As the aging population in China continues to accelerate, there is a rising demand for prevention of chronic diseases and health management. For example, patients with cardiovascular diseases may experience difficulty in oxygen supply due to factors such as blood viscosity, coronary artery sclerosis, or narrowed blood vessels. Additionally, respiratory system diseases may cause breathing difficulties, leading to insufficient oxygen intake for patients. If severe hypoxia occurs and timely oxygen supplementation is not received, there could be a risk to life. Therefore, such populations require long-term monitoring of blood oxygen levels to prevent dangerous situations. The fingertip pulse oximeter is a household health device with a "preventive" attribute.

The measurement principle of the fingertip pulse oximeter is based on the different absorption capacities of reduced hemoglobin (Hb) and oxygenated hemoglobin (HbO2) for red and infrared light. By irradiating the fingertip with two beams of light of different wavelengths (red and infrared) and obtaining measurement signals through a photosensitive component, the blood oxygen saturation value can be analyzed.

According to the schematic diagram of the fingertip pulse oximeter, the application of MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) is crucial, and the requirements for MOSFETs vary among different modules.

Analog switches are used to switch red and infrared LED lights. The LED lights emit currents up to tens of mA, so the analog switch must have low on-resistance characteristics. The red and infrared LED lights are driven by constant current circuits composed of general-purpose operational amplifiers and transistors, ensuring high current accuracy and low temperature drift. On the photosensitive receiver side, operational amplifiers are used to achieve photoelectric conversion, and low-noise operational amplifiers are typically chosen to improve system resolution. In terms of power topology, one method is to use two dry cells for power supply, stepping up through a DC-DC converter, and then stepping down through an LDO (Low Drop-Out) regulator for various circuits in the system; another method is to use lithium batteries for power supply, allowing for cyclic charging and use.

For circuits using low-voltage MOSFETs, the VB2290 model in SOT23-3 package is a suitable choice. This model is particularly suitable for portable electronic products powered by batteries.