With the rapid development and popularization of digital products, the internal devices 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 need display modules, the design of WLED backlight driving power supply has attracted more and more attention. Its performance will directly affect the display effect, the service life of the display module and the standby time of the battery.
Recently, BCD has launched a new WLED drive chip ap3029, which has great advantages in the design and application of WLED drive power supply. On the premise of ensuring performance, it improves the working frequency, carries out higher integration, reduces the number and size of peripheral devices and reduces the cost of the system. The following will introduce the application scheme and key parameters of ap3029 in WLED driver.
1、 Ap3029 introduction
Ap3029 is a boost converter with PWM (pulse width modulation) mode integrating switch and Schottky diode. It only drives the load WLED through several external devices – boost inductance, input and output capacitance and feedback resistance, and ap3029 has a high frequency (1.2MHz). In this way, it can also reduce the size of external inductance and save the space of the system.
Ap3029 has a wider input voltage range than other similar products, from 2.5V to 16V. The maximum input voltage of most similar products is only 6V, which can make the application range of the system wider.
When ap3029 works normally, it adopts output constant current control mode. Its output current is controlled by internal reference voltage and external feedback resistance. The internal reference voltage of ap3029 is low, only 200mV, which reduces the driving loss and improves the working efficiency to a certain extent.
The output voltage of ap3029 is determined by the number of load WLED in series. If the output WLED is open circuit, ap3029 will automatically set the output voltage to 27V to realize overvoltage protection.
Ap3029 has internal soft start function. When the system is started, the output voltage becomes very smooth and the input impulse current is limited.
Through the perfect and reliable protection function, the safety and practicability of the system can be greatly improved.
2、 Typical application scheme of ap3029
1. Ap3029 drive series WLED application scheme:
Generally, 2 ~ 6 wleds are required for backlight on a small display screen, and the ordinary WLED generally works at 3.2v/20ma. Figure 1 shows the driving circuit of 6 series wleds designed based on ap3029. The number of wleds can be adjusted according to different screen sizes.
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 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:
Table 1. Spec of WLED series output
2. Ap3029 drive parallel WLED application scheme:
With the increase of the display screen size of portable digital products, the number of WLED required by the display module also increases, which puts forward higher requirements for the performance and driving ability of WLED driver chip.
The strong driving ability of ap3029 has been brought into full play in the backlight driving design of large-size display screen. 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, more than 6 wleds are usually driven in parallel 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 parallel driven WLED.
Figure 2. Typical application diagram of ap3029 driving 10 wleds in parallel
The spec of the application scheme is shown in Table 2.
Table 2. Specifications of WLED parallel output
Table 3 is the relationship between output current and feedback voltage VFB when ap3029 drives 10 wleds at 85oc. If the driving capacity of ap3029 is sufficient, VFB will basically remain unchanged with the increase of output current.
Table 3. Relationship between output current and feedback voltage of ap3029 driving 10 wleds at 85oc
Under this typical application, the normal output current is 40mA. It can be seen from the data in Table 3 that there will be a certain margin for ap3029 to drive 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 of two rows of WLED. If two series wleds are directly connected in parallel, the topology determines the sum of the forward voltages of the two series wleds. It can be seen from Figure 3 that there is a certain difference in the forward voltage drop VF of WLED, and the dynamic impedance near its working point (if = 20mA) is very small. Therefore, there will be a large difference in the forward current of the two series WLED. Make the output current unmatched. This situation will lead to significant differences in the brightness of the two rows of WLED, which seriously affects the display effect.
Figure 3. Relationship between diode forward voltage and forward current
In order to solve the above problems, the PNP pair tube can be used at the output end to realize the matching of output current. The connection mode is shown in Figure 3. Here, it is recommended to use the integrated PNP pair tube with the amplification factor β〉〉 2. This can ensure that the characteristic parameters of the two PNP tubes are basically the same, reduce the error and achieve better current sharing effect.
Table 4 shows the current distribution of two rows of WLED (five WLED 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 the current distribution has been substantially improved after the current sharing design of the output current. Fully meet the current matching requirements in WLED backlight drive.
The two driving modes of ap3029 introduced above have been verified, the results are feasible, meet various requirements of customers, and are accepted and adopted by customers.
To sum up, as a backlight power supply for portable digital products, ap3029 has a broad market and can fully meet the different requirements of most customers. It is a design scheme with high performance price ratio.