JP5666173B2 - LIGHT EMITTING DIODE DRIVING CIRCUIT, DRIVE METHOD, LIGHT EMITTING DEVICE USING THE SAME, ELECTRONIC DEVICE, AND LIGHTING DEVICE - Google Patents

Description

  The present invention relates to a driving technique for a light emitting diode.

  As a backlight for liquid crystal panels, it replaces the conventional CCFL (Cold Cathode Fluorescent Lamp) and EEFL (External Electrode Fluorescent Lamp), and has excellent characteristics in terms of longer life, lower power consumption, and wider color gamut. A light emitting diode (hereinafter abbreviated as LED) is used.

  FIG. 1 is a circuit diagram showing a configuration example of a general LED drive circuit 200. The LED drive circuit 200 includes a power supply circuit 202, a current driver 204, a PWM (Pulse Width Modulation) dimming circuit 206, an LED terminal 208, and a protection circuit 210. The LED terminal 208 is connected to a cathode terminal of an LED string including a plurality of LEDs to be driven. The power supply circuit 202 supplies the drive voltage Vdrv to the anode of the LED string 10.

  A current driver 204 is connected to the LED terminal 208. The current driver 204 generates a drive current Idrv corresponding to the luminance of the LED string 10 and supplies it to the LED string 10.

  If a terminal on the LED drive path is grounded via the path 209 due to some failure or abnormality, an undesirably large current may flow through the power supply circuit 202, some LEDs, and the path 209. Therefore, the LED drive circuit 200 detects a circuit abnormality based on the potential of the LED terminal 208. Specifically, when the LED drive circuit 200 operates normally, the potential of the LED terminal 208 is stably maintained near a predetermined voltage. However, if the LED string 10 breaks down and becomes an open state, or if a terminal on the LED drive path is grounded or sky-faulted, the potential of the LED terminal 208 (also referred to as the detection voltage Vs) is different from the normal state. Value. The protection circuit 210 detects an abnormal state of the circuit by monitoring the detection voltage Vs of the LED terminal 208, and performs a predetermined protection process when an abnormality occurs.

JP 2004-32875 A JP 2002-252971 A JP 2007-028784 A JP 2007-173813 A JP 2008-136293 A JP 2006-325396 A

  For the purpose of controlling the brightness of the LED (dimming), the drive current Idrv flowing through the LED string 10 may be switched on (lighting period) and off (light-off period) with a duty ratio corresponding to the brightness. This is also referred to as PWM driving or PWM dimming. The PWM dimming circuit 206 acts on the current driver 204 and switches the drive current Idrv in accordance with the PWM signal SPWM.

  During the PWM drive extinguishment period or standby state, the drive current Idrv is off during the period when the potential of the LED terminal 208 is out of the predetermined range regardless of whether or not there is a failure. Proper circuit protection is prevented.

  The present invention has been made in view of the problems, and one of the exemplary purposes of an embodiment thereof is to provide a technique for detecting an abnormality of an LED during the LED extinguishing period.

  One embodiment of the present invention relates to a drive circuit for driving a light-emitting diode string including a plurality of light-emitting diodes connected in series. The drive circuit is a control circuit that controls a DC / DC converter for supplying a drive voltage to one end on the anode side of the light-emitting diode string, and a current that is provided on the cathode side on the current path of the light-emitting diode and generates a drive current. An abnormality detection circuit for monitoring a switching signal instructing on / off of a switching transistor of the DC / DC converter and detecting an abnormal state of the light-emitting diode string based on the switching signal when the driving current is off; .

  If the LED string is normal, the output current of the DC / DC converter is very small when the drive current is off. On the other hand, when a part on the LED string is short-circuited to an unexpected node, the output current of the DC / DC converter will be larger than that in the normal state even when the drive current is off, that is, in the extinguishing period. The magnitude of the output current of the DC / DC converter affects the pulse width and generation frequency of the switching signal. Therefore, according to this aspect, it is possible to detect an abnormal state when the drive current is off by monitoring the switching signal.

The DC / DC converter may be configured to operate in a discontinuous mode at light load conditions. The abnormality detection circuit may determine that the abnormality is present when the switching signal indicates a continuous mode in a state where the drive current is off.
If the LED string is normal, the output current of the DC / DC converter becomes very small when the drive current is off, and the DC / DC converter operates in a discontinuous mode. According to this aspect, it is possible to detect an abnormal state based on the fact that the DC / DC converter operates in the continuous mode despite the drive current being off.

The abnormality detection circuit may detect an abnormal state based on the switching signal and a clock signal having the same frequency as the switching signal in the continuous mode.
In the discontinuous mode, the switching signal generation frequency (frequency) is lower than the clock signal generation frequency (frequency). According to this aspect, the abnormal state can be detected based on the period or frequency of the clock signal and the switching signal.

The abnormality detection circuit may determine that an abnormal state has occurred when a pulse of the switching signal is detected a predetermined number of times or more in a predetermined detection period.
In the discontinuous mode, the frequency of switching signal pulses is reduced. If a switching signal pulse occurs more than a certain number of times in a certain detection period, it can be regarded as a continuous mode. Therefore, according to this aspect, an abnormal state can be detected.

The abnormality detection circuit may determine that the abnormality has occurred when the duty ratio of the switching signal is greater than a predetermined threshold value.
In an abnormal state, the output current of the DC / DC converter increases, so the duty ratio of the switching signal may increase. In this case, an abnormal state can be detected based on the duty ratio.

  Another embodiment of the present invention is a light-emitting device. This device includes a light-emitting diode string and a drive circuit according to any one of the above-described modes for driving the light-emitting diode string.

  Yet another embodiment of the present invention is an electronic device. This electronic apparatus includes a liquid crystal display panel and the above-described light emitting device provided as a backlight of the liquid crystal display panel.

  Yet another embodiment of the present invention is a lighting device. This lighting device includes the above-described light emitting device.

  It should be noted that any combination of the above-described constituent elements, and those in which constituent elements and expressions of the present invention are mutually replaced between methods, apparatuses, systems, and the like are also effective as an aspect of the present invention.

  According to an aspect of the present invention, an abnormal state can be detected during the turn-off period of the LED.

It is a circuit diagram which shows the structural example of a general LED drive circuit. It is a circuit diagram which shows the structure of the LED drive circuit which concerns on embodiment. 3A and 3B are time charts showing the operation of the LED drive circuit in a normal state and an abnormal state.

  The present invention will be described below based on preferred embodiments with reference to the drawings. The same or equivalent components, members, and processes shown in the drawings are denoted by the same reference numerals, and repeated descriptions are omitted as appropriate. The embodiments do not limit the invention but are exemplifications, and all features and combinations thereof described in the embodiments are not necessarily essential to the invention.

In this specification, “the state in which the member A is connected to the member B” means that the member A and the member B are physically directly connected, or the member A and the member B are in an electrically connected state. Including the case of being indirectly connected through other members that do not affect the above.
Similarly, “the state in which the member C is provided between the member A and the member B” refers to the case where the member A and the member C or the member B and the member C are directly connected, as well as an electrical condition. It includes the case of being indirectly connected through another member that does not affect the connection state.

  FIG. 2 is a circuit diagram showing a configuration of the LED drive circuit 100 according to the embodiment. FIG. 2 shows the entire light emitting device including the LED driving circuit 100 and the LED string 10. This light-emitting device can be used as various light sources using LEDs, such as a backlight of a liquid crystal panel, an illumination of a mobile phone, or a lighting device.

  The LED driving circuit 100 includes a DC / DC converter 40 and an LED driving IC 110, and drives the LED string 10 including at least one or a plurality of LEDs connected in series. The DC / DC converter 40 generates a drive voltage Vdrv required for driving the LED string 10 and supplies it to the anode of the LED string 10. FIG. 2 shows a diode rectification type DC / DC converter 40 including an inductor L1, a switching transistor M1, a rectifier diode D1, and an output capacitor Co. However, the topology is not particularly limited.

The LED driving IC 110 mainly has the following three functions.
(Function 1) A function for controlling the brightness by adjusting the drive current Idrv flowing through the LED string 10 (Function 2) A function for adjusting the drive voltage Vdrv by feedback controlling the DC / DC converter 40 (Function 3) Detecting an abnormal state Function to protect the circuit

Regarding function 1, the LED driver IC 110 includes a current driver 70 and a PWM dimming circuit 74.
Prepare. The current driver 70 is provided on the cathode side on the current path of the LED string 10 to be driven. The current driver 70 generates a drive current Idrv corresponding to the luminance, and controls the current flowing through the LED string 10. Adjustment of the luminance of the LED string 10 according to the magnitude of the drive current Idrv is referred to as current dimming (analog dimming).

  The LED drive IC 110 performs so-called PWM dimming in addition to current dimming. Specifically, the driving current Idrv is switched in a time-sharing manner with a duty ratio corresponding to the luminance, the time average of the driving current Idrv is adjusted, and the luminance is changed.

  The PWM dimming circuit 74 generates a dimming pulse SPWM that is pulse-modulated to have a duty ratio according to the luminance. The PWM dimming circuit 74 acts on the current driver 70 based on the dimming pulse SPWM to turn on and off the drive current Idrv. In FIG. 2, the high level period of the dimming pulse SPWM is assigned to the lighting period (on period Ton) of the LED string 10, and the low level period is assigned to the extinguishing period (Toff). In addition to the PWM dimming, the dimming pulse SPWM is also fixed at a low level when the current driver 70 is turned off to turn off the light.

  The LED driving IC 110 can optimize the luminance of the LED string 10 by using both current dimming and PWM dimming.

  Regarding the function 2, the LED driving IC 110 includes a control circuit 20. The control circuit 20 controls the DC / DC converter 40 so that the drive voltage Vdrv necessary for causing the LED string 10 to emit light with a desired luminance is obtained. The configuration of the control circuit 20 is not particularly limited, and a known or future usable one used for an LED driver may be used.

For example, the control circuit 20 receives the potential of the LED terminal connected to the cathode of the LED string 10 (hereinafter referred to as a detection voltage Vs), and performs DC / DC by feedback so that the detection voltage Vs matches a predetermined reference voltage Vref. The converter 40 is controlled. By this feedback control, the output voltage Vdrv of the DC / DC converter 40 is
Vdrv = VLED + Vref
Is adjusted to hold.

  Specifically, the control circuit 20 includes a pulse modulator 22 and a driver 24. The pulse modulator 22 is a voltage mode or current mode pulse width modulator or pulse frequency modulator, and generates a switching signal S4 having a duty ratio corresponding to an error between the reference voltage Vref and the detection voltage Vs. The driver 24 drives the gate of the switching transistor M1 of the DC / DC converter 40 connected to the SWOUT terminal according to the switching signal S4, and switches the switching transistor M1.

  Next, circuit protection as a third function will be described. Regarding the protection function, the LED driving IC 110 includes a protection circuit 50 and an abnormality detection circuit 60.

  The abnormality detection circuit 60 receives the switching signal S4 and the dimming pulse SPWM output from the driver 24. The abnormality detection circuit 60 monitors the switching signal S4 instructing on / off of the switching transistor M1 of the DC / DC converter 40 in a state where the drive current Idrv is off, that is, in the light-off period Toff of the LED string 10, and the switching signal An abnormal state of the LED string 10 is detected based on S4. The extinguishing period Toff can be detected when the dimming pulse SPWM is at a low level.

  The abnormality detection circuit 60 generates an abnormality detection signal S5 that is asserted (low level) in an abnormal state and negated (high level) in a normal state, and outputs it from the fail terminal FAIL.

  The protection circuit 50 cuts off the power supply to the DC / DC converter 40 when the abnormality detection signal S5 is asserted. Specifically, the protection circuit 50 includes a protection switch M50, resistors R50 and R52, and a control switch M52. The protection switch M50 is a P-channel MOSFET, and is provided in series with the inductor L1 of the DC / DC converter 40. The resistor R50 is provided between the gate and source of the protection switch M50. The resistor R52 and the control switch M52 are provided in series between the gate of the protection switch M50 and the ground terminal. An abnormality detection signal S5 is input to the gate of the control switch M52.

  During a period in which the abnormality detection signal S5 is negated (high level), the control switch M52 is turned on, and since the ground voltage is applied to the gate of the protection switch M50, the protection switch M50 is turned on and the input voltage Vin to the DC / DC converter 40 is turned on. Is supplied.

  When an abnormality is detected by the abnormality detection circuit 60 and the abnormality detection signal S5 is asserted (low level), the control switch M52 is turned off, and the input voltage Vin is applied to the gate of the protection switch M50 via the resistor R50 to protect it. The switch M50 is turned off. As a result, the supply of the input voltage Vin to the DC / DC converter 40 is cut off.

  The protection process by the protection circuit 50 is not particularly limited. When the DC / DC converter 40 is a synchronous rectification type, the protection circuit 50 may turn off a synchronous rectification transistor provided instead of the rectification diode D1.

The above is the configuration of the LED drive circuit 100. Next, the operation will be described.
If the LED string 10 is normal, the DC / DC converter 40 is in a no-load state in a state where the drive current Idrv is off, that is, the extinguishing period Toff, and the output current Iout becomes very small.

  On the other hand, when a part of the LED string 10 is short-circuited to an unexpected node such as a ground terminal, a large current flows from the DC / DC converter 40 to the ground terminal through a part of the LED string 10. As a result, even during the extinguishing period Toff, the DC / DC converter 40 will not be in a no-load state, and its output current Iout will be larger than in the normal state.

  The magnitude of the output current Iout of the DC / DC converter 40 affects the pulse width and occurrence frequency of the switching signal S4. Specifically, the pulse width and the frequency of occurrence increase as the output current Iout increases. Therefore, according to the abnormality detection circuit 60 of FIG. 2, it is possible to detect an abnormal state when the drive current Idrv is off by monitoring the switching signal S4.

  For example, the DC / DC converter 40 is configured to operate in a discontinuous mode at light load conditions. The abnormality detection circuit 60 may determine that an abnormal state occurs when the switching signal S4 indicates the continuous mode in a state where the drive current Idrv is off. The frequency of occurrence of the switching signal S4 in the continuous mode is higher than that in the discontinuous mode. 2 includes a counter 62 that counts the number of pulses of the switching signal S4.

3A and 3B are time charts showing the operation of the LED drive circuit 100 in a normal state and an abnormal state.
In addition to the switching signal S4, the counter 62 receives a clock signal CLK having the same frequency as the switching signal S4 in the continuous mode.

  First, the operation in the normal state will be described with reference to FIG. When the LED string 10 is normal, the output current Iout of the DC / DC converter 40 becomes very small during the extinguishing period Toff, and the DC / DC converter 40 operates in the discontinuous mode. In the discontinuous mode, the frequency of occurrence of the switching signal S4, that is, the frequency decreases.

  The counter 62 counts the cycle Tsw (frequency) of the switching signal S4 using the clock signal CLK. When the period of the switching signal S4 is larger than the predetermined threshold value or the frequency is smaller than the predetermined threshold value, it is determined that the discontinuous mode is set, and it can be detected that the LED string 10 is normal.

  Reference is made to FIG. In the abnormal state, the output current Iout of the DC / DC converter 40 does not become zero even though the drive current Idrv is zero. As a result, the DC / DC converter 40 does not shift to the discontinuous mode, the switching signal S4 is generated at the same frequency as in the normal load state, and the period (frequency) of the switching signal S4 and the clock signal CLK becomes equal. That is, when the period of the switching signal S4 is counted using the clock signal CLK, it becomes smaller than a predetermined threshold value. The abnormality detection circuit 60 can detect the continuous mode based on the count value of the counter 62.

  As described above, according to the LED drive circuit 100 of FIG. 2, it is possible to reliably detect an abnormal state based on the fact that the DC / DC converter 40 is operating in the continuous mode despite the drive current Idrv being off. Can do.

  The clock signal CLK may have a frequency that is unrelated to the frequency of the switching signal S4 in the continuous mode.

The operation of the abnormality detection circuit 60 can be grasped as follows.
The abnormality detection circuit 60 may determine that an abnormality has occurred when the pulse of the switching signal S4 is detected a predetermined number of times or more in a predetermined detection period.
In the discontinuous mode, the frequency of pulses of the switching signal S4 is reduced compared to the continuous mode. If a switching signal pulse occurs more than a certain number of times in a certain detection period, it can be regarded as a continuous mode.

  The present invention has been described based on the embodiments. This embodiment is an exemplification, and it will be understood by those skilled in the art that various modifications can be made to combinations of the respective constituent elements and processing processes, and such modifications are within the scope of the present invention. is there. Hereinafter, such modifications will be described.

Depending on the configuration of the DC / DC converter 40 and the control circuit 20, there may be a case where the driving current Idrv is off and the operation is performed in the continuous mode without shifting to the discontinuous mode. In this case, the abnormality detection circuit 60 may determine that the abnormality has occurred when the duty ratio of the switching signal S4 is greater than a predetermined threshold value. In other words, the abnormality detection circuit 60 may determine that an abnormal state occurs when the pulse width of the switching signal S4 corresponding to turning on of the switching transistor M1 is larger than a predetermined threshold value.
In the abnormal state, the output current Iout of the DC / DC converter 40 becomes larger than that in the normal state, so that the duty ratio of the switching signal S4 increases. Therefore, an abnormal state can be detected based on the duty ratio and the pulse width.

  The abnormality detection of the LED string 10 in the lighting period Ton of the LED string 10 can be performed by using a conventional abnormality detection circuit based on the detection voltage Vs.

  Although the present invention has been described based on the embodiments, it should be understood that the embodiments merely illustrate the principles and applications of the present invention, and the embodiments are defined in the claims. It goes without saying that many modifications and changes in arrangement are allowed without departing from the spirit of the present invention.

DESCRIPTION OF SYMBOLS 10 ... LED string, 20 ... Control circuit, 22 ... Pulse modulator, 24 ... Driver, 40 ... DC / DC converter, M1 ... Switching transistor, L1 ... Inductor, D1 ... Rectifier diode, Co ... Output capacitor, 60 ... Abnormality detection Circuit, 62 ... Counter, 50 ... Protection circuit, M50 ... Protection switch, S4 ... Switching signal, S5 ... Abnormality detection signal, 70 ... Current driver, 74 ... PWM dimming circuit, 100 ... LED drive circuit, 110 ... LED drive IC .

Claims (10)

A driving circuit for driving a light emitting diode string including a plurality of light emitting diodes connected in series,
A control circuit for controlling a DC / DC converter for supplying a driving voltage to one end of the light emitting diode string on the anode side;
A current driver that is provided on the cathode side on the current path of the light emitting diode and generates a drive current;
When the drive current is off, a switching signal that instructs on / off of the switching transistor of the DC / DC converter is monitored, and an abnormality detection signal indicating an abnormal state of the light-emitting diode string is generated based on the switching signal. An anomaly detection circuit to
Equipped with a,
The DC / DC converter is configured to operate in a discontinuous mode in which the frequency of the switching signal decreases in a light load state,
The abnormality detection circuit is configured to determine an abnormal state when the switching signal indicates a continuous mode in a state where the drive current is off,
The abnormality detection circuit counts the number of pulses of the switching signal, determines whether the mode is a continuous mode or a discontinuous mode according to the number of counts, and determines whether it is an abnormal state .   The drive circuit according to claim 1, wherein the abnormality detection circuit includes a counter that receives the switching signal and the clock signal and counts the number of pulses of the switching signal generated within a predetermined detection period.   On the input side of the DC / DC converter, there is provided a protection circuit capable of switching between interruption and conduction of power supply to the DC / DC converter,
  The abnormality detection circuit further includes a fail terminal for outputting the abnormality detection signal,
  3. The drive circuit according to claim 1, wherein the protection circuit is cut off when the abnormality detection signal output from the fail terminal indicates an abnormal state. 4. It said clock signal, the drive circuit according to claim 2, wherein the benzalkonium which having a same frequency as the switching signal in the continuous mode. 3. The drive circuit according to claim 2 , wherein the abnormality detection circuit determines an abnormal state when the pulse of the switching signal is detected a predetermined number of times or more in a predetermined detection period by the counter .   The drive circuit further includes a dimming circuit that sets the drive current by acting on the current driver based on a dimming pulse, and inputs the dimming pulse to the abnormality detection circuit,
  6. The drive circuit according to claim 1, wherein the extinguishing period of the light emitting diode string is detected when the dimming pulse is at a low level in the abnormality detection circuit. A light emitting diode string;
The drive circuit according to any one of claims 1 to 6 , which drives the light emitting diode string;
A light emitting device comprising: A liquid crystal display panel;
The light emitting device according to claim 7 provided as a backlight of the liquid crystal display panel;
An electronic device comprising:   An illumination apparatus comprising the light-emitting device according to claim 7.   A method of driving a light emitting diode string including a plurality of light emitting diodes connected in series,
  Controlling a DC / DC converter for supplying a driving voltage to one end of the light emitting diode string on the anode side;
  Generating a drive current by a current driver provided on the cathode side on the current path of the light emitting diode;
  Monitoring a switching signal instructing on / off of a switching transistor of the DC / DC converter in a state where the driving current is off, and detecting an abnormal state of the light-emitting diode string based on the switching signal;
  With
  The DC / DC converter is configured to operate in a discontinuous mode in which the frequency of the switching signal decreases in a light load state,
  The step of detecting the abnormal state includes:
  Counting the number of pulses of the switching signal, and determining whether the mode is continuous mode or discontinuous mode according to the number of counts;
  A step of determining an abnormal state when the switching signal indicates a continuous mode in a state where the drive current is off;
  A method comprising the steps of:

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