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Data Sheet

LTC9101-2A/LTC9102

APPLICATIONS INFORMATION

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Rev. 0 | 20 of 34

DEVICE CONFIGURATION

An LTC9101-2A can control between one and four LTC9102. Each

LTC9102 controls 12 ports. Thus, each LTC9101-2A can control up

to 48 ports.

As described later in Bus Addressing, each group of four ports

occupies a single I2C address.

OPERATING MODES

The LTC9101-2A/LTC9102 controls up to 48 independent ports,

each of which can operate in one of two modes: manual or semi-

auto. A third mode, shutdown, disables the port (see Table 8).

Table 8. Operating Modes

MODE

PORT

MODE

DETECT/CLASS

POWER-UP

Semi-Auto

10b

Host Enabled

Upon Request

Manual

01b

Once Upon Request

Upon Request

Shutdown

00b

Disabled

In manual mode, the port waits for instructions from the host system

before taking any action. When commanded by the host, the port

runs a single detection or classification cycle and reports the result

in its "Port Status" register. The host may command the port to

immediately apply or remove power with no requirement for valid

detection or classification results. Manual mode is not compliant

with IEEE specifications.

In semi-auto mode, the port repeatedly attempts to detect and

classify any PD attached to it. The port reports the status of these

attempts back to the host. When commanded to apply power, the

port verifies the presence of a compliant PD and applies power if a

compliant device is connected. The host must enable detection and

classification.

In shutdown mode, the port is disabled and does not detect or

power a PD.

Regardless of which mode it is in, the LTC9101-2A/LTC9102 re-

moves power automatically from any port that generates a fault.

It also automatically removes power from any port that generates

a disconnect event if disconnect detection is enabled. The host

controller may also command the port to remove power at any time.

DETECTION

Detection Overview

To avoid damaging network devices not designed to tolerate DC

voltage, a PSE must determine whether the connected device is

a valid PD before applying power. The IEEE 802.3 specification

requires that a valid PD has a common-mode resistance of 25k

±5% at any port voltage below 10V. The PSE must accept resistan-

ces that fall between 19k and 26.5k, and it must reject resistances

above 33k or below 15k (shaded regions in Figure 36). The PSE

may choose to accept or reject resistances in the undefined areas

between the must-accept and must-reject ranges. In particular, the

PSE must reject standard computer network interface cards (NICs),

many of which have 150Ω common-mode termination resistors that

are damaged if power is applied to them (the black region at the left

of Figure 36).

Figure 36. IEEE 802.3 Signature Resistance Ranges

Multipoint Detection

The LTC9101-2A/LTC9102 uses a multipoint method to detect PDs.

False-positive detections are rejected by checking for signature

resistance with both forced current and forced voltage measure-

ments.

Initially, two test currents are forced onto the port (through

the OUTn pin) and the resulting voltages are measured. The

LTC9101-2A calculates the difference between the two V-I points

to determine the resistive slope while removing offset caused

by series diodes or leakage at the port (see Figure 37). If the

forced current detection yields a valid signature resistance, two test

voltages are then forced onto the port and the resulting currents

are measured and subtracted. Both methods must report valid

resistances to report a valid detection. PD signature resistances

between 17k and 29k (typical) are accepted as valid and reported

as "Detect Valid" in the corresponding "Detection Status" register.

Values outside this range, including open and short circuits, are

also reported. If the port measures less than 1V during any forced

current test, the detection cycle aborts and "Short Circuit" is report-

ed. Table 9 shows the possible detection results.

Figure 37. PD Detection

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