CN201349354Y - Backlight source driving device combining analog and digital dimming - Google Patents

Abstract

The utility model relates to a combine simulation and digital backlight drive arrangement who adjusts luminance utilizes the switching formula dc-to-ac converter to convert input DC voltage to output AC voltage in order to open the backlight during pulse width modulation signal enable of adjusting luminance to do not transmit energy in order to close the backlight during pulse width modulation signal disable of adjusting luminance. When the duty cycle of the pulse width modulation dimming signal is greater than the critical duty cycle, the lamp current output by the backlight source is fixed at a first current value during the enabling period, and when the duty cycle of the pulse width modulation dimming signal is less than or equal to the critical duty cycle, the lamp current output by the backlight source is fixed at a second current value during the enabling period, and the second current value is less than the first current value. Therefore, the utility model discloses use digital dimming as the basis, further utilize the simulation to adjust luminance after the average brightness is low to a certain degree and reduce fluorescent tube electric current size, make average brightness further fall ground and improve luminance contrast value.

Description

Backlight driver combined with analog and digital dimming

technical field

The utility model relates to a driving device of a backlight source, in particular to a driving device of a backlight source combining analog and digital dimming.

Background technique

Since the liquid crystal itself does not have luminous properties, the liquid crystal display must be provided with a backlight behind the liquid crystal panel to provide light in order to achieve the display function. At present, most liquid crystal displays use cold cathode fluorescent lamps (Cold Cathode Fluorescent Lamp, CCFL for short) as the backlight source. CCFL has some special properties. Considering maximizing its efficiency, life and practicability, a driving device is needed to convert the power supply into an AC sine wave with a frequency of 40kHz to 80kHz to drive the CCFL. After the CCFL starts to enter the normal working state, the CCFL's The operating voltage is between 400Vrms and 1000Vrms, and the lamp current flowing through the CCFL is between 3mArms and 7mArms. The greater the lamp current, the brighter the brightness of the CCFL.

Please refer to FIG. 1 , which is a block diagram of a conventional CCFL driving device. The CCFL driving device 1 includes a switching inverter 11 , a step-up transformer 12 , a resonant circuit 13 , a feedback circuit 14 and a control circuit 15 . The switching inverter 11 is, for example, a half-bridge, full-bridge or push-pull inverter, which converts the input DC voltage Vdc into a square-wave AC voltage Vac1 by switching the power switch. The AC voltage Vac1 in the form of a square wave is boosted by the step-up transformer 12, and then filtered by the resonant circuit 13 to form an output AC voltage Vac2 in the form of a sine wave to drive the CCFL 2. The feedback circuit 14 detects the lamp current Ilamp flowing through the CCFL 2, and outputs a feedback voltage Vfb accordingly. The control circuit 15 outputs a control signal Vctrl according to the feedback voltage Vfb to control switching of the power switch of the switching inverter 11 to stabilize the brightness of the CCFL 2 . The control circuit 15 also receives the dimming signal Vdim, and the user can adjust the brightness of the CCFL 2 through the dimming signal Vdim. The existing ways to adjust the brightness of CCFL 2 can be divided into two types: analog dimming and digital dimming. Digital dimming is also called burst mode dimming or pulse width modulation (Pulse Width Modulation, referred to as PWM) dimming, which has the advantages of wide dimming range, good dimming linearity and easy circuit implementation, is currently the most common dimming method.

When the CCFL drive device 1 adopts analog dimming, its dimming signal Vdim is a signal in the form of DC, and the lamp current Ilamp of CCFL 2 is changed by changing the DC level of the dimming signal Vdim, so as to achieve dimming by adjusting the brightness of CCFL2 Function. In fact, one way is to fix the switching frequency of the power switch of the switching inverter 11, but change its duty cycle (duty cycle) according to the DC level of the dimming signal Vdim, and then change the amount of energy transmitted to CCFL2 to change Lamp current Ilamp size. Another way is to change the switching frequency of the power switch of the switching inverter 11 according to the DC level of the dimming signal Vdim, and then change the impedance of the resonant circuit 13 to change the magnitude of the lamp current Ilamp.

Please refer to FIG. 2(A) and FIG. 2(B), which are schematic diagrams of lamp current waveforms for conventional analog dimming. In this example, in Figure 2(A), the lamp current Ilamp is 7mArms; in Figure 2(B), the lamp current Ilamp is 3mArms. Therefore, the luminance of CCFL2 is higher in FIG. 2(A) than in FIG. 2(B).

When the CCFL drive device 1 adopts digital dimming, its dimming signal Vdim is a signal in the form of PWM, and because its frequency is much lower than the AC sine wave with a frequency of 40kHz to 80kHz driving CCFL 2, the dimming signal Vdim is called Low-frequency PWM dimming signal, or PWM dimming signal for short. The PWM dimming signal Vdim includes an enabling period and a disabling period in one period. During the enabling period, CCFL 2 is turned on to make it bright, and during the disabling period, CCFL 2 is turned off to make it dark. Another definition of its duty cycle is one The enable period is divided by a period and expressed as a percentage. Since the frequency of the PWM dimming signal Vdim is usually above 100Hz, under the influence of human visual persistence, the human eye cannot feel the CCFL 2 light up and down, but can only feel the average value of this change (that is, the average brightness). Therefore, Fix the lamp current Ilamp of CCFL 2, change the ratio of light and dark of CCFL 2 by changing the duty cycle of PWM dimming signal Vdim, and then change the average current of CCFL 2 to achieve the dimming function of adjusting the brightness of CCFL 2.

Please refer to FIG. 3(A) to FIG. 3(C), which are schematic diagrams of lamp current waveforms of conventional digital dimming. In this example, the fixed lamp current Ilamp is the rated value of 7mArms, but the average current of CCFL 2 is changed by using the PWM dimming signal Vdim with a frequency of 200Hz and different duty cycles, as shown in Figure 3(A), PWM dimming The duty cycle of the signal Vdim is 80%, and the average value of the lamp current Ilamp is 7mArms×80%=5.6mArms; in Fig. 3(B), the duty cycle of the PWM dimming signal Vdim is 40%, and the average value of the lamp current Ilamp is 7mArms ×40%=2.8mArms; in Figure 3(C), the duty cycle of the PWM dimming signal Vdim is 30%, and the average value of the lamp current Ilamp is 7mArms×30%=2.1mArms. Therefore, the average luminance of CCFL 2 is higher in Fig. 3(A) than in Fig. 3(B), and is higher in Fig. 3(B) than in Fig. 3(C).

An important parameter for display performance is the brightness contrast value of the screen. The brightness contrast value refers to the ratio of the measured data when the brightness is adjusted to the highest brightness and the lowest brightness. Displays with high brightness contrast are often more attractive to customers, because they can make the gray scale more delicate, and can bring sharper and clearer visual images to users. For a display (such as a liquid crystal display) that requires light from a backlight source to achieve a display function, adjusting the brightness of the picture means adjusting the brightness provided by the backlight source. Therefore, there is a need to improve the existing backlight dimming method to obtain a higher brightness contrast value.

Contents of the invention

The purpose of the utility model is to propose a backlight driving device combining analog and digital dimming, which has a higher brightness contrast value.

In order to achieve the above object and other objects, the utility model proposes a backlight driving device combining analog and digital dimming, which includes a switching inverter, a feedback circuit, a control circuit and a duty cycle detector, The switching inverter is coupled to a backlight, the feedback circuit is coupled to the backlight, the control circuit is coupled to the feedback circuit and the switching inverter, and the duty cycle detector is coupled to the feedback circuit; in addition, the switching The inverter receives an input DC voltage, and both the control circuit and the duty cycle detector receive a PWM dimming signal.

The switching inverter converts the input DC voltage into an output AC voltage to turn on the backlight when the PWM dimming signal is enabled, and does not transmit energy to turn off the backlight when the PWM dimming signal is disabled. The feedback circuit receives the lamp current output by the backlight source. When the duty cycle of the PWM dimming signal is greater than a critical duty cycle, the feedback circuit outputs a first feedback voltage according to the lamp current, and the control circuit controls the switching according to the first feedback voltage during the enabling period of the PWM dimming signal. The formula inverter fixes the current of the lamp tube at a first current value. When the duty cycle of the PWM dimming signal is less than or equal to the critical duty cycle, the duty cycle detector adjusts the impedance of the feedback circuit so that the feedback circuit outputs a second feedback voltage according to the current of the lamp tube, and the control circuit adjusts the pulse width modulation dimming signal to a second feedback voltage. During the enabling period of the light signal, the switching inverter is controlled according to the second feedback voltage to fix the current of the lamp tube at a second current value, and the second current value is smaller than the first current value.

The beneficial effects of the utility model are: the utility model is based on digital dimming, and the duty cycle detector is used to change the impedance of the feedback circuit to The analog dimming method reduces the current of the lamp tube, further reduces the average brightness and improves the brightness contrast value.

Description of drawings

Fig. 1 is the block diagram of a kind of existing CCFL driving device;

2(A) to 2(B) are schematic diagrams of current waveforms of lamp tubes for analog dimming;

3(A) to 3(C) are schematic diagrams of lamp current waveforms for conventional digital dimming;

4(A) to 4(C) are schematic diagrams of lamp current waveforms combining analog and digital dimming according to an embodiment of the present invention;

Fig. 5 is a block diagram of a CCFL driving device combining analog and digital dimming according to an embodiment of the present invention, which can realize the schematic diagram of the lamp current waveform shown in Fig. 4(A) to Fig. 4(C);

Fig. 6 is a specific implementation circuit diagram of the block diagram of the CCFL driving device 5 shown in Fig. 5;

FIG. 7 is a circuit diagram of another embodiment of the duty cycle detector 66 shown in FIG. 6 .

Explanation of reference signs: 1-CCFL driving device; 11-switching inverter; 12-boost transformer; 13-resonant circuit; 14-feedback circuit; 15-control circuit; 2-cold cathode fluorescent lamp (CCFL); 5 -CCFL driving device; 51, 61-switching inverter; 52,62-boost transformer; 53,63-resonant circuit; 54,64-feedback circuit; 55,65-control circuit; 56,66,76- Duty Cycle Detector; 651-Error Amplifier (EA); 652-Comparator (CMP); 653-Oscillator; 654-Output Driver; 761-Comparator (CMP); C1, C2-Capacitor; Cd, Cf- Filter capacitor; Cp-resonant capacitor; Db, Dd-diode; Dr1, Dr2-rectifier diode; Dm1, Dm2-body diode; I1-first current value; I2-second current value; Ilamp-lamp current; Llk- Resonant inductor; Mn1, Mn2-power switch; Mn3-switch; Rd1, Rd2-divider resistor; Rf1, Rf2, Rf3-feedback resistor; Rsen1, Rsen2-detection resistor; V1-first feedback voltage; V2-second feedback voltage; Vac1-AC voltage; Vac2-output AC voltage; Vctrl, Vg1, Vg2-control signal; Vdc-input DC voltage; Vdim-dimming signal; Verror-error voltage; Vfb-feedback voltage; Vpwm - pulse width modulation (PWM) dimming signal; Vramp - ramp voltage; Vref - reference voltage; Vth - threshold reference voltage.

Detailed ways

In order to make the above and other purposes, features and advantages of the present utility model more obvious and easy to understand, the preferred embodiments are specifically cited below, together with the accompanying drawings, and are described in detail as follows:

The brightness contrast value of the display screen refers to the ratio of the measured data when the brightness is adjusted to the highest brightness and the lowest brightness. For displays (such as liquid crystal displays) that require backlight to provide light to achieve display functions, their maximum brightness will be limited by the maximum brightness that the backlight can provide (such as fixing the lamp current to the rated value under digital dimming and PWM The duty cycle of the dimming signal is adjusted to 100%), it is not feasible to increase the maximum brightness. Therefore, the utility model is based on digital dimming, and after the average brightness is low to a certain extent (such as when the duty cycle of the PWM dimming signal is less than or equal to a critical duty cycle), it further uses analog dimming to reduce the current of the lamp tube, so that The average brightness is further reduced to increase the brightness contrast value.

Please refer to FIG. 4(A) to FIG. 4(C), which are schematic diagrams of lamp current waveforms combining analog and digital dimming according to an embodiment of the present invention. In this example, the critical duty cycle is 30%. When the duty cycle of the PWM dimming signal is greater than the critical duty cycle (30%), as shown in Figure 4(A) and Figure 4(B), the fixed lamp current Ilamp is the rated value of 7mArms, but the frequency is 200Hz and the duty cycle is different PWM dimming signal to change the average current of the backlight. When the duty cycle of the PWM dimming signal is less than or equal to the critical duty cycle (30%), as shown in Figure 4(C), use analog dimming to reduce and fix the lamp current Ilamp to 3mArms, and then use a frequency of 200Hz and different duty cycles The PWM dimming signal Vdim is used to change the average current of CCFL 2.

In Figure 4(A), the duty cycle of the PWM dimming signal is 80%, and the average value of the lamp current Ilamp is 7mArms×80%=5.6mArms; in Figure 4(B), the duty cycle of the PWM dimming signal is 40% , the average value of the lamp current Ilamp is 7mArms×40%=2.8mArms; in Fig. 4(C), the duty cycle of the PWM dimming signal is 30%, and the average value of the lamp current Ilamp is 3mArms×30%=0.9mArms. Therefore, the average brightness of the backlight is higher in FIG. 4(A) than in FIG. 4(B), and is higher in FIG. 4(B) than in FIG. 4(C). In addition, comparing the dimming method of the present invention shown in Figure 4(A) to Figure 4(C) and the existing digital dimming shown in Figure 3(A) to Figure 3(C), the PWM dimming signal responsibility When the cycle is less than or equal to the critical duty cycle (30%), the average brightness of the dimming mode of the present utility model is lower than the average brightness of the existing digital dimming; for example, the average current of the lamp tube shown in Fig. 4 (C) is 0.9mArms, while the average current of the lamp shown in Figure 3(C) is 2.1mArms.

Please refer to FIG. 5 , which is a block diagram of a backlight driving device combining analog and digital dimming according to an embodiment of the present invention, which can realize the lamp current waveform shown in FIG. 4(A) to FIG. 4(C) schematic diagram. In this example, the backlight 2 takes CCFL as an example, and the CCFL driving device 5 includes a switching inverter 51 , a step-up transformer 52 , a resonant circuit 53 , a feedback circuit 54 , a control circuit 55 and a duty cycle detector 56 . The control circuit 55 receives the PWM dimming signal Vpwm, and outputs a control signal Vctrl during the enabling period of the PWM dimming signal Vpwm, and controls the switching inverter 51 to convert the input DC voltage Vdc into a square wave AC voltage Vac1, and the AC voltage Vac1 Then, after step-up transformer 52 boosts the voltage, the output AC voltage Vac2 in the form of an approximate sine wave is filtered by resonant circuit 53 to open CCFL 2 and make it bright. The control circuit 55 outputs a control signal Vctrl during the period when the PWM dimming signal Vpwm is disabled, and controls the switching inverter 51 not to transmit energy to turn off the CCFL 2 to make it dark.

In addition, the duty cycle detector 56 is used to receive the PWM dimming signal Vpwm and detect its duty cycle, so that when the duty cycle of the PWM dimming signal Vpwm is less than or equal to the critical duty cycle, the analog dimming is further used to reduce the lamp current, The average brightness is further reduced to increase the brightness contrast value. When the duty cycle of the PWM dimming signal Vpwm is greater than the critical duty cycle, as shown in Fig. 4(A) and Fig. 4(B), the feedback circuit 54 receives the lamp current Ilamp and accordingly generates a feedback voltage Vfb whose value is the first feedback voltage V1 , and the control circuit 55 controls the switching inverter 51 according to the first feedback voltage V1 during the period when the PWM dimming signal Vpwm is enabled, so that the lamp current Ilamp is fixed at the first current value I1, such as 7mArms. When the duty cycle of the PWM dimming signal Vpwm is less than or equal to the critical duty cycle, as shown in FIG. The feedback voltage Vfb of the voltage V2, and the control circuit controls the switching inverter 51 according to the second feedback voltage V2 during the enabling period of the PWM dimming signal Vpwm so that the lamp current Ilamp is fixed at the second current value I2, such as 3mArms. Wherein, the second current value I2 is smaller than the first current value I1, and the first current value I1 is generally designed to be the rated value of the lamp current so that the CCFL 2 can provide maximum brightness.

Please refer to FIG. 6, which is a specific implementation circuit diagram of the block diagram of the CCFL driving device 5 shown in FIG. It is the period of enabling, and it is the period of disabling when it is at high level. Switching inverter 61 is a half-bridge inverter, which includes two power switches Mn1 and Mn2 implemented by N-channel metal-oxide-semiconductor field-effect transistors (MOSFETs), and the power switches Mn1 and Mn2 are drain-source There are anti-parallel body diodes (body diode) Dm1 and Dm2 between the poles, which can provide a reverse flow path. The two power switches Mn1 and Mn2 are controlled by the control signal Vctrl (including Vg1 and Vg2) received by their gates to control whether they are turned on or not, and they are turned on alternately during the enabling period of the PWM dimming signal Vpwm so as to reduce the input DC voltage Vdc The AC voltage Vac1 is converted into a square wave form, but is not turned on during the disabled period of the PWM dimming signal Vpwm so as not to allow the energy of the input DC voltage Vdc to pass through. The step-up transformer 62 receives the AC voltage Vac1 on the primary side, and generates a boosted AC voltage Vac1 on the secondary side. The resonant circuit 63 is in the form of a series resonant parallel load, the resonant inductor L1k and the resonant capacitor Cp are coupled in series and across the secondary side of the step-up transformer 62, and both ends of the resonant capacitor Cp are coupled to the load, that is, the CCFL 2.

The feedback circuit 64 includes a current-to-voltage circuit (realized by detection resistors Rsen1 and Rsen2), a rectifier circuit (realized by rectifier diodes Dr1 and Dr2), a first feedback resistor Rf1, a second feedback resistor Rf2, and a first filter capacitor Cf, wherein, the first end of the first feedback resistor Rf1 is coupled to the output of the rectifier circuit, the second end of the first feedback resistor Rf1 is coupled to the first end of the second feedback resistor Rf2 and the first filter capacitor Cf end, and the second end of the second feedback resistor Rf2 and the second end of the first filter capacitor Cf are both coupled to the ground end. The detection resistors Rsen1 and Rsen2 are respectively coupled in series with the corresponding CCFL 2 to receive the lamp current Ilamp output by the CCFL 2 and generate a voltage signal corresponding to the lamp current Ilamp, wherein, when the lamp current Ilamp is larger , the larger the voltage signal is. The voltage signal is then rectified by full-wave rectification diodes Dr1 and Dr2 to generate a voltage signal in the form of a pulsed DC voltage. The pulsed DC voltage signal is then divided by the first feedback resistor Rf1 and the second feedback resistor Rf2 and then passed through the first feedback resistor Rf2. Filtering by a filter capacitor Cf, a feedback voltage Vfb of the first feedback voltage V1 is generated on the first terminal of the first filter capacitor Cf.

The duty cycle detector 66 includes a diode Dd, a first voltage dividing resistor Rd1, a second voltage dividing resistor Rd2, a second filter capacitor Cd, a switch Mn3 (implemented by an N-channel MOSFET) and a third feedback resistor Rf3, wherein , the first end of the first voltage dividing resistor Rd1 is coupled to the cathode end of the diode Dd, the second end of the first voltage dividing resistor Rd1 is coupled to the first end of the second voltage dividing resistor Rd2, and the first end of the second filter capacitor Cd terminal and the switch Mn3 control terminal, and the second terminal of the second voltage dividing resistor Rd2 and the second terminal of the second filter capacitor Cd are both coupled to the ground terminal; in addition, the first terminal of the third feedback resistor Rf3 is coupled to the first The first terminal of the feedback resistor Rf1, the second terminal of the third feedback resistor Rf3 are coupled to the first terminal of the switch Mn3, and the second terminal of the switch Mn3 is coupled to the second terminal of the first feedback resistor Rf1. The anode terminal of the diode Dd receives the PWM dimming signal Vpwm. The PWM dimming signal Vpwm is filtered by the second filter capacitor Cd after being divided by the first voltage dividing resistor Rd1 and the second voltage dividing resistor Rd2. The first terminal of the filter capacitor Cd (or the control terminal of the switch Mn3) generates a control voltage signal corresponding to the duty cycle of the PWM dimming signal Vpwm, wherein the smaller the duty cycle of the PWM dimming signal Vpwm, the larger the control voltage signal. Therefore, it can be designed that when the duty cycle of the PWM dimming signal Vpwm is less than or equal to the critical duty cycle, the control voltage signal can make the gate-source voltage of the switch Mn3 greater than the threshold voltage to be turned on. Once the switch Mn3 is turned on, the third feedback resistor Rf3 is coupled in parallel with the first feedback resistor Rf1, at this time, a feedback voltage Vfb having a value of the second feedback voltage V2 is generated on the first terminal of the first filter capacitor Cf, and The second feedback voltage V2 is greater than the first feedback voltage V1.

The control circuit 65 includes an error amplifier 651 , a comparator 652 , an oscillator 653 and an output driver 654 . The positive input terminal of the error amplifier 651 is coupled to the output of the feedback circuit 64 to receive the feedback voltage Vfb, and also receives the PWM dimming signal Vpwm, and the negative input terminal of the error amplifier 651 receives the reference voltage Vref. The difference of the voltage Vref outputs an error voltage Verror. The positive input terminal of the comparator 652 is coupled to the output of the error amplifier 651 to receive the error voltage Verror, and the negative input terminal of the comparator 652 is coupled to the oscillator 653 to receive the ramp voltage Vramp (which includes a sawtooth wave or a triangular wave) output by the oscillator 653 Form voltage signal), so the comparator 652 compares the error voltage Verror and the ramp voltage Vramp, and outputs logic 1 or 0 accordingly. The output driver 654 is coupled to the comparator 652 and generates the control signal Vctrl according to the output of the comparator 652 . When the PWM dimming signal Vpwm is at a high level, the diode Db is turned on, and the value of the feedback voltage Vfb is about the aforementioned high level voltage and much larger than the reference voltage Vref, so that the error voltage Verror is always greater than the ramp voltage Vramp, so The comparator 652 always outputs a logic 1, and the output driver 654 outputs control signals Vg1 and Vg2 to control the power switches Mn1 and Mn2 to be non-conductive so as not to allow the energy of the input DC voltage Vdc to pass through. When the PWM dimming signal Vpwm is at low level, the diode Db is not conducting, and the value of the feedback voltage Vfb will be determined by the impedance of the feedback circuit 64 to be the first feedback voltage V1 or the second feedback voltage V2, so that the magnitude of the error voltage Verror is between the slope The voltage Vramp is between the maximum value and the minimum value and the comparator 652 outputs a signal in the form of PWM. At this time, the output driver 654 outputs control signals Vg1 and Vg2 in the form of PWM to control the power switches Mn1 and Mn2 to be turned on alternately so as to convert the input DC voltage Vdc AC voltage Vac1 in the form of a square wave.

Please refer to FIG. 7 , which is a circuit diagram of another embodiment of the duty cycle detector 66 shown in FIG. 6 . The critical voltage of the switch Mn3 realized by the N-channel MOSFET may drift due to factors such as temperature, so that the critical duty cycle drifts accordingly. In order to prevent the critical duty cycle from drifting with the threshold voltage of the switch Mn3, a comparator 761 is added before the control terminal of the switch Mn3 of the duty cycle detector 66 to form a duty cycle detector 76 as shown in FIG. 7 . The comparator 761 compares the control voltage signal on the first terminal of the second filter capacitor Cd with the critical reference voltage Vth, and outputs a logic 1 or 0 accordingly to control whether the switch Mn3 is turned on or not, wherein the logic 1 is, for example, a voltage of 5V and Logic 0 is, for example, 0V, so it will not be affected by the drift of the threshold voltage of the switch Mn3.

To sum up, the utility model is based on digital dimming, and uses the duty cycle detector to detect that the duty cycle of the PWM dimming signal is less than or equal to the critical duty cycle (equivalent to a low average brightness to a certain extent). When changing the impedance of the feedback circuit to reduce the lamp current in an analog dimming manner, the average brightness is further reduced and the brightness contrast value is increased.

The above description is only illustrative of the present utility model, rather than restrictive. Those of ordinary skill in the art understand that many modifications and changes can be made without departing from the spirit and scope defined by the following appended claims. , or equivalent, but all will fall within the protection scope of the present utility model.

Claims (8)

1. one kind in conjunction with the simulation and the backlight driving device of digital dimming, it is characterized in that it comprises:

One suitching type inverter, be coupled to a backlight, during an impulse width modulation and light adjusting signal enables, convert an input direct voltage to an output AC voltage to open described backlight, during described impulse width modulation and light adjusting signal forbidden energy, do not transmit energy to close described backlight;

One feedback circuit is coupled to described backlight, receives a lamp current of described backlight output;

One responsibility cycle detector, be coupled to described feedback circuit, receive described impulse width modulation and light adjusting signal, in described impulse width modulation and light adjusting signal responsibility cycle during greater than a critical responsibility cycle, described feedback circuit is exported one first feedback voltage according to described lamp current, when described impulse width modulation and light adjusting signal responsibility cycle was less than or equal to described critical responsibility cycle, the impedance that described responsibility cycle detector is adjusted described feedback circuit made described feedback circuit export one second feedback voltage according to described lamp current; And

One control circuit, be coupled to described feedback circuit and described suitching type inverter, receive described impulse width modulation and light adjusting signal, when described impulse width modulation and light adjusting signal responsibility cycle is greater than described critical responsibility cycle and during enabling, controlling described suitching type inverter according to described first feedback voltage makes described lamp current be fixed on one first current value, in the time of during described impulse width modulation and light adjusting signal responsibility cycle is less than or equal to described critical responsibility cycle and is enabling, control described suitching type inverter according to described second feedback voltage and make described lamp current be fixed on one second current value, wherein said second current value is less than described first current value.

2. the backlight driving device in conjunction with simulation and digital dimming according to claim 1 is characterized in that described feedback circuit comprises:

One electric current changes potential circuit, receives described lamp current and converts a voltage signal according to this to;

One rectification circuit, being coupled to described electric current changes potential circuit, and described voltage signal is carried out rectification to produce a pulsed dc voltage signal;

One first feedback resistor, its first end are coupled to described rectification circuit to receive described pulsed dc voltage signal;

One second feedback resistor, its first end are coupled to described first feedback resistor, second end, and its second end is coupled to an earth terminal; And

One first filtering capacitor, its first end are coupled to described first feedback resistor, second end and described second feedback resistor, first end and export described first feedback voltage, and its second end is coupled to described earth terminal.

3. the backlight driving device in conjunction with simulation and digital dimming according to claim 2 is characterized in that described responsibility cycle detector comprises:

One diode, its anode tap receive described impulse width modulation and light adjusting signal;

One first voltage grading resistor, its first end is coupled to described diode cathode end;

One second voltage grading resistor, its first end are coupled to described first voltage grading resistor, second end, and its second end is coupled to described earth terminal;

One second filtering capacitor, its first end are coupled to described first voltage grading resistor, second end and described second voltage grading resistor, first end, and its second end is coupled to described earth terminal;

One the 3rd feedback resistor, its first end are coupled to described first feedback resistor, first end; And

One switch, its first end are coupled to described the 3rd feedback resistor second end, and its second end is coupled to described first feedback resistor, second end, and its control end is coupled to described second filtering capacitor, first end.

4. the backlight driving device in conjunction with simulation and digital dimming according to claim 1 is characterized in that described control circuit comprises:

One error amplifier is coupled to described feedback circuit, exports an error voltage according to the difference of a described feedback voltage and a reference voltage;

One oscillator is exported a ramp voltage;

One comparator is coupled to described error amplifier and described oscillator, more described error voltage and described ramp voltage, and output logic 1 or 0 according to this; And

One output driver is coupled to described comparator, and the output of the described comparator of foundation produces described control signal.

5. the backlight driving device in conjunction with simulation and digital dimming according to claim 1 is characterized in that described suitching type inverter comprises a full-bridge type inverter.

6. the backlight driving device in conjunction with simulation and digital dimming according to claim 1 is characterized in that described suitching type inverter comprises a semi-bridge type inverter.

7. the backlight driving device in conjunction with simulation and digital dimming according to claim 1 is characterized in that described suitching type inverter comprises a push-pull dc-to-ac.

8. the backlight driving device in conjunction with simulation and digital dimming according to claim 1 is characterized in that described backlight comprises a cold-cathode fluorescence lamp.

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