Power over Ethernet (PoE) is a technology that allows network cables to deliver power to a powered device (PD) through power-sourcing equipment (PSE) or midspan, and has many advantages over traditional methods of delivering power. PoE allows power and data to be combined, removing the need for altering the AC mains infrastructure and can be installed by non-electricians. PoE is an intelligent system designed with protection at the forefront, preventing overload, underpowering, and installation errors, while allowing simple scalability and reliability.

The MAX17690 eliminates the need for an optocoupler or auxiliary transformer winding and achieves ±5% output voltage regulation over line, load, and temperature variations. The MAX17690 implements an innovative algorithm to accurately determine the output voltage by sensing the reflected voltage across the primary winding during the flyback time interval. By sampling and regulating this reflected voltage when the secondary current is close to zero, the effects of secondary-side DC losses in the transformer winding, the PCB tracks, and the rectifying diode on output voltage regulation can be minimized.

The MAX17690 also compensates for the negative temperature coefficient of the rectifying diode.

Key Features

• IEEE^® 802.3af/at Compliance
• Thermally Enhanced, 3mm × 3mm, 10-Pin TDFN Package (MAX5969B)
• 30V to 60V Input Voltage Range
• Programmable Switching Frequency from 50kHz to 250kHz
• Programmable Input Enable/UVLO Feature
• Programmable Input Overvoltage Protection
• Adjustable Soft-Start
• 2A/4A Peak Source/Sink Gate Drive Capability
• Hiccup Mode Short-Circuit Protection
• Fast Cycle-by-Cycle Peak Current Limit
• Thermal Shutdown Protection
• Space-Saving, 16-Pin, 3mm x 3mm TQFN Package
• -40°C to +125°C Operating Temperature Range

This board is not available for purchase.

Designed, Built, Tested

This document describes the hardware shown in Figure 1. It provides a detailed technical guide to designing a complete interface for a PD to comply with the IEEE 802.3af/at standard in a PoE class 3 system and an isolated no-opto flyback DC-DC converter using the MAX17690 controller and MAX17606 secondary-side synchronous MOSFET driver. The power supply has been built and tested.

Design Resources