Two kinds of analysis and comparison of LED backlight driving scheme

Ultra-thin, high-definition, energy saving, environmental protection is using LED backlight LCD TV is better than that of CCFL backlight highlights, as time goes on, the LED backlit LCD TV has been recognized by people and its time has come. Since September last year, just a few months time, the LED LCD TV has occupied more than 11% of the LCD market, there are forecasts suggest that 3 years CCFL backlight of liquid crystal TV will be completely replaced.

at the same time, along with the expansion of the market, LED backlight driving technology is mature, a backlight driving scheme is also more and more, plus ca change, however, in the field of LCD TV, LED backlight driver solution is still the mainstream of the inductor step-up DC/DC booster plus a constant current control circuit ( As shown in figure 1) 。 This article will contrast the two drivers also belong to this kind of program, the AP3039 + AP3608E ( Scheme 1) And AP3608 single chip solutions ( Scheme 2) 。

1 scheme comparison and testing of 1. 1 three chip simple

the table 1 shows that AP3068 is equivalent to a AP3039 on the function and a AP3608E integration. 1.

2 principle analysis on the working principle, two schemes are exactly the same, is to DC - first DC step-up conversion, and then on multi-channel output current collection and feedback, so as to realize constant current control, detailed work principle were expounded in many literature, go here, only lists the principle diagram of the two, for your reference.

　　1. 3 cost is

from figure 2, 3, the two solutions of peripheral devices almost the same, so the total cost depends on the difference between a few chips. Actually, AP3068 price less than the price of the AP3039 and AP3608E combined, that is to say, the second scheme in lower cost, more price advantage.

  　　1. 4 performance comparison

for LCD TV products, there are a few more concern in backlight technology indicators, one is the uniformity of backlight, are not allowed to appear different area of the LCD screen brightness changes, it also requires that each current of the LED string matching degree ( 当前匹配) To be higher. Secondly, the backlight can be adjusted to cope with the needs of different environment and individual consumers use habits, not allowed to adjust the unresponsive or unstable after adjustment. A third is temperature rise too much. Below is from the three aspects of the two schemes respectively to test and contrast.

　　1） Current match compared to

in the practical application, the size of the output capacitance of each channel currents flow effect has a great influence, if the condition allows, the capacitance value is bigger, the better. Mimo 46 v output voltage, the channel setting output current 60 ma, the total output current 0. 48 a, using the capacitance value of the 220 uf / 63 v. Table 2 for the two plans after the stable under the current test values of each channel ( Each Channel CH1 ~ CH8 said, unit: mA) :

calculated can be concluded that the current plan 1 is match to 1. 0%; Scheme 2 current matching degree is 0. 95%. Considering the errors in measuring instrument, device difference ( Such as sampling resistance is 1% accuracy) Factors such as, two schemes of current matching degree is approximately equal, also meet the requirements of specification (

　　2） Dimming test

backlight brightness control is an important function of the LCD TV products, not only requires the light output stability, but also require GuangXianXing degrees is better also. Of the two schemes are listed in the table below in a different light duty ratio, the average of the measured output current.

can be seen from the data in the table, the duty ratio of 100% at the time of the current value is not equal to the sum of the each channel currents in table 2, the main reason lies in the instrument measuring error caused by, this does not affect our switch light judgment performance. Output current waveform of test the two alternatives when shut off and opened without overshoot without shock. Therefore, the two schemes are able to the realization of the ideal dimming function.

　　3） Thermal performance contrast

temperature rise is an important technical index, because it is directly related to the service life of the product, the higher the temperature the shorter service life. But the device temperature standards vary, so there is no unified standard, only requires the lower the temperature rise, the better, usually at room temperature is 25 ℃ temperature rise below 50 ℃ in Ann specification is surrounded.

with thermal imager test, plan 1 is the highest temperature of device AP3608E, temperature of 47. 8℃； The highest temperature of the device is AP3068 plan 2, temperature of 52. 9 ℃, both differ about 5 ℃. Its reason has several aspects: ( 1) As mentioned earlier, AP3068 is equivalent to a AP3039 with a AP3608E, in addition to also bear AP3608E constant current loss, also undertake AP3039 drive current loss. （ 2) Scheme 2 is a single chip, heat concentration, cooling condition is better than those of plan 1. In addition, also have relationship with PCB Layout. But all in all, scheme 2 the highest device temperature rise slightly higher than those of plan 1.

through the above three aspects of comparison, the scheme 1 and 2 is not much difference in the performance, the biggest difference in terms of temperature rise, but also just slightly different. Conclusions

2 for LCD TV LED backlight driver in two solutions: AP3039 + AP3608E solutions and AP3068 single chip, through the principle of work, cost and performance aspects of the comprehensive comparison, the AP3068 single-core scheme is cheaper, a slightly higher temperature, if the temperature is not demanding, it is a better choice.