Efficient Power over Ethernet solution reduces overall cost

Power over Ethernet (PoE), defined by the IEEE 802.3at 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 allows for the flexibility to install devices anywhere, without the need for nearby AC power and without the need for an electrician to perform installation. The original IEEE 802.3af PoE specification limited the power delivered to a powered device (PD) to 13W, which limited the range of applications for devices such as IP phones and basic security cameras. In 2009, the IEEE 802.3at specification increased the supported power to 25.5W. However, this still does not meet the needs of increasingly power-hungry PoE applications 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++™, which extends the PoE and PoE+ specifications to 90W power supply 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) adopts 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 operation. The use of external FETs 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 100m CAT-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, protect the Hot Swap™ 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 PD application circuits.

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 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 the isolation function without any optoisolators or dedicated isolated DC/DC converters

Reliable cable discharge protection function

It is important to consider the reliability of PoE designs, especially when dealing with large numbers of cables, high voltages, high currents, or high temperatures. Linear Technology has extensive experience in this area and has designed low-cost, high-throughput circuit protection solutions that can be flexibly adjusted to meet IEC61000 cable discharge voltage requirements. Only one TVS is required to protect the high-voltage analog power supply, and a pair of low-cost clamping diodes are used on each output port (Figure 2). The diodes on the ports direct harmful surges into the power rails, where they are absorbed by the surge suppressor and VEE bypass capacitors. The surge suppressor also has the advantage of protecting the PSE controller from VEE power supply transients. Linear Technology's PSE controllers also have 80V absolute maximum rating limits on all analog pins to protect against transients. 

Figure 2: Reliable cable discharge protection

Reduce power consumption

Linear Technology's fourth generation PSE and PD controllers are fully compliant with the IEEE 802.3at specification and offer LTPoE++ power up to 90W while reducing heat dissipation by using low RDS(ON) external MOSFETs and 0.25Ω sense resistors. This is important for high power systems where the cost of thermal design and power loss is very high, and for power constrained applications where the operating power is maximized within the power budget. PSE and PD controllers with integrated MOSFETs have high RDS(ON) parameters, making thermal design difficult because the heat is dissipated inside the device. Damage to one port can damage the entire chip.

The LT4275 (Figure 3) is the only PD controller on the market that can control an external MOSFET, greatly reducing the overall heat dissipation of the PD and improving power efficiency, which is very important for higher power applications. This innovative approach allows the user to adjust the MOSFET to meet the thermal and efficiency requirements of the application, allowing the use of low RDS(ON) MOSFETs on 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 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 signaling between the PSE and PD while maintaining backward compatibility with the IEEE 802.3at 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, the 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 limits powering to Class 1.

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 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 migrates 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 LTPoE++ capable. 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 IEEE 802.3at standard requires that a compliant Class 2 PD 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 powers.

LTPoE++ provides a safe and reliable plug-and-play solution, greatly reducing the engineering complexity of PSE and PD. The advantage of LTPoE++ over other power extension topologies is that only one PSE and PD are needed to provide 90W of power on a CAT-5e cable, which greatly saves space, reduces costs, and shortens development time. The LTPoE++ solution reduces the cost of materials and related components, and also provides the most efficient end-to-end solution currently available, significantly reducing the total cost of ownership, increasing the power supply for practical applications, while reducing heat dissipation and not requiring a high-cost heat sink design.

The most striking point about LTPoE++ is that it does not require the Link Layer Discovery Protocol (LLDP) defined by the IEEE PoE+ specification for software-level power negotiation. LLDP requires extensions to the standard Ethernet stack and requires a significant software development effort. LTPoE++ PSE and PD autonomously negotiate power requirements as well as hardware-level capabilities while maintaining full compatibility with LLDP-based solutions. This gives LTPoE++ system designers the choice of whether to implement LLDP. Dedicated end-to-end systems may choose to forgo LLDP support. This provides time-to-market advantages, as well as reduced BOM cost, board area, and complexity.

Advanced features of the fourth generation

Linear Technology's family of Ethernet Power-over-Power PSE controllers is very mature and professional in PoE experience, 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 1MHz I2C 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, as well as rugged ESD and cable discharge protection, reducing component count and enabling cost-effective designs. Combined with the LT4275 PD 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 PD heat dissipation and improve power efficiency, which is critical for all power levels. High absolute maximum voltage ratings on 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, helping system designers focus their design efforts on high-value applications.