The IEEE 802.3 PoE specification requires that power distribution devices (PDs) accept DC power supply voltages of any polarity at their Ethernet inputs. The space-efficient, low-power LT4321 active dual-bridge seamlessly rectifies and combines power from data and unused pairs to produce a single output with the correct polarity. This reduces circuit size and cost, as the increased power efficiency eliminates heat dissipation requirements. Power savings of 10x or more allow PDs to remain within PoE classification levels or add powerful functionality while maintaining their classification.
The LT4321's bias current, carefully designed to meet IEEE 802.3 standards, does not affect detection and classification. The controller, which operates with 2-pair or 4-pair Ethernet, is compatible with PoE, PoE+, and LTPoE++™ standards. An integrated charge pump provides gate control for the eight low-resistance N-channel MOSFETs without the need for external capacitors. Although designed for PoE PD, the LT4321's 20V to 80V operating range, with an absolute maximum of 100V, makes it suitable and robust for telecommunications applications using batteries or reversible power supplies. Dual polarity allows the pins to provide turn-off capability, reducing the bias current to 32µA from the 0.5mA operating current.
Specified for an ambient temperature range of −40°C to 125°C, the LT4321 is supplied in a compact 16-pin, 4mm x 4mm QFN package.
Main features: LT4321
• Low-loss replacement for dual diode OR bridges
• Drives eight N-channel MOSFETs
• Reduces heat generation for easier thermal design
• Maximizes available power and voltage
• Operates between 20V and 80V DC, 100V absolute maximum
• PoE/PoE+/LTPoE++ compatible
or Targeted at 2-pair and 4-pair PoE applications
or Does not affect IEEE 802.3 detection and classification
or Compliant with IEEE 802.3 when combined with a PD controller
• DC and OR polarity correction for telecommunications power supplies
• Quiescent current of 0.8mA (max.) and 60µA (max.) off current
• Guaranteed ambient temperature range between -40°C and +125°C
• 16-pin, 4mm x 4mm QFN package
