Small size LCD display module has already become an important part of handheld digital products. With the improvement of consumers’ visual requirements, the design of LCD display module becomes more and more important. How to display more information on the 1.8-inch to 2.8-inch LCD screen, improve the display quality and achieve better visual effect has become one of the key points of many handheld digital product designers. In addition to improving the resolution of LCD screen, reducing delay time and improving software, the backlight design of LCD screen also plays an important role.

In traditional design, small-size color LCD screen generally uses white LED as backlight. 1-4 white LEDs are mostly used for 1.8-2.8-inch LCD screens. In order to achieve good display effect, the brightness of white LED shall be consistent and the light and shade shall be adjustable; In order to protect the handheld equipment, it is required to have the protection function when some LEDs are open circuit, and the normal led operation cannot be affected; In order to prolong the standby time of handheld devices, the static current of LED driven IC is very small and the work efficiency is very high. In addition, handheld devices integrate many functional modules, so the interference between modules must be reduced to maintain the stability of the whole system, which is no exception for white LED drive IC.

The ap3605 introduced by BCD is an LED Driver IC with excellent performance. It is very suitable for single lithium battery power supply. It is suitable for most small-size LCD backlight designs with high cost performance. The following describes the performance, application scheme and characteristics of ap3605.

1、 Ap3605 introduction

Ap3605 is a white LED driving chip with constant current source in charge pump boost mode. Only through two 1uF ceramic capacitors, ap3605 realizes 1.5 times boost. It can fully drive four white LEDs within the working range of a single lithium battery, which greatly improves the efficiency compared with the IC in 2 times boost mode.

Ap3605 has four constant current sources, which ensures that the current of four LEDs in parallel is equal and the brightness is consistent; The current of the constant current source is set through an external resistance. Ap3605 supports PWM dimming, and the dimming frequency can be as high as 50KHz. The operating frequency up to 1MHz makes the ap3605 only need 1uF input and output and boost capacitor, and greatly reduces the output voltage ripple.

The power supply voltage of ap3605 is 2.7V to 5.5V. It is designed for single lithium battery. Its built-in soft start function greatly reduces the surge current during startup, protects the lithium battery and prolongs the battery life.

Ap3605 is packaged in qfn-3 * 3-16, and figure 1 is its internal function block diagram.

2、 Typical application circuit of ap3605 backlight

Figure 2 is a typical white LED driving circuit designed by ap3605. It is powered by a single lithium battery. Except ap3605, it is only composed of five low-cost peripheral devices.

1. Input, output and boost capacitors:

Ceramic capacitors of X7R or X5R are recommended for CIN, cout, C1 and C2. Since the working frequency of ap3605 is as high as 1MHz, 1uF capacitance is enough to ensure that ap3605 outputs 80mA current to drive four LEDs. If the required output current is smaller, boost capacitors (C1 and C2) of 0.68uf or 0.47uf can be used.

In order to reduce the output ripple, it is recommended that cout be close to the corresponding pin of ap3605, followed by C1, C2 and CIN.

2. Resistance RISET

RISET is used to set the current of LED. The current of each LED can be set through the formula iled = 36.9/riset. In order to accurately set the current, it is recommended to use a resistance of 1% or higher accuracy.

3. White LED

Ap3605 can drive up to 4 white LEDs, which are connected by common cathode. Customers can select 1 to 4 LEDs as backlight according to the screen size. When some LEDs are not needed, just hang their corresponding pins in the air.

3、 Ap3605 is an important performance of white LED Driver IC

1. Current matching

Current matching is an important performance of parallel LED Driver IC. LED is a current driven device. The brightness of LED is directly proportional to its forward current, and its forward voltage drop varies greatly under the same current. Therefore, in order to ensure uniform brightness, it is best to use constant current drive.

As shown in Figure 1, ap3605 is equipped with four constant current sources to drive four LEDs. When the forward voltage drop of the LED changes between 3V and 4V, ap3605 can ensure that the current deviation does not exceed ± 3%, and its current matching characteristics are not affected by the forward voltage drop of the LED. It is suitable for many types of white LEDs, which well realizes the consistency of LED brightness.

2. PWM dimming frequency up to 50KHz

PWM dimming is the most widely used dimming method. The brightness is adjusted by adjusting the time ratio of LED on and off. Because the dimming frequency is generally 200Hz or above, human eyes can not feel the LED on and off, but can only feel the brightness changing. The PWM dimming frequency often used in handheld digital products is 1kHz to 10kHz.

Using PWM dimming, it is not easy to cause color deviation and has good display effect. However, the common dimming frequency has the advantage that it is easy to produce noise in the audio range that can be heard by human ears. In addition, the low-frequency PWM dimming signal will sometimes lead to the “water ripple” phenomenon of CSTN screen, so it is best to improve the PWM dimming frequency.

Ap3605 realizes the dimming frequency of up to 50KHz through the internal standby module. And will not cause surge current. As shown in Figure 3, when the EN pin becomes low, the output Vout of the charge pump is delayed by about 1.7ms and then turned off, but the constant current source will turn off immediately following the en signal, and the LED current becomes 0; When the en signal is high, the constant current source will turn on immediately following the en signal. Since Vout still works normally, the LED current will immediately change to 20mA. Therefore, when the dimming frequency of PWM is higher than 1kHz, ap3605 can directly and quickly adjust the LED current through the en signal, so as to realize high-frequency PWM dimming.

The standby dimming mode of ap3605 has the following three advantages:

1) PWM dimming frequency up to 50KHz avoids audio noise.

2) Only the constant current source is switched, and the LED current is stable.

3) The LED current can quickly follow the PWM signal. The LED current is directly proportional to the PWM duty cycle, which is convenient for the linear adjustment of brightness.

3. Power interference reduction

As shown in Figure 1, there is an LDO module between the ap3605 internal charge pump and Vin. The module regulates the input voltage of the charge pump so that the charge pump always works in 1.5 times mode, reducing the output voltage ripple and LED current ripple.

In addition, the LDO module also plays the role of low-pass filtering, which effectively avoids the mutual interference between the charge pump and the input power supply. The backlight driving circuit of most handheld devices is directly powered by lithium battery, and the battery also directly powers the RF. In a poorly designed system, the switching noise of the backlight driving circuit will indirectly affect the RF part through the power supply. The built-in LDO module of ap3605 can effectively isolate the switching noise and avoid the influence of backlight driving circuit on RF part to the greatest extent.

4. Over temperature protection

Ap3605 has built-in over temperature protection module. If some misoperation causes the internal thermostat of ap3605 to rise to about 160 ℃, ap3605 will be over temperature shut off to protect ap3605 from burning. In the over temperature off state, ap3605 consumes less than 1uA of current. When the internal temperature of ap3605 drops to about 140 ℃ due to normal operation, ap3605 will automatically restart and enter the normal working mode.

4、 Summary

For the convenience of customers, BCD provides comprehensive technical support, including ap3605 data manual, application documents, demonstration board, samples and support for specific customer applications. As a white LED driver chip, ap3605 can fully meet the backlight needs of 1.8 to 2.8-inch LCD screen, and has won many customers for its high cost performance.

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