The shortcomings of the current electronic cigarette integrated control system MCU

The just-concluded IECIE vape exhibition has undoubtedly become the most popular industry event in the first half of 2019, with more than 1,500+ brands filling up 40,000 square meters of display space. Exhibitors and spectators from all over the world are immersed in this steam culture feast. The enthusiasm for heavy rains is also due to the increasing number of vape audiences. In order to increase market competitiveness, vape manufacturers are increasingly demanding control panels. As we all know, the electronic cigarette control system is mainly composed of the main control chip, battery, button, LED breathing lamp and atomizer. If E-liquid is the "soul" of vapes, then MCU is the "heart" of vapes. To become the leader in this industry, the choice of MCU is crucial.

Since the shape of the electronic cigarette is roughly the same as that of the real cigarette, the volume is small, and the number of components is relatively large. At the same time, it is generally powered by a small battery. The battery life should be as long as possible. Finally, the temperature control of the atomizer of the electronic cigarette should be precise, which requires the accuracy of the main chip ADC to be high. Therefore, these characteristics of vapes bring many challenges to developers.

First, short circuit protection

Vapes are products that are carried around and used by themselves. Safety is the first priority. During the use of electronic cigarettes, short circuits caused by accidents such as atomizer damage or accidental touch may be encountered. If not protected in time, battery explosions, fires and other safety accidents may occur.

In product design, there are often several ways to achieve short circuit protection:

1. Through the chip's own LVR (low voltage reset) function:

Principle: When a short circuit occurs, the battery voltage VBAT will be pulled lower than the LVR voltage, and the MCU will reset and stop working.

Disadvantages: Unreliable. When the MOS internal resistance is large and the battery load capacity is strong, VBAT cannot be pulled down to the LVR voltage, resulting in protection failure.

2. Short-circuit protection by detecting the MOS output voltage through the ADC

Principle: The ADC is used to detect the output voltage of the MOS and judge according to the voltage.

Disadvantages: slow response. The short-circuit protection function requires software to implement, and the response speed cannot be guaranteed. It is often necessary to add a protection IC on the battery side to improve the reliability of the short-circuit protection. Moreover, in the electronic cigarettes with a large amount of gold, increasing the protection IC and the voltage dividing resistor will increase the PCB area. 3. Short circuit protection through IO interrupt

Principle: When the MOS is turned on, if a short circuit occurs, the voltage of the GPIO is pulled low, and an interrupt is generated, which is protected by software.

Disadvantages: 1. Need to add a resistor and a diode to increase PCB area and cost. 2, the software is complex.

Second, resistance detection

During the use of vape products, there will be such a problem: when continuous smoking, E-liquid penetrates slowly and cannot penetrate into the atomizer in time, which will lead to too high atomization temperature, burnt smell, and poor user experience. It is impossible to automatically detect whether the E-liquid is used up or not, and it can only be observed by the human eye. In order to solve these problems, it is necessary to measure the working temperature by measuring the impedance of the atomizer.

The conventional method is to add a measurement path to calculate the impedance of the atomizer by measuring the voltage and current on the sampling resistor and further calculate the temperature during the half cycle of the working path.

Disadvantages:

1. It is necessary to add 2~3 resistors and 1 MOS, which is fatal in the electronic cigarette products of inch-inch gold, and increases the cost;

2. An additional 1 GPIO and 2 ADCs are required for a total of 3 pins, which requires a higher cost MCU.