Datasheet

LTC4274
20
4274fd
APPLICATIONS INFORMATION
current reaches the inrush current limit level (typically
450mA), at which point the GATE pin will be servoed to
maintain the specified I
INRUSH
current. During this inrush
period, a timer (t
START
) runs. When output charging is
complete, the port current will fall and the GATE pin will
be allowed to continue rising to fully enhance the MOSFET
and minimize its on-resistance. The final V
GS
is nominally
12V. If the t
START
timer expires before the inrush period
completes, the port will be turned back off and a t
START
fault reported.
Current Limit
The LTC4274 port includes two current limiting 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 voltage of the
main supply (V
EE
), the type of PSE (1 or 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 spec, the LTC4274 will allow the port cur-
rent 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. The t
LIM
timer starts when the
I
LIM
threshold is exceeded and current limit is active. 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. 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.
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 LTC4274 will automatically
set I
LIM
to 425mA (shown in bold in Table 5) during in-
rush at port turn-on, and then switch to the programmed
I
LIM
setting once inrush has completed. To maintain IEEE
compliance, I
LIM
should 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 5. 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 LTC4274 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 MOSFETs,