JP2014112996A - Light load detection circuit, switching regulator, and method of controlling the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To keep a load current at the time of control mode switching substantially constant irrespective of an operating temperature in a switching regulator configured to compare a feedback voltage corresponding to an output voltage with a light load detecting reference voltage and switch a control mode on the basis of the result of comparison.SOLUTION: A light load detection circuit 20 includes a light load detecting reference voltage generation circuit 50 for generating a light load detecting reference voltage V50 proportional to an on-resistance of a switching element N1, and a comparator 13 for comparing a feedback voltage Vfb with the light load detecting reference voltage V50 and generating a light load detection signal S20 indicative of the result of comparison to be output to a control logic circuit 100. The control logic circuit 100 PWM-controls the switching element N1 when the feedback voltage Vfb is higher than the light load detecting reference voltage V50, and on the other hand, turns off the switching element N1 when the feedback voltage Vfb is at the light load detecting reference voltage V50.

Description

  The present invention relates to a light load detection circuit for a switching regulator, a switching regulator including the light load detection circuit, and a control method therefor.

  In recent years, power saving of electronic devices has been demanded. In general, in order to save power, it is important to reduce power consumption of an electronic device and to suppress wasteful power consumption by improving the efficiency of the power supply circuit itself. For example, a switching regulator is used as a highly efficient power supply circuit used in a small electronic device. Here, the efficiency of the switching regulator is generally expressed as a ratio of the output power to the input power, and is a function of the output current flowing through the load, and deteriorates when the output current is small or the load is light.

  Patent Documents 1 to 20 propose a technique for improving the efficiency of a switching regulator at a light load. For example, Patent Document 1 discloses a change in output voltage at switching in a switching regulator that switches a control mode between a PWM (Pulse Width Modulation) control mode and a VFM (Variable Frequency Modulation) control mode according to a load current. Is suppressed by using a simple circuit configuration. Further, in Patent Document 2, not only switching at light load and heavy load and switching between PWM mode and VFM mode, but also the circuit operation of the phase compensation circuit can be switched according to the use of the electronic device. A switching regulator is disclosed.

  The switching regulator according to the prior art uses a comparator to compare the feedback voltage corresponding to the output voltage with a predetermined reference voltage for light load detection, and based on the comparison result, the control mode at light load and at heavy load Switching between control modes.

  8A is a timing chart showing the operation of the current mode control type switching regulator according to the prior art at normal temperature, and FIG. 8B is a timing chart of the current mode control type switching regulator according to the prior art at a high temperature. It is a timing chart which shows operation. 8A and 8B, the slope after the slope compensation is performed on the feedback voltage Vfb corresponding to the output voltage and the switching current detection voltage proportional to the switching current flowing through the switching element of the switching regulator. A voltage Vs and a predetermined reference voltage Vlimit for detecting a light load are shown. In the period from time t11 to time t19, the switching element is controlled to perform a switching operation based on a comparison result between the slope voltage Vs and the feedback voltage Vfb.

  Here, the switching current detection voltage is generally proportional to the on-resistance of the switching element. On the other hand, the on-resistance of the switching element has a positive temperature characteristic with respect to the operating temperature. Therefore, the switching current detection voltage has a positive temperature characteristic, and as shown in FIGS. 8A and 8B, the slope of the slope voltage Vs increases as the temperature rises. Therefore, when the reference voltage Vlimit is a constant value that does not change in temperature, timings t13, t16, and t19 when the slope voltage Vs becomes higher than the feedback voltage Vfb at a high temperature are timings t14 when the slope voltage Vs becomes higher than the feedback voltage Vfb at room temperature. , Earlier than t19. As a result, the switching period at high temperature is shorter than the switching period at normal temperature. For this reason, if it is going to maintain the steady state from which the output voltage of a switching regulator becomes constant, a switching frequency will raise at the time of high temperature. As a result, the level of the feedback voltage Vfb increases, and the timing for switching from the VFM control mode at light load to the PWM control mode at heavy load is earlier than at normal temperature. This means that the output current at the time of switching the control mode does not become a constant value depending on the operating temperature. When a switching element having a strong temperature dependency is used, the influence becomes more significant.

  Further, in Patent Document 2, when switching from the VFM control mode to the PWM control mode, a feedback voltage is compared with a reference voltage for light load detection using a comparator having hysteresis, and the VFM control mode is based on the comparison result. However, the relationship between the temperature characteristic of the reference voltage and the temperature characteristic of the on-resistance of the switching element is not mentioned.

  As described above, in the switching regulator according to the related art, when the temperature dependence of the on-resistance of the switching element is relatively strong, the output current at the time of switching from the VFM control mode to the PWM control mode varies depending on the temperature. Thereby, depending on temperature, the efficiency at the time of light load worsens.

  An object of the present invention is to solve the above problems, compare a feedback voltage corresponding to an output voltage with a reference voltage for light load detection, and switch a control mode based on the comparison result. It is an object of the present invention to provide a light load detection circuit capable of making a load current substantially constant regardless of an operating temperature, a switching regulator including the light load detection circuit, and a control method thereof.

The light load detection circuit according to the first invention is:
Switching current detection in which the switching element connected to the input terminal is proportional to the switching current flowing in the switching element so that the input voltage input through the input terminal is converted into a predetermined output voltage and output to the load. A control circuit that performs on / off control based on a voltage and a feedback voltage corresponding to the output voltage;
A switching current detecting circuit that detects the switching current by utilizing the change in the on-resistance of the switching element in response to a temperature change and generates a switching current detection voltage proportional to the detected current. In the light load detection circuit for the regulator,
A light load detection reference voltage generation circuit for generating a light load detection reference voltage proportional to the on-resistance of the switching element;
Comparing means for comparing the feedback voltage with the light load detection reference voltage, generating a light load detection signal indicating the comparison result, and outputting the light load detection signal to the control circuit.

The switching regulator control method according to the second invention comprises:
Switching current detection in which the switching element connected to the input terminal is proportional to the switching current flowing in the switching element so that the input voltage input through the input terminal is converted into a predetermined output voltage and output to the load. A control circuit that performs on / off control based on a voltage and a feedback voltage corresponding to the output voltage;
A switching current detecting circuit that detects the switching current by utilizing the change in the on-resistance of the switching element in response to a temperature change and generates a switching current detection voltage proportional to the detected current. In the regulator control method,
The light load detection circuit is
A light load detection reference voltage generation circuit for generating a light load detection reference voltage proportional to the on-resistance of the switching element;
Comparing means for comparing the feedback voltage with the light load detection reference voltage, generating a light load detection signal indicating the comparison result, and outputting the light load detection signal to the control circuit,
In response to a light load detection signal indicating that the feedback voltage is greater than the light load detection reference voltage, the control circuit controls the switching element in a predetermined first control mode, while the feedback voltage is The method includes a step of controlling the switching element in a predetermined second control mode in response to a light load detection signal indicating that the voltage is not more than the light load detection reference voltage.

  According to the light load detection circuit, the switching regulator, and the control method thereof according to the present invention, the light load detection circuit includes a light load detection reference voltage generation circuit that generates a light load detection reference voltage proportional to the on-resistance of the switching element. In a switching regulator that compares the feedback voltage corresponding to the output voltage with a reference voltage for light load detection and switches the control mode based on the comparison result, the load current when switching the control mode is substantially the same regardless of the operating temperature. Can be made constant.

1 is a circuit diagram showing a configuration of a switching regulator 200 according to a first embodiment of the present invention. FIG. 2 is a circuit diagram illustrating configurations of a switching current detection circuit 3 and a light load detection circuit 20 in FIG. 1. FIG. 3 is an explanatory diagram showing first to third patterns of combinations of a current Iref of FIG. 2 and resistance values of resistors R1 and R2. It is a timing chart which shows operation | movement of the switching regulator 200 of FIG. It is a timing chart which shows the slope voltage Vs of FIG. 1 at the time of the burst oscillation at the time of normal temperature and high temperature. It is a circuit diagram which shows the structure of 20 A of light load detection circuits which concern on the 2nd Embodiment of this invention. It is a circuit diagram which shows the structure of the light load detection circuit 20B which concerns on the 3rd Embodiment of this invention. (A) is a timing chart which shows the operation at normal temperature of the current mode control type switching regulator according to the prior art, and (b) is a timing which shows the operation at high temperature of the current mode control type switching regulator according to the prior art. It is a chart.

  Hereinafter, embodiments according to the present invention will be described with reference to the drawings. In addition, in each following embodiment, the same code | symbol is attached | subjected about the same component.

First embodiment.
FIG. 1 is a circuit diagram showing a configuration of a switching regulator 200 according to the first embodiment of the present invention, and FIG. 2 is a circuit diagram showing configurations of a switching current detection circuit 3 and a light load detection circuit 20 in FIG. It is. In FIG. 1, a switching regulator 200 inputs an input voltage VIN from a DC power source 1 via a primary winding W1 of a transformer T and an input terminal DRAIN. The switching regulator 200 converts the input voltage VIN to a predetermined output voltage VOUT by performing on / off control of the switching element N1 connected to the input terminal DRAIN, and outputs the input voltage VIN to the load 22 via the transformer T. Here, the switching element N1 is an N-channel MOS field effect transistor (hereinafter referred to as an nMOS transistor).

  In FIG. 1, a switching regulator 200 includes a switching element N1, a control logic circuit 100 (control circuit) that controls on / off of the switching element N1, a starting circuit 7, and an undervoltage lockout circuit (hereinafter referred to as a UVLO circuit) 8. A power supply circuit 17, a delay circuit 16, a switching current detection circuit 3, a feedback circuit 30, a light load detection circuit 20, a PWM comparator 2, an adder 19, an oscillator and slope compensation circuit 6, The overcurrent detection circuit 40 is configured to include an input terminal DRAIN, a ground terminal COM, a feedback terminal FB, and a power supply terminal VCC. The light load detection circuit 20 includes a comparator 13 which is a hysteresis comparator, and a light load detection reference voltage which generates a light load detection reference voltage V50 and outputs it to the inverting input terminal of the comparator 13 as will be described in detail later. And a generation circuit 50. Further, the feedback circuit 30 includes diodes D31 and D32 and resistors R31, R32, and R33. Here, the diodes D31 and D32 are connected in reverse series to the feedback terminal FB. The resistor R31 is connected between the anodes of the diodes D31 and D32 and the power supply circuit 17, and the resistors R32 and R33 are connected in series between the cathode of the diode D32 and the ground terminal COM. Further, the overcurrent detection circuit 40 includes the comparator 4 and a voltage source 41 that generates a predetermined overcurrent detection reference voltage V41 and outputs the reference voltage V41 to the inverting input terminal of the comparator 4. Further, the control logic circuit 100 includes RS latch circuits 10 and 15, an AND gate 11, an inverter 14, and an OR gate 18.

  Here, the positive electrode of the DC power supply 1 is connected to the drain of the switching element N1 via the primary winding W1 of the transformer T and the input terminal DRAIN, while the negative electrode of the DC power supply 1 is connected to the ground terminal COM. Furthermore, one end of the tertiary winding W3 of the transformer T is connected to the ground terminal COM and one electrode of the capacitor C2, while the other end of the tertiary winding W3 of the transformer T is connected to the other end of the capacitor C2 via the diode D2. And the power supply terminal VCC. Here, the tertiary winding W3, the capacitor C2, and the diode D2 constitute an external power supply circuit for supplying current to the switching regulator 200 via the power supply terminal VCC every time the switching element N1 is turned on / off. Further, one end of the secondary winding W2 of the transformer T is connected to one electrode of the load 22 and the smoothing capacitor C21 via the diode D21, while the other end of the secondary winding W2 is connected to the load 22 and the smoothing capacitor C21. Connected to the other electrode.

  When the power source of the switching regulator 200 is turned on, a current flows through the starting circuit 7 via the primary winding W1 and the input terminal DRAIN. On the other hand, when the voltage at the power supply terminal VCC is lower than the predetermined threshold voltage, the UVLO circuit 8 turns on the switch SW to charge the capacitor C2. Further, when the voltage at the power supply terminal VCC becomes equal to or higher than a predetermined threshold voltage, the UVLO circuit 8 turns off the switch SW. The power supply circuit 17 is a regulator that generates a predetermined constant voltage based on an input voltage from the power supply terminal VCC and supplies the constant voltage to each circuit in the feedback circuit 30 and the switching regulator 200.

  When the switching element N1 is turned on, a current flows from the DC power source 1 through the primary winding W1, the input terminal DRAIN, and the switching element N1, and magnetic energy is accumulated in the transformer T. Next, when the switching element N1 is turned off, the energy accumulated in the transformer T is transmitted to the load 22 side through the secondary winding W2. A current flows through the secondary winding W2 and the diode D21, is smoothed by the smoothing capacitor C21, and the output voltage VOUT is supplied to the load 22.

  In FIG. 1, an output voltage detection circuit 21 detects an output voltage VOUT and outputs a signal indicating the detection result to a feedback circuit 30 via a photocoupler PC and a feedback terminal FB. Specifically, when the output voltage VOUT becomes higher than a predetermined set voltage, the current flowing through the light emitting portion of the photocoupler PC increases and the voltage at the feedback terminal FB decreases. On the contrary, when the output voltage VOUT becomes smaller than the set voltage, the current flowing through the light emitting portion of the photocoupler becomes small and the voltage at the feedback terminal FB increases. The feedback circuit 30 generates a feedback voltage Vfb corresponding to the output voltage VOUT based on the voltage of the feedback terminal FB, and outputs it to the inverting input terminal of the PWM comparator 2 and the non-inverting input terminal of the comparator 13. .

  The comparator 13 generates a high-level light load detection signal S20 when the feedback voltage Vfb is higher than the light load detection reference voltage V50. On the other hand, when the feedback voltage Vfb is equal to or lower than the light load detection reference voltage V50, the comparator 13 generates a low level light load detection signal S20. Further, the light load detection signal S20 is output to the first input terminal of the AND gate 11.

  The delay circuit 16 delays the rising timing of the control signal EXT output from the control logic circuit 100 to the gate of the switching element N1 by a predetermined delay time, and delays the falling timing of the control signal EXT by a predetermined preceding time. The control signal EXTd is generated by making it precede. Then, the control signal EXTd is output to the switching current detection circuit 3. The delay circuit 16 is provided to prevent the switching current detection circuit 3 from malfunctioning due to noise generated when the switching element N1 is turned on.

  As shown in FIG. 2, the switching current detection circuit 3 includes an nMOS transistor N2 having one end connected to the input terminal DRAIN, and two connected in series between the other end of the nMOS transistor N2 and the ground terminal COM. It comprises resistors R4 and R5. The control signal EXTd from the delay circuit 16 is output to the gate of the nMOS transistor N2. The nMOS transistor N2 has a smaller size than the switching element N1, and is connected in parallel to the switching element N1. For this reason, a small current Isw2 proportional to the switching current Isw1 flowing through the switching element N1 flows through the nMOS transistor N2. The current Isw2 is converted into a switching current detection voltage Vd, which is a voltage at a connection point between the resistors R4 and R5, by the resistors R4 and R5. Further, the switching current detection voltage Vd is output to the adder 19 and the non-inverting input terminal of the comparator 4. That is, the switching current detection circuit 3 detects the switching current Isw1 when the switching element N1 is turned on in accordance with the control signal EXTd from the delay circuit 16, and generates the switching current detection voltage Vd that is proportional to the detected drain current Isw1.

  In FIG. 1, an oscillator and slope compensation circuit 6 generates a clock CLK having a predetermined frequency and outputs it to the set terminal S of the RS latch circuit 15, and also generates a predetermined slope signal slope for slope compensation. And output to the adder 19. The adder 19 adds the slope signal slope to the switching current detection voltage Vd, and outputs the slope voltage Vs after the addition to the non-inverting input terminal of the PWM comparator 2. The PWM comparator 2 generates a high-level PWM signal S2 when the slope voltage Vs is greater than the feedback voltage Vfb. On the other hand, when the slope voltage Vs is equal to or lower than the feedback voltage Vfb, the PWM comparator 2 generates a low-level PWM signal S2. Further, the PWM signal S <b> 2 is output to the first input terminal of the OR gate 18.

  The comparator 4 generates a high-level overcurrent detection signal S4 when the switching current detection voltage Vd is greater than the overcurrent detection reference voltage V41. On the other hand, when the switching current detection voltage Vd is equal to or lower than the overcurrent detection reference voltage V41, the comparator 4 generates a low level overcurrent detection signal S4. Further, the overcurrent detection signal S4 is output to the second input terminal of the OR gate 18.

  An output signal from the OR gate 18 is output to the set terminal S of the RS latch circuit 10. In addition, the control signal EXT is output to the reset terminal R of the RS latch circuit 10 via the inverter 14. Further, the output signal from the RS latch circuit 10 is output to the reset terminal R of the RS latch circuit 15, and the output signal from the RS latch circuit 15 is output to the second input terminal of the AND gate 11. The output signal from the AND gate 11 is output as a control signal EXT to the gate of the switching element N1, the delay circuit 16, and the inverter 14.

  Next, the operation at the time of overcurrent detection when the switching current detection voltage Vd is larger than the overcurrent detection reference voltage V41 will be described. When the switching current detection voltage Vd becomes larger than the overcurrent detection reference voltage V41, the comparator 4 generates a high level overcurrent detection signal S4 and outputs it to the OR gate 18. Therefore, the RS latch circuit 10 outputs a high level output signal to the reset terminal R of the RS latch circuit 15. In response to this, the RS latch circuit 15 outputs a low level output signal to the AND gate 11. Therefore, the low level control signal EXT is generated, and the switching element N1 is turned off. As a result, an excessive current larger than a predetermined overcurrent threshold value corresponding to the overcurrent detection reference voltage V41 is prevented from flowing through the switching element N1.

  In FIG. 2, a light load detection reference voltage generation circuit 50 includes a voltage source 51 that generates a reference voltage Vp that is substantially independent of temperature, a voltage-current conversion circuit 52 that converts the reference voltage Vp into a current I52, and a current A current mirror circuit CM for converting I52 into a current Iref at a predetermined mirror ratio; and a current-voltage conversion circuit 54 for converting the current Iref into a light load detection reference voltage V50 and outputting it to the inverting input terminal of the comparator 13. Configured. Here, the voltage-current conversion circuit 52 includes an operational amplifier 53 that is a voltage follower circuit, a resistor R3, a diode D50, and an nMOS transistor N4. The resistor R3 and the diode D50 are connected in series between the nMOS transistor N4 and the ground. The current mirror circuit CM includes P-channel MOS field effect transistors (hereinafter referred to as PMOS transistors) P1 and P2.

  Further, the current-voltage conversion circuit 54 includes resistors R1 and R2 connected in series between the output terminal of the current mirror circuit CM and the ground, and an nMOS transistor N3 connected in parallel to the resistor R2. The Here, the light load detection signal S20 from the comparator 13 is output to the gate of the nMOS transistor N3.

  In FIG. 2, a current I52 = (Vp−Vf) / R3 determined by the reference voltage Vp, the resistance value of the resistor R3, and the forward voltage Vf of the diode D50 flows through the resistor R3 (R3 is the resistance of the resistor R3). Value.) Further, the current I52 is converted into a current Iref by the current mirror circuit CM. Here, the forward voltage Vf of the diode D50 has a negative temperature characteristic. In the present embodiment, a resistor R3 having a negative temperature characteristic opposite to the temperature characteristic of the on-resistance of the switching element N1 is used. Therefore, the current Iref has a positive temperature characteristic. The temperature dependence of the current Iref can be adjusted to a desired temperature dependence by adjusting the resistance value of the resistor R3.

  In FIG. 2, the current-voltage conversion circuit 54 converts the current Iref into the reference voltage V50a = Iref × R1 in response to the high level light load detection signal S20 (where R1 is the resistance value of the resistor R1). is there.). On the other hand, in response to the low-level light load detection signal S20, the current-voltage conversion circuit 54 converts the current Iref into a reference voltage V50b = Iref × (R1 + R2) (however, R1 and R2 are resistors R1 and R2). Resistance value.) Accordingly, the light load detection reference voltage V50 is switched between the reference voltages V50a and V50b.

  Next, the temperature dependence of the reference voltages V50a and V50b will be described. In the present embodiment, reference voltages V50a and V50b having positive temperature characteristics are generated in the same manner as the temperature characteristics of the resistance of the switching element N1. FIG. 3 is an explanatory diagram showing first to third patterns of combinations of the current Iref of FIG. 2 and the resistance values of the resistors R1 and R2. In this embodiment, resistors R1 and R2 having substantially the same temperature characteristics are used. In the present embodiment, resistors R1 and R2 having positive temperature characteristics having the same polarity as the on-resistance polarity of the switching element N1 or resistors R1 and R2 that do not depend on temperature are used. The resistors R1 and R2 have substantially the same temperature characteristics. As shown in FIG. 3, each element value is set by selecting one of the first to third combinations of the temperature characteristics of the current Iref and the temperature characteristics of the resistance values of R1 and R2. Thus, the reference voltages V50a and V50b having desired positive temperature characteristics can be generated. In the present embodiment, the load current at the timing when the feedback voltage Vfb becomes larger than the light load detection reference voltage V50 and the timing when the feedback voltage Vfb becomes equal to or lower than the light load detection reference voltage V50 is substantially constant regardless of the temperature change. Each element value of the light load detection reference voltage generation circuit 50 is set so that.

  FIG. 4 is a timing chart showing the operation of the switching regulator 200 of FIG. 1 at normal temperature.

  In FIG. 4, the reference voltage V50b is generated in a period before time t1. When the output voltage VOUT decreases, the feedback voltage Vfb from the feedback circuit 30 starts to increase. When the feedback voltage Vfb becomes larger than the reference voltage V50b at time t1, a high level light load detection signal S20 is generated. Further, the light load detection reference voltage V50 is switched to the reference voltage V50a. When the voltage level of the light load detection signal S20 is high, when the clock CLK from the oscillator and slope compensation circuit 6 becomes high level, the voltage level of the output signal from the RS latch circuit 15 becomes high level. A level control signal EXT is generated. In response to this, the switching element N1 is turned on.

  When the switching element N1 is turned on and a predetermined delay time set by the delay circuit 16 elapses, the switching current detection circuit 3 detects the switching current Isw1 and generates the switching current detection voltage Vd. When the compensated slope voltage Vs obtained by slope compensation of the switching current detection voltage Vd becomes larger than the feedback voltage Vfb, the voltage level of the PWM signal S2 from the PWM comparator 2 becomes a high level. In response to this, the RS latch circuit 10 outputs a high level output signal to the reset terminal R of the RS latch circuit 15. Accordingly, the voltage level of the output signal from the RS latch circuit 15 becomes low level, and the low level control signal EXT is generated. In response to this, the switching element N1 is turned off. The control logic circuit 100 keeps the output voltage VOUT constant by switching the switching element N1 by repeating the above-described operation when the feedback voltage Vfb is a heavy load greater than the light load detection reference voltage V50. Hereinafter, the control mode of the switching element N1 under heavy load is referred to as a burst oscillation mode. The burst oscillation mode is a control mode in which the switching element N1 is subjected to pulse width modulation control at the frequency of the clock CLK to cause burst oscillation.

  When the switching element N1 starts burst oscillation at time t1, the output voltage VOUT starts to rise. When the output voltage VOUT becomes higher than a predetermined set voltage, the current flowing through the light emitting portion of the photocoupler PC increases, and as a result, the voltage at the feedback terminal FD decreases. Therefore, when the current flowing through the load 22 becomes small (at light load), the feedback voltage Vfb decreases. When the feedback voltage Vfb becomes equal to or lower than the reference voltage V50a at time t2 in FIG. 4, a low level light load detection signal S20 is generated. Further, the light load detection reference voltage V50 is switched to the reference voltage V50b. In response to the light load detection signal S20 at the low level, the control signal EXT from the AND gate 11 becomes the low level, and the switching element N1 is turned off. That is, the control logic circuit 100 performs control so that the burst oscillation of the switching element N1 is stopped and turned off at a light load when the feedback voltage Vfb is smaller than the reference voltage V50. Hereinafter, the control mode of the switching element N1 at a light load is referred to as a burst oscillation stop mode.

  After time t2, the output voltage VOUT decreases, and the feedback voltage Vfb starts to increase again. At time t3, when the feedback voltage Vfb becomes higher than the reference voltage Vref, the switching element N1 is controlled to burst. The operation described above is repeated after time t3. Here, since the comparator 13 operates as a hysteresis comparator, the efficiency can be improved by reducing the number of times of switching at light load.

  In the present embodiment, the light load detection reference voltage generation circuit 50 generates a light load detection reference voltage V50 that is proportional to the on-resistance of the switching element N1. More specifically, in the light load detection reference voltage generation circuit 50, the load current at the time of switching between the burst oscillation mode at the heavy load and the burst oscillation stop mode at the light load is substantially dependent on the temperature. A light load detection reference voltage V50 is generated so as to be constant.

  FIG. 5 is a timing chart showing the slope voltage Vs of FIG. 1 during burst oscillation at normal temperature and high temperature. With reference to FIG. 5, the temperature characteristic of the slope voltage Vs will be considered. The slope voltage Vs is a voltage obtained by adding the switching current detection voltage Vd and the slope signal slope. Here, the switching current detection voltage Vd is a drain voltage of the switching element N1 proportional to the switching current Isw1, and is proportional to the on-resistance of the switching element N1. In general, the on-resistance of the switching element N1 has a positive temperature characteristic with respect to the operating temperature. Therefore, as the temperature rises and the on-resistance of the switching element N1 increases, the slope of the slope voltage Vs increases.

  Therefore, in FIG. 5, when the feedback voltage Vfb is the voltage Vfb1, the switching element N1 is on-time from the time ta to the time tc at room temperature, whereas the on-time is from the time ta to the time tb at high temperature. The on-time is shortened. For this reason, the power transferred to the load 22 per switching operation of the switching element N1 at a high temperature is lower than that at a normal temperature. As a result, in order to realize a steady state in which the output voltage VOUT is kept constant, the same ON time as that at room temperature is required, so the feedback voltage Vfb starts to rise and changes from the voltage Vfb1 to the voltage Vfb2.

  Conventionally, instead of the light load detection reference voltage V50 according to the present embodiment, a temperature independent reference voltage has been used. For this reason, as shown in FIG. 5, when the feedback voltage Vfb increases at a high temperature, the load current at the time of switching between the burst oscillation mode at the heavy load and the burst oscillation stop mode at the light load changes. On the other hand, in this embodiment, the light load detection reference voltage generation circuit 50 generates a light load detection reference voltage V50 that is proportional to the on-resistance of the switching element N1. Therefore, the load current at the time of switching between the burst oscillation mode and the burst oscillation stop mode can be made constant regardless of the operating temperature.

Second embodiment.
FIG. 6 is a circuit diagram showing a configuration of a light load detection circuit 20A according to the second embodiment of the present invention. Compared to the light load detection circuit 20 according to the first embodiment, the light load detection circuit 20A according to the present embodiment replaces the light load detection reference voltage generation circuit 50 with a light load detection reference voltage generation circuit 50A. Only the point with is different. The light load detection reference voltage generation circuit 50A is different from the light load detection reference voltage generation circuit 50 only in that a voltage / current conversion circuit 52A is provided instead of the voltage / current conversion circuit 52. Further, the voltage-current conversion circuit 52A is obtained by removing the diode D50 from the voltage-current conversion circuit 52. Only the differences from the first embodiment will be described below.

  In FIG. 6, the voltage / current conversion circuit 52A converts the reference voltage Vp into a current I52 = Vp / R3. This embodiment has the same operational effects as the first embodiment.

Third embodiment.
FIG. 7 is a circuit diagram showing a configuration of a light load detection circuit 20B according to the third embodiment of the present invention. The light load detection circuit 20B is different from the light load detection circuit 20 according to the first embodiment only in that a light load detection reference voltage generation circuit 50B is provided instead of the light load detection reference voltage generation circuit 50. Is different. The light load detection reference voltage generation circuit 50B is different from the light load detection reference voltage generation circuit 50 only in that a current source 55 is further provided. Only the differences from the first embodiment will be described below.

  In FIG. 7, a current source 55 is connected in parallel to the current mirror circuit CM and generates a current Im that is substantially independent of temperature. Therefore, the current Im is added to the current Iref from the current mirror circuit CM, and the current-voltage conversion circuit 54 converts the added current (Iref + Im) into the light load detection reference voltage V50.

  According to the present embodiment, even when the rate of change of the ON resistance of the switching element N1 with respect to the temperature is relatively small, the light load detection reference voltage V50 can be easily adjusted compared to the first embodiment.

  In the above embodiments, the present invention has been described by taking the insulating switching regulator 200 as an example. However, the present invention is not limited to this, and a non-insulating switching regulator may be used.

2 ... PWM comparator,
3 ... switching current detection circuit,
13 ... Comparator,
20, 20A, 20B ... Light load detection circuit,
50, 20A, 50B ... Light load detection reference voltage generation circuit,
51 ... Voltage source,
52, 52A ... voltage-current conversion circuit,
54 ... Current-voltage conversion circuit,
100: control logic circuit,
200 ... switching regulator,
CM: Current mirror circuit,
N1 is a switching element.

JP 2010-063276 A JP 2011-250627 A JP 2009-033883 A JP 2010-259257 A JP 2010-220338 A JP 2010-136497 A Japanese Patent No. 4908019 JP 2010-154716 A Japanese Patent No. 4619822 JP 2011-142895 A Japanese Patent No. 4865531 Japanese Patent No. 4667836 JP 2009-136064 A JP2011-072101A JP 2011-030391 A JP 2008-178257 A JP 2010-088274 A Japanese Patent No. 4726531 JP 2010-142111 A JP 2011-024345 A

Claims (10)

Switching current detection in which the switching element connected to the input terminal is proportional to the switching current flowing in the switching element so that the input voltage input through the input terminal is converted into a predetermined output voltage and output to the load. A control circuit that performs on / off control based on a voltage and a feedback voltage corresponding to the output voltage;
A switching current detecting circuit that detects the switching current by utilizing the change in the on-resistance of the switching element in response to a temperature change and generates a switching current detection voltage proportional to the detected current. In the light load detection circuit for the regulator,
A light load detection reference voltage generation circuit for generating a light load detection reference voltage proportional to the on-resistance of the switching element;
A light load detection circuit comprising: comparing means for comparing the feedback voltage with the light load detection reference voltage, generating a light load detection signal indicating the comparison result and outputting the light load detection signal to the control circuit. The control circuit controls the switching element in a predetermined first control mode in response to a light load detection signal indicating that the feedback voltage is greater than the light load detection reference voltage, while the feedback voltage is In response to a light load detection signal indicating that the voltage is equal to or lower than the light load detection reference voltage, the switching element is controlled in a predetermined second control mode,
The light load detection reference voltage generation circuit generates the light load detection reference voltage so that the load current when switching between the first and second control modes is substantially independent of temperature. The light load detection circuit according to claim 1.   3. The control circuit according to claim 2, wherein the control circuit performs pulse width modulation control on the switching element in the first control mode, and controls the switching element to be turned off in the second control mode. Light load detection circuit. The light load detection reference voltage generation circuit is
A voltage-current conversion circuit that converts a predetermined reference voltage substantially independent of temperature into a current; and
A current-voltage conversion circuit comprising a first resistor, wherein the converted current is converted into the light load detection reference voltage by passing the converted current through the first resistor and output. The light load detection circuit according to any one of 1 to 3.   5. The light load detection circuit according to claim 4, wherein the first resistor has a temperature characteristic of the same polarity as a polarity of the temperature characteristic of the on-resistance of the switching element.   The voltage-current conversion circuit includes a second resistor having a temperature characteristic opposite in polarity to the temperature characteristic of the on-resistance of the switching element, and the reference voltage is applied to the second resistor by applying the reference voltage to the second resistor. The light load detection circuit according to claim 4, wherein the current is converted into a current.   7. The light load detection circuit according to claim 6, wherein the voltage-current conversion circuit further includes a diode connected in series with the second resistor.   A switching regulator comprising the light load detection circuit according to claim 1. Switching current detection in which the switching element connected to the input terminal is proportional to the switching current flowing in the switching element so that the input voltage input through the input terminal is converted into a predetermined output voltage and output to the load. A control circuit that performs on / off control based on a voltage and a feedback voltage corresponding to the output voltage;
A switching current detecting circuit that detects the switching current by utilizing the change in the on-resistance of the switching element in response to a temperature change and generates a switching current detection voltage proportional to the detected current. In the regulator control method,
The light load detection circuit is
A light load detection reference voltage generation circuit for generating a light load detection reference voltage proportional to the on-resistance of the switching element;
Comparing means for comparing the feedback voltage with the light load detection reference voltage, generating a light load detection signal indicating the comparison result, and outputting the light load detection signal to the control circuit,
In response to a light load detection signal indicating that the feedback voltage is greater than the light load detection reference voltage, the control circuit controls the switching element in a predetermined first control mode, while the feedback voltage is A switching regulator control method comprising the step of controlling the switching element in a predetermined second control mode in response to a light load detection signal indicating that the voltage is not more than the light load detection reference voltage.   10. The switching according to claim 9, wherein the control circuit includes a step of performing pulse width modulation control of the switching element in the first control mode, and turning off the switching element in the second control mode. Regulator control method.

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