Datasheet

LTC4274A/LTC4274C
22
4274acfc
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 (0.5 or 0.25), 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
I
LIM
(mA)
INTERNAL REGISTER SETTING (hex)
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.