Linear Technology Design Note 361 Simple Battery Circuit Extends PoE Peak Current

About this manual

Power over Ethernet (PoE) allows delivery of power via standard CAT5 cable, but standards limit the maximum draw. This document addresses scenarios where a Powered Device (PD) requires more power than PoE provides, such as during disk drive spin-up or RF transmission bursts.

The proposed solution utilizes a rechargeable battery to store energy when consumption is low and release it during high-demand periods. The circuit employs the LTC4267 for PoE detection and regulation, and the LTC4055 for PowerPath control and Li-ion charging. This combination enables seamless switching between PoE power, battery power, or a parallel configuration to augment peak load current without delay.

What's inside

• Introduction
• The PoE Circuit
• PowerPath and Charger Circuit
• High Transient Load or Continuous Current
• Load Operation
• Optimization Options

Frequently asked questions

What is the maximum PoE current allowed by the standard before the circuit breaks?

The standard calls for the Power Sourcing Equipment to break the circuit if the PoE current exceeds 400mA.

What voltage and current does the LTC4267 supply in this design?

The LTC4267 circuit supplies 5V at 1.8A.

What is the maximum charge current limit for the Li-ion battery in this example?

In the provided circuit, the charge current is limited to 900mA.