Methods to eliminate LED flicker caused by fluctuating AC input

    Power over Ethernet (PoE), as defined by the IEEE802.3at specification, is a method of securely transmitting application data and power over a single CAT-5 Ethernet cable . It has gained widespread adoption because it supports the flexibility to install devices anywhere, without the need for nearby AC power and without the need for electricians to install them. The original IEEE802.3af PoE specification limited the power delivered to powered devices (PDs) to 13W, which limited the range of applications for devices such as IP phones and basic security cameras. In 2009, the IEEE802.3at specification increased the supported power to 25.5W. However, this still did not meet the needs of PoE applications with increasing power requirements, such as picocell base stations, wireless access points, LED signage, and heated pan-tilt-zoom (PTZ) outdoor cameras. In 2011, Linear Technology released a new proprietary standard, LTPoE++™, that extends the PoE and PoE+ specifications to 90W power delivery while maintaining 100% compatibility with the IEEE PoE standard. It supports four different power levels (38.7W, 52.7W, 70W, 90W) to adjust the power supply according to application requirements.

    LTPoE++ power supply equipment (PSE) uses a smarter PSE isolation architecture to reduce the number of components and minimize the use of expensive external components. Comprehensive cable discharge protection and 80V absolute maximum pins ensure high reliability in field work. The use of external FET enables heat dissipation performance to meet application requirements, improves system efficiency, and enhances long-term reliability. The LTPoE++ architecture only requires one PSE and PD controller to provide 90W of power through 4 pairs of 100mCAT-5e cables.

    System Isolation Requirements

    Implementing Power over Ethernet requires careful selection of architecture and components to reduce system cost while improving performance and reliability. A successful design must meet IEEE isolation requirements to protect the HotSwap™ FET during short circuit and overcurrent events, or comply with IEEE specifications. The PoE specification clearly states the isolation requirements to ensure that ground loops are broken, Ethernet data integrity is maintained, and noise is reduced in the PD application circuit.

    Traditional PSE isolation architecture isolates the digital interface and power supply at the host to PSE controller interface. Digital isolation units such as optocouplers are inherently expensive and unreliable. ICs that can implement isolation functions are very expensive and do not support I2C high-speed transmission. In addition, isolated DC/DC converters that power the PSE logic increase board area and system cost.

    Easy isolation

    Linear Technology's 12-port (LTC4270/LTC4271) and 8-port PSE (LTC4290/LTC4271) chipsets take a different approach to PSE isolation, migrating all digital functions to the host side of the isolation boundary (Figure 1). This greatly reduces the cost and complexity of the required components. A separate isolated DC/DC power supply is no longer required; the LTC4271 digital controller can use the host's logic power supply. The LTC4271 uses a transformer-isolated communication method to control the LTC4290 or LTC4270. Low-cost and widely used Ethernet transformer pairs can replace six optocouplers. An I2C communication mechanism with port management, reset, and fast port shutdown functions is programmed into the protocol, reducing radiated energy and providing 1500V of isolation.

Figure 1: The LTC4290/LTC4271 chipset implements isolation functionality without any optoisolators or dedicated isolated DC/DC converters

    Reliable cable discharge protection function

    考虑PoE设计的可靠性非常重要，特别是处理大量的电缆、高电压、大电流或者高温的情况。凌力尔特公司在这方面经验丰富，设计了低成本、大吞吐量的电路保护方案，能够灵活的调整满足IEC61000电缆放电电压要求。只需要一个TVS来保护高电压模拟电源，而在每一个输出端口上采用一对低成本箝位二极管（图2）。端口上的二极管引导有害的浪涌进入电源轨中，它们被浪涌抑制器以及VEE旁路电容吸收掉。浪涌抑制器还有保护PSE控制器不受VEE供电瞬变影响的优点。凌力尔特的PSE控制器在所有模拟引脚上还有80V绝对最大额定限制，实现了对瞬变的保护。

Figure 2: Reliable cable discharge protection

    Reduce power consumption

    Linear Technology's fourth-generation PSE and PD controllers are fully compatible with the IEEE802.3at specification, and the LTPoE++ power reaches 90W, while reducing heat dissipation by using low RDS(ON) external MOSFETs and 0.25Ω sense resistors. This is very important for high-power systems, where the cost of heat dissipation design and power loss is very high, and for power-constrained applications, which require the maximum operating power within the power budget. PSE and PD controllers with integrated MOSFETs have high RDS(ON) parameters, and since the heat is dissipated inside the device, it is difficult to perform heat dissipation design. Damage to one port will cause damage to the entire chip.

    The LT4275 (Figure 3) is the only PD controller on the market that can control an external MOSFET, greatly reducing the total heat loss of the PD and improving power efficiency, which is very important for higher power applications. This innovative approach allows users to adjust the MOSFET to meet the thermal and efficiency requirements of the application, supporting the use of low RDS(ON) MOSFETs in the order of 30mΩ. The LT4275 can support up to 90W of power.

    A TVS and 100V absolute maximum port pins are sufficient to protect against cable discharge events. The LT4275 operates over a wide -40°C to 125°C temperature range and has an overtemperature protection function to protect the device during instantaneous overloads. With this more robust protection function, it is easy to experience reliable applications.

Figure 3: LTPoE++ PD controller uses external MOSFETs to improve power efficiency

    How LTPoE++ works

    LTPoE++ uses a 3-event classification mechanism to provide mutual identification signals between PSE and PD while maintaining backward compatibility with the IEEE802.3at standard. Through the PD's response to the 3-event classification mechanism, the LTPoE++PSE determines whether the PD is a Class 1 (PoE), Class 2 (PoE+), or LTPoE++ device. The LTPoE++PSE uses a 3-event classification mechanism to cause the ICUT and ILIM thresholds to be updated. The PSE uses the ICUT threshold to monitor the PD current consumption. In the event of severe current faults, ILIM is used as a hard current limit to protect the PSE power supply.

    On the other end, the LTPoE++ PD uses the classification event number it receives to determine whether it is connected to a Class 1, Class 2, or LTPoE++ PSE. If the LTPoE++ PSE measures the PD's first classification event current as Class 0, Class 1, Class 2, or Class 3, the LTPoE++ PSE powers the port as a Class 1 device. Otherwise, if Class 4 is identified in the first classification event, the LTPoE++ PSE proceeds with the second classification event defined by the PoE+ specification. This tells the PD that it is connected to a Class 2 or LTPoE++ PSE. The absence of a second classification event indicates that the PD is connected to a Class 1 PSE, which is limited to Class 1 powering.

    Class 2 PD physical layer classification is defined by the IEEE as two consecutive Class 4 results. An LTPoE++ PD must show two consecutive Class 4 results in the first and second classification events for the LTPoE++ PD to present itself as a Class 2 PD to a Class 2 PSE.

    In the first and second classification events, after a valid Class 4 measurement, the LTPoE++ PSE will migrate to the third classification event. After two successful Class 4 measurements, the third classification event is performed. The third classification event must transition to a level other than Class 4 to identify the PD as supporting LTPoE++. During the third classification event, the LTPoE++ PSE considers the PD that maintains Class 4 to be a Class 2 PD. For all classification events, the IEEE802.3at standard requires that compliant Class 2 PDs repeat the Class 4 response. The third classification event tells the LTPoE++ PD that it is connected to the LTPoE++ PSE. Table 1 shows the classification event combinations for various PD power levels.

　　LTPoE++ Plug and Play Solution

    LTPoE++提供安全和可靠的即插即用解决方案，极大的降低了PSE和PD的工程复杂性。LTPoE++相对于其他电源扩展拓扑的优点在于只需要一个PSE和PD便能够在一条CAT-5e电缆上提供90W功率，大幅度节省了空间，降低了成本，缩短了开发时间。LTPoE++解决方案减少了材料和相关的元器件成本，还提供目前功效最高的端到端解决方案，显著降低了总体拥有成本，增大了对实际应用的供电，同时降低了热耗，不需要高成本的散热器设计。

    The most striking point of LTPoE++ is that for software-level power negotiation, it does not require the use of the Link Layer Discovery Protocol (LLDP) specified by the IEEEPoE+ specification. LLDP requires extension of the standard Ethernet stack and requires a large software development investment. LTPoE++ PSE and PD autonomously negotiate power requirements and hardware-level capabilities while maintaining full compatibility with LLDP-based solutions. This allows LTPoE++ system designers to choose whether to implement LLDP. Dedicated end-to-end systems may choose to forgo LLDP support. This brings a time-to-market advantage, while also reducing BOM costs, board area and complexity.

    Advanced features of the fourth generation

    Linear Technology's Ethernet Power Supply PSE controller series is very mature and professional in PoE experience, and has been supported by more than 200 million ports shipped. New fourth-generation features include field-updated firmware that supports future-proof designs. Optional 1-second current averaging is another new feature that simplifies host power management functions. Advanced power management features include priority fast shutdown, 12-bit per-port voltage and current readback, 8-bit programmable current limit, and 7-bit programmable overload current threshold. A 1MHzI2C interface supports the host controller to digitally configure the IC or perform queue port read operations. A "C" library is provided to reduce engineering costs and enable products to be listed as soon as possible.

    in conclusion

    Linear Technology offers the industry's lowest power single-port, 4-port, 8-port and 12-port PSE controllers, and also provides robust ESD and cable discharge protection, reducing the number of components and enabling cost-effective designs. Combined with the LT4275PD controller, a complete plug-and-play LTPoE++ system is capable of delivering 90W of power while maintaining full compatibility with PoE+ and PoE standards. The entire solution uses external low RDS(ON) MOSFETs to greatly reduce the total heat dissipation of the PD and improve power efficiency, which is critical for all power levels. The high absolute maximum voltage ratings of all analog pins and cost-effective cable discharge protection ensure that the device is well protected from the most common Ethernet voltage surges. The LTPoE++ system simplifies power delivery and helps system designers focus their design efforts on high-value applications.