LTC4270/LTC4271
APPLICATIONS INFORMATION
Table 5. Example Current Limit Settings
INTERNAL REGISTER SETTING (hex)
The shaded areas in Table 5 indicate settings that may
require a larger external MOSFET, additional heat sinking,
or setting t Enable.
LIM
I
(mA)
R
SENSE
= 0.5Ω
R
SENSE
= 0.25Ω
LIM
53
88
MOSFET Fault Detection
106
159
213
266
319
372
08
89
80
8A
09
8B
88
LTC4270/LTC4271 PSE ports are designed to tolerate
significant levels of abuse, but in extreme cases it is pos-
sible for the external MOSFET to be damaged. A failed
MOSFET may short source to drain, which will make the
port appear to be on when it should be off; this condition
may also cause the sense resistor to fuse open, turning
off the port but causing the LTC4270 SENSE pin to rise
to an abnormally high voltage. A failed MOSFET may also
short from gate to drain, causing the LTC4270 GATE pin
to rise to an abnormally high voltage. The LTC4270 OUT,
SENSE and GATE pins are designed to tolerate up to 80V
faults without damage.
08
89
425
478
00
8E
92
CB
10
D2
40
4A
50
5A
60
52
80
531
8A
584
638
90
9A
C0
CA
D0
DA
E0
49
40
4A
50
5A
60
52
744
850
If the LTC4270/LTC4271 sees any of these conditions for
more than 180ꢀs, it disables all port functionality, reduces
the gate drive pull-down current for the port and reports
a FET Bad fault. This is typically a permanent fault, but
the host can attempt to recover by resetting the port, or
by resetting the entire chip if a port reset fails to clear the
fault. If the MOSFET is in fact bad, the fault will quickly
return, and the port will disable itself again. The remaining
ports of the LTC4270/LTC4271 are unaffected.
956
1063
1169
1275
1488
1700
1913
2125
2338
2550
2975
An open or missing MOSFET will not trigger a FET Bad
fault, but will cause a t
fault if the LTC4270/LTC4271
START
attempts to turn on the port.
I
Foldback
LIM
Port Current Readback
TheLTC4270/LTC4271featuresatwo-stagefoldbackcircuit
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,
even at extended 802.3at power levels. Current limit and
foldback behavior are programmable on a per-port basis.
The LTC4270/LTC4271 measures the current at each port
with an internal A/D converter. Port data is only valid when
the port power is on and reads zero at all other times. The
converter has two modes:
• 100ms mode: Samples are taken continuously and the
measured value is updated every 100ms
Table 5 gives examples of recommended I
settings.
register
LIM
• 1s mode: Samples are taken continuously; a moving 1
second average is updated every 100ms
The LTC4270/LTC4271 will support current levels well
beyond the maximum values in the 802.3at specification.
42701f
23
Linear [ Linear ]
