Linear Technology DC1788B Demo Manual page 3

theory of operation
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
valid after PWRGD is active. This indicator is not connected
on the DC1788B-C.
Table 4. Summary of T2P Signals Supported by the DC1788B
Assemblies
ASSEMBLY PSE TYPE
LTPoE ++
DC1788B-A
IEEE 802.3at (Type 2, PoE + )
IEEE 802.3af (Type 1, PoE)
DC1788B-B LTPoE++
IEEE 802.3at (Type 2, PoE + )
IEEE 802.3af (Type 1, PoE)
DC1788B-C Any PSE
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
T2P SIGNAL
840Hz, 50% Duty Cycle
Logic Low
Logic High
Logic Low
Logic Low
**Contact Linear for multi-port PSE demo board options.
Logic High
Not Supported
DEMO MANUAL DC1788B
DC1788 PoE POWER
ASSEMBLY LEVEL
DC1788B-A 90W
70W
52.7W
38.7W
DC1788B-B 25.5W
DC1788B-C 13W
COMPANION PSE DEMO
BOARD
DC1814A-D
DC1814A-C
DC1814A-B
DC1814A-A
DC1567A
DC981A/B
dc1788bfa
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