Linear Technology DC1788B, Demo Manual

1

dc1788bfa

DEMO MANUAL DC1788B

Description

LT4275 

LTPoE++, IEEE 802.3at/ 

IEEE 802.3af Compliant  

PD Controller

Demonstration circuit 1788B features the 

LT

®

4275

, a fourth 

generation powered device (PD) controller for Power over 

Ethernet (PoE) applications.
The DC1788B is available in DC1788B-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 

R

DS(ON)

 N-channel MOSFET to maximize power 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 

converter that is right for the application. Linear Technology 

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