Linear Technology DC1840A Demo Manual page 7

Demonstration circuit 1682a operation
AUTO and MID Jumpers
The AUTO and MID pins of the LTC4271 are set by
jumpers JP1 and JP2 respectively on the DC1682A
(Figure 10). Setting JP1 to HI enables the AUTO pin mode
in the LTC4270/LTC4271 chipset. J2 provides test points
for access to AUTO and MID.
In AUTO pin mode (JP1 high), the LTC4270/LTC4271
2
chipset internal I C registers default to the AUTO pin high
state after a software or hardware reset, or system power
on. The LTC4270/71 chipset autonomously detects, pow-
ers on and disconnects power to PDs without the need
2
for I C host control.
Setting JP1 to LO disables AUTO pin mode and sets the
LTC4270/LTC4271 chipset to a low current shutdown
2
mode. An I C host controller can then be used to con-
figure the LTC4270/LTC4271 chipset to semi-auto mode
for controlled PSE operation or to manual mode for test
purposes.
Setting JP2 to HI enables the midspan mode detection
backoff timer in the LTC4270/LTC4271 chipset. For end-
point PSEs, set JP2 to LO to disable midspan mode.
For quick PSE evaluation in AUTO pin mode with
MIDSPAN disabled, set JP1 HI and JP2 LO on the DC1682A.
C19
1µF
100V
V
EE
Figure 11. DC1682A, 1 of 12 Ports Outputs. S1B Protection Diodes Located on the DC1680A
AGND
D1
LTC4270
SMAJ58A
V
VSSK SENSEn
EE
C26 D26
0.1µF RSENSEn
B1100
4 × 1.00
DEMO MANUAL DC1840A
S1B Diodes Port Protection
Ethernet ports can be subject to significant ESD events
when long data cables, each potentially charged to thou-
sands of volts, are plugged into the low impedance of the
RJ45 jack. To protect against damage, each port requires
a pair of clamp diodes; one to AGND and one to V
(Figure 11). An additional surge suppressor is required
for each LTC4270 chip from V
the ports steer harmful surges into the supply rails, where
they are absorbed by the surge suppressor and the V
bypass capacitance. The surge suppressor has the addi-
tional benefit of protecting the LTC4270 from transients
on the V supply.
EE
S1B diodes work well as port clamp diodes. The two
S1B diodes per port are a part of the LTC4270/LTC4271
application. These components are not on the DC1682A
due to space constraints but are shown in Figure 11 for
completeness.
In addition to the S1B diodes, a SMAJ58A or equivalent
is recommended for the V
directly across the AGND and V
DC1682A SIDE
GATEn OUTn
Cn
0.22µF
X7R
100V
OUTn
Qn
FDMC3612
V
EE
to AGND. The diodes at
EE
surge suppressor placed
EE
pins on the LTC4270.
EE
34-PIN
CONNECTOR
DC1680A SIDE
S1B
PROTECTION
S1B
OUTn
TO
PORT
S1B
V
EE
DC1840A F11
EE
EE
dc1840afb
7