Smart watch EL boost system circuit

　　EL (Electroluminescence) luminescent screen is an electroluminescent material luminescent screen. Structurally, the electroluminescent material is sandwiched between two electrodes . Its upper electrode is a transparent conductive film called ITO film (Indium Tin Oxide film), which is used to transmit light, while the lower electrode is the second electrode. Its material is silver or carbon or aluminum electrode . The luminescent material EL is mainly There are zinc sulfide, calcium sulfide or strontium sulfide, and then mixed with other ingredients such as magnesium, samarium, europium or fluorescent dyes to adjust the brightness and color of the light. Changing the excitation frequency can also cause changes in the color of the light. Cold light backlight systems are mostly used to provide backlight for liquid crystal displays (LCDs). EL is often used in watches, and many watches have EL backlights. Usually, integrated EL driver circuits are used in watches. Because the integrated EL driver circuit is relatively simple, this article will not describe it. This article describes how to use the built-in cold light driver of the microcontroller to drive EL. As shown in Figure 1. It occupies two IOs of the microcontroller, among which ELP is used for charging and ELC is used for discharging (maintaining an alternating voltage). The charging circuit is a classic Boost circuit . The transistor can be any low-power transistor, and a switching transistor is better. Through the cyclic charging and discharging action, an alternating voltage is generated on the EL panel. Different voltage values ​​and frequencies can obtain various brightness and colors.

　　Circuit principle: The circuit needs to pay attention to R15. If this current limiting circuit is not added, the power consumption of the entire circuit will be very large. If the microcontroller does not have a cold light driver, it will use two timers to output square waves. The boosted voltage cannot be analyzed simply by referring to the Boost voltage. If analyzed according to the Boost voltage , the output voltage = input voltage/(1-duty cycle), and the calculated voltage is only 6V, which is obviously inconsistent with the actual result. It is necessary to consider the characteristics of EL, namely capacitive load. EL does not continuously consume power, but gradually stores energy while consuming power, so it cannot be converted with a general duty cycle. This circuit purely only takes high voltage. The setting of the duty cycle of ELP and ELC does not have a great impact on the voltage, but the setting of the duty cycle can make the EL light look more coordinated.