Method for realizing voltage detection based on TL431 simple circuit

Voltage detection is a critical function in fiber-optic communication equipment, serving as a key source of information for power-off warnings. This enables the system to alert the background server or network management when power is lost. While dedicated voltage detection chips are available, their use increases the cost of communication devices. As a result, there is a strong demand for more cost-effective and reliable alternative circuits that can reduce production costs and enhance the global competitiveness of such products. **1. Fiber Optic Communication Equipment** **1.1 Introduction to FTTH (Fiber to the Home)** Fiber to the Home (FTTH) refers to a broadband communication system that delivers high-end services like triple play, internet, and television to homes or businesses using fiber optic cables and optoelectronics. This technology is commonly abbreviated as FTTH. There are two primary wiring structures in FTTH: point-to-point and point-to-multipoint. In a point-to-point setup, a single fiber connects the central office directly to each user. In contrast, a point-to-multipoint structure uses a Passive Optical Network (PON), which reduces the number of optical transceivers and fibers required, thus saving space and lowering costs. **1.2 Fiber Optic Communication Layout** Based on the point-to-multipoint PON solution, the following diagram illustrates a typical passive optical network layout. In this configuration, an Optical Line Terminal (OLT) manages multiple end users (ONUs). The OLT is typically located in a central equipment room, while each optical module is placed at the user’s location. Data from all ONUs is aggregated at the OLT before being sent to the core switch and eventually to the broader network.  *Figure 1* **2. Circuit Design** **2.1 Voltage Detection Chip: MAX708** **Overview** The MAX708 is a reset control chip manufactured by Maxim Integrated. It features a microprocessor-based design capable of generating two types of reset signals—active high and active low. The chip can trigger a reset through three different methods: an external comparator, VCC voltage monitoring, or a manual reset input. This chip includes an internal voltage comparator that monitors an external input voltage signal. It outputs high or low levels depending on the comparison result and provides a voltage drop alarm for other CPU processors. It is widely used in power switching systems where early warning mechanisms are necessary, such as sending alerts to operators during power failures. The MAX708 is available in SO-8 or TSSOP-8 packages and offers three selectable threshold voltages. **Performance** - Reset signals: RESET (high) and RESET# (low) - Power failure/low-voltage warning threshold: 1.25V - Manual reset input option - Supply current: 100µA - Optional threshold values: 2.63V, 2.93V, 3.08V - Valid RESET signal output at VCC = 1V A typical application circuit is shown in Figure 2, with the internal equivalent circuit illustrated in Figure 3. Since it is similar to the MAX706, the application and internal schematic are represented based on the MAX706’s design.  *Figure 2*  *Table 1: Pin Definition* From the equivalent circuit (Figure 3), when the PFI input voltage exceeds 1.25V, the /PFO output goes low. If the PFI voltage drops below 1.25V, the /PFO output becomes high. The main board supplies a DC 12V input, which is divided to provide approximately 1.4V to the PFI pin. When the 12V power is cut off, and it drops to 10V, the PFI voltage reaches 1.25V, triggering the internal comparator. This causes the /PFO output to go low, signaling the processor. Even as the voltage continues to drop from 10V to 5V, the motherboard’s power supply ensures continued operation. During this time, the /PFO signal is processed immediately, allowing the system to issue a timely warning.

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