Design of Backlight Driving 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, because most portable digital products such as mobile phones, digital cameras, MP3, PMP and digital photo frames will require Display modules, among which WLED The design of the backlight driving power supply is getting 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” title=”BCD”>BCD has launched a new WLED driver chip-AP3029″ title=”AP3029″>AP3029, this chip has great advantages in the design and application of WLED driver power, it guarantees performance Under the premise of improving the operating frequency, a higher degree of integration is carried out, the number and size of peripheral devices are reduced, and the cost of the system is reduced. The following will introduce the application scheme and key parameters of AP3029 in WLED driver.

1. Introduction to AP3029

  AP3029 is a boost converter in PWM (Pulse Width Modulation) mode that integrates switch and Schottky diode. It only drives the load WLED through a few external components – boost Inductor, input, output capacitor, feedback resistor, and AP3029 has a high frequency (1.2MHZ), so it can also reduce the size of the external inductor, Save more system space.

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

AP3029 adopts output constant current control mode during normal operation, and its output current is controlled by internal reference voltage and external feedback resistance. The reference voltage inside the AP3029 is low, only 200mV, which reduces the driving loss to a certain extent and improves the working efficiency.

The output voltage of AP3029 is determined by the number of load WLEDs in series. If the output WLED is disconnected, AP3029 will automatically clamp the output voltage to 27V to achieve overvoltage protection.

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

Through the perfect and reliable protection function, the security and practicability of the system can be greatly improved.

2. Typical application scheme of AP3029

1. AP3029 drive series WLED application scheme:

Usually, 2~6 WLEDs are needed for backlighting on a small-sized display screen, and ordinary WLEDs generally work at 3.2V/20mA. Figure 1 shows the driving circuit of 6 WLEDs in series based on AP3029 design. The number of WLEDs can be adjusted according to the screen size.

Design of Backlight Driving Power Supply for Portable Digital Products Based on AP3029

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

AP3029 drives WLED with constant current through 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 Spec of this typical application scheme is shown in Table 1:

Design of Backlight Driving Power Supply for Portable Digital Products Based on AP3029

Table 1. Spec of WLED series output

2. AP3029 drive parallel WLED application scheme:

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

The feature of AP3029’s strong driving ability has been 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-cell lithium battery is usually 3.2V~4.2V. Therefore, driving more than 6 WLEDs usually adopts a parallel driving method to control the output voltage within a reasonable range. Ensure that the system has a better working state. Figure 2 shows the typical application scheme of AP3029 driving WLED in parallel.

Design of Backlight Driving Power Supply for Portable Digital Products Based on AP3029

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

The Spec of this application scheme is shown in Table 2.

Design of Backlight Driving Power Supply for Portable Digital Products Based on AP3029

Table 2. Spec of WLED parallel output

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

Design of Backlight Driving Power Supply for Portable Digital Products Based on AP3029
Table 3. The relationship between the output current and the feedback voltage of the 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 the AP3029 will have a certain margin for driving 10 WLEDs at 85OC. Therefore, AP3029 can fully meet the requirements of driving 10 WLEDs.

In the parallel scheme, the most noteworthy is the current matching problem 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. It can be seen from Figure 3 that there is a certain difference in the forward voltage drop VF of the WLED, 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. so that the output currents cannot be matched. This situation will lead to a significant difference in the brightness of the two rows of WLEDs, which will seriously affect the display effect.

Design of Backlight Driving Power Supply for Portable Digital Products Based on AP3029

Figure 3. Diode Forward Voltage vs Forward Current

In order to solve the above-mentioned problems, a PNP pair tube can be used at the output end to realize the matching of the output current. The connection method is shown in Figure 3. Here, it is recommended to use an integrated PNP pair tube, and the amplification factor β>>2, which can ensure that the characteristic parameters of the two PNP tubes are basically the same, reduce the error, and achieve a better current sharing effect.

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, Current sharing accuracy of output current

It can be seen in Table 4 that after the output current is designed by current sharing, the current distribution has been substantially improved. Fully meet the current matching requirements in WLED backlight drive.

3. Summary

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

To sum up, 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 with high cost performance.

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