JP2008146949A - Backlight driving device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize stable light control in extremely low luminance, in a backlight driving device using a switching current source generating a high current. <P>SOLUTION: A current output from a switching current source 3 becomes the fixed minimum Imin when luminance is not more than prescribed low luminance, a fixed-cycle pulse width current Ib corresponding to a luminance setting signal Vs flows in a light emitting element 4 by a switching means 6, and a dummy current Id having a phase opposite to the pulse width current flows in a constant current load 7. Since the duty ratio of the pulse width current Ib falls as the luminance setting voltage (s) is lowered, light can be stably controlled in extremely low luminance. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a backlight driving device used in a liquid crystal display device and the like, and more particularly to a backlight driving device that requires a wide light control range from high luminance to ultra-low luminance.

  In an aircraft cockpit liquid crystal display device or the like, dimming by a backlight drive device is generally possible according to the surrounding conditions.

  FIG. 3 is a block diagram showing an example of a conventional backlight driving device for a liquid crystal display device (hereinafter referred to as LCD). The error amplifier 1 is composed of a differential operational amplifier or the like, and compares the luminance setting voltage Vs input to the non-inverting input terminal with the luminance voltage Vb input to the inverting input terminal, and outputs a difference. The PWM generation circuit 2 receives the difference signal from the error amplifier 1 and converts it into a pulse width modulation signal having a corresponding pulse width. The switching current source 3 is switched with a constant current by the pulse width modulation signal input from the PWM generation circuit 2, smoothed, and then output as a DC current signal. The backlight light source 4 includes a plurality of LEDs constituting a light emitting element. And is driven by the switching current source 3. The current detector 5 is composed of a resistor or the like, and performs current / voltage conversion on the current flowing through the backlight light source 4.

The operation of the backlight driving apparatus having the above configuration will be described below.
When the luminance voltage Vb corresponding to the luminance of the backlight light source 4 is lower than the luminance setting voltage Vs, a positive difference signal is output from the error amplifier 1, and the pulse width of the pulse width modulation signal output from the PWM generation circuit 2 is To increase. The switching current source 3 increases the DC drive current Ib that flows to the LED of the backlight source 4 in response to an increase in the pulse width. This drive current Ib is subjected to current / voltage conversion by the current detector 5, and the increased luminance voltage Vb is applied to the inverting input terminal of the error amplifier 1.
When the luminance voltage Vb is higher than the luminance setting voltage Vs, a negative differential signal is output from the error amplifier 1, and the pulse width of the pulse width modulation signal output from the PWM generation circuit 2 is reduced. The switching current source 3 passes a reduced DC drive current Ib corresponding to the reduced pulse width to the LED of the backlight source 4. This drive current Ib is current / voltage converted by the current detector 5, and the reduced luminance voltage Vb is input to the inverting input terminal of the error amplifier 1.
When such a feedback operation is continued and the difference finally becomes zero, that is, when the luminance voltage Vb coincides with the luminance setting voltage Vs, the operation is stabilized. At this time, light having a luminance corresponding to the luminance setting voltage Vs is generated from the backlight 4. That is, according to the backlight driving device as shown in FIG. 3, the user can adjust the backlight to any different luminance by changing the luminance setting voltage Vs.

  Prior art documents related to the backlight driving apparatus as described above include the following.

JP 2004-281349 A

When the backlight driving apparatus as shown in FIG. 3 is applied to an LED backlight for an LCD, a switching current source that generates a large current is generally required. In this case, the brightness setting voltage increases when the LED has high brightness, and the brightness setting voltage decreases when the LED has low brightness. Therefore, when trying to achieve ultra-low brightness using a switching current source that generates a large current, the brightness setting voltage becomes very small and approaches 0V. In such a state, the following problems occur, and there is a problem that it is difficult to make the LED have an extremely low brightness.
(1) The error amplifier 1 is not normally detected.
(2) The PWM generation circuit 2 does not operate normally due to intermittent oscillation or the like.
(3) The MAX-MIN ratio (maximum / minimum ratio of the pulse width) of the switching current source 3 becomes excessive, and the switching current source 3 does not operate normally.

  An object of the present invention is to solve such a problem, and an object of the present invention is to realize stable dimming at ultra-low luminance in a backlight driving device using a switching current source that generates a large current.

In order to achieve such a problem, the invention according to claim 1 of the present invention is:
In the backlight drive device that controls the backlight drive current with an error amplifier so that the backlight drive current feedback signal and the luminance setting signal are equal,
When the backlight driving current becomes a certain minimum value, a control circuit for turning on and off the backlight driving current at a duty ratio corresponding to the luminance setting signal is provided.

The invention according to claim 2
The backlight driving device according to claim 1,
A variable current source that outputs a controlled direct current to the error amplifier;
Switch means for turning on and off the backlight drive current flowing from the variable current source;
An electronic load for turning on and off a dummy current flowing from the variable current source;
Detecting means for detecting a sum of the backlight driving current and the dummy current, and feeding back a detection signal to the error amplifier. When the backlight driving current becomes a certain minimum value, The switch means and the electronic load are driven in opposite phases with a duty ratio corresponding to the luminance setting signal.

The invention described in claim 3
The backlight driving device according to claim 2,
The control circuit is characterized in that when the backlight drive current does not reach a certain minimum value, the control circuit turns on the electronic load.

The invention according to claim 4
In the backlight drive device according to claim 2 or 3,
The electronic load is a constant current load that can be controlled by an external signal.

The invention according to claim 5
The backlight driving device according to claim 2, wherein:
The variable current source is:
A PWM generation circuit that converts a differential signal input from the error amplifier into a pulse width;
And a switching current source for converting the pulse width of the pulse width modulation signal input from the PWM generation circuit into a direct current.

As described above, according to the backlight driving device of the present invention, when the brightness setting signal is lowered and the output of the variable current source becomes a certain minimum value, the switch means and the electronic load are set as the brightness setting signal. By driving in the opposite phases with the corresponding duty ratio, it is possible to realize a backlight capable of stably adjusting light with ultra-low luminance.

  FIG. 1 is a configuration block diagram showing an example of a backlight driving apparatus according to an embodiment of the present invention. The same parts as those shown in FIG.

The error amplifier 1 is composed of a differential operational amplifier or the like, compares the luminance setting voltage Vs input to the non-inverting input terminal with the adder output Va input to the inverting input terminal, and outputs the difference signal. The PWM generation circuit 2 outputs a pulse width modulation signal having a pulse width corresponding to the difference signal input from the error amplifier 1.
The switching current source 3 switches the constant current by the pulse width modulation signal input from the PWM generation circuit 2, smoothes it, and then outputs it as a DC current signal.
Here, the PWM generation circuit 2 and the switching current source 3 constitute a variable current source 11 that generates a direct current corresponding to the differential signal from the error amplifier 1.

The backlight light source 4 is configured by connecting a plurality of LEDs constituting a light emitting element in series, and an anode side terminal thereof is connected to a current output terminal of the switching current source 3.
One end of the switch 6 is connected to the cathode side terminal of the backlight source 4 to turn on and off the current flowing through the pack light 4.
The first current detector 5 includes a current-voltage conversion resistor, and is connected between the switch 6 and the common.

The constant current load 7 has a current output terminal of the switching current source 3 connected to its input terminal, and forms a dummy load in parallel with the backlight 4. The load current of the constant current load 7 is controlled to be equal to the load current of the LED 41 by a constant current load control signal P2 output from the control circuit 9.
The second current detector 8 is formed of a current-voltage conversion resistor and is connected between the output terminal of the constant current load 7 and the common. Here, the resistance value is equal to that of the current detector 5.

The control circuit 9 receives the luminance setting voltage Vs and the low luminance threshold voltage Vt, sends the dimming pulse signal P1 to the switch 6, and sends it to the constant current load control signal P2 constant current load 7. Here, the low luminance threshold voltage Vt is the minimum value of the luminance setting voltage Vs that can be stably controlled by the variable current source 11 in the low luminance range. In the range where the luminance setting voltage Vs is lower than the low luminance threshold voltage Vt, the output current of the variable current source 11 becomes a certain minimum value.

The adder 10 adds the output signals of the current detector 5 and the current detector 8 and feeds back the added signal to the inverting input terminal of the error amplifier 1.

In the above configuration, the luminance setting voltage Vs constitutes a luminance setting signal, and the low luminance threshold voltage Vt constitutes a low luminance threshold signal.
The switch 6 constitutes a switch means, and the drive current (backlight drive current) Ib flowing through the backlight 4 constitutes a first current flowing from the variable current source 11.
The constant current load 7 constitutes an electronic load that can be controlled by an external signal, and the dummy current Id flowing into the constant current load 7 constitutes a second current flowing from the variable current source 11.
Further, the current detector 5, the current detector 8 and the adder 10 constitute detection means 12 for detecting the sum of the first current and the second current and feeding back a detection signal to the error amplifier. The output voltage Va of the adder 10 constitutes a detection signal of the detection means 12.

The operation of the backlight driving device of FIG. 1 will be described below.
Depending on the magnitude relationship between the luminance setting voltage Vs and the low luminance threshold signal Vt, the operation mode of the backlight driving device is roughly divided into two.
When the luminance setting voltage Vs is higher (larger) than the low luminance threshold signal Vt, the dimming pulse signal P1 output from the control circuit 9 is at the H level, so that the switch 6 is turned on and the constant current load control signal P2 is 0V. Therefore, the constant current value of the constant current load 7 is zero. Accordingly, since all the output current of the switching current source 3 flows to the backlight 4, the operation at this time is the same as in the case of FIG. 3 as shown below.

When the adder output Va is lower than the luminance setting voltage Vs, a positive differential signal is output from the error amplifier 1 and the pulse width (duty ratio) of the pulse width modulation signal output from the PWM generation circuit 2 is increased. As a result, the direct current drive current Ib increased corresponding to the pulse width modulation signal flows from the switching current source 3 into the LED 41 of the backlight source 4. This drive current Ib is subjected to current / voltage conversion by the current detector 5, and the adder output Va increased via the adder 10 is fed back to the inverting input terminal of the error amplifier 1.

When the adder output Va is higher than the luminance setting voltage Vs, a negative difference signal is output from the error amplifier 1, and the pulse width of the pulse width modulation signal output from the PWM generation circuit 2 is reduced. The LED of the backlight source 4 is supplied with a reduced DC driving current Ib corresponding to the reduced pulse width from the switching current source 3. This drive current Ib is subjected to current / voltage conversion by the current detector 5, and the reduced luminance voltage Vb is fed back to the inverting input terminal of the error amplifier 1.

When such a feedback operation is continued and the difference finally becomes 0, that is, when the adder output Va coincides with the luminance setting voltage Vs, the error amplifier 1 → the PWM generation circuit 2 → the switching current source 3 → the backlight. The loop of 4 → switch 6 → current detector 5 → adder 10 → error amplifier 1 converges. At this time, light having a luminance corresponding to the luminance setting voltage Vs is generated from the backlight 4. That is, the user can dim the backlight to any different luminance by changing the luminance setting voltage Vs (≧ low luminance threshold signal Vt).

FIG. 2 is a time chart showing the operation of the backlight driving device when the luminance setting voltage Vs is lower (smaller) than the low luminance threshold signal Vt.
When the luminance setting voltage Vs becomes lower (smaller) than the low luminance threshold voltage Vt, the output of the PWM generation circuit 2 is locked with a signal having a certain minimum pulse width, and accordingly, the output of the switching current source 3 is also the smallest constant current value. Locked to I = Imin. At this time, the output of the adder 10 becomes constant at a voltage equal to the low luminance threshold signal Vt, and the locked state is maintained by the negative difference signal output from the error amplifier 1.

At this time, the control circuit 9 outputs a dimming pulse signal P1 composed of a pulse width signal having a period T and a constant current load control signal P2 having an opposite phase (inverted waveform).

As shown in FIG. 2A, the dimming pulse signal P1 is at the H level for a time width t1 proportional to the luminance setting voltage Vs and turns on the switch 6, so that the luminance current Ib flows through the LED 41. During this time, the constant current load control signal P2 becomes 0V, so that the current value of the constant current load 7 becomes zero.

When the dimming pulse signal P1 becomes the L level, the constant current load control signal P2 becomes a voltage proportional to the luminance setting voltage Vs (FIG. 2B), and is controlled by this luminance setting voltage Vs to the constant current load 7. A constant current equal to the luminance current Ib of the LED 41 flows as the dummy current Id.

As a result, the current detector 5 outputs the luminance voltage Vb shown in FIG. 2C, and the current detector 8 outputs the dummy voltage Vd shown in FIG. The luminance voltage Vb and the dummy voltage Vd having waveforms with phases opposite to each other are added by the adder 10 to become a DC adder output voltage Va and fed back to the inverting input of the error amplifier 1 (FIG. 2E). Here, the output Va of the adder 10 is a DC voltage corresponding to the sum of the drive current Ib and the dummy current Id, that is, the output current Imin of the switching current source 3.

Therefore, a current proportional to t1 / T times the constant current Imin (the duty ratio of the dimming pulse signal P1) flows through the backlight 4. Therefore, even when the luminance setting voltage Vs is lowered to the low luminance threshold signal Vt or less, stable dimming with ultra-low luminance can be realized by changing the pulse width t1 of the dimming pulse width signal P1. .

In summary, the pack light 4 has all the DC current I of the variable current source 11 flowing when Vs> Vt (normal luminance range), but the variable current source when Vs ≦ Vt (low luminance range). A current obtained by further reducing the minimum direct current Imin of 11 in accordance with the duty ratio flows.

According to the backlight driving device having the above-described configuration, the backlight 4 can be operated at a low current within the stable operation range by controlling the duty ratio of the backlight 4 with the dimming pulse signal P1 when the luminance is low. Therefore, stable ultra-low luminance dimming can be realized while using a large current generation circuit as a switching current source, and thus a backlight driving device that satisfies both high luminance and ultra-low luminance can be realized.

In the above-described embodiment, the combination of the PWM generation circuit and the switching current source is used as the variable current source 11. However, the present invention is not limited to this, and any variable current source that generates a DC drive current corresponding to the differential output. Can be used.

In the above embodiment, the current detector 5 and the current detector 58 are used to detect the drive current Ib and the dummy current Id, respectively, and the addition signal of the adder 10 is fed back. The output terminal of the load 7 may be connected to a common current detector (such as a resistor) as detection means. In this case, since the output voltage of the current detector can be directly fed back to the error amplifier 1, the adder 10 can be omitted.

  In the above embodiment, a constant current load is used as the electronic load. However, any dummy load having the same load characteristics as the backlight 4 can be used.

In the above embodiment, the LED is used as the light emitting element. However, the present invention is not limited to this, and any light emitting element that can be PWM driven can be used.

Further, the switch 6 and the constant current load 7 do not require large capacity parts.

In the above embodiment, the resistance element is used as the current detector. However, the present invention is not limited to this.

In the above-described embodiment, the circuit is configured by individual elements. However, a part of the configuration including the control circuit 9 and the like may be configured by a computer and software.

1 is a configuration block diagram of an embodiment of a backlight drive device according to the present invention. It is a time chart which shows operation | movement of the backlight drive device which concerns on this invention. It is a block diagram of a conventional backlight driving device.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Error amplifier 2 PWM generation circuit 3 Switching current source 4 Backlight 6 Switch means 7 Electronic load 9 Control circuit 10 Adder 11 Variable current source 12 Detection means 41 Light emitting element Vs Brightness setting signal Vt Low brightness threshold signal

Claims (5)

In the backlight drive device that controls the backlight drive current with an error amplifier so that the backlight drive current feedback signal and the luminance setting signal are equal,
A backlight drive apparatus comprising a control circuit for turning on and off the backlight drive current at a duty ratio corresponding to the luminance setting signal when the backlight drive current has a certain minimum value. A variable current source that outputs a controlled direct current to the error amplifier;
Switch means for turning on and off the backlight drive current flowing from the variable current source;
An electronic load for turning on and off a dummy current flowing from the variable current source;
Detecting means for detecting a sum of currents of the backlight driving current and the dummy current, and feeding back a detection signal to the error amplifier. When the backlight driving current becomes a certain minimum value, 2. The backlight driving apparatus according to claim 1, wherein the switch means and the electronic load are driven in opposite phases with a duty ratio corresponding to the luminance setting signal. 3. The backlight drive device according to claim 2, wherein the control circuit turns on the switch means and turns off the electronic load when the backlight drive current does not reach a certain minimum value.   4. The backlight driving device according to claim 2, wherein the electronic load is a constant current load capable of controlling a load by an external signal. The variable current source is:
A PWM generation circuit that converts a differential signal input from the error amplifier into a pulse width;
5. The backlight driving apparatus according to claim 2, further comprising a switching current source that converts a pulse width of the pulse width modulation signal input from the PWM generation circuit into a direct current.

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