Pushing The Boundaries Of PoE

Power over Ethernet, or PoE, has been increasing in popularity due to its flexible and cost-effective method of delivering both power and data over a single Ethernet cable. This allows equipment to be installed almost anywhere, without the constraint of AC-power proximity or requiring installation by an electrician. The original IEEE 802.3af PoE specification limited the power delivered to the Powered Device to just 13W, which in turn limited the scope of applications to devices such as IP phones and basic security cameras. In 2009, the IEEE 802.3at specification increased this available power to 25.5W. However, this was still not enough to satisfy the growing number of power hungry PoE applications, such as picocells, 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 of delivered power, while maintaining 100% interoperability with the IEEE PoE standards.

LTPoE++ provides a safe and robust plug-n-play solution that dramatically reduces engineering complexity in power sourcing equipment (PSEs) and powered devices (PDs). The benefit of LTPoE++ over other power-extending topologies is that only a single PSE and PD is required to deliver up to 90W over 4 pair CAT5e cable, resulting in significant space, cost and development time advantages. Four different power levels are available (38.7W, 52.7W, 70W, 90W), allowing the power supply to be sized according to the application's requirements.


Figure 1. Typical PoE System

PoE At Work

Before a PSE is allowed to apply power to the line, it must check for an IEEE-mandated signature resistance with a power-limited probing source. To be considered a valid signature, the PD must look like 25kΩ ±5% in parallel with 120nF or less of capacitance. The PSE, in turn, must accept a somewhat wider range of 19kΩ to 26.5kΩ to account for parasitic series and parallel resistances in the system (Figure 2). The PSE must reject anything below 15kΩ or above 33kΩ, or anything with >10µF across its terminals.

The PD signature impedance is allowed to have a voltage offset of up to 1.9V (typically caused by up to two diodes in series), and a current offset of up to 10µA (typically caused by leakage in the PD). These terms complicate the PSE resistance measurement, since a single V-I point measurement will not account for these errors. As a result, the PSE is required to take at least two different V-I points, separated by at least 1V at the PD.

It then must calculate the difference between the two points to find the true resistive slope, subtracting out voltage and current offsets. Since CAT-5 cable is typically run in ceilings, walls, and other spaces where AC wiring is also present, 50/60Hz noise can be significant. Linear Technology PSE controllers handle this by using a proprietary dual-mode, four- point detection method which ensures optimum immunity from false positive or negative PD detection.


Figure 2: IEEE 802.3af Signature Resistance Ranges

PD Classification

Once the PSE has successfully detected a PD, it performs the power classification step. The PSE must keep track of how many PDs are connected and what their power classification levels are, and stop accepting PDs when its power budget is exhausted.

The classification signature is checked by forcing between 14.5V and 20V across the PD and measuring the current drawn by the PD. The PSE uses the measured current to determine which class the PD falls into.

LTPoE++ uses a 3-event classification scheme to provide mutual identification handshaking between the PSE and PD while maintaining backwards compatibility with the IEEE 802.3at standard. The LTPoE++ PSE determines if a PD is a Type 1 (PoE), Type 2 (PoE+), or LTPoE++ device by the PD response to the 3-event classification scheme. The LTPoE++ PSE uses the 3-event classification scheme result to update the ICUT and ILIM thresholds.

The PSE uses the ICUT threshold to police the PD current consumption. ILIM is used as a hard current limit to protect the PSE power supply during serious current faults.

On the other end, the LTPoE++ PD uses the number of classification events it receives to determine whether it is connected to a Type 1, Type 2, or LTPoE++ PSE. If the LTPoE++ PSE measures the PD’s 1st classification event current as Class 0, Class 1, Class 2, or Class 3, the LTPoE++ PSE will proceed to power on the port as a Type 1 device. Otherwise if Class 4 is identified in the 1st classification event, the LTPoE++ PSE will continue with a 2nd classification event, as defined in the PoE+ specification. This informs the PD that it is connected to either a Type 2 or LTPoE++ PSE. The absence of the 2nd classification event indicates the PD is connected to a Type 1 PSE that is limited to Type 1 power.

The Type 2 PD physical layer classification is defined by IEEE as two consecutive Class 4 results. An LTPoE++ PD must also display two consecutive Class 4 results in the 1st and 2nd classification events, making an LTPoE++ PD appear as a Type 2 PD to a Type 2 PSE.

The LTPoE++ PSE will move on to the 3rd classification event after valid Class 4 measurements in the 1st and 2nd classification events. After two successful Class 4 measurements a 3rd classification event is performed. The 3rd classification event must switch to a class other than Class 4 to recognise the PD as LTPoE++ capable. A PD that maintains Class 4 during the 3rd classification event is considered by the LTPoE++ PSE to be a Type 2 PD. The IEEE 802.3at standard requires compliant Type 2 PDs to repeat Class 4 responses for all class events. The 3rd classification event informs the LTPoE++ PD that it is connected to an LTPoE++ PSE. Table 1 shows the class events permutations for the various PD power levels.

DC Disconnect

Just as a PSE must only send power to valid PD, a PSE also must not leave power on after the powered device has been unplugged because a powered cable could subsequently be plugged into a device that doesn’t expect power. LTPoE++ uses the DC disconnect method to determine the absence of a PD based on the amount of DC current flowing from the PSE to the PD. When the current stays below a threshold IMIN (between 5mA and 10mA) for a given time tDIS (300ms to 400ms), the PSE assumes that the PD is absent and turns off the power.

Once a PSE has successfully detected and classified a PD, it then makes the decision whether to power it on. If the PSE's available power is adequate to power the PD, the PSE powers on the PD and begins monitoring the port for the DC disconnect condition.

The PSE now has the whole picture: the detection sequence tells it that there is a real PD attached to the port; the classification routine tells it how much power that PD will draw so it can allocate its power supply resources accordingly; and the DC disconnect method tells it that the PD is still present and operating normally. The PD, in turn, has a straightforward way to communicate to the PSE what it is, how much power it wants, and whether or not it wants that power to keep flowing. All of this occurs without affecting the data stream in any way.

One important distinction with LTPoE++ is that it does not require the use of the Link Layer Discovery Protocol (LLDP) that was mandated in the IEEE PoE+ specification for software-level power negotiation. LLDP requires extensions to standard Ethernet stacks and can represent a significant software development effort. LTPoE++ PSEs and PDs autonomously negotiate power level requirements and capabilities at the hardware level while remaining fully compatible with LLDP-based solutions. This gives LTPoE++ system designers the choice to implement or not implement LLDP. Proprietary end-to-end systems may choose to forgo LLDP support. This creates time-to-market advantages while further reducing BOM costs, board size and complexity.

Linear Technology offers single, quad, octal and 12-port LTPoE++ PSE controllers with lowest-in-industry power dissipation, robust ESD and cable discharge protection, low component count and cost effective designs. When paired with the LT4275 PD controller (Figure 3), a complete plug-n-play LTPoE++ system (no LLDP required) can deliver up to 90W while remaining fully compatible with PoE+ and PoE standards. The entire solution uses external low RDS(ON) MOSFETs to drastically reduce overall PD heat dissipation and maximise power efficiency, which is important at all power levels. High abs max ratings on all analog pins and cost effective cable discharge protection ensure the devices are well protected from the most common Ethernet line surges. In summary, LTPoE++ systems simplify power delivery, allowing system designers to concentrate their design efforts on their high value applications.


Figure 3: LT4275 90W PD Controller Uses External MOSFET For Increased Power Efficiency