Backlight LED drive circuit

A backlight LED drive circuit 20 includes a step-up DC/DC converter 22 for stepping up a DC power voltage based on a PWM signal and supplying the voltage to the anode of an LED device 12c, a voltage detector 24 for detecting a feedback voltage FBV based on a voltage at a terminal 22b coupled to the cathode of the LED device, a PWM control circuit 26 for outputting a PWM signal to the step-up DC/DC converter 22 so that the feedback voltage FBV may become a predetermined voltage, and a PWM stop circuit 28 for stopping the PWM signal when the feedback voltage FBV is below a second predetermined voltage set smaller than the predetermined voltage. Thereby, such backlight LED drive circuit causes no troubles even when terminal to which the LED device is connected is rendered open.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is related to the Japan Patent Application No. 2007-274227, filed Oct. 22, 2007, the entire disclosure of which is expressly incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a backlight LED drive circuit for driving LED (Light Emitting Diode) used for a backlight of a liquid crystal panel.

2. Description of the Related Art

A backlight LED drive circuit for driving LED used for a backlight of a liquid crystal panel is well known.FIG. 5is a block diagram showing a conventional backlight LED drive circuit. Referring toFIG. 5, a backlight LED drive circuit1includes a step-up DC/DC converter2, voltage detector3, and PWM control circuit4. The drive circuit1steps up a DC supply voltage based on a PWM signal output from the PWM control circuit4and supplies the stepped up voltage to an LED device5formed of a plurality of LEDs connected in series. At this time, a feedback voltage (FB voltage) based on the voltage on the cathode side of the LED device5is detected and the PWM signal is controlled so that the feedback voltage may become a predetermined voltage. For example, when the feedback voltage becomes higher than the predetermined voltage, the duty ratio of the PWM signal is decreased so that the stepped up voltage is lowered, and when the feedback voltage becomes lower than the predetermined voltage, the duty ratio of the PWM signal is increased so that the stepped up voltage is raised.

There are proposed various drive circuits for driving an LED device suitably. For example, there is disclosed, in JP-A 2007-96296 (hereinafter called Patent Document 1), a display apparatus capable of preventing continuous increase in its standby power when a light source is turned off, thereby improving its display characteristic. In JP-A 2007-13183 (hereinafter called Patent Document 2), there is disclosed a backlight LED drive circuit having a PWM control unit for ON/OFF controlling a switch with a switching pulse having a duty ratio determined in accordance with a predetermined internal reference voltage and a detected voltage detected by a voltage detecting resistor. Further, in JP-A H03-255684 (hereinafter called Patent Document 3), there is disclosed a drive circuit of a light emitting device having its oscillator circuit provided with a switch as a control means for controlling oscillation to be made and stopped, and thereby deterioration or breakage of the light emitting device is prevented and the current flowing through the light emitting device is switched at a high speed.

Now, in the backlight LED drive circuit1shown inFIG. 5, if for example the LED device5becomes unconnected with the connection terminals2a,2bor the LED device5becomes broken so as to render the connection terminals2a,2bopen, then a voltage will not be applied to the connection terminal2b. Hence the feedback voltage will become lower than the predetermined voltage to increase the duty ratio of the PWM signal, and thereby the stepped up voltage output from the connection terminal2amay be increased. At this time, even if the stepped up voltage rises, the feedback voltage remains lower than the predetermined voltage. Therefore, there arises a possibility for example that the duty ratio of the PWM signal is kept at its maximum value and the stepped up voltage at the connection terminal2acontinues to rise until it reaches the maximum voltage that the step-up DC/DC converter2can output.

The circuit elements constituting a step-up DC/DC converter2, in general, are designed to have some margin to sustain normal operations. However, in view of the balance between the margin and such a factor as an increase in cost and installation space, it is considered desirable not to use such circuit elements that can stand the above mentioned maximum voltage. Hence, when the connection terminals2a,2bhave been rendered open during the operation of the backlight LED drive circuit1, there has been a possibility of troubles occurring in the circuit elements making up the step-up DC/DC converter2.

Further, when the LED device5has been connected between the connection terminals2a,2bunder the condition where the stepped up voltage at the connection terminal2ais high, there has been a possibility of troubles occurring in the LED device5.

While the arts disclosed in the above mentioned patent documents are not such that prevent occurrence of those troubles, there has not yet been proposed any art to prevent occurrence of troubles when the connection terminals2a,2bare rendered open.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a backlight LED drive circuit causing no troubles in the LED device and the circuit for supplying a voltage to the LED device even when the terminals with which the LED device is connected are rendered open while it is in operation.

A backlight LED drive circuit for driving an LED device as a backlight of a liquid crystal display panel, the present invention is configured of a step-up DC/DC converter for stepping up a DC power voltage in accordance with a PWM signal and supplying the stepped up voltage to the anode side of the LED device, a voltage detector for detecting a feedback voltage based on a voltage at the terminal coupled to the cathode side of the LED device, a PWM control circuit for outputting the PWM signal to the step-up DC/DC converter so that the feedback voltage may become equal to a predetermined voltage, and a PWM stop circuit to stop the outputting of the PWM signal to the step-up DC/DC converter when the feedback voltage is lower than a second predetermined voltage which is set lower than the above mentioned predetermined voltage.

In the backlight LED drive circuit configured as described above, when the feedback voltage is lower than the second predetermined voltage set lower than the predetermined voltage, the outputting of the PWM signal to the step-up DC/DC converter is stopped, and therefore the DC power voltage, as the raw voltage to be stepped up by the step-up DC/DC converter during its operation, is prevented from being stepped up.

According to the present invention as described above, the DC power voltage, as the raw voltage to be stepped up by the step-up DC/DC converter during its operation, is not stepped up, and therefore an overvoltage is prevented from being applied to each part of the step-up DC/DC converter or the terminal with which the anode side of the LED device is connected. Thus, a backlight LED drive circuit, causing no troubles in the LED device and the step-up DC/DC converter for supplying a power voltage to the LED device, can be provided.

Preferably, such a configuration may also be made in which the second predetermined voltage is an open/closed determining voltage for determining that no voltage is applied to the above referred terminal while the step-up DC/DC converter is in operation.

By making such a configuration, the outputting of the PWM signal to the step-up DC/DC converter can be suitably stopped when no voltage is applied to the terminal while the step-up DC/DC converter is in operation.

According to the present invention, since the outputting of the PWM signal to the step-up DC/DC converter can be suitably stopped when no voltage is applied to the terminal while the step-up DC/DC converter is in operation, an overvoltage can be suitably prevented from being applied to each part of the step-up DC/DC converter or the terminal with which the anode side of the LED device is connected.

Further, the PWM stop circuit may preferably be configured to stop the outputting of the above described PWM signal to the step-up DC/DC converter also when the above described feedback voltage exceeds a third predetermined voltage set higher than the predetermined voltage. By having such a configuration, an overvoltage can be prevented, when the stepped up voltage cannot be lowered by PWM control, from being applied to each part of the step-up DC/DC converter or the terminal with which the anode side of the LED device is connected.

According to this invention, an overvoltage is prevented from being applied to each part of the step-up DC/DC converter or the terminal with which the anode side of the LED device is connected, and therefore troubles can be prevented from occurring in the LED device and the step-up DC/DC converter for supplying a power voltage to the LED device.

Further, the above mentioned third predetermined voltage may preferably be set to be a high-voltage determining voltage for determining that the above mentioned DC power voltage, as the raw voltage to be stepped up by the step-up DC/DC converter during its operation, is higher than a predetermined voltage. By having this configuration made, when the raw voltage to be stepped up by the step-up DC/DC converter during its operation is higher than the predetermined voltage, the outputting of the PWM signal to the step-up DC/DC converter can be suitably stopped.

According to this invention, since the outputting of the PWM signal to the step-up DC/DC converter can be suitably stopped when the above mentioned DC power voltage, as the raw voltage to be stepped up by the step-up DC/DC converter during its operation, is higher than the predetermined voltage, and therefore an overvoltage is suitably prevented from being applied to each part of the step-up DC/DC converter or the terminal with which the anode side of the LED device is connected.

Further, such a configuration may preferably be made in which the PWM control circuit and the PWM stop circuit are incorporated in a panel drive circuit for driving the liquid crystal panel.

Further, the panel drive circuit may preferably be configured as a liquid crystal driver IC mounted on a glass substrate forming the liquid crystal panel.

According to this invention, since the PWM control circuit and the PWM stop circuit are incorporated in the panel drive circuit for driving the liquid crystal panel, cost down and space-efficiency improvement can be attained. Further, since the panel drive circuit is configured of a liquid crystal driver IC mounted on a glass substrate constituting the liquid crystal panel, further cost down and improvement in the space efficiency can be attained.

Further, preferably, the step-up DC/DC converter may be configured of a chopper type DC/DC converter having a choke coil connected to the DC power voltage of 3.3V, a switching element connected to the choke coil for performing a switching operation based on the PWM signal, and a smoothing capacitor connected to the output side, and the PWM control circuit and the PWM stop circuit may be incorporated in a liquid crystal driver IC mounted on a glass substrate configuring the above liquid crystal panel to output the PWM signal to the chopper type DC/DC converter so that the feedback voltage may become the predetermined voltage set at 0.6V, and to stop the outputting of the PWM signal to the step-up DC/DC converter when the feedback voltage is below the second predetermined voltage set at 0.2V to determine that no voltage is applied to the terminal while the chopper type DC/DC converter is in operation, or when the feedback voltage is above a third predetermined voltage set at 1.1V to determine that the DC power voltage, as the raw voltage to be stepped up by the chopper type DC/DC converter during its operation, is higher than the specified 3.3V. By having the configuration made as described above, the outputting of the PWM signal to the step-up DC/DC converter is suitably stopped when no voltage is applied to the terminal while the step-up DC/DC converter is in operation. Further, also when the stepped up voltage cannot be lowered by PWM control because the DC power voltage, as the raw voltage to be stepped up by the step-up DC/DC converter during its operation, is higher than the specified voltage, the outputting of the PWM signal to the step-up DC/DC converter can be suitably stopped, so that an overvoltage can be prevented from being applied to each part of the step-up DC/DC converter or the terminal with which the anode side of the LED device is connected.

According to this invention, since the outputting of the PWM signal to the step-up DC/DC converter is suitably stopped when no voltage is applied to the terminal while the step-up DC/DC converter is in operation, the outputting of the PWM signal to the step-up DC/DC converter is suitably stopped, an overvoltage is suitably prevented from being applied to each part of the step-up DC/DC converter or the terminal with which the anode side of the LED device is connected. Further, also when the stepped up voltage cannot be lowered by PWM control because the DC power voltage, as the raw voltage to be stepped up by the step-up DC/DC converter during its operation, is higher than a predetermined voltage, the outputting of the PWM signal to the step-up DC/DC converter can be suitably stopped, and therefore an overvoltage is prevented from being applied to each part of the step-up DC/DC converter or the terminal with which the anode side of the LED device is connected. Accordingly, no troubles are caused in the LED device and the step-up DC/DC converter for supplying a voltage to the LED device. Further, since the PWM control circuit and the PWM stop circuit can be incorporated in liquid crystal driver IC mounted on the glass substrate constituting the liquid crystal panel, further cost down and improvement in the space efficiency can be attained.

DETAILED DESCRIPTION OF THE INVENTION

Below will be described an embodiment of the present invention according to the following items:

(1) Schematic configuration of a liquid crystal monitor apparatus;

(2) Configuration of a backlight LED drive circuit;

(1) Schematic Configuration of a Liquid Crystal Monitor Apparatus

Referring toFIG. 1, a schematic configuration of the liquid crystal monitor apparatus10including a backlight LED drive circuit20(herein after called “LED drive circuit20”, refer toFIG. 2) to which the present invention is applied will be described.FIG. 1is a block diagram of the liquid crystal monitor apparatus10. InFIG. 1, the liquid crystal monitor apparatus10includes a video circuit11, liquid crystal module12, power supply circuit13, microcomputer14, and the LED drive circuit20.

The power supply circuit13receives a supply of a power voltage (AC) from an external commercial power source or the like on one hand, and on the other hand, supplies the received power voltage to the microcomputer14and other circuits such as the LED drive circuit20. The power supply circuit13converts, as needed, the voltage to be supplied to each circuit from AC to DC.

The microcomputer14is electrically connected with each part constituting the liquid crystal monitor apparatus10; a CPU14a. as an internal component part of the microcomputer14, controls the entirety of the liquid crystal monitor apparatus10in accordance with programs written in a ROM14band RAM14c, which are also internal component parts of the microcomputer14.

The video circuit11performs, on received digital image data consisting of RGB (red, green, and blue) signals, a scaling process adapted to number of pixels arranged in a matrix array on a liquid crystal panel12a(horizontal to vertical ratio m:n) to thereby generate image data for one screen to be displayed on the liquid crystal panel12a. Further, the same, after performing various processes such as brightness compensation, contrast adjustment, and saturation compensation on the image data, outputs the processed image data to the liquid crystal module12. Incidentally, the digital image data formed of the RGB signals may be image data generated by matrix conversion processing based on the luminance signal and color-difference signal extracted from the video signal as the basis to express a given picture image or such image data as generated by a microcomputer or the like. Further, the above referred video signal is for example such a video signal extracted by a known tuner circuit from a television broadcast signal received through a known antenna or a video signal output from a video reproducing apparatus.

The liquid crystal module12is made up of the liquid crystal panel12a, panel drive circuit12b, and LED device12c. The liquid crystal panel12ais for example a panel of an active matrix drive system. The panel drive circuit12bis controlled on the basis of image data output from the video circuit11to drive the liquid crystal panel12aand thereby allows an image corresponding to the image data to be displayed on the liquid crystal panel12a. The LED device12cconstitutes a light source for illuminating the liquid crystal panel12afrom its back side, namely a backlight of the liquid crystal panel12a, and has for example a plurality of LEDs.

The LED drive circuit20drives the LED device12cused for the backlight of the liquid crystal panel12a.

(2) Configuration of a Backlight Led Drive Circuit

FIG. 2is a block diagram of the LED drive circuit20. InFIG. 2, the LED drive circuit20includes a step-up DC/DC converter22, voltage detector24, PWM control circuit26, and PWM stop circuit28.

The step-up DC/DC converter22, based on a PWM signal, steps up a DC power voltage supplied from the power supply circuit13and supplies the stepped up voltage to the terminal22acoupled to the anode side of the LED device12cto illuminate the LED device12c.

The voltage detector24detects a feedback voltage FBV based on a cathode-side voltage at the terminal22boupled to the cathode side of the LED device12c.

The PWM control circuit26outputs a PWM signal to the step-up DC/DC converter22so that the feedback voltage FBV may become a predetermined voltage.

The PWM stop circuit28, when the feedback voltage FBV is below the second predetermined voltage set lower than the predetermined voltage, causes the outputting of the PWM signal to the step-up DC/DC converter22to be stopped. Also when the feedback voltage FBV is above the third predetermined voltage set higher than the predetermined voltage, the PWM stop circuit28causes the outputting of the PWM signal to the step-up DC/DC converter22to be stopped.

The above mentioned predetermined voltage may for example be a previously established proper voltage of the feedback voltage FBV to provide a proper stepped up voltage that will appropriately illuminate the LED device12c.

The second predetermined voltage may for example be a previously established open/closed determining voltage for determining that no cathode-side voltage is applied to the terminal22bwhile the step-up DC/DC converter22is in operation.

The third predetermined voltage may for example be a previously established a high-voltage determining voltage for determining that the DC power voltage supplied from the power supply circuit13, as the raw voltage to be stepped up by the step-up DC/DC converter22during its operation, is higher than a specified voltage.

By having the LED drive circuit20configured as described above, when no voltage is applied to the terminal22bcoupled to the cathode side of the LED device12cand the feedback voltage FBV is below the second predetermined voltage while the step-up DC/DC converter22is in operation, the outputting of the PWM signal to the step-up DC/DC converter22is stopped. Therefore, the DC power voltage, as the raw voltage to be stepped up by the step-up DC/DC converter22during its operation, will not be stepped up.

Also, when the stepped up voltage is unable to be lowered by PWM control because the DC power voltage, as the raw voltage to be stepped up by the step-up DC/DC converter22during its operation, is higher than the predetermined voltage, the outputting of the PWM signal to the step-up DC/DC converter22is suitably stopped. Therefore, an overvoltage is prevented from being applied to each part of the step-up DC/DC converter22or the terminal22awith which the anode side of the LED device12cis connected.

FIG. 3is a diagram showing a concrete example of a circuit configuration of an LED drive circuit20.

Referring toFIG. 3, the step-up DC/DC converter22is a chopper type DC/DC converter having a choke coil L1connected for example to a DC power voltage of 3.3V, a transistor TR1connected to the choke coil L1and serving as a switching element to be switched by a PWM signal input from the terminal22d, and a smoothing capacitor C1connected with the choke coil L1through a zener diode ZD1for smoothing a predetermined voltage pulse generated on the output side of the choke coil L1by switching operation of the transistor TR1and connected to the terminal22aon the output side. In the step-up DC/DC converter22configured as described above, a voltage stepped up from 3.3V, controlled to be lowered when the duty ratio of the PWM signal is decreased, or to be raised when the duty ratio of the PWM signal is increased, is output from the terminal22aand supplied to the LED device12c.

The voltage detector24is provided for example with a voltage dividing resistor R24, of which one end is connected with the terminal22c, in connection with the terminal22bto which the cathode-side voltage is input, and the other end is connected to ground GND. Resistance values of the voltage dividing resistor24are set, for example, such that the feedback voltage FBV at the voltage dividing point P24may become 0.6V when a proper stepped up voltage is supplied to the LED device12c. Hence, the above mentioned predetermined voltage is set at 0.6V.

The PWM control circuit26is for example configured of a step-up DC/DC controller IC27. This step-up DC/DC controller IC27functions, for example, to output a PWM signal from Pin No.5to the terminal22din order that the feedback voltage FBV input to Pin No.1(FB) becomes the predetermined voltage. 0.6V. For example, when the feedback voltage FBV becomes higher than the predetermined voltage, 0.6V, the duty ratio of the PWM signal is decreased and when conversely the feedback voltage FBV becomes lower than the predetermined voltage, 0.6V, the duty ratio of the PWM signal is increased. Further, the step-up DC/DC controller IC27outputs a PWM signal from Pin No.5while a high signal is input to Pin No.3(CE), but when, on the other hand, a low signal is input to Pin No.3(CE), it stops the outputting of the PWM signal from Pin No.5.

The PWM stop circuit28is configured of a comparator and an AND circuit and outputs a high signal to Pin No.3(CE) of the step-up DC/DC controller IC27when the feedback voltage FBV is within the range between 0.2V and 1.1V and when, on the other hand, the feedback voltage FBV is below 0.2 V as the second predetermined voltage, or when the feedback voltage FBV is higher than 1.1 V as the third predetermined voltage, outputs a low signal to Pin No.3(CE) of the step-up DC/DC controller IC27. Thus, when the feedback voltage FBV is within the range between 0.2V and 1.1V as the proper range for performing PWM control, a high signal is input to Pin No.3(CE) of the step-up DC/DC controller IC27, so that a PWM signal is output to the step-up DC/DC converter22and a proper stepped up voltage is supplied by the step-up DC/DC converter22to the LED device12c. When, on the other hand, the feedback voltage FBV is below 0.2V or above 1.1V, a low signal is input to Pin No.3(CE) of the step-up DC/DC controller IC27so that the outputting of the PWM signal to the step-up DC/DC converter22is stopped, and thus an overvoltage is prevented from being applied to each part of the step-up DC/DC converter22or the terminal22awith which the anode side of the LED device12cis connected.

Although, in the above described embodiment, the LED drive circuit20has been provided separate from the liquid crystal module12including the panel drive circuit12b, such a configuration may for example be made in which the voltage detector24, PWM control circuit26, and PWM stop circuit28of the LED drive circuit20are incorporated in the panel drive circuit12b. The step-up DC/DC converter22may for example be mounted on the same circuit board on which the microcomputer14and the like are mounted and externally attached to the liquid crystal module12.

FIG. 4is a schematic diagram showing a mounted state of each circuit of the liquid crystal monitor apparatus10.

Referring toFIG. 4, one or a plurality of substrates mounting the video circuit11, power supply circuit13, microcomputer14, and step-up DC/DC converter22thereon are connected to the liquid crystal module12through a flat cable or connector. On the side of the liquid crystal module12, there are shown known circuits to form the panel drive circuit12b, such as a source circuit (source driver)32and gate circuit (gate driver)34, mounted on a glass substrate30constituting the liquid crystal panel12a.

Here, the source circuit32and gate circuit34are, for example, liquid crystal driver ICs33,35, each thereof formed of a one-chip IC provided with terminals for wiring and are integrally secured, while being electrically connected, to the glass substrate30, by having the wiring terminals bonded onto the glass substrate30.

The liquid crystal driver IC33has the voltage detector24, PWM control circuit26, and PWM stop circuit28incorporated therein.

According to the present embodiment, as described above, when no voltage is applied to the terminal22bwhile the step-up DC/DC converter22is in operation, outputting of a PWM signal to the step-up DC/DC converter22is suitably stopped, so that a DC power voltage, as a raw voltage to be stepped up by the step-up DC/DC converter22during its operation, is not stepped up. Accordingly, application of an overvoltage to each part of the step-up DC/DC converter22(for example the transistor TR1and smoothing capacitor C1) or the terminal22awith which the anode side of the LED device12cis connected can be suitably prevented. Thus, an LED drive circuit20can be provided which, even when the terminals22a,22bwith which the LED device12cis connected are rendered open during its operation, will not cause any troubles in the LED device12cand the step-up DECO/DC converter22supplying a voltage to the LED device12c.

Further, according to the present embodiment, outputting of a PWM signal to the step-up DC/DC converter22is suitably stopped also when the DC power voltage, as the raw voltage to be stepped up by the step-up DC/DC converter22during its operation, is higher than a specified voltage, and hence, the stepped up voltage cannot be lowered by PWM control. Accordingly, application of an overvoltage to each part of the step-up DC/DC converter22or the terminal22awith which the anode side of the LED device12cis connected can be prevented, and therefore, any troubles will not be caused in the LED device12cand the step-up DC/DC converter22for supplying a voltage to the LED device12c.

Further, according to the present embodiment, the PWM control circuit26, PWM stop circuit28, and the like are incorporated in the panel drive circuit12bfor driving the liquid crystal panel12a, further cost down and improvement in the space efficiency can be attained.

While the present invention has been described above in detail on the basis of the accompanying drawings, the present invention may also be applied to other modes.

For example, though the voltage detector24, PWM control circuit26, and PWM stop circuit28have been incorporated, together with the source circuit (source driver)32, in the liquid crystal driver IC33in the above described embodiment, the same may be incorporated, together with the gate circuit (gate driver)34, in the liquid crystal driver IC35.

Further, the source circuit32and the gate circuit34may be incorporated, together with the voltage detector24, PWM control circuit26, and PWM stop circuit28, in a one-chip IC.

Further, though the PWM stop circuit28has been described in the above embodiment to stop outputting of a PWM signal to the step-up DC/DC converter22when the feedback voltage FBV is below the second predetermined voltage and, in addition, when the feedback voltage FBV is above the third predetermined voltage, it is enough if the PWM stop circuit28is configured to stop the outputting of the PWM signal to the step-up DC/DC converter22when at least the feedback voltage FBV is below the second predetermined voltage.

The above description being of a preferred embodiment, the invention may be carried out in various modifications and improvements based on the knowledge of persons skilled in the art.

While the invention has been particularly shown and described with respect to preferred embodiments thereof, it should be understood by those skilled in the art that the foregoing and other changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined in the appended claims.