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With the rapid development and rapid spread 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 driving power supplies has received increasing attention. Its performance will directly affect the display effect, display module life and battery standby time.
To this end, many companies have launched their own WLED driver chip. Here, we will use BCD's AP3029 as an example to introduce the relevant application of WLED driver.
Introduction to AP3029
The AP3029 is a PWM (Pulse Width Modulation) mode boost converter that integrates a switching transistor and a Schottky diode. It only drives the load WLED through a few external components—boost inductor, input and output capacitors, and feedback resistors. Moreover, the AP3029 has a higher frequency (1.2MHz), which also reduces the size of the external inductor and saves space in the system.
The input voltage of the AP3029 is 2.5~16V. In normal operation, the output constant current control mode is adopted. The output current is controlled by the internal reference voltage and an external feedback resistor. Its internal reference voltage is low, only 200mV, which reduces drive loss to a certain extent and improves work efficiency.
The output voltage of the AP3029 is determined by the number of series connected to the WLED. If the output WLED is disconnected, the AP3029 automatically clamps the output voltage to 27V for overvoltage protection.
The AP3029 has a soft-start function inside. When the system is started, the output voltage becomes smooth and the input surge current is also limited.
Typical application
1 drive series WLED application program
Usually, 2 to 6 WLEDs are required for backlighting on a small-sized display screen, and an ordinary WLED generally operates at 3.2V/20mA. Figure 1 shows six series-connected WLED driver circuits based on the AP3029. The number of WLEDs can be adjusted depending on the screen size.
Figure 1 Typical application diagram of AP3029 driving 6 WLEDs in series
The AP3029 drives the WLED with constant current through the feedback resistor R1, ILED=VFB/R1. The output voltage is the forward voltage of six WLEDs, which is 6×3.2+0.2=19.4V, and the output power is 388mW.
The parameters of this typical application scheme are shown in Table 1.
2 drive parallel WLED application program
As the display size of the portable digital product increases, the number of WLEDs required by the display module also increases, thus placing higher demands on the performance and driving capability of the WLED driver chip.
The AP3029's powerful drive capability is fully utilized in the backlight drive design of large-size displays. Since most portable digital products are powered by lithium batteries, the output voltage range of a single-cell lithium battery is usually 3.2 to 4.2V. Therefore, driving more than 6 WLEDs usually adopts a parallel driving mode to control the output voltage within a reasonable range. Figure 2 shows the typical application of the AP3029 parallel drive WLED.
Figure 2 Typical application diagram of AP3029 driving 10 WLEDs in parallel
The parameters of this application scheme are shown in Table 2.
Table 3 shows the relationship between the output current and the feedback voltage VFB when the AP3029 drives 10 WLEDs at 85 °C. If the AP3029 has sufficient drive capability, VFB will remain essentially the same as the output current increases.
In this typical application, the normal output current is 40mA. As can be seen from the data in Table 3, the AP3029 will have a certain margin when driving 10 WLEDs at 85 °C. Therefore, the AP3029 can fully meet the requirements of driving 10 WLEDs.
In the parallel scheme, the most notable is the current matching problem of the two rows of WLEDs. If the two strings of WLEDs are directly connected in parallel, the topology determines the sum of the forward voltages of the two strings of WLEDs. As can be seen from Fig. 3, there is a certain difference in the forward voltage drop VF of the WLED, and the dynamic impedance near the operating point (IF = 20 mA) is small. Therefore, there will be a large difference in the forward current of the two strings of WLEDs, so that the output current cannot be matched. This situation will cause a significant difference in the brightness of the two rows of WLEDs, which seriously affects the display effect.
In order to solve the above problem, the PNP pair tube can be used at the output to achieve the matching of the output current. The connection method is shown in Figure 3. Here, it is recommended to use the 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 better current sharing effect.
Figure 3 diode forward voltage forward current relationship
Table 4 shows the current distribution of two rows of WLEDs (five WLEDs per row) before and after the current sharing design at different input voltages.
It can be seen that after the output current is designed by the current sharing, the current distribution situation is substantially improved, and the current matching requirement in the WLED backlight driving is fully satisfied.
The AP3029 two driving methods described above have been verified and the results are feasible.
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