Design of backlight driver power supply for portable digital products based on AP3029

With the rapid development and popularization of digital products, the internal components of digital products are also facing higher requirements. Since most portable digital products such as mobile phones, digital cameras, MP3, PMP and digital photo frames require display modules, the design of WLED backlight driver power supply has attracted more and more attention. Its performance will directly affect the display effect, the life of the display module and the standby time of the battery.

Recently, BCD has launched a new WLED driver chip - AP3029. This chip has great advantages in WLED driver power design and application. It improves the operating frequency, integrates more highly, reduces the number and size of peripheral devices, and reduces system costs while ensuring performance. The following will introduce the application solutions and key parameters of AP3029 in WLED driving.

1. AP3029 Introduction

AP3029 is a PWM (pulse width modulation) mode boost converter that integrates the switch tube and Schottky diode. It drives the load W LED through only a few external devices - boost inductor, input, output capacitors, feedback resistors , and AP3029 has a higher frequency (1.2MHZ), which can also reduce the size of the external inductor and save more system space.

AP3029 has a wider input voltage range than other similar products, from 2.5V to 16V. Most similar products have a maximum input voltage of only 6V, which makes the system more widely used.

AP3029 adopts output constant current control mode when working normally. Its output current is controlled by internal reference voltage and external feedback resistor. The internal reference voltage of AP3029 is low, only 200mV, which reduces driving loss to a certain extent and improves working efficiency.

The output voltage of AP3029 is determined by the number of series-connected WLED loads. If the output WLED is short-circuited, AP3029 will automatically set the output voltage to 27V to provide overvoltage protection.

AP3029 has an internal soft-start function. When the system starts, the output voltage becomes very smooth and the input inrush current is also limited.

Through complete and reliable protection functions, the safety and practicality of the system can be greatly improved.

2. Typical application solutions of AP3029

1. AP3029 driving series WLED application solution:

Usually, a small-sized display screen requires 2 to 6 WLEDs as backlight. Ordinary WLEDs generally work at 3.2V/20mA. Figure 1 is a 6-series WLED driver circuit designed based on AP3029. The number of WLEDs can be adjusted according to the screen size.

Figure 1. Typical application diagram of AP3029 driving 6 WLEDs in series

AP3029 drives the WLED with a constant current through the feedback resistor R1, ILED=VFB/R1. The output voltage is the forward voltage of the six WLEDs, 6*3.2+0.2=19.4V. The output power is 388mW.

The specifications of this typical application solution are shown in Table 1:

Table 1. Specifications of WLED series output

2. AP3029 driving parallel WLED application solution:

As the display screen size of portable digital products increases, the number of WLEDs required in the display module also increases, which puts higher requirements on the performance and driving capability of the WLED driver chip.

The strong driving capability of AP3029 is fully utilized in the backlight driving design of large-size display screens. Since most portable digital products are powered by lithium batteries, the output voltage range of a single lithium battery is usually 3.2V~4.2V. Therefore, driving more than 6 WLEDs usually adopts parallel driving mode to control the output voltage within a reasonable range. Ensure that the system has a better working state. Figure 2 is a typical application solution of AP3029 parallel driving WLED.

Figure 2. Typical application diagram of AP3029 driving 10 WLEDs in parallel

The specifications of this application solution are shown in Table 2.

Table 2. Specifications of WLED parallel output

Table 3 shows the relationship between the output current and the feedback voltage VFB when AP3029 drives 10 WLEDs at 85OC. If the driving capability of AP3029 is sufficient, VFB will remain basically unchanged as the output current increases.

Table 3. Relationship between output current and feedback voltage of AP3029 driving 10 WLEDs at 85OC

In this typical application, the normal output current is 40mA. From the data in Table 3, it can be seen that AP3029 has a certain margin when driving 10 WLEDs at 85OC. Therefore, AP3029 can fully meet the requirements of driving 10 WLEDs.

In the parallel scheme, the most noteworthy issue is the current matching of the two rows of WLEDs. If two strings of WLEDs are directly connected in parallel, this topology determines the sum of the forward voltages of the two strings of WLEDs. As shown in Figure 3, there is a certain difference in the forward voltage drop VF of WLEDs, and the dynamic impedance near its operating point (IF=20mA) is very small. Therefore, there will be a large difference in the forward current of the two strings of WLEDs. This makes the output current unmatched. This situation will cause a significant difference in the brightness of the two rows of WLEDs, seriously affecting the display effect.

Figure 3, Relationship between diode forward voltage and forward current

In order to solve the above problems, a PNP pair of tubes can be used at the output end to achieve output current matching. The connection method is shown in Figure 3. Here, it is recommended to use an integrated PNP pair of tubes with an amplification factor of β>>2, so that the characteristic parameters of the two PNP tubes can be basically consistent, the error can be reduced, and a better current sharing effect can be achieved.

Table 4 shows the current distribution of two rows of WLEDs (five WLEDs in each row) before and after adding the current sharing design under different input voltages.

Table 4, Output current sharing accuracy

It can be seen from Table 4 that after the output current is designed for equalization, the current distribution is substantially improved, which fully meets the current matching requirements in WLED backlight driving.

Conclusion

The two driving modes of AP3029 introduced above have been verified, the results are feasible, and they meet the various requirements of customers and are accepted and adopted by customers.

In summary, AP3029 has a broad market as a backlight power supply for portable digital products and can fully meet the different requirements of most customers. It is a design solution with a high performance-price ratio.