LTC4292 datasheet(10/22 Pages) AD

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Data Sheet

LTC9105

THEORY OF OPERATION

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Rev. A | 10 of 22

OVERVIEW

The LTC9105 is an IEEE 802.3af/at/bt-compliant powered device

(PD) interface controller, allowing up to 90W operation while main-

taining backwards compatibility with existing power sourcing equip-

ment (PSE). The I2C interface on the LTC9105 provides a simple

and standard interface for the read back of the PD interface control-

ler's input state, including PSE allocated power, auxiliary power

supply presence, input voltage, and system current. Further, the

LTC9105 can be configured through I2C. Two LTC9105s, or a single

LTC9105 and an optional auxiliary supply, can provide hot backup

applications with automatic, seamless fail-over. An LTC9105 can

also be configured not to apply power to downstream circuitry when

the attached PSE provides insufficient power for the PD application.

The LTC9105 controls a low RDS(ON) N-channel MOSFET to max-

imize efficiency and delivered power. System current is sensed

across a 50mΩ resistor. Analog Devices also provides a complete

family of PSE, PD, and ideal bridge solutions. See Related Parts

section at the end of this data sheet for more information.

MODES OF OPERATION

Detection Signature

During detection, the PSE looks for a 25kΩ signature resistor, which

identifies the device as a PD. The PSE applies two voltages in the

range of 2.7V to 10.1V and measures the corresponding currents.

Figure 15 shows the detection voltages. The PSE calculates the

signature resistance using a ∆V/∆I measurement technique.

The LTC9105 presents the external signature resistor by connect-

ing the RSIG pin to RTN, allowing the PSE to recognize that the PD

is present and requesting power to be applied. A value of 24.9kΩ is

recommended for the external signature resistor.

IEEE 802.3bt Single-Signature vs Dual-Signature PDs

IEEE 802.3bt defines two PD topologies: single-signature and du-

al-signature. The LTC9105 primarily targets single-signature PD

topologies, eliminating the need for a second PD controller. All

PD descriptions and IEEE 802.3 standard references in this data

sheet are limited in scope to single-signature PDs. The LTC9105

may also be deployed in dual-signature PD applications. For more

information, contact Analog Devices.

Figure 15. Type 3 or 4 PSE, Detection and 1-Event Class Sequence

IEEE 802.3bt Physical Classification, Mark, and Demotion

IEEE 802.3af/at/bt defines physical layer classification to allow a

PD to request a power allocation from the connected PSE and to

allow the PSE to inform the PD of the PSE’s available power. A

PSE, after a successful detection, may apply a classification probe

voltage of 14.5V to 20.5V and measure the PD classification signa-

ture current. Once the PSE applies a classification probe voltage,

the PSE may return the PD voltage to the mark voltage range

before applying another classification probe voltage, or powering up

the PD. An IEEE 802.3bt PSE may apply as many as five events

before powering up the PD.

IEEE 802.3af/at/bt defines nine PD classes and four PD types. The

LTC9105 requested class is configured by connecting RCLSA and

RCLSB to RTN with 1% resistors, as shown in Table 6.

Table 6. IEEE 802.3bt Single-Signature Classification, Power Levels, and

Resistor Selection

PD Re-

quested

Class

PD Re-

quested

Power (W)

PD Type

Nominal

Class Cur-

rent (mA) RCLSA (Ω) RCLSB (Ω)

Type 1

2.5

487

Open

3.84

Type 1 or 3

10.5

97.6

Open

6.49

Type 1 or 3

18.5

54.9

Open

Type 1 or 3

35.7

Open

25.5

Type 2 or 3

24.9

Open

Type 3

40 / 2.5

487

26.1

Type 3

40 / 10.5

97.6

33.2

Type 4

40 / 18.5

54.9

45.3

71.3

Type 4

40 / 28

35.7

78.7

A PD is demoted when the requested power level is not available

from the PSE. If demoted, the PD must operate in a lower power

state. The PSE allocated power is determined by the PD requested

Class and the pse_mark field, as shown in Table 7.

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