Circuit Note

CN-0575

analog.com

Rev. 0 | 4 of 7

SINGLE-PAIR POWER OVER ETHERNET

Single-pair power over Ethernet is an Ethernet network implemen-

tation that uses a unique PHY transceiver over a twisted pair of

wires. It provides power over the existing data cabling structure and

is defined to work with the 10BASE-T1L 10 Mbps protocol.

SPoE is an extension of the power over data line (PoDL) standard

originally defined for use in automotive networks. The new standard

adds power classes 10 to 15, intended for use in industrial networks

that have either 24 V or 55 V nominal DC power available. These

power classes that standardize the power sourcing equipment

(PSE) output voltage range and the maximum power that can be

delivered to a PD, as shown in Table 2.

Table 2. SPoE Power Classes

Class

10

11

12

13

14

15

VPSE

(V, min/typ/max)

20/24/30

50/55/58

ICABLE

(mA, max)

92

240

632

231

600

1579

PPD

(W, max)

1.23

3.2

8.4

7.7

20

52

RCABLE

(Ω, max)

65

25

9.5

65

25

9.5

This Ethernet capability builds on traditional power over Ethernet

(PoE) and can be easily installed and maintained. The SPoE

compliant switches and endpoints ensure network additions and

upgrades are simple and fast. Moreover, it reduces system cost,

weight, wiring complexity, and power when compared to traditional

Ethernet cabling.

SPOE POWERED DEVICE CONTROLLER

An LTC9111 SPoE PD controller is used to communicate with a

PSE via the serial communication classification protocol (SCCP)

and negotiate the power allocation for the CN0575. The PD power

class advertised by the LTC9111 is determined by the logic levels

of its CLASSV and CLASSC pins, which can be configured to three

different states (shorted to GND, shorted to STBY voltage, or left

floating).

The CN0575 is designed to operate as either a Class 12 or Class

13 PD, with the class selection being implemented using jumpers

connected to the configuration pins.

INPUT POLARITY CORRECTION

IEEE 802.3cg requires polarity insensitive PDs to facilitate fool-

proof field installation with screw terminals. The LTC9111 drives

external MOSFETs (M2 and M3 in Figure 5) for polarity correction

on the low-side and relies on external Schottky diodes (DIN1 and

DIN2) to rectify the high-side voltage. Active low-side rectification

minimizes power losses and also helps overcome circuit start-up

challenges posed by the constraints of the standard. The drive

voltage at the LGATE1 and LGATE2 pins is compatible with

modern standard-level power MOSFETs. These MOSFETs do not

experience high voltage and high current simultaneously, so safe

operating area (SOA) performance is not critical.

Figure 5. LTC9111 SPoE PD Controller Circuit

External MOSFET Switch and Inrush Control

The external N-channel MOSFET switch (M1) functions as an

inrush controller and load switch, isolating the bulk capacitance

from the connector during classification and ramp up. Once the

operating voltage exceeds the device threshold for the selected

power class (17.5 V Typ. for Class 12; 46 V Typ. for Class 13), an

internal 10 µA (14 µA maximum) charge pump is enabled after a

100 ms delay. M1 then acts as a source follower, and CBULK voltage

linearly increases until M1 is fully enhanced.

Use Equation 1 to ensure that the inrush current does not exceed

the maximum PD current for the corresponding class.

CGATE≫2×CBULK×IGATEUPMAX

ICLASSMAX

(1)

where:

CGATE is the capacitance between RGATE and GND.

CBULK is the capacitance between OUT and GND.

IGATEUP(MAX) is the maximum GATE pull-up current of the LTC9111.

ICLASS(MAX) is the maximum PD current for the selected power

class.

During inrush, there can be significant transient power dissipation

in the external MOSFET; SOA capability must be considered in

selecting the pass device. Any load present at the OUT pin before

inrush is complete also contribute to the total PD current observed

at the connector; this may necessitate slower inrush (that is, larger

CGATE). The MOSFET used for this switch in CN0575 is able to

withstand the maximum current drain current of 11.5 A of the

selected power class, as well as the transient power dissipation of

60 V, 1 A for at least 100 µs.

A resistor, RGATE, in series with CGATE, is recommended for robust-

ness. As an example, a fault which shorts the OUT pin to ground

instantaneously while power is applied will pull the source of the

external MOSFET to ground while CGATE maintains high voltage at

the GATE pin, causing stress on the gate of the external MOSFET.

A resistor value of RGATE = 10 k will effectively decouple the gate of

the MOSFET from the capacitor in such a scenario.

The EN logic output is used to ensure that the downstream switch-

ing regulator (or other application circuitry) does not draw significant

current until inrush is complete. The EN pin is driven high once

GATE exceeds IN by 6.5 V, at which point M1 is fully enhanced and

full load current can be drawn.