Linear Technology DC1788B Guida rapida alla configurazione

dc1788bfa

DEMO MANUAL DC1788B

Description

LT4275

LTPoE++, IEEE 802.3at/

IEEE 802.3af Compliant

PD Controller

Demonstrationcircuit1788BfeaturestheLT

4275,afourth

generation powered device (PD) controller for Power over

Ethernet (PoE) applications.

TheDC1788B isavailable inDC1788B-A, DC1788B-B,and

DC1788B-C versions to meet the power level required by

the PD application. The DC1788B-A features the LT4275A

PD controller. This controller supports the IEEE 802.3at

(Type 2, PoE+), IEEE 802.3af (Type 1, PoE) and LTPoE++™

specification. LTPoE++ adds four power levels to the ex-

isting IEEE standard with 38.7W, 52.7W, 70W, and 90W

of delivered PD power at the RJ45 jack. The DC1788B-B

features the LT4275B PD controller and is compliant with

the IEEE 802.3at and IEEE 802.3af specifications. The

DC1788B-C features the LT4275C PD controller and is

compliant with the IEEE 802.3af specification.

The DC1788B includes an RJ45 Ethernet port, a high

power Ethernet magnetic, a discrete active bridge recti-

fier, a surge protector, an external N-channel FET, and the

LT4275. The discrete active bridge rectifier is used to

achieve higher efficiency than a conventional diode bridge

rectifier. Many of the main features from the previous gen-

eration PD controller are included in this generation PD

L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and

LTPoE++ and Hot Swap are trademarks of Linear Technology Corporation. All other trademarks

are the property of their respective owners.

controller. These include a power good indicator, a power

sourcing equipment (PSE) type indicator, and support

for an auxiliary power input. The major difference from

the previous generation PD controller is that the LT4275

drives an external N-channel Hot Swap™ FET at the PoE

high side voltage rail. This allows the user to choose a low

RDS(ON) N-channelMOSFETto maximizepower efficiency,

reduce heat dissipation, and ease thermal design. An LED

status indicator is included to indicate the Hot Swap FET

is fully turned on and the PSE is powering the PD. A suf-

ficient load to sink more than 10mA is also included to

assure the PSE maintains power to the PD and to meet

the DC maintain power signature current required by the

IEEE 802.3at/IEEE 802.3af specification.

Simply connect the output of the DC1788B to the DC/DC

converterthatisrightfortheapplication.LinearTechnology

offers a variety of DC/DC converter solutions that can be

used with the DC1788B (eg DC894, DC1317, etc).

Design files for this circuit board are available at

http://www.linear.com/demo

Table 1. Summary of Features Supported by the DC1788B Assemblies

ASSEMBLY PoE STANDARD

MAXIMUM POWER

LEVEL

POWER GOOD

INDICATOR (PWRGD)

PSE TYPE INDICATOR

(T2P)

AUXILIARY SUPPLY

SUPPORT

DC1788B-A LTPoE++

PoE+

PoE

90W Yes Yes No

DC1788B-B PoE+

PoE 25.5W Yes Yes Yes

DC1788B-C PoE 13W Yes No Yes

dc1788bfa

DEMO MANUAL DC1788B

Table 2. DC1788B Performance Summary

PARAMETER CONDITION VALUE

PD Input Voltage After Start-Up (VPORT) At the PD Ethernet Port

LTPoE++ 38.7W 49.8V to 57V

LTPoE++ 52.7W 47.8V to 57V

LTPoE++ 70W 45.1V to 57V

LTPoE++ 90W 41.0V to 57V

IEEE 802.3at (Type 2, 25.5W) 42.5V to 57V

IEEE 802.3af (Type 1, 13W) 37V to 57V

Efficiency DC1788B-A, VPORT = 48V, ILOAD = 1.1A 99.1% (Typical)

DC1788B-B, VPORT = 48V, ILOAD = 600mA 98.9% (Typical)

DC1788B-C, VPORT = 48V, ILOAD = 350mA 99.1% (Typical)

PoE Type Switching Frequency An LTPoE++ PSE is Powering a DC1788B-A 840Hz

performance summary

theory of operation

WhenanLTPoE++PSEis connected tothe DC1788Bdemo

board RJ45 connector, J1, via a CAT5e or CAT6 Ethernet

cable, the PSE initiates the detection process. During this

process, the discrete active bridge acts as a standard

diode bridge and the PD solution presents a 25k resistive

load to the PSE. After the PD has passed the detection

process, the PSE uses an IEEE 802.3at/IEEE 802.3af or

LTPoE++ classification method to determine the power

level the PSE can deliver to the PD. The PSE turns on the

port and provides power to the PD if the classification is

successful. During turn on, the MOSFETs of the discrete

active bridge are fully enhanced with low RDS(ON) in the

highcurrentpath.Thisreducesthepowerandheatdissipa-

tion compared to the conventional diode bridge. Once the

LT4275 detects the PSE has turned on the port, it drives

an external N-channel MOSFET to ramp up the output

voltage. After a successful PD controller output turn on,

the LT4275 drives a power good indicator (PWRGD) and

a PSE Type indicator (T2P) (DC1788B-A and DC1788B-B

only).The PWRGDindicator signals the loadto startdraw-

ing current and the T2P indicator signals the load the type

of PSE powering the PD.

Classification Signaling

The resistors at RCLS and RCLS++ determine the clas-

sification signature and sequence. The DC1788B-A in-

cludes user-selectable jumpers to support IEEE 802.3af,

IEEE 802.3at, and LTPoE++ power levels from 3.84W to

90W. Refer to Table 3 for the power levels. If an LTPoE++

power level is selected, an IEEE 802.3af or IEEE 802.3at

compliant PSE will classify this PD as a Class 4 PD. The

DC1788B-BincludesapreselectedRCLS resistortosupport

IEEE802.3at2-eventclassificationandthisboarddoesnot

contain jumper blocks. The DC1788B-C includes multiple

jumpers to support all IEEE 802.3af power levels. Table 3

showseachshuntpositionanditsassociatedclassnumber

and power level for the DC1788B-A and -C boards. Use

Table3 as a selection guide to choose a suitable power

level for the load.

Table 3. DC1788B-A and DC1788B-C Shunt Positions for PoE

Power Levels

ASSEMBLY

PoE

CLASS

POWER LEVEL AT

THE PD INPUT RCLASS JUMPERS

DC1788B-A & -C 0 13W JP1 JP2

DC1788B-A & -C 1 3.84W JP3 JP4

DC1788B-A & -C 2 6.49W JP5 JP6

DC1788B-A & -C 3 13W JP7 JP8

DC1788B-A 4 25.5W JP9 JP10

DC1788B-A 4* 38.7W JP11 JP12

DC1788B-A 4* 52.7W JP13 JP14

DC1788B-A 4* 70W JP15 JP16

DC1788B-A 4* 90W JP17 JP18

*An LTPoE++ PD will be classified as Class 4 by an IEEE 802.3at/af

compliant PSE.

dc1788bfa

DEMO MANUAL DC1788B

Power Good Indicator and Power Supply Start-Up

The LT4275 limits the inrush current to the output bulk

capacitor by controlling the output voltage slew rate

during turn-on. The slew rate is preprogrammed via a

resistor and a capacitor at the LT4275 HSGATE pin on the

DC1788B to limit the inrush current to the bulk capacitor

on board. If more capacitance is desired at the output,

refer to Inrush and Powered On section in the LT4275

data sheet to recalculate the inrush current and ensure it

is below the IEEE requirement of approximately 100mA.

The inrush current limit will cause startup problems if an

attached load draws more than 100mA of current during

inrush. Therefore, it is strongly recommended to use the

PWRGD indicator on the DC1788B to interface to a load

such as a DC/DC converter as shown in Figures 1, 2 and 4.

PSE Type Indicator

Refer to Table 4 for the summary of T2P indicator signals

supported by the DC1788B assembly. The T2P signal is

validafterPWRGDisactive.Thisindicatorisnotconnected

on the DC1788B-C.

Table 4. Summary of T2P Signals Supported by the DC1788B

Assemblies

ASSEMBLY PSE TYPE T2P SIGNAL

DC1788B-A LTPoE++ 840Hz, 50% Duty Cycle

IEEE 802.3at (Type 2, PoE+) Logic Low

IEEE 802.3af (Type 1, PoE) Logic High

DC1788B-B LTPoE++ Logic Low

IEEE 802.3at (Type 2, PoE+) Logic Low

IEEE 802.3af (Type 1, PoE) Logic High

DC1788B-C Any PSE Not Supported

theory of operation

Auxiliary Supply Support

The DC1788B-B and DC1788B-C versions support an

auxiliary supply input. The auxiliary supply input has pri-

ority over the PoE input. When the PD controller detects

a valid voltage from the auxiliary supply input, it turns off

the N-channel MOSFET (Q2) to cut the power from the

PSE and lets the auxiliary supply take over powering the

load. The PSE may detect the disconnected PD and turn

off the port. The PWRGD indicator outputs a logic high

signal when an auxiliary supply is present. The T2P indi-

cator outputs a logic low signal on the DC1788B-B board

when an auxiliary supply is present. The auxiliary supply

input is an assembly option on the DC1788B-A board.

DC1788B Companion PSE Demo Boards

Linear Technology offers a variety of PSE solutions to

evaluate with DC1788B. Refer to Table 5 to select a PSE

demo board based on the DC1788B assembly and the

application power requirement.

Table 5. Selection of Companion PSE Demo Board depending

on the DC1788 Assembly and the PoE Power Level

DC1788

ASSEMBLY

PoE POWER

LEVEL

COMPANION PSE DEMO

BOARD

DC1788B-A 90W DC1814A-D

70W DC1814A-C

52.7W DC1814A-B

38.7W DC1814A-A

DC1788B-B 25.5W DC1567A

DC1788B-C 13W DC981A/B

**Contact Linear for multi-port PSE demo board options.

dc1788bfa

DEMO MANUAL DC1788B

Dc1788B-a quick start proceDure

NOTE: Handle the DC1788B-A by the edge of the board.

Power over Ethernet Input

1. Refer to Figure 1 to evaluate the DC1788B-A with a

DC/DCconverter.Ifaresistiveloadisusedtoevaluatethe

DC1788B-A, connect the load by hand between VOUT+

and VOUT– after the LED (D3) is lit. It is recommended

to disconnect the resistive load before powering down

theDC1788B-A,soatthenextturnontheloadisdiscon-

nected. If the load is not connected and disconnected

by hand properly, the PD may not power on. Contact

field applications engineers for further details.

2. Default class select shunt positions are at JP1 and JP2

onthe DC1788B-Aboard.Inthisconfiguration,anyPSE

will turn on this PD. Choose a power level from Table3

and select the corresponding shunt positions.

3. Check the power delivery capability of the LTPoE++

PSE to ensure it can power the PD and the load. Do

not select a jumper position corresponding to a higher

power level than the LTPoE++ PSE can provide. Oth-

erwise, the LTPoE++ PSE will not turn on the PD after

classification.

4. Connect the output of the PSE to the RJ45 connector

(J1) on the DC1788B-A board with a CAT5e or CAT6

Ethernet cable.

5. After connection has been established, verify that the

LED (D3) is lit. This indicates the PSE has successfully

detected and powered the PD

Figure 1. Setup Diagram for the DC1788B-A with a DC/DC Converter, a Microprocessor, and an LTPoE++ PSE

dc1788bfa

DEMO MANUAL DC1788B

Dc1788B-B quick start proceDure

NOTE: Handle the DC1788B-B by the edge of the board.

Power over Ethernet Input

1. Refer to Figure 2 to evaluate the DC1788B-B with a

DC/DCconverter.Ifaresistiveloadisusedtoevaluatethe

DC1788B-B, connect the load by hand between VOUT+

and VOUT– after the LED (D3) is lit. It is recommended

to disconnect the resistive load before powering down

theDC1788B-B,soatthenextturnontheloadisdiscon-

nected. If the load is not connected and disconnected

by hand properly, the PD may not power on. Contact

field applications engineers for further details.

2. Connect the output of the PSE to the RJ45 connector

(J1) on the DC1788B-B board with a CAT5e or CAT6

Ethernet cable.

3. After connection has been established, verify that the

LED (D3) is lit. This indicates the PSE has successfully

detected and powered the PD.

Auxiliary Supply Input

1. Refer to Figure 3 to evaluate the DC1788B-B with a

DC/DC converter and an auxiliary DC power supply.

2. Connect the auxiliary supply to the AUX+ to AUX– in-

puts. Check to make sure the voltage polarity is correct

before turning on the auxiliary power supply.

3. Turn on the auxiliary power supply and verify that the

LED (D3) is lit.

Figure 2. Setup Diagram for the DC1788B-B with a DC/DC Converter, a Microprocessor, and a Type 2 PSE

dc1788bfa

DEMO MANUAL DC1788B

Dc1788B-B quick start proceDure

Figure 3. Setup Diagram for the DC1788B-B with a DC/DC Converter, a Microprocessor, and an Auxiliary DC Power Supply

dc1788bfa

DEMO MANUAL DC1788B

NOTE: Handle the DC1788B-C by the edge of the board.

Power over Ethernet Input

1. Refer to Figure 4 to evaluate the DC1788B-C with a

DC/DCconverter.Ifaresistiveloadisusedtoevaluatethe

DC1788B-C, connect the load by hand between VOUT+

and VOUT– after the LED (D3) is lit. It is recommended

to disconnect the resistive load before powering down

theDC1788B-C,soatthenextturnontheloadisdiscon-

nected. If the load is not connected and disconnected

by hand properly, the PD may not power on. Contact

field applications engineers for further details.

2. Connect the output of the PSE to the RJ45 connector

(J1) on the DC1788B-C board with a CAT5e or CAT6

Ethernet cable.

3. After connection has been established, verify that the

LED (D3) is lit. This indicates the PSE has successfully

detected and powered the PD.

Auxiliary Supply Input

1. Refer to Figure 5 to evaluate the DC1788B-C with a

DC/DC converter and an auxiliary DC power supply.

2. Connect the auxiliary supply to the AUX+ to AUX– in-

puts. Check to make sure the voltage polarity is correct

before turning on the auxiliary power supply.

3. Turn on the auxiliary power supply and verify that the

LED (D3) is lit.

Figure 4. Setup Diagram for the DC1788B-C with a DC/DC Converter and a Type 1 PSE

Dc1788B-c quick start proceDure

dc1788bfa

DEMO MANUAL DC1788B

Dc1788B-c quick start proceDure

Figure 5. Setup Diagram for the DC1788B-C with a DC/DC Converter and an Auxiliary DC Power Supply

dc1788bfa

DEMO MANUAL DC1788B

quick start proceDure

Figure 6. DC1788B-A Efficiency at Various PoE Load Currents (without LED D3)

Figure 7. Thermal Image. DC1788B-A with 90W Load. Top View

dc1788bfa

DEMO MANUAL DC1788B

quick start proceDure

Figure 8. Thermal Image. DC1788B-A with 90W Load. Bottom View

Indice

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