Device description and operation

PM8800

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Device description and operation

The PM8800 is a monolithic device embedding an IEEE 802.3af compliant PD interface

together with a current mode pulse width modulator to be used in all Power over Ethernet

Powered Devices.

In addition to the standard .3af features, PM8800 anticipates some features of the

forthcoming .3at standard, specifically targeting appliances or systems requiring higher

power with respect to the 12.95W allowed by standard PSE.

The PD interface integrates the 24.5k

Ω signature resistor used in detection and disabled

during the rest of operating modes. Classification is done through an external resistor

detached when classification is over, in order to save power.

The PM8800 integrates standard compliant UVLO thresholds to determine normal operating

mode (UVLO rising) or recognize disconnection (UVLO falling).

A graphical representation of the voltage thresholds and hysteresis during all the operating

phases is depicted in the following figure:

Figure 4.

State diagram of the PM800 interface depending on the input voltage

Note:

For input voltages in the range 1.5 to 11.5V, PM8800 exposes a 24.5k

detection is over, the internal resistor is disabled and the external classification resistor is

presented. When classification is over, the external resistor is disconnected and the

PM8800 wait for the input voltage to overpass the UVLO voltage.

The hot-swap MOSFET is specifically designed to have a low RDSON to contain the con-

duction losses and sustain up to 800mA. A constant dissipated power method is used to

limit the current in the in-rush phase. The integrated in-rush current limit controls in a safe

manner the current flowing through the MOSFET, shortening the duration of the hot-swap

event itself. Designers have the possibility to further limit the current in the in-rush by acting

on the proper programming resistor.

Designers have the possibility to set the limit of the current through the interface during nor-

mal operation. For non standard application, this limit exceeds the 350mA foreseen by the

802.3af and can reach up to 800mA.

1.5V

Classification

Detection

Powered

11.5V

10.3V

38.5V

31V

Input voltage

Wait for turn-on

23V

21V