LTC4274C_15 Datasheet, PDF (22/30 Pages) Linear Technology – Single PoE/PoE/LTPoE PSE Controller

LTC4274A/LTC4274C
APPLICATIONS INFORMATION
Current Limit
The LTC4274A/LTC4274C port includes two current limit-
ing thresholds (I_CUT and I_LIM), each with a corresponding
timer (t_CUT and t_LIM). Setting the I_CUT and I_LIM thresholds
depends on several factors: the class of the PD, the volt-
age of the main supply (V_EE), the type of PSE (Type 1 or
Type 2), the sense resistor (0.5Ω or 0.25Ω), the SOA of
the MOSFET, and whether or not the system is required
to implement class enforcement.
Per the IEEE specification, the LTC4274A/LTC4274C will
allow the port current to exceed I_CUT for a limited period
of time before removing power from the port, whereas it
will actively control the MOSFET gate drive to keep the port
current below I_LIM. The port does not take any action to
limit the current when only the I_CUT threshold is exceeded,
but does start the t_CUT timer. If the current drops below
the I_CUT current threshold before its timer expires, the
t_CUT timer counts back down, but at 1/16 the rate that it
counts up. If the t_CUT timer reaches 60ms (typical) the
port is turned off and the port t_CUT fault is set. This allows
the current limit circuitry to tolerate intermittent overload
signals with duty cycles below about 6%; longer duty cycle
overloads will turn the port off.
The I_LIM current limiting circuit is always enabled and
actively limiting port current. The t_LIM timer is enabled
only when the programmable t_LIM field is non-zero. This
allows t_LIM to be set to a shorter value than t_CUT to provide
more aggressive MOSFET protection and turn off a port
before MOSFET damage can occur. The t_LIM timer starts
when the I_LIM threshold is exceeded. When the t_LIM timer
reaches 1.7ms (typ) times the programmable t_LIM field the
port is turned off and the port t_LIM fault is set. When the
t_LIM field is zero, t_LIM behaviors are tracked by the t_CUT
timer, which counts up during both I_LIM and I_CUT events.
I_CUT is typically set to a lower value than I_LIM to allow the
port to tolerate minor faults without current limiting.
Per the IEEE specification, the LTC4274A/LTC4274C will
automatically set I_LIM to 425mA (shown in bold in Table 6)
during inrush at port turn-on, and then switch to the
programmed I_LIM setting once inrush has completed.
To maintain IEEE compliance, I_LIM should be kept at 425mA
for all Type 1 PDs, and 850mA if a Type 2 PD is detected.
I_LIM is automatically reset to 425mA when a port turns off.
Table 6. Example Current Limit Settings
INTERNAL REGISTER SETTING (hex)
I_LIM (mA)
R_SENSE = 0.5Ω
R_SENSE = 0.25Ω
53
88
106
08
88
159
89
213
80
08
266
8A
319
09
89
372
8B
425
00
80
478
8E
531
92
8A
584
CB
638
10
90
744
D2
9A
850
40
C0
956
4A
CA
1063
50
D0
1169
5A
DA
1275
60
E0
1488
52
49
1700
40
1913
4A
2125
50
2338
5A
2550
60
2975
52
I_LIM Foldback
The LTC4274A/LTC4274C features a two-stage foldback
circuit that reduces the port current if the port voltage falls
below the normal operating voltage. This keeps MOSFET
power dissipation at safe levels for typical 802.3af MOS-
FETs, even at extended 802.3at power levels. Current limit
and foldback behavior are programmable. Table 6 gives
examples of recommended I_LIM register settings.
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