December 02, 2019 by Sam Holland
Analog Devices’ DC 2911A evaluation board, which features the LT4321 PoE (Power over Ethernet) ideal diode bridge controller and the LT4293 PD (Powered Device) interface controller, is an IEEE 802.3bt-compliant and LTPoE++ (Linear Technology Power Over Ethernet++)-interoperable PoE Powered Device.
The LT4321 provides control for eight low RDS(ON) N-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) to both ease the thermal design and optimise the efficiency of end-to-end power delivery. Combining it with eight N-channel MOSFETs can replace the eight diodes required in a typical passive PoE bridge rectifier. The LT4321 maximises available voltage while also minimising power dissipation.
The LT4293, on the other hand, provides IEEE 802.3bt and LTPoE++-compliant interfacing. The interface controller supports the LT4320/LT4321 ideal bridges and uses a low RDS(ON) N-channel MOSFET for hot swapping (the replacement of a device while its corresponding computer system remains in operation), which helps to improve product efficiency.
Analog Devices’ DC2911A evaluation board. Image Credit: Analog Devices.
The demonstration circuit 2911A can accommodate up to 90W of power supply from power sourcing equipment (PSE) through a Registered Jac-45 (RJ45) connector. It is also compatible with local 48V DC power supply through its auxiliary input; however, when both supplies are connected, auxiliary supply input dominates the Power over Ethernet input.
The LT4293 provides an integrated signature resistor, under-voltage lockout, surge protection, power good (PWRGD) output, and overtemperature protection. It features an external Hot Swap N-Channel MOSFET for increased efficiency and lower power dissipation.
This PD interface controller supports auxiliary power override through the AUX pin. An external capacitor allows adjustment to the required start-up inrush current. PWRGD output, which can enable an isolated power supply, hints that the LT4293 is providing power to the downstream circuitry. The Trust-to-Trust protocol (aka T2P) output differentiates between IEEE 802.3bt and LTPoE++ PSE; plus, this signal communicates the power allocated from IEEE 802.3bt or LTPoE++-compliant PSEs to the downstream load.
Easy migration between IEEE 802.3bt and LTPoE++-compliant PDs is possible via pin-for-pin compatibility with the LT4294 and LT4275A/B/C family of PD interface controllers. The LT4293 is configurable to support all possible power levels for 802.3af/at/bt with external component changes. The LT293 has a −40 to 125°C junction temperature range and 5-Event Classification Sensing and can differentiate between LTPoE++ and IEEE 802.3bt PSEs.
The LTF321 supports PoE, PoE+, and LTPoE++ and works with 2-pair and 4-pair PoE applications (‘pair’ here referring to ‘pairs of Ethernet wires’). It is IEEE 802.3-compliant when paired with a PD controller and eases thermal design challenges—allowing for maximum power efficiency and power delivery.
This integrated bridge controller has less than 800μA quiescent operating current and high impedance pins to prevent reverse current. The LT4321 features an internal charge pump to substitute for larger and expensive P-type metal-oxide-semiconductor (PMOS) switches. A fast turn-off reduces reverse current transients in case the source of power is shorted (or fails outright).
The LT4321 can be utilised in PoE/PoE+/LTPoE++-powered devices and for DC polarity correction. The LT4293 is ideal for high-temperature applications, such as outdoor security cameras, high-power wireless data systems, and commercial and public information displays.
With both components in place, the user has a versatile board that can be installed on voice over internet protocol (aka VoIP) phones, internet protocol cameras, and wireless access points.
