Datasheet

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1366A
IEEE802.3AF/AT POWER-OVER-ETHERNET POWER SOURCING EQUIPMENT CONTROLLER
LTC4266
5
If the AUTO pin is low then:
■ All ports come up in shutdown mode. The
software must take action in order to power up
any PDs.
■ The current-sense resistors are assumed to be
0.50Ω; the software must change this to
0.25Ω for correct operation of the DC1366A.
(This feature allows the LTC4266 to be a drop-
in replacement for the older LTC4259 with no
software changes.)
■ High power is disabled. If the software were
to simply change the operating mode to AUTO,
a class 4 PD would be treated as class 0, i.e.
limited to 15.4W in accordance with the origi-
nal 802.3af standard.
Endspan vs. Midspan
The LTC4266 can be configured either for endspan
or midspan operation without software intervention
by setting the MID pin high or low respectively.
(You must reset the LTC4266 or cycle the power for
the MID pin to be sensed.) The only difference in
the behavior of the LTC4266 is that the detection
back-off timer is enabled when midspan operation is
selected.
Each port can be configured individually as either
endspan or midspan via I
2
C commands.
The DC1366A board is wired for Alternative-A, MDI-
X (power is injected on the data pairs of the CAT5/6e
cable; positive on pins 3 and 6 of the RJ45 connec-
tor, and negative on pins 1 and 2). The original
802.3af standard required all midspans to use Alter-
native-B, but 802.3at now allows midspans to use
Alternative-A.
Disconnect Sensing
The LTC4266 employs dc disconnect sensing only.
For the sake of software backward compatibility with
the older LTC4259, the LTC4266 includes register
bits for enabling ac disconnect sensing, but these
bits simply enable the dc disconnect sensing.
Pushbutton Switches
The DC1366A board includes several pushbutton
switches to facilitate experimentation with the
LTC4266.
■ The RESET button (S6) resets all ports just as
if the power supplies were cycled.
■ The Masked Shut-Down (MSD) button (S7)
will turn off any ports that have their corre-
sponding mask bit set in the MSD register.
■ Each port has an individual shut down switch
(S2 through S5 for ports 1 through 4 respec-
tively).
Masked Shut Down
The MSD register can be used to pre-assign low-
priority to selected ports so they can be shut down
quickly when needed.
A PSE system design can utilize the MSD feature in
various ways. For example, a PSE system may in-
clude a circuit that monitors the V
EE
supply; if it be-
comes overloaded and the voltage begins to sag, the
system can dump low-priority loads by asserting the
MSD pin. Shedding excess load quickly may allow
the V
EE
voltage to recover before it reaches the UVLO
threshold, thus avoiding shutting down higher-
priority loads.
I
2
C Addressing
The 7-bit I
2
C address of the LTC4266 is
010A
3
A
2
A
1
A
0
b, where A
3
through A
0
are determined
by pins AD3 through AD0 respectively. On the
DC1366A board these pins are controlled by the
quad DIP switch, S1. The LTC4266 has internal