More and more portable consumer electronics products are equipped with color displays, such as mobile phones, digital cameras, PDAs, MP3 players, PMP players, etc., in which mobile phones account for the majority of this market, resulting in small and medium-sized in the past two years. The rapid development of the size display industry chain. Depending on the application, there will be different types of displays, such as TFT-LCD, CSTN-LCD and O LED display . From the market application, the O LED display is only on the secondary screen of the folding mobile phone and the MP3 market. It has a certain share, and the mainstream of the market is still TFT and CSTN. These two types of LCD screens account for the vast majority of existing small and medium size display shipments. This article focuses on an analysis of the backlight drive solution for small and medium size LCD displays.
Technical analysis of backlight drive
The LCD display itself does not emit light. In order to clearly see the contents of the LCD display, a certain white backlight is required. In small and medium size LCD displays, white LEDs are generally used as the backlight for the display. The white LED backlight power supply consists of several white LEDs. For example, mobile phones and digital cameras generally only need 2 to 3 white LEDs, while PDAs and PMPs may require 3 to 6 LEDs depending on the area of ​​their display screens. The requirements for the backlight driver circuit are:
1. Meet the brightness requirements of the backlight;
2. The brightness of the entire display is uniform (there is no need to have one part brighter and the other part darker);
3. Brightness can be easily adjusted;
4. The drive circuit takes up less PCB space;
5. High work efficiency;
6. Low overall cost;
7. Little interference to other modules of the system.
Depending on the application, the focus of the system designer may vary. For example, for a low-cost product solution, the cost of the entire driver circuit may be placed first. For mobile applications, is the white driver circuit for other modules? The occurrence of EMI interference is a key factor to consider; in MP3 applications, it is possible to be less concerned about EMI interference.
White LED driver basically has two driving modes: 1. It is driven by the principle of inductive step-up DC/DC boost converter. All LEDs are connected in series, which is also called series driving mode. 2. Using boost The charge pump drive circuit generates a voltage of 5V/4.5V or a voltage that is adaptively determined according to the forward voltage of the LED. All the LEDs are connected in parallel and are generally called parallel drive.
Series drive circuit
From the perspective of technological development, tandem-type drives appear earlier and are more mature in technology. Taking the CP2126 of Qipan Microelectronics as an example, a typical series-type drive circuit is shown in Figure 1.
Taking CP2126 as an example, in general, the current flowing through each LED is the same in series, the uniformity of illumination is good; and because of its boosting principle, the voltage generated depends on when the LED conducts the specified current. The required voltage, the feedback voltage CS is internally set to 95mV, and it can be calculated that when 15mA LED current is required, the value of R1 should be:
Whether driving 2 LEDs or up to 5 LEDs, you can flexibly set the brightness of the LED by changing the resistance value of R1. Apply a certain duty cycle PWM control signal to the SHDN pin to make the brightness of the LED never Lights up to a stepless change between full brightness.
Two issues were noted in the design of the CP2126:
1. Avoid EMI interference problems
CP2126 avoids the EMI interference problem caused by the inductance and high current switch in the general series LED driver circuit. For applications such as MP3 and PMP, this problem may have little effect, but in mobile applications, EMI interference will The receiving sensitivity of the mobile phone is deteriorated. The CP2126 avoids this problem by optimizing the design of the internal circuitry. Table 1 compares the effects of the two series driver chips on the receiving sensitivity of the mobile phone under the same application conditions.
It can be seen that the operation of the CP2126 has little effect on the receiver sensitivity of the mobile phone. When the chip X is working, the receiving sensitivity of the mobile phone is reduced due to EMI interference. At the same time, the CP2126 can achieve 83% conversion efficiency under the typical application of inputting 3.6V to drive 3 white LEDs.
PCB design also has a large impact on circuit performance. In general, the traces of the high-frequency part should be as short and thick as possible, and the vias to the ground should be as large and large as possible. This is described in more detail in the relevant data of CP2126.
2. Built-in output open circuit protection circuit
In the application, there may be an open circuit fault condition of the LED. In this case, since the feedback voltage of the CS pin is always 0, if there is no protection circuit, the boost type circuit will continue to boost until the internal The switch tube is broken down and damaged. Therefore, a chip without an open-circuit protection circuit would require an external Zener diode to be used to protect the internal switch tube by its breakdown. The protection circuit is shown in Figure 2.
Obviously, such a protection circuit increases the cost of the system and the area of ​​the PCB. Another protection method is to add a pin and sample and test the voltage of VOUT in a package such as SOT23-6L. The CP2126 is designed to ensure that the peripheral application circuit does not need to be modified. The chip is still not damaged when the LED is open. When the fault condition is removed, the chip can work normally.
Parallel drive circuit
Although the series drive circuit has the advantage of high efficiency, the overall solution requires an inductor and a Schottky diode, which adds additional system cost, so that the final overall cost is not necessarily superior to parallel. At the same time, the size of the chip inductor is large, generally 5.2 × 5.2mm2 size, and there may be EMI interference.
Fixed mode parallel drive circuit
The early parallel drive circuit only solved the voltage problem required by the LED. It boosted the battery voltage through the charge pump to a fixed voltage of 5V or 4.5V, and then each LED passed a certain resistance value in series. To control the current of the LED.
A fundamental disadvantage of the charge pump circuit is that the conversion efficiency varies greatly with input voltage changes for a given output voltage requirement. In theory, the maximum efficiency that can be achieved by a double charge pump circuit is:
For example, when VIN=3.1V, VOUT=5V, the efficiency can reach 83.3%, and it can generally reach 80% or more due to the loss of internal devices. However, when VIN = 4.2V and VOUT = 5V, the theoretical efficiency is only 59.5%.
As can be seen from Figure 3, the CP2128 peripheral components only need three capacitors. Depending on the number of LED lamps to be driven, 1 to 5 resistors are required. Compared with the series driver circuit, although it has the disadvantage of inefficiency, the periphery The cost of components and the PCB area are relatively small, and it can be said to be a relatively low cost solution.
Mixed mode parallel drive circuit
In order to be able to compete with the series drive, the parallel drive circuit has to focus on two problems: one is efficiency and the other is current matching. In order to improve efficiency, it is very necessary to introduce a new working mode. In this case, the output voltage generated by the driver circuit is no longer a fixed value, but a voltage value that is appropriate and can drive the LED.
Generally speaking, when the working current is 20 mA, the forward voltage of the white LED is generally about 3.1 to 3.5V. The rated voltage of the lithium battery is 3.6V or 3.7V. The voltage after charging is generally 4.2V~4.3V. The lithium ion battery allows deep discharge to 2.7V. However, in practical applications, the forced shutdown voltage of the general mobile phone is 3.6. V or so (the forced shutdown voltage set by different machine manufacturers may be different). After the battery is fully charged, this voltage is sufficient to drive the LED directly. In this case, the charge pump circuit does not work. The battery voltage goes through a switch directly to VOUT and then drives the LED. As the battery is discharged, the battery voltage will gradually decrease. When the voltage is reduced to a certain extent to directly drive the LED, the charge pump circuit starts to work. Therefore, the integration of multiple drive modes has become the mainstream solution for backlighting, the so-called integrated 1x mode and 1.5x mode, and in the case of as much as possible, let the circuit work in 1x pass-through mode. Kai Pan's new CP2130/1/3 solves these two problems very well. The CP2130 can drive up to 5 LEDs, the CP2131 can drive up to 3 LEDs, and the CP2133 can drive up to 4 LEDs. Figure 4 is a schematic diagram of the mixed mode control.
Similarly, for system applications, the most important indicator is still efficiency and LED current matching. The so-called efficiency, working as much as possible in the 1x mode can significantly improve the power conversion efficiency of the entire battery operating voltage range.
Efficiency
For the efficiency problem of concern, the CP2130/1/3 implements two modes of adaptive dynamic switching (ie, based on the battery voltage and the actual conduction voltage drop of the LED), rather than a fixed voltage point switching (ie, only considering the battery voltage). This significantly increases efficiency. As long as the battery voltage is 350mV to 550mV higher than the operating voltage of the LED (depending on the pin settings of EN1 and EN2), the CP2130/1/3 can operate in a relatively efficient double mode.
Figure 5 shows the operating efficiency of the CP2130/1/3 for typical applications. It can be seen that the efficiency is a two-segment line chart according to the working mode. 80% of the battery energy is between 3.6V and 4.1V, and an average efficiency of more than 80% can be obtained in this voltage range.
2. Current matching
Compared with the series drive, an important problem to be solved by the parallel drive is the current matching between the LEDs. Since the brightness of the LED is related to its operating current, different currents may cause uneven brightness of the display. For parallel-driven LEDs, in practical applications, LEDs have different forward voltage drops under the same operating current due to batch and individual differences, and may have voltage differences of 50mV to 200mV. This difference needs to be considered when designing the internal current control circuit. The CP2130/1/3 adopts the Auto-Mirror technology applied for national patent protection, which can make the current matching degree of each parallel white LED almost change with the difference of the white LED conduction voltage drop. Even if the conduction voltage drop is between 50mV and 200mV, the current matching of the LED can still reach 2%.
Figure 6 shows a typical application. The CP2130/1/3 is packaged in a QFN 3*3mm2 16-pin package. The peripheral components are quite simple and require only four capacitors.
3. Support PWM dimming
At present, there are two main dimming methods. One is to adjust the brightness by changing the DC operating current of the LED. For example, some chips directly set the DC operating current of the LED by setting internal registers to achieve different brightness levels. The disadvantage of this approach is that color shifts may occur. The so-called white LED is actually a short-wavelength violet blue light emitted by a blue LED as its die, which excites the fluorescent material coated on the inner wall of the output optical lens, thereby generating a white composite light with a wide spectrum. In the case of non-rated current operation, the spectrum produced by the LED will change, resulting in a final color shift of white light.
Table 1: Comparison test of the influence of LED driver circuit on mobile phone signal receiving sensitivity
The other way is PWM dimming, which uses the human eye's visual pause principle to periodically control the on-current of the white LED to switch back and forth between zero current and rated operating current with a certain frequency and duty cycle. Adjust the brightness, this dimming method will not produce color shift. In application, in order to ensure that the LED cycle is not visible to the human eye, the frequency of PWM dimming is generally greater than 100Hz, CP2130/1/3 can support the dimming frequency range of 0~50kHz, which greatly facilitates the design of the system. . And because of the superior loop control characteristics of the chip, the smoothing process is ensured and the potential noise interference is eliminated.
At the same time, for the first dimming method, CP2130/1/3 also sets three current levels of full scale, 2/3 range and 1/3 range to realize DC dimming mode.
Summary of this article
This article compares the two commonly used backlight drive solutions. The series drive circuit has the advantage of high efficiency, but there are many external components, and the parallel drive circuit has been continuously developed, and has been changed from the original double charge pump mode voltage control type circuit to the adaptive mode selection plus current control circuit. Such a circuit has the advantages of a small number of peripheral components and a low overall cost, and also avoids the EMI interference problem that the serial drive circuit usually has. At present, there are more and more schemes for parallel drive circuits, and gradually become the mainstream solution for backlight drive circuits.
Author: Yu Xingzhi
Analog IC Design Project Manager
Qipan Microelectronics (Shanghai) Co., Ltd.
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