STE12PS datasheet(11/44 Pages) STMICROELECTRONICS

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STE12PS

Functional description

11/44

By default, the STE12PS will recognize a valid signature with the following characteristics:

–

an inverse slope of the port current vs. voltage (I-V) characteristic measuring

between 19 and 26.5 k

Ω (Rdl and Rdh),

–

a port capacitance of less than 4µF.

If required, the STE12PS can also perform a custom, resistive detection search – modifying

the acceptance window. This can be easily performed by changing the Rdh and Rdl limits or

by changing Gdl and Gdh via the logic interface.

In Midspan applications, where power is applied via spare wires, when the PSE fails to

detect a PD, the port remains in high-impedance (Hi-Z) for at least two seconds. If the

signature resistance is greater than 500k

Ω, then the two second wait is avoided.

Transition rates of the port voltage between the two probing levels can be adjusted with

capacitance Cdetslow.

3.2.2

Classification

Once a valid signature is detected, the port is probed for classification in order to perform

smart-power management (if enabled).

Port probing is performed by forcing a DC voltage in the range of 16V to 18 V (one DC

generator multiplexed between the channels) and monitoring current Iclass. The

measurement is repeated and stored in the Channel Monitor Classification registers to

ensure a coherent classification. The PD power class is defined as shown in

Table 4 above.

The detected class is then stored in the Channel Status registers.

Note:

The power absorbed in a link is calculated considering the actual value of the battery

voltage in order to arrive at a true power measurement result.

Table 4.

PD power classification

Class

Usage

Maximum power level at

PSE output (Pall)

Power level at PD input

Iclass

Default

15.4W (programmable)

0.44 to 12.95W

Iclass < Ithcl0

Optional

4W (programmable)

0.44 to 3.84W

Ithcl0 < Iclass < Ithcl1

Optional

7W (programmable)

3.84 to 6.49W

Ithcl1< Iclass < Ithcl2

Optional

15.4W (programmable)

6.49 to 12.95W

Ithcl2 < Iclass < Ithcl3

Optional

Reserved

Ithcl3 < Iclass < Ithcl4

Default

15.4W (programmable)

0.44 to 12.95W

Ithcl4 < Iclass

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