Development trend of dot matrix LED driver

In mobile phone and home appliances and other civilian machine in the global competition environment, the personalized design has become an important factor. Therefore, the display and the device created by way of every hue used by people, especially recently with the development of led to small and thin parts, installed on the LED dot matrix display electronic display device, mobile phone and mobile phone keyboard shell plus the superb LED decoration, more and more electronic device the design of improved. In addition, in the field of white goods, to a considerable extent, mature Chinese and India in the market to the development of high-performance, many use the LED display and decoration to make people feel out of the ordinary in vision, so demand is expanding.

Development trend of LED driver

In the above context, the requirement for LED to provide current to the lighting of the decorative LED driver IC, can be more flexible lighting mode to control the number of more LED.

However, the past is used for LCD backlight of the constant current LED driver to take 1 of the 1 LED constant current circuit element structure, it is necessary to increase the number of LED lamps increases chip area, increase the cost, so often at a distance. However, in recent years, LED has made progress in the high brightness, with a small current value can also make the LED with enough brightness to light, and the constant current circuit itself does not increase. LED power (positive) side of the built-in PMOS transistor in turn to drive the time-sharing light, which can control the number of more LED (Figure 1). As shown in the figure, the traditional LED driver with 6ch constant current circuit to make 6 LED light, and dot matrix driver with 3CH SW circuit and 3CH constant current circuit can be separately on the 9 LED components for lighting control.

Figure 1 circuit structure of traditional LED driver and dot matrix LED driver

Control 119 LED components

In general, the human visual perception of the frequency of less than 100Hz, even at a faster speed LED flash, will also be seen as the ratio of its ON/OFF (= Duty) average brightness. It is called PWM (=Pulse Width Modulation) control mode, is widely used as a LED driver dimming mode. The constant current control by constant current circuit ON Duty can adjust the value of DC (brightness = LED) (for example, the constant current value is 20mA, the ON Duty was 50%, and the uninterrupted constant current through 10mA is the same). The dot matrix LED driver can be controlled separately by means of matrix connections and combinations in Figure 1, which can be used to control the LED components of a number equal to the SW circuit * constant current circuit (Figure 2). Dot matrix driver is connected to the positive and negative of the LED components into a matrix, the SW circuit on the positive side of the ON. The constant current circuit on one side of the negative electrode drives the constant current value of each LED element to be set by each ON sequence, and the LED light is switched on. ROHM successful development of the BD26502GUL built-in SW circuit 7ch, constant current circuit 17ch, a total of LED components can be controlled (Figure 3).

BD26502GUL with a period of about 4msec (= 250Hz) for time-sharing light, because the SW circuit has 7ch, so the lighting time of each LED component is the cycle of 1/7. Therefore, the average amount of LED current in time is equal to the constant current setting value of 1/7. For BD26502GUL type products, even at the maximum value of the constant current circuit set (= 20mA) 20mA/7 = 2.9mA calculation is so big, but the luminous efficiency of LED is improved, to ensure that there is sufficient for high brightness display and decorative purposes.

Figure 2. Example of working on a dot matrix LED driver (Figure 1)

Figure 3. The LED control circuit of BD26502GUL

BD26502GUL features

In addition, it is not only simple to make a number of LED lights, how to increase the control without the use of CPU load on the premise of achieving a "compelling" and "beautiful" lighting mode is also an important issue. BD26502GUL built-in 13 modes of tilt function and automatic scrolling of the 8 direction, do not increase the load to the CPU can achieve a rich performance of the LED lighting effect (Figure 4, 5). Also, also can be equipped with a double matrix storage of 7x17 data on RAM, and by the side of these alternate display data erasing edge, to achieve the dynamic effects such as animation.

The BD26502GUL has a control clock input and output port, and it can also work with multiple blocks of this kind of IC (Figure 6). Tilt and rolling function is to control the time oscillation circuit generates a clock built in, but the internal oscillation circuit because of the multiple IC deviation each in frequency, so can not form a light pattern completely uniform in time. By using the IC, the clock can be switched to the external input from the pin, and the same clock can be used to make the lighting timing and the tilt and rolling time synchronization between the multiple IC. As shown in Figure 6, taken from the clock source to each block of IC parallel input clock, or through the IC clock input / output ports are connected in series, can all use the same IC synchronization clock signal. Using this function, is the number of LED components to achieve 7 x 17=119 more than the application can also achieve full synchronization without the slightest bit of uncoordinated lighting control.

[figure 4] the tilt function of the 13 modes that can be set for each LED element (reducing the load of CPU)

[figure 5] example of BD26502GUL's rolling function

[figure 6] multiple