Design of LED backlight drive scheme for medium-sized display screen

LED backlight is becoming a trend, pouring into people's lives. From the debut of LED backlight laptops in 2008 to the grand debut of LED backlight LCD TVs in 2009, they have attracted people's attention. For LED backlights of medium and large-sized displays, several major technical problems must be solved, among which the most direct problem is the consistency of the brightness of the light between LEDs. The number of backlight LEDs required for medium and large-sized displays can reach dozens to thousands. If the brightness of the LEDs is inconsistent, the backlight will be uneven, which will seriously affect the display effect.

We know that the brightness of LEDs is determined by the current flowing through them, so the way to ensure consistent brightness is to ensure that the current flowing through all LEDs is exactly the same. This means that when multiple LEDs are used for backlighting, the series connection method is more conducive to achieving consistent brightness than the parallel connection method. In this case, it is important to control the current accuracy of the LED string, which can ensure that the brightness of the backlight LED string is consistent between customer products.

But the number of LEDs connected in series cannot be too large. Under normal luminous brightness, the voltage drop across the white light LED is about 3.5V, which means that the voltage drop of 10 LEDs in series can reach 35V. For a 14" display, more than 40 LEDs are needed. If all are connected in series, the voltage drop will reach about 140V or even higher, not to mention larger displays. Therefore, for medium and large-sized displays, a completely series connection is unrealistic, and a combination of series and parallel connections must be used. When the system drives the backlight LED string group in this way, it may not be able to ensure the backlight effect just because each current meets the accuracy requirements. The current accuracy of ±1% means that the maximum difference between each channel can reach 2%. Unless the current accuracy index is raised to an exceptionally high level, the current difference between each channel must also be limited. Current matching is usually used to indicate the maximum difference level between multiple parallel outputs on the same system, and its definition is as follows:

Imatch = (Imax-Imin)/Iavg*100%

That is, the percentage of the difference between the maximum and minimum currents in each channel to the average value of each channel.

Because the LED backlight of medium and large-sized display screens has the above basic requirements, it is necessary to use a dedicated LED driver chip to help the system realize the drive management of the backlight LED, simplify the system design, and provide more control and protection functions. The PT4112 newly launched by China Resources Siwei Technology is such a product.

PT4112 Technical Features

PT4112 is a 6-channel white LED driver integrated circuit. Each channel can connect up to 10 white LEDs in series, so a single chip can drive up to 60 white LEDs. PT4112 not only has excellent current accuracy and current matching, ideal conversion efficiency, and a wide operating voltage range, but also integrates rich control functions and comprehensive protection functions. The main functions and technical features of PT4112 are as follows:

1. Ultra-high precision. Using patented design technology and advanced analog process, the current accuracy of PT4112 is as high as ±2%, and the current matching degree between the 6 circuits can be as high as ±1.5%. This is not only second to none among local original products, but also can fully meet the requirements of high-quality image applications such as high-end notebook computers.

2. High conversion efficiency. By adopting patented design technology and advanced analog process, the conversion efficiency of PT4112 can reach over 90%.

3. Wide operating voltage range. The operating voltage range of 5 to 25V allows engineers to easily select PT4112 in more applications without worrying about the impact of the operating voltage.

4. Support PWM brightness dimming, the PWM signal duty cycle range is 0 ~ 100%. Inputting the PWM signal to the BRT port of PT4112 can directly control the on and off of the LED light. By maintaining the circuit state during dimming shutdown, the AC ripple on the output capacitor can be kept small within a large PWM duty cycle range to reduce potential audio noise.

5. Complete protection functions. PT4112 has open circuit overvoltage protection, current limiting function by switching cycle, output circuit break protection, over-temperature protection, input voltage undervoltage lockout function, and soft start function. In addition, PT4112 also has a unique FAULT protection function. By providing a drive signal to the external PFET, just connect this PFET between the input and the inductor. When a serious error occurs in the system, this PFET can isolate the power supply (for portable applications, usually a lithium battery pack) and the WLED, thereby preventing the battery from being damaged due to large leakage current, making the entire system safe and reliable.

6. Unique circuit-breaking automatic brightness compensation function. This is a unique design of PT4112, an optional function, set through the IFB port of PT4112. When the IFB port is left unconnected, if one of the 6 outputs stops working, the current of the remaining 5 outputs will automatically increase by 20%. This function can be turned off by grounding the IFB port.

7. Good EMI characteristics. By adopting special design technology, the EMI of PT4112 is effectively suppressed.

Typical application circuit design and precautions

The PT4112 application circuit design method is as follows:

1. Current setting

The maximum current of each of the 6 output current paths can reach 25mA. The output current can be set by R1 connected to the Iset port. The required output current can be obtained by selecting different R1 resistance values:

R1 is the set resistance, the unit is KΩ, Iset is the required current, the unit is mA. The resistance
value corresponding to 20mA is 62KΩ.

2. Selection of inductor

The choice of inductor will be an important factor in determining whether the entire system can work stably. The characteristics of inductor mainly include three aspects: inductance, DCR and saturation current. The recommended inductor range for PT4112 is 4.7-10μH. If the size and package allow, it is recommended to use 10μH inductor, because such a selection system can obtain higher efficiency and smaller ripple. In addition, under the condition of certain output current limit, 10μH inductor can also help to obtain a larger output current. In the boost circuit, the calculation of inductance is usually calculated according to the following formula:

The peak-to-peak value of the inductor current ripple can be calculated as follows:

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Therefore, the peak value of the inductor current is:

In order to ensure stable operation of the system, the saturation current should usually be 1.5 times the peak current.

3. PWM dimming

PWM dimming can be achieved by applying a PWM chip to the BRT port of PT4112. PWM dimming is effective for all channels at the same time. PT4112 does not support PWM dimming for a single channel. The frequency of the PWM signal
is more suitable in the range of 100Hz to 1kHz, as shown in Figure 2.

4. PCB layout considerations

For high-frequency and high-current power supply systems, PCB layout is a very important link, so the following precautions should also be followed
:

1) The Iset terminal resistor should be as close to the chip as possible to effectively reduce the impact of noise on current setting;

2) The input capacitor Cin and the output capacitor Cout should be as close to the chip as possible to ensure that the system can work stably.

3) There will be a maximum current of 3 amperes on the routing from the input to the SW terminal and from the SW terminal to the output, so the routing needs to ensure the corresponding current density.