Although the LCD back light has many kinds of ways, for example LED, EL, CCFL and so on, but regarding the great size LCD screen, high becomes the mainstream back light photo source as a result of the CCFL luminous efficiency. The traditional LCD screen mainly uses in the notebook or the table model computer, the back light module in has under certain brightness premise, but also wants the size to be small and weight light and so on, mostly selects the side back light method (the CCFL tube to install nearby screen two or all around, through leads worn out fur woman with no pubic hair light scattering to the entire screen behind); But LCD TV is great size LCD TV specially, because to demonstrated that brightness (450cd/m2), the angle of view wide-angle (170°), the image contrast gradient (500:1) and so on has a higher request, thus needs to use the photo source use factor higher vertical back light technology.
1 vertical back light
The vertical back light, as the name suggests, is the CCFL tube immediate packing behind the LCD screen, and will diverge using the entire reflection membrane to other direction optical fiber reflects to the screen light-emitting area, enhances the photo source to the use factor. Although the vertical back light has solved large screen LCD questions and so on TV brightness, angle of view breadth, but because is mainly depending on increases the tube number to realize, the tube number from several to several dozens different, thus has the following problem:
(1) the uniformity is low, this is mainly because the tube number is the result of much, finally causes the screen to present the light and shade stripe, if various tubes brightness is inconsistent, the phenomenon is more obvious.
(2) the screen chromatic aberration, the tube number are many, although has promoted brightness and brightness, but also causes the back light module’s calorific capacity to increase, but liquid crystal member, because is high to the temperature sensitivity, easy to cause under the high temperature the screen color to be unusual; In addition, the CCFL tube itself will also reduce under the high temperature life.
Taking this into consideration, designs when the CCFL inversion power source needs to control each tube carefully the electric current, guarantees brightness to be consistent, raises the contravariant power source’s transfer efficiency in the tube definite situation to reduce the calorific capacity as far as possible.
Figure 2
2 multi-channel CCFL controllers
In order to reduce the cost, some CCFL inversion power source uses the plan which shown in Figure 1. It regarding the tube number is few, and selects the side back light method small screen LCD is one kind of low cost, the light-sized optimal plan, may eliminate the tube brightness inconsistent flaw through the beacon diffusion function extremely, because tube few (2~4), the entire bridge electric circuit average MOSFET electric current not too has been big, is also small to the converter efficiency influence.
Regarding great size LCD TV, to increase brightness, enhances the contrast gradient, tube quantity increases greatly, sometimes possibly achieves more than 30, if uses the above electric circuit, each tube’s brightness through the exterior parameter control, will present the serious stripe disturbance phenomenon with difficulty. Not only that because this time bridge connection electric current is very big, will cause the breakover to lose (I2×RON) increases suddenly along with the tube quantity, for instance 16 tube’s breakovers will lose will be 2 tube 64 times; When the tube is 32, its loss will be 2 tube 256 times, for single tube’s 1024 times, causes the serious discovery phenomenon, causes the back light module’s temperature to rise suddenly, will cause the LCD color to be unusual, will also cause the CCFL tube loss of life. Certainly may use many 1 to tow 2 or 1 tows 4 plans to reduce the calorific capacity greatly, but the stripe disturbance still existed, moreover as a result of each power source module difference, will also introduce the massive disturbance.
3 monolithic multi-channel CCFL controllers
In view of the above question, DALLAS/MAXIM Corporation has promoted DS3998 the high density monolithic CCFL controller, it integrated 8 CCFL controllers, carries on the monitoring separately to 8 CCFL tube’s electric current, causes 8 tubes the electric currents basic consistent, thus realizes brightness to control evenly, eliminates the screen stripe phenomenon. As shown in Figure 2, because each tube has the independence actuation and the control loop, because thus avoided shown in Figure 1 the electric circuit broke down to cause many tubes not to be bright, was advantageous for the fast service; Moreover, because each push-pull circuit only actuates a tube, winds through the electric current is small, the MOSFET breakover loses is very small, also avoided giving off heat the question, but the cost only slightly increased (pair of low power MOSFET). DS3998 not only well has solved the back light source brightness uniformity and gives off heat the question, but also below brings the extra advantage:
(1) the channel phase control technology reduces AC/DC the current surge
The plan or many independent control contravariant power sources with shown in Figure 1 is different, integrated the channel phase control function, as shown in Figure 3, each channel’s MOSFET electronics grid breakover time in 8 channels between equal distribution, reduced the surge current and simplified the AC/DC design requirements, reduced heating element’s radiating surface, the reduced volume, reduced the power source cost.
(2) E2PROM register and 2 serial interfaces
DS3988 uses the standard 2 serial interfaces and the internal E2PROM disposition register and the user memory block carries on the correspondence. Disposes the register to start the special piece register softly including 4 with 2 control registers (CR1/2), enables the user to be possible from to define DS3988 the parameter, like the soft start slope, the tube and adjust the optical frequency rate source, the breakdown monitor option and the channel enable/to be forbid. 8 byte non-volatile user memory blocks use for to save the product data, like the date code, the series number either the product marking number, this brings enormously for the product manufacturing debugging or the maintenance convenient.
(3) supports the more tube controls through the simple cascade
The DS3998 tube frequency has two kind of disposition options. When DS3988 as tube main engine, establishes in CR1 LFSS=0, chooses on the board the oscillator, and establishes the frequency through the LOSC input end’s exterior resistor (40kHz~80kHz). In this case, the tube frequency code from LSYNC the I/O pin output, provides the synchronized signal for other tubes from machine DS3988. As the tube from the machine-hour, through establishes in CR1 LFSS=1, is forbid the LOSC input, by must provide the exterior 40kHz~80kHz clock signal to LSYNC I/O. This signal may obtain from tube main engine DS3988 LSYNC I/O, may also obtain from other supply oscillators. Therefore, many DS3988 are very easy to realize surpasses 8 tubes the large screen LCD back light controls.
(4) precise tube beam control
The DS3988 use digit pulse-duration modulation ((DPWM) the signal (22.5Hz~440Hz) provides the high efficiency and the precise tube transfer. As shown in Figure 4, in the DPWM cycle’s high level stage, the tube frequency actuation tube which designated (40kHz~80kHz). Because zai zhei duan shijian li the tube frequency appears suddenly, therefore is called this time section “arises suddenly” the stage. In the DPWM cycle’s low level stage, the controller forbids the MOSFET electronics grid to actuate, therefore the tube is not actuated, by now the electric current no longer flowed through the tube, but the time is very short, will not cause the tube to present the complete ion to draw back stirs up. Adjusts the light (is also modulation) arises suddenly the stage through the adjustment the dutyfactor to realize the increasing and decreasing progressively. Because its DPWM signal has had many kinds of ways, therefore has provided the very big flexibility for the application. The soft start characteristic further reduced has input power source’s current surge.
4 concluding remark
Compares with the small screen liquid-crystal display, the large screen liquid crystal to the monitor brightness, the angle of view breadth and the demonstration contrast gradient set a higher request, therefore needs to use the vertical back light technology, simultaneously increases the CCFL tube to satisfy the request massively. DS3998 is aims at the high density which this application develops specially monolithic to integrate the CCFL controller, has solved the chromaticity unusual problem which well the multi-tube LCD screen stripe disturbance and gives off heat high causes, improved the video image quality, its leggy breakover control function also reduced the AC/DC power source’s design requirements, reduced the power source volume.
