# Patent

## Title
Switching converter, control circuit and control method thereof, and lighting device and electronic apparatus using the same

## Abstract

A control circuit of a switching converter includes a current detection comparator for comparing a detection voltage corresponding to a voltage drop of a detection resistor with a reference voltage and generating a peak current detection signal asserted when the detection voltage reaches the reference voltage, a driving logic unit for generating a pulse signal indicating a turn-on/off operation of a switching transistor and changing the pulse signal to an OFF level indicating the turn-off operation of the switching transistor when the peak current detection signal is asserted, a driver for driving the switching transistor based on the pulse signal, and a reference voltage setting unit for measuring time (TRECT) for which a current flows through a secondary coil and a switching period (T) of the switching transistor and adjusting the reference voltage (VREF) according to an equation: VREF=K×T/TRECT where K is a coefficient.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-210244, filed on Oct. 7, 2013, the entire contents of which are incorporated herein by reference.
TECHNICAL FIELD
The present disclosure relates to a switching converter.
BACKGROUND
A switching converter is used to step up or step down a DC voltage or convert a DC voltage into an AC voltage and vice versa. Various types of switching converters are available. Among these, there is a constant current-controlled switching converter for detecting a current flowing into a load and/or detecting a current flowing through a switching element of the switching converter and stabilizing the detected current to a predetermined value through a feedback control.
FIG. 1 is a circuit diagram of a flyback type switching converter. A switching converter 100r, to which an input voltage VIN from a power supply (not shown) is input, generates and supplies an output voltage VOUT to a load 502 by stepping down the input voltage VN while stabilizing a current ILED (referred to as a load current or a driving current) flowing through the load 502 to a target current value TREF. For example, the load 502 is a light emitting diode (LED) string and the switching converter 100r sets the load current ILED to the target current value TREF based on the target brightness of the LED string.
The switching converter 100r includes an output circuit 102 and a control circuit 200r. The output circuit 102 includes a smoothing capacitor C1, a rectifying diode D1, a switching transistor M1, a primary coil LP, an auxiliary inductor LZT and a detection resistor RCS.
In an ON period of the switching transistor M1, a current flowing through the switching transistor M1 is flown through the detection resistor RCS. A voltage drop (detection voltage) VCS of the detection resistor RCS is fed back to a current detection (CS) terminal of the control circuit 200r. 
A voltage VZT produced in the auxiliary inductor LZT is divided by resistors and the divided voltage is input to a ZT terminal of the control circuit 200r. 
The control circuit 200r includes a current detection comparator 202, a zero-cross detection circuit 204, a driving logic unit 206 and a driver 208.
FIG. 2 is an operation waveform diagram of the switching converter 100r of FIG. 1.
In an ON period of the switching transistor M1, a current (primary current) IP flows through the primary coil LP of a transformer T1, the switching transistor M1 and the detection resistor RCS. The primary current IP increases with time and the detection voltage VCS rises accordingly. The current detection comparator 202 compares the detection voltage VCS with a target voltage VREF set corresponding to the target current value IREF. The current detection comparator 202 asserts a peak current detection signal S1 (for example, the signal S1 is at a high level) if the detection voltage VCS reaches the target voltage VREF, that is, if the current IP reaches a peak current IPEAK (=VREF/RCS). In the ON period, energy stored in the transformer T1 increases. During the ON period, no current flows through a secondary coil LS of the transformer T1.
The driving logic unit 206 changes a pulse signal S2 to an OFF level (for example, a low level) corresponding to a turn-off operation of the switching transistor M1 when the peak current detection signal S1 is asserted. The driver 208 turns off the switching transistor M1 in response to the pulse signal S2.
In an OFF period of the switching transistor M1, a current (secondary current) IS flown through the secondary coil LS of the transformer T1 decreases with time and a voltage VZT decreases accordingly. The zero-cross detection circuit 204 compares the voltage VZT of the auxiliary inductor LZT with a threshold voltage VZERO set to about zero. Based on the comparison result, the zero-cross detection circuit 204 detects that the secondary current IS is zeroed (zero-cross), and asserts a zero-cross detection signal S3.
The driving logic unit 206 changes the pulse signal S2 to an ON level (for example, a high level) corresponding to a turn-on operation of the switching transistor M1 when the zero-cross detection signal S3 is asserted. The driver 208 turns on the switching transistor M1 in response to the pulse signal S2.
The control circuit 200r repeats the above operation. The load current ILED is a current obtained by smoothing the secondary current IS by means of the smoothing capacitor C1 and is expressed by the following equation (1).

ILED=1/2×IPEAK′×TOFF/T  (1)

Where, IPEAK′ is a peak of the secondary current IS and T is a switching period (TON+TOFF). Equation (1) may be rewritten into the following equation (2). Here, a forward voltage drop of the rectifying diode D1 is ignored.

ILED=IPEAK2×LS/VOUT  (2)

As can be seen from Equation (2), the driving current ILED depends on the output voltage VOUT, that is, a forward voltage of the load 502. That is, the switching converter 100r of FIG. 1 is problematic in that the driving current ILED is varied due to the variation of the forward voltage of the load (e.g., LED string) 502. In addition, it is also problematic that the driving current ILED is affected by a change in the inductance of the secondary coil LS of the transformer T1.
To solve the aforementioned problems, a method of connecting a resistor for detection of the driving current ILED in series to the load 502, feeding back a voltage drop of the resistor to the control circuit 200r, and controlling the driving current ILED to be equal to a target value has been proposed. However, applications requiring electrical isolation between a primary side and a secondary side need a photo coupler or a shunt regulator in order to feedback a detection voltage of the secondary side to the primary side, which may result in increased costs.
The above problems may also occur in a variety of loads other than the LED string. That is, in any load, if its impedance is varied or fluctuated, a voltage drop (the output voltage VOUT) across the load when the same load current flows is changed, which may cause a change in the load current ILED.
SUMMARY
The present disclosure provides some embodiments of a switching converter which is capable of reducing variation and fluctuation of a load and supplying a stable driving current to the load.
According to one embodiment of the present disclosure, a control circuit of a flyback type switching converter including a transformer having a primary coil and a secondary coil; a switching transistor connected to the primary coil; and a detection resistor arranged on a path of a current flowing through the switching transistor in an ON period of the switching transistor, includes a current detection comparator configured to compare a detection voltage corresponding to a voltage drop of the detection resistor with a reference voltage and generate a peak current detection signal asserted when the detection voltage reaches the reference voltage; a driving logic unit configured to generate a pulse signal indicating a turn-on/off operation of the switching transistor and transition the pulse signal to an OFF level indicating the turn-off operation of the switching transistor when the peak current detection signal is asserted; a driver configured to drive the switching transistor based on the pulse signal; and a reference voltage setting unit configured to measure time (TRECT) for which a current flows through the secondary coil and a switching period (T) of the switching transistor and adjust the reference voltage (VREF) according to an equation: VREF=K×T/TRECT where K is a coefficient.
According to this embodiment, it is possible to stabilize a driving current supplied to the load without monitoring and detecting a current of the secondary coil.
The control circuit may further include a zero-cross detection circuit configured to generate a zero-cross detection signal asserted when a current flowing through the secondary coil of the transformer is zeroed, in an OFF period of the switching transistor. The driving logic unit may change the pulse signal to an ON level indicating the turn-on operation of the switching transistor when the zero-cross detection signal is asserted.
The switching converter may further include an auxiliary inductor coupled to the transformer. The zero-cross detection circuit may generate the zero-cross detection signal based on a voltage across the auxiliary inductor.
The reference voltage setting unit may measure the time (TRECT) for which the current flows through the secondary coil and the switching period (T) of the switching transistor based on the peak current detection signal and the zero-cross detection signal.
The reference voltage setting unit may measure the time (TRECT) for which the current flows through the secondary coil and the switching period (T) of the switching transistor based on the pulse signal.
The load of the switching converter may include a light emitting device. The reference voltage setting unit may set the coefficient (K) based on a control signal indicating brightness of the light emitting device.
The control circuit may be integrated with a single semiconductor substrate. As used herein, the term “integrated” is intended to include both the case where all elements of a circuit are formed on a semiconductor substrate and the case where main elements of the circuit are integrated on the semiconductor substrate. In addition, some resistors, capacitors and the like for adjustment of a circuit constant may be provided outside the semiconductor substrate.
According to another embodiment of the present disclosure, a switching converter includes the above-described control circuit.
According to another embodiment of the present disclosure, a lighting device includes a light emitting unit including a plurality of light emitting diodes connected in series; a rectification circuit configured to smooth and rectify a general-purpose AC voltage to output a DC voltage; and a switching converter configured to receive the DC voltage from the rectification circuit as an input voltage, the light emitting unit being connected to the switching converter as a load, wherein the switching converter includes the above-described control circuit.
According to another embodiment of the present disclosure, an electronic apparatus includes a liquid crystal panel; and the above-described lighting device, wherein the lighting device is configured as a backlight configured to irradiate a light on a rear side of the liquid crystal panel.
According to another embodiment of the present disclosure, a control method of a flyback type switching converter including a transformer having a primary coil and a secondary coil; a switching transistor connected to the primary coil; and a detection resistor arranged on a path of a current flowing through the switching transistor in an ON period of the switching transistor, includes comparing a detection voltage corresponding to a voltage drop of the detection resistor with a reference voltage and generating a peak current detection signal asserted when the detection voltage reaches the reference voltage; generating a pulse signal indicating a turn-on/off operation of the switching transistor and changing the pulse signal to an OFF level indicating the turn-off operation of the switching transistor when the peak current detection signal is asserted; driving the switching transistor based on the pulse signal; measuring at least one of time (TRECT) for which a current flows through the secondary coil and a switching period (T) of the switching transistor; and adjusting the reference voltage (VREF) based on at least one of the measured time (TRECT) and the measured switching period (T).
Adjusting the reference voltage (VREF) may include adjusting the reference voltage (VREF) inversely proportional to the measured time (TRECT).
Adjusting the reference voltage (VREF) may include adjusting the reference voltage (VREF) proportional to the measured switching period (T).
Any combinations of the above-described elements or changes of the representations of the present disclosure between methods and apparatuses are effective as embodiments of the present disclosure.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a circuit diagram of a flyback type switching converter in the related art.
FIG. 2 is an operation waveform diagram of the switching converter of FIG. 1.
FIG. 3 is a circuit diagram of a switching converter according to an embodiment.
FIG. 4 is an operation waveform diagram of the switching converter of FIG. 3.
FIG. 5 is a block diagram of a lighting device including the switching converter of FIG. 3.
FIGS. 6A to 6C are exemplary views of the lighting device.
FIG. 7 is an operation waveform diagram of a switching converter operating in a discontinuous mode.



DETAILED DESCRIPTION
Preferred embodiments of the present disclosure will now be described in detail with reference to the drawings. Throughout the drawings, the same or similar elements, members and processes are denoted by the same reference numerals and explanation of which will not be repeated. The disclosed embodiments are provided for the purpose of illustration, not limitation, of the present disclosure and all features and combinations thereof described in the embodiments cannot be necessarily construed to describe the spirit of the present disclosure.
In the specification, the phrase “connection of a member A and a member B” is intended to include direct physical connection of the member A and the member B as well as indirect connection thereof via another member as long as the other member has no substantial effect on the electrical connection of the member A and the member B. Similarly, the phrase “interposition of a member C between a member A and a member B” is intended to include direct connection of the member A and the member C or direct connection of the member B and the member C as well as indirect connection thereof via another member as long as the other member has no substantial effect on the electrical connection of the member A, the member B and the member C.
FIG. 3 is a circuit diagram of a switching converter 100 according to an embodiment. The switching converter 100 includes an output circuit 102 and a control circuit 200. As shown in FIG. 3, the switching converter 100 is configured as a flyback type converter and supplies a driving voltage VOUT across a load 502.
The load 502 is a constant current-driven device and the switching converter 100 stabilizes a current flowing through the load 502 to a target value. For example, the load 502 may be a LED string including a plurality of light emitting diodes (LEDs) connected in series. The switching converter 100 stabilizes a current ILED flowing through the load 502 to a target current value IREF depending on the target brightness.
The output circuit 102 includes a transformer T1, a smoothing capacitor C1, a rectifying diode D1, a switching transistor M1 and a detection resistor RCS. A primary coil LP of the transformer T1, the switching transistor M1 and the detection resistor RCS are connected in series between an input line 104 and a ground line. A secondary coil LS of the transformer T1 and the rectifying diode D1 are connected in series to the load 502 to form a closed loop. Specifically, an anode of the rectifying diode D1 is connected to one end of the secondary coil LS and a cathode thereof is connected to an output line 106. One end of the smoothing capacitor C1 is connected to the cathode of the rectifying diode D1 and the output line 106 and the other end thereof is connected to the other end of the secondary coil LS.
The control circuit 200 is configured as a functional IC (Integrated Circuit) integrated with a single semiconductor substrate. The control circuit 200 has an output (OUT) terminal, a current detection (CS) terminal, an auxiliary (ZT) terminal, a power (VCC) terminal, a dimming (DIM) terminal and a ground (GND) terminal.
The GND terminal is grounded and the VCC terminal is supplied with a power supply voltage VCC. The OUT terminal is connected to a gate of the switching transistor M1 and the CS terminal is input with a detection voltage VCS indicative of a voltage drop of the detection resistor RCS. The DIM terminal is input with a control signal SDIM indicating the brightness of the load 502. The ZT terminal is input with a voltage VZT indicative of a voltage at one end of an auxiliary coil LZT.
The control circuit 200 includes a current detection comparator 202, a zero-cross detection circuit 204, a driving logic unit 206, a driver 208 and a reference voltage setting unit 240.
The current detection comparator 202 compares the detection voltage VCS with a reference voltage VREF and generates a peak current detection signal S1 based on the comparison result. When the detection voltage VCS reaches the reference voltage VREF, the current detection comparator 202 asserts the peak current detection signal S1 (for example, outputs the signal S1 at a high level). In some examples, a mask circuit may be disposed in a subsequent-stage of the zero-cross detection circuit 204. The mask circuit is configured to invalidate the comparison result of the current detection comparator 202, that is, the peak current detection signal S1, during a mask time TMSK from turn-on time of the switching transistor M1. The mask circuit may suppress noise or an effect of the noise.
The zero-cross detection circuit 204 generates a zero-cross detection signal S3 asserted when a current IS flowing through the secondary coil LS is zeroed, in an OFF period of the switching transistor M1. For example, the zero-cross detection circuit 204 includes a comparator for comparing the voltage VZT of the ZT terminal with a predetermined threshold voltage VZERO and may assert the zero-cross detection signal S3 when the voltage VZT becomes lower than the threshold voltage VZERO.
The driving logic unit 206 generates a pulse signal S2 indicating a turn-on/off operation of the switching transistor M1. When the peak current detection signal S1 is asserted, the driving logic unit 206 changes the pulse signal S2 to an OFF level (for example, a low level) that indicates the turn-off operation of the switching transistor M1. In addition, when the zero-cross detection signal S3 is asserted, the driving logic unit 206 changes the pulse signal S2 to an ON level (for example, a high level) that indicates the turn-on operation of the switching transistor M1.
For example, the driving logic unit 206 may be configured as a RS flip-flop having a set terminal input with the zero-cross detection signal S3 and a reset terminal input with the peak current detection signal S1. The configuration of the driving logic unit 206 has been described above, but the present disclosure is not limited thereto. The driver 208 drives the switching transistor M1 based on the pulse signal S2.
The reference voltage setting unit 240 generates the reference voltage VREF based on the control signal SDIM. The reference voltage setting unit 240 measures a time TRECT for which the secondary current IS flows through the secondary coil LS (hereinafter referred to as a rectification time) and a switching period T of the switching transistor M1. The reference voltage setting unit 240 sets the reference voltage VREF according to the equation: VREF=K×T/TRECT. Where, K is a real number coefficient. For example, K may be IREF/RCS. Here, IREF is a target current value of the load current ILED determined based on the control signal SDIM.

VREF=IREF/RCS×T/TRECT  (3)

The reference voltage setting unit 240 includes a measuring part 242, a calculating part 244 and a D/A converter 246. The measuring part 242 measures the rectification time TRECT and the switching period T. For example, the measuring part 242 may measure the rectification time TRECT and the switching period T based on the peak current detection signal S1 and the zero-cross detection signal S3. For example, the measuring part 242 may measure a time taken from an assertion of the peak current detection signal S1 to the next assertion thereof, as the switching period T, and may measure a time taken until the zero-cross detection signal S3 is asserted after the peak current detection signal S1 is asserted, as the rectification time TRECT.
The calculating part 244 calculates the reference voltage VREF according to Equation (3) and outputs a calculated value DREF. The D/A converter 246 converts the digital calculated value DREF into the reference voltage VREF.
The configuration of the switching converter 100 has been described above. FIG. 4 is an operation waveform diagram of the switching converter 100 of FIG. 3.
In a high level period of an output SOUT of the driver 208, the switching transistor M1 is turned on and a current IP of the primary coil LP increases with time, which in turn, increases the detection voltage VCS with time. When the detection voltage VCS reaches the reference voltage VREF generated by the reference voltage setting unit 240, the peak current detection signal S1 is asserted and the pulse signal S2 is changed to a low level.
When the pulse signal S2 is changed to the low level and the switching transistor M1 is turned off, the secondary current IS begins to flow through the secondary coil LS at the secondary side. The secondary current IS has the maximum value immediately after the switching transistor M1 is turned off. Thereafter, the secondary current IS decreases with time and the auxiliary voltage VZT decreases accordingly. When the secondary current IS is substantially zeroed, the zero-cross detection signal S3 is asserted and the pulse signal S2 is changed to a high level accordingly.
The control circuit 200 supplies the driving current ILED to the load 502 by repeating the above operation. During this switching operation, the switching period T and the rectification time TRECT of the switching transistor M1 are measured by the reference voltage setting unit 240 and the reference voltage VREF is determined by the measured switching period T and rectification time TRECT according to Equation (3).
As shown in FIG. 4, as the output voltage VOUT of the switching converter 100, that is, a forward voltage of the load 502, increases, a slope of the secondary current IS increases in the OFF period of the switching transistor M1 and, accordingly, a ratio of the switching period T to the rectification time TRECT (T/TRECT) increases. In accordance with Equation (3), the reference voltage VREF and the peak current IPEAK of the primary current IP increases as the ratio T/TRECT increases. Consequently, the load current IFED obtained by smoothing the secondary current IS can be maintained at a constant level.
With this switching converter 100, without monitoring the secondary current IS of the transformer T1, the load current ILED can be stabilized to a current value based on the control signal SDIM. In addition, it is possible to suppress an increase in costs without requiring expensive components such as a photo coupler or a shunt regulator.
Subsequently, applications of the switching converter 100 will be described. FIG. 5 is a block diagram of a lighting device 500 including the switching converter 100 of FIG. 3. The lighting device 500 includes a rectification circuit 504, a smoothing capacitor 506 and a microcomputer 508 in addition to a light emitting unit as the load 502 and the switching converter 100. The rectification circuit 504 and the smoothing capacitor 506 convert a general-purpose AC voltage VAC into a DC voltage VDC by rectifying and smoothing the AC voltage VAC. The microcomputer 508 generates a control signal SDIM indicating the brightness of the load 502. The switching converter 100 receives the DC voltage VDC as an input voltage VIN and supplies a driving current LED based on the control signal SDIM to the load 502.
FIGS. 6A to 6C are exemplary views of the lighting device 500. All elements are not shown but some are omitted in FIGS. 6A to 6C. A lighting device 500a is a direct tube type LED illuminator as shown in FIG. 6A. A plurality of LED elements constituting a LED string as the load 502 is laid on a board 510. The rectification circuit 504, the control circuit 200 and the output circuit 102 are mounted on the board 510.
A lighting device 500b is a bulb type LED illuminator as shown in FIG. 6B. A LED module as the load 502 is mounted on a board 510. The rectification circuit 504 and the control circuit 200 are mounted inside a housing of the lighting device 500b. 
As shown in FIG. 6C, a lighting device 500c is a backlight equipped in a liquid crystal display (LCD) apparatus 600. The lighting device 500c irradiates a light on a rear side of a liquid crystal panel 602.
The lighting device 500 may be used in a ceiling light. As described above, the lighting device 500 of FIG. 5 may be utilized in various applications.
The present disclosure is not limited to the above embodiments since these are illustrative only. It should be understood by those skilled in the art that various modifications to combinations of elements or processes may be made and such modifications fall within the scope of the present disclosure. Such modifications will be described below.
Modification 1
Although it has been illustrated in the above embodiments that the rectification time TRECT and the switching period T are measured based on the peak current detection signal S1 and the zero-cross detection signal S3 and the reference voltage VREF is adjusted based on the measurements, the present disclosure is not limited thereto. Instead, the following process may be performed.
As described in the above embodiments, in the modulation scheme of detecting the zero-cross of the secondary current IS, when the bottom of a down slope of the waveform of the secondary current IS is zeroed, the pulse signal S2 is immediately changed to an ON level (high level) and the switching transistor M1 is turned on. Accordingly, the length of time for which the pulse signal S2 has a low level, that is, OFF time of the switching transistor M1, is equal to the length of the rectification time TRECT for which the secondary current flows (which is referred to as a critical mode). Therefore, the measuring part 242 may measure the rectification time TRECT and the switching period T Based on the pulse signal S2 or the switching pulse SOUT.
Modification 2
The switching converter 100 may operate in a discontinuous mode instead of the critical mode. FIG. 7 is an operation waveform diagram of the switching converter 100 operating in a discontinuous mode. For example, the discontinuous mode is realized by inputting a clock signal CLK having a predetermined cycle T, instead of the zero-cross detection signal S3, to the set terminal of the RS flip-flop of the driving logic unit 206.
In the discontinuous mode, the OFF time of the switching transistor M1 is not equal to the rectification time TRECT. In such a case, the rectification time TRECT may be measured based on the zero-cross detection signal S3 which is generated by the zero-cross detection circuit 204.
Modification 3
Although it has been illustrated in the above embodiments that both the switching period T and the rectification time TRECT are measured, the present disclosure is not limited thereto. For example, in a situation where one of them is regarded as constant, only the other may be measured and used in calculating the reference voltage VREF. More specifically, in the discontinuous mode using the clock signal CLK of an oscillator as shown in FIG. 7, since the switching period T is known, only the rectification time TRECT may be measured.
Modification 4
The modulation scheme of the control circuit 200 is not limited to those described in the above embodiments. For example, it has been illustrated in the above embodiments that the switching converter is operated in a boundary between a continuous mode and a discontinuous mode, that is, in the critical mode where the bottom of an inductor current is zeroed. To this end, the zero-cross of the auxiliary inductor LZT is detected to detect that energy stored in the secondary coil LS of the output circuit 102 is zeroed. However, other schemes may be employed. As an alternative, a modulator operated in the continuous mode or the discontinuous mode instead of the critical mode may be employed. For example, a scheme of turning on the switching transistor M1 every predetermined period may be employed. In addition, a scheme using an error amplifier for amplifying an error between an output voltage or an output current, and a target value may be employed.
Modification 5
Although it has been illustrated in the above embodiments that the load 502 is a LED string, the type of the load is not particularly limited.
Modification 6
In the above embodiments, logical values of the high level and the low level for use in logic circuits may be changed, for example by appropriately inverting these levels by means of an inverter or the like.
According to the present disclosure in some embodiments, it is possible to stabilize a load current. In some embodiments, it is possible to stabilize the load current without monitoring and detecting a current of the secondary coil.
While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the disclosures. Indeed, the novel methods and apparatuses described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the disclosures. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the disclosures.

## Claims

1. A control circuit of a flyback type switching converter including:
a transformer having a primary coil and a secondary coil;
a switching transistor connected to the primary coil; and
a detection resistor arranged on a path of a current flowing through the switching transistor in an ON period of the switching transistor, the control circuit comprising:
a current detection comparator configured to compare a detection voltage corresponding to a voltage drop of the detection resistor with a reference voltage and generate a peak current detection signal asserted when the detection voltage reaches the reference voltage;
a driving logic unit configured to generate a pulse signal indicating a turn-on/off operation of the switching transistor and change the pulse signal to an OFF level indicating the turn-off operation of the switching transistor when the peak current detection signal is asserted;
a driver configured to drive the switching transistor based on the pulse signal; and
a reference voltage setting unit configured to measure time (TRECT) for which a current flows through the secondary coil and a switching period (T) of the switching transistor and adjust the reference voltage (VREF) according to an equation: VREF=K×T/TRECT where K is a coefficient.

2. The control circuit of claim 1, further comprising a zero-cross detection circuit configured to generate a zero-cross detection signal asserted when a current flowing through the secondary coil of the transformer is zeroed, in an OFF period of the switching transistor,
wherein the driving logic unit is configured to change the pulse signal to an ON level indicating the turn-on operation of the switching transistor when the zero-cross detection signal is asserted.

3. The control circuit of claim 2, wherein the switching converter further includes an auxiliary inductor coupled to the transformer, and
wherein the zero-cross detection circuit is configured to generate the zero-cross detection signal based on a voltage across the auxiliary inductor.

4. The control circuit of claim 2, wherein the reference voltage setting unit measures the time (TRECT) for which the current flows through the secondary coil and the switching period (T) of the switching transistor based on the peak current detection signal and the zero-cross detection signal.

5. The control circuit of claim 1, wherein the reference voltage setting unit measures the time (TRECT) for which the current flows through the secondary coil and the switching period (T) of the switching transistor based on the pulse signal.

6. The control circuit of claim 1, wherein a load of the switching converter includes a light emitting device, and
wherein the reference voltage setting unit sets the coefficient (K) based on a control signal indicating brightness of the light emitting device.

7. The control circuit of claim 1, wherein the control circuit is integrated with a single semiconductor substrate.

8. A switching converter comprising a control circuit of claim 1.

9. A lighting device comprising:
a light emitting unit including a plurality of light emitting diodes connected in series;
a rectification circuit configured to smooth and rectify a general-purpose AC voltage to output a DC voltage; and
a switching converter configured to receive the DC voltage from the rectification circuit as an input voltage, the light emitting unit being connected to the switching converter as a load,
wherein the switching converter includes a control circuit of claim 1.

10. An electronic apparatus comprising:
a liquid crystal panel; and
a lighting device of claim 9, wherein the lighting device is configured as a backlight configured to irradiate a light on a rear side of the liquid crystal panel.

11. A control method of a flyback type switching converter including a transformer having a primary coil and a secondary coil; a switching transistor connected to the primary coil; and a detection resistor arranged on a path of a current flowing through the switching transistor in an ON period of the switching transistor, the control method comprising:
comparing a detection voltage corresponding to a voltage drop of the detection resistor with a reference voltage and generating a peak current detection signal asserted when the detection voltage reaches the reference voltage;
generating a pulse signal indicating a turn-on/off operation of the switching transistor and changing the pulse signal to an OFF level indicating the turn-off operation of the switching transistor when the peak current detection signal is asserted;
driving the switching transistor based on the pulse signal;
measuring at least one of time (TRECT) for which a current flows through the secondary coil and a switching period (T) of the switching transistor; and
adjusting the reference voltage (VREF) based on at least one of the measured time (TRECT) and the measured switching period (T).

12. The control method of claim 11, wherein the adjusting the reference voltage (VREF) includes adjusting the reference voltage (VREF) inversely proportional to the measured time (TRECT).

13. The control method of claim 11, wherein the adjusting the reference voltage (VREF) includes adjusting the reference voltage (VREF) proportional to the measured switching period (T).