the second terminal of the switching transistor is output as the threshold voltage. 5.   5. The control circuit according to claim 1, wherein a hysteresis is set in the threshold current according to the normal mode and the light load mode.   The second controller is
  A first comparator that generates a first comparison signal that is asserted when a current flowing through the switching transistor exceeds a predetermined second threshold current during an ON period of the switching transistor;
  A second comparator that generates a second comparison signal that is asserted when a current flowing through the synchronous rectification transistor becomes lower than a predetermined third threshold value during an on period of the synchronous rectification transistor;
  A third comparator that generates a third comparison signal that is asserted when a feedback voltage corresponding to the output voltage of the switching regulator is lower than the target value;
  The control circuit according to claim 1, comprising:   The control circuit according to claim 1, wherein the control circuit is integrated on a single semiconductor substrate. A switching transistor and a synchronous rectification transistor provided in series between the input terminal and the ground terminal in order,
An inductor provided between a connection point of the switching transistor and the synchronous rectification transistor and an output terminal;
An output capacitor provided between the output terminal and the ground terminal;
The control circuit according to any one of claims 1 to 7, which drives the switching transistor and the synchronous rectification transistor;
A step-down switching regulator comprising: A battery that outputs battery voltage;
The step-down switching regulator according to claim 8, wherein the battery voltage is stepped down and supplied to a load.
An electronic device comprising:

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