LTC4258 datasheet(15/28 Pages) LINER

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LTC4258

4258p

The LTC4258 checks for the signature resistance by

forcing two test currents on the port (via the DETECT

pins) in sequence and measuring the resulting voltages. It

then subtracts the two V-I points to determine the resistive

slope while removing voltage offset caused by any series

diodes or current offset caused by leakage at the port (see

Figure 12). The LTC4258 will typically accept any PD

resistance between 17k and 29k as a valid PD and report

Detect Good (100 binary) in the Detect Status bits (bits 2

through 0) of the corresponding Port Status register.

Values outside this range, including open and short cir-

cuits, are also reported in the Detect Status bits. Refer to

Table 1 for a complete decoding of the Detect Status bits.

The first test point is taken by forcing a test current into

the port, waiting a short time to allow the line to settle and

measuring the resulting voltage. This result is stored and

the second current is applied to the port, allowed to settle

and the voltage measured. Each point takes 100ms to

measure, and an entire detection cycle takes 200ms.

The LTC4258 will not report Detect Good if the PD has

more than 5

µF in parallel with its signature resistor.

The port’s operating mode controls if and when the

LTC4258 runs a detection cycle. In manual mode, the port

will sit idle until a Restart Detection (register 18h) com-

mand is received. It will then run a complete 200ms

detection cycle on the selected port, report the results in

the Detect Status bits in the corresponding Port Status

received. In Semiauto mode, the LTC4258 autonomously

tests valid PDs connected to the ports but it will not apply

power until instructed to do so by the host controller. It

repeatedly queries the port every 320ms and updates the

Detect Status bits at the end of each cycle. If a Detect Good

is reported, it will advance to the classification phase and

report that result in the Port Status register. Until in-

structed to do otherwise, the LTC4258 will continue to

repeat detection on the port. Behavior in Auto mode is

similar to Semiauto; however, after a Detect Good is

reported, the LTC4258 performs the classification phase

and then powers up the port without further intervention.

The signature detection circuitry is disabled when the port

is in Shutdown mode, powered up or the corresponding

Detect Enable bit is cleared.

CLASSIFICATION

A PD has the option of presenting a “classification signa-

ture” to the PSE to indicate how much power it will draw

when powered up. This signature consists of a specific

constant current draw when the PSE port voltage is between

15.5V and 20.5V, with the current level indicating the power

class to which the PD belongs. Per the IEEE 802.3af speci-

fication, the LTC4258 identifies the five classes of PD listed

in Table 2. During classification, the LTC4258 controls and

APPLICATIO S I FOR ATIO

Figure 12. PD Detection

Table 2. IEEE 802.3af Powered Device Classes

IEEE 802.3af

CLASSIFICATION

MAXIMUM

MINIMUM PSE

CLASS

CURRENT AT PSE

PD POWER

OUTPUT POWER

CLASS DESCRIPTION

0mA to 5mA

12.95W

15.4W

PD Does Not Implement Classification, Unknown Power

8mA to 13mA

3.84W

Low Power PD

16mA to 21mA

6.49W

Medium Power PD

25mA to 31mA

12.95W

15.4W

High or Full Power PD

35mA to 45mA

12.95W

15.4W

Reserved, Power as Class O

FIRST

DETECTION

POINT

SECOND

DETECTION

POINT

VALID PD

25k

Ω SLOPE

275

165

0V-2V

OFFSET

VOLTAGE

4258 F12

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