Analysis and comparison of two LED backlight driving solutions

Ultra-thin, high-definition, energy-saving and environmentally friendly are the highlights of LCD TVs that use LED backlights over CCFL backlights. As time goes by, LED backlight LCD TVs have been recognized by people and their time has come. Since September last year, in just a few months, LED LCD TVs have occupied more than 11% of the LCD market share. It is predicted that CCFL backlight LCD TVs will be completely replaced within three years.

At the same time, with the continuous expansion of the market, LED backlight driving technology has become more mature, and there are more and more backlight driving solutions. However, in the field of LCD TV, the mainstream solution for LED backlight driving is still the inductive boost DC/DC boost conversion plus constant current control circuit (as shown in Figure 1). The two driving solutions to be compared in this article also belong to this category, namely AP3039+AP3608E (Solution 1) and AP3608 single-chip solution (Solution 2).

1 Comparison and testing of solutions

1.1 A brief description of the three chips

As can be seen from Table 1 above, the AP3068 is functionally equivalent to the integration of an AP3039 and an AP3608E.

1.2 Principle Analysis

In terms of working principle, the two schemes are exactly the same. Both schemes first perform DC-DC boost conversion, and then collect and feedback the multiple output currents to achieve constant current control. The detailed working principles are explained in many literatures and will not be repeated here. Only the schematic diagrams of the two are listed for reference.

1.3 Cost Comparison

As shown in Figures 2 and 3, the peripheral devices of the two solutions are almost the same, so the total cost mainly depends on the price difference between several chips. In fact, the price of AP3068 is less than the sum of the prices of AP3039 and AP3608E, which means that Solution 2 is lower in cost and has a greater price advantage.

1.4 Performance Comparison

For LCD TV products, there are several indicators that attract more attention in terms of backlight technology. The first is the uniformity of the backlight. It is not allowed to have obvious brightness changes in different areas of the LCD screen, which requires that the current matching degree (Current Match) of each LED light string should be high. The second is that the backlight can be adjusted to meet the needs of different environments and consumers' personal usage habits. It is not allowed to have no response to adjustment or instability after adjustment. The third is that the temperature rise of the device cannot be too high. The following tests and compares these two solutions from these three aspects.

1) Current matching comparison

In practical applications, the size of the output capacitor has a great influence on the current sharing effect of each channel. If conditions permit, the larger the capacitance value, the better. The output voltage of the solution in this article is 46V, the output current of each channel is set to 60mA, the total output current is about 0.48A, and the capacitance value of 220uF/63V is used. The following table 2 shows the current test values ​​of each channel of the two solutions after stabilization (CH1~CH8 represents each channel, unit: mA):

After calculation, it can be concluded that the current matching degree of Scheme 1 is 1.0%; the current matching degree of Scheme 2 is 0.95%. Taking into account factors such as instrument measurement errors and device differences (such as the sampling resistor has an accuracy of 1%), the current matching degrees of the two schemes are approximately equal and both meet the requirements of the specification (<1.5%).

2) Dimming test

Backlight brightness adjustment is an important function of LCD TV products, which requires not only stable output during dimming, but also good dimming linearity. The following table shows the average output current measured under different dimming duty cycles for these two solutions.

From the data in the table, we can see that the current value when the duty cycle is 100% is not equal to the sum of the currents of each channel in Table 2. The main reason is due to the instrument test error, which does not affect our judgment of the dimming performance. After testing, the output current waveforms of these two solutions have no overshoot or oscillation when turning off and on. Therefore, these two solutions can achieve the dimming function very ideally.

3) Thermal performance comparison

Device temperature rise is an important technical indicator because it is directly related to the service life of the product. The higher the temperature rise, the shorter the service life. However, different devices have different temperature rise standards, so there is no unified standard. It is only required that the lower the device temperature rise, the better. Generally, when the room temperature is 25℃, the device temperature rise is below 50℃ and is within the safety range.

Using a thermal imager to test, the highest temperature device in Solution 1 is AP3608E, with a temperature of 47.8°C; the highest temperature device in Solution 2 is AP3068, with a temperature of 52.9°C, a difference of about 5°C. There are several reasons for this: (1) As mentioned above, AP3068 is equivalent to an AP3039 and an AP3608E. In addition to bearing the constant current loss of AP3608E, it also bears the driving current loss of AP3039. (2) Solution 2 is a single-chip solution with concentrated heat sources and poorer heat dissipation conditions than Solution 1. In addition, it is also related to PCB Layout. But in general, the highest device temperature rise in Solution 2 is slightly higher than that in Solution 1.

Through the comparison of the above three aspects, it can be seen that there is not much difference in electrical performance between Schemes 1 and 2. The biggest difference is in the temperature rise of the device, but it is only slightly different.

2 Conclusion

A comprehensive comparison of the two solutions for LED backlight driving in LCD TVs: the AP3039+AP3608E solution and the AP3068 single-chip solution in terms of working principle, cost and performance shows that the AP3068 single-chip solution has a lower cost and a slightly higher temperature rise. If the temperature rise requirement is not stringent, it is a better choice.